Dopant-free dicyanofluoranthene-based hole transporting material with low cost enables efficient flexible perovskite solar cells

Low cost, highly efficient dopant-free hole transporting materials (HTMs) are highly desirable for the commercialization of perovskite solar cells (PVSCs). Herein, a facile synthetic route is designed to prepare two new D-A-D-type HTMs (BTF5 and BTF6) with low lab synthetic costs by making dicyanofluoranthene as the key intermediate. The rational structure modifications of donor subunits enable a significant optimization of photophysical, charge transporting properties and final device performance for resulting HTMs. BTF6 with metamethoxyl substitutions exhibits more matched energy levels with perovskites and much enhanced hole mobility in comparison to BTF5 with para-methoxyl substitutions, thereby leading to significantly distinct device efficiencies as dopant-free HTMs in inverted PVSCs, 20.34% for BTF6 vs. 11.42% for BTF5. In addition, the unsatisfactory crystallinity of perovskite films atop BTF5 is found to be another major reason for its significantly poor device efficiency. More encouragingly, it is further demonstrated that BTF6 can be suitable for fabricating dopant-free flexible PVSCs towards a promising PCE over 18% with a low hysteresis, which is the highest value for flexible devices based on small molecule HTMs, and even can be comparable to the best PCEs reported from self-doped PEDOT:PSS.