Effect of substituted side chain on donor-acceptor conjugated copolymers

The effects of substitute side chain (alkyl or alkoxy) on optical and electrical properties of low-band-gap conjugated copolymers were investigated using poly(2,7-dihydroindeno[2,1-a]indene-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole), poly(2,7-dihydroindeno[2,1-a]indene-co-4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole) (PININE-DHTBT), and poly(2,7-dihydroindeno[2,1-a]indene-co-4,7-bis[3-(hexyloxy)-2-thienyl]-2,1,3-benzothiadiazole) (PININE-DHOTBT). Alkyl introduced PININE-DHTBT exhibits blueshifted absorption spectrum, while alkoxy introduced PININE-DHOTBT exhibits redshifted absorption spectrum. Because of steric hindrance of alkyl or alkoxy side chain, highest occupied molecular orbitals of PININE-DHTBT and PININE-DHOTBT highly localized on donor segments, thereby decreasing hole mobilities of PININE-DHTBT and PININE-DHOTBT. Consequently, despite the spectral advantage of PININE-DHOTBT for photovoltaic cells, the actual solar cell property of PININE-DHOTBT was not enhanced.