Fullerene-free polymer solar cells enabled with a PhI-based wide band gap donor polymer: promoting efficiencies via acceptor screening and device engineering

Abstract

The development of a wide bandgap polymer for the preparation of high-performance fullerene-free polymer solar cells (PSCs) has been a hot topic. PBDT-PhI is a typical wide band gap polymer, which has been initially reported in our previous study (Polym. Chem., 2013, 4, 2174) and recently applied in fullerene-free PSCs, affording the efficiency of 8.31% (Macromolecules, 2017, 50, 8928). In this study, by judiciously selecting acceptors and systematic device optimizations, we promoted the efficiencies of PBDT-PhI-based fullerene-free PSCs beyond 10% for the first time. We first selected a series of commercial acceptors, e.g., IT-4F, ITIC, ITCT, and IEICO-4F, to match PBDT-PhI. Good complementary absorption was observed between PBDT-PhI and the four acceptors. Ultraviolet photoelectron spectroscopy (UPS) tests revealed suitable energy level alignments between PBDT-PhI and the acceptors, except for IEICO-4F; primary device preparation was carried out using chloroform (CF) as the spin-coating solvent and 1,8-diiodoocane (DIO) as the additive, and the efficiencies of 8.18%, 7.99%, 8.86%, and 3.90% for IT-4F, ITIC, ITCT, and IEICO-4F, respectively, were obtained. Based on PBDT-PhI/ITCT, further device optimizations were conducted using high-boiling point chlorobenzene (CB) and CB/CF mixed solvents to spin-coat the active layers, and the enhanced efficiencies of 9.17% and 10.17%, respectively, were obtained. The best performance of the PBDT-PhI/ITCT-based devices prepared using the CB/CF mix solvent was ascribed to the highest charge dissociation rates and the weakest recombination losses.