Replacing alkyl side chain of non-fullerene acceptor with siloxane-terminated side chain enables lower surface energy towards optimizing bulk-heterojunction morphology and high photovoltaic performance

Green-Solvent-Processed All-Polymer Solar Cells with Enhanced Efficiency and Stability through Molecular Design and Side-Chain Engineering

Green-solvent-processed all-polymer solar cells (AP-SCs) are regarded as an excellent candidate for renewable energy due to their better stability and eco-friendly features. Two polymers, PYF-U and PYF-BO, have been designed by introducing a Y-series derivative with difluoro-substituted dicyanindenone units and a difluorobenzotriazole derivative as the first and second electron-deficient (A) units, respectively. The introduction of two additional F atoms on dicyanindenone units leads to a more coplanar backbone because of noncovalent interactions. Compared with the polymer PYF-U with undecyl chains on thiophene, the polymer PYF-BO with 2-butyloctyl chains exhibits stronger intermolecular aggregation during the film-forming process, more dominant face-on molecular packing, and higher crystallinity in films. Therefore, the PM6:PYF-BO AP-SC achieves an efficiency of 15.38%, outperforming that of the PM6:PYF-U device (14.27%). Moreover, the former exhibits a longer T80 lifetime (1789 h) than the latter (826 h) under thermal aging at 65 °C because of better molecular packing and morphology. Our research demonstrates that combining noncovalent interactions to enhance the coplanarity of the polymeric backbone with side-chain engineering to optimize molecular packing and blend-film morphology is one of the efficient strategies for developing high-performance polymer acceptors.

Polymerized-Small-Molecule Acceptors Featuring Siloxane-Terminated Side Chains for Mechanically Robust All-Polymer Solar Cells

Flexible and stretchable organic solar cells (OSCs) show great promise in wearable and stretchable electronic applications. However, current high-performance OSCs consisting of polymer donors (PDs) and small-molecule acceptors (SMAs) face significant challenges in achieving both high power conversion efficiency (PCE) and excellent stretch-ability. In this study, we synthesized a new polymerized-small-molecule acceptor (P-SMA) PY-SiO featuring siloxane-terminated side chains and compared its photovoltaic and mechanical performance to that of the reference PY-EH with ethylhexyl-terminated side chains. We found that the incorporation of siloxane-terminated side chains in PY-SiO enhanced the molecular aggregation and charge transport, leading to an optimized film morphology. The resultant of all-polymer solar cells (all-PSCs) based on PBDB-T/PY-SiO showed a higher PCE of 12.04% than the PY-EH-based one (10.85%). Furthermore, the siloxane-terminated side chains also increased the interchain distance and provided a larger free volume for chain rotation and reconfiguration, resulting in a higher film crack-onset strain (COS: 18.32% for PBDB-T/PY-SiO vs 11.15% for PBDB-T/PY-EH). Additionally, the PY-SiO-based stretchable all-PSCs exhibited an impressive PCE of 9.8% and retained >70% of its original PCE even under a substantial 20% strain, exceeding the performance of the PY-EH-based stretchable all-PSCs. Our result suggests the great potential of the siloxane-terminated side chain for achieving high-performance and stretchable OSCs.

Polymer Donor with a Simple Skeleton and Minor Siloxane Decoration Enables 19% Efficiency of Organic Solar Cells

Development of polymer donors with simple chemical structure and low cost is of great importance for commercial application of organic solar cells (OSCs). Here, side-chain random copolymer PMQ-Si605 with a simply 6,7-difluoro-3-methylquinoxaline-thiophene backbone and 5% siloxane decoration of side chain is synthesized in comparison with its alternating copolymer PTQ11. Relative to molecular weight (Mn) of 28.3 kg mol-1 for PTQ11, the random copolymer PMQ-Si605 with minor siloxane decoration is beneficial for achieving higher Mn up to 51.1 kg mol-1. In addition, PMQ-Si605 can show stronger aggregation ability and faster charge mobility as well as more efficient exciton dissociation in active layer as revealed by femtosecond transient absorption spectroscopy. With L8-BO-F as acceptor, its PMQ-Si605 based OSCs display power conversion efficiency (PCE) of 18.08%, much higher than 16.21% for PTQ11 based devices. With another acceptor BTP-H2 to optimize the photovoltaic performance of PMQ-Si605, further elevated PCEs of 18.50% and 19.15% can be achieved with the binary and ternary OSCs, respectively. Furthermore, PMQ-Si605 based active layers are suitable for processing in high humidity air, an important factor for massive production of OSCs. Therefore, the siloxane decoration on polymer donors is promising, affording PMQ-Si605 as a high-performing and low cost candidate.

Fine-Tuning of Siloxane Pendant Distance for Achieving Highly Efficient Eco-Friendly Nonfullerene Solar Cells

Side-chain engineering has been proved to show profound impact on polymer properties and blend morphology. Herein, the critical role of siloxane-terminated alkoxy side chains was revealed by a tiny decorating method through which the molar content of siloxane-terminated alkoxy side chain was fixed to 5 %, and its alkyl linker was the only tuning factor. The pentylene, heptylene, and nonylene linkers were designed and used to synthesize wide-bandgap polymers PQSi505, PQSi705, and PQSi905, respectively. Interestingly, the siloxane pendant of combinatory side chain exhibited a distinct impact on molecular packing when its branching position shifted slightly. Among the three polymers, PQSi705 had the strongest aggregation and the highest packing order. Finally, toluene-processed organic solar cells (OSCs) based on PQSi705 and Y6-BO achieved the best power conversion efficiency of 15.77 %. This work suggests that siloxane pendant can serve as a powerful modulator to enhance the photovoltaic performance of OSCs.

Introducing Siloxane-Terminated Side Chains in Small Molecular Donors for All-Small-Molecule Organic Solar Cells: Modulated Molecular Orientation and Enhanced Efficiency

In this work, three small molecular donors (SMDs) S35, S35-1Si, and S35-2Si, with 3,5-difluorophenyl-substituted benzodithiophene as the central 2-dimensional unit to combine different numbers of siloxane-terminated side chain, were synthesized for all-small-molecule organic solar cells (ASM-OSCs). The three SMDs showed comparable film absorption peaks at 570 nm and optical band gaps of 1.8 eV. Relative to S35 and S35-1Si with symmetric alkyl side chains and asymmetric side chains on the central unit, respectively, the S35-2Si carrying two symmetric siloxane-terminated side chains displayed largely elevated melting and crystalline temperatures, lowered surface energy, and modulated molecular orientation. The three SMDs possessed edge-on dominated molecular orientations of their neat films; however, a big difference was found for their blend films with nonfullerene acceptor Y6. The S35:Y6 and S35-1Si:Y6 blends exhibited edge-on and face-on bimodal orientations but the S35-2Si:Y6 blend showed pure face-on orientation, indicating quite different donor:acceptor intermolecular interactions. Some large domains existed in the S35:Y6 and S35-1Si:Y6 blends, but could be suppressed by the S35-2Si:Y6 blend, leading to a more balanced charge transport. In ASM-OSCs, the two S35:Y6 and S35-1Si:Y6 active layers showed comparable power conversion efficiencies (PCE) of ∼12% but a much higher efficiency of 13.50% could be achieved with the S35-2Si:Y6 active layer. Our results suggest that the siloxane-terminated side chain is promising to regulate crystalline ability of a SMD, paving a way for high performance ASM-OSCs.