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Tuning the morphology through processing additives represents one of the most promising strategies to boost the performance of organic solar cells (OSCs). However, it remains unclear how oligothiophene-based solid additives influence the molecular packing and performance of OSCs. Here, two additives namely 2T and 4T, are introduced into state-of-the-art PM6:Y6-based OSCs to understand how they influence the film formation process, nanoscale morphology, and the photovoltaic performance. It is found that the 2T additive can improve the molecular packing of both donor polymer and non-fullerene acceptor, resulting in lower Urbach energy and reduced energy loss. Furthermore, the blend film with 2T treatment displays enhanced domain purity and a more favorable distribution of the acceptor and donor materials in the vertical direction, which can enhance charge extraction efficiency while simultaneously suppressing charge recombination. Consequently, OSCs processed with 2T additive realize a promising efficiency of 18.1% for PM6:Y6-based devices. Furthermore, the general applicability of the additive is demonstrated, and an impressive efficiency of 18.6% for PM6:L8-BO-based OSCs is achieved. These findings highlight that the uncomplicated oligothiophenes have excellent potential in fine-adjustment of the active layer morphology, which is crucial for the future development of OSCs. © 2024 Wiley-VCH GmbH.

At present, high-performance organic photovoltaics mostly adopt a bulk-heterojunction architecture, in which exciton dissociation is facilitated by charge-transfer states formed at numerous donor-acceptor (D-A) heterojunctions. However, the spin character of charge-transfer states originated from recombination of photocarriers allows relaxation to the lowest-energy triplet exciton (T-1) at these heterojunctions, causing photocurrent loss. Here we find that this loss pathway can be alleviated in sequentially processed planar-mixed heterojunction (PMHJ) devices, employing donor and acceptor with intrinsically weaker exciton binding strengths. The reduced D-A intermixing in PMHJ alleviates non-geminate recombination at D-A contacts, limiting the chance of relaxation, thus suppressing T-1 formation without sacrificing exciton dissociation efficiency. This resulted in devices with high power conversion efficiencies of >19%. We elucidate the working mechanisms for PMHJs and discuss the implications for material design, device engineering and photophysics, thus providing a comprehensive grounding for future organic photovoltaics to reach their full promise. Organic solar cells with a bulk-heterojunction architecture suffer from photocurrent loss driven by triplet states. Now, Jiang et al. show that sequentially deposited donor-acceptor planar-mixed heterojunctions suppress triplet formation, enabling efficiencies over 19%., Funding Agencies|City University of Hong Kong [9380086]; Innovation and Technology Commission of Hong Kong [GHP/018/20SZ, MRP/040/21X]; Environment and Ecology Bureau of Hong Kong under the Green Tech Fund [202020164]; US Office of Naval Research [N00014-20-1-2191, N000141712204, N000142012155]; Research Grants Council of Hong Kong [11307621, 11316422, C6023-19GF]; Hong Kong Postdoctoral Fellowship Scheme; Guangdong Major Project of Basic and Applied Basic Research [2019B030302007]; Guangdong-Hong Kong-Macao Joint Laboratory of Optoelectronic and Magnetic Functional Materials [2019B121205002]; National Key R&D Program of China [2017YFA0303703, 2018YFA0209100]; Fundamental Research Funds for the Central Universities [0204-14380177]; National Natural Science Foundation of China [22225305, 21922302, 21873047, 52002393, 51873160]; Hong Kong Baptist University

In this paper, a strategy of asymmetric alkyl and alkoxy substitution is applied to state-of-the-art Y-series nonfullerene acceptors (NFAs), and it achieves great performance in organic solar cell (OSC) devices. Since alkoxy groups can have a significant influence on the material properties of NFAs, alkoxy substitution is applied to the Y6 molecule in a symmetric manner. The resulting molecule (named Y6-2O), despite showing improved open-circuit voltage (Voc), yields extremely poor performance due to low solubility and excessive aggregation properties, a change that is due to the conformational locking effect of alkoxy groups. In contrast, asymmetric alkyl and alkoxy substitution on Y6, yields a molecule named Y6-1O that can maintain the positive effect of Voc improvement and obtain reasonably good solubility. The resulting molecule Y6-1O enables highly efficient nonfullerene OSCs with 17.6% efficiency and the asymmetric side-chain strategy has the potential to be applied to other NFA-material systems to further improve their performance. © 2020 Wiley-VCH GmbH

