Mechanisms for Spontaneous Generation of Interlayers in Organic

Review pubs.acs.org/cm

Mechanisms for Spontaneous Generation of Interlayers in Organic Solar Cells Jane Vinokur, Basel Shamieh, Igal Deckman, Avni Singhal, and Gitti L Frey* Department of Materials Science and Engineering, Technion − Israel Institute of Technology, Haifa 32000, Israel

ABSTRACT: The structure of bulk heterojunction organic photovoltaic devices generally includes interlayers, thin films positioned between the active layer and one or both of the electrodes, intended to enhance the performance and/or stability. Interlayers can consist of organic or inorganic materials. They play different roles, such as enhancing adhesion and reducing energetic barriers, and are deposited using techniques such as spin coating and thermal deposition. Here we focus on interlayers that do not require a distinct deposition process but rather spontaneously form at the film surface or the active layer/electrode interface. This review paper identifies the underlying mechanisms that induce the interlayer self-generation, and classifies the relevant studies based on (reported or realized) driving forces for interlayer formation. By doing so, this perspective not only offers directive tools for the design of interlayers at surfaces and buried interfaces, but also provides insights on thermodynamic and/or kinetic aspects of processes that occur spontaneously during device fabrication and/or testing but have been overlooked until now.

1. INTRODUCTION Organic solar cells (or organic photovoltaics, OPVs) have recently reached a certified record of 11.5%1 in powerconversion efficiency, which demonstrates their feasibility for becoming lightweight, flexible, and low-cost alternatives to inorganic solar cells.2,3 The structure of the active layer in most OPV devices is based on the bulk heterojunction (BHJ), a nanoscale intermixed continuous network of electron-donor and electron-acceptor materials.4,5 Although attaining control of morphology has led to a steady increase in device efficiency, the generally moderate performance of OPVs indicates that seminal processes are still limited and not fully understood.6 One such process is the extraction of the photoexcited charge carriers from the active layer to the electrodes.7−10 Enhancing charge extraction and reducing recombination losses related to charge accumulation at the active layer/contact interface are expected to significantly enhance the short circuit photocurrent density (Jsc) and the fill factor (FF).11,12 Furthermore, a Schottky barrier at the metal−organic semiconductor junction increases the overall series resistance and reduces the built-in potential, which is translated to lower open circuit voltage (Voc).13 Because the device efficiency is directly proportional to FF, Voc, and Jsc, the deterioration of these parameters has a negative effect on the device performance. It is well established that formation of ohmic contacts requires adjustment of the chemical interactions and electronic band alignment at the active layer/electrode interface. Adjust© XXXX American Chemical Society

ment of chemical interactions can improve the adhesion between the active layer and the electrode, while tuning the interfacial electronic alignment is necessary for charge transfer across the interface. In the direct architecture, the top contact (cathode) should attain a work function suitable for electron injection/collection to/from the LUMO of the acceptor, and the bottom contact (anode) should attain a work function suitable for hole injection/collection to/from the HOMO of the donor. Therefore, it is necessary to balance the transparency and doping of the bottom anode to attain high conductivity and suitably high work function value.14 The relatively low work function required for the cathode, on the other hand, inherently imposes poor environmental stability due to its low reduction potential.15 Indeed, the limited lifetime associated with OPVs is often attributed to oxidation of the metal electrode and not necessarily degradation of the organic components.16 Cathodes with higher work functions, although more stable, impose barriers for charge injection and/or poorselectivity for charge carriers, which limit device performance. In the case of an inverted device architecture, an inert metal (i.e., with high work function) is required for the top contact (anode). In such structures, it is necessary to introduce interlayers to reduce the work function of the (now) bottom Received: September 6, 2016 Revised: November 1, 2016

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DOI: 10.1021/acs.chemmater.6b03770 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Schematic illustrations of the mechanisms that induce additive segregation to form interlayers in OPVs: (a) surface energy, (b) chemical interactions, (c) solubility, and (d) density or a concentration gradient.

cathode substrate.17 Therefore, chemically and energetically tuning the active layer/electrode interface through interlayers could enhance the two major drawbacks of OPVs: performance and stability. A plethora of organic and inorganic materials has been suggested as interlayers for tailoring the active layer/electrode interface. To date, the most widely used inorganic interlayer materials are metal oxides, such as TiO2, ZnO, NiO, WO3, V2O5, Al2O3, and MoO3; and salts, e.g., LiF, Cs2CO3, and CuSCN.18−23 The variety of organic interlayer materials is even broader and includes polymers, polymer blends (e.g., poly(3,4ethyl-enedioxythiophene):polystyrenesulfonate (PEDOT:PSS)), organic acids, small molecules, and conjugated compounds.20,22−25 The interlayer materials can be classified as n-type or p-type based on their utilization as either cathode or anode interlayers, respectively. The types of interlayer materials, different processing techniques, and their effects on device performance have been extensively reviewed.20−25 Generally, interlayers are deposited in a distinct processing step, such as thermal deposition in vacuum or spin coating from an orthogonal solvent. These methods are suitable as batch processes for laboratory research, however, they are technologically inefficient and in some cases even incompatible with large area rollto-roll (R2R) processing.26 Recently, spontaneous segregation of materials to the bottom/top surface has been harnessed to direct and form desired interlayers.27−30 To do so, the interlayer material is added as an additive to the solution and collectively deposited with the BHJ. The additive then segregates to either of the active layer/electrode interfaces to generate the desired interlayer. Typically, enhancement in device performance is correlated with increasing the content of the interlayer additive up to an optimal value that corresponds to the formation of a continuous interlayer at the interface, usually a few nanometers thick. Further increase in additive content results in additive residues in the BHJ or at the other interface, consequently deteriorating the device properties. Importantly, the conditions for complete interlayer formation vary from one additive to another and depend on concentration, chemical composition, and processing parameters.31−33 Another aspect to consider when introducing the additive to the OPV blend is its impact on the BHJ morphology. Not every combination of interlayer-forming additive and active components would be beneficial for OPV efficiency. However, the experience and knowledge on processing additives could be harnessed to generate guidelines for choosing suitable interlayer

additives that also contribute to the formation of the desired BHJ morphology.34 Under such conditions, the segregation of judiciously selected additives from the BHJ to form the interlayer can potentially offer additional benefits such as improved morphology and induced crystallinity in the BHJ and result in advantageous performances and stability over corresponding devices with a sequentially deposited interlayer.35,36 Indeed, in several studies, the incorporation of interlayer-forming additives into a BHJ blend showed a positive impact on the device efficiency and/or ambient stability.32,37 The spontaneous interlayer generation method is mainly advantageous because it reduces the number of processing steps and allows the formation of interlayers at buried interfaces that are not directly accessible during processing. Recently, this approach was shown to be specifically beneficial for printing OPV devices, demonstrating its significance and potential application in transition to R2R compatible processing and large-scale production.38,39 We believe that the demand for techniques compatible with large-scale fabrication and recent demonstrations utilizing spontaneous interlayer formation in printed OPVs will encourage research and promote substantial progress in this field. In this perspective paper, we review the studies that report on the segregation of additives to form interlayers spontaneously in OPVs. However, rather than addressing the materials and the roles of the interlayers, we focus this paper on the mechanisms that induce additive segregation from the BHJ to the desired interface. By doing so, we provide directive tools for additive interlayer selection based on physical properties and chemical reactivity. We classify driving forces based on thermodynamic and kinetic considerations. Thermodynamic considerations drive the system to reduce its overall free energy. Therefore, for additive segregation to occur spontaneously, it must reduce the free energy of the system. Parameters that can be harnessed to reduce the overall free energy by additive segregation include surface energy, electrochemical potential, specific masses (gravitational potential), and immiscibility between components (enthalpy of mixing). On the other hand, the very rapid film drying and centrifugal force during spin coating make kinetic considerations also of great significance. More specifically, the relative solubility of blend components plays a crucial role in directing the final morphology and vertical phase separation, which often deviate from the thermodynamically stable state. Figure 1 schematically summarizes the mechanisms that induce additive segregation from the BHJ to the desired interface in OPVs and are discussed in this review. B


