Oligomeric Acceptor: A "Two-in-One" Strategy to Bridge Small Molecules and Polymers for Stable Solar Devices

Simultaneous enhancement of efficiency, stability and stretchability in binary polymer solar cells with a three-dimensional aromatic-core tethered tetrameric acceptor

Polymer solar cells (PSCs) leverage blend films from polymer donors and small-molecule acceptors (SMAs), offering promising opportunities for flexible power sources. However, the inherent rigidity and crystalline nature of SMAs often embrittle the polymer donor films in the constructed bulk heterojunction structure. To address this challenge, we improved the stretchability of the blend films by designing and synthesizing a tethered giant tetrameric acceptor (GTA) with increased molecular weight that promotes entanglement of individual SMA units. The key to this design is using tetraphenylmethane as the linking core to create a three-dimensional and high C2 symmetry structure, which successfully regulates their aggregation and relaxation behavior. With GTA as the acceptor, its blend films with polymer donor PM6 exhibit significantly improved stretchability, with nearly a 150% increase in crack onset strain value compared to PM6:Y6. Moreover, the PSCs achieve an increased efficiency of up to 18.71% and demonstrate outstanding photostability, maintaining >90% of their initial power conversion efficiency after operating for over 1000 hours. Our findings demonstrate that by specifically designing three-dimensional tethered SMAs and aligning their molecular weights more closely with those of polymer counterparts, we can achieve enhanced stretchability without compromising morphological stability or device efficiency.

Construction of Linear Tetramer-Type Acceptors for High-Efficiency and High-Stability Organic Solar Cells

Thanks to the development of non-fullerene acceptor (NFA) materials, the photovoltaic conversion efficiency (PCE) of organic solar cells (OSCs) has exceeded 20 %, which has met the requirements for commercialisation. In the current stage, the main focus is to balance the performance and stability. It has been shown that all-polymer formulation can improve device stability, however, PCE is not in satifsfaction, and the batch-to-batch variation leads to quality control issues. In this work, we constructed monodispersed tetramer NFA materials named G-1 and G-2, to best integrate the merits of small molecule and polymer. Density functional theory (DFT) calculations and experimental results showed that different connecting units at the centre could significantly affect the molecular planarity and thin film morphology. The alkene-bonded tetramer G-1 had a more regioregular structure, which leads to better molecular planarity, and more ordered packing in thin film. More importantly, the oligomeration induced a favourable face-on orientation, achieved a lower binding energy and exhibited a higher photoluminescence yield. As a result, the exciton and charge carrier kinetics was optimized with reduced non-radiative energy loss. The OSC based on PM6 : G-1 achieved a PCE of 19.6 %, which is the highest PCE reported so far for oligomer-based binary OSC. In addition, the device stability was largely improved, showing a lifetime over 10000 hours in the inverted OSC device.

Local Structure-Induced Selective Interactions Enables High-Performance and Burn-in-Free Organic Photovoltaics

Oligomeric acceptors (OAs) have attracted considerable attention in the organic photovoltaics (OPV) field owing to their capacity in balancing the merits from both monomeric and polymeric acceptors. A delicate control over the distortion between blocks of OAs usually determines the performance and stability of relevant OPV devices. However, it imposes great complexity to realize a controllable degree of distortion by tuning the skeleton of blocks and the position of linker between blocks. Herein, we developed a facile strategy to rationally control the geometry distortion of OAs via a straightforward substitution of alkoxy side-chains on their blocks. This helps elucidate the integrated influences of molecular distortion and non-bonded contacts on the selective interactions between OA molecules and between OA and host acceptor in ternary blend. We demonstrate the alkoxy-OA molecules having stronger self-interactions would mitigate their interactions with host acceptor, therefore alleviating the kinetic diffusion and excessive aggregation of total acceptors. Combining with a composite-interlayer strategy by introducing a phenyl-substituted self-assembled monolayer to enhance the doping with polyoxometalate, an impressive efficiency of 20.1 % is achieved accompanied by a negligible burn-in loss against physical aging. This study demonstrates the validation of tuning of selective interactions towards high-performance and burn-in-free OPV.

Halogen-Atom Engineering on Aromatic-Core in Tethered Small Molecule Acceptors for High-Performance Polymer Solar Cells

Tethered small molecular acceptors (SMAs), where multiple SMA-subunits are connected to the aromatic core via flexible chains, are proposed to suppress thermodynamic relaxation when blended with polymer donors to construct stable polymer solar cells (PSCs). However, optimizing their chemical structure to further enhance device performance remains a challenge, requiring careful fine-tuning between molecular aggregation and photovoltaic efficiency. In this study, the photovoltaic properties of tethered dimers are effectively modulated simply through halogen-atom engineering on the aromatic core. Specifically, DY-Cl with a chlorine atom and DY-Br with a bromine atom are designed. The study revealed the chloride acceptor enhances the intermolecular interaction, promotes charge transport, and optimizes the morphology of the active layer compared with its bromide counterpart. Notably, DY-Cl based PSCs achieves   a power conversion efficiency of 18.72%, maintaining over 80% of initial PCE after operating for 1000 h. These findings underscore the potential advantages of halogen-atom engineering on tethered acceptors as a straightforward yet effective method to achieve high efficiency and stable PSCs.

Progress in research on organic photovoltaic acceptor materials

In the past two decades, organic solar cells (OSCs) have begun to attract attention as the efficiency of inorganic solar cells gradually approaches the theoretical limit. In the early development stage of OSCs, p-type conjugated polymers and n-type fullerene derivatives were the most commonly used electron donors and acceptors. However, with further research, the shortcomings of fullerene materials have become increasingly apparent. In recent years, non-fullerene acceptor materials, including small molecules and polymers, have emerged as promising alternatives to fullerene derivatives. This review summarizes various types of acceptor materials in OSCs and analyzes the advantages and disadvantages of each.

Linker-Mediated Delocalized Excited States in Dimeric Acceptors Enable Efficient Exciton Dissociation with Negligible Energy-Level Offsets

Efficient exciton dissociation at low energy offsets is key to overcoming voltage losses in organic solar cells. In this work, we developed two dimeric acceptors, i-YT and o-YT, by precisely controlling the position of an asymmetric electron-donating linker. It induced the foldamer conformation of i-YT with a para linkage (relative to the dicyano groups), while retaining the unfold conformation for o-YT. This subtle structural modification influenced the molecular assembly properties, enabled near-zero energy offset exciton dissociation and power conversion efficiencies exceeding 18 % for i-YT based organic solar cells. Detailed excitonic dynamics further revealed that the linker position critically influences three processes: the formation of delocalized singlet excited states, ultrafast charge transfer (~5 ps) in solid blends, and the suppression of exciton recombination. Additionally, devices based on i-YT demonstrated outstanding long-term stability, retaining 85 % of their initial efficiency after 1,400 hours of continuous illumination. These findings introduce a new class of dimeric acceptors that combine high efficiency with exceptional stability, offering a promising pathway toward low-energy-loss organic photovoltaics.

