Organic solar cells (OSCs) have advanced rapidly due to the development of new photovoltaic materials. However, the long-term stability of OSCs still poses a severe challenge for their commercial deployment. To address this issue, a dimer acceptor (dT9TBO) with flexible linker is developed for incorporation into small-molecule acceptors to form molecular alloy with enhanced intermolecular packing and suppressed molecular diffusion to stabilize active layer morphology. Consequently, the PM6:Y6:dT9TBO-based device displays an improved power conversion efficiency (PCE) of 18.41% with excellent thermal stability and negligible decay after being aged at 65 °C for 1800 h. Moreover, the PM6:Y6:dT9TBO-based flexible OSC also exhibits excellent mechanical durability, maintaining 95% of its initial PCE after being bended repetitively for 1500 cycles. This work provides a simple and effective way to fine-tune the molecular packing with stabilized morphology to overcome the trade-off between OSC efficiency and stability.

Dimer Acceptor Adopting a Flexible Linker for Efficient and Durable Organic Solar Cells.

With the emergence of ADA'DA‐type (Y‐series) non‐fullerene acceptors (NFAs), the power conversion efficiencies (PCEs) of organic photovoltaic devices have been constantly refreshed and gradually reached 20% in recent years (19% for single junction and 20% for tandem device). The acceptors possess specific design concept, which greatly enrich the NFA types and have excellent compatibility with many donor materials. It is gratifying to note that the previously underperforming donor materials combine with these regulated acceptors to shine again. Nowadays, the concept of modular design is widely used in the research of acceptors and donors, injecting new vitality into the field of organic photovoltaics. Furthermore, these acceptors also promote the research of multicomponent devices, tandem devices, bilayer devices, processing solvent engineering, and additive engineering. Herein, the latest progresses of polymer solar cells with efficiency over 17% are briefly reviewed from the aspects of active material design, interface material development, and device technology. At last, the opportunities and challenges of organic photovoltaic commercialization in the future are discussed.

Recent Developments of Polymer Solar Cells with Photovoltaic Performance over 17%

The nonradiative energy loss (∆Enr) is a critical factor to limit the efficiency of organic solar cells. Generally, strong electron-phonon coupling induced by molecular motion generates fast nonradiative decay and causes high ∆Enr. How to restrict molecular motion and achieve a low ∆Enr is a sticking point. Herein, the free volume ratio (FVR) is proposed as an indicator to evaluate molecular motion, providing new molecular design rationale to suppress nonradiative decay. Theoretical and experimental results indicate proper proliferation of alkyl side-chain can decrease FVR and restrict molecular motion, leading to reduced electron-phonon coupling while maintaining ideal nanomorphology. The reduced FVR and favorable morphology are simultaneously obtained in AQx-6 with pinpoint alkyl chain proliferation, achieving a high PCE of 18.6% with optimized VOC, JSC and FF. Our study discovered aggregation-state regulation is of great importance to the reduction of electron-phonon coupling, which paves the way to high-efficiency OSCs. 

Suppressing electron-phonon coupling in organic photovoltaics for high-efficiency power conversion

Organic solar cells (OSCs) have potential for applications in wearable electronics. Except for high power conversion efficiency (PCE), excellent tensile properties and mechanical stability are required for achieving high-performance wearable OSCs, while the present metrics barely meet the stretchable requirements. Herein, we propose a facile and low-cost strategy for constructing intrinsically stretchable OSCs by introducing a readily accessible polymer elastomer as a diluent for organic photovoltaic blends. Remarkably, record-high stretchability with a fracture strain of up to 1000% and mechanical stability with elastic recovery>90% under cyclic tensile tests were realized in the OSCs active layers for the first time. Specifically, the tensile properties of best-performing all-polymer photovoltaic blends were increased by up to 250 times after blending. Previously unattainable performance metrics (fracture strain >50% and PCE >10%) were achieved simultaneously for the resulting photovoltaic films. Furthermore, an overall evaluation parameter y was proposed for the efficiency-cost-stretchability balance of photovoltaic blend films. The y value of our dilute-absorber system was two orders of magnitude greater than those of prior state-of-the-art systems. Additionally, intrinsically stretchable devices were prepared to verify the mechanical stability. Overall, this work offers a new avenue for constructing and comprehensively evaluating intrinsically stretchable organic electronic films. This article is protected by copyright. All rights reserved.

