We simulated device characteristics of a-Si:H single junction, pc-Si:H single junction and a-Si:H/mu c-Si:H tandem solar cells with the numerical device simulator Advanced Semiconductor Analysis (ASA). For this purpose we measured and adjusted electrical and optical input parameters by comparing measured and simulated external quantum efficiency, current - voltage characteristic and reflectivity spectra. Consistent reproducibility of experimental data by numerical simulation was achieved for all types of cells investigated in this work. We also show good correspondence between the experimental and simulated characteristics for a-Si:H/mu c-Si:H tandem solar cells with various absorber thicknesses on both Asahi U-type SnO2:F and sputtered/etched (Julich) ZnO:Al substrates. Based on this good correlation between experiment and theory, we provide insight into device properties that are not directly measurable like the spatially resolved absorptance and the voltage-dependent carrier collection. These data reveal that the difference between tandem solar cells grown on Asalti U-type and Mich ZnO substrates primarily arises from their optical properties. In addition, we find out that the doped layers do not contribute to the photocurrent except for the front p-layer. We also calculated the initial efficiencies of a-Si:H/mu c-Si:H tandem solar cells with different combinations of a-Si:H and pc-Si:H absorber layer thicknesses. The maximum efficiency is found at 260 nm/1500 nm for tandem solar cells on Asalti U-type substrates and at 360 nm/850 nm for tandem solar cells on Julich ZnO substrates. (C) 2011 Elsevier B.V. All rights reserved.