The effect of alkyl substitution position of thienyl outer side chains on photovoltaic performance of A-DA′D-A type acceptors

The side chain configuration has shown significant influence on the intermolecular interaction and aggregation morphology of organic small molecule acceptors (SMAs). In this work, we designed and synthesized two isomeric A-DA ' D-A type SMAs with thienyl outer side chains attaching a 2-ethylhexyl substituent at the alpha- or beta-position (named o-TEH and m-TEH respectively) for investigating the effect of thienyl conjugated outer side chains and the alkyl substitution position of thienyl outer side chains on the properties of SMAs. Compared to o-TEH with alpha-substitution, beta-substituted m-TEH shows closer pi-pi stacking, stronger intermolecular interaction and higher electron mobility. In addition, the blend films of m-TEH with the PBQ6 polymer donor possess more suitable phase separation, enhanced molecular packing, higher and more balanced hole and electron mobilities, longer charge carrier lifetime and less charge carrier recombination than PBQ6:o-TEH based blend films. Organic solar cells (OSCs) based on PBQ6:m-TEH achieved a power conversion efficiency (PCE) of 18.51%, which is significantly higher than the PCE (16.22%) of PBQ6:o-TEH based OSCs. Up to now, 18.51% is one of the highest PCEs reported for binary OSCs, indicating that m-TEH with 2-ethylhexyl beta-substituted thienyl outer side chains is an excellent high-performance SMA for OSCs.

Automated summary

The side chain configuration of organic small molecule acceptors (SMAs) has been found to significantly impact their intermolecular interaction and aggregation morphology. In this study, two isomeric A-DA 'D-A type SMAs with thienyl outer side chains were designed and synthesized to investigate the effects of thienyl conjugated outer side chains and the alkyl substitution position on SMA properties. It was found that the beta-substituted m-TEH SMA exhibited stronger intermolecular interaction and higher electron mobility compared to the alpha-substituted o-TEH SMA. Furthermore, the m-TEH blend film with the PBQ6 polymer donor showed more suitable phase separation, enhanced molecular packing, and improved hole and electron mobilities compared to the o-TEH blend film. Organic solar cells based on PBQ6:m-TEH achieved a significantly higher power conversion efficiency (PCE) of 18.51% compared to PBQ6:o-TEH based solar cells. This study demonstrates that m-TEH with 2-ethylhexyl beta-substituted thienyl outer side chains is an excellent high-performance SMA for organic solar cells.