Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic

Review Cite This: Chem. Rev. 2018, 118, 3447−3507

pubs.acs.org/CR

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells Guangye Zhang,†,‡,∥ Jingbo Zhao,†,∥ Philip C. Y. Chow,†,‡,∥ Kui Jiang,†,‡,∥ Jianquan Zhang,†,‡ Zonglong Zhu,† Jie Zhang,§ Fei Huang,§ and He Yan*,†,‡,§ †

Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration & Reconstruction, Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China ‡ HKUST-Shenzhen Research Institute, No. 9 Yuexing first RD, Hi-tech Park, Nanshan, Shenzhen 518057, China § Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China ABSTRACT: The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure−property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

CONTENTS 1. Introduction 2. Brief Introduction to OPV Working Principle 2.1. Light Absorption and Exciton Generation 2.2. Charge-Transfer State and Free Ccarrier Generation 2.3. Charge Transport 3. Molecular Design Principle 4. Rylene Diimide-Based NFAS 4.1. PDI-Based NFAs 4.1.1. PDI Monomers: Requirement for Twisting 4.1.2. Twisting Strategy-PDI Dimers 4.1.3. Twisting Strategy-PDI Trimers and Tetramers 4.1.4. Discussion: How Much Twisting Is Needed? 4.1.5. Fused-Ring PDI 4.1.6. α-Substituted PDI 4.2. Other Rylene-Based NFAs 4.3. Summary and Challenges

© 2018 American Chemical Society

5. A-D-A Type Acceptors 5.1. Early Reports of A-D-A Type Acceptors 5.2. A-D-A Type Acceptors with a Core Unit of IDT or Its Derivatives 5.2.1. Design of the Core Units 5.2.2. Side Chain Effects 5.2.3. Effect of the Spacer Unit 5.2.4. Design of the “A” Unit 5.3. A-D-A Type Acceptors with Other Core Units 5.4. Summary 6. Other NFAS 6.1. DPP-Based NFAs 6.2. Others 6.2.1. Fused-Ring Aromatics 6.2.2. Linear Oligomers 6.2.3. Twisted Dimers 6.2.4. Three Dimensional Structures 7. Comparison among Different Core Units 8. Nonfullerene All Small Molecule OPV

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Received: September 5, 2017 Published: March 20, 2018 3447

DOI: 10.1021/acs.chemrev.7b00535 Chem. Rev. 2018, 118, 3447−3507

Chemical Reviews 9. Ternary Blend Nonfullerene OPV 9.1. Motivations to Use Ternary Blend 9.2. Benefits of NFAs in Ternary Blend OPVs 9.3. Utilizing NFAs in Ternary Blend OPVs 9.3.1. Pair of Miscible NFAs in Ternary Blend 9.3.2. Blending NFA with a Pair of Polymer Donors 9.3.3. Combining NFA and Fullerene in a Ternary Blend 10. Enhanced Device Stability of Nonfullerene OPV 10.1. Enhanced Stability of BHJ Solar Cells with IDT Based A-D-A Type NFAs 10.2. Enhanced Stability of BHJ Solar Cells with PDI or DPP Based NFAs 10.3. Enhanced Stability of Ternary Blend Nonfullerene Solar Cells 10.4. Semitransparent Nonfullerene Solar Cell with Enhanced Stability 10.5. Summary 11. Reducing Voltage Loss 12. Challenges and Perspectives 12.1. Perspectives on Molecular Design Strategy and Synthetic Complexity 12.1.1. Molecular Design to Enhance Light Absorption 12.1.2. Molecular Design to Balance Morphology and Charge Transport 12.1.3. Synthetic Complexity 12.2. Studying Charge Transport 12.3. Understanding Morphology of Nonfullerene Active Layers 12.3.1. Quantitative Morphology-Performance Correlations 12.3.2. Molecular Miscibility 12.3.3. Solvent Additive Effect 12.3.4. Vertical Phase Segregation 12.4. Multijunction NFA OPVs 12.5. Device Structure Engineering and New Interfacial Layers 12.6. Thick Active Layer, Large-Area Devices, Environmentally Friendly Processing, and Roll-to-Roll Printing 13. Concluding Remarks Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments References

Review

fabricated using high-throughput processes at low temperatures, making them considered as a promising candidate for the next generation PV technologies. Most OPV devices are fabricated using the blend of a conjugated polymer and a fullerene derivative as the active components, 7−9 with the maximum power conversion efficiency (PCE) over 11%.10,11 The majority of these achievements are enabled by the design of new polymer donors. In contrast, the archetypal fullerene derivative, PC61BM,12 and its C70 analog, PC71BM,13 have been dominantly used as the electron accepting materials ever since the application of the former in 1995.7 Electronically, fullerenes have a proficient electron withdrawing ability14 and a high electron mobility.15 The physical shape of fullerenes has also offered them three-dimensional electron transport property and the capability of forming favorable blend morphology that balances charge generation and transport.16 Despite these beneficial properties, fullerenes possess considerable limitations. First, fullerenes do not absorb strongly in the visible region of the solar spectrum. Meanwhile, chemical modification of their backbone is not straightforward, rendering a low structural flexibility and an elevated difficulty in tuning the electronic/optical properties. This not only increases the synthetic complexity but also makes fullerenes less likely to gain complementary light harvesting to the polymers. When made into devices, fullerenes have been shown to have poor photostability in air in both pristine and blend films.17 The demand for replacing fullerenes in OPVs has led to the rapid development of nonfullerene acceptors (NFAs), including both polymeric and small molecular organic acceptors. In the past few years, dramatic progress has been made in the field of both branches. If not specified, NFAs throughout this review refer to the small molecular organic nonfullerene electron accepting materials. Unlike fullerenes,18 NFAs have excellent synthetic flexibility with more readily available source materials, which affords easily tunable optical/electronic properties and improved solubility. In conjunction with their high absorption coefficients, NFAs can be easily tailored to work with novel polymers in terms of both optical complementarity and energetic compatibility, allowing the solar cell to attain broad solar spectrum coverage. In addition, the low-cost cores, facile synthesis and simplified purification can significantly reduce the production cost of NFAs. As a result of these advantageous properties, huge efforts have been devoted to designing novel NFAs and applying them in organic solar cells. Particularly, dramatic progress has been made in the NFA-based OPV field with over 400 publications in the past four years (Figure 1). Over 10% PCEs have been reported for organic solar cells based on different combinations of polymer and NFA, with the best PCE up to 13.1% for singlejunction device19 and 13.8% for tandem structure.20 Besides the high and fast-growing PCE (Figure 1), OPV devices based on NFAs manifest other distinct features that have not been widely recognized in fullerene based OPVs. For example, the fraction of publications on NFA-based OPVs that mention a reduction in voltage loss is much higher than that on fullerene-based OPVs. 21−23 Evidence suggests that the commonly believed energetic offset for generating free charge carriers in OPVs may not be a constraint in NFA-based systems.23,24 Such low voltage loss could significantly improve the overall performance of OPVs. Besides low voltage loss, enhanced device stability has also been reported for NFA solar cells.19,25−37

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1. INTRODUCTION The growing demand for renewable energy sources has caused the rapid development in photovoltaic (PV) technologies. Solar energy is the largest carbon-neutral energy source available today1 and is the fastest-growing form of renewable energy.2 Among various PV technologies, organic photovoltaics (OPVs), whose building blocks are based on earth abundant nontoxic materials, have demonstrated a short energy payback time (EPBT)3,4 and a great potential to reduce the levelized cost of energy (LCOE).5,6 In addition, OPVs are flexible and can be 3448


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goal of sections 4−6 is to compare the effects of different molecular design strategies on device performance, to generate insights from previous work, and to outline directions for further improvement, which are captured by a discussion section (section 7). With sections 4−7 focusing on the polymer:NFA based devices, section 8 summarizes the limited but promising result on all small-molecule nonfullerene solar cells, i.e., BHJ solar cells based on a small molecule donor and a nonfullerene small molecule acceptor. In section 9, we extend our discussion beyond binary single-junction BHJ and summarize the role of NFA in ternary blend OPVs. As the maximum reported PCE reached above 13%, OPV device’s operational stability becomes increasingly vital for the objective of commercialization. Therefore, section 10 summarizes the results on the improved stability of NFA-based binary or ternary BHJ solar cells with the topic semitransparent OPV also covered. Section 11 delineates the voltage loss in OPV and highlights the potential of using NFA to achieve a low voltage loss system. Section 12 provides perspectives on molecular design and synthetic complexity of NFAs and highlights the problems and challenges facing the field, mainly from the morphological and device physics points of view, where topics such as energy loss, active layer morphology, multijunction device, interface engineering, and roll-to-roll printing are discussed, with perspectives and outlook provided. Concluding remarks are given in section 13.

Figure 1. Nonfullerene organic solar cell publications each year (via Web of Science) and the maximum power conversion efficiency reported each year up to Aug 2017.

Despite these exciting results, problems and challenges remain for this young field. For instance, molecular twisting is introduced to prevent the molecules from overaggregation, but it presumably hinders charge transport.23,35,38−43 This paradox is prominent in NFAs as they are not as diffusive as fullerenes and they do not have the three-dimensional charge transport capability in general. A thorough understanding of both intramolecular and intermolecular interactions for NFAs in the pristine form as well as in the blend film is needed. Besides, there lack systematic morphological studies on the general morphology forming process and device physics studies on exciton dissociation at the polymer/NFA interface, which are key issues to be addressed for further advancement of photovoltaic performance. By the means of summarizing material property and photovoltaic performance, this review aims to provide a balanced assessment of the state-of-the-art molecular design strategies, to facilitate discussions on how to better understand the structure−property relationship for further optimizing active layer morphology, to highlight the fundamental material/device properties that lack understanding, and to delineate major challenges facing the NFA-based OPV field. The emphasis of this review is the solution-processed BHJ organic solar cell application of nonfullerene molecular electron accepting materials. Consequently, polymeric electron acceptors44−53 and the design of novel polymer donors to be compatible with NFAs are beyond the scope of this review. Similarly, the application of NFAs in other organic electronics, such as organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), and organic rectifiers, are not covered in this review. This review begins with a brief introduction to OPV working principle (section 2), where light absorption, exciton dissociation, charge generation, and charge transport are covered and the concept of bulk-heterojunction is discussed. As a consequence of the limited exciton diffusion length, BHJ solar cells with specific morphology requirement are necessary for realizing the full potential of the materials. In section 3, an overview of the molecular design strategy is provided. Then we categorize NFAs based upon structural similarities and provide detailed material/device property summaries for each category (sections 4−6), with a particular focus on recent materials representing top-of-the-line performance in OPV devices. The

2. BRIEF INTRODUCTION TO OPV WORKING PRINCIPLE 2.1. Light Absorption and Exciton Generation

In order to harvest sunlight for the generation of electrical energy, a solar cell has to convert photons into free charge carriers and be able to transport these carriers to the electrodes to give photovoltage and photocurrent. Molecular electronic materials such as π-conjugated polymers are ideal solar lightharvesting systems due to their (1) large extinction coefficients as a result of the large wavefunction overlap between the electronic ground state and the lowest excited state54,55 and (2) broad absorption bands as a result of the significant geometry relaxations that take place in the excited state.55−57 The intense absorption bands over a broad wavelength range enable a good matching with a sizable portion of the solar spectrum for efficient light harvesting in relatively thin layers (∼100−200 nm). In a neutral organic molecule, the highest occupied molecular orbital (HOMO) in the ground state configuration contains two electrons with opposite spins. The absorption of an incident photon promotes an electron in the HOMO into the lowest unoccupied molecular orbital (LUMO), leaving behind a hole to form an electron−hole pair (Figure 2). In an ideal photovoltaic system, the electron−hole pair can easily separate to form long-lived, free charge carriers with high quantum yields, such that a large photocurrent is created when they are collected by the electrodes. However, in practice such efficient free carrier generation requires the electron−hole pair to overcome their mutual Coulomb attraction, V, which is given by V=

e2 4πε0εrr

(1)

where e is the electronic charge, ε0 is the permittivity of vacuum, εr is the dielectric constant of the surrounding 3449


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the material by random diffusion. This incoherent hopping process is typically driven by both Förster resonance energy transfer (FRET) and Dexter energy transfer and is unaffected by electric fields due to the electrical neutrality of excitons. This energy transfer process occurs either intramolecularly or intermolecularly following a downhill energy gradient and may lead to trapping of the exciton at trap or defect sites that leads to inhomogeneous broadening of the density of states. Further migration of the exciton energy relies on thermal fluctuation. Organic films typically have short singlet exciton lifetimes before they decay to the ground state, and thus they have relatively short diffusion lengths (in the range of ∼5−20 nm). Therefore, a compromise with regards to the film thickness is required to optimize both exciton dissociation (favoring thin layers) and light absorption (favoring thick layers). A key breakthrough that overcomes the limitation of short exciton diffusion lengths of organic materials was the introduction of the bulk heterojunction.7,64 Bulk heterojunctions comprise a bicontinuous interpenetrating network of the donor and acceptor material, which can be formed by solution processing or by coevaporation of the two materials. Due to the large interfacial area between donor and acceptor, the distance that the excitons have to travel before reaching an interface is significantly decreased, and efficient harvesting of exciton energies can therefore be achieved when the nanoscale phase segregation between the two materials matches with the corresponding exciton diffusion lengths. For planar heterojunctions, exciton harvesting can also be improved by means of enhancing exciton diffusion lengths and/or using Förster resonance energy transfer (FRET) in multilayer structures.65,66 Nevertheless, we will focus on bulk heterojunction device structures in this review.

Figure 2. Electronic state diagram describing the photoinduced charge-carrier formation mechanism in an organic solar cell. Adapted with permission from ref 55. Copyright (2009) American Chemical Society.

medium, and r is the distance between the electron−hole pair. In solar cells based on inorganic semiconductors, such as silicon p−n junctions, electron−hole pairs can easily overcome their Coulomb attraction to generate free charges due to the high dielectric constant (εr ≈ 12 in silicon) and the highly delocalized nature of the photoexcited states (corresponds to a large r in eq 1).56,58,59 However, overcoming this Coulomb attraction between electron−hole pairs is much more challenging in organic materials due to the low dielectric constant (εr ≈ 2−4) and the more localized nature of the photoexcited states. As a result of the strong Coulomb interaction between electrons and holes, as well as strong electron−lattice and electron−electron interactions, photoexcitation of organic conjugated materials generates a tightly bound electron−hole pair known as an exciton. These primary photoexcitations have singlet (spin-zero) character due to the conservation of spin. In order to facilitate efficient charge generation, it is necessary to separate these singlet excitons by overcoming its binding energy. Typical binding energies of singlet excitons in organic materials have been reported to be ∼0.5 eV.55 This binding energy is significantly greater than the thermal energy kBT at room temperature (25 meV), and thus an additional driving energy is required to create free charges. The ubiquitous pathway to effectively separate excitons into free carriers is by using a heterojunction between an electrondonating (donor) and electron-accepting (acceptor) material. The differing electron affinity (and/or ionization potentials) between the two materials creates an energetic offset at their interface and thereby provides a driving energy for exciton dissociation. In 1986, Tang reported the first example of an OPV device based on a vacuum-deposited CuPc/perylene derivative planar bilayer heterojunction.60 However, despite achieving relatively efficient exciton dissociation, the overall efficiency of such a planar heterojunction device is limited by the requirement of exciton migration to the donor−acceptor interface. This is because the rate of photoinduced electron transfer decreases exponentially with the separation between the donor and acceptor molecule, and therefore excitons must migrate to the interface in order to dissociate into charges.61−63 Due to the localized nature of excited states in organic materials, photogenerated excitons typically migrate through

2.2. Charge-Transfer State and Free Ccarrier Generation

Although the charge generation process following exciton dissociation is fundamental to the operation of organic solar cells, its mechanism has remained a disputed topic in the literature. In this section, we briefly summarize the key experimental findings and theoretical models that help shed light on this topic. Upon reaching a donor−acceptor interface, excitons can dissociate following the transfer of an electron (hole) from the donor (acceptor) material to the acceptor (donor) material. Historically, this charge transfer process of OPV is described within a modified Marcus framework, which considers the tunnelling of point like charges.56,67 This initial electron (or hole) transfer process generates an electron−hole pair across the interface, which is commonly known as a charge-transfer state (CTS). Despite being on separate material domains, the electron and hole are still 0.5−1 nm apart such that they remain bound by Coulomb binding energy. Once formed, the CTS can either overcome its binding energy and separate into free charge carriers or undergo relaxation and recombine geminately to the ground state resulting in energy loss. The dissociation of CTS has often been described using the Onsager−Braun framework, which considers hopping of charges within a disordered density of states by thermal activation. However, recent studies have challenged the application of such a theoretical framework to describe charge generation processes in high performance OPV devices. These include experimental studies showing that charge generation yield is independent of electric field and temperature, and dissociation 3450


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While charge generation in a wide range of OPV systems based on fullerene acceptors has been studied over the years, careful examination of those based on nonfullerene acceptors has remained limited. This research topic is an area of great interest driven by the rapid improvement of device efficiencies reported with nonfullerene based systems in recent years. It is possible that the charge generation process in fullerene systems may not directly apply to nonfullerene systems. For instance, recent studies have shown that very high charge dissociation yields can be achieved in nonfullerene systems even in the absence of a significant driving energy.87,88 These findings represent a major advancement for OPV technology because the requirement of a driving energy to generate free carriers in turn limits the open-circuit voltage. A detailed discussion about voltage loss in OPVs is discussed in section 11. We note that the driving energy should not be simply determined by taking the LUMO energy difference between the donor and acceptor material because the energy of the frontier orbital is the property of isolated material, which does not account for exciton binding energy and other interfacial effects69 and are typically estimated by electrochemical measurements. Instead, to evaluate the energy loss, a meaningful driving energy should be evaluated spectroscopically (e.g., PDS)89 or electrically (e.g., FTPS-EQE)69 using the blend film or better an actual OPV device. (PDS and FTPS-EQE refer to photothermal deflection spectroscopy and Fourier-transform photocurrent spectroscopy external quantum efficiency, respectively.) Due to the relatively weak and narrow absorption of fullerene derivatives (the archetypal acceptors used in a BHJ), historically, most discussions on charge generation focus on the photoinduced electron transfer from the “donor” to the “acceptor”, where the donor refers to the species with lower ionization potential and the acceptor refers to that with higher electron affinity. This charge generation process is termed channel I. However, the mirror process, namely the channel II charge generation, also occurs in a BHJ system, where the photoexcitation of the acceptor is followed by a hole transfer from the acceptor to the donor. With the rapid development of nonfullerene acceptors, numbers of new small molecular or polymeric acceptors have been realized with strong and complementary absorption to the donor materials. Consequently, equal attention needs to be placed on channel II charge generation. Experimental methods for probing channel I/II in a BHJ OPV system include device internal quantum efficiency measurements, photoluminescence quenching measurements, transient absorption spectroscopy measurements, etc. For a detailed discussion on the charge generation pathways, readers can refer to the review article by Stoltzfus et al.90 It is important to note that CTS can also travel along the donor−acceptor interface. Recent work by Deotare et al. shows that the CTS can move over distances of 5−10 nm in a model donor−acceptor system, driven by energetic disorder and diffusion to lower energy sites.91 Such distance is comparable to the exciton diffusion lengths in many organic materials and phase segregation of BHJ blends, and therefore it is possible that CTS diffusion also plays an important role in the charge generation process in OPV. For instance, CTS formed in blends with large density of low energy sites at the interface may have a greater chance of being trapped in those sites, followed by recombination. We will review the various charge recombination pathways in OPV in the next section.

may happens on ultrafast generation time scales (within 100 fs).68 The difficulty in developing a general framework for charge generation in OPV is in part due to the sheer variation in material systems and blend combinations that have different energetics, structural and morphological properties. In the absence of a comprehensive theoretical framework for CTS and its dissociation, material design of OPV has been primarily driven by empirical design rules.69,70 It is generally believed that a significant driving energy (>0.2 eV) provided by the interfacial energy offset is required to facilitate efficient charge generation. This rule was developed based on the observation that many OPV material systems (particular those based on fullerene acceptors) suffer from low charge generation yields in the absence of this energetic offset.71 This general observation has led to the debate over whether charge generation happens primarily through vibronically “hot” or “cold” CTS. Given the driving (or excess) energy involved in the initial charge transfer process, it is often thought that a hot CTS may facilitate charge separation more easily compared to a cold (relaxed) CTS by thermalizing into more loosely bound electron−hole pairs. These hot states should intuitively be less prone to geminate recombination compared to more tightly bound, cold CTS, provided that they can separate before they relax vibronically. Jailaubekov et al. have shown that the relaxation time of hot CTS in a planar heterojunction of copper phthalocyanine and fullerene acceptor happens within ∼1 ps.72 This concept implies that the dissociation yield should occur on an ultrafast time scale and correlate with the amount of excess energy involved in the charge transfer reaction, and this notion is supported by a number of studies.73−76 However, this concept has also been challenged by studies showing that the charge generation yield is independent of the energy of the absorbed photon, whether in the donor or acceptor, and thus the excess energy does not play a significant role in promoting charge generation.77,78 In particular, Vandewal et al.77 have shown that the same charge generation yield can be achieved even when the low-energy cold CTS is directly excited, which strongly implies that free carriers are generated from the relaxed intermolecular states without requiring any excess energies. Apart from energetics, the importance of structural morphology at the donor−acceptor interface on charge generation is also highlighted in several studies. For instance, for OPV blends based on fullerene acceptors it has been proposed that charge separation efficiency is improved in the presence of fullerene aggregates near the interface.79−82 These aggregates promote the delocalization of electronic wave functions, which may have a significant role on the charge separation mechanism of OPV blends. In particular, Gelinas et al.83 concluded that the control of the availability of states in the fullerene phase by wavefunction delocalization can lead to an ultrafast coherent electron motion in the fullerene via ballistic transport, resulting in long-range electron−hole separation (∼4 nm within 50 fs). Such a long-range charge separation mechanism would also be less sensitive to interfacial energetic offset than that predicted by the Marcus/Onsager framework.84 However, while CTS delocalization favors charge generation, excessive delocalization may also lead to reduced device open-circuit voltage by providing more pathways through which recombination can occur.85,86 This implies that there may exist a trade-off between charge generation and voltage loss and optimizing the delocalization of CTS is necessary to balance photocurrent and photovoltage. 3451


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state. Although triplet excitons have long lifetimes (up to μs) and may be able to thermalize back into charges if given enough time, it was found that, once formed, these triplet excitons are rapidly quenched by surrounding charges (triplet-charge annihilation) which subsequently lead to energy loss. It is most likely that triplet recombination occurs in blends where low-energy, relaxed CTS are formed.97−99 In the absence of these relaxed CTS, the separation of CTS into free charge carriers is kinetically more favorable compared to triplet exciton formation, and thus triplet recombination is suppressed. On the other hand, in systems where triplets are allowed to form, bimolecular triplet recombination on subnanosecond time scales represents a major loss pathway that limits device efficiency. It is thus clear that recombination events in OPV are governed by microscopic details of the interface between donor and acceptor molecules, and improved device performance can be achieved by understanding the origins of these processes and developing strategies to limit them.

