Manipulation of PEDOT:PSS with Polar and Nonpolar Solvent Post-treatment for Efficient Inverted Perovskite Solar Cells

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely used as a hole-conducting polymer in many optoelectronic devices including perovskite solar cells. However, its electrical and surface properties are not well controlled during the conventional ambient annealing. Herein, we apply the solvent post-treatments, including toluene vapor annealing and ethylene glycol (EG) washing, to modify not only the electrical conductivity and work function but also, importantly, the surface composition and morphology of PEDOT:PSS thin films. We show that annealing PEDOT:PSS films in a nonpolar toluene vapor environment results in a slightly enhanced electrical conductivity and increased work function while maintaining the surface composition and morphology. The CH3NH3PbI3 perovskite solar cells using the toluene vapor-annealed PEDOT:PSS hole transporting layers (HTLs) yield a 31.8% increase in power conversion efficiency (PCE) from the control devices with the ambient condition-annealed PEDOT:PSS HTLs. All photovoltaic parameters are increased because of reduced trap states at the perovskite/HTL interface, as well as efficient and balanced charge generation, transport, and extraction rates. In contrast, washing PEDOT:PSS films with the polar EG solvent removes the PSS on the surface, increases the surface roughness, and dramatically increases the electrical conductivity by 5 orders of magnitude but slightly decreases the work function. Consequently, the CH3NH3PbI3 perovskite solar cells with EG-washed PEDOT:PSS HTLs result in a 28.6% decrease in PCE from the control devices because of the increased trap states at the perovskite/HTL interface, which leads to an inefficient hole extraction. The charge accumulation at the perovskite/HTL interface also reflects in a serious hysteresis of J–V curves in the reversed bias region. This work highlights the importance of controlling both electronic and surface properties of PEDOT:PSS HTLs for the improvement of perovskite solar cell performance.