Highly Efficient and Stable All-Polymer Solar Cells Enabled by Near-Infrared Isomerized Polymer Acceptors

One of the most promising approaches to achieving high-performance all-polymer solar cells (all-PSCs) is to develop near-infrared polymer acceptors (PAs) constructed from n-type fused-ring electron acceptors (FREAs) as their fundamental building blocks. However, the effects of regioisomerized structures on the molecular and photovoltaic properties of the PAs have never been reported. In this work, we designed and synthesized three isomeric FREA-based PAs (namely, PYTT-1, PYTT-2, and PYTT-3) based on different isomeric thiophene-fused ending-groups and systematically investigated the effects of the isomeric molecular geometry on the optoelectronic properties, charge transport, molecular aggregation packing order, and morphological properties. Matched with the polymer donor poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)] (PBDB-T), these all-PSCs achieved power conversion efficiencies (PCEs) of over 12%, while the PBDB-T:PYTT-2 all-PSCs achieved an impressively high PCE of up to 14.32% due to the improved exciton dissociation and charge generation, more balanced charge-transport properties, less nonradiative recombination loss, faster charge extraction, and optimized active-layer morphology than PYTT-1 and PYTT-3 systems. Importantly, the photostability and thermal stability of these three systems were also investigated, with the PYTT-2 system being the most stable one. Our results provide an effective way to develop FREA-based PAs by optimizing thiophene-fused ending-groups for next-step high-performance all-PSCs.