The tandem device showed an orange color but also remained semi-transparent. Absorption spectra were recorded to evaluate the optical properties of all fabricated solar cells. The absorption spectra were obtained for both the dye-sensitized films and the complete solar cells and are shown in Figure . In the dye-sensitized film, we can clearly see the characteristic and complementary absorption of PB6 and D35 on film and in the final devices, as well as in the IPCE (SI, Figure ). Upon comparing the absorption spectra of the pure sensitized films to those of the functioning solar cell, we observed that the dyes remained stable during the cell fabrication process. In the absorption spectra of the n-ssDSC, we can clearly see the contributions of Spiro-OMeTAD HTM which absorbs below 400 nm. Furthermore, it is evident that there is a baseline shift in the absorption spectra for PB6-based solar cell. This shift can primarily be attributed to the scattering of light by the ZnO/ITO interfacial layer, a phenomenon that could also be visually perceived. Interestingly, the increase of Jsc with light intensity (I), that is generally described by Jsc ∝ I α , shows a quasi linear behavior for all solar cells (Figure ). A value of α = 1 indicates that all separated free carriers are extracted before recombination, and α < 1 indicates that Jsc depends on bimolecular recombination (i.e., carriers recombine before being extracted). Notably, the obtained α = 0.96 from the slope of the log(Jsc) versus log(I) plot for the p-ssDSC is much better than in a previous study with NiO-PB6 cells that only used a ZnO layer from Zn(acac)2. This indicates that the ALD ZnO layer achieves a good pore filling and reduces the Langevin recombination among the photo-generated charge carriers, likely due to a better charge extraction and transporting abilities. The spread of J-V parameters of the manufactured devices for p-ssDSC, n-ssDSC and the tandem devices are shown in Figure Notably, as expected, the series resistance of the tandem cell (400 Ωcm 2 ) is close to that of the sum of the individual devices. However, there is still some additional resistance originating from purely pressing the two cells together in the tandem device.