Polyfluorene derivatives are high-performance organic hole-transporting materials for inorganic-organic hybrid perovskite solar cells

Photovoltaics based on organic-inorganic perovskites offer new promise to address the contemporary energy and environmental issues. These solar cells have so far largely relied on small-molecule hole transport materials such as spiro-OMeTAD, which commonly suffer from high cost and low mobility. In principle, polyfl uorene copolymers can be an ideal alternative to spiro- OMeTAD, given their low price, high hole mobility and good processability, but this potential has not been explored. Herein, polyfl uorene derived polymers- TFB and PFB, which contain fl uorine and arylamine groups, are demonstrated and can indeed rival or even outperform spiro-OMeTAD as efficient holeconducting materials for perovskite solar cells. In particular, under the one-step perovskite deposition condition, TFB achieves a 10.92% power conversion efficiency that is considerably higher than that with spiro-OMeTAD (9.78%), while using the two-step perovskite deposition method, about 13% efficient solar cells with TFB (12.80%) and spiro-OMeTAD (13.58%) are delivered. Photoluminescence reveals the efficient hole extraction and diffusion at the interface between CH ₃ NH ₃ PbI ₃ and the hole conducting polymer. Impedance spectroscopy uncovers the higher electrical conductivity and lower series resistance than spiro-OMeTAD, accounting for the signifi cantly higher fi ll factor, photocurrent and open-circuit voltage of the TFB-derived cells than with spiro-MeOTAD.