2.3. Charge Transport

Free charge carriers are generated upon the dissociation of excitons and/or CTS at the donor−acceptor interface, and they move toward their respective electrodes driven by the applied electric field. The efficiency at which the charges are transported depends on their mobilities. While the ordered nature of inorganic semiconductors lead to high charge mobilities (typically ∼102 cm2 V−1 s−1), organic semiconductors tend to exhibit relatively low charge mobilities as a result of disorder effects, weak electronic couplings, and large electron-vibration couplings (leading to polaron formation).55 Charge transport thus relies on polarons hopping from site to site, with carrier mobilities strongly depending upon morphology that can range over several orders of magnitude from 10−6−10−3 cm2 V−1 s−1 (highly disordered amorphous films) to above 1 cm2 V−1 s−1 (highly ordered materials). Photocurrent is formed when the charge reaches the electrode, and efficient charge extraction at the electrode is required to suppress recombination.92,93 During the charge transport process, free electrons and holes may encounter near an interface and form a CTS, which then either recombines (through vibronic coupling to the groundstate) or dissociates back into free carriers. This recombination process is detrimental to OPV operation by limiting both photocurrent and photovoltage. In the limit of Langevin recombination, as in the case of organic light-emitting diodes (OLED), every encounter leads to recombination to the ground state with no probability of separating once again into free carriers. However, it is well-known that the rate of recombination in OPV is several orders of magnitude slower than predicted by Langevin theory, leading to the need of introducing a “reduced Langevin factor” to correctly model charge kinetics in OPV.92 This raises the question of whether Langevin theory mispredicts the recombination rate in OPV because it overestimates the frequency of the electron−hole encounter or because only a small fraction of encounters eventually lead to recombination. As explained by Burke et al.,86 understanding the origin of this reduction factor has important consequences for OPV design. For instance, will increasing carrier mobility in order to improve fill-factor in turn reduce open circuit voltage (VOC) by making carriers recombine quicker? While it remains difficult to precisely control mobility, higher OPV efficiencies are typically achieved with higher carrier mobilities because the fill-factor improves without a corresponding loss in VOC.94 This implies that the limiting factor for recombination is not the frequency of electron−hole encounters but instead the rate at which the CTSs recombine to the ground state and how quickly they can dissociate back into free carriers.77,86,95 Recombination can also occur through spin-triplet states. Similar to excitons, CTS may have spin-singlet or spin-triplet properties, with the difference being that the energy difference (or exchange energy) between singlet and triplet states of CTS is much smaller than that typically found for excitons. This is due to the weak electronic coupling of CTS.75,96 The triplet CTS states may be populated either geminately (following intersystem crossing on nanosecond time scales) or bimolecularly (following encounter of free carriers with opposite charges). The detrimental role of triplet CTS on charge dissociation yield of OPV is well documented.56 Previous studies have shown that, in blends where there exists a lowerlying triplet exciton state (say in the donor polymer domain), the energy of the CTS can be transferred to the triplet exciton

3. MOLECULAR DESIGN PRINCIPLE Historically, the main light absorber in a polymer/fullerene BHJ blend is the p-type polymer because of the low absorptivity of the fullerenes, which makes the use of the words “donor” and “acceptor” specific to electrons. We follow this nomenclature as it was widely recognized by the field. However, many NFAs contain strong dye-based chromophores that could make them an even stronger light absorber than polymers and achieve significant photocurrent generation through hole transfer to the electron donor material after photoexcitation.90 This is one of the original motivations of utilizing NFAs, which could significantly enhance the photocurrent as both channels (donor and acceptor) can contribute to free carrier generation. To maximize this effect, the BHJ should be designed using donors and acceptors that have broad and complementary spectral coverage. Thanks to the development of the polymeric and small molecular donor materials in the past decades, there exists a large library of donor materials to choose from or modify to this end. The design rules of conjugated polymers are also valuable for the development of novel NFAs. Many approaches can be employed to tailor the optical and electrochemical properties such as adjusting the conjugation lengths to tune the spectral response, using fluorination to alter the frontier energy levels, and tuning the extent of the HOMO−LUMO overlap to modify the extinction coefficients. As described in previous sections, there has been evidence suggesting the traditionally believed >0.2 eV excess energy might not be a strict restriction for efficient free carrier generation. Therefore, BHJ design should not be limited to a pair of materials with a specific LUMO−LUMO (or HOMO−HOMO) offsets. Here, we should note that we use the term LUMO level to indicate the electron affinity energy, which follows the standard usage in discussing organic electronics, but it needs to bear in mind that the occupation of this level will cause the energy to change due to a combination of Coulomb forces, exchange, and vibronic interactions.100 In contrast to energetics, designing a BHJ with favorable nanoscale phase separation is far more complicated. A planar geometry with extended conjugation is typically beneficial for light absorption of the molecule, but it could induce a strong aggregation tendency that leads to excessively large domains with an inadequate interface between the donor and acceptor 3452


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OPVs, OFETs,112,113 light harvesting arrays,114 and even sodium-ion batteries as organic electrodes.115 Compared to PDI, its higher homologues such as terrylene and quaterrylene tetracarboxdiimides (TDIs and QDIs), have relatively less flexible synthesis and a much higher propensity to crystallize (Figure 3b−d) into a large domain with sizes of hundreds of micrometers,103 which make them candidates for organic transistor applications116 but less suitable for BHJ-type OPV device applications. In terms of OPV application, naphthalene diimides (NDIs) and PDIs are the most widely studied rylene derivatives. The earliest OPV application of PDIs can be traced back to the pioneering work by Tang.60 Compared to PDIs, NDIs are less crystalline in general and have relatively weaker absorption in the visible, which make them more applied as building blocks in polymeric acceptors.44−47,51−53 This section summarizes the development of PDIs, NDIs, and other rylene derivatives as electron accepting materials in BHJ type OPVs, with a particular focus on the most recent publications.

for efficient exciton dissociation. Therefore, a molecular design strategy, e.g., reducing the coplanarity, must be considered to introduce enough miscibility and solubility to the molecule. However, reducing the coplanarity of the molecule is typically accompanied by a reduction of the charge carrier mobility. Therefore, in addition to energetics and absorption, the essence of molecular design is to judiciously control the molecular geometry to balance exciton dissociation and charge carrier transport in a BHJ blend. Furthermore, chemical modification needs to retain the chemical, thermal, and photostability of the molecule. Meanwhile, enabling solution processability, preferentially in environmentally friendly solvents, is another design consideration for the eventual commercialization of this technology.

4. RYLENE DIIMIDE-BASED NFAS Rylene diimides are a robust, versatile category of polycyclic aromatic n-type (or ambipolar) materials with remarkable electron mobility, high electron affinity, high absorption coefficients, and outstanding oxidative/thermal stability,101,102 which make them ideal candidates for application in a wide range of organic electronics. Among various rylene dyes, perylene dyes have been known for over a century. Perylene diimides (PDIs) have generally been used as pigments and dyes. Owing to their rigid aromatic core with multiple grafting sites, PDIs show strong propensity to self-assemble into ordered bars, spheres, or ordered structures in other shapes.104−109 Thanks to their exceptional electronic and optical properties (Figure 3a) along with their outstanding thermal, oxidative, and photostability, PDI-based semiconductors have been widely applied to OLEDs,110,111

4.1. PDI-Based NFAs

For BHJ OPV applications, shown in Figure 3b, the PDI monomer still has too strong an aggregation for achieving domain sizes small enough for efficient exciton dissociation. Therefore, functionalization of PDI on various positions is introduced to make the molecule twisted, which is essential for reducing aggregation. Depending on the position of functionalization, the PDI based NFAs can be categorized into bay, α, and imide functionalized derivatives (Figure 4). Among them,

Figure 4. Functionalization positions on a PDI.

the bay position is the most explored grafting site due to the feasible synthesis and effectiveness to reduce intermolecular aggregation. Connecting different numbers of PDIs through a central core has also been proved an effective approach to introduce twisting to the molecule. These approaches have caused differences in intramolecular twisting and/or intermolecular interaction between neighboring PDIs, whereas an expected outcome of the twisting approach is the hindered charge transport within the less crystalline network. Natural questions raised are how much twisting is needed? Were any of the molecules overtwisted? Would there be alternative method to maintain good intermolecular interaction while reducing the domain size? These are concerns to bear in mind at this stage of PDI research. On the other hand, α position functionalization and fused-ring strategies emerge as promising approaches to address the twisting-coplanarity paradox. Subsection 4.1 begins with recapping the early application of PDI-based NFAs in OPVs such as the monomeric PDIs, followed by summarizing

Figure 3. (a) Absorption spectra of PDI and its higher homologues. Adapted with permission from ref 102. Copyright (2010) American Chemical Society. 2D-WAXS patterns of (b) PDI 1, (c) TDI 2, and (d) QDI 3. The numbers after PDI, TDI, and QDI refer to the original names of the molecules given by the authors of ref 103. (b−d) Adapted with permission from ref 103. Copyright (2006) American Chemical Society. 3453


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various twisting strategies, focusing on the bay-position functionalized PDIs. Fused-ring strategies and α position functionalization are then discussed. Chemical structures of selected PDI-based materials are shown in each subsection with their material properties and OPV device parameters summarized in Table 1. 4.1.1. PDI Monomers: Requirement for Twisting. Pioneering work employed unsubstituted PDI dyes in OPVs. To enable solution processability, only imide-functionalized PDIs were used in the beginning (Figure 5).117,118 SchmidtMende et al. demonstrated using discotic liquid crystal hexaperihexabenzocoronene in combination with a monomeric PDI dye to produce a thin film with vertically segregated donor and acceptor materials.119 An EQE >34% was demonstrated for a thickness of almost 490 nm. The low PCE, particularly the low short circuit current (JSC) indicates the strong aggregation problem. In 2013, Sharenko et al. reported the use of pDTS(FBTTh2)2 and PDI to fabricate all-small-molecule solar cells with a PCE up to 3.0%.120 In a later report, mixing PDI with PBDTTT-C-T (Figure 6) resulted in a PCE of 3.7%, which was the best performance of PDI monomers functionalized only at the imide position.121 To alleviate the overaggregation, substitutions at other positions were introduced.122−126 One representative report was a slip-stacked PDI by introducing substituents at the α positions.127 The best-performing acceptor phenyl-PDI (Figure 5, molecule 4.3) was obtained by adding a phenyl group at each α position, and the steric hindrance of the substituents caused a slipped stack of the PDI planes. As a result, a PCE up to 3.67% was obtained by combining with a large bandgap donor polymer PBTI3T (Figure 6). In another report, introducing phenyl groups at the bay region was also reported.128 The acceptor TP-PDI (Figure 5, molecule 4.4) achieved a PCE of 4.1% when blended with PTB7-Th (Figure 6). Despite increased PCE, the overaggregation is still one of the key issues limiting the device performance in monomeric PDIbased BHJ solar cells. To reduce the aggregation tendency, various molecular design strategies have been developed that led to dramatic improved photovoltaic performance ever since. 4.1.2. Twisting Strategy-PDI Dimers. Other than functionalizing a PDI monomer, one straightforward approach to reduce the planarity is to connect two PDIs together through a single bond or the aid of a spacer unit (Figure 7). Pioneering research in this area was reported by Rajaram et al., who linked two PDIs with a single bond between the two nitrogen atoms at each PDI’s imide region,129 forming a twisted PDI dimer (Per 1). Compared to the PDI monomer (Per 2, see Figure 5, molecule 4.2), the reduced planarity of Per 1 (Figure 7, molecule 4.5) suppressed the excessive aggregation and boosted the photocurrent dramatically (Figure 8). A high PCE of 2.77% was obtained for Per 1 with PBDTTT-C-T as a lowbandgap donor polymer. The similar strategy was also used by Zhang et al., who presented a PDI dimer linked at the bay region with a thiophene used as the spacer (Bis-PDI-T-EG, Figure 7, molecule 4.6).130 Such dimerization, together with the side chains at the bay region, suppressed the intermolecular interaction and aggregation of PDI. An outstanding PCE of 4.03% was achieved using PBDTTT-C-T as the donor polymer. In a follow-up study, a new acceptor with shortened side chains at the bay region enabled a higher PCE of 4.34% in combination with the same donor polymer through tuning the fraction of additive in the solvent mixture.131

A simple PDI dimer linked at the bay region through a single bond was reported later by Jiang et al.132 Among the PDI dimers connected through one, two or three singles bonds, the one with a single bond between two PDIs (s-diPBI, Figure 7, molecule 4.7) resulted in a more twisted molecular structure than those with two or three single bonds. Consequently, an OPV device based on s-diPBI exhibited a high PCE of 3.63% with PBDTTT-C-T as the donor polymer and a mixture of 1,8diiodooctane (DIO)/1-chloronaphthalene (CN) as solvent additive whereas PDI dimers linked by two or three single bonds displayed significantly lower device performance. Further improvement of the device performance was achieved by Zang et al. through combining s-diPBI with PTB7-Th. A high JSC of 11.98 mA cm−2 and a high fill factor (FF) of 59% contributed to an excellent PCE of 5.9%.133 Later, Sun et al.134 and Meng et al.135 reported two derivatives of s-diPBI with bay regions at both ends of the dimer end-capped by either sulfur (SdiPBI-S, Figure 7, molecule 4.8) or selenium (SdiPBI-Se, Figure 7, molecule 4.9), rendering the PDI unit a planar structure. Compared with s-diPBI, SdiPBI-S and SdiPBI-Se exhibited larger dihedral angles between the two PDI units, as well as larger optical bandgaps. When combined with a medium bandgap donor polymer PDBT-T1 (Figure 6), SdiPBI-Se achieved a high PCE of 8.42% while SdiPBI-S showed a PCE of 7.16%. In terms of synthetic cost, Hendsbee et al. demonstrated the gram-scale synthesis of three N-annulated PDI compounds and a PDI dimer linked via a single bond at the bay position (Figure 7, molecule 4.10), without the need for purification using column chromatography.136 One of the PDI dimers exhibited a PCE of 7.55% in BHJ solar cell. In addition to connecting two PDIs through a single bond, to further tune the morphology of the active layer, Yan et al. studied the effect of spacers on aggregation and thus phase separation between PDI and polymer. A series of spacers including thiophene, benzene, bithiophene and spirobifluorene were inserted between the two PDI units.137 It was found that the planar dimer with a phenyl ring as the spacer showed the largest domain size and the worst solar cell performance due to strong intermolecular interactions. In contrast, the best PDI dimer in this report was the one with spirobifluorene as the spacer (SF-PDI2), which led to a small domain size of the blend film and a PCE of 2.35% with P3HT as the donor polymer. In 2016, Liu et al. designed a new conjugated polymer, P3TEA (Figure 6), and combined it with SF-PDI2 (Figure 7, molecule 4.11) in a BHJ solar cell.87 The nonfullerene solar cell demonstrated not only a 9.5% efficiency with nearly 90% internal quantum efficiency but also an extremely high VOC (1.11 V) and a decent FF (0.64). The authors quantified the energy loss of the device by taking the difference between the bandgap of the donor (obtained using the crossing point between the absorption and emission spectra) and the VOC of the device, and a low voltage loss (0.61 V) was determined. Noticeably, subgap-EQE and electroluminescence measurements exhibited almost overlapped spectra between the pure polymer and the polymer:SF-PDI2 blend (Figure 9a,b), which indicated a nearly zero driving force for charge separation in this highly efficient OPV system. In addition, the authors employed another polymer:NFA system with greater driving force (PffBT4T-2DT:SF-PDI2) as a control group, and demonstrated that P3TEA:SF-PDI2 showed roughly identical exciton dynamics, indicating an ultrafast charge separation in spite of its minimal driving force. Moreover, through analyzing 3454


−6.05 −6.09d −6.0b −5.71c −5.53a −5.66a −5.55c −6.16c −5.94a −5.40a −6.01b −6.0b −6.03a −5.77c −5.97c −6.00c −5.86c −5.96c −5.97a −5.97a −5.46a −6.04d

−5.98d −6.36b −5.48b −6.26d −5.77c

−5.97c −6.21d

−5.95b −5.74a −5.60b −5.71d

−3.85 −3.87d −3.8b −3.71b −3.83a −3.72a −3.57b −3.98a −3.83a −3.70a −3.93b −3.86b −3.81a −3.72b −4.00b −3.75b −3.76b −3.82b −3.78a −3.69a −3.68a −3.77d

−3.77d −4.03b −3.75b −3.91d −3.76b

−3.80b −3.75d

−3.73b −3.58a −3.78b −3.89d

4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 4.24 4.25 4.26 4.27 4.28 4.29

4.30 4.32 4.34 4.35 4.36

4.37 4.38

4.39 4.40 4.41 4.42

SdiPBI-S SdiPBI-Se 7b SF-PDI2 IDT-2PDI P1TP i-Me2T2-PDI2 CP-V F2B-T2PDI S(TPA-PDI) H-tri-PDI B(PDI)3 Ta-PDI TPE-PDI4 Tetra-PDI TPC-PDI4 TPPz-PDI4 Me-PDI4 SF-PDI4 P4N4 PBI-Por helical PDI 1

FPDI-T Ph2a FITP hPDI4 hPDI3-PyrhPDI3 TPH-Se βTPB6-C

Fused-TriPDI FTTB-PDI4 αPBDT TPB

c

−5.95a

−3.87a

4.7

a

2.01 1.89 2.068 1.69

−6.02c −5.69a −6.12d −5.65b

−4.01b −3.82a −4.06b −3.84b

4.3 4.4 4.5 4.6a

phenyl-PDI TP-PDI Per 1 Bis-PDI-TEG s-diPBI

3455

2.20 1.88 2.18f 2.14f

2.17 2.14f

2.22 2.10f 1.77f 2.00f 2.01

2.20 2.22 2.2 2.03f 1.54 1.8 1.98 2.18 1.77 1.76 2.09 2.03f 2.05 2.05 1.97 2.25 2.10 2.14 2.05 2.11 1.48 2.14f

2.08

Eg,opt (eV)

internal ref

HOMO (eV)

acceptor

LUMO (eV)

PTB7-Th P3TEA PTB7-Th PTB7-Th

PDBT-T1 PTB7-Th

PTB7-Th PTB7-Th PTB7-Th PTB7-Th PTB7-Th

PDBT-T1 PDBT-T1 P3TEA P3HT BDT-2DPP PBDTTT-C-T PffBT4T-2DT PPDT2FBT PTB7-Th PBDTTT-C-T PBDT-TS1 PTB7-Th PTB7-Th PTB7-Th PTB7-Th PffBT4T-2DT PffBT-T3(1,2)-2 PBDTTT-C-T PV4T2FBT PDBT-T1 PBDB-T PTB7-Th

PBDTTT-C-T

PBTI3T PTB7-Th PBDTTT-C-T PBDTTT-C-T

donor

1:1.5 1:1.5 1:1.5 1:1

1:1 1:1.5

1:2 1:2.25 1.2:1 1:1 1:1

1:1 1:1 1:1.5 1:1 1:1 1:1 1:1.4 1:2 1:1.2 1:1 1:1 1:1.5 1:1 1:1.4 1:1.2 1:1.5 1:1.5 1:1 1:0.8 1:1 1:1 3:7