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−CF3.46,49 Therefore, fluorinated additives are good candidates for applications that require segregation to the film surface. Analogous to fluorinated materials, low surface energy is also characteristic of silane-based molecules. In fact, fluorinated and silanated compounds are the two main types of additives utilized for spontaneous generation of interlayers in OPVs due to their low surface energy via self-segregation to the surface during spin coating. Wei et al. were the first to utilize surface energy considerations as an approach for inducing spontaneously forming cathode interlayers in OPV devices.28 They designed a fullerene derivative with a fluorocarbon chain, F-PCBM, and used it as an additive in P3HT:PCBM blends. X-ray photoelectron spectroscopy (XPS) depth profile analysis revealed that during spin coating an F-PCBM layer of about 2 nm was formed at the film surface. A schematic illustration of the surface-segregated monolayer formation is shown in Figure 2. These results were later confirmed by Sum Frequency

The various thermodynamic and/or kinetic considerations can interfere with, or contribute to, each other. Therefore, in this perspective we target each of the above mechanisms separately, focusing on the dominating force in each example. For each mechanism, we also summarized the reported values of relevant device performances in a table. The tables compare the performances of devices with spontaneously formed interlayers to those with no interlayer (reference device) or with a separately deposited interlayer. In the context of device performances, it is important to note that many publications lack adequate statistical treatments and, hence, not all tabulated values have sufficient statistical power.40−42 Therefore, the information in the tables should be treated more qualitatively than quantitatively. We emphasize the contribution of the spontaneous interlayers to the performance only when there is additional supporting spectroscopic and microscopic evidence. Today, the OPV community pays more attention to this issue and publication requires reproducible data, proper statistical treatment, and reasonable explanations for outlier results.

2. SURFACE ENERGY DIRECTED SEGREGATION When a multicomponent organic blend is applied to form a thin film on a substrate such as glass, silicon, ITO, etc., thermodynamic considerations dictate that the lowest surface energy component will enrich the film surface to reduce the overall free energy of the system. In contrast, the highest surface energy component will migrate away from the surface and either phase separate to form domains inside the film or accumulate on a high-surface-energy substrate.43 This thermodynamically induced vertical phase separation process can be utilized to generate an interlayer concurrently with the active layer deposition in OPVs. To harness this spontaneous process in OPV fabrication, it is necessary to understand the parameters that control the surface energy of the different components. Surface free energy is defined as the work involved in increasing the surface by unit area. The most common approach to experimentally estimate the surface energy of a thin film is measuring the contact angle of a liquid drop on the solid substrate and relating it to the interfacial energy between the solid/liquid and liquid/vapor through Young’s equation.44 Since the solid/liquid interfacial energy can not be measured directly, some assumptions must be made. Hence, several models have been developed over the years to provide useful tools to assess the surface energy based on contact angle data.45,46 However, the different models are not completely consistent with each other and estimated values often significantly differ from one model to another. Therefore, one must be very cautious when comparing surface energies estimated by different models. These models do however provide some rule-of-thumb principles to estimate surface energy based on chemical composition. Generally, the more a component exhibits polarity and hydrogen bonding, i.e. hydrophilicity, the higher its surface energy. Similarly, hydrophobic nonpolar components exhibit low surface energies.47,48 For a more detailed discussion regarding the origin of the surface energies of materials and the fundamental phenomenon governing wetting behavior, which is beyond the scope of this review, the reader is directed to related literature, such as ref 44. Segregation of low-surface-energy additives to the film surface. In a general study on the correlation between chemical composition and surface energy, Zisman et al. reported that, for hydrophobic components, surface energies follow the order −CH2 > −CH3 > −CF2 > −CF2H >

Figure 2. (a) Schematic representation of surface-segregated monolayer formation. (b) Schematic energy diagrams of the interface between PCBM and the top Al electrode without (left panel) and with (right panel) a dipole moment layer. ϕ and ϕ′ are the electron injection barriers with and without the F-PCBM layer, respectively. The chemical structure of PCBM is shown between the energy levels. Adapted with permission from refs 33 and 28. Copyright 2009 the American Chemical Society and 2008 Wiley-VCH.

Generation (SFG) spectroscopy.33 The device was then completed by evaporating a top Al cathode. The OPV characterization showed a significant decrease in series resistance and, hence, FF and PCE improvement (see values in Table 1). The enhanced device performance was attributed to the presence of the spontaneously generated F-PCBM interlayer and to the dipole moment it induced at the organic/ electrode interface. These dipole moments, along the oriented fluorocarbon chains of the additive, are directed with the positive charge toward the organic layer, and the negative charge toward the metal contact (Figure 2). Consequently, the ionization potential (IP) of the organic film increased, as shown by cyclic voltammetry (CV), triggering a decrease of the metal work function and hence reduction of the injection barrier. Furthermore, Ultraviolet Photoelectron Spectroscopy (UPS) measurements revealed that the dipole moment at the organic/ Al interface could be manipulated by the length of the additive’s fluorinated chain. It was found that the longer the fluorocarbon C


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Table 1. Performance Comparison for Devices with Spontaneously Generated Interlayers due to Low Surface Energy (Termed Spont. in Table), and Corresponding Reference Devices with either Conventional or No Interlayera

a

TA = thermal annealing; SA = solvent annealing; CIL = cathode interlayer; AIL = anode interlayer; WF = work function.

chain, i.e. the higher the number of fluorinated groups, the higher the IP of the organic surface and hence a further reduction of the injection barrier.31,33 The surface segregating properties of fluorinated additives were utilized by Yao et al. for spontaneous formation of anode interlayers in an inverted device configuration.32 In this work, the fluorinated additive used was the fluoroalkyl side-chain

diblock copolymer P3HT-b-P3FAT (1:3 P3HT:PFAT blocks ratio), which was blended with P3HT:PCBM. Driven by the low surface energy of the fluoroalkyl chain, the additive selfassembled on the surface during spin coating to form a ∼4 nm thick anode interlayer, as characterized by XPS depth profiles. Comparing the devices with the P3HT-b-P3FAT anode interlayer to those with the commonly used PEDOT:PSS D


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Figure 3. (a) Synthetic route for functionalized ZnOF NPs and fabrication process of the polymer solar cells with self-assembled ZnOF NPs as the cathode buffer layer. (b) Energy-level diagram for the device shown in part a. Adapted with permission from ref 58. Copyright 2015 the Royal Society of Chemistry.

