Dissertation

Improving the Stability of Halide Perovskites for Photovoltaic Applications and Solar Fuel Generation

Poli, Isabella

2019

Overview

Halide perovskites have become a popular material to fabricate photovoltaic devices for the conversion of sunlight into electricity. Perovskite solar cells have shown extraordinary performance, reaching power conversion efficiencies of over 24% in less than a decade. Some of the reasons for their success are the high light absorption and the possibility of using low cost solution-based fabrication. Perovskite devices are notably efficient, but they still suffer from instabilities that challenge their commercialisation. Indeed, perovskite solar cells degrade when exposed to moisture, high temperature and light, causing irreversible loss of efficiency. In this work, different strategies for improving the stability of perovskite solar cells have been studied. Humidity is one of the most invasive factors that affects perovskite solar cell stability. Water molecules easily penetrate through the extraction layers reaching the perovskite structure and irreversibly degrading the absorber film. Moisture-induced degradation of perovskite solar cells can be minimised by modifying the top perovskite surface with interfacial hydrophobic thin layers and by passivating the grain boundaries. In this work, for example, the perovskite surface is treated with CH3NH3PbI3 nanocrystals capped with long chain ligands (oleic acid and oleylamine), significantly enhancing the film’s hydrophobicity. Bulkier and larger organic cations, like tetrabutylammonium, are also incorporated through the absorber perovskite thin film, passivating grain boundaries and improving the stability of perovskite solar cells when exposed to ambient conditions without encapsulation (relative humidity higher than 50%). Another method to limit the moisture-induced degradation of perovskite solar cells is through encapsulation. Here, a flexible and robust graphite sheet is used to protect halide perovskite devices from water. Instabilities are also manifested as light-induced defect formation and ion migration through the perovskite film, which can lead to material degradation. It has been shown that under-coordinated surface sites and grain boundaries act as defect reservoirs in perovskite films. Photo-instabilities can therefore be limited by passivating the surface of perovskite films. Here, 5-aminovaleric acid iodide is used to effectively passivate the surface defects of methylammonium lead iodide, photostabilising the perovskite film under continuous illumination, eliminating the formation of lightinduced defects. Light-induced ion migration also causes halide segregation in mixed halide (I and Br) perovskite compositions. Here, it is shown that by incorporating 5-aminovaleric acid iodide in triple cation mixed halide perovskites to passivate grain boundaries, halide segregation is arrested for over 1 h under continuous illumination. Finally, perovskite structures are unstable when exposed to high temperatures. Previous reports have shown that methylammonium lead iodide films tend to degrade over time when heated at 85 °C in ambient air through volatilization of the organic cation. In this work, the use of fully inorganic perovskite materials obtained by substituting Cs for the methylammonium cation considerably improve the thermal stability of perovskite films, which are found to be stable during heating in air at temperatures higher than 350 °C. Not only the MAPbI3 perovskite film itself, but also the solar cell device as a whole is particularly unstable when exposed to high temperature. The organic hole extraction layers are indeed prone to thermal instability and undesirable side reactions with metal contacts tend to be induced at high temperatures. To avoid degradation of the hole transporting materials and side reactions with metals, hole-conductor free architectures made by infiltrating porous layers of carbon, ZrO2 and TiO2 are used. In this work, carbon solar cells with photovoltages as high as 1.45V are achieved by using CsPbBr3 as infiltrating absorber material.

• Publisher: ProQuest Dissertations & Theses
• Subject: Grain boundaries / High temperature / Humidity / Photovoltaic cells