1:1

1:1 1:1 1:1 1:1

D:A ratio

Table 1. Summary of Representative Rylene Diimide-Based Acceptors

DCB+0.75%DIO CB+2.5%DIO +2.5%DPE CB TMB+2.5%ODT CB+3%CN CB+8%DPE

DCB+1.5%DIO +1.5%CN CB+0.75%DIO CB+0.25%DIO TMB+2.5%ODT DCB DCB DCB+3%DIO CB CF+DPE NR DCB+5%DIO CB+7%DPE CB+3%CN CB+0.5%CN CB DCB+3%CN CB CB+DCB(7:3) DCB+3%DIO CB+2%DIO DCB+0.5%DIO CB+1%DIO CB+1%DIO+1% CN CB+2%CN CF+1%DIO CB+2%CN CB+1%DIO CB

CF+0.5%DIO DCB+1%CN CB DCB+5%DIO

processing solvent

6.19 10.58 4.92 8.47

9.28 7.69

6.72 3.89 7.33 8.27 7.6

7.16 8.42 7.55 2.35 3.12 3.61 4.1 5.28 5.05 3.32 7.25 5.65 9.15 5.53 3.54 4.3 7.1 2.73 5.98 5.71 7.4 6.05

3.63

3.67 4.1 2.78 4.03

PCE (%)

0.91 1.13 0.81 0.79

1.0 0.92

0.94 0.93 0.99 0.802 0.80

0.90 0.96 1.13 0.61 0.95 0.89 0.91 0.87 0.84 0.88 0.732 0.83 0.78 0.91 0.86 0.96 0.987 0.77 0.90 0.958 0.78 0.803

0.73

1.024 0.87 0.76 0.85

VOC (V)

12.39 13.89 12.74 18.40

12.99 14.9

12.48 7.68 13.24 15.1 15.1

11.98 12.49 11.03 5.92 7.75 7.78 8.0 10.04 10.60 11.92 16.52 13.12 17.10 11.7 8.39 9.2 12.5 7.83 12.02 9.40 14.5 13.3

10.58

6.56 10.1 7.9 8.86

JSC (mA cm‑2)

55 65.9 46 58

71.5 56

58 54.3 56 68.2 62.9

66.1 70.2 61 65 42.4 52.1 56 60.16 57 33.6 60.03 52 68.5 52 49 49 56 45.0 54.2 63.4 66 56.6

46.80

54.59 46.4 46 54.1

FF (%)

1.2 × 10−3 3.6 × 10−3 NR NR 2.0 × 10−5 2.0 × 10−5 1.8 × 10−3 1.07 × 10−4 9.8 × 10−5 7.17 × 10−4 1.2 × 10−4 1.748 × 10−4 3.6 × 10−4 NR 1.88 × 10−3 NR NR 5.55 × 10−5 2.46 × 10−4 1.21 × 10−3 NR 2.9 × 10−4 5.92 × 10−2 NR 5.60 × 10−4 1.2 × 10−4 3.9 × 10−4

NR 2.8 × 10−3 4.8 × 10−3 >10−7 7.1 × 10−5 2.3 × 10−6 2.4 × 10−4 8.3 × 10−5 1.39 × 10−5 3.4 × 10−5 2.32 × 10−5 1.4 × 10−5 4.20 × 10−5 2.7 × 10−4 NR 8.66 × 10−5 NR NR 1.78 × 10−6 1.93 × 10−5 1.90 × 10−3 NR 3.4 × 10−4 1.63 × 10−4 4.6 × 10−5 3.66 × 10−4 1.5 × 10−5 7.1 × 10−4 2.2 × 10−3 4.67 × 10−5 2.83 × 10−4 1.1 × 10−4 8.00 × 10−4 6.10 × 10−6

3.21 × 10−5e 3.2 × 10−3 6.4 × 10−3 NR NR NR NR NR NR NR 3.0 × 10−5 NR NR NR 1 × 10−3 1.4 × 10−3e 2.8 × 10−4 2.3 × 10−3 NR NR 2.69 × 10−3 1.0 × 10−2e NR NR NR NR NR NR 3.2 × 10−2e NR 1.26 × 10−3e 2.2 × 10−4 NR NR

4.17 × 10−4 1.5 × 10−4 1.79 × 10−5 1.08 × 10−5

1.7 × 10−3 2.67 × 10−4

NR

NR NR NR 3.0 × 10−3

NR NR NR 1.0 × 10−3

2.8 × 10−3e NR 8 × 10−3e 3.9 × 10−3e

H (blend)

E (blend)

E (neat)

mobility (cm2 V‑1 s‑1)

inverted inverted inverted inverted

conventional inverted

inverted inverted inverted inverted inverted

conventional conventional inverted inverted conventional conventional inverted inverted inverted conventional inverted inverted inverted inverted inverted inverted inverted inverted inverted conventional inverted inverted

conventional

inverted inverted inverted conventional

device structure

ref

164 165 166 167

162 163

156 157 158 160 161

134 135 136 137 138 139 140 141 142 143 144 145 38 146 147 148 35 149 150 151 152 155

132

127 128 129 130

Chemical Reviews Review


ref

Review a Measured by CV using NFA films. bMeasured by CV in solutions. cCalculated using measured LUMO/HOMO and the optical bandgap. dDetailed method was not reported. eMeasured using OFET devices. fDetermined from the film absorption onset by the authors of this review; NR Not reported. CF: chloroform; DIO:1,8-diiodooctane; DCB: 1,2-dichlorobenzene; CN: 1-chloronaphthalene; CB: chlorobenzene; TMB: 1,2,4-trimethylbenzene; ODT: 1,8-octanedithiol; DPE: diphenyl ether; CV: cyclic voltammetry; E (neat): electron mobility in NFA neat films; E (blend): electron mobility in donor:NFA blend films; H (blend): hole mobility in donor:NFA blend films.

inverted conventional inverted NR NR 3.40 × 10−5 2.18f 2.39 1.68 −5.9c −6.6b −5.71a 4.43 4.44 4.45 2,2′-Bi(PDI) BiNDI BTDI3

−3.7a −4.0b −3.88a

PTB7-Th PTB7 PBDT-TS1

1:1.5 1:1 1:1

CB+2%DIO DCB+0.5%DIO CB

5.43 2.41 3.64

0.78 0.75 0.73

12.57 5.47 10.77

52.36 59.1 46

NR 0.365e NR

2.2 × 10−5 1.73 × 10−5 1.59 × 10−7

device structure H (blend) E (blend) Eg,opt (eV) HOMO (eV) internal ref acceptor

LUMO (eV)

donor

D:A ratio

processing solvent

PCE (%)

VOC (V)

JSC (mA cm‑2)

FF (%)

E (neat)


Table 1. continued

168 173 174

Chemical Reviews

Figure 5. Chemical structures of monomeric PDIs.

the electroluminescence external quantum efficiency (EL-EQE) of four combinations of polymer:NFA with different driving forces (Figure 9c), the authors showed that a decreased driving force corresponded to an increased EL-EQE. The increased ELEQE can be further translated into a small nonradiative recombination loss, which also contributed to the overall low energy loss. Further details of the detailed balance theory that the authors employed to delineate energy loss are discussed in section 11). These results were the first example of efficient charge separation on a small driving force in a nonfullerene organic solar cell, which manifested the great potential of using NFAs to achieve a low energy loss system. The spacer approach stimulated the design of many other PDI dimers with different spacers. For instance, when a bulky IDT unit was utilized as the spacer, the PDI dimer named IDT2PDI138 (Figure 7, molecule 4.12) demonstrated different performance in combination with different donors: IDT-2PDI exhibited a higher electron mobility in the P3HT blend, which resulted in an excellent FF of 67%. When a molecular donor, BDT-2DPP (Figure 6), was employed, a higher PCE (3.12%) was achieved due to a more compatible electronic structure between the donor and acceptor. In another report by Wang et al., the authors studied the structure−property relationship among PDI dimers containing oligothiophene spacers with different lengths.139 A longer spacer provided the PDI dimers with lower bandgaps, stronger absorption, and higher energy levels. Using the same donor polymer PBDTTT-C-T, the dimer with one thiophene (P1TP, Figure 7, molecule 4.13) exhibited the best OPV performance (3.61% PCE). It was hypothesized that the dimer with no spacer had a highly twisted structure and a large domain size. On the other hand, too long a spacer led to an overmixed morphology that was detrimental to charge transport. In another report, Zhao et al. synthesized PDI dimers containing methyl substituted bithiophene spaces (Figure 7, molecule 4.14) and compared the effect of substitution position on film morphology.140 The dimer whose substitution offered a head-to-head geometry exhibited a better blend morphology with a smaller domain size and thus a higher PCE (4.1%) in the BHJ device with PffBT4T-2DT (c) as the donor polymer. Park et al. reported a V-shape PDI dimer (CP-V, Figure 7, molecule 4.15) where the two PDIs were connected through the imide position.141 Compared to an Mshape dimer connected through the bay position, CP-V exhibited a larger bandgap, a higher electron mobility, and a better solar cell device performance (PCE up to 5.28%). Hadmojo et al. inserted a 2,5-difluorobenzene unit into a bithiophene-bridged PDI dimer and investigated the effect of this unit on material property.142 The resultant acceptor F2BT2PDI (Figure 7, molecule 4.16) exhibited a larger bandgap, a 3456


Chemical Reviews

Review

Figure 6. Chemical structure of selected p-type polymers used in combination with NFAs.

4.1.3. Twisting Strategy-PDI Trimers and Tetramers. To further explore the twisting strategy and design NFAs that

stronger twisting between the two PDI units, and a better device performance than the original dimer. 3457


Chemical Reviews

Review

Figure 7. Chemical structures of selected PDI dimers.

which leads to a higher electron mobility in the blend. With PTB7-Th as the donor polymer, the Ta-PDI based OPV device showed an outstanding PCE of 9.18% with a high JSC of 17.1 mA cm−2 and a high FF of 68.5%. Besides trimeric PDIs, tetrameric PDIs were developed by connecting four PDI arms to a central core. Liu et al. designed a tetrameric PDI molecule with a tetraphenylethylene core (TPEPDI4, Figure 10, molecule 4.21).146 As a result of the highly twisted nature of the TPE core, the molecule exhibited a weak aggregation, which enabled a suitable phase separation and thus a high PCE of 5.53% with PffBT4T-2DT as the donor polymer. Akin to this approach, a PDI tetramer based on a tetraphenylsilane core was developed (Figure 10, molecule 4.22).147 The compact core made the PDIs interlocked, and a proof-of-concept device showed a PCE of 3.54% when blended with PTB7-Th. Another study investigated the effect of the central atom of the tetrahedron core on film morphology and device performance, including carbon (TPC-PDI4, Figure 10, molecule 4.23), silicon (TPSi-PDI4), and germanium (TPGePDI4).148 It was shown that, using the same donor polymer PffBT4T-2DT, both TPC-PDI4 and TPSi-PDI4 could achieve a high efficiency of >4%, while the germanium-based one showed an inferior performance (1.6%). Motivated by the PDI tetramer TPE-PDI4,146 Lin et al. made a further modification to the molecule by using a tetraphenylpyrazine core, which reduced the extent of twisting compared with TPE-PDI4, enhanced the

could form three-dimension (3D) or quasi-3D molecular structures, other routes such as combining three or more PDIs in a molecule were explored (Figure 10). A pioneering study was reported by Lin et al., who synthesized a PDI trimer named S(TPA-PDI) (Figure 10, molecule 4.17), in which three PDI units were connected by a triphenylamine core.143 Due to the existence of a sp3 hybridized nitrogen atom at the center, the whole molecule possesses a quasi-3D nonplanar structure. Due to the suppressed intermolecular interaction and molecular aggregation, an efficiency of 3.32% was achieved when the PDI tetramer was combined with a low bandgap polymer PBDTTTC-T. However, the electron mobility of S(TPA-PDI) in the blend film was only 2.32 × 10−5 cm2 V−1 s−1, which may explain the low FF (33.6%) of the device. In another study, a trimer by elongation of Per 1 (H-tri-PDI, Figure 10, molecule 4.18) achieved a PCE of 7.25% and an excellent JSC of 16.52 mA cm−2 with a high-performance donor polymer PBDT-TS1 (c).144 Later, a trimeric PDI (B(PDI)3, Figure 10, molecule 4.19) with the three PDI arms connected to a central benzene core was designed by Li et al.145 The nonplanar molecule, when combined with the prototypical polymer PTB7-Th, exhibited a PCE of 5.65%. In a more recent report by Duan et al., a PDI trimer with triazine as the core unit (Ta-PDI, Figure 10, molecule 4.20) was reported and compared with a benzenecore analogue.38 It was found that the triazine-based trimer had a less twisted structure and thus higher aggregation propensity, 3458


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was used as the core by Zhang et al. to construct a PDI tetramer PBI-Por (Figure 10, molecule 4.28), which had a significantly lower bandgap and weaker crystallinity than most other PDI acceptors.152 A polymer donor PBDTBDD with complementary absorption was used and a PCE of 7.4% was achieved. 4.1.4. Discussion: How Much Twisting Is Needed? The structure−property relationship discussed in refs 38 and 35 has stimulated the demand to reexamine the twisting strategy. Specifically, through examining the effect of the twisting angle on device performance for the series of structurally similar PDI tetramers35 (Figure 11), it is not difficult to conclude that the enlarged nonplanarity does not necessarily correspond to an enhanced device performance. These results indicate that when the nanoscale phase separation is within a reasonable range, i.e., not too large for efficient exciton dissociation, the essential intermolecular π−π stacking that could dramatically change the local or long-range charge transport property starts to play a dominant role in determining the overall device performance. This indication bears substantial importance to the PDI-based NFA research, where the majority of effort has been focused on introducing nonplanarity in the past. Similarly, regarding the comparison between Ta-PDI and PhPDI in ref 38, the Ph-PDI molecule bears resemblance to a “three-wing propeller” with large twisting angles (76°, 54°, and 42°) between the PDI units; in contrast, two of the three PDI units in Ta-PDI displayed a more coplanar geometry (19°) while the third PDI unit showed a large twisting angle with respect to the other two (83° and 74°; Figure 12). Nonetheless, the blend films did not exhibit significant variation in terms of phase separation, but the Ta-PDI demonstrated higher electron mobility and considerably higher PCE than Ph-PDI when combined with the same polymer. 4.1.5. Fused-Ring PDI. Ring fusion is an effective route to enhance coplanarity of a molecule,153,154 which could lead to enhanced molecular order that is beneficial for charge carrier transport. As the previous work has demonstrated that the chemists were able to introduce sufficient (in some cases even too much) twisting,35,38 a proper extent of ring-fusion within the molecule could potentially improve charge transport while maintaining adequate donor/acceptor interface for charge generation. Chemical structures of selected fused-ring PDIs are shown in Figure 13. An example was given by Zhong et al., who designed a helical PDI dimer with an ethylene spacer

Figure 8. (a, b) Optical images illustrating the overaggregation of monomeric PDI (Per 2, molecule 4.2) and the reduced stacking of a dimerized PDI (Per 1). (c) J−V characteristics of BHJ solar cells made with PBDTTT-C-T:Per 1 (Figure 7, molecule 4.5) and PBDTTT-CT:Per 2. Adapted with permission from ref 129. Copyright (2012) American Chemical Society.

electron mobility of the acceptor TPPz-PDI4 (Figure 10, molecule 4.24), and improved the efficiency to 7.1% with PffBT-T3(1,2)-2 (Figure 6) as the donor polymer.35 In addition to connecting the PDI units at the bay region, four PDIs connected to a tetraphenylcarbon core unit via the imide region was also reported (Me-PDI4), which showed a PCE of 2.35% combined with the donor polymer PBDTTT-C-T.149 Another PDI tetramer with spirobifluorene as the core unit (Figure 10, molecule 4.26) was also developed, exhibiting a PCE of 5.98% when PV4T2FBT (Figure 6) was used as the donor polymer.150 Liu et al. reported another PDI tetramer with 2,3,7,8-tetraphenylpyrazino[2,3-g]quinoxaline as the core unit (Figure 10, molecule 4.27),151 which enabled a PCE up to 5.71% with PDBT-T1 as the donor polymer. Later, a porphyrin

Figure 9. (a) Normalized FTPS-EQE spectra of pure P3TEA and blend P3TEA:SF-PDI2 blend devices. (b) Normalized electroluminescence spectra pure P3TEA and P3TEA:SF-PDI2 blend devices. (c) EL-EQE of P3TEA:SF-PDI2 (blend A), PffBT4T-2DT:SF-PDI2 (blend B), P3TEA:diPDI (blend C), and PffBT4T-2DT:diPDI (blend D)-based solar cells at different voltages. The value of the driving force for blend A is negligible; for blend B it is 160 meV; for blend C it is 200 meV; and for blend D it is 370 meV. Adapted with permission from ref 87. Copyright 2016 Nature Publishing Group. 3459


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Figure 10. Chemical structures of trimerized and tetramerized PDIs.

ring-fusion between the spacer and PDI increased the intramolecular electronic coupling, which, in general, resulted in an upshifted LUMO and therefore a blue-shifted absorption with sharp peaks (Figure 15). Along with increased electron mobilities, the molecule with a fused benzene spacer achieved the highest PCE of 3.89%, with PTB7-Th as the donor polymer. In a later report, two PDI units were fused with an IDTT bridge, which resulted in a fully planar dimer (Figure 13, molecule 4.34).158 Nevertheless, the PCE when combined with PTB7-Th reached 7.33%, which was likely the result of the suppressed aggregation caused by the four bulky alkylphenyl side chains. In addition to PDI dimers, linear PDI oligomers were reported and have been used to further improve the photovoltaic device performance. In 2014, Zhong et al. synthesized a series of PDI helical structures by fusing the

(Figure 13, molecule 4.29). The rigid yet nonplanar structure enabled an appropriate phase separation when blended with PTB7-Th.155 An ultrafast electron transfer from the donor to the acceptor and hole transfer from the acceptor to the donor (∼0.2 ps) were revealed (Figure 14). As a result, an excellent efficiency of 6.05% was achieved. In another report, three fused analogue PDI dimers were developed using furan, thiophene, or selenophene as the spacer.156 The dimer with thiophene as the spacer was found to have the strongest intermolecular packing, the highest electron mobility, and the best performance (6.72%) with PTB7-Th. Hartnett et al. studied the effect of ring-fusing between the PDI unit and the spacer for three series of PDI dimers with different spacers, namely thiophene (Figure 13, molecule 4.30), benzene (Figure 13, molecule 4.31−4.33), and thienothiophene.157 They found that, compared to nonfused PDI dimers, 3460


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Figure 11. Structures of TPC-PDI4, TPE-PDI4, and TPPz-PDI4 with a decreasing extent of intramolecular twisting. Reprinted with permission from ref 35. Copyright 2016 John Wiley and Sons.

but also introduced more absorption bands with a signature intense absorption peak in the higher-energy region, e.g., peak at ∼385 nm for the dimer 2. Through DFT calculations and femtosecond transient absorption spectroscopical studies, the high-energy absorptions were attributed (at least partially) to the electron being promoted from the olefinic bridges to the LUMO situated on the PDI framework and/or the electron being promoted from the HOMO situated on the PDI subunits to the olefinic bridges. The elongation of helical PDI 1 to fused PDI trimer and tetramer increased the efficiency to over 8%.160 Similar to the dimer, the trimer and tetramer were also nonplanar and rigid, and their bandgaps decreased slightly with increasing number of PDI units due to an extended conjugation. The strong absorption and high mobility of h-PDI4 (Figure 13, molecule 4.35) enabled a PCE up to 8.3% with PTB7-Th as the donor. Recently, a linear PDI oligomer with a pyrene connecting two PDI trimers was developed (hPDI3-Pyr-hPDI3, Figure 13, molecule 4.36).161 The ribbon had a molecular length of ∼5 nm and showed broad and strong absorption in the range of 300−650 nm, and the initial test with PTB7-Th achieved a PCE of 7.6%. Recently, a PDI trimer that combined design strategies including (i) spacer fusion, (ii) functionalization of the end bay regions, and (iii) construction of a 3D structure was reported by Meng et al. (Figure 13, molecule 4.37).162 The three PDI units, with selenium-annulated bay regions, were fused to a benzene core. Due to a strong steric hindrance, the acceptor TPH-Se exhibited a highly twisted three-bladed propeller structure (Figure 17). The strong intermolecular interaction contributed to a high electron mobility, which further led to an excellent FF (71.5%) in the device when blended with PDBTT1. In addition, a broad and strong absorption enabled a high JSC of 12.99 mA cm−2, and properly matched energy levels contributed to a high VOC (1.0 V). Finally, an outstanding PCE of 9.28% was achieved. Two other reports used a similar strategy. The first example was an acceptor βTPB6-C (Figure 13, molecule 4.38) with four PDI units fused to a BDT-Th unit, which also showed a 3D structure.163 Compared to its nonfused analogue acceptor, βTPB6-C exhibited a larger bandgap, higher VOC and JSC, and a higher PCE with PTB7-Th as the donor polymer. The second example was the fused PDI trimer with benzotrithiophene as the core unit developed by Wang et al.164 It was shown that the fused-TriPDI (Figure 13, molecule 4.39)

Figure 12. Calculated optimal molecular geometries of Ta-PDI and Ph-PDI (top). Bottom: (a,b) atomic force microscopy (AFM) height images and (c,d) phase images of PTB7-Th:Ta-PDI (a,c) and PTB7Th:Ph-PDI films. (b,d) Image size: 5 × 5 μm2. Reprinted with permission from ref 38. Copyright 2017 John Wiley and Sons.

neighboring PDIs.159 Shown in Figure 16 below, different ribbon lengths corresponded to different conformations. For instance, the PDI tetramer showed two conformers while the PDI tetramer had four isoenergetic conformations. Electronically, ring-fusion not only narrowed the bandgap (Figure 16g,h) 3461


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Figure 13. Chemical structures of selected fused-ring PDIs. The blue color highlights the nonfused structure, while the green highlights the fused structure.

molecule 4.40) with a “double-decker” molecular shape, which demonstrated an outstanding PCE of 10.58% when combined with the polymer P3TEA.165 Compared to the nonfused PDI tetramer, ring-fusion provided FTTB-PDI4 with a strong intermolecular stacking, a high electron mobility, a blue-shifted absorption (more complementary with the polymer), and energy levels that are more compatible to the polymer. This result further highlights the effectiveness of the ring-fusion strategy in modulating the intramolecular property. 4.1.6. α-Substituted PDI. The α-position, or sometimes referred as the nonbay or ortho-position, is another grafting site that has been shown to have a different effect on the coplanarity of the functionalized molecule in comparison to the bay-

Figure 14. Exciton generation and charge separation in the PTB7:1 blend at high excitation energy. Adapted with permission from ref 155. Copyright (2014) American Chemical Society.

exhibited a larger bandgap, higher mobility, and better solar cell device performance than the nonfused analogue molecule. More recently, Zhang et al. used the ring-fusion strategy to obtain a novel PDI tetramer named FTTB-PDI4 (Figure 13, 3462


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addition, 2,2′-bi(PDI) exhibited a higher LUMO level and thus a higher VOC of its BHJ solar cell. The overall PCEs were similar for the two NFAs, when PTB7-Th was used as the donor polymer. 4.2. Other Rylene-Based NFAs

In addition to PDI, its lower homologue NDI and higher homologue such as TDI have also been employed to build NFAs.169−172 A recent NDI-based NFA example was reported by Liu et al. in 2015.173 The authors connected two NDI units with a vinyl group. The NDI dimer, named BiNDI (Figure 19, molecule 4.44), exhibited a near-planar structure and achieved a PCE of 2.41% with PTB7 as the donor polymer. In 2017, Feng et al. reported the first application of terrylene derivatives as NFAs in organic solar cells.174 Similar to the twisting strategy used to decrease the coplanarity of PDI-based NFAs, a dimerized TDI and a trimerized TDI were synthesized with a best PCE of 3.64% achieved using the BTDI3 (Figure 19, molecule 4.45) as the acceptor and PBDT-TS1 as the donor polymer. Compared to PDI, NDI-based NFAs typically exhibit a low absorption coefficient in the visible as well as low electron mobilities due to the shortened conjugated backbone. Therefore, NDI is often used as the electron-withdrawing moiety to design polymeric acceptors. On the other hand, TDI consists of more conjugated aromatic rings than PDI, which typically leads to red-shifted, intense absorption, and a high electron mobility. However, too strong an aggregation tendency may be detrimental for efficient free carrier generation. Besides, the synthetic complexity may be another reason limiting TDI’s application.