contribution occurred exclusively after thermal annealing, suggesting that film formation kinetics stall ATMB’s migration so that its diffusion to form the interlayer requires postdeposition thermal assistance (vide inf ra). The segregation of low-surface-energy additives to the surface during spin coating has also been harnessed to generate commonly used inorganic interlayers. Inspired by Wei et al., Tan et al. modified ZnO nanoparticles (NPs) with fluoroalkyl chains to obtain functionalized ZnOF NPs.58 The modified ZnOF NPs were blended and spun with the P3HT:PCBM active layer. Driven by the surface segregation of the fluoroalkyl chains, due to their low surface energy, ZnOF NPs migrated from the P3HT:PCBM active layer to the surface during a solvent annealing process to form a cathode interlayer, as schematically illustrated in Figure 3. Indeed, OPVs comprising an optimal P3HT:PCBM:ZnOF NPs blend ratio and annealing conditions achieved performances higher than those obtained for equivalent devices with ZnO interlayers deposited in a separate processing step, as listed in Table 1. It is important to note that spontaneously generated interlayers incorporating fluorinated groups can offer an additional contribution to the device performance, i.e. improved stability under ambient conditions. Carle et al. showed that fluorination of the side chains of a donor molecule leads to enhanced photochemical stability with comparison to the unfluorinated donor.37 The spontaneous generation of fluorinated interlayers also enhanced the photochemical stability of devices, as reported by Yao et al. 32 They demonstrated that incorporating even a small amount of fluorinated diblock copolymer (P3HT-b-P3FAT) as additive to P3HT:PCBM, and its segregation to the surface, significantly enhanced the PCE stability under ambient condition (as mentioned above). The spontaneously generated interlayer was shown to have selectivity for hole collection. Hence, the photochemical stability was attributed to the eliminated PEDOT:PSS layer, which is known for its problematic ambient stability.59,60 Tan et al. mentioned the positive effect of the fluorinated ZnOF buffer layer on the devices stability, but the actual ambient stability characterization was not reported.58 The utilization of fluorinated compounds is well established in the field of OFET because it has been shown to be effective in forming a kinetic barrier against the diffusion of oxygen and humidity into the semiconductor films.61 Hence, further research on the beneficial effect of spontaneously segregated fluorinated additives to form interlayers and their contribution to the stability of the OPV devices is needed. As mentioned above, silanated compounds are known to exhibit low surface energies and hence have also been suggested

anode interlayer revealed a remarkable 50% PCE increase, as listed in Table 1. Indeed, the IP of the organic films was found to increase as a function of the copolymer additive concentration until a saturation value was reached, associated with full coverage of the surface. Here too, it was shown that the magnitude of the IP shift can be varied by controlling the relative amount of the fluorinated groups on the P3HT-b-PFAT polymer. Although the IP shift was largest for blends with the PFAT homopolymer, the corresponding devices were not as efficient, probably due to poor interaction between the interlayer and the active layer. The improved device performance was associated with the fluorinated interlayer, suggesting a reduction of the injection barrier at the anode/organic interface. Indeed, UPS measurements presented in this study reported a correlation between the PFAT content in the film and the Ag anode work function. However, in contrast to the earlier reports mentioned above,28 in this case the fluorinated interlayer induced an increase in the work function of the adjacent metal.32 A work function increase generally indicates a net interfacial dipole moment directed by the positive charge toward the metal and the negative charge toward the organic film.50−52 Therefore, the orientation of the dipole moment in this study is opposite to that reported for the F-PCBM additives, which reduced the metal work function.28 This inconsistency, although not discussed in the study, might be settled by assuming a reorganization of the interlayer stimulated by the metal deposition process. Indeed, it is well established that the direction of a net interfacial dipole monolayer “sandwiched” between a metal and a semiconductor depends on the nature of the metal and the method of its deposition.51,53 Furthermore, it was previously reported that metal−molecule polarization and partial charge redistribution between the metal and the molecules could completely invert the dipole direction.53−56 A n o t h e r p e r flu o r i n a t ed c o m p o u n d , 4 - a m i n o -2 (trifluoromethyl)benzonitrile (ATMB), was suggested as an additive for the P3HT:PCBM system by Jeong et al.57 They found that an optimal concentration of the additive enhanced the photocurrents and FF, hence improving the PCE from 4 to 5%, as shown in Table 1. XPS depth profiling of annealed P3HT:PCBM:ATMB films revealed that the additive molecules were predominantly positioned close to the film surface (within the upper 25 nm). The authors speculated that the ATMB-rich layer, which was in contact with the LiF/Al electrode, enhanced the FF by reducing charge recombination losses and leakage currents. They also reported that the ATMB enhanced P3HT crystallinity and hence hole mobility by 2 orders of magnitude compared to reference devices. Interestingly, the ATMB E


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Figure 4. (a) Tandem cell structure (left) and illustration of the PEI:BHJ nanocomposite self-organization on the PEDOT:PSS and ITO surfaces. (b) Energy-level diagram of the tandem solar cell shown in part a. Adapted with permission from ref 67. Copyright 2015 Wiley.

generally impose dipole moments at the organic/metal interface, which increase the IP of the organic film and reduce the work function of the adjacent metal electrode. Accordingly, fluorinated additives are suitable for cathode interlayers. On the other hand, the silanated compounds do not affect the electrode work function but rather passivate trap states at the organic/electrode interface, effectively suppressing charge recombination and enhancing charge transport. So far, segregation of silanated additives was demonstrated for cathode interlayers only. However, we believe that the electrostatically inert characteristics of silanated additives offer their use also for trap passivation at the organic/anode interface. Segregation of high-surface-energy additives to the bottom substrate. High-surface-energy additives, such as hydrophilic molecules, are repelled from the organic/air surface during film deposition. The distribution of a high-surfaceenergy additive in a blend film generally depends on the phase separation between the blend components. However, in the case of very thin films, such as those used in OPVs, the underlying substrate also has a significant effect on the distribution of the additive in the film. Typically, the substrates used in OPVs, i.e. ITO, MoOx, ZnO, and PEDOT:PSS, attain high surface energies and hence encourage the segregation of high-surface-energy additives toward them.43 Amine-based molecules are known to reduce the work function of a variety of metals and, hence, are commonly used as cathode interlayers in OPV devices.17,65,66 In parallel, the tendency of amines to attain high surface energy, due to their hydrophilic properties, suggests they could be suitable additives for segregation to the bottom organic/substrate interface during device processing. Accordingly, several studies reported the use of amine-based additives to spontaneously generate cathode interlayers in inverted OPV devices. Kang et al. demonstrated the use of spontaneous vertical phase separation of polyethylenimine (PEI).29,67 The high surface energy of PEI relative to that of the BHJ components, evaluated using wetting measurements, directs its vertical migration toward the highsurface-energy ITO substrate, thus minimizing the system’s free energy, as illustrated in Figure 4a. Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) depth profiling of ITO/PEI/ BHJ and ITO/PEI:BHJ samples confirmed that the PEI effectively separates from the blend and segregates to form a distinct interlayer on the ITO substrate. The PEI interlayers induce interfacial dipoles, forming a strong electrostatic force between the positively charged protonated amines of the PEI

