Understanding the environmental degradation of methylammonium lead iodide Perovskite
Creators: name: Aristidou, Nicholas Paul
Publisher: name: Imperial College London
Year: 2019

description: Hybrid lead halide perovskite semiconductors have accelerated to the forefront of pho- tovoltaics. These materials possess highly desirable features including, fast charge trans- port, high extinction coefficients, large spectral overlap and solution process-ability. As a result of these, low-cost devices have emerged boasting impressive power conversion ef- ficiencies in excess of 20%. The rapid development of this technology is in part due to the materials versatility allowing numerous device configurations and fabrication techniques to be employed. Unfortunately, the excitement surrounding perovskites is hampered by their inability to withstand environmental stress. These systems have been found to ex- hibit significant performance losses and undergo irreversible material degradation when ex- posed to oxygen and light. Highlighted from these initial findings is that under these condi- tions, the reactive oxygen species superoxide can form and breakdown the perovskite crystal. A greater understanding of the mechanistic action leading to the generation of superoxide has been achieved through a powerful combination of experimental and computational results. The work has examined the role of material selection in the fabrication of devices. In addition the role of morphology of the perovskite has also been examined, where electron extraction from the perovskite layer is critical in achieving long term stability. The driving force for separation and the velocity at which electrons can be extracted are critical components in the effective- ness of an electron extraction layer in aiding stability enhancements. Rapid oxygen diffusion and iodide vacancies have been identified as key contributors to the mechanistic formation of superoxide. In order to achieve this a unique combination of isothermal gravimetric analysis and Time-of-Flight secondary ion mass spectrometry were employed. Critically these showed the rapid uptake of oxygen and the ubiquitous presence of these species after exposure to air. Inspired by these results, new methods have been developed to generate perovskite solar cells with increased performance life-time. The work herein, has also identified the impact of the selection of the organic cation and exchanging the halide upon the stability towards oxygen and light. Furthermore, the consequence of introducing moisture into the equation has been consid- ered and revealed greater detail about the mechanistic formation of superoxide from these species. The generation of superoxide in perovskite materials for photovoltaic applications is highly undesirable and persists as a key issue regarding their commercial employment. However, inspired by the fact photo-absorbers can generate superoxide a new application where the generation of the species could be used in a productive way is explored. To this end, the generation of superoxide from the organic polymer P3HT is explored. The production of superoxide from films is then harnessed to react with another species in a solution media. This simulation, leads to potential application where a contaminated solution, for example with a biological species, could be cleaned by addition of a P3HT film, oxygen and light. Here, the superoxide species would form from the film and then react and denature the contaminant.