Figure 15. UV−vis absorption of Ph1, Ph2a, and Ph2b in ref 157. Reproduced from ref 157 with permission from the Royal Society of Chemistry.

position.147 Introducing substitution at the α-positions has therefore been considered as an alternative route to address the twisting-coplanarity problem and develop efficient PDI-based NFAs (Figure 18). Two pairs of PDI dimers, with benzodithiophene (BDT) or pyrene diimides (PID) as the spacers connected at either bay or α-position, were synthesized and studied by Zhao et al.166 For both cases, the α-substituted PDIs (Figure 18, molecule 4.41) showed a stronger aggregation, a higher electron mobility, and finally a better PV performance. The best efficiency (4.92%) was achieved with αPBDT when PTB7-Th was used as the donor polymer. Recently, a BDT based core named TPB (Figure 18, molecule 4.42) was employed by Wu et al. to construct α-substituted PDI tetramers.167 Due to a complementary absorption and carefully optimized film morphology, the JSC of the device surpassed 18 mA cm−2. Although the low mobilities led to a moderate FF, the overall efficiency reached 8.47%. In another report, Fan et al. compared PDI dimers connected directly to each other via different positions.168 DFT calculations showed that the dimer connected at the α-position (2,2′-bi(PDI), Figure 18, molecule 4.43) had a higher dihedral angle between the two PDI units than that of the dimer connected at the bay-position (1,1′-bi(PDI)). The absorption spectrum of 1,1′-bi(PDI) was broader than that of 2,2′bi(PDI), indicating a stronger inter-PDI electronic coupling. In

4.3. Summary and Challenges

Rylene diimides and their derivatives are the earliest electron acceptors applied in organic solar cells and are still being widely studied by many research groups. Among various rylene diimides, PDI is the workhorse material to construct small molecular NFAs owing to their capability of being functionalized into derivatives that could be solution-processed into nanoscale structures while retaining a relative high electron mobility and a strong absorption coefficient. Most PDI

Figure 16. DFT molecular geometries of PDI helical ribbons for the PDI dimer 2 (a), trimer 3 (b, c), and tetramer 4 (d, e, f) from ref 159. UV−vis absorption (g) and photoluminescence spectra (h) for 2−4 along with a PDI monomer in solution. Adapted with permission from ref 159. Copyright (2014) American Chemical Society. 3463


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Figure 17. (a) X-ray molecular structures of TPH 4b and TPH-Se 6a (top view and side view). (The alkyl chains and hydrogen atoms are omitted for clarity.) (b) The slipped 3D stacking mode of TPH 4b (top) and TPH-Se 6a (down). (c) Se···O (3.0 Å) interactions between different PBI subunits of TPH-Se 6a. Adapted with permission from ref 162. Copyright (2016) American Chemical Society.

acceptors have bandgaps over 1.9 eV, and therefore low bandgap donor polymers were generally used to complement the absorption. Electrically, electron mobilities of recent highperformance PDI acceptors are typically on the order of 10−5 to ∼10−3 cm2 V−1 s−1, which, despite further enhancement still being required, are able to enable a high FF in solar cell devices in general. Another advantage of PDI-based acceptors is the facile funtionalization of PDI. Substitution at different positions has been realized to construct different acceptors. Nevertheless, an important consideration of PDI acceptors is to control the coplanarity, which is important to balance electron mobility and

exciton dissociation. Meanwhile, most PDI-based organic solar cells (OSCs) need to be processed with solvent additives. Several reports showed that the device performance was sensitive to the amount of additives, which is an issue to be addressed since solvent additives may affect the large-scale production and device longevity.

5. A-D-A TYPE ACCEPTORS In addition to the development of rylene-based NFAs, the successful strategy of using a conjugated “push-pull” structure in designing semiconducting polymers has also been applied to 3464


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easily extended to the near-infrared (NIR) that could substantially improve photocurrent generation and in the meantime allows the design of semitransparent devices. To tune the solubility and film morphology, alkyl or aromatic side chains are typically introduced on the central fused-rings. Overall, the synthetic flexibility, easily tuned electronic/optical property, and rigid backbone make the A-D-A type small molecules promising acceptor materials for nonfullerene OPV applications. Chemical structures and properties of representative materials are summarized in Figure 21 and Table 2. 5.1. Early Reports of A-D-A Type Acceptors

One of the first NFAs of this type, named FEHIDT (Figure 21, molecule 5.1), was reported in 2013 by Winzenberg et al., where the “A” and “D” units are indan-1,3-dione and fluorene, respectively, and a thiophene was used as the spacer.177 The spacer thiophene and the flanking indan-1,3-dione are coplanar, but they have a dihedral angle of ∼20° with the central fluorene unit. The acceptor molecule has a large bandgap similar to that of PDI, with a strong absorption in the range of 400−600 nm. A PCE of 2.43% was obtained by using FEHIDT in combination with P3HT (Figure 6) as the donor polymer. In a later study by Kim et al., the indan-1,3-dione (“A” unit) of FEHIDT was replaced by 3-ethylrhodanine, and a new acceptor named Flu-RH (Figure 21, molecule 5.2) was obtained.178 In addition, the alkyl chains on the central fluorene unit were noctyl instead of 2-ethylhexyl. Overall, Flu-RH showed a similar optical bandgap and absorption range to FEHIDT, but a higher LUMO level offered it a high VOC of 1.03 V with P3HT, which led to a best efficiency of 3.08% despite a lower FF (0.52). Another early work on push−pull NFA was represented by the report of FBR (Figure 21, molecule 5.3) by Holliday et al. FBR was obtained by replacing the thiophene spacers in FluRH by benzothiadiazole units.179 The bandgap and absorption range of FBR was similar to those of Flu-RH. The spacer, benzothiadiazole, and flanking rhodanine were coplanar, while a dihedral angle of ∼35° between them and the central fluorene unit was exhibited. The nonplanar structure could provide the molecule with nonanisotropic electron transport and reduced tendency to overaggregate, which contributed to an excellent PCE of 4.1% with P3HT as the donor polymer. A major breakthrough in the design of A-D-A type acceptor is the introduction of an indacenodithiophene (IDT) building block as the central “D” unit. The first example, a NFA named DC-IDT2T (Figure 21, molecule 5.4), was reported by Bai et al., where the “A” unit was 1,1-dicyanomethylene-3-indanone (DCI).180 A small optical bandgap with a strong absorption extended to the NIR region was demonstrated. The overall device performance with a low bandgap donor polymer PBDTTT-T-C was 3.93%. Two milestone A-D-A type acceptors were soon reported by modifying DC-IDT2T. The first one, IEIC (Figure 21, molecule 5.5), reported by Lin et al., was obtained by adding a 2-ethylhexyl alkyl chain on each spacer thiophene.181 Properly matched energy levels and strong absorption ensured a high VOC of 0.97 V and a good JSC of 13.55 mA cm−2, which led to an excellent PCE of 6.31% when IEIC was blended with the low bandgap polymer PTB7-Th. Later, Lin et al. reported another milestone molecule named ITIC (Figure 21, molecule 5.6), where the IDT core was fused with the thiophene spacer in DC-IDT2T.182 This modification slightly downshifted the energy levels and enlarged the bandgap. In combination with PTB7-Th, the BHJ solar cell (PTB7-Th:ITIC) exhibited PCE

Figure 18. Chemical structures of selected α-substituted PDIs.

Figure 19. Chemical structures of the NDI and TDI-based NFAs.

constructing NFAs. The electron-rich and electron-deficient moieties combined can extend conjugation and reduce bandgap. An A-(π)-D-(π)-A backbone (hereafter denoted as A-D-A) is typically employed when designing novel NFAs, where “A” and “D” represent the electron-withdrawing and electron-donating moieties, respectively, which are sometimes linked by a π conjugated spacer unit or a second “A” or “D” moiety, to further extend the conjugation (Figure 20). Despite

Figure 20. Cartoon representing a chemical structural template for designing the A-D-A type push−pull NFAs.

the fact that other types of NFAs have also been developed, e.g., a D-A-D small molecular acceptor,175,176 most high performance NFAs are synthesized using this structural template. Besides the easily tuned absorption for spectral breadth and the rigid backbone for reduced reorganization energy that benefits charge conduction, the A-D-A molecules have the electron-rich and electron-deficient moieties located at different parts of the molecule, allowing possible contact between the electron-deficient part of the donor material and the LUMO of the NFA to facilitate charge transfer. Owing to their strong intramolecular charge transfer, the A-D-A backbone typically exhibits strong and broad absorption that can be 3465


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Figure 21. Chemical structures of A-D-A type NFAs. 3466


1.52h 1.67 1.58 2.03 1.82 1.63 2.02 2.0 2.05 1.56

−5.91a

−5.66a −5.52a −5.51a −5.50b −5.50b −5.32a −5.44a −5.58d −5.52a

−5.74a

−5.66c −5.47a −5.62b −5.68b −5.67b −5.43b −6.05b −5.75a −5.93a

−5.40b

−3.83a

−3.93a −3.82a −3.78a −3.63b −3.90b −3.95a −4.19a −3.98a −4.02a

−4.01a

−4.14c −3.76a −3.96b −3.62b −3.79b −3.71b −4.13b −3.70a −3.86a

−3.83b

5.15 5.16 5.17 5.18 5.19 5.20 5.21 5.22 5.23

5.24

5.25 5.26 5.27 5.28 5.29 5.30 5.31 5.32 5.33

5.34

ITIC-Th1

IT-4F ITCC ITCPTC DTBTF DICTF FDICTF CBM IDFBR SFBRCN

NFBDT

−5.45a −5.69a

−3.79a −3.91a

5.12 5.13

3467

1.55

1.60 1.58 1.63 1.54 1.32 1.34 1.24 1.60 1.48

1.70

1.52 1.62

1.52

5.14

−5.41a

−3.81a

5.11

1.68 1.63 1.53

−5.52a −5.51d −5.42a

−3.69a −3.88a −3.82a

IDT-2BR O-IDTBR IDTIDTIC IDTIDSeIC IDSe-T-IC IC-C6IDTIC IC-1IDTIC ITIC-Th m-ITIC IDT-BOC6 ATT-1 ATT-2 IEICO IEICO-4F IT-M INIC3

5.8 5.9 5.10

1.55 1.57 1.59 1.54

−5.43a −5.42a −5.48a −5.5a

−3.85a −3.82a −3.83a −3.8a

5.4 5.5 5.6 5.7

DC-IDT2T IEIC ITIC IDTT-2BM

2.00 2.10 2.14

−5.95c −5.58a −5.70b

−3.95d −3.53a −3.57b

Eg,opt (eV)

HOMO (eV)

LUMO (eV)

5.1 5.2 5.3

internal ref

FEHIDT Flu-RH FBR

acceptor

PBDB-T

PBDB-T-SF PBDB-T PBT1-EH DR3TSBDT PTB7-Th PBDB-T PTB7-Th P3HT PTB7-Th

FTAZ

PDBT-T1 J61 PBDB-T PTB7-Th PTB7-Th PBDTTT-E-T PTB7-Th PBDB-T FTAZ

PDBT-T1

J51 PDBT-T1

J51

P3HT P3HT PTB7-Th

PBDTTT-C-T PTB7-Th PTB7-Th PBDTTT-C-T

P3HT P3HT P3HT

donor

1:0.8

1:1 1:1 1:1 1:0.5 1:1.4 1:1.2 3:7 1:1 1:1.2

1:1.5

1:1 1:1 1:1 1:1.5 1:1.8 1:1 1:1.5 1:1 1:1.5

1:1

1:1 1:1

1:1

1:0.6 1:1 1:1.5

1.2:1 1:1.5 1:1.3 1.5:1

1.2:1 1:1.5 1:1

D:A ratio

processing solvent

CF+1%CN CF DCB+1%DIO CB+1%DIO CB+2%CN CB+2%DIO CB CB+1%DIO CF+0.25% DIO CF+0.25% DIO CB+0.5%DIO CB+1%DIO CF CF CF CB+0.3%DIO CB+2%DIO CB CB+0.75% DIO CF

CF

CF CF

CF

DCB DCB CF+DCB (4:1) DCB+15%CF NR NR DCB+CF (6:4)+3% DIO DCB+3%CN CB DCB

Table 2. Summary of Representative A-D-A Type Acceptors Materials

10.42

13.0 11.4 11.8 3.84 7.93 10.06 5.3 4.5 10.12

12.1

9.6 11.77 9.60 10.07 9.58 8.4 10.0 12.05 11.5

7.39

8.58 8.71

8.02

5.12 6.30 6.48

3.93 6.31 6.80 4.81

2.43 3.08 4.11

PCE (%)

0.868

0.88 1.01 0.95 1.15 0.85 0.94 0.88 0.89 0.90

0.849

0.88 0.912 1.01 0.87 0.73 0.82 0.739 0.94 0.852

0.92

0.91 0.89

0.91

0.84 0.72 0.94

0.90 0.97 0.81 0.851

0.95 1.03 0.82

VOC (V)

17.85

20.50 15.9 16.5 7.42 16.33 15.81 10.6 7.4 17.25

19.33

16.24 18.31 17.52 16.48 20.75 17.7 22.8 17.40 19.68

13.39

15.20 15.05

15.16

8.91 13.9 14.49

8.33 13.55 14.21 9.87

3.82 5.70 7.95

JSC (mA cm‑2)

67.2

71.9 71 75.1 45 55 66 53 68 65.2

73.73

67.1 70.55 54 70 63 58 59.4 73.5 68.5

60

62.0 65

58.0

68.1 60 47.5

52.3 48 59.1 57.2

67 52 63

FF (%)

10−3 10−4 10−5 10−4

2.7 × 10−2 3.25 × 10−4 NR 1.71 × 10−3 1.14 × 10−4 3.82 × 10−4 3.37 × 10−5 1.0 × 10−4 NR 1.5 × 10−4 3.68 × 10−4

7.6 × 10−3 4.32 × 10−4 6.74 × 10−4 2.69 × 10−3 4.13 × 10−5 1.93 × 10−4 2.40 × 10−5 1.9 × 10−6 NR 2.2 × 10−4 1.38 × 10−4

NR 5.05 × 10−4 9.26 × 10−4 3.2 × 10−3 NR NR NR NR NR NR NR

3.0 × 10−4 1.54 × 10−4 5.31 × 10−4 5.13 × 10−4 5.10 × 10−4 1.5 × 10−3 NR 3.33 × 10−4 2.0 × 10−4

4.2 × 10−4 1.30 × 10−4 4.99 × 10−4 2.40 × 10−4 3.69 × 10−4 4.6 × 10−4 NR 1.10 × 10−4 1.4 × 10−4

6.1 × 10−4 2.45 × 10−4 NR NR NR 1.40 × 10−4 1.14 × 10−4 NR 1.7 × 10−4

3.4 × 10−5

1.4 × 10−4

4.5 × 10−4

NR 1.1 × 10−3

8.25 × 10−5 5.1 × 10−5

× × × × 2 × 10−4 NR NR

2.0 4.5 4.3 4.1

7.72 × 10−5 2.9 × 10−4

10−3 10−4 10−4 10−5

7.21 × 10−5

× × × ×

7.87 × 10−5

1.5 1.0 1.1 1.3

1.27 × 10−5

10−4 10−4 10−4 10−5

NR NR NR

H (blend)

2.6 × 10−4 3−6 × 10−6 4.54 × 10−5

× × × ×

NR NR 2.6 × 10−5

E (blend)

3.4 × 10−4 NR NR

3.3 2.1 3.0 1.0

NR NR NR

E (neat)


conventional

inverted conventional conventional conventional conventional conventional inverted inverted inverted

inverted

inverted conventional inverted conventional inverted conventional conventional inverted inverted

inverted

conventional inverted

conventional

conventional inverted conventional

conventional conventional conventional conventional

conventional conventional inverted

device structure

ref

204

19 197 198 199 200 201 202 26 203

33

190 191 192 190 191 193 194 195 196

189

187 188

186

184 25 185

180 181 182 183

177 178 179



Review

up to 6.8%, comparable to PC61BM-based OSCs. A remarkable improvement was recently reported by using a large bandgap polymer PBDTBDD for ITIC.29 Benefiting from suitable energy levels, complementary absorption, and high mobilities, a high VOC of 0.899 V, an excellent JSC of 16.81 mA cm−2, and a FF of 74.2% were obtained, respectively, which resulted in an outstanding efficiency of 11.21%.

a Measured by CV using NFA films. bMeasured by CV in solutions. cMeasured by other methods. dCalculated using measured HOMO/LUMO and the optical bandgap. eDetailed method was not reported. fMeasured using OFET devices. hDetermined from the film absorption onset by the authors of this review; NR Not reported.

ref

205 conventional 1.07 × 10−3 1.86 × 10−3 2.17 × 10−3 −5.50a 5.35 DTCC-IC

−3.87a

1.59

PTB7-Th

1:1

CF+1%CN

6.0

0.95

11.23

56.2

E (blend) E (neat) HOMO (eV) LUMO (eV) internal ref acceptor

Table 2. continued

Eg,opt (eV)

donor

D:A ratio

processing solvent

PCE (%)

VOC (V)

JSC (mA cm‑2)

FF (%)


H (blend)

device structure

Chemical Reviews

5.2. A-D-A Type Acceptors with a Core Unit of IDT or Its Derivatives

5.2.1. Design of the Core Units. Following the success of IEIC and ITIC, new acceptors based on IDT and its derivatives have been developed. On the basis of ITIC, an NFA with indacenodithieno[3,2-b]thiophene (IDTT) as the core unit, named IDTT-2BM (Figure 21, molecule 5.7) was reported by Bai et al.183 Compared to ITIC, the flanking units were changed to 2-(benzo[c][1,2,5]-thiadiazol-4-ylmethylene)-malononitrile (BM), leading to a reduced bandgap and a rigid, coplanar backbone along the entire molecule. However, the electron mobility of IDTT-2BM appeared to be lower than that of ITIC, which caused a largely imbalanced hole/electron mobility and could partially explain the modest FF (∼57%) and PCE (4.81%) with PBDTTT-C-T as the donor polymer. The IDT core has also been introduced to FBR, and the resulting NFA was named IDT-2BR (Figure 21, molecule 5.8).184 It adopted a planar backbone, while the hexylphenyl side groups displayed a dihedral angle of 115° (Figure 22), which facilities charge transport while preventing large phase separation in the blend film. With P3HT as the donor polymer, an excellent performance (5.12%) was achieved.

