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Fluorine side chain functionalization of non‐fullerene acceptors (NFAs) represents an effective strategy for enhancing the performance of organic solar cells (OSCs). However, a knowledge gap persists regarding the relationship between structural changes induced by fluorine functionalization and the resultant impact on device performance. In this work, varying amounts of fluorine atoms were introduced into the outer side chains of Y‐series NFAs to construct two acceptors named BTP‐F0 and BTP‐F5. Theoretical and experimental investigations reveal that side‐chain fluorination significantly increase the overall average electrostatic potential (ESP) and charge balance factor, thereby effectively improving the ESP‐induced intermolecular electrostatic interaction, and thus precisely tuning the molecular packing and bulk‐heterojunction morphology. Therefore, the BTP‐F5‐based OSC exhibited enhanced crystallinity, domain purity, reduced domain spacing, and optimized phase distribution in the vertical direction. This facilitates exciton diffusion, suppresses charge recombination, and improves charge extraction. Consequently, the promising power conversion efficiency (PCE) of 17.3 % and 19.2 % were achieved in BTP‐F5‐based binary and ternary devices, respectively, surpassing the PCE of 16.1 % for BTP‐F0‐based OSCs. This work establishes a structure‐performance relationship and demonstrates that fluorine functionalization of the outer side chains of Y‐series NFAs is a compelling strategy for achieving ideal phase separation for highly efficient OSCs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Numerous previous reports on the sequential deposition (SD) technique have demonstrated that this approach can achieve a p‐i‐n active layer architecture with an ideal vertical composition gradient, which is one of the critical factors that can influence the physical processes that determine the photovoltaic performance of organic solar cells. Herein, a commonly used photovoltaic system comprised of PM6 as a donor and Y6 as an acceptor is investigated with respect to sequential blade‐processing deposition to comprehensively explore the morphology characteristics as a function of baseplate temperature. A systematic study of the temperature‐dependent blend morphology elucidates the SD‐processed configuration merits and device physics behind temperature‐controlled degree of vertical composition gradient, and constructs the temperature‐microstructure‐property relationship for the corresponding photovoltaic parameters. The result shows, as the temperature increases, the morphology of the active layer has undergone a distinct evolution from the pseudo‐bulk heterojunction to a pseudo‐planar heterojunction and then to a pseudo‐planar bilayer, leading to a non‐monotonic correlation between baseplate temperature and device performance. This investigation not only reveals the importance of precisely controlling baseplate temperature for gaining vertical morphology control, but also provides a path toward rational optimization of device performance in the lab‐to‐fab transition. [ABSTRACT FROM AUTHOR]

The environmental impact of solution processed organic solar cells (OSCs) can be mitigated by introducing so‐called green solvents during the fabrication processes. However, the effects of such green solvents on the molecular‐level structures and optoelectronic properties lack in‐depth characterization. Here, insights into the structure–processing–property correlation of a PPDT2FBT:PC61BM bulk‐heterojunction (BHJ) system processed from a green solvent, ortho‐xylene (o‐XY), is investigated in comparison with the same blend processed from a traditional halogenated solvent, chlorobenzene (CB). The BHJ blends are characterized with various techniques probing at difference length scales, and an increased donor:acceptor (D:A) interfacial area as well as well‐mixed features in the bulk morphologies of the active layer are observed for the o‐XY processed BHJ blend. Furthermore, molecular‐level differences in the D–A intermolecular interactions at the BHJ interfaces in o‐XY and CB cast films are elucidated by 2‐dimensional solid‐state nuclear magnetic resonance (ssNMR) measurements and analysis. These results are consistent with the device properties, suggesting that the green‐solvent‐processed devices have longer charge carrier lifetimes and faster charge carrier extraction. The optimized PPDT2FBT:PC61BM devices processed from o‐XY can achieve a noteworthy higher power conversion efficiency (PCE) owing to a higher short‐circuit current density and fill factor. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The effect of the cross‐coupling catalyst tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) on the performance of a model organic bulk‐heterojunction solar cell composed of a blend of poly([2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene]{3‐fluoro‐2[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl}) (PTB7‐Th) donor and 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IOTIC‐4F) non‐fullerene acceptor is investigated. The effect of intentional addition of different amounts of Pd(PPh3)4 on morphology, free charge carrier generation, non‐geminate bulk trap‐ and surface trap‐assisted recombination as well as bimolecular recombination and charge extraction is quantified. This work shows that free charge carrier generation is affected significantly, while the impact of Pd(PPh3)4 on non‐geminate recombination processes is limited because the catalyst does not facilitate efficient trap‐assisted recombination. The studied system shows substantial robustness towards the addition of Pd(PPh3)4 in small amounts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Thermal transition of organic solar cells (OSCs) constituent materials are often insufficiently researched, resulting in trial‐and‐error rather than rational approaches to annealing strategies to improve domain purity to enhance the power conversion efficiency. Despite the potential utility, little is known about the thermal transitions of the modern high‐performance acceptors Y6 and N3. Here, by using an optical method, it is discovered that the acceptor N3 has a clear solid‐state aggregation transition at 82 °C. This unusually low transition not only explains prior optimization protocols, but the transition informs and enables a double‐annealing method that can fine‐tune aggregation and the device morphology. Compared with 16.6% efficiency for PM6:N3:PC71BM control devices, higher efficiency of 17.6% is obtained through the improved protocol. Morphology characterization with x‐ray scattering methods reveals the formation of a multilength scale morphology. Moreover, the double‐annealing method is illustrated and easily transferred and validated with Y6‐based devices, using the transition of Y6 at 102 °C. As a result, the PCE improved from 16.0% to 16.8%. Design of high‐performance acceptors with yet lower aggregation transitions might be required for OSCs to successfully transition to low thermal budget industrial processing methods where annealing temperatures on plastic substrates have to be kept low. [ABSTRACT FROM AUTHOR]

