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More than Conformational “Twisting” or “Coplanarity”: Mo-lecular Strategies for Designing High-efficiency Non-fullerene Organic Solar Cells Chuanlang Zhan, and Jiannian Yao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04339 • Publication Date (Web): 08 Mar 2016 Downloaded from http://pubs.acs.org on March 13, 2016

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Chemistry of Materials

More than Conformational “Twisting” or “Coplanarity”: Molecular Strategies for Designing High-efficiency Non-fullerene Organic Solar Cells Chuanlang Zhan* and Jiannian Yao Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China ABSTRACT: The power conversion efficiencies (PCEs) of organic solar cells are lower than that of recently emerging perovskite solar cells. Can a PCE of >12% be achieved with single-junction organic solar cells? To achieve a high PCE, much effort has been focused on the design and synthesis of electron-donor materials, including polymers and small molecules, and on innovative solar cell device structures. In this perspective, we focus on a different approach  replacing traditional fullerene acceptors with nonfullerene organic acceptors. This method is an interesting and powerful alternative for achieving more efficient organic solar cells because the molecular structures of organic acceptors can be easily chemically modified and their optoelectronic properties and aggregation behaviors are tunable. However, the film morphology affects charge separation, transport and collection and must therefore be considered when improving the electrical performance of non-fullerene organic solar cells (NF-OSCs). Herein, we discuss molecular strategies for obtaining high-efficiency non-fullerene organic acceptors, with particular focus on small molecules. We also highlight the challenges and opportunities in this field, namely selecting novel donor-acceptor combinations, enhancing the material absorptivity to capture more solar photons, controlling the donor-acceptor interface structure to improve the charge separation, and tailoring the π-π−stacking structures and orientations to enhance mobile carrier transport and collection. NF-OSCs that are more efficient than traditional fullerene-based organic solar cells should be obtained by tailoring the organic acceptor structure and developing appropriate film-processing techniques, because these approaches are expected to produce NF-OSCs with a higher open-circuit voltage than their fullerene counterparts and comparable short-circuit current densities and fill factors.

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Content 1.

Introduction

2.

Factors limiting the efficiency of an organic solar cell and related countermeasures

3.

Molecular strategy overview

4.

Twisted small-molecule approach 4.1. Molecular strategies for obtaining a twisted conformation and its effect on the film morphology 4.2. Adjusting the light absorption and energy levels

5.

Coplanar small-molecule approach

6.

Organic polymer acceptors

7.

Film-processing techniques efficiency NF-OSCs

8.

Challenges and opportunities

9.

Conclusions

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obtaining

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erene OSCs. The development and commercialization of highefficiency OSCs in which an organic n-type molecule replaces the fullerene-based acceptor material are attractive because they can be constructed from many different organic donoracceptor combinations and can be produced cheaply on a large scale via readily accessible organic synthesis methods. An NF-OSC utilizes a non-fullerene organic acceptor (polymer and/or small molecule) and organic donor (polymer and/or small molecule) blend as the photoactive material that captures solar photons. In this perspective, we focus on recent advances in non-fullerene organic acceptors, giving particular attention to the concepts of and strategies for designing efficient small-molecule acceptors. Several reviews of smallmolecule and polymeric organic acceptors were published before 2014,10-17 and more reviews have since appeared in the literature.18-22 In addition, this topic has been discussed in other recent reviews.1,23-26 If interested, readers can access and read these articles for more information.

high2. Factors limiting the efficiency of an organic solar cell and related countermeasures

1. Introduction New generation solar cells1 include organic, inorganicorganic hybrid (e.g., inorganic–organic semiconductors),2 quantum dot,3 dye-sensitized, and perovskite (e.g., organic– lead halide hybrid) solar cells, among others. Recent research on perovskite solar cells has made great progress; the solar-toelectrical power conversion efficiency (PCE) of the optimal perovskite solar cell has reached a certified value of 20.1%,4 which is approximately 2 times higher than that of state-ofthe-art single-junction organic solar cells (OSCs).5,6 Achieving a higher OSC efficiency is challenging, and their efficiency affects their competitiveness and commercial potential. Traditional OSCs utilize fullerene derivatives, typically, [6,6]phenyl-C61 (or C71)-butyric acid methyl ester (PC61BM or PC71BM, respectively; unless otherwise noted, PCBM is used to denote both PC61BM or PC71BM) as the electron-acceptor material. PCBM is a good electron acceptor and electrontransporting material. Its blends with electron-donor materials have ideal film morphologies for balancing charge dissociation and transport. However, the fullerene backbone cannot be easily chemically modified, and hence, the fullerene’s absorption and energy levels cannot be readily tuned, although several PCBM analogues with higher lowest unoccupied molecular orbital (LUMO) energies have been reported, demonstrating their potential for use as acceptor materials.7 Organic acceptors have more potential than fullerene derivatives because of their diverse molecular structures and easily tunable absorption and energy levels. Recently, much research has focused on developing highly efficient non-fullerene organic acceptors, which are expected to replace PCBM acceptors. In 2013, the PCE of single-junction non-fullerene organic solar cells (NFOSCs) was only 4%8 and has now reached a certified value of 8.3%,9 making NF-OSCs more competitive to traditional full-

A solar cell device converts solar photons into electrons. This conversion process involves four sub-processes (Figure 1): (1) solar photon absorption and exciton generation (ηg), (2) exciton diffusion and subsequent separation at the donoracceptor interface (ηs), (3) mobile electron and hole transport (ηt), and (4) mobile charge collection (ηc) and electrical power output. The total photon-to-electron conversion efficiency is the product of the four sub-process efficiencies: ηtotal = ηg ⋅ ηs ⋅ ηt ⋅ ηc.

Figure 1. Diagram showing a typical laboratory-scale solar cell model and the working principles of photon-to-electron conversion in a typical solar cell: the numbers 1, 2, 3 and 4 represent the exciton generation, exciton dissociation, charge transport and charge collection sub-processes, respectively. The dashed lines represent the possible recombination loss processes. TCO denotes the transparent conducting oxide.

Solar photons are captured by the photoactive layer and subsequently separated at the donor-acceptor interface due to the differences in the donor and acceptor LUMO energies (∆ELUMO = ELUMO,donor − ELUMO,acceptor) and highest occupied molecular orbital (HOMO) energies (∆EHOMO = EHOMO,donor − EHOMO,acceptor). In general, ∆ELUMO and ∆EHOMO should be larger

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than 0.3 eV for efficient exciton separation. The energy difference between the acceptor LUMO and donor HOMO (∆EDA = ELUMO,acceptor − EHOMO,donor) is related to the open-circuit voltage (Voc) via the following empirical relationship: Voc = (∆EDA − E0)/q, where E0 is an empirical factor with a typical value of 0.3−0.4 eV, and q is the electron charge (−1.602×10−19 C).27-29 Nevertheless, recent studies found that experimental Voc values can be linearly correlated to the energy level of the chargetransfer state (CT state) generated at the donor-acceptor interface.30-33 These results suggest that additional factors other than ∆EDA influence Voc. As observed for many donor-acceptor systems, a higher acceptor LUMO energy and lower donor HOMO energy generally lead to a higher Voc. In short, the light absorption and energy levels of the donor-acceptor system are the two primary considerations in designing highefficiency photovoltaic materials (Figure 2).

Figure 2. Illustration of five prerequisites for obtaining efficient photovoltaic materials.

