The research presented in this thesis focuses on the use of B-g CsSnI3 perovskite as the light harvesting semiconductor in discrete layer photovoltaic (PV) devices. Chapters 1 and 2 give a brief introduction with relevant theory, and experimental techniques respectively. Chapter 3 describes the use of B-g CsSnI3 in PV devices based on a CuIj CsSnI3j fullerene architecture, showing how device Voc is strongly dependent on the energetics at the perovskite fullerene interface, and that using excess SnI2 in CsSnI3 preparation greatly improves device stability. Chapter 4 describes the effect that different tin halides have on stabilising films of B-g CsSnI3 and on the performance of PV devices. SnCl2 was found to be the most beneficial source of excess Sn, with the champion device achieving a power conversion efficiency of over 3.5% combined with remarkable stability. Spontaneous n-type doping of the fullerene layer by SnCl2 is shown to be the reason for high device efficiency. In Chapter 5 the effect of different substrate electrodes on the stability of PV devices based on CsSnI3:SnCl2 films is described. It is shown that the stability of thin films of B-g CsSnI3 perovskite towards oxidation in air depends strongly on the choice of substrate electrode and that unencapsulated devices using ITO or semi-transparent Au as the hole-extracting electrode, without an HTL, are more stable than those using an HTL. PV devices using ITO only as the hole-extracting electrode exhibit the highest stability, with a 30% reduction in efficiency only after 20 hours testing in air for the champion device. Chapter 6 describes an investigation of A and B site substitution in CsSnI3, with particular focus on Rb partial A-site substitution. It was found that increasing the Rb content reduced lm stability, but significantly increased device Voc due to an increase in the perovskite ionisation potential.