Recent research efforts on solution‐processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation–recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.

Organic–inorganic perovskite solar cells have experienced a remarkable development. In a short period of time power conversion efficiencies have jumped to values of more than 22%. However, the stability of these devices is an important subject. The stability of CH₃NH₃PbI₃ perovskite films is investigated using visible and ultraviolet light in oxygen atmosphere and in vacuum. Illumination in O₂ atmosphere results in a swift degradation. Oxygen acts as a catalyst decomposing methylammonium ions (CH₃NH₃⁺) into CH₃NH₂ and hydrogen. In vacuum, another degradation mechanism is observed. Prolonged illumination of the samples with photons from blue and UV light‐emitting diodes also results in dissociation of the methylammonium ion into CH₃NH₂ and hydrogen. In both cases the resulting molecules are highly mobile at room temperature and diffuse out of the samples. The light‐induced dissociation of CH₃NH₃⁺ is accompanied by the generation of localized defects in the band gap of the perovskite. Furthermore, the experimental data clearly show that the molecular orbitals of CH₃NH₃⁺ are not in resonance with the energy bands of the perovskite lattice.