Wide-Bandgap Small Molecular Acceptors Based on a Weak Electron-Withdrawing Moiety for Efficient Polymer Solar Cells

Narrow-bandgap small molecular acceptors (SMAs) with absorption extending into the near-infrared spectral region such as ITIC derivatives are widely investigated, while the development of their wide-bandgap counterparts remains largely unexplored. Wide-bandgap non-fullerene acceptors (NFAs) are highly desirable and beneficial for constructing efficient device layouts such as ternary blend and tandem solar cells that require multiple light-harvesting materials with different regions of absorption. In this contribution, the design and synthesis of two wide-bandgap SMAs (IDT-TBA and IDDT-TBA), consisting of a weak electron-withdrawing moiety (1,3-diethyl-2-thiobarbituric acid, TBA) is presented. Compared to ITIC, this molecular design strategy results in energetically down-shifted HOMO levels and hence much enlarged bandgaps of 1.91eV for IDT-TBA and 1.78eV for IDDT-TBA, respectively. Further photovoltaic performance evaluation demonstrates power conversion efficiencies (PCEs) of 6.5% for IDT-TBA and 7.5% for IDDT-TBA, respectively, when using PBDB-T as the electron donor polymer. In addition, time-delayed collection field (TDCF) experiments suggest that both IDT-TBA and IDDT-TBA based cells exhibit field-independent charge generation with external charge generation efficiencies exceeding 90%, implying negligible geminate recombination losses. The results demonstrate that TBA units are promising and attractive building blocks as weak electron-withdrawing acceptors to construct wide-bandgap high-efficiency SMAs for efficient organic photovoltaic devices.