Optimizing molecular conformation and photovoltaic property of simple asymmetrical SMAs by tuning electron-donating central subunit

Abstract

To overcome complex synthetic routes of multiple-fused-ring small-molecule acceptors (SMAs), it is crucial to develop simple fused-ring and non-fused-ring SMAs and explore structure-property relationship for high-efficiency and low-cost organic solar cells (OSCs). For this purpose, a novel SMA named BIO-4Cl was designed and synthesized, which consist of an asymmetric dual electron-donating D-D′ central block of alkoxy- indenothiophene (INT) and benzodithiophene (BDT), as well as two electron-accepting (A) terminal group of 2-(5,6-dichloro-3-oxo-2,3-dihydro-1 H-inden-1-ylidene)malononitrile. The effect of the electron-donating D-D′ central block on molecular conformation, absorp- tion, energy level, mobility, morphology and photovoltaic performance of the SMAs was primarily studied by integrating its analogue IOEH-4Cl with the cyclopentadithiophene (DTC) and INT central block. By altering the electron-donating D′ subunit in the D-D′ central block, it is found that BIO-4Cl and IOEH-4Cl behave different molecular conformation with the S-type and C-type, respectively. As a result, BIO-4Cl exhibits a blue-shifted absorption with a decreasing molar extinction coefficient, as well as a low HOMO and LUMO energy level in comparison with IOEH-4Cl. The IOEH-4Cl blend film displays better crystallinity and film morphology than the BIO-4Cl blend film using polymer PM6 as donor material. The IOEH-4Cl-based OSCs display better photovoltaic property than the BIO-4Cl-based OSCs, in which the power convention efficiency (PCE) was increased 4.75 times to 13.7%. This work indicates that fine-tuning the electron-donating subunit in an asymmetric dual electron-donating D-D′ central block is crucial to optimize molecular conformation and PCE for simple-structure asymmetric SMAs.