Halide perovskite semiconductors find use in a broad range of optoelectronic applications including photovoltaic solar cells and light-emitting diodes. In such devices the semiconductor is sandwiched in between interlayers for charge transport, extraction, or injection. When it comes to hole transport layers, the conducting polymer PEDOT:PSS has become an ubiquitous material. The halide perovskite thin film is commonly obtained by crystallization of precursors using solution processing on top of PEDOT:PSS. It is demonstrated here that such a widely spread technique is actually affecting the electrical properties of the underlying conducting polymer. The halide perovskite layer and precursors are drastically doping the PEDOT:PSS, its conductivity being increased by two orders of magnitude from 0.2 to 20 S cm?1. The depth of penetration of halide dopants is determined to be higher than 150 nm, superior to the usual thickness of PEDOT:PSS films. This phenomenon has important impact on diode leakage currents, on emission patterns of perovskite LEDs and on overestimated photocurrent density in perovskite solar cells embedding the PEDOT:PSS interlayer. © 2021 Wiley-VCH GmbH