Physical and Chemical Approaches to Improving the Stability of Perovskite Solar Cells

Abstract

In recent years, perovskite solar cells (PSCs) have emerged as one of the most promising photovoltaic technologies. Their compatibility with low cost, simple fabrication techniques, high performance, and industrial scalability make them attractive for commercialization. However, moisture can cause serious damage to PSCs, resulting in complete device failure in a period of hours to days. Improved lifetimes are necessary for their future success. This thesis focuses on my efforts to improve the moisture stability of PSCs using two different approaches. The first approach focuses on applying hydrophobic barrier layers as hole-transport layers (HTLs) to improve the lifetime of PSCs, whereas the second approach is to find a stable perovskite composition. The first section of this thesis focuses on polythiophene-based HTLs. According to previous studies, hydrophobic HTLs like poly(3-hexylthiophene) improve the stability of the underlying perovskite layer by blocking moisture ingress. This section discusses the synthesis of four poly(3-alkoxythiophenes) with different side chains having different degrees of hydrophobicity. The effect of the side chain is discussed in terms of its ability to protect thin films of MAPbI3. The second section builds on this work. Here, the problems with the poor device performance of the polythiophene HTLs are addressed. A new device architecture is introduced which uses a poly(3-hexylthiophene) nanowire network in a poly(methyl methacrylate) matrix as the HTL. Due to the incorporation of the poly(methyl methacrylate) matrix, there was a large increase in the stability of the device towards both liquid and vapor-phase water. The third portion of this thesis investigates the decomposition processes of different well-known perovskite compositions. It focuses on the perovskites themselves and screens different perovskites for their moisture stability using in situ absorption spectroscopy and in situ grazing iii incidence wide angle x-ray scattering. It provides a better understanding of perovskite degradation processes and takes us one step closer to PSCs with longer lifetimes. This thesis discusses two approaches to improve the lifetime of PSCs. As moisture instability is an intrinsic problem of the PSCs, a stable perovskite composition is necessary for longer-lived devices. Similarly, it is also essential to develop better barrier layers to prevent moisture ingress.