Indacenodithiophene-based conjugated polymers incorporating alkylthiophene side chains: Improvement of organic solar cell performance

Abstract

Varying the structures of conjugated side chains can be an efficient approach toward tailoring the optoelectronic properties of conjugated polymers. In this study, we synthesized four indacenodithiophene (IDT)-based side chain conjugated donor–acceptor alternating polymers. We introduced four different acceptor units (difluorobenzothiadizole: PIDTHT-FBT; difluoroquinoxaline: PIDTHT-QF; dicyanoquinoxaline: PIDTHT-QCN; diketopyrrolopyrrole: PIDTHT-DPP) to study the effects of the conjugated side chains on the optoelectronic properties (absorption spectra, energy levels, hole mobility) of the polymers and their resulting organic photovoltaic (OPV) devices. The energy levels of the highest occupied molecular orbitals for the new polymers were deeper when they featured alkylthiophene side chains. Films of these materials blended with PC71BM exhibited sufficiently high space-charge-limited current (SCLC) hole mobilities (>1.2 × 10−5 cm2 V−1 s−1) for efficient charge carrier extraction. The greatest OPV performance was that of the device incorporating PIDTHT-QF, with a power conversion efficiency of 5.15% under illumination with AM 1.5G solar light (1000 W m−2). Thus, side chain conjugated (two-dimensional) IDT polymers incorporating optimized acceptor groups are prospective materials for OPV applications.