Ductile Oligomeric Acceptor-Modified Flexible Organic Solar Cells Show Excellent Mechanical Robustness and Near 18% Efficiency

Control of molecular aggregation structures towards flexible organic photovoltaics

Flexible organic photovoltaics (OPVs) utilizing conjugated polymers have shown considerable promise in the field of wearable electronic devices. Although active-layer materials featuring extensive conjugated structures demonstrate good electron and optical properties, they often suffer from brittleness, which poses a significant challenge to the advancement of flexible OPVs. The aggregation structure of molecules within the active layer is pivotal in determining its mechanical properties, particularly its stretchability. Recently, researchers have employed a variety of strategies to manipulate the molecular aggregation structure within the active layer to enhance its tensile properties. This review first categorizes the aggregation structures of molecules across different scales, ranging from small to large (including molecular arrangement, chain entanglement, crystallization, phase separation, and semi-interpenetrating networks) and elucidates the mechanisms by which tensile performance can be improved. Subsequently, it summarizes the methodologies for regulating the molecular aggregation structures at various scales. Finally, the review discusses the ongoing development of flexible OPVs to provide valuable insights for researchers in the field.

A Strain Relaxation Modulation for Printing High-performance Flexible Pseudo-Planar Heterojunction Organic Solar Cells

The rational toughening of photosensitive films is crucial for the development of robust and flexible organic solar cells (F-OSCs), which are always influenced by mechanical strain and thermodynamic relaxation within the films. Nevertheless, the potential determinants of these properties and quantitative metrics modulating the overall performance of flexible devices have not been thoroughly defined. Herein, a fine-grain strengthening strategy is demonstrated for mitigating the excessive aggregation or crystallization in small-molecule acceptor films, the secondary thermal relaxation of side chains in polyethylene oxide (PEO) local motion restricts the free fluctuation volume through hydrogen-bonding interactions, thereby suppressing the non-ideal thermodynamic behavior and residual-enriched state. These contribute to an increase in yield strength and a reduction in microcracks while enhancing the fracture energy at the donor/acceptor interface. Finally, the optimal F-OSCs demonstrate champion PCEs of 19.12% (0.04 cm2) and 16.92% (1.00 cm2), and maintain 80% of their initial efficiency after heating at 85 °C for 2600 h. Besides, the flexibility and mechanical robustness of devices are also optimized, the elastic modulus and stiffness are decreased by 50.68% and 5.71%. This work provides interesting references for the synergistic enhancement of efficiency, mechanical and environmental stability in flexible organic photovoltaics.

Oligomerized Electron Acceptors with Alkynyl Linkages to Suppress Electron-Phonon Coupling for Low-Energy-Loss Organic Solar Cells

A novel series of alkynyl-linked oligomerized electron acceptors have been synthesized via Sonogashira coupling. The alkynyl linkages can enhance molecular planarity and aggregation, suppress electron-phonon coupling, and reduce non-radiative losses. Binary organic solar cells (OSCs) achieved an efficiency of 17.90%, with a non-radiative loss of 0.185 eV, while ternary OSCs reached a remarkable efficiency of 19.52%. Oligomerized electron acceptors, featuring molecular weights akin to polymers and well-defined chemical structures, have emerged as promising candidates for organic solar cells (OSCs) due to their consistent batch-to-batch reproducibility and improved thermal stability. In this study, we developed a series of oligomerized electron acceptors incorporating alkynyl linkages via an efficient Sonogashira coupling reaction between alkyne-substituted Y-type precursors and multi-substituted iodobenzenes. This method produced monomeric (S-Alkyne-YF), dimeric (D-Alkyne-YF), and trimeric (T-Alkyne-YF) configurations, enabling systematic control over molecular size and substituent arms. The alkynyl linkages, characterized by high bond strength and planar geometry, enhanced molecular planarity and aggregation in films, thus facilitating precise control over morphology and phase separation in the photoactive layers. Notably, the inclusion of these linkages effectively suppressed electron-phonon coupling, resulting in reduced non-radiative energy losses and elevated photocarrier lifetime. OSCs based on PM6:T-Alkyne-YF achieved a power conversion efficiency of 17.90%, a low non-radiative energy loss of 0.185 eV, and an open-circuit voltage of 0.943 V. Furthermore, integrating T-Alkyne-YF into the D18:N3 blend yielded an exceptional PCE of 19.52%. These results underscore the potential of alkynyl-linked oligomerized acceptors in advancing highly efficient and stable OSCs, offering a viable pathway for reducing electron-phonon coupling and enhancing device performance.