Moreover, the photoactive layer film morphology plays a vital role in the photon-to-electron conversion efficiency. As a solid film is formed from a dilute solution, the compounds in the solution form aggregates. Consequently, depending on the π−electron couplings of the organic semiconductor molecules in the film, the film absorption spectrum can shift hypsochromically or bathochromically relative to the solution spectrum, and the absorption coefficient, band shape and band gap also change. The exciton diffusion, exciton dissociation and charge transport efficiencies all depend on the π−electron couplings. It was observed that increasing the solid-state order of the donor and acceptor domains, for example, by using a high boiling point additive solvent, can dramatically reduce the geminate recombination loss of bound electron-hole pairs, e.g., the bound CT states.34 The reduction in the geminate losses might result from the fact that the local crystallinity of the electron-acceptor domains can promote the delocalization of the geminate electron-hole pair states, thereby facilitating charge dissociation at the donor-acceptor interface.35 Moreover, it has been noted that the solid-state order affects the energy levels and order of the frontier molecular orbitals, for example, the HOMO.5,36 After donor-to-acceptor phase separation is achieved, the phase purity strongly affects the recombination losses of the mobile carriers. “Designing” the film mor-

phology is one goal of the OSC community. It is well known that weak noncovalent forces, such as van de Waals, ππ−stacking, dipolar, and hydrogen-bonding interactions, thermodynamically and kinetically control the aggregation and donor-to-acceptor phase separation behaviors of organic semiconductor molecules. A subtle change in the molecular conformation and structure, either in the conjugated backbone or side chains, generally leads to a change in the weak noncovalent forces. For example, the interaction strength and modes, which affect the aggregation behavior and aggregate structure, can be varied as widely demonstrated by molecular selfassembly studies. Engineering the noncovalent forces between photovoltaic-responsive molecules is thus crucial for controlling the film morphology and improving the photon-toelectron conversion efficiency. It is the third prerequisite for designing high-efficiency photovoltaic materials (Figure 2). The molecular conformation, conjugated backbone and side chains are three important considerations in designing photovoltaic materials, not only for improving the solution processability but also for controlling the film morphology. In principle, the effective exciton diffusion length of an organic semiconductor film is very small, typically 5–20 nm. This length is much smaller than the photoactive film thickness, which is typically 100–300 nm. Bulk heterojunction (BHJ) structures37 cleverly bridge the gap between the small exciton diffusion length and large film thickness. These structures can be obtained by easy-handling, cheap solutionprocessing methods. For example, it can be obtained by spincoating a donor-acceptor solution on a surface-modified transparent electrode, or by ink-printing and roll-to-roll (R2R) techniques. In any solution-processing methods, the donor-toacceptor phase separation and subsequent formation of interpenetrating nanoscale networks are both mainly controlled by kinetic mechanisms, which typically lead to the formation of a metastable state. Because of the weak and noncovalent nature of the intermolecular forces, not only the aggregation and phase-separation behaviors but also the metastable nanoaggregate domain structures depend on the film-forming kinetic conditions. Film processing is generally necessary for obtaining an optimal film morphology and is the fourth factor that must be considered in the design of high-efficiency photovoltaic materials (Figure 2). The film morphology can be optimized by changing the host processing solvent, utilizing an additive solvent,38,39 and/or employing thermal-,40 solvent-,41 or solvent-vapor-annealing techniques.42 In addition to the photoactive layer, the interface between the photoactive layer and electrode and the solar cell device structure play critical roles in the efficiency of photovoltaic materials. Interfacial engineering has been shown to be an efficient method for improving the electrical performance of OSCs. This engineering approach includes synthesizing new materials for the electron and hole transport layer43-49 and engineering the energy levels of the photoactive layer and cathode/anode interlayer to make them comparable.50 Moreover, it was found that normal and inverted cell structures have different solar cell performances, partially due to the vertical phase separation of the blended donor-acceptor materials.51 General-

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ly, a higher efficiency can be obtained with a tandem cell structure than with the corresponding single-junction solar cell.52-54 In this perspective, we focus on the synthesis of organic acceptor materials and on the photoactive layer film morphology. The amorphous nature of organic semiconductor films and the energy barriers between aggregate domains also affect charge transport across the photoactive film. Light absorption and charge transport have opposite dependences on the photoactive layer thickness in an OSC. Such a trade-off between the light absorption and charge transport leads to a thinner film thickness, which is normally 100 nm for most of the optimal OSC devices. Recently, several studies reported a series of crystalline polymer donors for which the photoactive layer in the optimal PCBM-based OSCs were up to 300 nm thick.6,55,56 This result might be because the carrier mobility is less dependent on the film thickness in these systems.55 A thicker film is required not only for capturing more solar photons but also for manufacturing large-area solar cells by the R2R technique. Unfortunately, the photoactive layer in recently reported efficient NF-OSCs is generally approximately 100 nm thick. In this context, the discovery of organic acceptors that can be used to fabricate a single-junction photoactive blend film up to 300 nm thick for use in a solar cell with a satisfactory electrical performance is urgent. Moreover, both monomolecular and bimolecular recombination losses occur in OSCs. They include (1) non-geminate losses from trapping in deep states and from bimolecular recombination of mobile electrons and holes57 and (2) geminate losses originating from bound electron-hole pairs. The latter losses include the decay and recombination of donor and acceptor excitons and charge-transfer excitons formed at the donor-acceptor interfaces (e.g., CT states) (shown in Figure 1 by the dashed lines).58 Non-geminate losses can be reduced by optimizing the film properties, such as the phase purity and energy levels, and by passivating the organic-to-electrode interface. Geminate losses are associated with the donoracceptor interface structure, phase size, energy levels and internal field strength. The geminate losses determine the number of excitons lost before full dissociation to form mobile carriers, whereas the non-geminate losses determine the number of the mobile carriers lost before collection by the electrode.22 Additionally, geminate recombination affects the loss in Voc.59 The fill factors (FFs) of NF-OSCs are typically only 60%, and their short-circuit current densities (Jsc) are generally lower than those of the corresponding fullerene OSCs. The lower FF suggests that in NF-OSCs, the non-geminate recombination losses are large, or the electron-to-hole mobilities are unbalanced. The lower FF could be also caused by fielddependent charge generation.60 Accordingly, it is necessary to find new film-processing methods to optimize the film morphology. 3. Molecular strategy overview Of the five prerequisites, the light absorption and energy levels are associated with molecular optoelectronic properties and can be rationally designed to some extent based on mod-

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ern chemistry. The solution processability and film-processing technique both depend on the solubility of an organic molecule in a given solvent. According to the “like dissolves like” principle, the side chains on the conjugation backbone can be modified to improve the solubility. Moreover, the conformations of and π-π interactions between the conjugated backbones determine their stacking strength and mode, which are important structural factors affecting the solubility, film morphology and even kinetic conditions of the film-processing method.

Figure 3. Diagrams showing the different morphologies formed by a typical perylene diimide molecule and the widely used PC71BM. Perylene diimide tends to form crystalline structures (a),65 whereas PC71BM tends to form nanoscale aggregates (black domains) when blended with a donor material, such as an esterterminated diketopyrrolopyrrole molecule (b).68 The TEM images were adapted with permission from Ref. 65 (a), Copyright 2013, The Royal Society of Chemistry, and Ref. 68 (b), Copyright 2014, John Wiley and Sons.

It is well known that the perylene diimide (PDI) chromophore is an excellent n-type semiconductor moiety. Its absorption coefficient in solution is on the order of 104 M-1·cm-1. Its absorption band occurs in the visible region, typically between 400 and 600 nm, and can be red-shifted or blue-shifted, depending on electron-donating or electron-accepting substitutions on the bay region. The absorption band of a PDIderivative thin film can also be shifted relative to that of its solution; the shift direction depends on the perylene π-electron interactions, which lead to the formation of either an H- or Jtype aggregate.61 The LUMO and HOMO energy levels of the PDI motif are approximately −4.0 and −6.0 eV, respectively, showing that this motif has a good electron affinity. Both the LUMO and HOMO levels can be tailored by the substituent groups on the bay region. Its solubility and aggregation behaviors can be controlled by the substituent groups on the bay region and/or on the nitrogen atoms. Studies have clearly demonstrated that the electron mobility in a PDI film measured by organic field-effect transistor or organic thin-film transistor techniques can be greater than 1 cm2/(V·s).62 Altogether, the PDI chromophore has excellent optoelectronic

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properties that are suitable for constructing non-fullerene organic acceptors. However, studies of the molecular selfassembly of PDI derivatives23,63,64 show that traditional PDI derivatives such as that65 shown in Figure 3a tend to form crystalline aggregates of up to hundreds of nanometers in size. This size is much larger than the exciton diffusion length (typically 5-20 nm), and therefore, excitons can be effectively quenched before they reach the donor-acceptor interface.66,67 In contrast, when PCBM is blended with a donor material, it tends to form nanoscale aggregates that are similar in size to the exciton diffusion length (Figure 3b).68 Therefore, the use of PDI chromophores as effective non-fullerene acceptors is severely limited by its aggregation behavior, and its solution processability must be improved to obtain nanoscale domains when it is blended with a donor material.

