The sun provides the most abundant source of clean energy and has the potential to provide three orders of magnitude more energy than the world energy demand. However, while state of the art commercial technology is capable of harvesting solar energy with 25% power conversion efficiency (PCE), it does so at a high cost. In order to incentivize the use of solar generated electricity and thus reduce our global carbon footprint, it is therefore necessary to develop more affordable and comparably efficient technologies. Organic bulk-heterojunction and perovskite solar cells are two promising solution-processed technologies with single junction record efficiencies of 14% and 22.7%, respectively. Nevertheless, stability is the biggest concern limiting commercialization of these technologies. The first part involves developing new design criteria for stabilizing organic solar cells. Solution-processed small molecules were used as a model system because of their narrow polydispersity index, ease of purification, and PCE exceeding 10%. The operational stability of six different high performance small molecule solar cells were assessed under 1 sun intensity at maximum power in an inert environment. Two types of degradation typically seen in organic solar cells -- burn-in and linear -- were observed and resulted in a TS80 stabilized lifetime of 2-3 years. Burn-in degradation was induced by heat and light, while linear degradation was caused only by heat. From these results, we concluded that the stability of small molecule solar cells can be improved by designing molecules with higher molecular weight for thermal stability and higher crystallinity for photostability. The second part covers optimization of perovskite solar cell encapsulation for environmental stability. While perovskite solar cells are efficient, they are known to decompose in the presence of heat and moisture. They are also mechanically fragile because of mismatches in the thermal expansion coefficients between multiple layers. I will provide my insights on a holistic design of glass-glass encapsulation utilizing commercially available materials, which for the first time enabled perovskite solar cells to pass the IEC standard's damp heat, temperature cycling, and UV-exposure tests. Having demonstrated that properly packaged perovskite solar cells can be environmentally stable, I pave the way for long-term stability of perovskites in the field and bring solution-processed perovskite solar cells one step closer to commercialization.