Coplanar Dimeric Acceptors with Bathochromic Absorption and Torsion-Free Backbones through Precise Fluorination Enabling Efficient Organic Photovoltaics with 18.63% Efficiency

Giant dimeric acceptors (GDAs), a sub-type of acceptor materials for organic solar cells (OSCs), have garnered much attention due to the synergistic advantages of their monomeric and polymeric acceptors, forming a well-defined molecular structure with a giant molecular weight for high efficiency and stability. In this study, for the first time, two new GDAs, DYF-V and DY2F-V are designed and synthesized for OSC operation, by connecting one vinylene linker with the mono-/di-fluorinated end group on two Y-series monomers, respectively. After fluorination, both DYF-V and DY2F-V exhibit bathochromic absorption and denser packing modes due to the stronger intramolecular charge transfer effect and torsion-free backbones. Through precise fluorination, the DYF-V-based devices exhibit the highest performance of 18.63% among the GDA-based OSCs, outperforming its non-fluorinated counterpart, DY-V-based ones (16.53%). Theoretical and morphological results demonstrate that proper fluorination in DYF-V-based devices strengthens intra/intermolecular interactions for enhanced crystallinity, superior phase segregation, and less energy disorder, which is beneficial for fast exciton dissociation, rapid carrier transport, and suppressed charge recombination. The work demonstrates that proper fluorination on GDAs with rigid coplanar backbones is effective for broader photon harvesting, stronger packing, and robust stability in GDA-based OSCs.

A dendritic hexamer acceptor enables 19.4% efficiency with exceptional stability in organic solar cells

To achieve the commercialization of organic solar cells (OSCs), it is crucial not only to enhance power conversion efficiency (PCE) but also to improve device stability through rational molecular design. Recently emerging giant molecular acceptor (GMA) materials offer various advantages, such as precise chemical structure, high molecular weight (beneficial to film stability under several external stress), and impressive device efficiency, making them a promising candidate. Here, we report a dendritic hexamer acceptor developed through a branch-connecting strategy, which overcomes the molecular weight bottleneck of GMAs and achieves a high production yield over 58%. The dendritic acceptor Six-IC exhibits modulated crystallinity and miscibility with the donor, thus better morphology performance compared to its monomer, DTC8. Its charge transport ability is further enhanced by additional channels between the armed units. Consequently, the binary OSCs based on D18:Six-IC achieves a cutting-edge efficiency of 19.4% for high-molecular weight acceptor based systems, as well as decent device stability and film ductility. This work reports high-performance OSCs based on dendritic molecule acceptor with a molecular weight exceeding 10000 g/mol and shares the understanding for designing comprehensively high-performing acceptor materials.

Non-Fused Star-Shape Giant Trimer Electron Acceptors for Organic Solar Cells with Efficiency over 19

Organic solar cells (OSCs) based on giant molecular acceptors (GMAs) have attracted extensive attention due to their excellent power conversion efficiency (PCE) and operation stability. However, the large conjugated plane of GMAs poses great challenges in regulating the solubility, over-size aggregation and yield, which in turn further constrains their development in commercial products. Herein, we employ a non-fused skeleton strategy to develop novel non-fused star-shape trimers (3BTT6F and 3BTT6Cl) for improving device performance. Single-bond linkage can break the rigid planarity to form a 3D architecture, generating multidimensional charge transfer pathways. Importantly, the non-fused skeleton strategy can not only significantly improve solubility and synthesis yield, but also effectively suppress molecular excessive aggregation. Consequently, due to the optimized film-forming process and charge dynamics, 3BTT6F-based binary device obtains a high PCE of 17.52 %, which is significantly higher than the reported fully fused trimers. Excitingly, 3BTT6F-based ternary device even obtains a top-level PCE of 19.26 %. Furthermore, the non-fused star-shape configuration also endows these acceptors with enhanced intermolecular interaction in the active layer, demonstrating excellent operational stability. Our work emphasizes the potential of non-fused star-shape trimers, providing a new pathway for achieving highly efficient and stable OSCs.

Progress in the Stability of Small Molecule Acceptor-Based Organic Solar Cells

Significant advancements in power conversion efficiency have been achieved in organic solar cells with small molecule acceptors. However, stability remains a primary challenge, impeding their widespread adoption in renewable energy applications. This review summarizes the degradation of different layers within the device structure in organic solar cells under varying conditions, including light, heat, moisture, and oxygen. For the photoactive layers, the chemical degradation pathways of polymer donors and small molecule acceptors are examined in detail, alongside the morphological stability of the bulk heterojunction structure, which plays a crucial role in device performance. The degradation mechanisms of commonly used anode and cathode interlayers and electrodes are addressed, as these layers significantly influence overall device efficiency and stability. Mitigation methods for the identified degradation mechanisms are provided in each section to offer practical insights for improving device longevity. Finally, an outlook presents the remaining challenges in achieving long-term stability, emphasizing research directions that require further investigation to enhance the reliability and performance of organic solar cells in real-world applications.

Dual Additive Strategy with Quasi-Planar Heterojunction Architecture Assisted in Morphology Optimization for High-Efficiency Organic Solar Cells

Achieving high-performance and stable organic solar cells (OSCs) remains a critical challenge, primarily due to the precise optimization required for active layer morphology. Herein, this work reports a dual additive strategy using 3,5-dichlorobromobenzene (DCBB) and 1,8-diiodooctane (DIO) to optimize the morphology of both bulk-heterojunction (BHJ) and quasi-planar heterojunction (Q-PHJ) based on donor D18 and acceptor BTP-eC9. The systematic results reveal that the dual additive strategy significantly promotes phase separation while inhibiting excessive aggregation, which, in turn, improves molecular order and crystallization. As a result, BHJ and Q-PHJ OSCs processed with dual additive DIO + DCBB achieve impressive power conversion efficiencies of 17.77% and 18.60%, respectively, the highest reported values for dual additive-processed OSCs. The superior performance is attributed to improved charge transport and reduced recombination losses, as evidenced by higher short-circuit current densities (JSC) and fill factors (FF). Importantly, Q-PHJ OSCs processed with either DCBB or DIO + DCBB, in comparison to BHJ OSCs, exhibit exceptional shelf-stability, maintaining 80% of their initial power conversion efficiency after 2660 and 2193 h, respectively. These findings underscore the potential of dual additive strategies to advance the development of stable, high-efficiency OSCs suitable for large-area fabrication, marking a significant step forward in renewable energy technology.

Cascade Effect of a Dimerized Thermally Activated Delayed Fluorescence Dendrimer

Thermally activated delayed fluorescence (TADF) emitters with a high horizontal orientation are highly essential for improving the external quantum efficiency (EQE) of organic light-emitting diodes; however, pivotal molecular design strategies to improve the horizontal orientation of solution-processable TADF emitters are still scarce and challenging. Herein, a phenyl bridge is adopted to connect the double TADF units, and generate a dimerized TADF dendrimer, D4CzBNPh-SF. Compared to its counterpart with a single TADF unit, the proof-of-the-concept molecule not only exhibits an improved horizontal dipole ratio (78 %) due to the π-delocalization-induced extended molecular conjugation, but also displays a faster reversed intersystem crossing rate constant (6.08×106 s-1) and a high photoluminescence quantum yield of 95 % in neat film. Consequently, the non-doped solution-processed device with D4CzBNPh-SF as the emitter achieves an ultra-high maximum EQE of 32.6 %, which remains at 26.6 % under a luminance of 1000 cd/m2. Furthermore, when using D4CzBNPh-SF as a sensitizer, the TADF-sensitized fluorescence device exhibits a high maximum EQE of 30.7 % at a luminance of 575 cd/m2 and a full width at half maximum of 36 nm.

Delicate Regulation of Central Substituents Boosts Organic Photovoltaic Performance of Dimeric Acceptors

Dimeric acceptors are expected to satisfy both excellent power conversion efficiency (PCE) and operational stability of organic solar cells (OSCs). However, comparing to highly planar and symmetrical monomer-like acceptors, the quite different steric/spatial configurations of dimeric acceptors affect device outcomes greatly. Herein, on basis of the same dimeric molecular platform that constructed by bridging central units of two monomer-like acceptor, diverse substituents (─OCH3 for D1, ─CH3 for D2, and ─CF3 for D3) are grafted on central units to regulate the three dimensions (3D) geometries of dimeric acceptors delicately. A systematic investigation reveals the substituent-dependent variation of energy level, absorption, and molecular packing behavior. Consequently, D2 acceptor, characteristic of more favorable configuration, affords a superior film morphology and charge transfer/transport dynamics in resulting OSCs, thus yielding an excellent PCE of 17.50% along with a good long-term stability. This work manifests the crucially important role of central substituents in constructing high-performance dimeric acceptors.