Figure 22. (a, b) Optimized molecular geometries of IDT-2BR. (c, d) Molecular orbitals of IDT-2BR. Reproduced from ref 184 with permission. Copyright 2015 Royal Society of Chemistry.

Later, Holliday et al.25 reported another effort to introduce IDT core to FBR with n-octyl as the side chains on the IDT core instead of the n-hexylphenyl used in IDT-2BR. This acceptor, O-IDTBR (Figure 21, molecule 5.9), exhibited a redshifted absorption and a more ordered nanostructure than both FBR and IDT-2BR due to an enhanced intermolecular packing, especially after thermal annealing, which emphasized the importance of side chains. The downshifted LUMO level resulted in a smaller VOC of 0.72 V with P3HT, but the broadened absorption boosted the JSC to 13.9 mA cm−2. An overall PCE of 6.30% was achieved for P3HT:O-IDTBR, which was much higher than P3HT:PCBM based solar cells. Li et al. showed an acceptor, IDTIDT-IC (Figure 21, molecule 5.10), with a fused IDTIDT as the central donor unit 3468


Chemical Reviews

Review

general, the “D” moiety consists of multiple conjugated aromatic rings and is typically larger in size than the “A” moiety. Therefore, in most high-performance A-D-A type NFAs, the side chains are substituted on the “D” moiety, to prevent excessive steric hindrance between the “A” units of the acceptor and the electron deficient fragment of the polymer. Drawing on the accomplishment of the IDT-based backbones, engineering the side chains could further improve the device performance and/or offer wider applicability to more donor materials. For instance, Lin et al. replaced the alkylphenyl chains of ITIC with alkylthienyl chains (ITIC-Th, Figure 21, molecule 5.15).190 Compared to phenyl, the thienyl side chains could enhance the nanoscale molecular order induced by the sulfur−sulfur intermolecular interaction, which was beneficial for electron transport. As a result, a PCE as high as 9.6% was achieved in an optimized PDBT-T1:ITIC-Th BHJ, with an excellent JSC of 16.24 mA cm−2 and a high FF of 67.1%. In addition to using structurally different side chains, the position of the side chain can also dramatically vary material properties and device performance. For example, Yang et al. relocated the alkyl chains on the phenyl rings of ITIC, forming an isomer named m-ITIC (Figure 21, molecule 5.16).191 The isomerization caused little effect on the electronic structure of the acceptor but enhanced its intermolecular self-assembly. In the two-dimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) patterns (Figure 23), sharper and

and DCI as the end-capping.185 The 10-heterocyclic fused ring backbone of IDTIDT-IC extended its absorption into the NIR (λedg ∼ 810 nm) region, which gave rise to a high JSC of 14.49 mA cm−2 in combination of PTB7-Th. In spite of the high JSC, the BHJ device was able to achieve a high VOC of 0.94 V with a low energy loss (∼0.59 eV, calculated by subtracting eVOC from the optical bandgap of IDTIDT-IC) and a high IQE (>80%) throughout nearly the entire absorption range. The electron mobility for IDTIDT-IC in the blend film was lower than that of IEIC or ITIC, which was likely the source of a relatively low FF (47.5%). Overall, the best efficiency of PTB7-Th:IDTIDTIC reached 6.48%. On the basis of this work, Li et al. replaced the two sulfur atoms at the end of the IDTIDT core with selenium (Figure 21, molecule 5.11).186 The large Se atoms increased the ground state quinoid resonance character, which improved the electron mobility and lowered the bandgap. When a medium bandgap donor polymer (J51; Figure 6) with an intense absorption in the range of 400−600 nm was used to complement the absorption, and a high efficiency of 8.02% was achieved. The strategy of replacing sulfur with selenium has also been applied to modifying IEIC, which produced a planar acceptor IDSe-T-IC (Figure 21, molecule 5.12) with a slightly lower bandgap than IEIC.187 In combination with J51, a high efficiency up to 8.58% was demonstrated. One marked advantage of the A-D-A type NFA is their facile synthesis. The “A” and “D” units could be distinctly prepared before being connected together. As an example, Lin et al. synthesized a facile planar fused ring molecule named ICC6IDT-IC (Figure 21, molecule 5.13), where an IDT core was directly flanked with DCI units, and the side chains were simple alkyl instead of alkylphenyl groups.188 The BHJ of PDBTT1:IC-C6IDT-IC, where the polymer and NFA showed complementary absorption and compatible electronic property, provided a PCE as high as 8.71%. To understand the structure−property relationship, the authors synthesized a series of structurally similar derivatives with 1−3 IDT as cores and studied the morphological evolution of the series.189 An acceptor IC-1IDT-IC (Figure 21, molecule 5.14) was first obtained by removing the alkyl thiophene spacer in IEIC, and two analogues with 2 or 3 IDT in the cores were developed. It was shown that, with increasing number of IDT in the core, the bandgap as well as the tendency to crystallize of the acceptor decreased, which was consistent with the trend for the electron mobility. Two donor polymers, PTB7-Th and PDBT-T1, were used to construct 6 different BHJs, among which, the best performance (7.39% efficiency) was obtained with PDBTT1:IC-1IDT-IC due to a fine balance among energy level, carrier mobility and film morphology. The aforementioned ICC6IDT-IC was achieved by replacing the alkylphenyl side chains with simple alkyl chains on the IDT core unit. Compared to IC-nIDT-IC (n = 1−3), IC-C6IDT-IC showed a bathochromic shift in its absorption, ordered crystal packing, and an improved electron mobility of 1.1 × 10−3 cm2 V−1 s−1 in neat films and 2.9 × 10−4 cm2 V−1 s−1 in the blend with PDBTT1. These improvements resulted in a high FF and JSC, and an excellent overall performance of 9.20%. It is also worth noting that all of these devices were fabricated from as-cast films without solvent additives in the solution or solvent/thermal annealing after film formation. 5.2.2. Side Chain Effects. Akin to its role in the design of polymeric donor materials, the side chains of A-D-A type NFA are critical in determining the solubility and morphology. In

Figure 23. (a) Line cuts of the 2D-GIWAXS patterns of neat m-ITIC and ITIC films. 2D-GIWAXS patterns of (b) m-ITIC film and (c) ITIC film. Adapted with permission from ref 191. Copyright (2016) American Chemical Society.

more intense π−π stacking peaks were observed in the out-ofplane direction for m-ITIC in both pure and blend films with a medium bandgap polymer (J61; Figure 6). The enhanced intermolecular interaction resulted in an increased electron mobility, which in turn gave rise to an outstanding OPV performance with a PCE of 11.77%, a VOC of 0.912 V, a JSC of 18.31 mA cm−2, and a FF of 70.55%. In addition, the authors demonstrated that the PCE of the device was less sensitive to thickness of the active layer, e.g., a PCE of >8% was maintained 3469


Chemical Reviews

Review

(Eopt g − eVOC) of ∼0.5 eV, indicating a high potential of the PCE. The increasingly smaller energy loss observed in these recent NFA-based systems triggered research to further reduce the energetic offsets between the donor and the NFA. Due to the high PCE and wide applicability of the polymer PBDB-T in the ITIC family NFAs, in order to further reduce energy loss, one facile idea would be to slightly increase the energy levels of the NFA on the basis of ITIC. By introducing one or two methyl groups as the substituents to the end-capping groups (dicycanovinylindan-1-one) of ITIC, Li et al. were able to obtain two novel methyl-modified NFAs named IT-M (Figure 21, molecule 5.22) and IT-DM, respectively.195 Compared to ITIC, incorporation of the methyl groups was found to slightly upshift both the HOMO and LUMO levels, which enhanced the VOC with the same donor polymer and improved the nanoscale morphology in the meantime. Taking all of these effects into account, the best device performance was achieved using PBDB-T:IT-M, which showed a PCEmax of 12.05%, with a VOC of 0.94 V, a JSC of 17.40 mA cm−2, and a FF of 73.5%. Inspired by the success of IDTT as the core and fluorinated DCI as the “A” moiety, Dai et al. constructed a series NFAs using 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2,3-d]thiophene) (IBDT) as the core, which had a further extended, rigid and coplanar structure, as well as a stronger electron donating ability than IDT and IDTT. DCI or fluorinated DCI were used as the end-capping groups.196 The resulting acceptor INIC had a larger rigid plane and stronger electron-donating ability, which enhanced the optical absorption and electron transport. Fluorination on the DCI end groups further enhanced charge transport ability, and lowered the LUMO levels without significantly affecting the HOMO levels. With two fluorine atoms on each DCI unit, the acceptor INIC3 (Figure 21, molecule 5.23) showed the strongest tendency to crystallize, which led to the tightest polymer packing during the cocrystallization of FTAZ (Figure 6) and INIC3. More importantly, the extended coherence length did not result in an excessively large phase separation and a reasonably small domain size in the FTAZ:INIC3 blend was obtained. All of these factors contributed to excellent JSC and FF of 19.68 mA cm−2 and 68.5%, respectively. Although the lower LUMO level led to a relatively smaller VOC of 0.852 V among the acceptors, the overall efficiency of FTAZ:INIC3 reached 11.5%. Furthermore, fluorination of the DCI unit was also applied to ITIC-Th and ITIC.33 Similar to the previous reports, a monofluorinated acceptor, namely ITIC-Th1 (Figure 21, molecule 5.24), exhibited lower energy levels, slightly redshifted absorption, improved intermolecular interaction, and better electron mobility than ITIC-Th. When FTAZ was used as the donor polymer, the blend FTAZ:ITIC-Th1 showed a slightly higher crystallinity and a smaller domain size than FTAZ:ITIC-Th. As a result, a high JSC of 19.33 mA cm−2 and a FF of 73.73% of FTAZ:ITIC-Th1 led to an outstanding efficiency of 12.1%, which was much higher than that base on ITIC-Th. In a recent report, difluorination of each DCI unit of ITIC led to a new acceptor IT-4F (Figure 24).19 To match the downshifted energy levels of IT-4F, a derivative of PBDB-T was developed by fluorinating the BDT building block. In comparison to fullerene-based organic cells where the newly designed polymers must meet the energy level requirement set by PCBM, the capability of synergistically adjusting the energy levels for both donor and acceptor clearly shows the design

even at a thickness of 360 nm, which further illustrated the strong impact of a seemingly minor structural modification. 5.2.3. Effect of the Spacer Unit. Besides creating large fused ring systems, another strategy to rigidify the backbone of the acceptor was to use “conformational locks”, which utilizes noncovalent interaction such as intramolecular hydrogen bonding to lock the coplanar conformation. This strategy has previously been employed to design polymers with high charge carrier mobilities. Since the neighboring aromatic rings are connected by a single bond, the synthesis of such conformational locked NFAs should be more facile than ring fusion. The first attempt was reported by Liu et al.,192 who used dihexyloxybenzene as the spacers to replace the thiophene in IEIC. The new NFA, IDT-BOC6 (Figure 21, molecule 5.17), was compared with an analogue acceptor without oxygen (IDTBC6). It was shown that IDT-BOC6 possessed a more planar structure than IDT-BC6, which offered it a smaller bandgap, a higher electron mobility, and a higher fluorescence quantum yield. When PBDB-T (Figure 6) was used as the donor polymer, a high PCE of 9.6% was achieved. Liu et al. designed two acceptors with thieno-[3,4-b]thiophene as the spacer to replace the thiophene in IEIC, namely ATT-1 (Figure 21, molecule 5.18)190 and ATT-2 (Figure 21, molecule 5.19).191 ATT-2 had the same electron accepting moiety (DCI) as IEIC, while ATT-1 featured a new “A” moiety, 2-(1,1-dicyanomethylene)rhodanine. In addition, the direction of the thieno-[3,4-b]thiophene spacer was different from each other for ATT-1 and ATT-2. Due to a stronger electron-withdrawing ability of DCI, ATT-2 exhibited a smaller optical bandgap, with absorption edge extended to ∼900 nm. Both acceptors were combined with PTB7-Th and achieved PCEs of ∼10% in their BHJ solar cells. In addition, benefiting from efficient photon harvesting in the NIR region, semitransparent PTB7-Th:ATT-2 devices were fabricated and a very promising PCE of 7.74% was achieved, while maintaining an average transmittance of 37% in the visible with good color rendering property. The attempt to further reduce the optical bandgap of these IDT-based A-D-A acceptors and harvesting more photons in the NIR region was reported by replacing alkyl side chains with alkoxy chains on the spacer units of IEIC.193 The new acceptor IEICO (Figure 21, molecule 5.20) has an absorption edge of ∼925 nm due to an upshifted HOMO level compared with IEIC. Consequently, when combined with a low bandgap donor polymer PBDTTT-E-T, IEICO achieved a much higher JSC (17.7 mA cm−2) and PCE (8.4%) than IEIC (JSC = 11.7 mA cm−2 and PCE = 4.9%). Using the optical bandgap of IEICO, the energy loss of this system (Eopt g − eVOC) was as low as 0.52 eV. 5.2.4. Design of the “A” Unit. In addition to the central “D” moiety, the side chains, and the spacer unit, the property of the end-capping “A” moieties is essential for determining the overall electron affinity and bandgap. To be better energetically compatible with narrow bandgap donor polymers, the energy levels of IEICO were downshifted by introducing fluorine atoms on the end-capping DCI units, which also narrowed its optical bandgap to 1.24 eV as a result of the enhanced intramolecular charge transfer effect.194 When the new acceptor, IEICO-4F (Figure 21, molecule 5.21), was combined with the low bandgap polymer (PTB7-Th), an outstanding JSC of 22.8 mA cm−2 was achieved, resulting in an overall efficiency of 10.0%. Similar to IEICO, the device based on PTB7-Th:IEICO-4F also demonstrated a small energy loss 3470


Chemical Reviews

Review

al. used the weak electron-donating unit fluorene as the building block and introduced thiobarbituric acid as the endcapping “A” unit to replace the rhodanine in Flu-RH.199 The acceptor DTBTF (Figure 21, molecule 5.28) showed a large optical bandgap with strong absorption in the range of 400− 600 nm. When blended with a medium bandgap molecular donor DR3TSBDT (Figure 6), the BHJ device demonstrated a high VOC of 1.15 V benefiting from the high LUMO level of the acceptor. However, both the JSC and FF of the optimized device were relatively low, which could be attributed to the relatively low electron mobility and large domains observed in the transmission electron microscopy (TEM) images, leading to an efficiency of 3.84%. In a later report by Li et al., the “A” unit was further replaced by DCI, and a new acceptor DICTF (Figure 21, molecule 5.30) was obtained. The DCI end-group lowered the LUMO level without significantly altering the HOMO level.200 With PTB7-Th as the donor material, the blend PTB7-Th:DICTF showed a higher JSC and FF than those of DR3TSBDT:DTBTF. Overall, the PCE was improved to 7.93%. On the basis of these NFAs, Qiu et al. synthesized a new acceptor FDICTF by rigidifying the core unit and the spacer in DICTF, forming a ladder type large core unit.201 FDICTF possessed two additional alkyl chains on each “lock” sitting between the original core unit and spacer of DICTF. This structure evolution flattened the molecule, slightly up-shifted the LUMO level, and significantly elevated the HOMO level, together causing the absorption to red-shift by ∼65 nm. To complement the red-shifted absorption and modified energy levels, the donor polymer PBDB-T was chosen to form a BHJ. Compared to PBDB-T:DICTF, a significantly improved efficiency up to 10.06% was achieved by PBDB-T:FDICTF. Using 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM) as the end-capping groups, Wang et al. designed A-D-A NFAs with three core units including fluorine, carbazole, and cyclopenta-[2,1-b:3,4-b′]dithiophene.202 The three acceptors were named FBM, CBM (Figure 21, molecule 5.31), and CDTBM, respectively. They exhibited comparable solar cell performance when PTB7-Th was used as the donor polymer, with CBM showing the best PCE of 5.3%. Baran et al. replaced the IDT core in O-IDTBR with indacenodibenzene, which enlarged the bandgap.26 Although the new acceptor IDFBR (Figure 21, molecule 5.32) showed only a 4.5% PCE with P3HT, it showed strong and complementary absorption to O-IDTBR. The ternary blends using IDFBR and IDTBR as the acceptors and P3HT or PTB7Th as the donor demonstrated largely improved photovoltaic performance (see section 9.3.1 for details) and device stability (see section 10.1 for details). Recently, Zhang et al. introduced spirobifluorene as the core unit to FBR. Flanked with BT, the new acceptor, namely SFBRCN (Figure 21, molecule 5.33), contained a strong electron withdrawing unit, 2-(1,1-dicyanomethylene)rhodanine, as the end-capping “A” moiety.203 The acceptor maintained a similarly large bandgap (2.03 eV) to FBR, while the strong electron deficient nature of BT and the end-groups afforded SFBRCN a deep LUMO. Furthermore, the authors showed that the 3D structure of the BT flanked spirobifluorene core prevented the molecule from overaggregation. In combination with the low bandgap polymer PTB7-Th, which provided a complementary absorption to SFBRCN, the BHJ device showed a high PCE of 10.12%. Replacing the benzene unit in the IDT core by a BDT was reported by Kan et al.204 The acceptor NFBDT (Figure 21,

Figure 24. (a) Chemical structures of ITIC and PBDB-T, along with their fluorinated derivatives IT-4F and PBDB-T-SF, respectively. (b) Absorption spectra of neat polymer and NFA films. (c) Energy diagrams of the materials. Adapted with permission from ref 19. Copyright (2017) American Chemical Society.

flexibility of materials for nonfullerene organic solar cells. Compared to ITIC, the red-shifted absorption edge of IT-4F allowed photon harvesting in the NIR, which resulted in a JSC of 20.50 mA cm−2 for the PBDB-T-SF:IT-4F BHJ and a benchmark PCE of 13.0%. In addition, the device exhibited a remarkable PCE of >12% at an active layer thickness of 200 nm. Besides fluorination, Yao et al. reported another strategy to modify the “A” unit by tailoring the aromatic ring of the DCI building block.197 The new acceptor ITCC (Figure 21, molecule 5.26) had a fused thiophene ring, instead of a phenyl ring (ITIC), at each end of the molecule. The stronger electron donating nature of the thiophene than phenyl provided ITCC with a higher lying LUMO level than ITIC. Morphological characterization revealed a denser π−π stacking for ITCC than ITIC, which led to a higher electron mobility. With PBDB-T as the donor polymer, the blend of PBDB-T:ITCC exhibited a higher domain purity than PBDB-T:ITIC, which was responsible for the suppressed bimolecular recombination and enhanced FF of the former blend. Overall, the higher FF and VOC led to a PCE of 11.4% for the PBDB-T:ITCC BHJ device, despite a slightly reduced JSC due to the narrow absorption range relative to ITIC. In another report by Xie et al., the thiophene ring on the “A” unit was fused with the cyclopentane ring at its 3- and 4-positions, which altered the double bond arrangement and thus the electronic structure of the whole molecule.198 The molecule ITCPTC (Figure 21, molecule 5.27) exhibited a slightly smaller bandgap and higher mobility than ITIC. In combination with a large bandgap donor polymer PBT1-EH (Figure 6), the overall PCE reached 11.8%. 5.3. A-D-A Type Acceptors with Other Core Units

In addition to IDT and IDTT, A-D-A type molecules with other core units were also developed as promising NFAs. Ni et 3471


Chemical Reviews

Review

Figure 25. Chemical structures of DPP-based NFAs.

NFAs with other cores and shapes had never halted. For example, the diketopyrrolopyrrole (DPP) based NFAs continue to emerge, and the subphthalocyanine (SubPc) type NFAs have demonstrated planar heterojunction OPV devices with PCEs over 8%.65 We note that phthalocyanine (Pc) dyes are widely employed to design NFAs due to their high extinction coefficient and relatively long exciton diffusion length. Most of the Pc-based NFAs were deposited using vacuum deposition techniques, and the OPV devices were based on planar heterojunction structures. Since this review focuses on solutionprocessed BHJ solar cells, the details of the Pc-based nonfullerene planar heterojunction solar cells are not discussed. Reviews on this topic are available.39,43,206−208 Nonetheless, other NFAs with different structures can inspire the design and application of new moieties in the state-of-the-art molecules. This section surveys these NFA classes and summarize their performance in BHJ OPVs.

molecule 5.34) possessed the same conjugation length as IDTT. Consequently, they showed similar bandgaps. When PBDB-T was used as the donor polymer, a high PCE of 10.42% was achieved by performing solvent vapor annealing to the active layer prior to electrode deposition. In another report by Cao et al., the IDT core in IC-1IDT-IC was replaced by DTCC, which had seven fused rings and five side chains.205 The acceptor DTCC-IC (Figure 21, molecule 5.35) had four alkoxyphenyl and one linear octyl side chains. DTCC-IC had a smaller bandgap than IC-1IDT-IC and showed a high SCLC mobility (∼10−3 cm V−1 s−1), which gave rise to a 6.0% PCE in its BHJ solar cell with PTB7-Th as the donor. 5.4. Summary

Although A-D-A type NFAs were developed much later than PDIs, tens of new structures with excellent performance have been reported to date. The best performance of this type has surpassed those of both fullerenes and PDIs. Similar to PDIs, A-D-A type NFAs are easy to synthesize and modify. The functionalization of the end-groups, spacers, core units, and side chains could be independently realized, which enables numerous structure possibilities. In addition, the HOMO and LUMO levels of A-D-A type NFAs can be easily tuned by adjusting the electron rich and deficient strength of the “D” and “A” building blocks, respectively. As a result, a large number of NFAs with absorption edges in the NIR region have been developed by using strong electron donating “D” units and/or withdrawing “A” units, which is a challenging objective for PDIbased NFAs.

