Due to the promising application of clean and renewable energy sources, thin-film solar cells have attracted much attention. To date, encouraging efficiencies of 19% for organic solar cells have been achieved, benefiting from the development of materials and the control of solid-state behavior. However, relative to the rapid growth of device performance, the development of device stability has lagged, making them still far from being commercially available. My thesis is to investigate the impact of solid-state behavior related to device performance and stability of organic solar cells. Chapter 1 introduces the background, methods, and progress of the organic solar cells. Chapter 2 describes modified ZnO nanoparticles as electron transport layer to improve the performance and stability of organic solar cells. Chapter 3 describes the structure and property relationship of some non-fullerene acceptors for organic solar cells. Chapter 4 describes morphological studies of non-fullerene acceptor-based organic photovoltaic active layers by X-ray scattering and solid-state nuclear magnetic resonance. Chapter 5 describes the use of X-ray scattering and solid-state nuclear magnetic resonance to understand the morphology degradation of non-fullerene acceptor-based organic photovoltaic active layers.