Control of Phase Separation of Benzothienoisoindigo-Benzodithiophene Copolymer for Organic Photovoltaics

Abstract

A new low-bandgap polymer composed of benzothienoisoindigo (BTIDG) and benzodithiophene (BDT) was designed and synthesized. By virtue of unsymmetrical BTIDG moiety, BDT-BTIDG copolymer exhibits moderate electrochemical properties in both bandgap (1.5 eV) and the highest occupied molecular orbital (HOMO, -5.4 eV). They are 0.4 eV wider and 0.5 eV deeper than those of previous symmetric thienoisoindigo (TIDG)-BDT analogue, respectively. Although the BDT-BTIDG is highly soluble in common organic solvents like chlorobenzene, the blend film of phenyl-C₆₁-butyric-acid-methyl ester (PCBM) caused a large phase separation over 200 nm spatial scale, leading to a structural color due to light interference. We have mitigated the phase separation to some extent using chloroform and solvent additive, giving the PCE of 0.83% for BDT-BTIDG:PCBM = 1 : 2. This work indicates that solubility of copolymer is not directly relevant to optimal bulk heterojunction morphology.