Thin-film solar cells based on organic and perovskite semiconductors have emerged as promising energy generation technologies in recent years. However, their poor stability must be addressed before they can be widely deployed. This thesis explores the degradation mechanisms of several state-of-the-art perovskite and organic photovoltaic cells and develops strategies to improve their stability by introducing new materials, device structures, and protective coatings.

First, we studied the photostability of organic photovoltaics comprising Y-series non- fullerene acceptors—a ubiquitous class of electron acceptors among state-of-the-art high- efficiency organic photovoltaics. We found that UV light photochemically degrades these materials and induces trap formation, which causes solar cell degradation. To address this issue, we demonstrated that UV-absorbing down-conversion layers can effectively block UV radiation from solar cells and extend their lifetimes.

Next, we investigated the operational stability of UV-absorbing photovoltaics that have potential utility for transparent applications such as solar windows. Among UV-absorbing organic photovoltaics containing contorted hexabenzocoronene derivatives, we found that morphological degradation of halogenated acceptors deteriorates the active layer/electrode interface which reduces charge extraction and photovoltaic performance. This degradation was not readily mitigatable without new materials choices. We thus developed an all-inorganic UV-absorbing perovskite, CsPbCl2.5Br0.5, using thermal co-evaporation of CsCl, CsBr and PbCl2. Transparent photovoltaics employing this absorber were found to be highly stable while also demonstrating record-high average visible transmittance, a near-perfect color-rendering index, sufficient power output for low-power applications that prioritize aesthetics, large-area scalability, and high yields

Finally, we extended our study of inorganic perovskite stability to broadband-absorbing CsPbI3. We revealed that interfacial strain in CsPbI3 at the perovskite/electron-transport layer interface accelerates CsPbI3 polymorphic transformation, which is the primary degradation mode for CsPbI3 solar cells. By introducing a flexible alkyltrimethoxysilane layer at this interface, we eliminated the interfacial strain, leading to improved phase/device stability and enhanced interfacial charge transfer for higher device power-conversion efficiencies.

Collectively, this thesis elucidates the degradation processes of a variety of organic and perovskite photovoltaics and introduces strategies to mitigate their degradation. This comprehensive understanding of degradation is instructive for future accelerated aging methods on emerging thin-film solar cells to evaluate their stability prior to commercialization.

Adviser: Lynn Loo

Zoom Link: https://princeton.zoom.us/j/9449718911