The last two decades have seen unprecedented research progress made in the fabrication and testing of organic solar cell (OSC) devices due to, among other things, rapid growth of interest in the development of organic materials for photovoltaic applications, the ease of processing, and the prospect of achieving high power conversion efficiency (PCE) cost effectively. The effects of impurity doping at the ppm level in photovoltaic organic semiconductors, including: (i) Seven-nines purification of organic semiconductors, (ii) pn-control of single and co-deposited organic semiconductors by impurity doping, (iii) ionisation sensitisation of doping showing the doping efficiency of 100%, (iv) ppm-doping effects in the simplest n+p-homojunction organic photovoltaic cells, and (v) the Hall effect of bulk-doped organic single crystals, are discussed in Chapter One. In Chapter Two the fabrication and characterisation of perovskite-type solar cells are reviewed and summarised such as CH3NH3PbI3, [HC(NH2)2]PbI3, and CsSnI3, which are expected for solar cell materials. Chapter Three proposes an experimental method to tailor SDE and optimise the power conversion efficiency (PCE), based on the electrical transport curve. Chapter Four provides a brief history of organic photovoltaic cell devices, factors limiting stability and power conversion efficiency, fundamental parameters that have been reported to improve the general performance of the devices, and recent developments in organic solar cell devices.