Acridine-Substituted-Centronucleus Nonfullerene Acceptors Enables Organic Solar Cells with Over 20% Efficiency with Low Nonradiative Recombination Loss

In this work, we propose a novel strategy of introducing luminescent acridine units for central nuclear substitution in quinoxaline-based acceptor molecules (named AQx-o-Ac and AQx-m-Ac) to enhance their photoluminescence quantum yields (PLQY), which can effectively improve the electroluminescent quantum efficiency (EQEEL) of OSCs and thereby suppress ΔEnr. In addition, the substituted acridine unit accelerates molecular aggregation and optimizes molecular crystallization, effectively alleviating the static disorder of acceptor molecules and facilitating charge extraction and transport in OSCs. As a result, the PM6:AQx-m-Ac binary OSCs achieve an excellent PCE of 18.64% with an exceptionally low ΔEnr of 0.166 eV. To the best of our knowledge, a ΔEnr of 0.166 eV represents the lowest value reported for OSCs achieving PCEs over 18 %. Finally, the acceptor AQx-m-Ac is incorporated into PM6:eC9 blend as the third component, and the optimal ternary device produces a superior PCE of 20.28%. This work highlights the potential of promoting luminescence for suppressing nonradiative energy loss and charts a viable path for upcoming breakthrough in high-efficiency organic photovoltaics.

Molecular Design of Dimeric Acceptor Enables Binary Organic Solar Cells with 19.78% Efficiency and Enhanced Stability

The development of organic solar cells (OSCs) with high efficiency and stability is highly desirable to facilitate its commercial applications. Although dimeric acceptors with distinctive advantages have been widely studied, high-performance binary OSCs based on such molecules have rarely been achieved. In this work, a new dimeric acceptor (DY-FL) is constructed by simultaneously optimizing the linking sites and units, as well as the building blocks. Thanks to the effective molecular design, DY-FL provides improved molecular stacking for fibrous morphology with favorable exciton/charge dynamics. Consequently, DY-FL-based binary OSCs render a superior power conversion efficiency (PCE) of 19.78%, representing a record-breaking efficiency for binary OSCs based on dimeric acceptors. Importantly, DY-FL-based devices display significantly enhanced operational stability under external stimuli such as light and heat, in comparison to their small molecule acceptor (Y-F)-based counterpart. These findings highlight the significance of building blocks and linking modes, providing insight into the effective molecular design strategy of dimeric acceptors for state-of-the-art 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.

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.

U-Shaped Dimeric Acceptors for Balancing Efficiency and Stability in Organic Solar Cells

Despite significant improvements in power conversion efficiencies (PCEs) of organic solar cells (OSCs), achieving excellent stability remains a great challenge to their commercial feasibility. Here, U-shaped dimeric acceptors (5-IDT and 6-IDT) with different molecular lengths are introduced into the binary OSCs as a third component, respectively. The introduction of the third component effectively reduces the energetic disorder and non-radiative voltage losses and improves the exciton dissociation and charge transport of the devices. Consequently, the PCEs of the 6-IDT- and 5-IDT-treated OSCs are significantly improved to 19.32% and 19.96%, respectively, which is the highest PCE for oligomeric acceptors-based ternary OSCs to date. Meanwhile, the thermal stability of the treated devices is dramatically improved, with the initial efficiency retention of the 6-IDT- and 5-IDT-treated devices increasing from 18% to 32% and 75%, respectively, after 1000 h of thermal stress. This is mainly attributed to the ability of the smaller molecular length of 5-IDT to stabilize the phase-separated morphology of the polymeric donor and small molecular acceptor, rather than the high glass transition temperature and low diffusion coefficient.

