Dithieno[3,2-a:2?,3?-c]phenazine based hole-transporting materials for efficient perovskite solar cells: Effects of donors numbers

Incorporation of electron-deficient polycyclic aromatics into molecular skeleton has been proved to be an effective strategy for improving the device performance of organic photovoltaics. In this context, three low-cost hole-transporting materials (HTMs) endowed with dithieno[3,2-a:2 ',3 '-c]phenazine core were successfully syn-thesized and employed for perovskite solar cells (PSCs). A comparative evaluation in relation to the numbers of peripheral donors was systematically investigated by measurement of their photophysical, electrochemical and photovoltaic performance. It is revealed that the low-symmetrical WH04 featuring three triphenylamine (TPA) donors exhibited a deeper HOMO level, a higher hole-transporting capacity and smoother film morphology than the molecules with two or four terminal donors. As a result, the WH04-based PSCs realized the highest power conversion efficiency of 20.52%, accompanied with excellent long-term device stability, which is competitive with spiro-OMeTAD based devices. We believe that molecular engineering of donors numbers is envisioned as an effective strategy for constructing highly efficient D-A-D type HTMs for PSCs.