In this paper, a difluoro-monobromo end group is designed and synthesized, which is then used to construct a novel polymer acceptor (named PY2F-T) yielding high-performance all-polymer solar cells with 15.22% efficiency. The fluorination strategy can increase the intramolecular charge transfer and interchain packing of the previous PY-T based acceptor, and significantly improve photon harvesting and charge mobility of the resulting polymer acceptor. In addition, detailed morphology investigations reveal that the PY2F-T-based blend shows smaller domain spacing and higher domain purity, which significantly suppress charge recombination as supported by time-resolved techniques. These polymer properties enable simultaneously enhanced JSC and FF of the PY2F-T-based devices, eventually delivering device efficiencies of over 15%, significantly outperforming that of the devices based on the non-fluorinated PY-T polymer (13%). More importantly, the PY2F-T-based active layers can be processed under ambient conditions and still achieve a 14.37% efficiency. They can also be processed using non-halogenated solvent o-xylene (no additive) and yield a decent performance of 13.05%. This work demonstrates the success of the fluorination strategy in the design of high-performance polymer acceptors, which provide guidelines for developing new all-PSCs with better efficiencies and stabilities for commercial applications.

Developing high-performance donor polymers is important for nonfullerene organic solar cells (NF-OSCs), as state-of-the-art nonfullerene acceptors can only perform well if they are coupled with a matching donor with suitable energy levels. However, there are very limited choices of donor polymers for NF-OSCs, and the most commonly used ones are polymers named PM6 and PM7, which suffer from several problems. First, the performance of these polymers (particularly PM7) relies on precise control of their molecular weights. Also, their optimal morphology is extremely sensitive to any structural modification. In this work, a family of donor polymers is developed based on a random polymerization strategy. These polymers can achieve well-controlled morphology and high-performance with a variety of chemical structures and molecular weights. The polymer donors are D–A1–D–A2-type random copolymers in which the D and A1 units are monomers originating from PM6 or PM7, while the A2 unit comprises an electron-deficient core flanked by two thiophene rings with branched alkyl chains. Consequently, multiple cases of highly efficient NF-OSCs are achieved with efficiencies between 16.0% and 17.1%. As the electron-deficient cores can be changed to many other structural units, the strategy can easily expand the choices of high-performance donor polymers for NF-OSCs. © 2020 Wiley-VCH GmbH

With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non-fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room-temperature (RT) diffusion coefficients of 3.4 × 10−23 and 2.0 × 10−22 are measured for ITIC-2Cl and ITIC-2F, two state-of-the-art non-fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC-2F and ITIC-2Cl. The differences as observed by secondary-ion mass spectrometry, differential scanning calorimetry (DSC), grazing-incidence wide-angle X-ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC-4F and ITIC-4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability. © 2020 Wiley-VCH GmbH

Inner side-chain engineering on Y6 has been proven successful in improving short-circuit current density (JSC) through fine-tuning aggregated structures of acceptors. However, it fails in tuning the lowest unoccupied molecular orbital level (LUMO) and open-circuit voltage (VOC). In this paper, we turn to focus on engineering the outer side chains on the flanking thienothiophene units with 4-hexylphenyl (PhC6) and 6-phenylhexyl (C6Ph) chains. Use of PhC6 enhances the steric effect between the attached phenyl and the ending group, which in combination with the additional conjugation effect provided by the linking phenyl leads to upshifted energy levels and increased VOC as a result. Again, substitution with the bulkier PhC6 unprecedentedly improves film-morphology with reduced paracrystalline disorder and long period and increased root-mean-square composition variations as well, leading to increased electron and hole mobilities and suppressed monomolecular recombination with JSC and fill-factor (FF) simultaneously enhanced. The PM6:BTP-PhC6-based devices yield a higher efficiency value of 16.7% than the PM6:BTP-C6Ph-based one (15.5%). Therefore, this study shows a conceptual advance in materials design towards reducing the conflict between VOC and JSC in binary blended organic solar cells, which can be achieved by introducing bulkier chains to twist the backbone and simultaneously enhance the packing order.

Core engineering of small molecule acceptors (SMAs) is crucially important for enhancing device efficiency for nonfullerene organic solar cells (NF-OSCs). The most commonly used SMAs (e.g., ITIC) utilize indacenodithieno[3,2-b]thiophene (IDTT) as the central core, which has restricted their absorption ranges due to the weak electron-donating ability and short conjugation length. Here, we fused two electron-rich units, namely cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) and dithieno[3,2-b:2′,3′-d]pyran (DTPR), into the cores for constructing low-bandgap SMAs. The resulting CPDT-4Cl and DTPR-4Cl molecules exhibit extended nonacyclic central cores and strengthened intramolecular transfer (ICT) effect, resulting in red-shifted absorption (up to ∼950 nm) and up-shifted HOMO levels compared with IDTT-4Cl. Consequently, the NF-OSCs based on PTB7-Th:CPDT-4Cl and PTB7-Th:DTPR-4Cl achieved higher PCEs of 12.15% and 10.75%, respectively, than those of the PTB7-Th:IDTT-4Cl ones (7.70%). Notably, high short-circuit current densities (JSC) of 23–25 mA cm−2 were obtained by the CPDT-4Cl and DTPR-4Cl-based devices, indicating the great potential of the electron-donating CPDT and DTPR as promising building blocks to construct high-performance low-bandgap SMAs.