Asymmetric small-molecule acceptor enables suppressed electron-vibration coupling and minimized driving force for organic solar cells

Minimizing the energy loss, particularly the non-radiative energy loss (ΔEnr), without sacrificing the charge collection efficiency, is the key to further improve the photovoltaic performance of organic solar cells (OSCs). Herein, we proposed an asymmetric molecular design strategy, via developing alkyl/thienyl hybrid side chain based asymmetric small molecule acceptors (SMAs) BTP-C11-TBO and BTP-BO-TBO, to manipulate the intermolecular interactions to realize enhanced luminescence efficiency and reduced energy loss. Theoretical and experimental results indicate that compared to the three symmetric SMAs BTP-DC11, BTP-DTBO and BTP-DBO, the asymmetric SMAs BTP-C11-TBO and BTP-BO-TBO exhibit repressed electron-vibration coupling and reduced ΔEnr. Moreover, the asymmetric nature of BTP-BO-TBO allows the formation of multiple D:A interfacial conformations and interfacial energies, which have made the charge-transfer state energies closer to that of the strongly absorbing (and emitting) local-exciton state, thus gaining the low ΔEnr while maintaining efficient exciton dissociation. Consequently, the PM6:BTP-BO-TBO-based OSCs achieve a higher power conversion efficiency of 19.76%, with a high open circuit voltage of 0.913 V and an efficient fill factor of 81.17%, profiting from the more improved and balanced charge mobility and longer carrier lifetime. This work provides molecular design ideas to suppress nonradiative decay and paves the way to obtain high-performance OSCs.

Synergistic Multimodal Energy Dissipation Enhances Certified Efficiency of Flexible Organic Photovoltaics beyond 19

All-polymer organic solar cells (OSCs) have shown unparalleled application potential in the field of flexible wearable electronics in recent years due to the excellent mechanical and photovoltaic properties. However, the small molecule acceptors after polymerization in still retain some mechanical and aggregation properties of the small molecule, falling short of the ductility requirements for flexible devices. Here, based on the multimodal energy dissipation theory, the mechanical and photovoltaic properties of flexible devices are co-enhanced by adding the thermoplastic elastomer material (polyurethane, PU) to the PM6:PBQx-TF:PY-IT-based active layer films. The construction of multi-fiber network structure and the decrease of films' residual stresses contribute to the enhancement of carrier transport properties and the decrease of defect state density. Eventually, the PCE (power conversion efficiency) of 19.40% is achieved on the flexible devices with an effective area of 0.102 cm2, and the third-party certified PCE reaches 19.07%, which is the highest PCE for flexible OSCs currently available. To further validate the potential of this strategy for large-area module applications, the 25-cm2-based flexible and super-flexible modules are prepared with the PCEs of 15.48% and 14.61%, respectively, and demonstration applications are implemented.

Perspective on Flexible Organic Solar Cells for Self-Powered Wearable Applications

The growing advancement of wearable technologies and sophisticated sensors has driven the need for environmentally friendly and reliable energy sources with robust mechanical stability. Flexible organic solar cells (OSCs) have become promising substitutes for traditional energy solutions thanks to their remarkable mechanical flexibility and high power conversion efficiency (PCE). These unique properties allow flexible OSCs to seamlessly integrate with diverse devices and substrates, making them an excellent choice for powering various electronic devices by efficiently harvesting solar energy. This review summarizes recent achievements in flexible OSCs from the perspective of self-powered wearable applications. It discusses advancements in materials, including substrates and transparent electrodes, evaluates performance criteria, and compares the PCEs of flexible OSCs to their rigid counterparts. Subsequently, novel applications of flexible OSCs in self-powered wearable applications are explored. Finally, a summary and perspectives on the current challenges and obstacles facing flexible OSCs and their applications in self-powered wearables are provided, aiming to inspire further research toward practical implementations.

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.