Figure 4. Diagrams showing two typical conjugated backbone conformations (twisted and planar) of non-fullerene smallmolecule acceptors and the possible π-π stacks formed by these backbones. The arrows show the possible π-π−stacking and electron transport directions.

Studies of the self-assembly of PDI derivatives showed that as the number of substituent groups at the 1-, 6-, 7-, and 12positions (bay region) increases from one to four, the perylene core becomes increasingly twisted due to steric effects between the substituent groups.69,70 Consequently, the aggregate size decreases to the nanoscale.71,72 This result suggests that breaking the coplanarity of the conjugated backbone can improve the solution processability of organic molecules with a large π system (Figure 4). However, studies of the electron mobility in PDI derivatives indicated that high electron mobility requires long-range, ordered π−system packing. Therefore, single crystals generally have much higher electron mobilities (in the π-π−stacking direction) than amorphous thin films.62,73 The twisted conformation can reduce aggregation, thereby reducing excimer formation74 and leading to nanoscale domain formation, which facilitates exciton dissociation. However, organic molecules with twisted conformations are less likely to form compact, long-range π-π stacks than planar organic molecules. Therefore, it is important that newly designed molecules with twisted π systems can form chargetransport favorable π-π stacks and also charge-dissociation favorable nanoscale aggregates during processing. In contrast, coplanar organic acceptor molecules form compact, longrange π-π stacks, such as slipped stacks or so-called J-type aggregates, relatively easily. The simultaneous achievement of nanoscale aggregate domain formation and charge-transport

favorable π-π-stacking structures is necessary in the coplanar approach. Beyond conformational “twisting” or “coplanarity”, the conjugated backbone must be judiciously designed to achieve the desired light absorption, and the noncovalent side-chain interactions must be tailored to meet the solution processability and film-morphology criteria for obtaining efficient nonfullerene small-molecule acceptors. 4. Twisted small-molecule approach 4.1. Molecular strategies for obtaining a twisted conformation and its effect on the film morphology A monoimide product can be obtained from perylene dianhydride by several procedures,75,76 and imidization of the monoimide product, for example, with hydrazine, leads to the formation of twisted PDI dimers, in which the two PDI planes have a dihedral angle of approximately 90° due to strong steric effects.77,78 Narayan and coworkers reported that the N-N linked PDI dimer M1 (Figure 5) had a PCE of approximately 3% when a conjugated PBDTTT-C-T polymer (a conjugated polymer of alternating 4,8-bis(2-ethylhexyloxy)benzo[1,2b:4,5-b′]dithiophene and alkylcarbonyl-substituted thieno[3,4-b]thiophene units) was used as the donor, whereas the control PDI monomer (M2) had a PCE value of only 0.13%. Near-field scanning optical microscopy revealed that the monomer blend formed micron-sized aggregate domains, whereas the dimer blend did not. It was proposed that the crystal packing of the control monomer could be disrupted by introducing a twisted conformation, which originated from the non-planar conformation.79,80

Figure 5. Chemical structures and proposed packing of a PDI monomer (M2) and dimer (M1) and the J–V curves of the best cells constructed from blends of these acceptors with the PBDTTT-C-T donor material. The figures are adapted with permission from Ref. 79, Copyright 2012, American Chemical Society.

Zhan and coworkers observed that one of the bromine atoms in 1,7-dibrominated PDI can be selectively substituted by an alkyl alcohol when K2CO3 and DMF were used as the base

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Figure 6. (a) Chemical structures and optimal conformations of M3 and M4 (the 2-ethylhexyl carbon atoms and all hydrogen atoms are omitted for clarity). (b) Absorption spectra of the M3 (dimer) and M4 (monomer) neat films and their PBDTTT-C-T blended films (1:1, wt%) with 5% DIO. All the films were prepared from 1,2-dichlorobenzene solutions. (c) J-V curves of the most efficient solar cells with and without 5% DIO as an additive (solid and dashed lines, respectively). (d) Optical microscopy images (grey) and atomic force microscopy (AFM) phase images of the M4-based (without DIO) and M3-based (with or without 5% DIO) solar cell blends. (e) Depiction of the improved phase separation and enhanced aggregation of the dimeric molecules. The figures were adapted with permission from Ref. 8, Copyright 2013, John Wiley and Sons.

and reaction solvent, respectively. The yield of the unsymmetrical 1-bromo-7-alkoxyl product ranged from 40% to 95%, depending on the nucleophilic strength of the alkyl alcohol. These results can be explained as follows. Upon formation of the electron-rich alkoxyl group at the 1-position, electrons are delocalized on the electron-deficient perylene core, which leads to a decrease in the electrophilicity of the carbonbromine (C-Br) bond at the 7-position. This decrease in electrophilicity deactivates the nucleophilic substitution of the bromine atom at the 7-position by the weakly nucleophilic alkyl alcohol.81 Zhan, Yao and coworkers synthesized a series of PDI dimers in which the PDI units were covalently linked through an oligothienyl aromatic bridge in the bay region by reacting 1bromo-7-alkoxyl PDI and 2,5-bis(tributylstannyl) oligothiophene via Stille coupling.82 Then, collaborating with Hou et al, they blended a thienyl-bridged PDI dimer, bis-PDI-T-EG (M3, 1,1′-bis(2-methoxyethoxyl)-7,7′-(2,5-thienyl)bis-PDI), with a small band-gap conjugated polymer (PBDTTT-C-T) and studied the photovoltaic properties of the blend. The absorption spectra of M3 and the control PDI monomer (M4)71 were similar, and their thin-film absorption spectra were complementary to the PBDTTT-C-T donor film absorption spectrum (Figure 6). The donor-to-acceptor ∆ELUMO and ∆EHOMO values were 0.59 and 0.54 eV, respectively. The PDI monomer blend

formed micron-sized aggregate domains. However, the PDI dimer blend formed nanoscale phase-separated domains with an average size of 12 nm, clearly demonstrating that the twisted conformation significantly reduced the aggregation tendency. The PDI monomer and dimer had PCE values of 0.03% and 0.77%, respectively, showing that a higher PCE could be achieved with a more twisted dimeric backbone than with a less twisted monomeric perylene. In addition, 5% DIO was used as an additive solvent to promote donor-to-acceptor phase separation and enhance the PDI dimer aggregation. The resulting dimer had a PCE of 4.03%, which was a record value at that time, Jsc of 8.85 mA/cm2, Voc of 0.85 V and FF of 54.1%. For comparison, the monomer had a PCE of only 0.13% when 5% DIO was used as an additive solvent,8 and the PBDTTT-C-T:PC71BM blend had a PCE of 7.6%.83 The thienyl bridge in M3 was replaced with a selenophenyl bridge to give a PDI dimer with PCE values of 1.31% and 4.01% when blended with poly(3-hexylthiophene) (P3HT) and PBDTTT-C-T donor materials, respectively. The significant increase in the PCE was partially due to the higher absorbance of solar photons by the low band-gap polymer donor.84 To obtain twisted conformations, two PDI units can be covalently linked by a bridge, such as spirobifluorene (M5) or a single carbon-carbon bond (M6), or they can be fused by a carbon-carbon double bond (M7). Zhao, Pei and coworkers

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Figure 7. Chemical structures of several non-fullerene organic small-molecule acceptors with twisted conformations.

