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

by Jeffrey Arnold Christians

Institution: University of Notre Dame
Department: Chemical Engineering
Degree: PhD
Year: 2015
Keywords: solar cells; photovoltaics; hole transfer; spectroscopy; perovskite; methylammonium lead iodide
Record ID: 2058865
Full text PDF: http://etd.nd.edu/ETD-db/theses/available/etd-03252015-114911/


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 Sb<sub>2</sub>S<sub>3</sub> ETA solar cells, and improving the stability of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> 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 Sb<sub>2</sub>S<sub>3</sub> ETA solar cells. Hole transfer out of Sb<sub>2</sub>S<sub>3</sub> is found to be strongly dependent on Sb<sub>2</sub>S<sub>3</sub> 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 TiO<sub>2</sub>/Sb<sub>2</sub>S<sub>3</sub>/CuSCN solar cells. Solar cells employing the hybrid perovskite material CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> 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 CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> 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 CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> in the presence of atmospheric moisture. It is shown that H<sub>2</sub>O is able to complex with CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>, 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.