Perovskite-based tandem solar cells can increase the power conversion efficiency (PCE) of conventional single-junction photovoltaic devices. Here, we present monolithic perovskite/CIGSe tandem solar cells with a perovskite top cell fabricated directly on an as-grown, rough CIGSe bottom cell. To prevent potential shunting due to the rough CIGSe surface, a thin NiO x layer is conformally deposited via atomic layer deposition on the front contact of the CIGSe bottom cell. The performance is further improved by an additional layer of the polymer PTAA at the NiO x /perovskite interface. This hole transport bilayer enables a 21.6% stabilized PCE of the tandem device at ∼0.8 cm 2 active area. We use TEM/EDX measurements to investigate the deposition uniformity and conformality of the NiO x and PTAA layers. By absolute photoluminescence measurements, the contribution of the individual subcells to the tandem V OC is determined, revealing that further fine-tuning of the recombination layers might improve the tandem V OC . Finally, on the basis of the obtained results, we give guidelines to improve monolithic perovskite/CIGSe tandems toward predicted PCE estimates above 30%.