synthesized M5 by Suzuki coupling. Its LUMO and HOMO energies were −3.71 and −5.71 eV, respectively, and its film absorption band was observed between 400 and 600 nm. When commercial P3HT was used as the blend donor, a PCE of 2.4%, Jsc of 5.92 mA/cm2, Voc of 0.61 V and FF of 65% were obtained.85 Yan and coworkers reported that a PC71BMbased single-junction solar cell with a high-efficiency difluorobenzothiadiazole-based polymer donor (PffBT4T2OD) had a PCE of 10.8%.6 They then replaced the 2octyldodecyl side chains with 2-decyltetradecyl and blended the resulting polymer (PffBT4T-2DT) with M5 to fabricate NF-OSCs. The LUMO and HOMO energies of PffBT4T-2DT were −3.71 and −5.36 eV, respectively, and the M5 LUMO and HOMO energies were reported to be −3.83 and −5.90 eV, respectively. The M5:PffBT4T-2DT blend had an average PCE of 6.0%, average Jsc of 10.7 mA/cm2, Voc of 0.98 V and FF of 57%.86 The increase in the Voc is consistent with the fact that the HOMO energy of PffBT4T-2DT is lower than that of P3HT. Wang and coworkers synthesized the PDI dimer M6 by the one-step homocoupling of halogenated PDIs in dry DMSO in the presence of a copper powder catalyst. Its LUMO energy was −3.91 eV. In collaboration with Hou et al, they reported that the best PCE of M6:PBDTTT-C-T blend was 3.63%. The other cell parameters were Jsc = 10.58 mA/cm2, Voc = 0.73 V, and FF = 46.8%. If the two M6 PDI units were fixed through two or three carbon-carbon single bonds, the blends of the resulting acceptors with the same polymer had PCE values of only 1.54% and 1.36%, respectively.87 Wang, Jen and coworkers reported that the M6:PTB7-Th (poly[[2,6′-4,8-di(5ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene][3-fluoro2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) blend

had a PCE of 5.9% in an inverted cell, in which the ZnO surface was modified with a self-assembling C60 monolayer.88 Subsequently, Wang, Hou and coworkers used PBDTBDD, a low band-gap polymer, as the donor, and the resulting blend had a PCE of 4.4%.89 They also used PBDTTPD (poly(benzo[1,2-b:4,5-b′]dithiophene−thieno[3,4-c]pyrrole4,6-dione)), PTB7-Th, PSBTBT (poly[(4,4′-bis(2ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3benzothiadiazole)-4,7-diyl]), and PDPP3T (a conjugated polymer with alternating diketopyrrolopyrrole and terthiophene units) as the donor with the M6 acceptor and reported Voc values of 1.04, 0.77, 0.68, and 0.71 V, respectively. The trend in the Voc values was consistent with the trend in the donor HOMO levels, which were −5.52, −5.21, −5.07, and −5.17 eV, respectively. The Jsc values of the corresponding solar cells were 6.8, 11.5, 6.4 and 3.5 mA/cm2, respectively.90 The donoracceptor conbinamtion could be varied to tune the donoracceptor interactions and crystallization process, which affect the film morphology and consequently the charge separation efficiency, carrier mobility, recombination losses, and Jsc values. Recently, Wang, Sun and coworkers reported two PDI dimers in which two sulfur or selenium atoms were fused onto the PDI core bay regions. These dimers had a higher LUMO energy than the parent dimer (−3.85 vs. −3.92 eV). The conjugated polymer PDBT-T1 (poly{dithieno[2,3-d:2′,3′d′]benzo[1,2-b:4,5-b′]dithiophene-co-1,3-bis(thiophen-2-yl)benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione})91 was used as the blend donor material. The blend film morphology was optimized using 1,8-diiodooctane (DIO) as an additive solvent. For the sulfur-fused molecule, Voc slightly decreased from 0.92 to 0.90 V, the average Jsc value increased from 11.23 mA/cm2

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to 11.65 mA/cm2, and the average FF value increased from 57.1% to 65.5% as the DIO content increased from 0% to 0.75%. It was proposed that the use of DIO as an additive could enhance the aggregation of the twisted acceptor and lead to a preferred out-of-plane orientation of the polymer donor. Consequently, the average PCE increased from 5.85% to 6.90%.92 Similarly, a PCE of 8.4% was achieved using the selenium-fused molecule.93 Normally, the M1, M3, M5 and M6 twisted conformations are somewhat flexible, whereas the twisted conformation of M7, which was synthesized by Nuckolls and coworkers, is fixed by a two-carbon bridge. M7 was synthesized via the Stille coupling of trans-1,2-bis(tributylstannyl)ethene and

monobrominated PDI, followed by Mallory photocyclization under irradiation with a 450 W medium-pressure mercury lamp.94 Its LUMO and HOMO energies were −3.77 and −6.04 eV, respectively. The photovoltaic properties of its blends with PTB7 (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5b′]dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) and PTB7Th were measured. Both blends had a Voc of approximately 0.79 V, but the PTB7-Th blend had a higher Jsc than the PTB7 blend (13.5 vs. 10.9 mA/cm2) and hence a higher PCE (6.05% vs. 4.81%). Transient absorption spectroscopy data indicated that both the electron transfer from PTB7 to the PDI acceptor and the hole transfer from PDI to PTB7 were ultrafast, on the time scale of 0.2 ps.95 Recently, PCE values of 7.9% and 8.3% were measured for a PDI trimer and tetramer, respectively, in which three and four PDI units are fused by two-carbon bridges.9 Jen et al reported that the blend of a thiophene-fused PDI dimer with the PTB7-TH donor material had a maximum PCE of 6.72%. The other three parameters were Jsc = 11.95 ± 0.33 mA/cm2, Voc = 0.93 ± 0.01 V, and FF = 58 ± 1%.96 The concept of using a twisted conformation to achieve nanoscale phase-separated aggregate domains can be extended to non-PDI derivatives. Jenekhe and coworkers reported the synthesis and photovoltaic properties of M8. This structure consisted of two tetraazabenzodifluoranthene diimide (BFI) units covalently linked through a thienyl bridge. The BFI planes were twisted relative to each other with a dihedral angle of 33°. The LUMO and HOMO energies of M8 were −3.8 and −5.8 eV, respectively. This dimer exhibited an absorption band between 300 and 450 nm and a weak band between 450 and 600 nm. This acceptor was blended with a low band-gap polymer donor, PSEHTT (poly[(4,4′-bis(2ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,5bis(3-(2-ethylhexyl)thiophen-2-yl)thiazolo[5,4-d]thiazole)]). The blend had a PCE of 5%, Jsc of 10.14 mA/cm2, Voc of 0.86 V, and FF of 58%.97 This work is interesting because it shows that other large π systems with appropriately twisted conformations are good candidates for efficient small-molecule acceptors. Sauvé and coworkers reported another example of organic acceptor, M9 (bis[2,6-diphenylethynyl-1,3,7,9tetraphenylazadipyrromethene] zinc(II)). M9 had an absorption band between 580 and 800 nm, which is complementary

