Research activities in organic and perovskite solar cells have recently been growing very fast due to their merits such as low cost of fabrication processes, flexibility, light weight and large-scale production. In this thesis, theoretical studies on the operation mechanisms of bulk heterojunction (BHJ) organic solar cells (OSCs) and planar perovskite solar cells (PSCs) have been carried out, with the view of enhancing their photovoltaic performance and stability. In this thesis, after the introductory chapter 1, in chapter 2 are presented the studies of efficient light absorption, exciton generation, and recombination rate in BHJ OSCs with conventional and inverted configurations. In chapter 3, BHJ OSCs with four different Urbach energies are simulated, and the combined influence of Urbach energy and charge carrier mobility on the power conversion efficiency (PCE) of OSCs is investigated. In chapter 4, the diffusion length of charge carriers in the active layer of a PSC of the structure Glass/PEDOT: PSS/ CH3NH3PbI3/PC60BM/Al is modelled, and the combined influence of applied voltage, temperature and Urbach energy on the diffusion length of charge carriers is investigated. In chapter 5, a mathematical model is developed to calculate the operating temperature of PSCs. The model has shown that by passivating the interfaces and hence reducing the density of tail states, the operating temperature of a PSC can be decreased significantly at higher applied voltages which may lead to the reduction in degradation and prevention of phase transition. We have developed a model to calculate the operating temperature of an illuminated PSC under the forward bias and that of a shaded one under the reverse bias at different ambient conditions in chapter 6. In chapter 7, the experimental works that have been done at the Hong Kong Baptist University, Charles Darwin University and University of New South Wales are presented. The results presented in this thesis are expected to provide useful insights in fabricating OSCs and PSCs with higher efficiency and stability.