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

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.

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 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.