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to the P3HT film absorption band. Its LUMO and HOMO energies were −3.85 and −5.60 eV, respectively. The M9:P3HT blend had a PCE of 4.1%, Jsc of 9.1 mA/cm2, Voc of 0.77 V, and FF of 59%. Transmission electron microscopy revealed the formation of nanoscale fibril domains. As a control, the PCBM:P3HT-based solar cell was fabricated and had a PCE of 3.55%.98 This work shows that organic-metal coordinated complexes might have potential for use as alternative non-fullerene acceptors. Recently, Wan, Chen and coworkers reported the use of a diketopyrrolopyrrole (DPP)-based organic acceptor, M10, in which four DPP moieties are covalently linked through a spirobifluorene core. Its LUMO and HOMO energies were −3.60 and −5.26 eV, respectively. Its absorption spectrum was similar to those of most DPP-based small molecules with a localized band between 480 and 680 nm, which is complementary to the absorption of P3HT. The M10:P3HT blend had a PCE of 3.63%, Jsc of 6.96 mA/cm2, Voc of 1.10 V, and FF of 47.5%.99 Recently, these researchers reported another structure similar to M10, in which each DPP unit was capped by a phenyl unit. This new acceptor was blended with P3HT to give a PCE of 5.16%.100 This study indicates that DPP derivatives with carefully designed molecular structures can be used as efficient non-fullerene organic acceptors. The M11 and M12 acceptors represent an alternative strategy for obtaining twisted non-fullerene organic acceptors. Zhan, Yao and co-workers synthesized M11. This acceptor had a band gap of only 1.62 eV due to the intramolecular chargetransfer (ICT) absorption from the peripheral 4,8-bis(2ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (2T-BDT) unit to the perylene core. Its LUMO and HOMO energies were −3.95 and −5.57 eV, respectively. A PCE of 1.7% was obtained when M11 was blended with the P3HT polymer donor.101 Introducing four phenyl groups at the 1-, 6-, 7-, and 12positions can lead to a twisted perylene core conformation, as demonstrated by Liu, Zhu and coworkers.102 The resulting M12 acceptor was blended with the PTB7-Th polymer donor to give a PCE of 4.1%, Jsc of 10.1 mA/cm2, Voc of 0.87 V, and FF of 46%, as reported by Sun and coworkers. Absorption spectral data indicated that the M12 absorption band of the thin film was not obviously red-shifted or blue-shifted relative to that of the solution, suggesting that the aggregation of the PDI molecules was reduced.103 The degree of twisting can be tuned by the bridge structure. Jenekhe and coworkers demonstrated that the dihedral angle between two BFI planes could be tuned from 33° to 62° by bridging the BFI units with thienyl, selenophenyl, 3,4dimethylthienyl, or 3,6-dimethyl-2,5-thienothiophene. Of these structures, the 3,4-dimethylthienyl-bridged dimer, which had a dihedral angle of 62°, had the highest Jsc value (12.56 mA/cm2) and PCE (6.37%). In this case, 3% 1,8-diiodooctane was used as a solvent additive to treat the blend film, which was then aged at room temperature for 2 days.104 Recently, these researchers reported another BFI dimer with 3,4ethylenedioxythienyl as the bridge. The optimal dihedral angle between the two BFI planes was 76°. With this molecule as the non-fullerene acceptor and PSEHTT as the donor, a PCE of 8.1% was achieved. The PCE was increased to 8.5% when

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PTB7-Th and PSEHTT were used as binary donor materials. The Jsc, Voc, and FF values were 15.67 mA/cm2, 0.91 V, and 60%, respectively. In this case, the blend film was optimized by thermal annealing at 175°C for 10 min in an argon-filled glovebox.105 Jen and coworkers noted that the miscibility of the twisted PDI dimer in PTB7-Th was significantly improved compared to that in PTB7 due to the two-dimensional conjugated side chains in PTB7-Th. The relatively rigid dimer geometry originating from the N-N covalent bond not only increased its miscibility in PTB7-Th but also ensured that its aggregate domains were appropriately sized. In contrast, the relatively flexible geometry of M6 disrupted the PTB7-Th π-π stacking. Consequently, the N-N covalently bonded dimer had higher Jsc (13.12 vs. 12.86 mA/cm2), FF (60% vs. 54%) and PCE values (6.41% vs. 5.45%) than M6.106 The bridging structure might affect the solution processability of twisted acceptors considerably. For example, Zhan, Yao and coworkers demonstrated that changing the aromatic bridge from 4,8-bis(2-ethylhexloxy) BDT to 2T-BDT dramatically reduced the aggregation of the PDI dimers and improved their solution processability.107 Zhao, Pei and coworkers reported that replacing the spirobifluorene bridge with other aromatic groups or changing the covalent linking positions from 2,7- to 2,2′- on the spirobifluorene-diyl bridge affects the electrical performance of the corresponding NF-OSCs.85 Of course, the nature of the side chains should affect the film morphology of organic blends significantly, as widely observed in organic solar cell studies. For example, as the number of 2-methoxylethoxyl side chains on M3 was increased from 0 to 2 and 4, the solution processability improved, and the PCE of its blend with the P3HT donor increased from 0.41% to 0.76% and 1.54%, respectively.108 However, recent work is still mainly focused on engineering the backbone conjugation and conformation, and side-chain engineering has rarely been investigated to date, although it is very important. The effects of the side chains on the film morphology and consequently on the electrical performance of NF-OSCs still needs to be investigated systematically.

Both the perylene diimide HOMO and LUMO are delocalized over the perylene core, but they have nodes at the imide nitrogen positions. Thus, substituents at the nitrogen positions have a smaller effect on the energy levels than substitutions elsewhere on the perylene core. In contrast, structural modifications to the bridge or side chains on the bay region enable the facile tuning of the light absorption and energy levels of PDI acceptors. The LUMO (HOMO) energy of a conjugated semiconductor molecule is related to the coupling and LUMO (HOMO) energies of the covalently linked π systems. The formation of a twisted conformation weakens this coupling and reduces the π-electron conjugation. The LUMO energy of dimeric PDIs is typically less sensitive than the HOMO energy to changes in the aromatic bridge structure; the HOMO energy can be obviously affected by the electron-donating bridge.82,109,110,115 Moreover, a change in the electron-donating or electron-accepting ability of the peripheral side chains on the bay region can affect both the LUMO and HOMO energies of dimeric PDIs.108 Similar phenomena are also observed for BFI dimers.104 Fusing a PDI dimer via a thienyl group instead of connecting the monomers via a thienyl bridge could enhance the π-electron delocalization, resulting in a change in the LUMO energy of the twisted molecule.96 During the synthesis of twisted organic acceptors, a narrow band gap could be achieved by introducing electron-donating side chains on the perylene core bay region and/or by using an electrondonating aromatic bridge to increase the HOMO energy. In this strategy, it is important to maintain the high-lying LUMO of the target molecule to achieve a high Voc. 5. Coplanar small-molecule approach

Three and four PDI units were covalently linked by triphenylamine,109,110 tetraphenylethylene,111 tetraphenylmethane,112,113 or spiro-bifluorene114 bridges to produce PDI trimers and tetramers, respectively. The PCE of their blends with the PBDTTT-C-T, PffBT4T-2DT, PTB7-Th and P4T2FBT polymer donors ranged from 1.9% to 6.0%. 4.2. Adjusting the light absorption and energy levels An organic semiconductor molecule generally exhibits an absorption band upon excitation with light. Organic acceptors and donors can both absorb solar photons, generating excitons. As discussed previously, donor-acceptor pairs should be chosen such that their absorption bands are complementary to enhance the ability of the blended film to capture solar photons. Moreover, the frontier molecular orbital energy levels of the organic acceptor strongly affect exciton dissociation.

Figure 8. Chemical structures of several non-fullerene organic acceptors with coplanar conformations.

Incorporating bulky side chains at the imide nitrogen positions is a traditional strategy for obtaining soluble PDI derivatives such as M13 (Figure 8). Alternatively, the perylene core coplanarity can be perfectly maintained by substitutions at the 2-, 5-, 8-, and 12-positions. Marks, Wasielewski and coworkers demonstrated that the twisting of the coplanar perylene

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core relative to the phenyl plane in M14 is beneficial for its solution processability. The slipping angle between packed perylene planes was larger when the 2-, 5-, 8-, and 12substituents were phenyl groups than when they were phenethyl and hexyl groups. The LUMO (HOMO) energy could be tuned by the substituents as demonstrated by the LUMO (HOMO) energies of −3.83 (−5.90) eV, −3.91 (−5.92) eV and −4.01 (−6.14) eV of the PDI derivatives with the hexyl, phenethyl and phenyl substituents, respectively. Consequently, the Jsc value increased from 1.51 to 2.44 and 6.56 mA/cm2, the FF increased from 39.3% to 48.5% and 54.6%, and the PCE increased from 0.65% to 1.20% and 3.67% for the hexyl, phenethyl and phenyl substituents, respectively. All three PDIs had a Voc of approximately 1.0 V. However, the best PCE value of 3.67% was obtained with a 60 nm-thick photoactive thin film.116

had a PCE of 5.44%, Voc of 0.85 V, Jsc of 9.68 mA/cm2 and FF of 66%.126 These authors also reported a maximum PCE of 7.64% for a blend of a structure similar to M18 and the benzothiadiazole-based conjugated polymer PPDT2FBT (poly[(2,5bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)]).55 The Jsc, Voc and FF values were 11.88 mA/cm2, 1.03 V and 63%, respectively.127 These studies strongly indicate that the use of several conjugated electron-accepting moieties might also be an interesting strategy for obtaining efficient non-fullerene smallmolecule acceptors, because this strategy might lead to suitable LUMO and HOMO energy levels that facilitate electron transfer from the blend donor to the acceptor and hole transfer from the blend acceptor to the donor. In particular, coplanar backbones enable the formation of π-π stacks that facilitate mobile carrier transport.