as additives for self-segregated interlayers in OPVs. Yamakawa et al. showed that PDMS-b-PMMA forms a surface-segregated layer during the solution coating process, due to the low surface energy of the PDMS block.62 The spontaneously formed cathode interlayer was verified by XPS depth profiling and was shown to improve the Jsc and Voc of the P3HT:PCBM OPV device. As PDMS-b-PMMA itself does not attain an inherent molecular dipole, it was not expected to affect the energy level alignment at the organic/metal interface. Therefore, the enhanced performance (see values in Table 1) was attributed to the passivation of trap states at the interface, which prevents charge recombination. However, this mechanism was not fully verified. Another silane-based additive, (3-chloropropyl)trimethoxysilane (CP3MS), was used by Farahat et al. in a dialkylated diketopyrrolopyrrole chromophore (SMDPPEH)based molecular BHJ solar cell.63 The additive, CP3MS, spontaneously migrated from the SMDPPEH:PC60BM BHJ to the surface, forming an ultrathin interlayer at the organic/Al cathode interface. The presence of the interlayer, which was confirmed by XPS depth profiling and contact angle measurements, suppressed charge recombination and enhanced charge transport at the interface, consequently improving device performance (Table 1). A conjugated random copolymer with no fluorinated or silane groups, poly(3-hexylthiophener-3-((hexyloxy)methyl)thiophene) (P(3HT-r-3HOMT)), was used as an additive by Li et al.64 The authors concluded, based on the characteristic XPS C−O peak, that the additive segregated to the surface, presumably due to its lower surface energy. When the additive was utilized in a P3HT:ICBA BHJ system, it caused a 20% improvement in the device PCE (Table 1) and a gradual increase of Voc with additive concentration. The authors speculated that the (P(3HT-r-3HOMT))-rich layer at the organic/cathode interface suppresses surface recombination and improves charge collection efficiency. Kelvin probe force microscopy measurements indicated that the IP of the organic film increases (in magnitude) with the additive, relative to the P3HT:ICBA reference film. Therefore, the performance improvement could be associated with a reduction in the charge injection barrier. In this section, we reviewed the segregation of low-surfaceenergy additives during film processing to spontaneously form interlayers in OPVs. Two main types of additives were used, fluorinated and silanated compounds. The fluorinated additives F


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Table 2. Performances Comparison for Device with Spontaneously Generated Interlayers due to High Surface Energy (Termed Spont. in Table), and Corresponding Reference Devices with either Conventional or No Interlayera

G


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Chemistry of Materials Table 2. continued

a

TA = thermal annealing; CIL = cathode interlayer; WF = work function.

devices prepared with a separate PEI cathode interlayer (the values are summarized in Table 2). The use of PEI self-segregation properties was further extended to simplify the fabrication of tandem cells. The tandem cell structure consists of two serial identical PTB7Th:PC70BM BHJ subcells separated by a PEDOT:PSS recombination layer. The PEI additive was blended with the

and the negatively charged oxygen anions of the ITO. This selfsegregated PEI interlayer was utilized in the P3HT:ICBA, PTB:PC70BM, and small molecule-based DTS-F:PC70BM systems, suggesting its wide applicability for OPVs of inverted architecture. All the ITO/BHJ:PEI/MoOx/Ag devices fabricated in a single coating process showed performances comparable to those of equivalent ITO/BHJ/PEI/MoOx/Ag H


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Figure 5. (a) Device structure illustration; (b) J−V characteristics; and (c) schematic energy level diagram of the inverted printed ITO/ZnO/PhenNaDPO:pDPP5T-2:PC61BM/MoO3/Ag device. Adapted with permission from ref from 38. Copyright 2016 The Royal Society of Chemistry.

Figure 6. (a) Schematic illustration of film deposition and interlayer formation. (b) current density−voltage (J−V) characteristics of devices with PCBDAN (amount indicated) blended in P3HT:PCBM. (c) Energy band diagram of devices with a PCBDAN interlayer. Adapted with permission from ref 42. Copyright 2014 the American Chemical Society.

Zhang et al. for printed inverted OPV devices based on diketopyrrolopyrrole-quinquethiophene alternating copolymer (pDPP5T-2) and PC61BM blends.38 The small molecule additive, (2-(1,10-phenanthrolin-3-yl)naphth-6-yl)diphenylphosphine oxide (Phen-NaDPO), was blended with pDPP5T-2:PC61BM and collectively printed on ITO/ZnO substrates. The Phen-NaDPO additive was found to have higher surface energy (49.81 mJ/m2) than that of pDPP5T-2 (28.34 mJ/m2) and PCBM (34.78 mJ/m2). The relatively high surface energy and strong interaction with the cathode material drive the strong attraction of Phen-NaDPO to the ZnO interface and the subsequent formation of the selective cathode interlayer. The existence of such a Phen-NaDPO enriched layer at the bottom contact was verified by water contact angle measurement at the top and bottom surfaces. As evidenced from Kelvin Probe measurements, the Phen-NaDPO interlayer reduces the effective work function of the ITO/ZnO cathode substrate and, hence, increases the built in potential in the cell. This results in higher Jsc and FF relative to reference devices with no additive and, consequently, in device PCE enhancement from 4% to 5.22%. Notably, the printed devices with a self-organized interlayer showed superior performance over equivalent optimized ITO/ZnO/Phen-NaDPO/pDPP5T2:PC61BM/MoO3/Ag printed devices (PCE of 4.8%), in which Phen-NaDPO solution was bladed as the cathode interlayer atop ZnO in a discrete processing step (values are listed in Table 2). It is also noteworthy that the Voc values are

BHJ components and spun on the respective electron collectors (bottom ITO and PEDOT:PSS recombination layer). Again, ∼2-nm-thick ionically self-assembled phase-separated PEI interlayers were formed at both ITO and PEDOT:PSS interfaces, as indicated by TOF-SIMS analysis, inducing favorable interfacial dipoles. As a result, the work functions of the ITO cathode and PEDOT:PSS recombination layer were reduced, facilitating electron collection from both cells. This provided desirable ohmic contacts in the ITO/PEI:BHJ/ PEDOT:PSS/PEI:BHJ/MoOx/Ag tandem structure, yielding high built-in fields within the subcells and energy conversion efficiency of 10.8% (see values in Table 2). Recently, the spontaneous segregation of PEI was taken onestep further and utilized for printing a tandem cell. This demonstration reveals the conceptual compatibility of the spontaneous interlayer formation with roll-to-roll fabrication.39 For the optimized printed device, with 9% efficiency, the authors used a low molecular weight PEI additive (1300 g/mol instead of 750 000 g/mol used in spin coated devices) because the diffusion of the conventional PEI was limited by its high Mw (as verified by TOF-SIMS and XPS). The significant difference between the coating dynamics of printing and spin coating was attributed to the existence of a strong centrifugal force during spin coating that assists the vertical segregation of high-Mw PEI additive to the substrate. The utilization of spontaneous interlayer formation in printed OPV technology was also recently demonstrated by I


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and PTB7:PC70BM, demonstrating a general approach to improve performances of inverted structure OPVs. The segregation of high-surface-energy additives to the bottom substrate can also be used to spontaneously generate inorganic interlayers. Jung et al. applied this principle to fabricate a common inverted OPV device comprising a P3HT:PCBM active layer and a ZnO interlayer at the organic/substrate interface.70 This was done by blending a ZnO sol−gel precursor ((Zn(CH3COO)2·2H2O) in the active layer, as shown by the schematic illustration in Figure 7. The