6.1. DPP-Based NFAs

DPP, with a high polarity and a strong electron-withdrawing ability, has been commonly used to construct NFAs. Conjugated NFAs based on DPP typically features (i) easy synthesis and modification, (ii) strong absorption profiles and readily tunable energy levels, (iii) a strong tendency to crystallize, and (iv) high carrier mobilities. Here we summarize recent reports on DPP-based NFA materials (Figure 25 and Table 3). It is worth noting that most of the DPP-based NFAs were studied in combination with P3HT in BHJ solar cells, partially due to the NFAs’ high-lying LUMO levels. Similar to the development of PDI dimers, structure engineering of the spacers of DPP dimers is an effective method to construct different acceptor materials. The first DPP-based acceptor with a PCE above 2% was reported in 2013 by Lin et al. with a structure of 5H-dibenzo[b,d]silole (DBS) flanked by two DPP units (DBS-2DPP, Figure 25,

6. OTHER NFAS Despite the fact that most top-of-the-line PCEs were achieved by either PDI-based or IDT/IDTT-based NFAs, research on 3472


3473

6.4 6.5 6.6 6.7

6.8

6.9 6.10 6.11

6.12

6.13 6.14 6.15

6.16

6.17 6.18 6.19 6.20

6.21 6.22

DPP 1b SF-DPPEH SF(DPPB)4 DTDfBT(TDPP)2

DTI

Cor-NI DIR-2EH BNIDPA-BO

SubPc (2)

BT(TTI-n12)2 NIDCS S4

NTz-Np

Ph-MH Zn(WS3)2 DBFI-T DBFI-DMT

DBFI-EDOT SF-OR

1.83 1.77g 1.64

3.04 1.97 2.22 2.03g 1.89 2.22 1.74 1.73

−5.30a −5.21b −5.31b −5.29b −5.26b −5.26b −5.85e −6.1e −6.28d −5.38b −5.8d −5.6c −5.99e −5.90b −5.61b −6.01c −6.01d −5.60b −5.8a −5.82a −5.72a −5.50b

−3.28a −3.39b −3.65b

−3.53b −3.60b −3.51b −4.33d

−3.9e

−3.24b −3.30b −3.6a

−3.5c

−3.53e −3.42b −3.73b

−3.60b

−3.31b −3.85b −3.8a −3.66a

−3.65a −3.25b 1.70 2.15

2.70 1.55 1.73 1.91

2.21g

1.70 1.79 1.77g 1.52

Eg,opt (eV)

HOMO (eV)

LUMO (eV)

PSEHTT P3HT

P3HT P3HT PSEHTT PSEHTT

DPP-Py P3HT pDTS(FBTTh2)2 P3HT

PTB7

P3HT P3HT PTB7

PBDTTPD

P3HT P3HT P3HT PTB7

P3HT P3HT P3HT

donor

1:2 1:0.5

1:1 1:0.7 1:2 1:2

1:1

1:2 1:2.5 1:1

1:1.5

1:1 1:4 1:3

1:1

2:1 1:1 2:1 1:2

1.2:1 1:1 1:3

D:A ratio DCB CF CF+0.4% DIO NR CF CF DCB+1% CN CB+2% CN NR DCB CB+DCB (9:1) DCB+1% DIO CF CF CB+0.4% DIO CB+DCB (4:1) CF DCB CF CB+3% DIO CB CF

processing solvent

8.10 4.66

2.05 4.10 5.04 6.4

2.81

2.40 2.71 1.93

3.51

1.03 3.05 3.08

1.47

2.69 3.63 5.16 5.00

2.05 3.17 2.37

PCE (%)

0.93 0.96

0.76 0.77 0.86 0.92

0.90

1.05 0.73 0.87

0.935

0.82 1.22 0.98

1.051

0.90 1.10 1.14 0.81

0.97 1.18 0.97

VOC (V)

13.82 7.44

5.59 9.1 10.14 12.56

5.18

3.72 8.04 5.27

7.8

2.75 4.29 8.13

4.6

5.88 6.96 8.29 12.10

4.91 5.35 6.25

JSC (mA cm‑2)

63 65.0

48 59 58 55

60

60 46 42

48

46 58.2 39

30

51 47.5 55 51

43 50.2 39

FF (%)

3.8 × 10−5 1.1 × 10−5 2.1 × 10−4 2.8 × 10−4 1.57 × 10−4 7.53 × 10−3 8.49 × 10−5

1.6 × 10−5 3.3 × 10−6 1.9 × 10−4 1.2 × 10−4 3.32 × 10−2 6.80 × 10−2 6.71 × 10−6

NR 5.0 × 10−7 NR 0.006f 0.056f NR NR

NR 2.00 × 10−5 NR

NR

NR NR 2.4 × 10−4

1.32 × 10−4 1.26 × 10−4 5.2 × 10−5 NR

NR

NR 5.65 × 10−8 NR

NR NR NR

NR

NR NR NR

NR

NR

5.87 × 10−4 NR 1.480 × 10−4 3.1 × 10−3

2.05 × 10−5 NR 1.292 × 10−4 8.2 × 10−4

NR NR NR NR

6.1 × 10−4 NR 9.49 × 10−5

2.8 × 10−5 NR 2.68 × 10−3

3.3 × 10−4 2.8 × 10−4 NR

H (blend)

E (blend)

E (neat)


inverted conventional

conventional inverted inverted inverted

conventional

conventional conventional inverted

conventional

inverted conventional inverted

conventional

conventional conventional conventional inverted

conventional conventional conventional

device structure

ref

226 228

222 227 224 225

221

175 218 220

207

215 216 217

214

211 212 36 213

209 210 439

a Measured by CV using NFA films. bMeasured by CV in solutions. cMeasured by other methods. dCalculated using measured LUMO/HOMO and the optical bandgap. eDetailed method was not reported. fMeasured using OFET devices. gDetermined from the film absorption onset by the authors of this review; NR = not reported.

6.1 6.2 6.3

internal ref

DBS-2DPP F(DPP)2B2 F8-DPPTCN

acceptor

Table 3. Summary of Other Representative NFAs



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Figure 26. Chemical structures of selected NFAs with other core units.

molecule 6.1).209 The high LUMO level of DBS-2DPP led to a high VOC (0.97 V) for the P3HT:DBS-2DPP device, and an optical bandgap of 1.83 eV was beneficial for harvesting the low-energy photons. By changing the spacer to dipropyl fluorene and adding a phenyl end-group at each side of the dimer, a new DPP acceptor F(DPP)2B2 (Figure 25, molecule 6.2) was developed.210 F(DPP)2B2 had a similar absorption range with DBS-2DPP. With an even higher VOC of 1.18 V, the overall PCE reached 3.17%. Later, Li et al. used cyanothiophene to replace the benzene as end-group and used octyl alkyl chains to replace the propyl ones on the central fluorene core.439 The new acceptor F8-DPPTCN (Figure 25, molecule 6.3) showed a lower LUMO level and a narrower bandgap than F(DPP)2B2. As a result, in comparison with P3HT:F(DPP)2B2, an increased JSC but a decreased VOC and FF were observed in P3HT:F8-DPPTCN, leading to a slightly reduced PCE. Constructing tetramers with a 3D or quasi-3D structure was also reported for DPP-based NFAs. For example, a two-layer structured DPP tetramer with [2,2]paracyclophane as the core

unit (1b, Figure 25, molecule 6.4) was developed by Yang et al., showing a PCE of 2.7% with P3HT.211 Another DPP tetramer with spirobifluorene as the core unit was reported later by Wu et al.212 Among several acceptors with different alkyl chains, SF-DPPEH (Figure 25, molecule 6.5) with a branched alkyl chain showed the highest crystallinity after annealing and a PCE of 3.63%. In a later study, a phenyl ring was added at the end of each DPP unit in SF-DPPEH.36 The new acceptor SF(DPPB)4 (Figure 25, molecule 6.6) had an upshifted LUMO level but a reduced optical bandgap compared to SF-DPPEH. Therefore, both the JSC and VOC were enhanced, and an excellent performance of 5.16% was achieved with P3HT. The morphological stability was also demonstrated to be better than the fullerene-based devices by a long-time thermal annealing test. Jung et al. synthesized two DPP-based NFAs. DTBT(TDPP) 2 had a 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT) core flanked by two DPP units, and DTDfBT(TDPP)2 (Figure 25, molecule 6.7) had a fluorinated core.213 3474


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The two NFAs both displayed a bandgap of ∼1.5 eV, but fluorination provided DTDfBT(TDPP)2 with downshifted HOMO and LUMO levels and stronger intermolecular interaction. In combination with the relatively low-crystallinity low-bandgap polymer PTB7-Th, DTBDfT(TDPP)2 showed more crystalline features with reasonably small domains than DTBT(TDPP)2 in their respective blends. The higher hole and electron mobilities measured in the PTB7-Th:DTDfBT(TDPP)2 blend gave rise to an improved PCE (5.0%) over the BHJ based on the nonfluorinated molecule (3.0%). It is worth noting that both efficiencies were obtained through the aid of 1.0% CN as solvent additive in the dichlorobenzene (DCB) solution. All of the devices fabricated without CN showed excessively large domains (PCEs below 1.0%), which indicated that the strong aggregation tendency of the NFAs favored a largely separated phase even with the low crystallinity polymer. This is consistent with the crystalline nature of DPP and suggests that solvent additive or other methods to kinetically lock the morphology during film drying are needed to achieve the small domain sizes.

important for tuning the solubility and crystallinity. Using a molecular donor DPP-Py (Figure 6) to complement the absorption, a high VOC of 1.05 V and a PCE of 2.40% were demonstrated. Another example of linear oligomer was reported by Kwon et al. The molecule NIDCS (Figure 26, molecule 6.14) consisted of a phenyl ring as the core unit, naphthalimides as the flanking units and a thiophene-ethylene spacer.218 Three acceptors with different side chains on the phenyl rings were studied. It was shown that the spacers and the central phenyl ring were nearly coplanar, while a dihedral angle of ∼45° was observed between the thiophene and the flanking naphthalimide unit. All three acceptors showed large bandgaps, and the one without any side chains on the central unit exhibited the best OPV performance (2.71%) with P3HT as the donor. The low electron mobility in the blend (5.65 × 10−8 cm2 V−1 s−1) was the limiting factor for achieving high FF in these blends. The other two derivatives were further studied with different donors. Kwon et al., in a later study, utilized a polymer donor PPDT2FBT (Figure 6) and a NIDCS derivative with longer alkoxy side chains.219 With more compatible energy levels between the donor and acceptor, the VOC was improved to 1.03 V. A thermal annealing step at 90 °C was also found crucial for achieving the high performance, which was explained by the increased crystallinity of the acceptor. The best efficiency was 7.64% benefiting from a high JSC of 11.88 mA cm−2. Another linear acceptor (Figure 26, molecule 6.15) with chlorinated isoindigo (IID) as the central unit, thiophene as the spacer, and naphthalimide as the flanking unit was later reported by McAfee et al.220 The design strategy was that the twisted naphthalimide unit could suppress the overcrystallization tendency, and chlorination on the IID unit could improve charge transport. With a molecular donor p-DTS(FBTTh2)2 (Figure 6), the best efficiency was 1.93%. Chatterjee et al. synthesized a linear acceptor with a naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole as the core, thiophene-ethyne as the spacer and naphthalimide as the end-capping groups (Figure 26, molecule 6.16).221 With P3HT as the donor polymer, the best efficiency was 2.81%. Jinnai et al. demonstrated another example with a simple chemical structure (Figure 26, molecule 6.17).222 The benzothiadiazole-based acceptor had ethyne as the spacer and phthalimide as the flanking unit. The effect of alkyl chains on the phthalimide unit was investigated by comparing five different alkyl chains. The alkyl chains were found to have a strong influence on the energy levels and the crystallinity of the materials and in turn the PV device performance. Noticeably, the overall structural similarity between the molecules allowed the authors to study the structure−property relationship, and a good correlation between JSC and the dispersion component of the surface energy (γd) of the acceptor molecule was observed. The authors hypothesized that the branched alkyl chain could enhance the fraction of the amorphous region and thus expose more conjugated planes to the D:A interface. The best performance was achieved using the acceptor with special 2methylhexyl alkyl chains. When P3HT was used as the donor polymer, an efficiency of 2.05% was achieved. 6.2.3. Twisted Dimers. Similar to the strategy used in constructing PDI dimers, other twisted dimers based on other electron withdrawing building blocks were also developed. Spacers were frequently used to connect the building blocks. In 2013, the synthesis of a novel building block named tetraazabenzodifuoranthene diimides (BFI), which had a large planar backbone structure, was reported by Li et al.223 Different

6.2. Others

Besides the NFAs above, many other structures were also explored (Figure 26). Here we categorize them into fused-ring aromatics, linear oligomers, twisted dimers, and 3D structures. Their properties along with device performance are listed in Table 3. 6.2.1. Fused-Ring Aromatics. Decacyclene triimides (DTI, Figure 26, molecule 6.8) were fused-ring aromatics reported by Pho et al. in 2013, which consisted of a benzene core and three fused naphthalimide units.214 The best efficiency was 1.47% with PBDTTPD (Figure 6) as the donor polymer. A possible factor limiting the efficiency was the large bandgap of DTI, which constrained light harvesting in the blue region. In a later report, an naphthalimide unit connecting to a corannulene (Cor-NI, Figure 26, molecule 6.9) was synthesized by Lu et al.215 The main absorption of Cor-NI was also in the UV range, and the overall performance was 1.03% with P3HT. In another study, Chen et al. reported a rubicence derivative named DIR2EH (Figure 26, molecule 6.10), which had a strong absorption in the range of 400−600 nm.216 Due to a high-lying LUMO level, the VOC was as high as 1.22 V with P3HT, and the PCE reached 3.05% at a low D:A ratio of 1:4. Another acceptor with two naphthalimide units fused onto a diphenylanthrazoline unit was reported by Li et al. with a name of BNIDPA-BO (Figure 26, molecule 6.11).217 BNIDPA-BO had a planar backbone with an absorption edge at ∼550 nm, but the main absorption was in the 300−400 nm region. When PTB7 was used as the donor polymer, the VOC was as high as 0.98 V but the FF was only 39%, which resulted in an overall efficiency of 3.08%. Ebenhoch et al. reported a derivative of SubPc in solutionprocessed BHJ OSCs.207 The acceptor named 2 (Figure 26, molecule 6.12) was obtained by connecting SubPc with a phenoxy group, which had an absorption edge of ∼620 nm. Using PTB7 as the donor polymer, a PCE of 3.5% was achieved. 6.2.2. Linear Oligomers. Linear oligomers can be developed by connecting different building blocks together. One example was BT(TTI-n12)2 (Figure 26, molecule 6.13), where a thienoimide unit was connected to each side of the benzothiadiazole unit via a thiophene spacer.175 This acceptor had an almost planar structure with an absorption edge at ∼650 nm. The linear alkyl chain on the imide unit was proved to be 3475


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of 4.10%, which was higher than that of P3HT:PCBM. Another SF-core based 3D acceptor (SF-OR) was recently reported by Qiu et al., which had a high LUMO level and a high VOC of 0.96 V was achieved with P3HT as the donor polymer.228 Despite a low mobility, the FF still reached 65%, and a high PCE of 4.66% was thus obtained.

spacers were employed to construct dimers with the BFI building block. The first example was DBFI-T (Figure 26, molecule 6.19). With thiophene as the spacer, the two BFI units showed a twisted structure.224 The acceptor had a large bandgap and a deep LUMO level, and achieved a PCE of 5.04% with PSEHTT (Figure 6) as the donor polymer. In a later study, Li et al. explored four BFI dimers with different spacers.225 The one with dimethylthiophene as the spacer (DBFI-DMT, Figure 26, molecule 6.20) showed the largest dihedral angle between the two BFI units, and the best PCE of 6.4% with the same donor polymer PSEHTT. The improved PCE for DBFI-DMT over DBFI-T was attributed to the enhanced electron mobility of DBFI-DMT and its upshifted LUMO level and high EQEs. A subsequent research by Hwang et al. demonstrated a dimer named DBFI-EDOT, where EDOT was used as the spacer to further enlarge the dihedral angle to 76° between the two BFI units.226 With a further enhanced electron mobility and higher energy levels, the overall performance for DBFI-EDOT reached 8.1% with PSEHTT. 6.2.4. Three Dimensional Structures. Constructing a 3D structure is a challenging but promising strategy to develop high-performance acceptors due to the success of fullerenes. An azadipyrromethene dimer with an zinc atom as the core (Zn(WS3)2) was reported by Mao et al. in 2014 (Figure 27),

7. COMPARISON AMONG DIFFERENT CORE UNITS PDI-based NFAs exhibit outstanding thermal and chemical stabilities, and they could achieve extraordinary electron mobilities, proved by their successful application in OTFTs. However, to be solution processable into nanoscale phase separated grains, approaches such as molecular twisting are introduced to overcome the overaggregation caused by excessively strong intermolecular interaction, which dramatically reduced the intrinsic high mobility of PDI. Broadly speaking, most PDI-based NFAs show an electron mobility in the range of 10−6 to 10−4 cm2 V−1 s−1, compared to 10−4 to 10−3 cm2 V−1 s−1for the IDT-based counterparts in blend films with reasonable domain sizes for BHJ solar cell applications. For DPP-based NFAs, an important limitation of their application is their relatively high-lying LUMO levels, which is not compatible with most low-bandgap donor polymers. Regarding the recent rapid development of the A-D-A type NFAs, besides their own distinctive advantages, the other nontrivial reason is the existence of a large library of highperformance large bandgap polymers as electron donor/hole acceptor that could be chosen to match with the NFAs, which generally have strong absorption in the red or even NIR region of the solar spectra. Moreover, the design strategy of large/ medium bandgap polymers is relatively flexible. In contrast, there are less high performance narrow bandgap polymers, especially those with absorption in the NIR region, which limits the maximum current and increases the difficulty in finding the best match with novel PDI-based NFAs. Besides PDIs and A-D-A type NFAs, many other structures were also explored by different groups. Among them, only the BFI-based dimers and the two 3D acceptors achieved PCEs higher than 4%. However, one should not underestimate the potential of these new structures because PDIs and A-D-A type NFAs also struggled from limited efficiencies only a few year ago. Further molecular design of the new acceptors could lead to continuous improvement of their performance. In addition, the structure−property relationships obtained in these NFAs

Figure 27. 3D structure of Zn(WS3)2. Reprinted with permission from ref 227. Copyright 2014 John Wiley and Sons.

showing a 3D structure.227 In spite of the amorphous nature of the molecule, the electron mobility in the blend with P3HT could reach 1.9 × 10−4 cm2 V−1 s−1. With an absorption edge at ∼800 nm, Zn(WS3)2 had a complementary absorption to P3HT. The BHJ solar cell of P3HT:Zn(WS3)2 showed a PCE

Figure 28. (a) Computed deviation from the molecular plane versus experimental PCE. (b) Computed difference between LUMO+1 and LUMO energy versus experimental PCE for a device containing a given electron acceptor. The acceptors are classified as described in ref 229. (c) (Lack of) correlation between molecular size and measured PCE for the considered data set. Reproduced from ref 229 with permission. Copyright 2017 Royal Society of Chemistry. 3476