Zhan and coworkers reported the synthesis of M15.117 In this structure, the 2-(3-oxo-2,3-dihydroinden-1ylidene)malononitrile (INCN, electron-accepting unit) and indacenodithieno[3,2-b]thiophene (IDTT, electron-donating moiety) units are coplanar, and the two peripheral substituent groups on each sp3 carbon atom are bulky p-(n-hexyl)-aryl groups that were introduced to tailor the solution processability.118,119 The LUMO and HOMO energies of M15 were −3.83 and −5.48 eV, respectively, and its absorption band was observed between 500 and 800 nm. The M15:PTB7-Th blend had a PCE of 6.80%, Jsc of 14.2 mA/cm2, Voc of 0.81 V, and FF of 59.1%.117 Alternatively, if 3-(n-hexyl)thienyl bridges were used to connect two electron-accepting INCN units to indaceno[1,2-b:5,6-b0]dithiophene (IDT),118 the blend of the resulting molecule (IEIC) with PTB7-Th had a PCE of 6.3%.120 Very recently, Yan and coworkers blended IEIC with a large band-gap polymer (PffT2-FTAZ-2DT), and the resulting material had a PCE of 7.3%, Jsc of 12.2 mA/cm2, Voc of 0.998 V, and FF of 59%.121 An A-A-D-A-A molecule, M16, was reported by Holliday and coworkers. In this molecule, an electron-accepting unit (benzothiadiazole) was placed between rhodanine substituents and a central fluorene unit, resulting in a high-lying LUMO and HOMO (−3.5 and −5.70 eV, respectively). The dioctyl side chains at the 9,9′-positions enhanced the solution processability. The corresponding NF-OSC had a PCE of 4.11%, Jsc of 7.95 mA/cm2 and FF of 63%. The Voc was 0.82 V, which is very large, when P3HT was used as the blend donor.122 M17, an A-π-A-π-A type molecule, in which two DPP units are covalently linked to the central benzothiadiazole through thienyl bridges, was synthesized by Jo and coworkers. This structure had low-lying LUMO and HOMO energies (−4.33 and −5.85 eV, respectively) and exhibited an absorption band between 450 and 800 nm. A PCE of 5% was obtained for the M17:PTB7 blend. The Jsc, Voc and FF values were 12.1 mA/cm2, 0.81 V and 51%, respectively.123 As a solar cell is fabricated with a blend of a small-molecule donor and a small-molecule organic acceptor as the photoactive layer, the resulting solar cell is denoted as an all small-molecule OSC (all SMSCs).22,124,125 M18 is a small-molecule nonfullerene acceptor, which was reported by Park and coworkers. It was employed with a low band-gap small-molecule donor (p-DTS(FBTTh2)2). The resulting all small-molecule system

Although several types of coplanar organic acceptors have been reported in the literatures, these acceptors have not been studied systematically. Future studies will require judicious design of their absorption and energy levels and also sidechain engineering. Again, a deep understanding of the relationship between the structure and electrical performance has not yet been formulated. In addition to solution-processed small molecules, subphthalocyanines (SubPcs) are another interesting type of nonfullerene small-molecule acceptor. Recently, Heremans and coworkers reported a PCE of 8.4% for a three-layered, vacuum-deposited organic solar cell. In this OSC, a cascade energy transfer from a hexathiophene donor layer to an M19 layer and then to an M20 layer was demonstrated to improve the solar cell electrical performance.128 6. Organic polymer acceptors

Figure 9. Chemical structures of several non-fullerene polymer acceptors.

In addition to organic small-molecule acceptors, n-type organic polymers are another type of non-fullerene acceptor. They are typically constructed from conjugated electronaccepting (A) and electron–donating (D) moieties in an A-D pattern. The electron-accepting units can be PDI,129,130 naphthalene diimide (NDI)131 or other molecules.132,133 Of the reported polymer acceptors, NDI-based conjugated polymers have been shown to be the most efficient. The chemical struc-

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tures of the four most efficient polymers are shown in Figure 9. Jenekhe and coworkers replaced the thiophene unit in polymer P1,134 which was synthesized by Kim and coworkers, with selnophene to synthesize P2. This polymer had a smaller band gap (−1.76 vs. −1.85 eV) and lower-lying LUMO (−3.84 vs. −3.79 eV) than P1. When PTB7-Th was used as the donor, the P2 blend had a higher PCE than the P1 blend (7.73% vs. 5.96%), which was mainly due to the significant increase in the electron mobility (the Jsc value increased from 13.46 to 18.80 mA/cm2).135 Polyera produces the commercial polymer P3, which has a PCE of 6.4%.16 Recently, Li et al used P3 with a bifluorated-benzothiazole-(2T-BDT) conjugated polymer donor to achieve a PCE of 8.27%.136 The corresponding Jsc, Voc and FF values were 14.18 mA/cm2, 0.83 V and 70.24%, respectively. Interestingly, this system had a high FF value.137 Jen and coworkers fluorinated the bithiophene unit in P3 to produce P4. The P4:PTB7-Th blend had a PCE of 6.7%, Voc of 0.81 V, Jsc of 13.53 mA/cm2 and FF of 62%.138 7. Film-processing techniques for obtaining high-efficiency NF-OSCs To utilize the photovoltaic properties of a synthesized acceptor molecule, an appropriate donor material and suitable solar cell structure are required. In addition, the film morphology must be optimized to achieve the highest possible photonto-electron conversion. The molecular structure is the primary factor controlling the blend film morphology. However, the photoactive blend film must be processed to obtain the optimal morphology. As illustrated in Figure 1, the donor and acceptor excitons dissociate at the donor-acceptor interface via so-called electron and hole transfer paths, and the generated mobile electrons and holes are transported through the bicontinuous acceptor and donor phases, respectively. In an all-organic system, the acceptor-to-donor hole transfer might become more efficient than the donor-to-acceptor electron transfer, as observed by Fréchet and coworkers for a benzothiadiazole-thienoimide acceptor and DPP-pyrene donor.139 The total electrical current is the sum of the mobile electron and hole currents, i.e., J = Je + Jh. PCBM acceptors enable isotropic electron transport. In contrast, the charge transport in an organic small-molecule crystal is highly anisotropic; the charge mobility along the ππ−stacking direction is high, whereas those along the edge-toedge packing directions are much lower. On one hand, the anisotropic electron transport implies that the morphology of the organic acceptor domain has a considerable impact on the electrical performance of an NF-OSC. For example, Keivanidis and coworkers found that the M13 excimer photoluminescence could be efficiently quenched when the PDI column length was comparable to the excimer diffusion length. The well-ordered PDI domains resulted in poor electronic connectivity, which decreased the electron transport with increasing PDI domain phase size.140,141 On the other hand, the anisotropic electron transport suggests that the film morphology of all-organic blends must be optimized to achieve the ideal acceptor-phase morphology,

and, also ideal donor-phase morphology. Using an additive solvent, which has been proven to be efficient for fullerene:organic blend systems, is a facile way to obtain the optimal film morphology. In 2008, Heeger et al reported that selectively dissolving the PCBM component in an additive solvent allows the film morphology to be optimized.142 In 2013, Zhan, Yao and coworkers demonstrated that slowly evaporating a high boiling point host/additive solvent from the wet film could facilitate the self-organization of the dissolved organic component. Consequently, the hole mobility was improved, leading to an increase in the Jsc, FF and hence PCE values.143 These researchers also recently observed that the organiccomponent solubility and the phase size of the organic acceptor domains decreased when the additive solvent was changed from 1-chloronaphthalene (CN) to 1,8-octanedithiol (ODT) and 1,8-diiodooctane (DIO). Additionally, the phase size decreased as the DIO content ([DIO]) was decreased from 1% to 0.15%. In this case, less residual DIO remained in the wet film to solubilize the organic acceptor, enabling the film morphology to be optimized. In both cases, i.e., when the DIO content was decreased or the additive solvent was replaced by a solvent with a lower organic-component solubility, the amount of the organic component dissolved in the residual additive solvent in the wet film decreased, which affected the phase size, i.e., the film morphology.144 In addition, these researchers observed that the experimental Jsc value could be correlated to the organic acceptor domain size144,145 and proposed that the acceptor excitons could be utilized in the energy transfer from the organic acceptor to the low band-gap small-molecule donor. This energy transfer would occur simultaneously with the normal hole transfer path to exploit the acceptor excitons.144 Similarly, Keivanidis and coworkers suggested that the energy transfer from the PDI acceptor to the polymer donor contributed to acceptor exciton harvesting.146