independent of the Phen-NaDPO concentration (which reflect the Vbi), as expected in devices with highly selective contacts. The schematic energy level alignment, device structure, and characteristics are shown in Figure 5. The high surface energy and dipole moment of the amine group can also be used to induce segregation of a fullerene acceptor to the bottom interface and concurrently tune the interfacial energy alignment between the active layer and the bottom cathode. Ma et al. used an amine-based fullerene derivative, [6,6]-phenyl-C61-butyric acid 2-((2-(dimethylamino)-ethyl) (methyl) amino) ethyl ester (PCBDAN), as a spontaneously cathode interlayer forming additive in the P3HT:PCBM system.42 Driven by high surface energy, 30.4 mJ/m2, PCBDAN segregated toward the buried ITO substrate, as shown schematically in Figure 6a, and reduced the effective work function from 4.7 eV to approximately 4.1 eV, as measured by a Kelvin Probe. Meanwhile, P3HT enriched the organic/air interface, creating a preferable vertical morphology for an inverted organic solar cell. Blank devices with no PCBDAN performed poorly, while the addition of PCBDAN led to devices of 3.7% efficiency. This value is comparable to the 3.85% efficiency obtained by the reference devices with the PCBDAN interlayer deposited in a distinct process. The authors also reported similar results with another amine-based cathode interfacial compound, poly-[(9,9-bis(30-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9 dioctylfluorene)] (PFN), implying that self-organization may be a general phenomenon in ternary polymer blends. Indeed, Peng et al. showed that an inverted PTB7:PC70BM device with a PFN cathode interlayer can be fabricated by a single step process via blending PFN with the BHJ components.68 The obtained device performance was similar to that of a reference device fabricated by a standard two-step process, as shown in Table 2. The surface energy of the PTB7:PC70BM BHJ, 19 mJ/m2, estimated from contact angle measurements, is significantly lower than that of PFN, 49.8 mJ/ m2. Hence, the additive molecules segregate toward the underlying high-surface-energy ITO substrate, 55 mJ/m2, to minimize the free energy of the system. The PFN interlayer decreases the ITO effective work function, as measured by scanning Kelvin Probe force microscopy, which results in Voc enhancement and hence improved device performance. The benefits of dipole layers as electrode work function modifiers encouraged Fu et al. to use an ionic liquid (IL), BenMeln-Cl, as an additive intended to segregate to the highsurface-energy ITO substrate.69 The surface energy of BenMeln-Cl IL, 38.67 mJ/m2, is higher than that of most commonly used donor molecules, 24−28 mJ/m2, suggesting that during spin coating the IL would enrich the organic/ITO interface. Indeed, contact angle measurements, XPS, and TOFSIMS depth profile analysis confirmed the formation of a selfassembled BenMeln-Cl IL layer at the organic/ITO interface. The BenMeln-Cl IL formed an interfacial dipole layer oriented with the positive charge pointing toward the organic film and the negative charge pointing toward the ITO cathode. This alignment shifted the band edge of ITO closer to the vacuum level, effectively reducing the work function of the ITO from 4.8 to 4.4 eV. The improved energy level alignment at the organic/ITO interface promoted efficient charge transfer. The obtained device performance was similar to that of a bilayer device, as shown in Table 2. This concept of spontaneously generated IL interlayers was introduced in several donor:acceptor systems, i.e. P3HT:PCB60M, PBDTTT-C:PC70BM,

Figure 7. Schematic illustration showing the one-step fabrication process of an inverted P3HT:PCBM device with a ZnO interlayer. Adapted with permission from ref 70. Copyright 2015 Elsevier.

surface energy of zinc acetate is significantly higher than that of P3HT and PCBM, and therefore, it spontaneously segregates to the ITO substrate. Using SIMS depth profiles to investigate the vertical component distribution, the authors confirmed the spontaneous formation of a 20 nm thick ZnO interlayer at the organic/substrate interface during spin coating and sequential thermal annealing processes. The performances of the devices were similar to those obtained for devices with a ZnO interlayer deposited in a distinct fabrication step, as listed in Table 2. XPS analysis of the ZnO interlayer revealed oxygen deficiencies, probably due to the sol−gel process occurring inside the organic film. The authors speculated that the oxygen deficient regions, as well as some other defect sites, caused a light soaking effect, which was overcome by a 5 min UV irradiation treatment (for further discussion see the Post-treatments and optimization section). In-situ conversion of the sol gel precursors inside the organic matrix can result in byproducts and impurities in the film and on the oxide surfaces that could degrade the device performances and hence should be seriously considered. In this section, we reviewed the studies utilizing segregation of high-surface-energy additives to spontaneously form interlayers at the substrate/organic interface in OPVs. The most popular high-surface-energy interlayer additives are amine-based compounds. The interactions between the amine group and the oxygens of the metal oxide substrates induce dipoles that reduce the electrode’s work function. Accordingly, amine-based additives are highly suitable for spontaneous formation of cathode interlayers in devices of inverted configuration. The segregation of high-surface-energy inorganic additives was also utilized for interlayers by blending ZnO sol− gel precursors in the active layer. We believe the selfsegregation of sol−gel precursors to the bottom substrate could be extended to other known oxide interlayers such as molybdenum, titanium, nickel, vanadium, tungsten, etc. J


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Table 3. Performance Comparison of Devices with Spontaneously Generated Interlayers Driven by Chemical Affinity to the Electrode (Termed Spont. in Table), and Corresponding Reference Devices either with Conventional Interlayer or No Interlayera

a

TA = thermal annealing; CIL = cathode interlayer; AIL = anode interlayer; WF = work function.

group.30,35,71−73 In this case, additives are blended in the BHJ and segregate to the organic/metal interface during metal deposition to reduce the overall free energy of the system. The additive−metal interactions have two significant contributions to the OPV: (I) modification of the effective metal work function to improve charge extraction to the electrode, and (II)

3. INTERLAYER FORMATION INDUCED BY CHEMICAL INTERACTIONS Recently, a new mechanism for spontaneous formation of interlayers in OPVs, harnessing the chemical interactions between selected additives and the evaporated metal as the driving force for migration, was realized and studied in our K


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Chemistry of Materials enhancement of device stability and lifetime extension.30,73−76 In many cases, the additive−metal interactions are strong enough to overcome the free surface energy considerations mentioned in the previous sections. Therefore, understanding the parameters that control this mechanism offers an alternative approach to interlayer generation that is not limited to additives with low/high surface energy. The migration of additives toward the organic/metal interface induced by chemical interactions was first realized by Dekman et al. for P3HT:PCBM BHJ using a short polyethylene glycol (PEG) additive and an Al cathode.30 The presence of a PEG interlayer is known to modify the work function of the adjacent cathode, dramatically improving device performance.77 A recent study showed that a PEG cathode interlayer is also useful in inverted configuration devices when inserted between the ITO cathode and active layer.78 This effect was demonstrated on a variety of organic active layers, establishing that PEG is a generic cathode interlayer material. For example, in the case of PBDTTT-EFT:PC71BM based devices, the presence of a PEG interlayer increased the PCE values from ∼2% to ∼8.5%. Spontaneous formation of PEG interlayers was achieved by blending PEG within the BHJ solution. Since PEG’s surface energy, ∼43 mJ/m2, is significantly higher than that of P3HT (∼27 mJ/m2) and PCBM (∼38 mJ/m2), it does not segregate to the BHJ/air interface during spin coating. Indeed, XPS analysis corroborated the absence of PEG at the organic/air interface. Yet, the device performance was similar to that reported for devices with a PEG interlayer deposited in a separate processing step, as listed in Table 3 and shown in Figure 8.75,77 Therefore, it was inferred and confirmed that the PEG segregated to the BHJ/Al interface during the Al deposition.30,35,71−73

Figure 9. Illustration of the suggested spontaneous interlayer formation by PEG migration due to PEG−metal interactions. Adapted with permission from ref 71. Copyright 2015 the American Chemical Society.

