To date, substrate-configuration metal-halide perovskite solar cells (PSCs) fabricated on opaque substrates such as metal foils provide inferior efficiencies compared with superstrate-configuration cells on transparent substrates such as glass. Herein, a substrate-configuration PSC on planarized steel is presented. To quantify the differences between the two configurations, a 15.6%-efficient n–i–p superstrate-configuration PSC is transformed step wise into a substrate-configuration cell. Guided by optical modeling, the opaque Au electrode is replaced by a transparent MoO3/thin Au/polystyrene dielectric–metal–dielectric electrode. The semitransparent device affords efficiencies of 15.4% and 11.4% for bottom and top illumination, respectively. Subsequently, substrate-configuration PSCs with a metal bottom electrode are fabricated on glass and planarized steel, using a thin MoO3 interlayer between the Au bottom electrode and the SnO2 electron transport layer. The glass-based substrate-configuration cell provides 14.0% efficiency with identical open-circuit voltage and fill factor as the superstrate cell. The cell on planarized steel reaches 11.5% efficiency due to a lower fill factor. For both substrate-configuration cells, the lower short-circuit current density limits the efficiency. Optical modeling explains this quantitatively to be due to absorption and reflection by the top electrode and absorption by the organic hole transport layer.