Semi-planar-semi-twisted selenophen-containing narrow bandgap small molecules for efficient polymer solar cells

Polymer solar cells (PSCs) containing small molecule electron acceptors (SMAs) with twisted backbones offer many advantages over flat molecules, including the ability to move molecular energy levels, enhanced charge extraction properties, higher extinction coefficients, longer carrier lifetimes, and lower recombination rates. Collectively, these advantages contribute to the power conversion efficiency (PCE) of PSCs. However, the restricted absorption spectra of twisted SMAs present a challenge to further performance improvement. To address this issue, three acceptor-π-donor-π’-acceptor-type SMAs with indacenodiselenophene (IDSe) segment as the electron-giving unit and halogenated end groups were designed and synthesized. Taking advantage of the strong electron donating ability of the IDSe fusing ring core and the introduced isomerized π bridged segments, all SMAs exhibit red-shifted absorption in the near-infrared region and successfully generate a semi-planar semi-twisted backbone. When blended with J52, the devices of the two halogenated SMAs (h-IDSe-4F and h-IDSe-4Cl) showed preferable exciton dissociations, low recombination rates, balanced and high charge mobilities, leading to high efficiencies of 9.33 % and 9.03 %, higher than the 5.07 % of the h-IDSe-based device. This work demonstrates that tuning the planarity of the conjugated backbone, fine-tuning the core unit, and halogenation substitutions in the end groups have a significant impact on regulating the light-harvesting and performance of organic photovoltaic devices.