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developed, the PCE of this device platform has increased from ∼5% to >10% within 3 years.199,233−241 In 2013, Sharenko et al. combined the molecular donor pDTS(FBTTh2)2 with a monomeric PDI acceptor and obtained a PCE of 3.0% for the solution processed BHJ solar cell using 0.4 vol % DIO as solvent additive in the chlorobenzene solution.120 Later, by tuning the solvent additive in the solvent mixture, Chen et al. achieved a PCE of 3.7% using a combination of DTS(FBTTh2)2 and bis-PDI-T,241 while the efficiency was boosted to 5.4% by Kwon et al. using the same molecular donor and an naphthalimide based NFAs (NIDCSMO).240 The same NFA has been employed by Min et al. and a PCE of 5.3% was demonstrated with a BDTT based p-type molecular donor (BDTT-S-TR).238 The PCE was obtained by surveying a series of solvent additive at various concentrations. With the emergence of NFAs such as IEIC and O-IDTBR, the PCE of all SMSC has been further enhanced.239 As an example, Badgujar et al. used O-IDTBR and a rhodanine donor BDT3TR to obtain device with a PCE >7% and an enhanced thermal stability.237 Recently, the rapid PCE growth in IDT/IDTT-based polymer:NFA solar cell field has also promoted the development of relevant all SMSCs. Yang et al. synthesized an A-D-A molecular donor (DRTB-T) using an alkylthienyl-substitued BDT trimer as the core unit capped with 3-ethylrhodanines.235 The wide bandgap donor was combined with IC-C6IDT-IC,188 a narrow bandgap NFA, to obtain an efficient BHJ device with efficiency up to 9.08%. The high PCE was enabled by a solvent vapor annealing (SVA) process: a 30 s dichloromethane SVA increased the PCE from 5.03% (as-cast) to 7.47% and a 60 s dichloromethane SVA further enhanced the PCE to 9.08%. This indicated a dramatic morphology evolution during SVA and was confirmed by morphological characterization, which revealed a clear increase in crystallinity of the blend film, mainly caused by the enhanced intermolecular order of the acceptor. In a more recent work by Bin et al., the authors designed a pair of small molecular donors inspired by the success of the BDTT-alt-FBTA D−A copolymers. The two small molecular donors have different conjugation lengths enabled by the difference in their side chains. Donor H11 had thienyl conjugated side chains while the other donor H12 had alkoxy side chains on the BDT unit. In combination with a highly crystalline low bandgap NFA IDIC, the blend of H11:IDIC and H12:IDIC yielded the best PCEs of 9.73% and 5.51%, respectively, with a thermal annealing at 120 °C for 10 min performed prior to electrode deposition. Compared to the PCEs of the as-cast samples (5.37% for H11:IDIC and 1.83% for H12:IDIC), thermal annealing caused a remarkable enhancement to the device. Through a series of morphological characterizations (Figure 29), thermal annealing has been shown to induce prominent self-aggregation of the small molecules and cause enhanced and more balanced hole and electron mobilities for both material sets. Overall, the extended conjugation of H11 and the extra crystallinity introduced by thermal annealing provided a huge enhancement to the performance of the device, which suggested a lack of phase separation between the components in these two systems originally. Furthermore, on the basis of this work, Qiu et al. designed another molecular donor using cyano end-capping groups and improved the donor:IDIC photovoltaic device performance to 10.11%.234 Most of these reports utilize solvent additives or posttreatments such as SVA or thermal annealing to tune the film

may also be valuable for the development of any other highperformance acceptor materials. As there are increasingly more NFAs emerging each year, it is essential to systematically investigate the common features shared by the high-performance molecules and seek the underlying chemical and physical principle to guide future molecular design. In early 2017, Kuzmich et al. published an analysis article by constructing a database of 80 NFAs (with PCE ≥ 3%) with different core units and molecular geometries and investigated whether common electronic and geometric properties of the NFAs such as nonplanarity and molecular size show any correlation with the overall device performance.229 In line with our discussion, the authors revealed that, despite that the highest PCEs for each NFA class were achieved at an RMSDp between ∼1 and 2, there lacked a direct correlation (Figure 28a) between the PCE and the nonplanarity of the NFA (represented by the root-mean-square distance of the sp2 carbon atoms from the best plane passing across them, RMSDp). Although there were only ∼13 data points with high nonplanarity (RMSDp ≥ 2), the fact that all of them showed PCE ≤ ∼6% suggests that excessive molecular twisting is detrimental to charge transport that limits the overall PCE. In addition, there appears to be no correlation between molecular size (represented by the total number of atoms in a molecule) and device PCE (Figure 28b). However, the authors calculated the difference between LUMO and LUMO+1 (ΔLUMO) for the neutral molecule and found that all of the 22 NFAs with PCE >6% showed 40% increase in PCE compared to the polymer:fullerene binary device. Difference in molecular interaction between NFA and fullerene in a ternary blend could cause variations in phase separation. One report by Chen et al.284 employed the organic alloy formation between the NFA (TPE-4PDI) and PC71BM to explain the tunable VOC in the ternary blend PTB7-Th:TPE4PDI:PC71BM, despite the drastically different chemical nature between the two acceptors. The authors used DFT calculations to simulate the composition dependence of VOC. At a mass electron DOS ratio of 2:1 between PC71BM and TPC-4PDI, the calculated VOC curve fit well with the experimental data, which provides support to their model. In another ternary system reported by Liu et al.,253 where ITIC-Th and PC71BM were blended together with a wide bandgap polymer (PDBTT1), the quasi-linear relationship between VOC and ITIC-Th weight ratio was interpreted by a parallel-like BHJ model. The authors carried out grazing-incidence X-ray diffraction

significantly enhanced JSC compared to either binary device based on the constituent components. In addition, despite the structural dissimilarity between the two NFAs, i.e., one based on perylene bisimide while the other containing an IDT core, the two NFAs demonstrate decent miscibility confirmed by the morphological characterizations, which led to a linear relationship between VOC and film composition. As a result of both improved JSC and tunable VOC, the best ternary device demonstrated a PCE ∼10.3%, much higher than the binary counterparts (∼8.2% and ∼6.4%). In line with these work, Yu et al. employed two NFAs with IDT cores, IT-M and IEIC-O,194 whose bandgaps were 1.58 and 1.37 eV, respectively, in a ternary blend in combination with a large bandgap polymer, J52.276 The similar LUMO levels between IDT and IT-M offered their binary BHJ devices, when each was blended with J52, almost identical VOCs, but the elevated HOMO level of IEIC-O caused a red-shifted absorption, which provided more complementary solar spectrum coverage and was the main source of the improvement in JSC (from binary 17.1 to ternary 19.7 mA/cm2) and thus PCE (from 9.4% binary to 11.1% ternary) at a IT-M:IEICO weight ratio of 4:1. Besides, the structural similarity provides the NFAs with compatible morphology, i.e., the blend morphology of IT-M and IEIC-O can be viewed as the compositional average of morphology formed by the individual material. This is another factor for maintaining the high FF in the ternary blend device. Furthermore, Jiang et al. systematically studied the device performance of a series of polymer:NFA1:NFA2-based ternary blends, where NFA1 and NFA2 are two different nonfullerene acceptors.255 By comparing four different combinations of NFA1 and NFA2 including the structurally similar ITIC-Th and IEIC-Th, the structurally dissimilar ITIC-Th and SF-PDI4 or TPE-PDI4, the authors not only confirmed the role of surface tension in influencing device behavior (tuned vs pinned VOC) but also provided a morphology control method for constructing nonfullerene ternary blends. It was found that the morphology of the ternary blend could be effectively controlled through the use of two NFAs with a small interfacial tension and a polymer with a temperature dependent aggregation property. An enhanced PCE (11.2%) was achieved despite the similar absorption ranges of the two NFAs, which was attributed to the enhanced electron transport through (higher FF) and elevated LUMO level of the third component (higher VOC). Su et al. reported a ternary blend consisting ITIC and IDIC as acceptors and a large bandgap polymer PSTZ as donor.277 However, ITIC and IDIC have almost identical absorption range. The authors attribute the device performance enhancement to the improved morphology and charge transport by tuning the ratio of the two NFAs. These results imply that in addition to broadening absorption range and increase VOC, blending two NFAs also has a potential to finetune the blend morphology. This clearly is another advantage for this device platform as tuning the morphology278 in a controllable fashion has significant implication for charge generation and transport of OPVs. 9.3.2. Blending NFA with a Pair of Polymer Donors. The idea of broadening spectral range using NFA has been extended to all-polymer solar cells. Ding et al. added 6 wt % ITIC into a polymer donor:polymer acceptor blend (PTP8:P(NDI2HD-T)).279 Compared to both polymers, ITIC has a red-shifted absorption. Besides, the addition of ITIC played another critical role of balancing hole and electron mobility 3481


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blends. Additionally, studies on ternary device stability are needed to figure out whether the introduction of fullerenes in NFA OPVs decrease the lifetime of the devices.

10. ENHANCED DEVICE STABILITY OF NONFULLERENE OPV As the maximum reported PCE reached above 13%, OPV device’s operational stability becomes increasingly vital for the objective of commercialization. Possible degradation routes of OPVs are manifold.17,285−287 For example, the morphology optimized for device efficiency might not be in thermodynamical equilibrium, which causes the nanoscale morphology to change during operation, particularly for the fullerene involved systems due to their high diffusivity.288−291 In addition, dimerization of fullerenes caused by photochemical reaction further increases device instability.288−291 Besides, molecular oxygen has been shown to cause both reversible and irreversible effects on performance of the devices based of π-conjugated systems.17,286 As fullerenes play a role in most of these degradation pathways, the replacement of fullerenes with NFAs could be beneficial for device longevity. 10.1. Enhanced Stability of BHJ Solar Cells with IDT Based A-D-A Type NFAs

On the basis of the improved device longevity for P3HT:OIDTBR demonstrated by Holliday et al.25 (section 5.2.1), Baran et al. further revealed that an 800 h exposure to air made the P3HT:PC71BM BHJ solar cell nonfunctional, whereas the device retained 70% of the efficiency under the same condition once the fullerene was replaced with IDTBR (Figure 35).26 The same trend had been observed in these devices under 1.5 AM illumination, unencapsulated in air. Later, Gasparini et al. compared the “burn-in” effect (a rapid decay in the early stage followed by a flattened slower decay at longer times) between P3HT:PC71BM and P3HT:IDTBR devices.27 It was concluded that fullerene dimerization was the primary cause of the photoinduced burn-in. Replacing PC71BM eliminated not only dimerization but also the disorder induced voltage loss. In a later work by Wadsworth et al., the stability of nonfullerene solar cells fabricated using a low bandgap polymer (PffBT4T-2DT) and an A-D-A type acceptor (EH-IDTBR) was systematically compared among various processing solvents for the active layer.28 The authors showed that the devices whose active layers were processed from hydrocarbon solvents such as o-xylene, mesitylene, and trimethylbenzene showed more reproducible device performance. In addition, the mesitylene processed nonfullerene device maintained ∼92% of its initial PCE after 4000 h storage in a N2 atmosphere and ∼80% initial PCE after 250 h under illumination, which outperformed the devices processed from chlorobenzene. The workhorse A-D-A type NFA, ITIC, showed excellent thermal stability in the BHJ solar cells with PBDB-T as the polymer donor.29 In comparison to PBDB-T:PC71BM, PBDBT:ITIC based device exhibited little PCE decline after being thermally annealed at 100 °C for 250 h, while the former dropped by >40% of its initial value (Figure 36). In line with these reports, decent thermal stabilities were also presented in other A-D-A type NFA based solar cells.30,31 Choi et al. synthesized a novel conjugated polymer, 3MT-Th, and evaluated its performance in BHJ solar cells with ITIC as the electron acceptor.32 The active layer was fabricated using nonhalogenated solvent. Compared to PTB7-Th:ITIC, 3MT-

Figure 34. (a) Electron spin resonance (ESR) response of the PBDTTT-E-T:Bis-PC71BM:IEICO ternary blends. (b) Schematic of charge transfer in ternary blend films. (c) EQE spectra of PBDDTTTE-T:IEICO binary and PBDTTT-E-T:Bis-PC71BM:IEICO ternary blend devices. Reprinted with permission from ref 281. Copyright 2017 John Wiley and Sons.

(GIXRD) and R-SoXS studies and concluded that ITIC-Th and PC71BM form their own transport network. In general, NFA and fullerenes have rather different chemical structures and electronic and material properties. Different miscibility between NFA and fullerene may cause a fundamentally different working principle of ternary blend solar cells. For example, both organic alloy and parallel linkage models have been used to explain device behavior in different systems, indicating dramatically different interaction between fullerene and NFA in the blends. Meanwhile, miscibility and morphology characterization are highly demanded in conjunction with the energy diagram and recombination analysis when the model of charge transfer cascade is used to explain device performance since the energetically favorable transition may be spatially forbidden (or vice versa). Broadly speaking, different mechanisms may coexist in determining the overall device performance. Therefore, interpreting device result in NFA/fullerene blends requires a better understanding on the molecular interaction among polymer, fullerene, and NFA. Furthermore, systematic investigation of the voltage loss mechanism is also needed to clarify the role of NFA in such 3482


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Figure 37. Evaluation of device performance stability under ambient conditions up to 1100 h storage. (a) 3MT-Th:ITIC processed from toluene with 0.25% diphenyl ether (DPE) and (b) PTB7-Th:ITIC processed from toluene with 0.25% DPE. The average photovoltaic parameters were obtained from four devices. Reprinted with permission from ref 32. Copyright 2017 John Wiley and Sons.

Figure 35. (a) Shelf storage lifetime (dark, in air) comparison of P3HT:IDTBR:IDFBR device efficiencies with other polymer:fullerene systems. Devices were exposed to ambient conditions over a 1200 h duration or until devices no longer showed any diode behavior. (b) Photostability of P3HT:IDTBR:IDFBR device and polymer:fullerene solar cells (in air, unencapsulated, under AM1.5 illumination at 1 sun) for 90 h. Adapted with permission from ref 26. Copyright 2017 Nature Publishing Group.

In the most recent highly efficient nonfullerene solar cells by Zhao et al.33 and Zhao et al.,19 the ITIC derivatives ITIC-Th1 and IT-4F outperform PC71BM in terms of not only device efficiency but also device longevity. The same observation has also been made by Choi et al. on PTB7-Th:IDT(DCV)2 nonfullerene solar cells compared with the fullerene counterparts.34 In a later report by Guo et al., the authors presented a 10.5% PCE for nonfullerene solar cells based on PTZ1:IDIC. Preliminary evaluation revealed decent thermal stability for the unencapsulated devices with a device area of 0.81 cm2. After a 10 h thermal annealing at 100 °C, the PCE of device still retains 81% of the original value. 10.2. Enhanced Stability of BHJ Solar Cells with PDI or DPP Based NFAs

Figure 36. PCEs of the PBDB-T:PC71BM and PBDB-T:ITIC devices against time of thermal annealing at 100 °C. Reprinted with permission from ref 29. Copyright 2016 John Wiley and Sons.

Since most of the top-of-the-line PCEs of nonfullerene solar cells were achieved using ITIC or other IDT core based NFAs, there are relatively fewer studies on device longevity with acceptors based on PDI or other core units. However, rylene diimides and DPP based molecules are well-known for their pronounced thermal and chemical stabilities. 103,143 As improved PCEs are being progressively published for PDIbased NFAs, systematic studies on PDI based nonfullerene devices are expected to emerge. The work shown below manifest the huge potential of using PDIs to improve device longevity. Lin et al. have evaluated the ambient stability of nonfullerene solar cells based on a series of three-dimensional PDI based acceptors.35 It was shown that the intramolecular twisting of these acceptors not only produced a dramatic difference in

Th:ITIC based devices exhibited superior shelf lifetime with no obvious PCE decay after 1000 h (Figure 37), which was attributed to the robust morphology formed between 3MT-Th and ITIC. By measuring the UV−vis spectra of both pure and blend films under illumination, the authors observed a fast decay of the 0−0 vibronic peak in the neat PTB7-Th film, which was not observed for 3MT-Th. In addition, from the variation of the 649 and 703 nm peaks of the UV−vis spectra upon illumination time (in air), the authors attributed enhanced stability of 3MT-Th:ITIC to mainly the better photostability of the polymer in the blend. 3483


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Figure 38. (a) Normalized UV−vis−NIR absorption spectra of PTB7-Th and ATT-2 in thin films. (b) The standard human eye sensitivity spectra. (c) Shelf stability of ATT-2:PTB7-Th in ambient conditions. The inset shows photographs of the institute abbreviation without (top) and with (bottom) the semitransparent device. (a−c) Reprinted with permission from ref 292. Copyright 2017 John Wiley and Sons. (d) Absorption spectra of PTB7-Th and IHIC pure films and PTB7-Th: IHIC blended films. (e) Visible transmission spectrum of the optimized semitransparent device with the architecture of indium tin oxide (ITO)/zinc oxide (ZnO)/PTB7-Th:IHIC(100 nm)/MoO3/Au(1 nm)/Ag(15 nm). Inset shows the structure of the acceptor molecule. (f) Photographs of Bo Ya Pogoda in Peking University taken with exactly the same camera settings on a sunny day: (left) without any filter; (right) with semitransparent device based on the architecture of ITO/ZnO/PTB7-Th:IHIC(100 nm)/MoO3/Au(1 nm)/Ag(15 nm). (d−f) Reprinted with permission from ref 293. Copyright 2017 John Wiley and Sons.

surprising considering the more thermally stable and less diffusive nature of the A-D-A NFA.262 Shastry et al. demonstrated another avenue to improve device stability.37 By adding high mobility single-walled carbon nanotubes as the third component in a nonfullerene blend formed by PTB7 and a helical PDI dimer(hPDI2), the ternary device showed reduced bimolecular recombination and enhanced stability at both 1 sun and concentrated illumination.

device performance but also resulted in a significant variation in device shelf stability in air. The efficiency of the device based on TPC-PDI4, which was the NFA with the most intramolecular twisting, decreased to a level of 76.8% of the original PCE after 2 weeks. In contrast, the less twisted core offered TPPz-PDI4 and TPE-PDI4-based devices with excellent shelf stability by maintaining over 90% of the original PCE, which outperformed that of the fullerene-based one. This result might be caused by the excessive twisting of the TPC core that could facilitate molecular diffusion in the blend, which is detrimental for morphological stability. Li et al. investigated thermal stability for nonfullerene solar cell devices consisting of P3HT and a DPP based NFA, SF(DPPB) 4.36 After 3 h of thermal annealing at 150 °C, while the nonfullerene device showed no evidence of PCE decay, the fullerene based counterpart displayed an efficiency below 1%.

10.4. Semitransparent Nonfullerene Solar Cell with Enhanced Stability

In addition to stability, the readily tunable optical property of NFAs made them ideal candidates for fabricating semitransparent photovoltaic devices. In ref 292, the authors demonstrated a semitransparent OPV device fabricated using PTB7-Th:ATT-2, whose main absorption located in the range of 600−850 nm, which allowed high transmittance of the blue photons (Figure 38a−c). The normal device with thick electrode showed a high PCE of 9.58% with decent ambient stability, i.e., >8.5% PCE after 14 days in air without encapsulation. By reducing the Ag electrode thickness to below 20 nm, the semitransparent device showed a high PCE of 7.74%. A more recent publication by Wang et al. demonstrated the synthesis of a novel NFA, IHIC, which exhibited strong absorption in the NIR region (∼600−900 nm) with a superior electron mobility.293 The authors exploited the narrow bandgap of IHIC (1.38 eV) and blended it with the archetypal narrow bandgap polymer, PTB7-Th (bandgap ∼1.58 eV). The resultant BHJ solar cell manifested not only a high PCE of

10.3. Enhanced Stability of Ternary Blend Nonfullerene Solar Cells

In the work mentioned in section 10.1,26 in addition to the enhanced stability obtained by replacing fullerene with IDTBR, interestingly, the authors found that the ternary device based on a blend of P3HT:IDTBR:IDFBR (1:0.7:0.3 weight ratio) demonstrated further enhanced dark and photostability compared to the binary devices. 80% initial performance was retained after 800 h exposure to air and 85% was retained after 90 h in air under illumination for the ternary device. Wang et al. reported that using m-ITIC as the third component could significantly enhance the thermal stability compared to the PTB7-Th:PC71BM binary blend, which is not 3484


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9.77% but also a high average visible transmittance of 36% due to the minimal absorption of the blend film in the green-blue wavelength region (Figure 38d−f). Furthermore, an initial stability test revealed that the unencapsulated semitransparent device retained ∼97% of its initial PCE after a continuous illumination for 130 min (AM 1.5G, 1 sun).

qΔV = Eg − qVOC SQ SQ rad rad = (Eg − qV OC ) + (qV OC − qVOC ) + (qVOC − qVOC) rad,belowgap SQ non − rad = (Eg − qV OC ) + qΔVOC + qΔVOC

= ΔE1 + ΔE2 + ΔE3 (2)

10.5. Summary

where q is the elementary charge, ΔV is the voltage loss, VSQ OC is the maximum voltage attainable according to the Shockley− Queisser limit (which assumes no absorption below the optical gap of the cell),300 Vrad OC is the open-circuit voltage when only radiative recombination is present, ΔVrad,belowgap is the voltage OC loss of radiative recombination from sub-bandgap absorption, and ΔVnon−rad is voltage loss due to nonradiative recombinaOC tion. The first term of the energy losses in eq 2 (Eg − qVSQ OC) corresponds to radiative recombination originating from absorption above the bandgap. This loss is unavoidable for any type of solar cell and is typically between 0.25 and 0.30 eV. The second term (qΔVrad,belowgap ) corresponds to additional OC radiative recombination from absorption below the bandgap. For inorganic and perovskite cells, this term is negligible due to steep absorption edges and thus can be neglected from the energy diagram shown in Figure 39.297 In contrast, this loss is typically much higher for OPVs due to the presence of CTS at the donor−acceptor heterojunction as evident from the redshifted absorption spectrum.301 As discussed in section 2, OPVs typically rely on an energy offset between the electron affinity of the donor and acceptor materials to provide a driving force for overcoming the binding energy of the excitons to form charge carriers. The requirement of this energy offset (typically >0.2 eV) to enable efficient charge generation leads to significant voltage loss. For examples, this loss is as large as 0.67 V for P3HT:PCBM blend and 0.2 V for the state-of-the-art PTB7:PCBM blend. In order to minimize this loss in OPVs, it is necessary to reduce the energy difference between the singlet exciton on the donor and/or acceptor and the interfacial CTS without reducing charge generation efficiency. Previous attempts to achieving this goal using blends of donor polymer and fullerene acceptors have generally been unsuccessful, with the gain in PCE due to increased VOC compromised by a large reduction in photocurrent.71 However, recent studies have reported OPV blends of novel donor polymers and nonfullerene acceptors that exhibit highly efficient charge generation efficiencies despite having small donor−acceptor energy offset. Examples of these novel systems include P3TEA:SF-PDI2,87 PTB7Th:IEICO-4F,19 and PvBDTTAZ:O-IDTBR.88 These blends have steep absorption edges as typically found in inorganic cells, and, as a result, the voltage loss corresponding to CTS absorption in these blends is much smaller than that observed in typical high-performance OPVs. The third term (qΔVnon−rad = −kT ln (ηEL)) is due to OC nonradiative recombination, where k is the Boltzmann constant, T is the temperature and ηEL is the electroluminescence (EL) quantum efficiency of the diode when charge carriers are injected into the device in the dark.298,302 Inorganic solar cells typically have ΔVnon−rad in the range of 0.25 V, while this value OC is typically larger for OPVs (0.38−0.44 V).303 The large difference is due to the very low ηEL of bulk-heterojunction blends (typically ∼10−6−10−8) compared to that typically found in silicon or perovskite cells (typically ∼10−2−10−3). It is thus clear that increasing the ηEL of OPVs can lead to lower

Despite the limited number of publications on device stability so far, substitution of fullerenes with NFAs appears to fulfill the expectation of stability improvement. However, other degradation routes still affect device longevity strongly, such as the oxidative degradation of the donor as well as the interfacial layer degradation. Intriguingly, the strategy of designing ternary blend nonfullerene OPVs may not be necessarily detrimental to device longevity. Future research on NFAs should not only focus on their photovoltaic PCE but also on the device longevity and other factors that affect eventual commercialization. Introducing industrial figure of merit is necessary in the near future to evaluate the full potential of a new material.294 Considering that most OPVs in the literature were not optimized for device longevity, there is a huge potential for chemists and device engineers to design more stable NFAs and further improve device stability to essentially reduce the efficiency-stability-cost gap.