Achieving Record-high Stretchability and Mechanical Stability in Organic Photovoltaic Blends with a Dilute-absorber Strategy.

A simultaneous further increase in the power conversion efficiency (PCE) and device stability of organic solar cells (OSCs) over the current levels needs to be overcome for their commercial viability. Herein, a bay-area benzamide-functionalized perylene diimide-based electron transport layer, namely H75 is developed, to obtain the aforementioned characteristics. The advantages of H75-employed OSCs include a notable PCE up to 18.26% and outstanding device stabilities under conditions of varying severity (>95% PCE retention after 1500 h upon long-term aging and exceptional T80 lifetimes (the time required to reach 80% of initial performance) of over 1000 h in light-soaking, 500 h in thermal stress at 85 °C, 72 h in 85% high relative humidity, and 100 h in atmospheric-air conditions without encapsulation in conventional architecture). The excellent performance of H75-employed OSC can be attributed to its various beneficial features derived from the bay-area benzamide functionalities (e.g., excellent film-forming ability, suitable energy level, reduced aggregation, and intrinsic high structural stability). The findings of this work provide further insights into the molecular design of electron transport layers for realizing more efficient and stable OSCs. 

Over 18.2%‐Efficiency Organic Solar Cells with Exceptional Device Stability Enabled by Bay‐Area Benzamide‐Functionalized Perylene Diimide Interlayer

With the great potential of the all‐polymer solar cells for large‐area wearable devices, both large‐area device efficiency and mechanical flexibility are very critical but attract limited attention. In this work, from the perspective of the polymer configurations, two types of terpolymer acceptors PYTX‐A and PYTX‐B (X = Cl or H) are developed. The configuration difference caused by the replacement of non‐conjugated units results in distinct photovoltaic performance and mechanical flexibility. Benefiting from a good match between the intrinsically slow film‐forming of the active materials and the technically slow film‐forming of the blade‐coating process, the toluene‐processed large‐area (1.21 cm2) binary device achieves a record efficiency of 14.70%. More importantly, a new parameter of efficiency stretchability factor (ESF) is proposed for the first time to comprehensively evaluate the overall device performance. PM6:PYTCl‐A and PM6:PYTCl‐B yield significantly higher ESF than PM6:PY‐IT. Further blending with non‐conjugated polymer donor PM6‐A, the best ESF of 3.12% is achieved for PM6‐A:PYTCl‐A, which is among the highest comprehensive performances.

Regulation of Polymer Configurations Enables Green Solvent‐Processed Large‐Area Binary All‐Polymer Solar Cells With Breakthrough Performance and High Efficiency Stretchability Factor

Selective doping of a single conjugated polymer (CP) to obtain p‐type and n‐type conductive materials would be highly attractive for organic thermoelectric applications, because it will greatly reduce the time and costs of synthesizing different types of CPs. However, this strategy has rarely been investigated. In this study, two CPs are synthesized, designated PTQDPP‐T and PTQDPP‐2FT, based on a newly developed quinoidal unit with thienoisatin as the termini and a thiophene‐flanked diketopyrrolopyrrole (ThDPP) unit as the quinoidal core. The electron‐rich thiophene rings in thienoisatin and the electron delocalization induced by thienoisatin resulted in polymers with high‐lying highest occupied molecular orbital, and the electron‐deficient nature of ThDPP unit and its quinoidal backbone endowed the polymers with low‐lying lowest unoccupied molecular orbitals. As a result, both polymers can be p‐type and n‐type doped. Because of its high mobility, doped PTQDPP‐2FT performed better in organic thermoelectric devices than the doped PTQDPP‐T. After being doped with FeCl3 and N‐DMBI, PTQDPP‐2FT showed p‐type and n‐type power factors of 278.2 and 2.37 µW m−1 K−2, respectively. These are the best for bipolar (p‐type and n‐type) performances that obtained by selective doping of a single polymer.

