A double hole-transport layer strategy toward efficient mixed tin-lead iodide perovskite solar cell

Mixed tin (Sn)-lead (Pb) iodide perovskite is considered the most promising low-bandgap photovoltaic material for pursuing both low toxicity and high performance of perovskite solar cells (PSCs). However, their performance is still lagging behind the full Pb-based analogues. In PSCs, the hole transport layer (HTL) is one of the most important fundamentals to affect the device performance. Herein, the (FASnI3)0.6(MAPbI3)0.4 PSCs with an inverted structure were successfully fabricated, where a bilayer structure of CuI/PEDOT:PSS was employed as HTL. It is observed that the CuI/PEDOT:PSS film has a smooth morphology with good interfacial contact and the introduced CuI can partly affect the crystallization behavior of (FASnI3)0.6(MAPbI3)0.4 to facilitate high-quality perovskite film growth. Simultaneously, the cascading energy alignment in the device together with the high mobility of CuI provided the efficient hole extraction and transport from perovskite to anode. Besides, the photovoltaic performance was observed to be significantly dependent on the concentration of CuI that determines its thickness, as evidenced by the electrochemical impedance spectroscopy measurement. Consequently, by using a double-layer CuI/PEDOT:PSS as HTL with the CuI concentration of 10 mg ml−1, the fabricated PSCs based on 60% Sn perovskite lead to an improved champion power conversion efficiency of ~15.75% with negligible J-V hysteresis. Importantly, the double HTL of CuI/PEDOT:PSS makes the PSCs show improved stability, which is a critical characteristic for the advancement of PSCs.