the film (left panel of Figure 9 (t0)). The first evaporated Al atoms that reach the organic film interact with PEG molecules that are close to the surface to reduce the interfacial energy. Further evaporation and metal−PEG interactions generate a gradient of PEG concentration that induces the diffusion of PEG molecules from the film to the metal/organic interface (t1 in the scheme). This migration could also include molecules from the bottom PEG layer, but the concentration gradient pulling PEG to the upper interface is modulated by its affinity to the bottom substrate (vide inf ra). Finally, once a PEG monolayer at the BHJ/metal interface is completed, the driving force for migration is terminated. Modifying the end-group of ethylene oxide oligomers revealed that the interaction between the end-group of PEG and the metal is responsible for PEG migration to the interface. More specifically, acidic- or amineterminated PEG molecules segregated to the thermally evaporated Al atoms, while PEG molecules with ester- or ether-terminated groups did not.71 Therefore, the extent of migration increases with the acidity of the end-groups. The organometallic reaction reduces the interfacial energy and hence is the driving force for PEG migration to the blend/metal interface. Identifying the interactions between PEG and Al as the driving force for PEG migration allows us to explain earlier studies on PEG interlayer formation. For example, a PEGmodified fullerene, PEG-C60, was blended with P3HT:PCBM and improved the device performance and stability, as listed in Table 3.76 The authors identified an 8−10 nm PEG-C60 interlayer on the top of the organic film by XPS depth profile, and suggested that its formation occurred during spin coating. However, the additive is expected to have a high surface energy inhibiting its segregation during spin coating. Notably, the XPS measurements were conducted after the Al deposition. Therefore, these results are in full agreement with the mechanism of PEG migration and interlayer formation induced by PEG-Al interactions. Surprisingly, introducing PEG-C60 into devices with highwork-function cathodes, Au and Cu, also showed a significant increase in Voc and device performances (Table 3).76 However, these metals are unlikely to attract PEG molecules due to their positive reduction potentials. Indeed, we have shown that PEG does not segregate to an Au electrode from a P3HT:PEG blend even when a large amount of PEG is used (20%wt), as evident

Figure 8. Current density−voltage (J−V) curves under illumination and in the dark (inset) of a P3HT:PCBM:PEG(25 wt %) device (red triangles) and a corresponding P3HT:PCBM reference device (black squares). Adapted with permission from ref 30. Copyright 2013 the Royal Society of Chemistry.

A suggested mechanism for PEG migration toward the organic/metal interface during the deposition of metals with a low reduction potential, i.e. Ca or Al, is illustrated in Figure 9. It proposes that during spin coating of the BHJ the high surface energy of PEG directs its segregation to the bottom organic/ substrate interface. Some PEG molecules, up to its solubility limit in P3HT, are retained and homogeneously dispersed in L


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Chemistry of Materials from XPS analysis.35 Therefore, the effect of PEG-C60 on highwork-function electrodes remains unclear. The tendency of ethylene glycol to segregate to the organic/ metal interface was utilized by Shi et al. to generate interlayers of a block copolymer (BCP) with triethylene glycol side chains, P3HT-b-P3TEGT.79 The BCP was used as an additive in the P3HT:PCBM and PBDTTT-C-T:PC70BM systems in inverted device configuration (see performance values in Table 3). P3HT-b-P3TEGT was blended in the BHJs, and the XPS depth profile revealed that it spontaneously self-segregated to the film surface. The authors attributed the vertical migration of P3HTb-P3TEGT to the triethylene glycol side chain. However, measurements reported in the same study showed that the contact-angle of water on the films decreases with the additive content, pointing to an increase in surface energy. Therefore, spontaneous surface segregation would be unlikely to occur unless other driving forces are involved, or considerations for segregation are rather kinetic than thermodynamic. Generally, segregation and phase separation of block copolymers involve many complex considerations that often depend on the preparation method and are beyond the scope of this review. It is also noteworthy that the P3HT-b-P3TEGT diblock copolymer was shown to induce an increase of the Ag effective work function, while interlayers of additives containing ethylene glycol backbones (HEG-DT, PEG, PEG-C60) have generally been shown to reduce the Ag or Al effective work functions.30,73−76 The work function increase in this study might indicate that the direction of the interlayer net dipole moment was reversed compared to that generally obtained in ethylene oxide-based interlayers. The migration of the additive to the top organic/metal interface during metal evaporation, due to the interaction between the additive and deposited metal, could be modulated by the affinity of the additive to the underlying substrate.80 Several studies showed that the surface energy of the substrate, and in particular its polar component, determines the strength of the interaction between the substrate and the polar hydroxylterminated PEG groups.35,74 Comparing ITO, PEDOT:PSS, and MoO3 (γMoO3 > γPEDOT:PSS > γITO) reveals the strongest attraction of PEG to MoO3 and the weakest to ITO. In fact, the surface energy of MoO3 is about 5 times greater than those of the other mentioned substrates. Therefore, in the case of an underlying MoO3 layer, the attraction between PEG to MoO3 is so significant that PEG does not migrate to the organic/Al interface during metal deposition. Indeed, including both a MoO3 anode interlayer and PEG additives results in the total loss of OPV performance, as shown in Figure 10 and Table 3.74

Figure 11. Current density−voltage (J−V) curves of glass/ITO/ PEDOT:PSS/P3HT:PCBM:HEG-DT/Ag OPV devices with various contents of HEG-DT. Adapted with permission from ref 73. Copyright 2016 the Royal Society of Chemistry.

Figure 10. Current density−voltage (J−V) curves of devices fabricated with and without 10 wt % PEG-400 on (a) PEDOT:PSS/ITO glass and (b) MoO3 (10 nm)/ITO glass substrates. Adapted with permission from ref 74. Copyright 2011 the American Chemical Society.

Furthermore, formation of the interlayer not only modified the Ag work function and enhanced charge injection, but also significantly extended the environmental stability, due to the hygroscopic character of the ethylene-glycol backbone. These examples demonstrate that tuning additive−metal interactions is a useful tool for inducing the segregation of additives and that the segregation process can be used to form interlayers and passivate interfaces. This section summarizes the studies on spontaneous interlayer formation due to chemical interactions between additives and evaporated electrodes. To date, this approach was demonstrated for derivatives of PEG, a known cathode interlayer material. The chemical reaction between the additive’s end-groups and the evaporated metal is responsible

In certain cases, the competitive affinity of the substrate and top electrode to the additive can have beneficial outcomes. Li et al. demonstrated this by harnessing PEG to concurrently improve the conductivity of the bottom PEDOT:PSS anode interlayer and generate a top cathode interlayer.81 In this case, PEG was solution deposited on the PEDOT:PSS layer, followed by BHJ spin coating and Al evaporation. The PEG not only penetrated the PEDOT:PSS layer and improved its conductivity, but due to the relatively weak PEDOT:PSS−PEG interactions, also migrated upward to the BHJ/Al interface through the entire active layer to spontaneously form a cathode interlayer. The double-role PEG solar cells demonstrated a performance better than typical P3HT:PCBM devices with no PEG, as listed in Table 3. While PEG is a good interlayer for OPVs due to the dipole and interaction it induces with the deposited Al, it does not interact with Ag and hence would not migrate to a silver electrode. Therefore, for OPVs with Ag cathodes, we suggested an additive with an ethylene glycol backbone to form the dipole interlayer, while capped with thiol groups to initiate migration to the Ag electrode.73 A blend of P3HT:PCBM and a hexa(ethylene glycol) dithiol (HEG-DT) additive was spun onto an ITO/PEDOT:PSS substrate, followed by Ag cathode evaporation. XPS analysis confirmed the absence of the additive at the BHJ/air interface and its presence at the BHJ/Ag interface. The HEG-DT interlayer led to improved photovoltaic device performances, as can be seen in Figure 11 and Table 3.