Performance Enhancement by Integrating the Ionic Thermoelectric Generator with a Photovoltaic Cell

The global solar market is booming with a rapid growth in installed integrated devices, while photovoltaic (PV) systems are suffering from waste heat, which causes the decline of the photovoltaic conversion efficiency (PCE). This study presents the seamless integration of the ionic thermoelectric generator (iTEG) layer with traditional PV modules, facilitating the exploitation of waste heat and augmenting the overall power output. Experimental results validate the effectiveness of the iTEG, demonstrating substantial power generation and a consistent energy output. Specifically, a power output of 515 mW/m2 is achieved at a temperature difference of 20 K, while an energy density of 229.7 J/m2 is recorded at a temperature difference of 10 K with an external resistance of 1 kΩ. Notably, this system maintains continuous electricity generation over 100 cycles. Furthermore, the iTEG effectively reduces the operating temperature of the solar panel by 2 °C, which is beneficial in minimizing PCE losses attributed to the temperature coefficient. This research holds substantial practical implications, particularly for large-scale power generation leveraging iTEG technology in conjunction with solar cells.

Regulating Electron-Phonon Coupling by Solid Additive for Efficient Organic Solar Cells

Strong electron-phonon coupling can hinder exciton transport and induce undesirable non-radiative recombination, resulting in a shortened exciton diffusion distance and constrained exciton dissociation in organic solar cells (OSCs). Therefore, suppressing electron-phonon coupling is crucially important for achieveing high-performance OSCs. Here, we employ the solid additive to regulating electron-phonon coupling in OSCs. The planar configuration of SA1 confers a significant advantage in suppressing lattice vibrations in the active layers, reducing the scattering of excitons by phonons. Consequently, a slow but sustained hole transfer process is identified in the SA1-assisted film, indicating an enhancement in hole transfer efficiency. Prolonged exciton diffusion length and exciton lifetime are achieved in the blend film processed with SA1, attributed to a low non-radiative recombination rate and low energetic disorder for charge carrier transport. As a result, a high efficiency of 20 % was achieved for ternary device with a remarkable short-circuit current. This work highlights the important role of suppressing electron-phonon coupling in improving the photovoltaic performance of OSCs.

A butterfly-shaped acceptor with rigid skeleton and unique assembly enables both efficient organic photovoltaics and high-speed organic photodetectors

It remains challenging to design efficient bifunctional semiconductor materials in organic photovoltaic and photodetector devices. Here, we report a butterfly-shaped molecule, named WD-6, which exhibits low energy disorder and small reorganization energy due to its enhanced molecular rigidity and unique assembly with strong intermolecular interaction. The binary photovoltaic device based on PM6:WD-6 achieved an efficiency of 18.41%. Notably, an efficiency of 19.42% was achieved for the ternary device based on PM6:BTP-eC9:WD-6. Moreover, the photodetection device based on WD-6 demonstrated an ultrafast response speed (205 ns response time at λ of 820 nm) and a high cutoff frequency of -3 dB (2.45 MHz), surpassing the values of most commercial Si photodiodes. Based on these findings, we showcased an application of the WD-6-based photodetection device in high-speed optical communication. These results offer valuable insights into the design of organic semiconductor materials capable of simultaneously exhibiting high photovoltaic and photodetective performance.

Molecular Design of Active Layer for High-Performance Stretchable Organic Solar Cells

Stretchable organic solar cells (SOSCs) have advanced rapidly in the last few years as power sources required to realize portable and wearable electronics become available. Through rational material and device engineering, SOSCs are now able to retain their photovoltaic performance even when subjected to repeated mechanical deformations. However, reconciling a high efficiency and an excellent stretchability is still a huge challenge, and the development of SOSCs has lagged far behind that of flexible OSCs. In this perspective article, recent strategies for imparting mechanical robustness to SOSCs while maintaining high power conversion efficiency are reviewed, with emphasis on the molecular design of active layers. Initially, an overview of molecular design approaches and recent research advances is provided in improving the stretchability of active layers, including donors, acceptors, and single-component materials. Subsequently, another common strategy for regulating photovoltaic and mechanical properties of SOSCs, namely multi-component system, is summarized and analyzed. Lastly, considering that SOSCs research is in its infancy, the current challenges and future directions are pointed out.

