Optimizing the Alkyl Side-Chain Design of a Wide Band-Gap Polymer Donor for Attaining Nonfullerene Organic Solar Cells with High Efficiency Using a Nonhalogenated Solvent

Molecular design is one of the fundamental strategies to improve the photovoltaic performance of organic solar cell (OSC) devices. The alkyl side chains of materials play an essential role in optimizing the molecular packing and blend morphology for the improvements of device performance. Herein, two new polymer donor materials, namely, PBSF-A12 and PBSF-D12, were designed and synthesized, based on PBDB-T-SF by extending the length of the alkyl chains on the donor and acceptor units, respectively. The influence of the alkyl side chain on the photophysical properties of materials and device performance has been studied in detail. Both PBSF-A12 and PBSF-D12 exhibit excellent solubility in nonhalogenated as well as common solvents compared with PBDB-T-SF. Furthermore, PBSF-D12 exhibited superior face-on molecular packing that is more conducive to charge transport. As a result, the OSCs based on PBSF-D12:IT-4F blend film exhibited a power conversion efficiency (PCE) of 13.4% vs 10.9% for PBSF-A12:IT-4F-based devices. More importantly, the active layer thickness and the area have little effect on the device performance. The device can still maintain a satisfactory PCE when the thickness changes from 70 to 250 nm and the active area changes from 0.04 to 1.00 cm2. Thus, we believe that PBSF-D12 is an encouraging donor material and the PBSF-D12:IT-4F blend film is a promising competitor for future large-area printing using nonhalogenated solvents for attaining high-performance OSCs.