Significant enhancement of the photovoltaic performance of organic small molecule acceptors via side-chain engineering

To achieve efficient polymer solar cells (PSCs), it is important to increase the optical absorption coefficient and charge mobility of photovoltaic materials for obtaining a high short-circuit current density (J_sc) and fill factor (FF) in the devices without sacrificing the open-circuit voltage (V_oc). Herein, we designed and synthesized two novel narrow bandgap n-type organic semiconductor (n-OS) acceptors named POIT-M and MOIT-M by modifying the side-chains of IT-M from para-hexylphenyl to para-hexyloxylphenyl and then to meta-hexyloxylphenyl. Due to the synergistic effects of introducing oxygen atoms and varying substitution positions on the phenyl side-chains, MOIT-M shows a significantly improved absorption coefficient, stronger intermolecular π-π stacking interaction, increased crystallinity and higher electron mobility in comparison with IT-M and POIT-M, which helps to gain higher J_sc and FF in PSCs. These special features combined with the complementary absorption of the MOIT-M acceptor and wide bandgap polymer PTZ1 donor resulted in a high power conversion efficiency (PCE) of 11.6% with a V_oc of 0.96 V, a J_sc of 17.5 mA cm^-2 and a FF of 68.8% for the PSCs processed with simple thermal annealing at 120 °C for 10 min, which is one of the highest PCEs reported for additive-free PSCs and significantly higher than those of the PSCs based on PTZ1:IT-M (9.1%) or PTZ1:POIT-M (9.7%). Our results indicate that side-chain engineering is an effective way to further improve the photovoltaic performance of n-OS acceptors in PSCs.