Trifluoromethylation Enables Compact 2D Linear Stacking and Improves the Efficiency and Stability of Q-PHJ Organic Solar Cells

Compared to the bulk heterojunction (BHJ) devices, the quasiplanar heterojunction (Q-PHJ) exhibits a more stable morphology and superior charge transfer performance. To achieve both high efficiency and long-term stability, it is necessary to design new materials for Q-PHJ devices. In this study, QxIC-CF3 and QxIC-CH3 are designed and synthesized for the first time. The trifluoromethylation of the central core exerts a modulatory effect on the molecular stacking pattern, leveraging the strong electrostatic potential and intermolecular interactions. Compared with QxIC-CH3, the single crystal structure reveals that QxIC-CF3 exhibits a more compact 2D linear stacking behavior. These benefits, combined with the separated electron and hole transport channels in Q-PHJ device, lead to increased charge mobility and reduced energy loss. The devices based on D18/QxIC-CF3 exhibit an efficiency of 18.1%, which is the highest power conversion efficiency (PCE) for Q-PHJ to date. Additionally, the thermodynamic stability of the active layer morphology enhances the lifespan of the aforementioned devices under illumination conditions. Specifically, the T80 is 420 h, which is nearly twice that of the renowned Y6-based BHJ device (T80 = 220 h). By combining the advantages of the trifluoromethylation and Q-PHJ device, efficient and stable organic solar cell devices can be constructed.

Chlorine-mediated strategy for organic photovoltaics

Organic solar cells (OSCs), a nascent technology in the photovoltaic field, have attracted considerable research interest. Recently, the power conversion efficiency (PCE) of OSCs has significantly improved, thereby demonstrating substantial potential for commercialization. To achieve this, it is crucial to enhance the performance and stability of OSCs, necessitating the development of novel materials and devices. This feature article presents a review of chlorine-mediated photovoltaic materials in our group. By carefully controlling energy levels, molecular stacking and aggregation behavior, significantly improved performance was achieved. Furthermore, single-crystal analysis facilitated a profound comprehension of the influence of chlorine-mediated interactions on molecular stacking. This has enabled the design and synthesis of a series of high-performance non-fullerene acceptors (NFAs) with three-dimensional network stacking structures. Building upon these materials, we developed quasi-planar heterojunction (Q-PHJ) devices with a significant stability advantage. To sum up, the chlorine-mediated materials and the Q-PHJ devices provide valuable guidance and reference for the development of efficient and stable organic solar cells.

Unveiling the Influence of Linkers on Conformations of Oligomeric Acceptors for High-Performance Polymer Solar Cells

Conformational isomerism of organic photovoltaic materials has a profound impact on their molecular packing and therefore performance of polymer solar cells (PSCs). However, the conformations of oligomeric acceptors (OAs) are mostly predicted by simulations rather than experimental determinations. Herein, the stereochemical S-shaped structure of two dimeric-type acceptor molecules, V-DYIC and V-DYIC-4F, is first confirmed with different end groups (IC for V-DYIC and IC-2F for V-DYIC-4F), incorporating vinylene linkage for connecting the distinct state-of-the-art small molecule acceptor Y-segments. Through the synthetic control of fluorination sites adjacent to the vinyl-linker, S-shaped the conformation by NMR experiments is validated. Compared to the O-shaped dimer, S-shaped conformation results in enhanced lamellar order and reduced nonradiative recombination losses. The optimal acceptor, V-DYIC-4F, achieved a champion efficiency of 18.10% with the lowest energy loss of 0.556 eV in its devices paired with PM6 due to their efficient carrier transport, and suppressed recombination compared to other devices, being attributed to the synergistic effect of conformation and end group fluorination. The insights gained in this work contribute valuable knowledge of both synthetic control and structural determination of OAs, providing strategic design guidelines for the future development of dimeric acceptors toward high-efficiency PSCs.

Giant dimeric donors for all-giant-oligomer organic solar cells with efficiency over 16% and superior photostability

Increasing the molecular weight while maintaining mono-dispersity has been proved crucial in innovating high-performance photovoltaic materials in giant oligomeric acceptors. However, developing efficient giant oligomeric donors to replace the batch-varied polymers remains challenging due to a lack of design principles. Here, by designing two unique isomeric rhodanine-based linkers, we successfully regulate the assembly behaviors of giant dimeric donors (G-Dimer-Ds) and fabricate the first all-giant-oligomer OSCs pairing with giant dimeric acceptor DY. Multiple characterizations demonstrate the small homo-molecular interaction with strong thermal-driven assembly capability in G-Dimer-D2 simultaneously facilitates reducing energetic disorder, improving charge transport and obtaining stable morphology, resulting in a satisfactory efficiency of 15.70% and long-term photostability with an extrapolated T80 of ca.10,000 hours, and further enhancing thermal-driven assembly promotes efficiency of 16.05%. Our results provide construction approaches on efficient giant donors, and propose a promising type of OSC with completely definite structures, high efficiency and superior stability.

Oligomerized Acceptors with High Efficiency and Superior Stability in Organic Solar Cells

Oligomerized small molecular acceptors (OSMAs) have rapidly become a research hotspot in the field of organic solar cells due to their advantages of effective combination with definite structure of small molecules and high viscosity and glass transition temperature of polymers. From this perspective, we classify and summarize the representative OSMAs from the three binding sites of end, core, and wing, and we summarize the exploration of new synthesis methods to improve the plights, such as low synthesis yield and difficult purification. Finally, the existing challenges and future research directions are concluded and prospected.

Isomeric Dimer Acceptors for Stable Organic Solar Cells with over 19 % Efficiency

The strategy of isomerization is known for its simple yet effective role in optimizing molecular configuration and enhancing the power conversion efficiency (PCE) of organic solar cells (OSCs). However, the impact of isomerization on the design of dimer acceptors has been rarely investigated, and the relationship between the chemical structure and optoelectronic property remains unclear. In this study, we designed and synthesized two dimer acceptor isomers named D-TPh and D-TN, which differ in the positional arrangement of their end capping groups. Compared to D-TN, D-TPh exhibited enhanced backbone planarity, elevated lowest unoccupied molecular orbital energy level, and more ordered molecular stacking. Consequently, the OSC device based on PM6 : D-TPh achieved a PCE of 19.05 %, higher than that (PCE=18.42 %) of the device based on PM6 : D-TN. Large-area PM6 : D-TPh devices (1 cm2) yielded a PCE of 18.00 %. More importantly, the extrapolated T80 lifetime of the PM6 : D-TPh device is over 2800 h with MPP tracking under continuous one-sun illumination. These results suggest that isomerization strategy is an effective way to optimize the molecular configuration of dimer acceptors for the fabrication of high-efficiency and stable OSCs.