Designing a Highly Crystalline Polymer Donor with Alkylsilyl and Fluorine Substitution to Achieve Efficient Ternary Organic Solar Cells

Adopting a ternary strategy is an effective approach to enhance the power conversion efficiency (PCE) in organic solar cells (OSCs). Previous research on highly efficient ternary systems has predominantly focused on those based on highly crystalline dual small molecule acceptors. However, limited attention has been given to ternary systems utilizing dual polymer donors. Herein, by incorporating the fluorine and alkylsilyl substitution, a new polymer donor named PX1 is developed, which demonstrates strong crystallinity and excellent miscibility with polymer PM6. Moreover, PX1 broadens and enhances the absorption properties of the PM6:L8-BO blends, and its molecular orbital energy level is situated between those of PM6 and L8-BO, highlighting its suitability as a third component. Introducing 20% PX1 into the PM6:L8-BO system resulted in a high PCE of 18.82%. PX1 effectively suppresses charge recombination and reduces energy losses, while also serving as a morphology modulator that enhances the crystallization and improves the molecular packing order of the active layer by shortening the π-π stacking distance and extending crystalline coherence length. These factors collectively contribute to the performance improvements in ternary devices. This study demonstrates that employing a dual polymer donor strategy is a promising approach for achieving high-performance ternary OSCs.

20.4% Power conversion efficiency from albedo-collecting organic solar cells under 0.2 albedo

Highly efficient bifacial organic solar cells (OSCs) have not been reported due to limited thickness of the active layer in conventional configurations, not allowing for efficient harvesting of front sunlight and albedo light. Here, bifacial OSCs are reported with efficiency higher than the monofacial counterparts. The incorporation of pyramid-based asymmetrical optical transmission (AOT) array to a transparent silver electrode suppresses the escaping of front sunlight without sacrificing the harvesting of albedo light. Parasitic absorption induced by the excitation of surface plasmons in an AOT electrode is further reduced by doping organic emitter in electron transport layer and capping high dielectric constant film to silver. The rear electrode achieves a front transmittance of 7% and a rear transmission of 86%. At a conventional albedo of 0.2, the synergistic effect of AOT and minimized optical loss endow the bifacial OSCs with power conversion efficiency of 20.4%. This work paves the way for the utilization of albedo light in OSCs.

A Novel Upside-Down Thermal Annealing Method Toward High-Quality Active Layers Enables Organic Solar Cells with Efficiency Approaching 20

The emerging non-fullerene acceptors with low voltage losses have pushed the power conversion efficiency of organic solar cells (OSCs) to ≈20% with auxiliary morphology optimization. Thermal annealing (TA), as the most widely adopted post-treatment method, has been playing an essential role in realizing the potential of various material systems. However, the procedure of TA, i.e., the way that TA is performed, is almost identical among thousands of OSC papers since ≈30 years ago other than changes in temperature and annealing time. Herein, a reverse thermal annealing (RTA) technique is developed, which can enhance the dielectric constant of active layer film, thereby producing a smaller Coulomb capture radius (14.93 nm), meanwhile, forming a moderate nano-scale phase aggregation and a more favorable face-on molecular stacking orientation. Thus, this method can reduce the decline in open circuit voltage of the conventional TA method by achieving decreased radiative (0.334 eV) and non-radiative (0.215 eV) recombination loss. The power conversion efficiency of the RTA PM6:L8-BO-X device increases to 19.91% (certified 19.42%) compared to the TA device (18.98%). It is shown that this method exhibits a superb universality in 4 other material systems, revealing its dramatic potential to be employed in a wide range of OSCs.