Realizing p‐Type and n‐Type Doping of a Single Conjugated Polymer via Incorporation of a Thienoisatin‐Terminated Quinoidal Unit

Dimerized small-molecular-acceptors (DSMA)-based organic solar cells have gained much progress but insufficient flexibility of the rigid DSMAs limits application in wearable electronics. By regulating dimer conformation, a series of DSMAs (2BOHD-T, 2BOHD-TCxT (x = 4, 6)) are synthesized through microwave-assist-reaction. Conformation evolution from 2-dimension planarity to 3-dimension architecture evoked by linkage engineering from rigidity to flexibility precisely controls the electronic structures, intermolecular interactions, film-forming processes, and flexibility of DSMAs. The rigid terminal-linked 2BOHD-T performs best in small-area devices with efficiency of 17.68%. The 3-dimensional flexible terminal-linked 2BOHD-TC4T achieves a record efficiency of 16.50% with a notable VOC of 0.980 V among the reported flexible DSMAs. 2BOHD-TC4T-based large-area printing device obtains the best efficiency (14.53%) among these acceptors. Furthermore, PM6:2BOHD-TC4T not only improves thermal stability close to the all-polymer system but also achieves an excellent crack onset strain, producing the large-area flexible device with excellent bending tolerance. An updated evaluation parameter, efficiency-stretchability-thermal stability-factor (ESSTF), is proposed. 2BOHD-TC4T-based device yields the highest ESSTF, demonstrating the most balanced large-area device efficiency, thermal stability, and mechanical robustness. 

Dimerized Small Molecular Acceptors: Regulation of Dimer Conformation Realizes Binary Organic Solar Cells with Highly Comprehensive Performance

Developing high-performance active layer with excellent comprehensive performance is very crucial for the commercialization of organic solar cells (OSCs). Here, we demonstrate the multi-function of ferroelectric polymer polyvinylidene fluoride (PVDF) in improving the device comprehensive performance including device efficiency, stability, green-solvent large-area printing and mechanical property. Addition of PVDF not only enhances the build-in field to promote charge kinetics, but also develops robust network through strong interaction and chain entanglement between polymer donor and PVDF. Importantly, such robust network effectively protects the active layer from excessive flushing/swelling between D/A component to optimize layer-by-layer (LBL) deposition, induce a favorable vertical phase distribution and prolong the film-forming process, consequently facilitating green-solvent blade-coating printing, improving device efficiency and stability, and enhancing device mechanical flexibility. Due to the multiple advantages of PVDF, the PM6/BTP-eC9 obtains an excellent efficiency of 18.35% for BTP-eC9-based LBL devices. By blade-coating printing with green solvent, one of the highest efficiencies of 16.40% is achieved for large-area (1.21 cm2) binary device. Particularly, toughness is used to comprehensively evaluate mechanical property of OSCs, showing significant improvement with simultaneously enhanced fracture strength and tensile strain, and enabling small molecule-based system even have comparable mechanical flexibility and comprehensive performance to the all-polymer system. 

Multi-functionally Ferroelectric Polymer Promotes Highly-efficient Large-area Organic Solar Cells with Excellent Comprehensive Performance

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Regulation of Polymer Configurations Enables Green Solvent‐Processed Large‐Area Binary All‐Polymer Solar Cells With Breakthrough Performance and High Efficiency Stretchability Factor

Realizing p‐Type and n‐Type Doping of a Single Conjugated Polymer via Incorporation of a Thienoisatin‐Terminated Quinoidal Unit

Dimerized Small Molecular Acceptors: Regulation of Dimer Conformation Realizes Binary Organic Solar Cells with Highly Comprehensive Performance

Multi-functionally Ferroelectric Polymer Promotes Highly-efficient Large-area Organic Solar Cells with Excellent Comprehensive Performance