Mesostructured Thin Film Solar Cells: Examining Hole Transfer Mechanisms and Device Stability

To meet their potential for abundant, cost-effective renewable energy production, photovoltaics must be developed which are simultaneously higher efficiency and lower cost. The rise of nanotechnology has enabled the design of new classes of solar cells, such as extremely thin absorber (ETA) and perovskite solar cells, which could ultimately meet this goal. This dissertation describes progress in the fundamental understanding of charge transfer processes in Sb₂S₃ ETA solar cells, and improving the stability of CH₃NH₃PbI₃ perovskite solar cells. ETA solar cells have the distinct advantage of very thin absorber layers which facilitate efficient charge extraction in materials with relatively low minority carrier diffusion lengths. Femtosecond transient absorption spectroscopy was employed to track minority carrier (hole) behavior in Sb₂S₃ ETA solar cells. Hole transfer out of Sb₂S₃ is found to be strongly dependent on Sb₂S₃ thickness, therefore we develop a model which allows for the comprehensive exploration of the mechanism of hole transfer and the elucidation of the rate limiting step. The broader implications of these results on photovoltaic performance are explored through measurements on TiO₂/Sb₂S₃/CuSCN solar cells. Solar cells employing the hybrid perovskite material CH₃NH₃PbI₃ have recently risen to the forefront due to their unparalleled rise in power conversion efficiency. Nonetheless, the so-called perovskite solar cells have substantial hurdles to overcome on the road to commercialization. Two issues which have risen to the forefront are the cost and stability of the hole transport material (HTM) and the stability of CH₃NH₃PbI₃ when exposed to moisture. We demonstrate that CuI, an inorganic HTM with dramatically lower cost than typical organic HTMs, shows promising stability, displays two orders of magnitude higher p-type conductivity, and reaches 75% of the performance of comparable devices featuring a typical organic HTM. Lastly, we systematically explore the degradation of CH₃NH₃PbI₃ in the presence of atmospheric moisture. It is shown that H₂O is able to complex with CH₃NH₃PbI₃, forming a hydrate. We characterize the optical and structural changes associated with this transformation, and subsequently study the deleterious effects of moisture on photovoltaic performance in light on these spectroscopic understandings in order to elucidate the mechanism of degradation.