Unraveling the Solution Aggregation Structures and Processing Resiliency of High-Efficiency Organic Photovoltaic Blends

The solution aggregation structure of conjugated polymers is crucial to the morphology and resultant optoelectronic properties of organic electronics and is of considerable interest in the field. Precise characterizations of the solution aggregation structures of organic photovoltaic (OPV) blends and their temperature-dependent variations remain challenging. In this work, the temperature-dependent solution aggregation structures of three representative high-efficiency OPV blends using small-angle X-ray/neutron scattering are systematically probed. Three cases of solution processing resiliency are elucidated in state-of-the-art OPV blends. The exceptional processing resiliency of high-efficiency PBQx-TF blends can be attributed to the minimal changes in the multiscale solution aggregation structure at elevated temperatures. Importantly, a new parameter, the percentage of acceptors distributed within polymer aggregates (Ф), for the first time in OPV blend solution, establishes a direct correlation between Ф and performance is quantified. The device performance is well correlated with the Kuhn length of the cylinder related to polymer aggregates L1 at the small scale and the Ф at the large scale. Optimal device performance is achieved with L1 at ≈30 nm and Ф within the range of 60 ± 5%. This study represents a significant advancement in the aggregation structure research of organic electronics.

Advances in Stretchable Organic Photovoltaics: Flexible Transparent Electrodes and Deformable Active Layer Design

Stretchable organic photovoltaics (OPVs) have attracted significant attention as promising power sources for wearable electronic systems owing to their superior robustness under repetitive tensile strains and their good compatibility. However, reconciling a high power-conversion efficiency and a reasonable flexibility is a tremendous challenge. In addition, the development of stretchable OPVs must be accelerated to satisfy the increasing requirements of niche markets for mechanical robustness. Stretchable OPV devices can be classified as either structurally or intrinsically stretchable. This work reviews recent advances in stretchable OPVs, including the design of mechanically robust transparent electrodes, photovoltaic materials, and devices. Initially, an overview of the characteristics and recent research progress in the areas of structurally and intrinsically stretchable OPVs is provided. Subsequently, research into flexible and stretchable transparent electrodes that directly affect the performances of stretchable OPVs is summarized and analyzed. Overall, this review aims to provide an in-depth understanding of the intrinsic properties of highly efficient and deformable active materials, while also emphasizing advanced strategies for simultaneously improving the photovoltaic performance and mechanical flexibility of the active layer, including material design, multi-component settings, and structural optimization.

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.

Multi-component Copolymerized Donors enable Frozen Nano-morphology and Superior Ductility for Efficient Binary Organic Solar Cells

Multi-component copolymerized donors (MCDs) have gained significant interest and have been rapidly developed in flexible organic solar cells (f-OSCs) in recent years. However, ensuring the power conversion efficiency (PCE) of f-OSCs while retaining ideal mechanical properties remains an enormous challenge. The fracture strain (FS) value of typical high-efficiency blend films is generally less than 8 %, which is far from the application standards of wearable photovoltaic devices. Therefore, we developed a series of novel MCDs after meticulous molecular design. Among them, the consistent MCD backbone and end-capped functional group formed a highly conjugated molecular plane, and the solubilization and mechanical properties were effectively optimized by modifying the proportion of solubilized alkyl chains. Consequently, due to the formation of entangled structures with a frozen blend film morphology considerably improved the high ductility of the active layer, P10.8/P20.2-TCl exhibited efficient PCE in rigid (18.53 %) and flexible (17.03 %) OSCs, along with excellent FS values (16.59 %) in pristine films, meanwhile, the outstanding FS values of 25.18 % and 12.3 % were achieved by P10.6/P20.4-TCl -based pristine and blend films, respectively, which were one of the highest records achieved by end-capped MCD-based binary OSCs, demonstrating promising application to synchronize the realization of high-efficiency and mechanically ductile flexible OSCs.

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.