Figure 10. Example showing that the packing order of twisted molecules (for example, M21) is enhanced by solvent annealing (SA) of the as-cast photoactive film. All figures were adapted with permission from Ref. 107, Copyright 2013, The Royal Society of Chemistry.

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In addition to using an additive solvent, solvent annealing (SA) also provides a means for controlling the film morphology. In this method, the wet blend film is placed in a lidcovered petri dish for a period of time, and then the lid is removed to allow the blended film to dry completely. If a solvent is added to the lid-covered petri dish before the as-cast blended film, this alternative approach is called solvent vapor annealing (SVA). Zhan, Yao and coworkers showed that the intensity of the 2T-BDT-bridged PDI dimer (M21, Figure 10) X-ray diffraction peak increased significantly because the packing of the twisted molecules was largely enhanced after SA. It was proposed that slowly evaporating the host and additive solvents from the wet film promoted PDI molecule aggregation. The promotion of PDI molecule aggregation was speculated under the assistance by the steric-pairing effects, which originated from the twisted conformations.107

Figure 11. Example showing the dramatic dependence of the electrical performance of an NF-OSC on the film-forming kinetics. The solar cell data were reused with permission from Ref. 148, Copyright 2015, American Chemical Society.

These researchers also demonstrated that increasing the amount of the DIO additive solvent could efficiently tune the donor-to-acceptor weight ratio in the air and bury photoactive blend surfaces. It was observed that the amount of donor molecules in the acceptor-rich buried surface critically affected the injection and extraction of electrons and holes from the buried contact. The donor abundance in the buried surface decreased with decreasing DIO amount, which was beneficial for electron extraction. In contrast, increasing the donor abundance had the reverse effect; hole extraction from the buried surface became more favorable. Consequently, the best solar cell performance was obtained with an inverted cell when a smaller amount of DIO was used, whereas a normal cell had the best performance when a large amount of DIO was employed.147 These researchers further reported that the photoactive film morphology could be finely tuned by controlling the filmforming kinetic parameters of the solvent annealing/solvent vapor annealing (SA/SVA) process. In particular, the photoactive film was placed in an open or fully covered petri dish (SA), or it was placed in a fully covered petri dish with 7.5 µL of pure 1,2-dichlorobenzene (o-DCB) or 15% DIO in o-DCB

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(solvent vapor annealing, SVA) (Figure 11). The resulting film morphology changed continuously as the processing conditions were varied. Specifically, the donor-to-acceptor phase separation improved, the aggregation of the twisted acceptor molecules was enhanced, and the acceptor abundance in the buried surface became more favorable for electron extraction. Consequently, the electron mobility increased dramatically, and the mobile carrier recombination losses decreased, leading to a dramatic increase in both the Jsc and FF values and ultimately in the PCE.148 8. Challenges and opportunities A BHJ OSC with a planar device structure requires mobile hole and electron transport along the out-of-plane direction (Figure 12a). So, face-on orientation of the π-π stacking of the donor and acceptor molecules is ideal for an OSC. The π-π stacking of PCBM molecules is isotropic, which can result in similar electron transport mobilities in all directions (Figures 12b and c). As bicontinuous networks form within a PCBM blended film, electron transport along the out-of-plane direction can be easily realized. However, an organic acceptor consists of planar π systems (in the figure, red and blue planes are used to represent the organic acceptor and donor π systems, respectively). The π-π stacking between organic π systems is anisotropic. Planar π systems can pack into H-type, J-type or probably helical aggregates (Figure 12d), and the stacking can be controlled by engineering the peripheral side chains and altering the experimental conditions.61,149,150 In a J-type stack, the slipping stack angle (θ) can theoretically vary from 0° to 90° (Figure 12d). Anisotropic π-π stacking leads to anisotropic electron transport with a high mobility in the π-π-stacking direction and dramatically lower mobilities along the edge-toedge directions. Controlling the organic donor orientation is essential but challenging for fullerene OSCs as yet.151 For NFOSCs, it is essential to control not only the organic donor molecule orientation but also the orientation of the organic acceptor molecule π-π stacks (Figure 12e). This fact implies that it might be more difficult to realize high-efficiency NF-OSCs than fullerene solar cells. Therefore, designing molecules and optimizing the film morphology for NF-OSCs will require more attention and patience. Judiciously designing organic acceptor molecules and modifying organic donor molecules are both necessary. In particular, film-processing techniques that differ from those used to optimize the film morphology of fullerene:donor blends are required to control the orientations of the acceptor and donor molecule π-π stacks in all-organic acceptor-to-donor blends. We can rationally speculate that the all organic system should have much higher charge mobilities than the fullerene-organic donor system if ideal face-on orientation of the π-π stacking of the donor and acceptor molecules can be both achieved via suitable film-processing techniques and via judicious engineering the molecular structure. Based on the equation J = NqµE, where N is the mobile carrier concentration, µ is the carrier mobility, and E is the internal electric field, the carrier mobility is one of the two experimental parameters (n and µ) that determines the experimental electrical current. The carrier mobility must be improved to

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realize high-efficiency OSCs. The electron mobility in PCBM acceptors is typically on the order of 10-4−10-2 cm2/(V·s).7 In contrast, the electron mobility in organic molecules, for example, PDI derivatives, can range from 10-4 cm2/(V·s) to greater than 1 cm2/(V·s), depending on the packing ordering and the π−electron coupling.62,73 Particularly, the electron mobility in organic crystals can be much higher because of the long-range, well-ordered π-π stacks along the entire crystal through which electrons can be transported. The π-π-stacking distance is another factor that influences the electron mobility in organic film materials. This distance can be tailored by introducing bulky side chains or using twisted conformations. Nevertheless, the electron mobilities in most organic acceptors reported to date are low, typically below 10-3 cm2/(V·s), hindering the realization of high-efficiency NF-OSCs. However, chemically modifying organic acceptors can increase the probability of synthesizing a solution-processable organic acceptor with an electron mobility of greater than 10-2 cm2/(V·s). While increasing the electron mobility is challenging, it is an avenue of research for realizing high-efficiency NF-OSCs.

and donor molecules on a substrate for NF-OSCs and fullerene OSCs.