M


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Table 4. Performance Comparison for Devices with Spontaneously Generated Interlayers Driven by Additive Solubility (Termed Spont. in Table), with Corresponding Reference Devices either with Conventional Interlayer or No Interlayera

a

TA = thermal annealing; CIL = cathode interlayer; AIL = anode interlayer; WF = work function.

for additive migration toward the electrode, while its backbone is accountable for the dipole and electronic properties of the interface. This method was used for cathode interlayers, but the separation between the migration-responsible and electronicfunctional groups makes it suitable for any type of interlayers.

was poor due to the photo-oxidation of P3HT by the NiO nanoparticles. Accordingly, the same strategy was applied using TiO2-covored-NiO, which lead to devices with improved performances and stability, as shown in Figure 12 and Table 4.

4. INTERLAYER GENERATION DUE TO ADDITIVE SOLUBILITY The spontaneous formation of interlayers could be kinetically generated, for example, by the difference between the solubility of the components. Due to rapid film drying during the spin coating, the less soluble components precipitate at the bottom, while the more soluble components enrich the top surface.82 Susarova et al. demonstrated this principle using a highly soluble fullerene derivative (HSFD) and low-soluble polymer, P3BT, as additives in a P3HT:PCBM blend.83 The difference in additive solubility was expected to induce a strong phase separation during spin coating, with a HSFD cathode interlayer on the top and a P3BT anode interlayer on the bottom, all in a single deposition step. The inferred formation of these interlayers enhanced the FF and hence the device performance, as listed in Table 4, although further investigation is required to confirm the presence of the interlayers. This technique was also used by Park et al. to generate an inorganic replacement to the commonly used PEDOT:PSS anode interlayer.84 To do so, they blended p-type NiO nanoparticles with a P3HT:PCBM blend and spun it on ITO. The low solubility of the nanoparticles resulted in the spontaneous formation of an inorganic anode interlayer at the bottom organic/substrate interface. The stability of the devices

Figure 12. (a) Current density−voltage (J−V) curves of P3HT:PCBM solar cells with a hole-collecting interlayer of bare NiO (gray squares), TiO2-covered NiO (black triangles) and a reference device without NiO (gray circles). (b) Schematic band diagram of the P3HT:PCBM device with the NiO hole-collecting interlayer. Adapted with permission from ref 84. Copyright 2012 the American Chemical Society.

A puzzling result was attained when poly(vinylpyrrolidone) (PVP) was used as an additive. Wang et al. reported that blending PVP with P3HT:PCBM enhanced the device performance and obtained results similar to those of devices with a separately deposited PVP top cathode interlayer, as listed in Table 4.85 The authors suggested that PVP segregates to the surface to form an interlayer spontaneously. However, the N


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Table 5. Performance Comparison of Devices with Spontaneously Generated Interlayers Driven by Additive Density (Termed Spont. in Table), with Corresponding Reference Devices either with Conventional Interlayer or No Interlayera

a

TA = thermal annealing; CIL = cathode interlayer; WF = work function.

surface energy and mass density of PVP are significantly higher than those of all other components in the blend. Although contact angle measurements were shown, no XPS or TOFSIMS profiling was provided to support the hypothesis of PVP surface enrichment. Furthermore, the devices show that the Voc progressively decreases with PVP content. However, the authors claim that a cathodic PVP interlayer should decrease the work function of the Al cathode, and hence, the Voc should increase with PVP content. Therefore, further investigation is required to understand PVP interlayer formation and its role in BHJ OPVs.

5. INTERLAYER FORMATION DUE TO ADDITIVE DENSITY, AMPHIPHILIC CHARACTER, OR CONCENTRATION GRADIENTS In a previous section, we showed how low-surface-energy additives segregate to the organic/air interface during spin coating. However, Yu et al. reported that PVDF, a low-surfaceenergy fluorinated additive, did not segregate to the P3HT:PCBM surface during spin coating, but rather accumulated at the bottom substrate.86 The authors attributed this behavior to the density of PVDF, which is the highest in the ternary blend. The PVDF layer at the bottom substrate has been used as a cathode interlayer in inverted devices and shown to improve device performance, as listed in Table 5. It is noteworthy that differences in materials density are rarely considered as the driving force for vertical separation in nanoscale films, and hence invoking this argument in OPV systems should be accompanied by suitable theory and/or calculations. Two different studies reported the segregation of oleamide, an amphiphilic surfactant, to the BHJ surface during spin coating and corresponding improved device PCE, as shown in Figure 13 and Table 5.87,88 Furthermore, surface Kelvin Probe measurements indicated that the surface ionization potential of films with oleamide was higher than that of P3HT:PCBM or

Figure 13. Solar cell structure with oleamide interlayer (inset), and current density−voltage (J−V) curves of PTB7:PC71BM BHJ with oleamide. Reproduced with permission from ref 88. Copyright 2016 The Japan Society of Applied Physics.

PTB7:PC71BM blends with no additive. The higher ionization potential is associated with a work function reduction of the adjacent Al cathode and improved electron collection. However, the surface energy of oleamide is higher than that of all other components in the blend, and hence, it is not expected to segregate to the BHJ/air interface. Therefore, the segregation of oleamide appears to involve other driving forces. One possible explanation is the amphiphilic character of oleamide, suggesting that, despite the overall high surface energy of the molecule, the hydrophobic chain induces segregation to the film surface. Alternatively, the mixture of O


Review


Figure 14. (a) Photopolymerization of PC60BAAB. (b) BHJ structure before (left panel) and after (right panel) exposure to UV light. Strong absorption creates a gradient of UV intensity along the light propagation direction inducing a PCBAAB concentration gradient (i.e., vertical phase separation) by photopolymerization. Adapted with permission from ref 92. Copyright 2014 Nature Publishing Group.

resulting in enhanced diffusivity. In parallel, heating enhances the mobility of the matrix molecules, further increasing additive diffusivity. Importantly, too intensive heat treatment leads to excessive growth of donor and acceptor domains, which is unfavorable for the device performance and thus must be avoided. However, if the difference in surface energies between the additive and the other components is sufficient, domain coarsening of the additive will occur at moderate temperatures, resulting in an ordered interlayer/BHJ bilayer while avoiding the negative temperature effects.43 Hence, thermal treatment can be utilized for effective improvement of interlayer continuity. For example, when a low-surface-energy perfluorinated additive, ATMB, was incorporated in P3HT:PCBM, thermal annealing was required to boost the interlayer generation and obtain improved performances.57 Thermal annealing is especially significant when an additive is subjected to several competitive forces, for example, attraction to the bottom substrate due to high surface energy concurrent with attraction to the top evaporated electrode. In such cases, the interplay between the opposing effects is governed by the sequence in which the thermal annealing and cathode deposition are performed. Our study on PEG additives in P3HT:PCBM films revealed that thermal annealing performed before Al deposition inhibits the migration of PEG to the organic/metal interface, suppressing the contribution of PEG to the performance. In contrast, annealing af ter Al deposition enhances PEG migration to the organic/metal interface and improves device performance, as shown in Figure 15.72 These results were confirmed by XPS analysis of the organic/metal interface of films annealed either before or after the metal deposition. Thermally annealing the P3HT:PCBM:PEG film prior to metal deposition accelerates PEG’s migration away from the surface toward the bottom substrate. The PEG molecules are then too far to be attracted to Al during the evaporation. However, when films annealed prior to the metal deposition are annealed again after the metal deposition, the Voc increases, reaching values usually obtained by devices with PEG interlayers. Therefore, this result could indicate that the second thermal treatment provides the energy required for PEG diffusion to the top organic/metal interface. Interestingly, a similar sequence of processes (i.e., thermal annealing after electrode evaporation) is suggested in the common P3HT:PCBM device fabrication. It is possible that there too, the metal deposition induces beneficial processes that

solvents used in these studies might have triggered other kinetic considerations.89−91 A unique approach to encourage segregation of an additive toward an interface is by inducing a concentration gradient in the BHJ. This was innovatively demonstrated by Zhang et al. for vertical phase separation of the BHJ92 but not yet shown for the generation of interlayers. In their study, a photopolymerizable fullerene derivative was used as the acceptor and the BHJ was exposed to UV illumination. In regions with higher UV intensity, the polymerizable molecules were consumed faster, which resulted in a concentration gradient. This drove the diffusion of acceptor molecules toward the intensely illuminated regions, effectively directing the desired BHJ phase separation, as illustrated in Figure 14. This approach was reported to be applicable for conventional and inverted device structures, depending on the UV illumination direction, and lead to a 20% increase in PCE of the photovoltaic devices. As mentioned above, this approach has yet to be harnessed for interlayer generation.