11. REDUCING VOLTAGE LOSS One of the most important factors that limit the efficiency of OPVs is the relatively large voltage loss from the bandgap (Eg) of the light absorber to the device open-circuit voltage (VOC). For example, one of the best performing OPVs reported to date, PffBT4T-2OD:PC71BM,295 has a VOC of 0.77 V for an Eg of 1.65 eV, corresponding to a voltage loss of 0.88 V. In contrast, state-of-the-art c-Si or perovskite cells have much lower voltage losses, typically in the range of 0.40−0.55 V.86,87,296 In order to understand the reason behind the large voltage loss suffered by OPV, we have to review the origins of voltage losses in a solar cell. Figure 39 illustrates the voltage loss pathways found in an inorganic (a) and an organic (b) solar cell. Based on detailed balance theory,87,297−299 the overall energy loss for any solar cell (both inorganic and organic) can be attributed to the three terms shown in eq 2.

Figure 39. Energy loss in inorganic and organic solar cells. Adapted with permission from ref 87. Copyright 2016 Nature Publishing Group. 3485


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voltage losses, with 1 order of magnitude increase in ηEL corresponding to 60 mV reduction in ΔVnon−rad . This is indeed OC the case in the model high-efficiency, low-voltage loss OPV blend, P3TEA:SF-PDI2,87 where a high ηEL of ∼10−4 is achieved. Unlike typical OPV blends based on fullerene acceptors, where the electroluminescence that arises from CTS emission is clearly red-shifted relative to the donor/ acceptor singlet emission, the EL spectrum of the P3TEA:SFPDI2 blend resembles that of the pure polymer (P3TEA) device. This indicates that the EL emission of P3TEA:SF-PDI2 is mainly from the singlet excitons in the polymer, which are most likely populated following electron transfer from the CTS enabled by the small donor−acceptor energy offset.304 This finding indicates that reducing the energetic offset at the interface can in principle improve VOC by reducing both ΔVrad,belowgap and ΔVnon−rad and is a promising way to improve OC OC OPV efficiency provided that efficient charge generation can be maintained. Such systems inspire the OPV community to reconsider whether the widely quoted exciton binding energy (∼0.3 eV) was based on a sound theoretical foundation. In general, this value was estimated by calculating the Columbic attraction of an electron and a hole assuming a close contact (e.g., 1 nm) between the two charges.305 However, recent studies have shown that the positive charge (i.e., a hole) on the chain of a polymer might have a large initial excitation volume approaching 20 nm3.306 Consider a polymer as a cylinder, assuming a 0.8 nm radius for the cylinder, the delocalization distance of the exciton on the polymer chain could be as large as 9 nm. Another study measured the electro-absorption signal in a fullerene system using transient absorption spectroscopy and estimated the electron−hole distance to be ∼4 nm, which would significantly reduce the exciton binding energy to 13% within ∼5 years. In addition to concerning the energetics and optical properties, due to the requirement for an interpenetrating bicontinuous network, the chemists need to make balanced assessment on the solubility, aggregation tendency, and molecular miscibility with the donor as well as the potential carrier mobility when designing new NFAs. Developing molecules to satisfy these demands would be the essential goal. Whereas, at the current stage, each material class demonstrates merits fulfilling some of these criterions but shows limitations in the others. Future molecular design should have balanced attentions paid on energetics (influencing charge separation and VOC, see section 11), light absorption, free carrier generation (morphology), charge transport, and synthetic flexibility. 12.1.1. Molecular Design to Enhance Light Absorption. A highly complementary absorption between the donor and acceptor is the prerequisite of a broad and strong EQE spectrum and is one of the most crucial reasons for the success of NFAs. PDI-based NFAs, in general, exhibit absorption coefficients of ∼105 M−1 cm−1 in solution and ∼104 cm−1 in a thin film while typical high performance IDT-based NFAs show absorption coefficients of ∼105 cm−1 in a thin film. Compared to the A-D-A type NFAs whose absorption profiles are more readily to tune, PDI molecules show relatively fixed absorption in the range 500−650 nm. Consequently, design strategies to enhance extinction coefficients and alternating the spectral range are strongly desired for PDI acceptors. As demonstrated in section 4.1.5, multiple cases have shown that the ring-fusion strategy can be used to significantly increase the absorption coefficients and therefore serves as the top candidate for solving this issue. Modifying the molecular geometry through connecting multiple PDI units together can also lead to changes in both absorptivity as well as absorption range. Furthermore, the higher homologue of PDI such as TDIs can be considered as they exhibit higher absorption coefficients and more red-shifted absorption spectra than PDIs. 3486


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intermolecular charge transfer while maintaining adequate solubility and miscibility are strongly needed. Historically, sacrificing aggregation for solubility/miscibility results in many amorphous PDI molecules, which can be employed as structural templates for modifications to occur. Approaches such as ring-fusion and ortho-position functionalization have been proved effective in enhancing the overly reduced planarity and should be further explored on more structures. Novel molecular shapes such as 3D geometries, “double-decker” configurations,165 or “slip-stacked” structures127 can also be further investigated. On the other hand, high performance A-D-A type NFAs typically exhibit an electron mobility in the range of 10−4 to 10−3 cm2 V−1 s−1. In addition, a large fraction of IDT-based NFAs show a preferentially face-on orientation in the blend film, which is a desirable property for charge transport in OPV applications. Since the “D”, “A”, and spacer units can be functionalized independently, the currently established methodologies can still be employed to realize more combination of these moieties. Extending the central conjugation length/area seems to be a useful approach to improve the optical and electronic property. Side chain engineering is another effective method to tune intermolecular interaction. Replacing the alkylphenyl side chain with an alkylthienyl one or changing the grafting site of the side chain have shown positive effects on device performance. Furthermore, in addition to the wellknown fluorination method, using a fused thiophene ring to substitute the phenyl ring can also be applied to tune the electron withdrawing ability and electron mobility when designing the “A” unit. Finally, considering that the PDI-based and IDT-based NFAs showed 9%) at an active layer thickness of 210 nm with a decent FF (66.8%; Figure 45b). It is worth noting that a 10.5% efficient solar cell at an active device area of 81 mm2 was demonstrated for PTZ1:IDIC (Figure 45c). The PCE of the PBDB-T-SF:IT-4F based device in ref 19 also demonstrated a weak dependence on the active layer thickness: a PCE >12% was obtained for active layers with thickness up to 200 nm and a PCE >10% was achieved using an 300 nm-thick active layer (Figure 45d). The authors also obtained an 11.1% efficient device at a device area of 1.00 cm2 in this work. Similarly, a thick active layer and efficient nonfullerene solar cells were achieved using NFAs including IDT-2BR,184 mITIC,191 IT-OM-2,427 di-PBI,133 ITIC,428 etc. The active layers of most efficient OPV devices are processed using chlorinated solvents such as chlorobenzene, o-dichlorobenzene, or chloroform, which are not environmentally friendly solvents. However, previous work have already shown that eco-friendlier solvents can be employed to process the organic layer based on polymer:fullerene blends.10 Similarly, nonchlorinated, environmentally friendlier solvents have also been utilized to process nonfullerene solar cells. A few

between the active layer and the cathode. By comparing the PFN-2TNDI with another interlayer (PFN), the authors delineated the difference of interfacial doping between a fullerene-based BHJ and a nonfullerene based BHJ (Figure 44). A clear interfacial n-doping was observed at the fullerene/ WSCP while the doping at the NFA/WSCP interface was not prominent. Therefore, in terms of reducing contact resistance and forming an ohmic contact with the NFA based active layer, the self-doped PFN-2TNDI outperformed PFN. Furthermore, the authors revealed that PFN-2TNDI could serve the role of dissociating the exciton of the polymer donor at the interface. A pronounced PCE value of 11.1% was achieved in nonfullerene solar cells. This research clearly demonstrates the importance of tailoring interfacial properties among different material systems. To better harvest the potential of the novel nonfullerene active medium, further understanding of the role of different interlayers is strongly demanded. 12.6. Thick Active Layer, Large-Area Devices, Environmentally Friendly Processing, and Roll-to-Roll Printing

The thickness of the active layer of an organic solar cell has a strong impact on its potential for industrial-scale production. In general, the thickness of the active layer of an polymer:fullerene based organic solar cell is from tens to hundreds of nanometers.295,424,425 In comparison, most of the nonfullerene BHJ organic solar cells reported contain an active layer with thickness around or below 100 nm due to the relatively low electron mobility of the NFA, which limits photon harvesting and could affect the overall robustness of the device, particularly 3493


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blocks and molecular design strategies, along with convergent and low-cost synthetic routes have been employed in practice to build over 100 NFAs with tunable spectroscopical and electrochemical properties. NFAs have been employed to combine with either polymer or molecular donors in a variety OPV device platforms including binary/ternary single-junction and multijunction solar cells, which demonstrated not only record power conversion efficiencies but also improved chemical, thermal, and photostability. At the current PCEadvancing pace, efficiencies over 15% can be forecasted for single-junction nonfullerene OPVs in the near future. More excitingly, environmentally friendly solvents, room-temperature processing, large area, and thick-active layer have been exercised by various groups to fabricate NFA-based organic solar cells. These accomplishments manifest the potential for industrial production and eventual commercialization of OPVs based on this material class. On the other hand, morphology and device physics studies have shown that, at the current stage, improving the nanoscale organization and charge carrier conduction of the acceptors by designing new or optimizing known NFAs to match with the high-performance polymers, whose emergence were based on the knowledge gained from the past decades, is still essential to further enhance device performance. Empirical design rules can be gained from analysis of the known material properties and device parameters, which can stimulate the development of new materials and/or discover better material combinations. Nevertheless, a better understanding of the physical processes governing device operation, such as the morphology forming process and charge generation and recombination mechanism, is required to guide future research.

typical examples include the use of anisole to make a 5.43% allpolymer solar cell,429 the use of a toluene/diphenylether mixture to fabricate a 9.73% polymer:NFA solar cell,32 the use of a mixture of THF and IPA to fabricate a highly efficient (11.34%) polymer:NFA solar cell,430 and the use of mesitylene as the active layer solvents to achieve a 11.1% (higher than that processed from chlorobenzene) polymer:NFA solar cell with long-term device stability.28 One of the unique features of organic solar cells is the capability of being roll-to-roll (R2R) printed into large area, flexible devices. Although most R2R based mass production of OPVs were based on fullerene acceptors,431−436 nonfullerene roll-coated devices have also been realized.382,437,438 For instance, Cheng et al. demonstrated roll-coated all polymer solar cells in 2014437 and Liu et. fabricated polymer:NFA based nonfullerene solar cells using a slot-die coating and flexographic printing methods.438 Recently, Liu et al. showed polymer:NFA (PTB7-Th:IEIC) solar cells fabricated via a roll-coating process under ambient conditions.382 The authors compared both flexible ITO and ITO-free substrates (Figure 46), which

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Zonglong Zhu: 0000-0002-8285-9665 Jie Zhang: 0000-0001-8536-5436 Fei Huang: 0000-0001-9665-6642 He Yan: 0000-0003-1780-8308 Author Contributions ∥

G.Z., J.Z., P.C.Y.C., and K.J. contributed equally to this work.

Notes

Figure 46. (a) PEDOT:PSS PH1000 and ZnO coated PET flexible substrate with an Ag grid (ITO-free substrate), (b) slot-die coating of the active layer of PTB7-TH:IEIC and top PEDOT:PSS layer on the mini roll-coater in sequence, (c) flexographic printing of the top Ag electrode, and (d) long stripes of roll-coated nonfullerene OSCs based on the ITO-free substrate. Reproduced from ref 437 with permission. Copyright 2015 Royal Society of Chemistry.

The authors declare no competing financial interest. Biographies Guangye Zhang received his Ph.D. degree in 2015 from University of California, Los Angeles for his study in the field of organic photovoltaics under the direction of Professor Benjamin Schwartz. He is currently a research assistant professor at HKUST. Besides organic and hybrid solar cells, his research interests include optoelectronic techniques such as transient photovoltage, charge extraction, and photocurrent spectral response.

exhibited PCEs up to 2.26% and 1.79%, respectively, comparable to the reference devices with fullerene acceptors under the same conditions. Noticeably, the nonfullerene devices displayed better dark shelf stability than the fullerenebased devices.

Jingbo Zhao received his B.Sc. degree in Peking University in 2012, after which he joined Professor He Yan’s group in HKUST and obtained his Ph.D. degree in 2015. He worked as a Postdoctoral Fellow in the same group and then moved to City University of Hong Kong as a Research Fellow in Professor Alex K.-Y. Jen’s group. His research interests include development of functional materials for organic and perovskite solar cells, understanding the structure-

13. CONCLUDING REMARKS The examples presented in this review clearly demonstrate that the nonfullerene acceptor is a promising class of materials to replace fullerenes in organic solar cells. Diversified building 3494


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performance relationship of organic semiconductors and fabrication of flexible electronic devices.

Technology Commission for the support through projects ITCCNERC14SC01 and ITS/083/15.

Philip C. Y. Chow obtained his Ph.D. degree from the University of Cambridge in 2016 for his studies of charge dynamics in organic solar cells. He is currently a Research Assistant Professor at HKUST with research interests in organic optoelectronic devices.

REFERENCES (1) Office of Science; U.S. Department of Energy. Basic Research Needs for Solar Energy Utilization; 2005. (2) U.S. Energy Information Administration. International Energy Outlook 2016; www.eia.gov/forecasts/ieo/pdf/0484(2016).pdf. (3) Espinosa, N.; Hösel, M.; Angmo, D.; Krebs, F. C. Solar Cells with One-Day Energy Payback for the Factories of the Future. Energy Environ. Sci. 2012, 5, 5117−5132. (4) Krebs, F. C.; Espinosa, N.; Hösel, M.; Søndergaard, R. R.; Jørgensen, M. 25th Anniversary Article: Rise to Power - OPV-Based Solar Parks. Adv. Mater. 2014, 26, 29−39. (5) Lacal-Arantegui, R. A. J.-W.; Andrei Bocin-Dumitriu, B. S.; Zubi, G.; Magagna, D.; Carlsson, J.; Moss, R.; Fortes, M.; del, M. P.; Lazarou, S.; Baxter, D.; Scarlat, N.; et al. Technology Map of the European Strategic Energy Technology Plan, 2013; http://setis.ec. europa.eu/system/files/2013TechnologyMap.pdf. (6) Mazzio, K. A.; Luscombe, C. K. The Future of Organic Photovoltaics. Chem. Soc. Rev. 2015, 44, 78−90. (7) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science (Washington, DC, U. S.) 1995, 270, 1789−1791. (8) Heeger, A. J. 25th Anniversary Article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26, 10−28. (9) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666−12731. (10) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (11) Jin, Y.; Chen, Z.; Xiao, M.; Peng, J.; Fan, B.; Ying, L.; Zhang, G.; Jiang, X.-F.; Yin, Q.; Liang, Z.; et al. Thick Film Polymer Solar Cells Based on Naphtho[1,2- c:5,6- c ]bis[1,2,5]thiadiazole Conjugated Polymers with Efficiency over 11%. Adv. Energy Mater. 2017, 7, 1700944. (12) Hummelen, J. C.; Knight, B. W.; Lepeq, F.; Wudl, F.; Yao, J.; Wilkins, C. L. Preparation and Characterization of Fulleroid and Methanofullerene Derivatives. J. Org. Chem. 1995, 60, 532−538. (13) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Efficient Methano[70]fullerene/MDMO-PPV Bulk Heterojunction Photovoltaic Cells. Angew. Chem., Int. Ed. 2003, 42, 3371−3375. (14) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science (Washington, DC, U. S.) 1992, 258, 1474−1476. (15) Liu, T.; Troisi, A. What Makes Fullerene Acceptors Special as Electron Acceptors in Organic Solar Cells and How to Replace Them. Adv. Mater. 2013, 25, 1038−1041. (16) Ganesamoorthy, R.; Sathiyan, G.; Sakthivel, P. Review: Fullerene Based Acceptors for Efficient Bulk Heterojunction Organic Solar Cell Applications. Sol. Energy Mater. Sol. Cells 2017, 161, 102− 148. (17) Fraga Domínguez, I.; Distler, A.; Lüer, L. Stability of Organic Solar Cells: The Influence of Nanostructured Carbon Materials. Adv. Energy Mater. 2017, 7, 1601320. (18) Huang, S.; Zhang, G.; Knutson, N. S.; Fontana, M. T.; Huber, R. C.; Ferreira, A. S.; Tolbert, S. H.; Schwartz, B. J.; Rubin, Y. Beyond PCBM: Methoxylated 1,4-bisbenzyl[60]fullerene Adducts for Efficient Organic Solar Cells. J. Mater. Chem. A 2016, 4, 416−424. (19) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148−7151. (20) Cui, Y.; Yao, H.; Gao, B.; Qin, Y.; Zhang, S.; Yang, B.; He, C.; Xu, B.; Hou, J. Fine-Tuned Photoactive and Interconnection Layers

Kui Jiang obtained his B.S. degree in Chemistry from Wuhan University with Professor Jingui Qin in 2011. After that, he moved to the Hong Kong University of Science and Technology. He is currently a Ph.D. candidate under the direction of Professor He (Henry) Yan. His current research focuses on the devices fabrication for photovoltaic solar cells. Jianquan Zhang received his B.S. degree in Polymer Science and Engineering from Zhejiang University, People’s Republic of China, in 2015. He is currently a Ph.D. student under the supervision of Professor He Yan. His interests lie in the synthesis of novel donor polymers and small molecular acceptors for nonfullerene organic solar cells. Zonglong Zhu received his Ph.D. degree in Hong Kong University of Science and Technology in 2015 and worked as a postdoc fellow in Materials Science & Engineering Department at the University of Washington until 2017. Currently, he works in Prof. Henry Yan’s group as a visiting scholar. His current research is in electronic devices based on inorganic/organic perovskites, especially for their applications in photovoltaics and light-emitting diodes. Jie Zhang received his B.S. degree in Chemistry from Northeast Normal University in 2007 and gained his Ph.D. degree in Materials Science from the South China University of Technology in 2012 under the supervision of Prof. Yong Cao and Prof. Fei Huang. His research interests are conjugated functional materials for organic/polymer optoelectronics. Fei Huang received his B.S. degree in Chemistry from Peking University in 2000 and gained his Ph.D. degree in Materials Science from the South China University of Technology in 2005 under the supervision of Prof. Yong Cao. After postdoctoral work at University of Washington with Prof. Alex K.-Y. Jen, he began his academic career in 2009 as a professor of South China University of Technology. His main interests are in the fields of organic functional materials and devices for optoelectronics. He (Henry) Yan received his B.Sc. degree from Peking University in 2000 and obtained his Ph.D. degree from Northwestern University in 2004. He spent most his research career at Polyera Corporation before joining HKUST in 2012. He is currently an associate professor in the department of chemistry at HKUST, an associate director of HKUST Energy Institute, and an adjunct professor at South China University of Technology. His main research interests focus on organic semiconducting materials for organic and perovskite solar cells.

ACKNOWLEDGMENTS The work described in this paper was partially supported by the National Basic Research Program of China (973 Program Project Numbers 2013CB834701 and 2014CB643501), the ShenZhen Technology and Innovation Commission (Project Number JCYJ20170413173814007), the Hong Kong Research Grants Council (Project Nos. T23-407/13 N, N_HKUST623/ 13, 16305915, 16322416, 606012, 16306117, and 16303917), HK JEBN Limited, HKUST president’s office (Project FP201), and the National Science Foundation of China (No. 21374090). We especially thank Hong Kong Innovation and 3495


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Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic

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