  Thin-film solar cells suffer from various types of recombination, of which
leakage current usually dominates at lower voltages. Herein, we demonstrate
first a three-order reduction of the shunt loss mechanism in planar
methylammonium lead iodide perovskite solar cells by replacing the commonly
used hole transport layer
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with a
better hole-selective polyarylamine. As a result, these cells exhibit superior
operation under reduced light conditions, which we demonstrate for the extreme
case of moonlight irradiance, at which open-circuit voltages of 530 mV can
still be obtained. By the shunt removal we also observe the VOC to drop to zero
after as long as 2 h after the light has been switched off. Second, at higher
illumination intensities the dominant losses in the PEDOT:PSS-based cell are
ascribed to surface recombination and are also proven to be substantially
minimized by instead employing the polyarylamine. We attribute the reduced
shunt and surface recombination to the far better suited semiconductor
character of the polyarylamine, compared to that of PEDOT:PSS, efficiently
blocking electrons from recombining at this electrode.