Dimerized Acceptors with Conjugate-Break Linker Enable Highly Efficient and Mechanically Robust Organic Solar Cells

Designing new acceptors is critical for intrinsically stretchable organic solar cells (IS-OSCs) with high efficiency and mechanical robustness. However, nearly all stretchable polymer acceptors exhibit limited efficiency and high-performance small molecular acceptors are very brittle. In this regard, we select thienylene-alkane-thienylene (TAT) as the conjugate-break linker and synthesize four dimerized acceptors by the regulation of connecting sites and halogen substitutions. It is found that the connecting sites and halogen substitutions considerably impact the overall electronic structures, aggregation behaviors, and charge transport properties. Benefiting from the optimization of the molecular structure, the dimerized acceptor exhibits rational phase separation within the blend films, which significantly facilitates exciton dissociation while effectively suppressing charge recombination processes. Consequently, FDY-m-TAT-based rigid OSCs render the highest power conversion efficiency (PCE) of 18.07 % among reported acceptors containing conjugate-break linker. Most importantly, FDY-m-TAT-based IS-OSCs achieve high PCE (14.29 %) and remarkable stretchability (crack-onset strain [COS]=18.23 %), significantly surpassing Y6-based counterpart (PCE=12.80 % and COS=8.50 %). To sum up, these findings demonstrate that dimerized acceptors containing conjugate-break linkers have immense potential in developing highly efficient and mechanically robust OSCs.

Three-in-One Strategy Enables Single-Component Organic Solar Cells with Record Efficiency and High Stability

Single-component organic solar cells (SCOSCs) with covalently bonding donor and acceptor are becoming increasingly attractive because of their superior stability over traditional multicomponent blend organic solar cells (OSCs). Nevertheless, the efficiency of SCOSCs is far behind the state-of-the-art multicomponent OSCs. Herein, by combination of the advantages of three-component and single-component devices, this work reports an innovative three-in-one strategy to boost the performance of SCOSCs. In this three-in-one strategy, three independent components (PM6, D18, and PYIT) are covalently linked together to create a new single-component active layer based on ternary conjugated block copolymer (TCBC) PM6-D18-b-PYIT by a facile polymerization. Precisely manipulating the component ratios in the polymer chains of PM6-D18-b-PYIT is able to broaden light utilization, promote charge dynamics, optimize, and stabilize film morphology, contributing to the simultaneously enhanced efficiency and stability of the SCOSCs. Ultimately, the PM6-D18-b-PYIT-based device exhibits a power conversion efficiency (PCE) of 14.89%, which is the highest efficiency of the reported SCOSCs. Thanks to the aggregation restriction of each component and chain entanglement in the three-in-one system, the PM6-D18-b-PYIT-based SCOSC displays significantly higher stability than the corresponding two-component (PM6-D18:PYIT) and three-component (PM6:D18:PYIT). These results demonstrate that the three-in-one strategy is facile and promising for developing SCOSCs with superior efficiency and stability.

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.

Terpolymer Containing a meta-Octyloxy-phenyl-Modified Dithieno[3,2-f:2',3'-h]quinoxaline Unit Enabling Efficient Organic Solar Cells

With the rapid development of small-molecule electron acceptors, polymer electron donors are becoming more important than ever in organic photovoltaics, and there is still room for the currently available high-performance polymer donors. To further develop polymer donors with finely tunable structures to achieve better photovoltaic performances, random ternary copolymerization is a useful technique. Herein, by incorporating a new electron-withdrawing segment 2,3-bis(3-octyloxyphenyl)dithieno[3,2-f:2',3'-h]quinoxaline derivative (C12T-TQ) to PM6, a series of terpolymers were synthesized. It is worth noting that the introduction of the C12T-TQ unit can deepen the highest occupied molecular orbital energy levels of the resultant polymers. In addition, the polymer Z6 with a 10% C12T-TQ ratio possesses the highest film absorption coefficient (9.86 × 104 cm-1) among the four polymers. When blended with Y6, it exhibited superior miscibility and mutual crystallinity enhancement between Z6 and Y6, suppressed recombination, better exciton separation and charge collection characteristics, and faster hole transfer in the D-A interface. Consequently, the device of Z6:Y6 successfully achieved enhanced photovoltaic parameters and yielded an efficiency of 17.01%, higher than the 16.18% of the PM6:Y6 device, demonstrating the effectiveness of the meta-octyloxy-phenyl-modified dithieno[3,2-f:2',3'-h]quinoxaline moiety to build promising terpolymer donors for high-performance organic solar cells.

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.