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 ππ 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. [ABSTRACT FROM AUTHOR]

Understanding the correlation between polymer aggregation, miscibility, and device performance is important to establish a set of chemistry design rules for donor polymers with nonfullerene acceptors (NFAs). Employing a donor polymer with strong temperature‐dependent aggregation, namely PffBT4T‐2OD [poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3″′‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2″′‐quaterthiophen‐5,5‐diyl)], also known as PCE‐11 as a base polymer, five copolymer derivatives having a different thiophene linker composition are blended with the common NFA O‐IDTBR to investigate their photovoltaic performance. While the donor polymers have similar optoelectronic properties, it is found that the device power conversion efficiency changes drastically from 1.8% to 8.7% as a function of thiophene content in the donor polymer. Results of structural characterization show that polymer aggregation and miscibility with O‐IDTBR are a strong function of the chemical composition, leading to different donor–acceptor blend morphology. Polymers having a strong tendency to aggregate are found to undergo fast aggregation prior to liquid–liquid phase separation and have a higher miscibility with NFA. These properties result in smaller mixed donor–acceptor domains, stronger PL quenching, and more efficient exciton dissociation in the resulting cells. This work indicates the importance of both polymer aggregation and donor–acceptor interaction on the formation of bulk heterojunctions in polymer:NFA blends. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: Despite the rapid development of nonfullerene acceptors (NFAs), the fundamental understanding on the relationship between NFA molecular architecture, morphology, and device performance is still lacking. Herein, poly[[4,8‐bis[5‐(2‐ethylhexyl)thiophene‐2‐yl]benzo[1,2‐b:4,5‐b0]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]‐thieno[3,4‐b]thiophenediyl]] (PTB7‐Th) is used as the donor polymer to compare an NFA with a 3D architecture (SF‐PDI4) to a well‐studied NFA with a linear acceptor–donor–acceptor (A–D–A) architecture (ITIC). The data suggest that the NFA ITIC with a linear molecular structure shows a better device performance due to an increase in short‐circuit current (  Jsc) and fill factor (FF) compared to the 3D SF‐PDI4. The charge generation dynamics measured by femtosecond transient absorption spectroscopy (TAS) reveals that the exciton dissociation process in the PTB7‐Th:ITIC films is highly efficient. In addition, the PTB7‐Th:ITIC blend shows a higher electron mobility and lower energetic disorder compared to the PTB7‐Th:SF‐PDI4 blend, leading to higher values of Jsc and FF. The compositional sensitive resonant soft X‐ray scattering (R‐SoXS) results indicate that ITIC molecules form more pure domains with reduced domain spacing, resulting in more efficient charge transport compared with the SF‐PDI4 blend. It is proposed that both the molecular structure and the corresponding morphology of ITIC play a vital role for the good solar cell device performance. [ABSTRACT FROM AUTHOR]

Abstract: Side‐chain engineering is an important strategy for optimizing photovoltaic properties of organic photovoltaic materials. In this work, the effect of alkylsilyl side‐chain structure on the photovoltaic properties of medium bandgap conjugated polymer donors is studied by synthesizing four new polymers J70, J72, J73, and J74 on the basis of highly efficient polymer donor J71 by changing alkyl substituents of the alkylsilyl side chains of the polymers. And the photovoltaic properties of the five polymers are studied by fabricating polymer solar cells (PSCs) with the polymers as donor and an n‐type organic semiconductor (n‐OS) m‐ITIC as acceptor. It is found that the shorter and linear alkylsilyl side chain could afford ordered molecular packing, stronger absorption coefficient, higher charge carrier mobility, thus results in higher Jsc and fill factor values in the corresponding PSCs. While the polymers with longer or branched alkyl substituents in the trialkylsilyl group show lower‐lying highest occupied molecular orbital energy levels which leads to higher Voc of the PSCs. The PSCs based on J70:m‐ITIC and J71:m‐ITIC achieve power conversion efficiency (PCE) of 11.62 and 12.05%, respectively, which are among the top values of the PSCs reported in the literatures so far. [ABSTRACT FROM AUTHOR]

Long device lifetime is still a missing key requirement in the commercialization of nonfullerene acceptor (NFA) organic solar cell technology. Understanding thermodynamic factors driving morphology degradation or stabilization is correspondingly lacking. In this report, thermodynamics is combined with morphology to elucidate the instability of highly efficient PTB7‐Th:IEICO‐4F binary solar cells and to rationally use PC71BM in ternary solar cells to reduce the loss in the power conversion efficiency from ≈35% to <10% after storage for 90 days and at the same time improve performance. The hypomiscibility observed for IEICO‐4F in PTB7‐Th (below the percolation threshold) leads to overpurification of the mixed domains. By contrast, the hypermiscibility of PC71BM in PTB7‐Th of 48 vol% is well above the percolation threshold. At the same time, PC71BM is partly miscible in IEICO‐4F suppressing crystallization of IEICO‐4F. This work systematically illustrates the origin of the intrinsic degradation of PTB7‐Th:IEICO‐4F binary solar cells, demonstrates the structure–function relations among thermodynamics, morphology, and photovoltaic performance, and finally carries out a rational strategy to suppress the degradation: the third component needs to have a miscibility in the donor polymer at or above the percolation threshold, yet also needs to be partly miscible with the crystallizable acceptor. [ABSTRACT FROM AUTHOR]