Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is frequently used as hole transport layer in planar p-i-n perovskite solar cells. Here we show that processing of a metal halide perovskite layer on top of PEDOT:PSS via spin coating of a precursor solution chemically reduces the oxidation state of PEDOT:PSS. This reduction leads to a lowering of the work function of the PEDOT:PSS and the perovskite layer on top of it. As a consequence, the solar cells display inferior performance with a reduced open-circuit voltage and a reduced short-circuit current density, which increases sublinearly with light intensity. The reduced PEDOT:PSS can be re-oxidized by thermal annealing of the PEDOT:PSS/perovskite layer stack in the presence of oxygen. As a consequence, thermal annealing of the perovskite layer in air provides solar cells with increased open-circuit voltage, short-circuit current density and high efficiency.