Dimeric Small Molecule Acceptors via Terminal-End Connections: Effect of Flexible Linker Length on Photovoltaic Performance

The dimerization of small molecule acceptors (SMAs) holds significant potential by combining the advantages of both SMAs and polymer acceptors in realizing high power conversion efficiency (PCE) and operational stability in organic solar cells (OSCs). However, advancements in the selection and innovation of dimeric linkers are still challenging in enhancing their performance. In this study, three new dimeric acceptors, namely DY-Ar-4, DY-Ar-5, and DY-Ar-6 are synthesized, by linking two Y-series SMA subunits via an "end-to-end" strategy using flexible spacers (octyl, decyl, and dodecyl, respectively). The influence of spacer lengths on device performance is systematically investigated. The results indicate that DY-Ar-5 exhibits more compact and ordered packing, leading to an optimal morphology. OSCs based on PM6: DY-Ar-5 achieves a maximum PCE of 15.76%, attributes to enhance and balance carrier mobility, and reduce carrier recombination. This dimerization strategy using suitable non-conjugated linking units provides a rational principle for designing high-performance non-fullerene acceptors.

Dimerized M-Series Acceptors with Low Diffusion Coefficients for Efficient and Stable Polymer Solar Cells

As the simplest oligomeric acceptors, dimerized acceptors (DAs) are easier to synthesize, and more importantly, they can retain good intermolecular interaction and photovoltaic properties of their parent small-molecule acceptors (SMAs). Nevertheless, currently most efficient DAs are derived from banana-shaped acceptors and they might suffer from inferior device stability with high diffusion coefficients. Herein, we design and synthesize two planar DAs (DMT-FH and DMT-HF) by bridging two linear-shaped M-series SMAs with a thiophene unit. The effects of fluorination position on the diffusion coefficients, power conversion efficiencies (PCEs) and stability of the DAs are systematically studied. Our results suggest that DMT-HF with fluorination on the ending indanone groups shows enhanced intermolecular interactions, improved PCE and stability compared with the counterpart (DMT-FH) with fluorination on the central indanone groups. Further optimization on the DMT-HF-based devices yields an outstanding PCE of 17.17 %, which is the highest among all linear-shaped SMA-based DAs. Notably, with the low diffusion coefficient (3.36×10-24 cm2 s-1) of DMT-HF, the resulting device retains over 93 % of the initial PCE after 5000 h of continuous heating at 85 °C, suggesting its excellent thermal stability. The results highlight the importance of intermolecular interaction and fluorination for achieving efficient and stable polymer solar cells.

Conjugation-Broken Dimer Acceptors Enable High-Efficiency, Stable, and Flexibility-Robust Organic Solar Cells

Dimer acceptors in organic solar cells (OSCs) offer distinct advantages, including a well-defined molecular structure and excellent batch-to-batch reproducibility. Their high glass transition temperature (Tg) aids in achieving an optimal kinetic morphology, thereby enhancing device stability. Currently, most of dimer acceptor materials are linked with conjugated units in order to obtain high power conversion efficiencies (PCEs). In this study, different from previous works on conjugation-linked dimer acceptors, a novel series of dimer acceptors are synthesized (named T1, T4, T6, and T12), each linked with different flexible alkyl linkers, and investigated their PCEs, device stability, and flexibility robustness. When blended with PM6, the T6-based device achieves a PCE of 17.09%, comparable to the fully conjugated T0-based device's PCE of 17.12%. The molecular dynamics simulations and density functional theory calculations suggested that flexible conjugation-broken linkers (FCBLs) promote intermolecular electronic couplings, thereby maintaining good electron mobilities of dimer acceptors. Notably, the T6-based device exhibits impressive long-term stability with a T80 lifetime of 1427 h, while in the T0-based device, T80 is only 350 h. The present work has thus established the relationship between the length of flexible alkyl linkers in such dimer acceptors and the performance and stability of OSCs, which is important to further designing new materials for the fabrication of efficient and stable OSCs.

What is the Limit Size of 2D Conjugated Extension on Central Units of Small Molecular Acceptors in Organic Solar Cells?

2D conjugated extension on central units of small molecular acceptors (SMAs) has gained great successes in reaching the state-of-the-art organic photovoltaics. Whereas the limit size of 2D central planes and their dominant role in constructing 3D intermolecular packing networks are still elusive. Thus, by exploring a series of SMAs with gradually enlarged central planes, it is demonstrated that, at both single molecular and aggerated levels, there is an unexpected blue-shift for their film absorption but preferable reorganization energies, exciton lifetimes and binding energies with central planes enlarging, especially when comparing to their Y6 counterpart. More importantly, the significance of well-balanced molecular packing modes involving both central and end units is first disclosed through a systematic single crystal analysis, indicating that when the ratio of central planes area/end terminals area is no more than 3 likely provides a preferred 3D intermolecular packing network of SMAs. By exploring the limit size of 2D central planes, This work indicates that the structural profiles of ideal SMAs may require suitable central unit size together with proper heteroatom replacement instead of directly overextending 2D central planes to the maximum. These results will likely provide some guidelines for future better molecular design.

The Development of Quinoxaline-Based Electron Acceptors for High Performance Organic Solar Cells

In the recent advances of organic solar cells (OSCs), quinoxaline (Qx)-based nonfullerene acceptors (QxNFAs) have attracted lots of attention and enabled the recorded power conversion efficiency approaching 20%. As an excellent electron-withdrawing unit, Qx possesses advantages of many modifiable sites, wide absorption range, low reorganization energy, and so on. To develop promising QxNFAs to further enhance the photovoltaic performance of OSCs, it is necessary to systematically summarize the QxNFAs reported so far. In this review, all the focused QxNFAs are classified into five categories as following: SM-Qx, YQx, fused-YQx, giant-YQx, and polymer-Qx according to the molecular skeletons. The molecular design concepts, relationships between the molecular structure and optoelectronic properties, intrinsic mechanisms of device performance are discussed in detail. At the end, the advantages of this kind of materials are summed up, the molecular develop direction is prospected, the challenges faced by QxNFAs are given, and constructive solutions to the existing problems are advised. Overall, this review presents unique viewpoints to conquer the challenge of QxNFAs and thus boost OSCs development further toward commercial applications.

Recent Progress of Oligomeric Non-Fullerene Acceptors for Efficient and Stable Organic Solar Cells

Organic solar cells (OSCs) have achieved remarkable power conversion efficiencies (PCEs) of over 19 % in the past few years due to the rapid development of non-fullerene acceptors (NFAs). However, the operational stability remains a great challenge that inhibits their commercialization. Recently, oligomeric NFAs (ONFAs) have attracted great attention, which not only can deliver excellent device performance, but also improve the thermal-/photo- stability of OSCs. This is attributed to the suppressed molecular diffusion of ONFAs associated with their high glass-transition temperature (Tg) and improved thermodynamic properties of ONFAs. Herein, we focus on investigating the correction between the ONFA chemical structure, material properties, device performance, and stability. In addition, we also try to point out the challenges in synthesizing ONFAs and provide potential directions for future ONFA designs.

Chlorinated Oligomers with Regulate Planarity Achieving Superior Photovoltaic Performance

Chlorine substitution, as an effective and low-cost modification strategy, has been applied in the design of donor and acceptor structures in organic solar cells. We synthesized a series of chlorinated dimerized acceptors to investigate the effect of chlorine numbers and locations on the photovoltaic properties. The results show that the planarity and morphology of DYV-γ-2Cl are greatly improved due to the appropriate numbers and positions of the substituted chlorine atoms. Therefore, the device based on PM6:DYV-γ-2Cl achieves a superior power conversion efficiency (PCE) of 15.54% among the three oligomeric acceptors with optimized molecular planarity and film morphology. This work demonstrated the positive effect of suitable numbers and the substitution positions of chlorines on the molecular arrangement and photovoltaic properties of the corresponding dimerized acceptors.

