A sodium citrate-modified-PEDOT:PSS hole transporting layer for performance enhancement in inverted planar perovskite solar cells

The properties of the hole transporting layer (HTL) play an important role in determining the performance of inverted planar perovskite solar cells (PSCs). High performance methylammonium lead iodide PSCs were demonstrated through controlled growth of the perovskite layer and suppression of charge recombination at the perovskite/electrode interfaces. In this work, the effect of the sodium citrate modified poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) HTL on the growth of the perovskite active layer, the charge extraction properties and hence the performance of the PSCs was investigated. A power conversion efficiency (PCE) of >11.50% was achieved for low-processing temperature solution-processable PSCs made with a sodium citrate modified PEDOT:PSS HTL, and is >20% higher than that of a structurally identical control PSC with a pristine PEDOT:PSS HTL (9.13%). Photoelectron spectroscopy measurements reveal that sodium citrate solution modification partially removes the PSS on the surface of the PEDOT:PSS HTL, leading to an almost two times increase in the ratio of PEDOT to PSS as compared to that measured for the pristine PEDOT:PSS HTL. The light intensity-dependent J-V characteristics of the PSCs were measured to analyze the performance enhancement of the PSCs. The results reveal that the sodium citrate-modified PEDOT:PSS HTL benefits the efficient operation of PSCs in two ways: (1) it assists the crystal growth to increase in the perovskite domain size, and (2) it favors the charge collection in the PSCs.