The world is facing immense challenges such as climate change and the depletion of non-renewable resources, making renewable sources of energy essential for a sustainable future. Organic solar cells are emerging as a promising technology; however, their stability requires significant improvement. The nanostructure of the active layer evolves over time, especially during heating, leading to a degradation in device performance. The focus of this thesis is to improve the thermal stability of the active layer.Firstly, the thesis studies the impact of mixing on glass formation by introducing the concept of kinetic fragility to organic semiconductors. Model systems of up to eight perylene derivatives are investigated that demonstrate an unprecedented ability to form a stable molecular glass due to aggregate formation. Next, the thesis discusses the impact of isomers on glass formation, which is illustrated with an anthradithiophene-based compound. Binary mixtures of isomers were also found to form aggregates that stabilize the liquid state. In addition, the thesis describes fragility studies of doped systems and establishes that chemical doping can affect the glass formation of a semiconducting polymer. The doped polymer shows a strong tendency for glass formation which is assigned to restricted motion of oxidized polymer chains. Furthermore, the thesis analyzes mixtures of organic photovoltaic acceptors. Binary mixtures of two indacenodithienothiophene-based acceptors are found to co-crystallize, while mixtures of three to five fused-ring non-fullerene acceptors exhibited a reduced tendency to crystallize. Finally, the thesis discusses the use of acceptor mixtures for improving the thermal stability of organic photovoltaic devices. Ternary solar cell devices with two acceptors are discussed that show a stable nanostructure and improved thermal stability compared to binary devices. The thesis also explores hexanary devices that consist of five acceptor molecules, which exhibit excellent thermal stability. Therefore, the use of multicomponent acceptor mixtures is found to be a powerful tool for creating thermally stable organic solar cells
