Low-Temperature Side-Chain Removal and Layer-by-Layer Processing for Enhanced Stability and Efficiency of D18-Based Organic Photovoltaics

The simultaneous achievement of high efficiency and high stability in organic photovoltaics (OPVs) is crucial for their commercial viability. However, insulating side chains, while vital for the solution processability of active layer components, severely limit the intrinsic stability of devices by increasing the molecular dynamics that drive the phase separation of bulk heterojunction (BHJ) morphologies. The removal of side chains from conjugated polymers via tertiary ester pyrolysis has previously enabled thermally stable morphologies; however, the requisite high temperatures (>200 °C) for rapid elimination have only yielded uncompetitive OPV performances. This study introduced a new design and processing rationale for thermocleavable side chains (TCS) to produce efficient and intrinsically stable nonfullerene acceptor (NFA)-based OPVs. D18-TCS, an isostructural derivative of the high-performance donor polymer D18-Cl with TCSs, was first synthesized and fully characterized. Its side chains can be cleaved at temperatures as low as 140 °C using an acid-catalyzed cleavage (ACC) method, in which a strong acidic additive (diphenyl phosphoric acid) is coprocessed into polymer films to catalyze pyrolysis, circumventing the high temperature (220 °C) required for conventional pyrolysis. After side-chain elimination, the newly formed D18-COOH retained optical properties similar to those of D18-Cl, while also displaying enhanced extrinsic stability in air as well as improved morphological stability to heat. In D18-TCS+Y6-based OPV devices, ACC processing consistently produced higher device efficiencies than traditional thermocleavage (TC), attributed to the milder temperature (140 °C) for cleaving side chains than thermal only (220 °C). When synergistic layer-by-layer processing was used to promote the vertical diffusion of the NFA during ACC, the performance of devices dramatically improved, with the side-chain-cleaved D18-COOH/Y6-based device to approach a 13% power conversion efficiency, with significantly higher intrinsic stability than the reference D18-Cl/Y6-based OPVs.