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, polyfluorene 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 fluorine and arylamine groups, are demonstrated and can indeed rival or even outperform spiro-OMeTAD as efficient hole-conducting 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 CH3NH3PbI3 and the hole conducting polymer. Impedance spectroscopy uncovers the higher electrical conductivity and lower series resistance than spiro-OMeTAD, accounting for the significantly higher fill factor, photocurrent and open-circuit voltage of the TFB-derived cells than with spiro-MeOTAD.