Dimeric Giant Molecule Acceptors Featuring N-type Linker: Enhancing Intramolecular Coupling for High-Performance Polymer Solar Cells

Giant molecular acceptors (GMAs) are typically designed through the conjugated linking of individual small molecule acceptors (SMAs). This design imparts an extended molecular size, elevating the glass transition temperature (Tg) relative to their SMA counterparts. Consequently, it effectively suppresses the thermodynamic relaxation of the acceptor component when blended with polymer donors to construct stable polymer solar cells (PSCs). Despite their merits, the optimization of their chemical structure for further enhancing of device performance remains challenge. Different from previous reports utilizing p-type linkers, here, we explore an n-type linker, specifically the benzothiadiazole unit, to dimerize the SMA units via a click-like Knoevenagel condensation, affording BT-DL. In comparison with B-DL with a benzene linkage, BT-DL exhibits significantly stronger intramolecular super-exchange coupling, a desirable property for the acceptor component. Furthermore, BT-DL demonstrates a higher film absorption coefficient, redshifted absorption, larger crystalline coherence, and higher electron mobility. These inherent advantages of BT-DL translate into a higher power conversion efficiency of 18.49 % in PSCs, a substantial improvement over the 9.17 % efficiency observed in corresponding devices with B-DL as the acceptor. Notably, the BT-DL based device exhibits exceptional stability, retaining over 90 % of its initial efficiency even after enduring 1000 hours of thermal stress at 90 °C. This work provides a cost-effective approach to the synthesis of n-type linker-dimerized GMAs, and highlight their potential advantage in enhancing intramolecular coupling for more efficient and durable photovoltaic technologies.

Recent progress and prospects of dimer and multimer acceptors for efficient and stable polymer solar cells

High power conversion efficiency (PCE) and long-term stability are essential prerequisites for the commercialization of polymer solar cells (PSCs). Small-molecule acceptors (SMAs) are core materials that have led to recent, rapid increases in the PCEs of the PSCs. However, a critical limitation of the resulting PSCs is their poor long-term stability. Blend morphology degradation from rapid diffusion of SMAs with low glass transition temperatures (Tgs) is considered the main cause of the poor long-term stability of the PSCs. The recent emergence of oligomerized SMAs (OSMAs), composed of two or more repeating SMA units (i.e., dimerized and trimerized SMAs), has shown great promise in overcoming these challenges. This innovation in material design has enabled OSMA-based PSCs to reach impressive PCEs near 19% and exceptional long-term stability. In this review, we summarize the evolution of OSMAs, including their research background and recent progress in molecular design. In particular, we discuss the mechanisms for high PCE and stability of OSMA-based PSCs and suggest useful design guidelines for high-performance OSMAs. Furthermore, we reflect on the existing hurdles and future directions for OSMA materials towards achieving commercially viable PSCs with high PCEs and operational stabilities.

Dimerized Nonfused Electron Acceptor Based on a Thieno[3,4-c]pyrrole-4,6-dione Core for Organic Solar Cells

In this work, the first dimerized nonfused electron acceptor (NFEA), based on thieno[3,4-c]pyrrole-4,6-dione as the core, has been designed and synthesized. The dimerized acceptor and its single counterpart exhibit similar energy levels but different absorption spectra due to their distinct aggregation behavior. The dimerized acceptor-based organic solar cells (OSCs) demonstrate a higher power conversion efficiency of 11.05%, accompanied by enhanced thermal stability. This improvement is attributed to the enhancement of the short-circuit current density and fill factor, along with an increase in the glass transition temperature. Characterizations of exciton dynamics and film morphology reveal that a dimerized acceptor-based device possesses an enhanced exciton dissociation efficiency and a well-established charge transport pathway, explaining its improved photovoltaic performance. All these results indicate that the dimerized NFEA as a promising candidate can achieve efficiency-stability-cost balance in OSCs.

Emerging Light-Harvesting Materials Based on Organic Photovoltaic D/A Heterojunctions for Efficient Photocatalytic Water Splitting

Photocatalytic water splitting to hydrogen is a highly promising method to meet the surging energy consumption globally through the environmentally friendly means. As the initial step before photocatalysis, harvesting photons from sunlight is crucially important, thus making the design of photosensitizers with visible even near-infrared (NIR) absorptions get more and more attentions. In the past three years, organic donor/acceptor (D/A) heterojunctions with absorptions extending to 950 nm, have emerged as the new star light-harvesting materials for photocatalytic water splitting, demonstrating exciting advantages over inorganic materials in solar light utilization, hydrogen yielding rate, etc. This Minireview firstly gives a brief discussion about the principle processes and determining factors for photocatalytic water splitting with organic photovoltaic D/A heterojunction as photosensitizers. Thereafter, the current progress is summarized in details by introducing typical and excellent D/A heterojunction-based photocatalytic systems. Finally, not only the great prospects but also the most challenging issues confronted by organic D/A heterojunctions are indicated along with a perspective on the opportunities and new directions for future material explorations.

Tethered Trimeric Small-molecular Acceptors through Aromatic-core Engineering for Highly Efficient and Thermally Stable Polymer Solar Cells

Polymer solar cells (PSCs) rely on a blend of small molecular acceptors (SMAs) with polymer donors, where thermodynamic relaxation of SMAs poses critical concerns on operational stability. To tackle this issue, tethered SMAs, wherein multiple SMA-subunits are connected to the aromatic-core via flexible chains, are proposed. This design aims to an elevated glass transition temperature (Tg) for a dynamical control. However, attaining an elevated Tg value with additional SMA subunits introduces complexity to the molecular packing, posing a significant challenge in realizing both high stability and power conversion efficiency (PCE). In this study, we initiate isomer engineering on the benzene-carboxylate core and find that meta-positioned dimeric BDY-β exhibits more favorable molecular packing compared to its para-positioned counterpart, BDY-α. With this encouraging result, we expand our approach by introducing an additional SMA unit onto the aromatic core of BDY-β, maintaining a meta-position relative to each SMA unit location in the tethered acceptor. This systematic aromatic-core engineering results in a star-shaped C3h-positioned molecular geometry. The supramolecular interactions of SMA units in the trimer contribute to enhancements in Tg value, crystallinity, and a red-shifted absorption compared to dimers. These characteristics result in a noteworthy increase in PCE to 18.24 %, coupled with a remarkable short-circuit current density of 27.06 mA cm-2. More significantly, the trimer-based devices delivered an excellent thermal stability with over 95 % of their initial efficiency after 1200 h thermal degradation. Our findings underscore the promise and feasibility of tethered trimeric structures in achieving highly ordered aggregation behavior and increased Tg value in PSCs, simultaneously improving in device efficiency and thermal stability.

Eliminating the Burn-in Loss of Efficiency in Organic Solar Cells by Applying Dimer Acceptors as Supramolecular Stabilizers

The meta-stable active layer morphology of organic solar cells (OSCs) is identified as the main cause of the rapid burn-in loss of power conversion efficiency (PCE) during long-term device operation. However, effective strategies to eliminate the associated loss mechanisms from the initial stage of device operation are still lacking, especially for high-efficiency material systems. Herein, the introduction of molecularly engineered dimer acceptors with adjustable thermal transition properties into the active layer of OSCs to serve as supramolecular stabilizers for regulating the thermal transitions and optimizing the crystallization of the absorber composites is reported. By establishing intimate π-π interactions with small-molecule acceptors, these stabilizers can effectively reduce the trap-state density (Nt) in the devices to achieve excellent PCEs over 19%. More importantly, the low Nt associated with an initially optimized morphology can be maintained under external stresses to significantly reduce the PCE burn-in loss in devices. This research reveals a judicious approach to improving OPV stability by establishing a comprehensive correlation between material properties, active-layer morphology, and device performance, for developing burn-in-free OSCs.