Even though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:non-fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses, ii) charge generation efficiencies, iii) non-geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. An extension to advanced morphological techniques, such as solid-state NMR (for atomic level insights on the local ordering and donor:acceptor pi-pi interactions) and resonant soft X-ray scattering (for donor and acceptor interfacial area and domain spacings), provide detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

Developing effective methods to make efficient bulk-heterojunction polymer solar cells at roll-to-roll relevant active layer thickness is of significant importance. We investigate the effect of fullerene content in polymer:fullerene blends on the fill factor (FF) and on the performance of thick-film solar cells for four different donor polymers PTB7-Th, PDPP-TPT, BDT-FBT-2T, and poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT). At a few hundreds of nanometers thickness, increased FFs are observed in all cases and improved overall device performances are obtained except for PDCBT upon increasing fullerene content in blend films. This fullerene content effect was studied in more detail by electrical and morphological characterization. The results suggest enhanced electron mobility and suppressed bimolecular recombination upon increasing fullerene content in thick polymer:fullerene blend films, which are the result of larger fullerene aggregates and improved interconnectivity of the fullerene phases that provide continuous percolating pathways for electron transport in thick films. These findings are important because an effective and straightforward method that enables fabricating efficient thick-film polymer solar cells is desirable for large-scale manufacturing via roll-to-roll processing and for multijunction devices.

Newly developed fused-ring electron acceptors (FREAs) have proven to be an effective class of materials for extending the absorption window and boosting the efficiency of organic photovoltaics (OPVs). While numerous acceptors have been developed, there is surprisingly little structural diversity among high performance FREAs in literature. Of the high efficiency electron acceptors reported, the vast majority utilize derivatives of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) as the acceptor moiety. It has been postulated that the high electron mobility exhibited by FREA molecules with INCN end groups is a result of close pi-pi stacking between the neighboring planar INCN groups, forming an effective charge transport pathway between molecules. To explore this as a design rationale for electron acceptors, we synthesized a new fused-ring electron acceptor, IDTCF, which has methyl substituents out of plane to the conjugated acceptor backbone. These methyl groups hinder packing and expand the pi-pi stacking distance by similar to 1 angstrom, but have little impact on the optical or electrochemical properties of the individual FREA molecule. The extra steric hindrance from the out of plane methyl substituents restricts packing and results in large amounts of geminate recombination, thus degrading the device performance. Our results show that intermolecular interactions (especially pi-pi stacking between end groups) play a crucial role in performance of FREAs. We demonstrated that the planarity of the acceptor unit is of paramount importance as even minor deviations in end group distance are enough to disrupt crystallinity and cripple device performance., Funding Agencies|NSF [CBET-1639429]; National Science Foundation, National Nanotechnology Coordinated Infrastructure, NNCI [ECCS-1542015]; ONR [N000141712204]; U.S. Department of Energy [DE-AC02-05CH11231]

Compared to the rapid development of nonfullerene organic solar cells (OSCs) based on the state-of-the-art indacenodithiophene (IDT)-based small molecule acceptors (SMAs), the progress for perylene diimide (PDI)-based electron acceptors has lagged behind owing to the lack of understanding on the structure-morphology-performance relationship of PDI SMAs. Given the ease of synthesis for PDIs and their high intrinsic electron mobility, it is crucial to identify key material parameters that influence the polymer:PDI blend morphology and to develop rational approaches for molecular design toward high-performance PDI-based SMAs. In this study, three pairs of PDI-based SMAs with and without ring-fusion are investigated and it is found that ring-fusion and domain purity are the key structural and morphological factors determining the fill factors (FFs) and efficiencies of PDI-based nonfullerene OSCs. This data shows that nonfullerene OSCs based on the ring-fused PDI-based SMAs exhibit much higher average domain purity and thus increased charge mobilities, which lead to enhanced FFs compared to those solar cells based on nonfused PDIs. This is explained by higher Florry Huggins interaction parameters as observed by melting point depression measurements. This study suggests that increasing repulsive molecular interactions to lower the miscibility between the polymer donor and PDI acceptor is the key to improve the FF and performance of PDI-based devices.