6. POST-TREATMENTS AND OPTIMIZATION OF SPONTANEOUSLY FORMED INTERLAYERS The general process for active layer deposition in organic electronic devices is spin coating; hence, their morphology is generally not thermodynamically stable. The common denominator of all additive migration mechanisms detailed above is the system’s free energy reduction. However, the rate at which the migration processes occur depends not only on the magnitude of the driving force, but also on the diffusivity and solubility of the additive, i.e. its permeability in the organic matrix. For example, a high Mw of an additive hinders its mobility and consequently its diffusivity, stalling the segregation process and resulting in no or noncontinuous interlayers. The device performance could critically depend on the partial interlayer coverage, as demonstrated by Chien et al. for PEG additives.74 In their study, PEG additives of different Mw values (chain lengths) were introduced into a P3HT:PCBM blend. They found that low-Mw PEG readily migrated to the interface and improved device performance, while high-Mw PEG did not improve the performance, indicating its trapping in the active layer. Increasing the migration rate toward the thermodynamically stable state is possible by performing post-treatments such as thermal annealing, solvent annealing, and light irradiation. Heat treatment provides thermal energy to overcome the activation barrier for diffusion of the additive in the active film, P


Review


top BHJ/metal interface. The first is segregation of additives with low surface energy, most commonly fluorinated or silanated molecules, to the BHJ surface during processing. The second is the migration of additives with metal-affinity, for example with OH end-groups to Al or thiol to Ag, to the BHJ/ metal interface during metal deposition. Likewise, we reviewed several strategies to spontaneously generate interlayers at the bottom BHJ/electrode interface. For example, additives with high surface energy, i.e. with highly hydrophilic and/or polar groups such as oxides or amines, segregate to the metal oxide undelaying substrates during film processing. Additives with specific affinity to the underlying substrate are expected to follow the same process and migrate to the bottom interface during film processing, but this has not been demonstrated thus far. In addition to thermodynamic considerations, kinetics can also induce interlayer formation. Namely, very rapid solvent evaporation and/or the centrifugal forces during spin coating can lead to kinetically directed metastable states. For example, low-solubility additives segregate to the bottom interface regardless of their surface energy. Nonetheless, most often the primal metastable configuration makes post-treatments necessary to achieve stability and optimization. These treatments generally include thermal or solvent annealing. In summary, classifying and demonstrating spontaneous interlayer formation based on the driving forces for additive segregations provides the tools for judicious design and selection of additive and processing condition to form desired interlayers at selected interfaces. It can be noticed, though, that most studies to date were performed on the “work horse” BHJ P3HT:PCBM, while those on other OPV systems are still scarce. Our recent studies, however, suggest that the mechanisms for interlayer spontaneous formation presented in this paper are generic and universal, and can be applied to any organic blend, including highly efficient small molecule systems.

Figure 15. Current density−voltage (J−V) curves under illumination of P3HT:PCBM:PEG devices with 10 wt % PEG. The films are either not annealed (black circles) or annealed at 110 °C for 10 min either before (blue squares), after (red triangles), or before and after (blue dots) the Al metal evaporation.

are suppressed by annealing, and hence it is preferable to perform the annealing process after metal deposition. Solvent annealing also assists the migration of additives mainly by “softening” the matrix, effectively enhancing additive permeability through it. Molecule diffusion through the matrix occurs exclusively in the free volume of the matrix.93,94 During annealing in a “good” solvent, the polymer chain−chain distance increases, exposing a higher free volume for diffusion of the additive. In contrast, in “bad” solvents, the polymer chains aggregate and condense, limiting the additive’s diffusion to “grain boundary” zones.95,96 In other words, solvent annealing in “good” solvents can increase the pre-exponential factor of the diffusion coefficient and consequently enhance the diffusion process. For example, it was previously shown that fluorinated ZnO particles blended with P3HT:PCBM segregate to the film surface during spin coating due to their low surface energy. After film deposition, annealing in a “good” solvent for the P3HT:PCBM, DCB, resulted in further diffusion of ZnOF particles to the surface, and consequently better device performance. In contrast, annealing in a “bad” solvent for P3HT:PCBM, acetone, resulted in a decrease in device performance (see values in Table 1).58 While annealing optimizes the formation process of the interlayers, other post-treatments can be utilized to optimize the interlayer itself. For example, in a recent study, ZnO interlayers were prepared by either spin coating a P3HT:PCBM:ZnO-nanoparticles blend (Figure 7) or separate deposition before the P3HT:PCBM active layer.70 Discrete ZnO interlayers were identified in both cases, but the device with the spontaneously formed ZnO interlayer was not as efficient as the device with the discretely processed interlayer. However, after 5 min of UV light irradiation, the parameters of the device with the self-generated interlayer recovered to the proper values. The response of the device to light soaking may result from saturation of trap sites on dangling bonds, which reduces the charge recombination and hence improves the performance.70,97

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

REFERENCES

(1) Stubhan, T.; Salinas, M.; Ebel, A.; Krebs, F. C.; Hirsch, A.; Halik, M.; Brabec, C. J. Increasing the Fill Factor of Inverted P3HT:PCBM Solar Cells Through Surface Modification of Al-Doped ZnO via Phosphonic Acid-Anchored C60 SAMs. Adv. Energy Mater. 2012, 2, 532−535. (2) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells. Nat. Commun. 2014, 5, 5293. (3) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar cell efficiency tables. Prog. Photovoltaics 2015, 23, 1−9. (4) Dennler, G.; Scharber, M. C.; Brabec, C. J. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2009, 21, 1323−1338. (5) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.; Li, G.; Yang, Y. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 2013, 4, 1446. (6) Peet, J.; Senatore, M. L.; Heeger, A. J.; Bazan, G. C. The Role of Processing in the Fabrication and Optimization of Plastic Solar Cells. Adv. Mater. 2009, 21, 1521−1527. (7) Hwang, J.; Wan, A.; Kahn, A. Energetics of metal−organic interfaces: New experiments and assessment of the field. Mater. Sci. Eng., R 2009, 64, 1−31. (8) Koch, N. Organic Electronic Devices and Their Functional Interfaces. ChemPhysChem 2007, 8, 1438−1455.

7. SUMMARY AND FUTURE PROSPECTS In this perspective, we reviewed the mechanisms and driving forces that cause the spontaneous formation of interlayers in OPVs by segregation of additives to interfaces. We presented two strategies for inducing the formation of interlayers at the Q


Review

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Mechanisms for Spontaneous Generation of Interlayers in Organic

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