Non-Fully Conjugated Dimerized Giant Acceptors with Different Alkyl-Linked Sites for Stable and 19.13 % Efficiency Organic Solar Cells

Achieving both high power conversion efficiency (PCE) and device stability is a major challenge for the practical development of organic solar cells (OSCs). Herein, three non-fully conjugated dimerized giant acceptors (named 2Y-sites, including wing-site-linked 2Y-wing, core-site-linked 2Y-core, and end-site-linked 2Y-end) are developed. They share the similar monomer precursors but have different alkyl-linked sites, offering the fine-tuned molecular absorption, packing, glass transition temperature, and carrier mobility. Among their binary active layers, D18/2Y-wing has better miscibility, leading to optimized morphology and more efficient charge transfer compared to D18/2Y-core and D18/2Y-end. Therefore, the D18/2Y-wing-based OSCs achieve a superior PCE of 17.73 %, attributed to enhanced photocurrent and fill factor. Furthermore, the D18/2Y-wing-based OSCs exhibit a balance of high PCE and improved stability, distinguishing them within the 2Y-sites. Building on the success of 2Y-wing in binary systems, we extend its application to ternary OSCs by pairing it with the near-infrared absorbing D18/BS3TSe-4F host. Thanks to the complementary absorption within 300-970 nm and further optimized morphology, ternary OSCs obtain a higher PCE of 19.13 %, setting a new efficiency benchmark for the dimer-derived OSCs. This approach of alkyl-linked site engineering for constructing dimerized giant acceptors presents a promising pathway to improve both PCE and stability of OSCs.

Stability of organic solar cells: toward commercial applications

Organic solar cells (OSCs) have attracted a great deal of attention in the field of clean solar energy due to their advantages of transparency, flexibility, low cost and light weight. Introducing them to the market enables seamless integration into buildings and windows, while also supporting wearable, portable electronics and internet-of-things (IoT) devices. With the development of photovoltaic materials and the optimization of fabrication technology, the power conversion efficiencies (PCEs) of OSCs have rapidly improved and now exceed 20%. However, there is a significant lack of focus on material stability and device lifetime, causing a severe hindrance to commercial applications. In this review, we carefully review important strategies employed to improve the stability of OSCs over the past three years from the perspectives of material design and device engineering. Furthermore, we analyze and discuss the current important progress in terms of air, light, thermal and mechanical stability. Finally, we propose the future research directions to overcome the challenges in achieving highly stable OSCs. We expect that this review will contribute to solving the stability problem of OSCs, eventually paving the way for commercial applications in the near future.

Noncovalent Interaction Boosts Performance and Stability of Organic Solar Cells Based on Giant-Molecule Acceptors

Designing giant-molecule acceptors is deemed as an up-and-coming strategy to construct stable organic solar cells (OSCs) with high performance. Herein, two giant dimeric acceptors, namely, DYV and DYFV, have been designed and synthesized by linking two Y-series derivatives with a vinyl unit. DYFV exhibits more red-shifted absorption, down-shifted energy levels, and enhanced intermolecular packing than DYV because the intramolecular noncovalent interaction (H···F) of DYFV leads to better coplanarity of the backbone. The D18:DYFV film owns a distinct nanofibrous nanophase separation structure, a more dominant face-on orientation, and more balanced carrier mobilities. Therefore, the D18:DYFV OSC achieves a higher photoelectron conversion efficiency of 17.88% and a longer-term stability with a t80 over 45,000 h compared with the D18:DYV device. The study demonstrates that the intramolecular noncovalent interaction is a superior strategy to design giant-molecule acceptors and boost the photovoltaic performance and stability of the OSCs.

Tethered Small-Molecule Acceptor Refines Hierarchical Morphology in Ternary Polymer Solar Cells: Enhanced Stability and 19% Efficiency

Polymer solar cells (PSCs) are promising for efficient solar energy conversion, but achieving high efficiency and device longevity within a bulk-heterojunction (BHJ) structure remains a challenge. Traditional small-molecule acceptors (SMAs) in the BHJ blend show thermodynamic instability affecting the morphology. In contrast, tethered SMAs exhibit higher glass transition temperatures, mitigating these concerns. Yet, they might not integrate well with polymer donors, causing pronounced phase separation and overpurification of mixed domains. Herein, a novel ternary device is introduced that uses DY-P2EH, a tethered dimeric SMA with conjugated side-chains as host acceptor, and BTP-ec9, a monomeric SMA as secondary acceptor, which respectively possess hypomiscibility and hypermiscibility with the polymer donor PM6. This unique combination affords a parallel-connected ternary BHJ blend, leading to a hierarchical and stable morphology. The ternary device achieves a remarkable fill factor of 80.61% and an impressive power conversion efficiency of 19.09%. Furthermore, the ternary device exhibits exceptional stability, retaining over 85% of its initial efficiency even after enduring 1100 h of thermal stress at 85 °C. These findings highlight the potential advantage of tethered SMAs in the design of ternary devices with a refined hierarchical structure for more efficient and durable solar energy conversion technologies.

Simple and Efficient Synthesis of Novel Tetramers with Enhanced Glass Transition Temperature for High-Performance and Stable Organic Solar Cells

Oligomer acceptors in organic solar cells (OSCs) have garnered substantial attention owing to their impressive power conversion efficiency (PCE) and long-term stability. However, the simple and efficient synthesis of oligomer acceptors with higher glass transition temperatures (Tg ) remains a formidable challenge. In this study, we propose an innovative strategy for the synthesis of tetramers, denoted as Tet-n, with elevated Tg s, achieved through only two consecutive Stille coupling reactions. Importantly, our strategy significantly reduces the redundancy in reaction steps compared to conventional methods for linear tetramer synthesis, thereby improving both reaction efficiency and yield. Furthermore, the OSC based on PM6:Tet-1 attains a high PCE of 17.32 %, and the PM6:L8-BO:Tet-1 ternary device achieves an even more higher PCE of 19.31 %. Remarkably, the binary device based on the Tet-1 tetramer demonstrates outstanding operational stability, retaining 80 % of the initial efficiency (T80 ) even after 1706 h of continuous illumination, which is primarily attributed to the enhanced Tg (247 °C) and lower diffusion coefficient (1.56×10-27  cm2  s-1 ). This work demonstrates the effectiveness of our proposed approach in the straightforward and efficient synthesis of tetramers materials with higher Tg s, thus offering a viable pathway for developing high-efficiency and stable OSCs.

A Pyrene-Fused Dimerized Acceptor for Ternary Organic Solar Cells with 19% Efficiency and High Thermal Stability

A pyrene-fused dimerized electron acceptor has been successfully synthesized and subsequently incorporated as the third component in ternary organic solar cells (OSCs). Diverging from the traditional dimerized acceptors with a linear configuration, this novel electron acceptor displays a distinctive "butterfly-like" structure, comprising two Y-acceptors as wings fused with a pyrene-based backbone. The extended π-conjugated backbone and the electron-donating nature of pyrene enable the new acceptor to show low solubility, elevated glass transition temperature (Tg ), and low-lying frontier energy levels. Consequently, the new dimerized acceptor seamlessly integrates as the third component into ternary OSCs, enhancing electron transporting properties, reducing non-radiative voltage loss, and elevating open-circuit voltage. These merits have enabled the ternary OSCs to show an exceptional efficiency of 19.07%, a marked improvement compared to the 17.6% attained in binary OSCs. More importantly, the high Tg exhibited by the pyrene-fused electron acceptor helps to stabilize the morphology of the photoactive layer thermal-treated at 70 °C, retaining 88.7% efficiency over 600 hours. For comparison, binary OSCs experience a decline to 73.7% efficiency after the same duration. These results indicate that the "butterfly-like" design and the incorporation of a pyrene unit is a promising strategy in the development of dimerized electron acceptors for OSCs.