In addition to solar photon absorption, exciton dissociation affects the generation efficiency of mobile carriers. PCBM acceptors can facilitate isotropic charge dissociation in all directions, but as noted by Morgante, Cvetko and coworkers, when the donor molecule has a shape that is complementary to the spherical PCBM, an extended interface might form between the donor and acceptor molecules, leading to faster electron transfer from the donor to PCBM.152 Moreover, Zhan and coworkers observed that the capping unit on the conjugated backbone might influence the charge separation efficiency of fullerene:donor blends. The charge separation between the donor and fullerene molecules might be enhanced when aromatic units cap the small-molecule conjugated backbone instead of alkyl chains, because the donor-acceptor compatibility is improved. In addition, the capping aromatic units lead to enhanced absorptivity, reduced non-geminate losses. Altogether, an increase in the Jsc value and hence in the PCE value are both obtained from the fullerene:aromatic-capping molecule.153 In planar organic π systems, the π-π−stacking interfacial area between the donor and acceptor molecules is adjustable. For example, a change in the structure from the H-type to the J-mode, which has different slipping degrees, could alter the charge separation efficiency. Moreover, the π-π−stacking distance between organic planar π systems can be tuned by chemically modifying the structure. Fréchet and coworkers showed that using a twisted octylphenyl side group instead of a linear alkyl group on a polythiophene backbone increased the donorto-acceptor π-π distance, which could lead to an increase in the energies of the thermally relaxed and excited charge-transfer states, thereby decreasing the charge separation barrier (Figure 13).154 Accordingly, facile chemical modifications of the organic acceptor also enable the synthesis of solutionprocessable organic acceptors with improved charge dissociation and increased exciton harvesting efficiency when combined with a suitable donor material. Nevertheless, the engineering and tailoring of the organic donor-acceptor interfacial structure and the effects of the interfacial structure on the charge separation are not well understood. In addition, more powerful tools and methods are needed to characterize the donor-acceptor interfacial structure and study the relationship between the interfacial structure and the electrical performance. All of these factors are significant challenges in this field.

Figure 12. (a) Model of carrier transport directions in a laboratory-scale planar solar cell. (b) π-π-stacking distance (for example, 3.5 Å) between organic molecule planes and between PCBM core spheres. (c) Possible π-π-stacking orientations of organic molecule planes and of PCBM core spheres on a substrate. The arrows show the π-π-stacking and electron transport directions. (d) Schematic models of face-to-face (cofacial), slipped (displaced), and helical stacks between organic planar π systems. e) Depictions of the ideal π-π-stacking orientations of both the acceptor

Figure 13. A larger donor-acceptor π-π distance could lead to good charge separation. Reprinted with permission from Ref. 154, Copyright 2011, American Chemical Society.

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The ability of a material to capture solar photons is related to its absorption band width, film absorptivity and band gap. Organic acceptors reported in the literatures have band gaps ranging from 1.5 to 3.0 eV, depending on the π-electron coupling in their conjugated backbones. For A−D−A acceptors, the absorption band width is extended via ICT absorption, and their absorptivities can be tuned by incorporating electrondonating and electron-accepting units connected by a π bridge and by enlarging the π-conjugation system. The film absorptivity of PDIs is on the order of 104−105 cm-1. Increasing the absorptivity can enable a material to capture more solar photons and improve the photocurrent generation. One goal of future research should be finding new structures with enhanced absorptivity on a per gram basis (not a per mole basis) to improve the efficiency of NF-OSCs. Organic materials typically have high exciton binding energies of 0.1-2 eV155,156 because of their small dielectric constants of 2-6.157 In contrast, the exciton binding energy of an organic–lead halide hybrid perovskite film was estimated to be between 1-50 meV because of its relatively larger dielectric constant.158,159 Jen at el showed that the charge separation can be improved by introducing polar nitrile side chains to increase the dielectric constant of the polymer donor.160 Accordingly, finding new organic structure with a higher dielectric constant is required for improving the charge separation, increasing the photocurrent, and gain reducing the Voc loss. To achieve highly efficient solar-to-electric power conversion, the donor-acceptor pair needs to be carefully selected to ensure that their absorption spectra are complementary and that their energy levels match. However, the film morphology depends largely on the donor and acceptor structures and the film-processing conditions. Only a few donor-acceptor combinations have been investigated to date. Given the diversity of possible organic donor-to-organic acceptor combinations, high-efficiency NF-OSCs should be achievable. However, the film-processing conditions might vary on a case-by-case basis, even after subtly changing the organic acceptor or donor molecule structure. Therefore, the tedious work of developing appropriate film-processing methods and techniques will require careful attention and patience. Altogether, the facile chemical modification of organic acceptors largely increases the possibilities for new donoracceptor combinations, enables improvements in exciton generation and charge separation and allows the tailoring of electron transport and π-π−stacking orientations. For a given donor material and cell structure, an organic acceptor with a higherlying LUMO level can reasonably supply a higher Voc. It is hypothesized that all organic blends with optimal film morphologies will have Jsc and FF values that are comparable to and even higher than those of their fullerene counterparts. Thus, NF-OSCs are expected to be more efficient than fullerene OSCs, based on the equation PCE = Jsc⋅Voc⋅FF/Pin, where Pin is the incident light intensity in mW/cm2. To date, Jsc and FF values of greater than 20 mA/cm2 and 70%, respectively, have been achieved with fullerene OSCs, whereas NF-OSCs have been shown to have Voc values of

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0.8−1.1 V. Based on these data, a PCE of greater than 12% can be expected for state-of-the-art single-junction NF-OSCs. Of course, tandem cell structures will afford much more efficient NF-OSCs. Furthermore, can the PCE be up to 20%? To achieve this goal, it should require rational design and synthesis of new organic donors and new organic acceptors with both donor and acceptor having enhanced absorption coefficients and the donor-acceptor system having a higher dielectric constant value than 6, which can facilitate high-efficiency charge separation. In addition, it should require new film-processing techniques suitable for optimizing the film-morphology of the all-organic system. 9. Conclusions Recent advances in the subfield of non-fullerene organic solar cells strongly indicate that organic n-type molecules have potential for replacing traditional fullerene acceptors. These acceptors are expected to exhibit superior electrical performance and higher efficiencies than traditional fullerene-based organic solar cells as efforts are made to judiciously design novel organic acceptors, select suitable donor-acceptor combinations, develop appropriate film-processing methods for the selected all-organic systems, engineer the interfaces between the photoactive layer and electrodes, and fabricate solar cells with a suitable device structure.

AUTHOR INFORMATION Corresponding Author [email protected] (C.Z.)

Notes The authors declare no competing financial interests. Biographies Chuanlang Zhan, Ph.D., received his Ph.D. degree (2000) from the Institute of Photographic Chemistry, Chinese Academy of Sciences (CAS), under the supervision of Prof. Duoyuan Wang. He then joined the Institute of Chemistry, CAS, as a postdoctoral research fellow at the CAS Key Laboratory of Organic Solids, Institute of Chemistry, CAS, and was later promoted to associate professor at this institute. He is now a professor of chemistry at the Institute of Chemistry, CAS. His research activities focus on the synthesis and selfassembly of functional molecular materials, solar energy conversion and solar cells. Jiannian Yao, Ph.D., received his Ph.D. degree under the supervision of Prof. Akira Fujishima at Tokyo University in 1993. He now serves as a professor of chemistry at the Institute of Chemistry, Chinese Academy of Sciences, the president of the Chinese Chemical Society, and the vice president of the National Natural Science Foundation of China. His research interests include organic and inorganic optofunctional nanomaterials.

ACKNOWLEDGMENTS Funding Sources The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (NSFC, Nos.

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91433202, 21327805, 91227112 and 21221002), Chinese Academy of Sciences (CAS, XDB12010200), and Ministry of Science and Technology of the People's Republic of China (MOST, 2013CB933503 and 2012YQ120060).

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(158) Lin, Q.; Armin, A.; Nagiri, R. C. R.; Burn, P. L.; Meredith, P. Electro-optics of perovskite solar cells. Nat. Photonics 2015, 9, 106–112. (159) D’Innocenzo, V.; Grancini, G.; Alcocer, M. J. P.; Kandada, A. R. S.; Stranks, S. D.; Lee, M. M.; Lanzani, G.; Snaith, H. J.; Petrozza, A. Excitons versus free charges in organo-lead trihalide perovskites. Nat. Commun. 2014, 5, 3586. (160) Cho, N.; Schlenker, C. W.; Knesting, K. M.; Koelsch, P.; Yip, H.-L.; Ginger, D. S.; Jen, A. K.-Y. High-Dielectric Constant Side-Chain Polymers Show Reduced Non-Geminate Recombination in Heterojunction Solar Cells. Adv. Energy Mater. 2014, 1301857.

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Coplanarity - American Chemical Society

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