Optimized Crystal Framework by Asymmetric Core Isomerization in Selenium-Substituted Acceptor for Efficient Binary Organic Solar Cells

Both the regional isomerization and selenium-substitution of the small molecular acceptors (SMAs) play significant roles in developing efficient organic solar cells (OSCs), while their synergistic effects remain elusive. Herein, we developed three isomeric SMAs (S-CSeF, A-ISeF, and A-OSeF) via subtly manipulating the mono-selenium substituted position (central, inner, or outer) and type of heteroaromatic ring on the central core by synergistic strategies for efficient OSCs, respectively. Crystallography of asymmetric A-OSeF presents a closer intermolecular π-π stacking and more ordered 3-dimensional network packing and efficient charge-hopping pathways. With the successive out-shift of the mono-selenium substituted position, the neat films give a slightly wider band gap and gradually higher crystallinity and electron mobility. The PM1 : A-OSeF afford favourable fibrous phase separation morphology with more ordered molecular packing and efficient charge transportation compared to the other two counterparts. Consequently, the A-OSeF-based devices achieve a champion efficiency of 18.5 %, which represents the record value for the reported selenium-containing SMAs in binary OSCs. Our developed precise molecular engineering of the position and type of selenium-based heteroaromatic ring of SMAs provides a promising synergistic approach to optimizing crystal stacking and boosting top-ranked selenium-containing SMAs-based OSCs.

Propeller vs Quasi-Planar 6-Cantilever Small Molecular Platforms with Extremely Two-Dimensional Conjugated Extension

Two exotic 6-cantilever small molecular platforms, characteristic of quite different molecular configurations of propeller and quasi-plane, are established by extremely two-dimensional conjugated extension. When applied in small molecular acceptors, the only two cases of CH25 and CH26 that could contain six terminals and such broad conjugated backbones have been afforded thus far, rendering featured absorptions, small reorganization and exciton binding energies. Moreover, their distinctive but completely different molecular geometries result in sharply contrasting nanoscale film morphologies. Finally, CH26 contributes to the best device efficiency of 15.41 % among acceptors with six terminals, demonstrating two pioneered yet highly promising 6-cantilever molecular innovation platforms.

Oligomerized Fused-Ring Electron Acceptors for Efficient and Stable Organic Solar Cells

Organic solar cells (OSCs) have attracted wide research attention in the past decades. Very recently, oligomerized fused-ring electron acceptors (OFREAs) have emerged as a promising alternative to small-molecular/polymeric acceptor-based OSCs due to their unique advantages such as well-defined structures, batch reproducibility, good film formation, low diffusion coefficient, and excellent stability. So far, rapid advances have been made in the development of OFREAs consisting of directly/rigidly/flexibly linked oligomers and fused ones. In this Minireview, we systematically summarized the recent research progress of OFREAs, including structural diversity, synthesis approach, molecular conformation and packing, and long-term stability. Finally, we conclude with future perspectives on the challenges to be addressed and potential research directions. We believe that this Minireview will encourage the development of novel OFREAs for OSC applications.

On the Conformation of Dimeric Acceptors and Their Polymer Solar Cells with Efficiency over 18

The determination of molecular conformations of oligomeric acceptors (OAs) and their impact on molecular packing are crucial for understanding the photovoltaic performance of their resulting polymer solar cells (PSCs) but have not been well studied yet. Herein, we synthesized two dimeric acceptor materials, DIBP3F-Se and DIBP3F-S, which bridged two segments of Y6-derivatives by selenophene and thiophene, respectively. Theoretical simulation and experimental 1D and 2D NMR spectroscopic studies prove that both dimers exhibit O-shaped conformations other than S- or U-shaped counter-ones. Notably, this O-shaped conformation is likely governed by a distinctive "conformational lock" mechanism, arising from the intensified intramolecular π-π interactions among their two terminal groups within the dimers. PSCs based on DIBP3F-Se deliver a maximum efficiency of 18.09 %, outperforming DIBP3F-S-based cells (16.11 %) and ranking among the highest efficiencies for OA-based PSCs. This work demonstrates a facile method to obtain OA conformations and highlights the potential of dimeric acceptors for high-performance PSCs.

Complete Peripheral Fluorination of the Small-Molecule Acceptor in Organic Solar Cells Yields Efficiency over 19

Due to the intrinsically flexible molecular skeletons and loose aggregations, organic semiconductors, like small molecular acceptors (SMAs) in organic solar cells (OSCs), greatly suffer from larger structural/packing disorders and weaker intermolecular interactions comparing to their inorganic counterparts, further leading to hindered exciton diffusion/dissociation and charge carrier migration in resulting OSCs. To overcome this challenge, complete peripheral fluorination was performed on basis of a two-dimensional (2D) conjugation extended molecular platform of CH-series SMAs, rendering an acceptor of CH8F with eight fluorine atoms surrounding the molecular backbone. Benefitting from the broad 2D backbone, more importantly, strengthened fluorine-induced secondary interactions, CH8F and its D18 blends afford much enhanced and more ordered molecular packings accompanying with enlarged dielectric constants, reduced exciton binding energies and more obvious fibrillary networks comparing to CH6F controls. Consequently, D18:CH8F-based OSCs reached an excellent efficiency of 18.80 %, much better than that of 17.91 % for CH6F-based ones. More excitingly, by employing D18-Cl that possesses a highly similar structure to D18 as a third component, the highest efficiency of 19.28 % for CH-series SMAs-based OSCs has been achieved so far. Our work demonstrates the dramatical structural multiformity of CH-series SMAs, meanwhile, their high potential for constructing record-breaking OSCs through peripheral fine-tuning.

Facile, Versatile and Stepwise Synthesis of High-Performance Oligomer Acceptors for Stable Organic Solar Cells

Oligomer acceptors have recently emerged as promising photovoltaic materials for achieving high power conversion efficiency (PCE) and long-term stability in organic solar cells (OSCs). However, the limited availability of diverse acceptors, resulting from the sole synthetic approach, has hindered their potential for future industrialization. In this study, we present a facile and effective stepwise approach that utilizes two consecutive Stille coupling reactions for the synthesis of oligomer acceptors. To demonstrate the feasibility of the novel approach, we successfully synthesize a trimer acceptor, Tri-Y6-OD, and further systematically investigate the impact of oligomerization on device performance and stability. The results reveal that this approach has significant advantages compared to the conventional method, including reduced formation of unwanted by-products and lower difficulties in purification. Remarkably, the OSC based on PM6 : Tri-Y6-OD achieves an impressive PCE of 18.03 % and maintains 80 % of the initial PCE (T80 ) for 1523 h under illumination, surpassing the performance of the corresponding small molecule acceptor Y6-OD-based device. Furthermore, the versatility of the synthetic strategy in obtaining diverse acceptors is further demonstrated. Overall, our findings provide a facile, versatile and stepwise way for synthesizing oligomer acceptors, thereby facilitating the development of stable and efficient OSCs.

Modular-Approach Synthesis of Giant Molecule Acceptors via Lewis-Acid-Catalyzed Knoevenagel Condensation for Stable Polymer Solar Cells

The operational stability of polymer solar cells is a critical concern with respect to the thermodynamic relaxation of acceptor-donor-acceptor (A-D-A) or A-DA'D-A structured small-molecule acceptors (SMAs) within their blends with polymer donors. Giant molecule acceptors (GMAs) bearing SMAs as subunits offer a solution to this issue, while their classical synthesis via the Stille coupling suffers from low reaction efficiency and difficulty in obtaining mono-brominated SMA, rendering the approach impractical for their large-scale and low-cost preparation. In this study, we present a simple and cost-effective solution to this challenge through Lewis acid-catalyzed Knoevenagel condensation with boron trifluoride etherate (BF3  ⋅ OEt2 ) as catalyst. We demonstrated that the coupling of the monoaldehyde-terminated A-D-CHO unit and the methylene-based A-link-A (or its silyl enol ether counterpart) substrates can be quantitatively achieved within 30 minutes in the presence of acetic anhydride, affording a variety of GMAs connected via the flexible and conjugated linkers. The photophysical properties was fully studied, yielding a high device efficiency of over 18 %. Our findings offer a promising alternative for the modular synthesis of GMAs with high yields, easier work up, and the widespread application of such methodology will undoubtedly accelerate the progress of stable polymer solar cells.

Poly(dimethylsiloxane)-block-PM6 Polymer Donors for High-Performance and Mechanically Robust Polymer Solar Cells

High power conversion efficiency (PCE) and stretchability are the dual requirements for the wearable application of polymer solar cells (PSCs). However, most efficient photoactive films are mechanically brittle. In this work, highly efficient (PCE = 18%) and mechanically robust (crack-onset strain (COS) = 18%) PSCs are acheived by designing block copolymer (BCP) donors, PM6-b-PDMSx (x = 5k, 12k, and 19k). In these BCP donors, stretchable poly(dimethylsiloxane) (PDMS) blocks are covalently linked with the PM6 blocks to effectively increase the stretchability. The stretchability of the BCP donors increases with a longer PDMS block, and PM6-b-PDMS_19k :L8-BO PSC exhibits a high PCE (18%) and 9-times higher COS value (18%) compared to that (COS = 2%) of the PM6:L8-BO-based PSC. However, the PM6:L8-BO:PDMS_12k ternary blend shows inferior PCE (5%) and COS (1%) due to the macrophase separation between PDMS and active components. In the intrinsically stretchable PSC, the PM6-b-PDMS_19k :L8-BO blend exhibits significantly greater mechanical stability PCE_80% ((80% of the initial PCE) at 36% strain) than those of the PM6:L8-BO blend (PCE_80% at 12% strain) and the PM6:L8-BO:PDMS ternary blend (PCE_80% at 4% strain). This study suggests an effective design strategy of BCP P_D to achieve stretchable and efficient PSCs.

Electron Transport in Organic Photovoltaic Acceptor Materials: Improving the Carrier Mobilities by Intramolecular and Intermolecular Modulations

High carrier mobility is beneficial to increase the active-layer thickness while maintaining a high fill factor, which is crucial to further improve the light harvesting and organic photovoltaic efficiency. The aim of this Perspective is to elucidate the electron transport mechanisms in prototypical non-fullerene (NF) acceptors through our recent theoretical studies. The electron transport in A-D-A small-molecule acceptors (SMAs), e.g., ITIC and Y6, is mainly determined by end-group π-π stacking. Relative to ITIC, the angular backbone along with more flexible side chains leads to Y6 having a closer stacking and enhanced intermolecular electronic connectivity. For polymerized rylene diimide acceptors, to achieve high electron mobilities, they need to simultaneously increase intramolecular and intermolecular connectivity. Finally, finely tuning the π-bridge modes to enhance intramolecular superexchange coupling is essential to develop novel polymerized A-D-A SMAs.

Highly Efficient Mixed Conduction in a Fused Oligomer n-Type Organic Semiconductor Enabled by 3D Transport Pathways

Tailoring organic semiconductors to facilitate mixed conduction of ionic and electronic charges when interfaced with an aqueous media has spurred many recent advances in organic bioelectronics. The field is still restricted, however, by very few n-type (electron-transporting) organic semiconductors with adequate performance metrics. Here, a new electron-deficient, fused polycyclic aromatic system, TNR, is reported with excellent n-type mixed conduction properties including a µC* figure-of-merit value exceeding 30 F cm^-1 V^-1 s^-1 for the best performing derivative. Comprising three naphthalene bis-isatin moieties, this new molecular design builds on successful small-molecule mixed conductors; by extending the molecular scaffold into the oligomer domain, good film-forming properties, strong π-π interactions, and consequently excellent charge-transport properties are obtained. Through judicious optimization of the side chains, the linear oligoether and branched alkyl chain derivative bgTNR is obtained which shows superior mixed conduction in an organic electrochemical transistor configuration including an electron mobility around 0.3 cm² V^-1 s^-1 . By optimizing the side chains, the dominant molecular packing can be changed from a preferential edge-on orientation (with high charge-transport anisotropy) to an oblique orientation that can support 3D transport pathways which in turn ensure highly efficient mixed conduction properties across the bulk semiconductor film.

Hybrid Cycloalkyl-Alkyl Chain-Based Symmetric/Asymmetric Acceptors with Optimized Crystal Packing and Interfacial Exciton Properties for Efficient Organic Solar Cells

Hybrid cycloalkyl-alkyl side chains are considered a unique composite side-chain system for the construction of novel organic semiconductor materials. However, there is a lack of fundamental understanding of the variations in the single-crystal structures as well as the optoelectronic and energetic properties generated by the introduction of hybrid side chains in electron acceptors. Herein, symmetric/asymmetric acceptors (Y-C10ch and A-C10ch) bearing bilateral and unilateral 10-cyclohexyldecyl are designed, synthesized, and compared with the symmetric acceptor 2,2'-((2Z,2'Z)-((12,13-bis(2-butyloctyl)-3,9 bis(ethylhexyl)-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″':4',5']thieno[2',3':4,5] pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10- diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (L8-BO). The stepwise introduction of 10-cyclohexyldecyl side chains decreases the optical bandgap, deepens the energy level, and enables the acceptor molecules to pack closely in a regular manner. Crystallographic analysis demonstrates that the 10-cyclohexyldecyl chain endows the acceptor with a more planar skeleton and enforces more compact 3D network packing, resulting in an active layer with higher domain purity. Moreover, the 10-cyclohexyldecyl chain affects the donor/acceptor interfacial energetics and accelerates exciton dissociation, enabling a power conversion efficiency (PCE) of >18% in the 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6) (PM6):A-C10ch-based organic solar cells (OSCs). Importantly, the incorporation of Y-C10ch as the third component of the PM6:L8-BO blend results in a higher PCE of 19.1%. The superior molecular packing behavior of the 10-cyclohexyldecyl side chain is highlighted here for the fabrication of high-performance OSCs.

Tethered Small-Molecule Acceptors Simultaneously Enhance the Efficiency and Stability of Polymer Solar Cells

For polymer solar cells (PSCs), the mixture of polymer donors and small-molecule acceptors (SMAs) is fine-tuned to realize a favorable kinetically trapped morphology and thus a commercially viable device efficiency. However, the thermodynamic relaxation of the mixed domains within the blend raises concerns related to the long-term operational stability of the devices, especially in the record-holding Y-series SMAs. Here, a new class of dimeric Y6-based SMAs tethered with differential flexible spacers is reported to regulate their aggregation and relaxation behavior. In their polymer blends with PM6, it is found that they favor an improved structural order relative to that of Y6 counterpart. Most importantly, the tethered SMAs show large glass transition temperatures to suppress the thermodynamic relaxation in mixed domains. For the high-performing dimeric blend, an unprecedented open circuit voltage of 0.87 V is realized with a conversion efficiency of 17.85%, while those of regular Y6-base devices only reach 0.84 V and 16.93%, respectively. Most importantly, the dimer-based device possesses substantially reduced burn-in efficiency loss, retaining more than 80% of the initial efficiency after operating at the maximum power point under continuous illumination for 700 h. The tethering approach provides a new direction to develop PSCs with high efficiency and excellent operating stability.