Snaith group

Abstract:

This thesis explores the development of narrow-bandgap, lead-tin (PbSn), perovskite solar cells (PSCs), focusing on the enhancement of performance and stability through two fabrication techniques: solution processing and vacuum thermal evaporation. It also covers how to build, optimise, and upgrade customised evaporator design and how to optimise co-evaporation recipe of perovskites. It covers several key aspects of the fabrication process, device characterisation, and stability testing.

The first chapter introduces the urgency of renewable energy, particularly photovoltaics, as a solution to global warming. The research focuses on perovskite solar cells, specifically the PbSn perovskites, which makes them ideal for tandem solar cells.

Chapter two provides a theoretical foundation for the research, explaining the working principles of photovoltaic cells, and focusing on perovskite materials, their crystal structures, and bandgap tunability. It also covers the two main fabrication methods-solution processing and vacuum evaporation-highlighting the advantages and disadvantages of each.

Chapter three describes all experimental methods and characterisations used throughout the entire thesis.

Chapter four presents an innovation in interface engineering, introducing alumina nanoparticles buffer layer for solution-processed PbSn perovskites. This buffer layer prevents direct contact between the perovskite absorber and the top electrode, mitigating detrimental effects on device performance and stability. The applied interface improves device maximum-power-point efficiency from 10.3% to 15.3% by enhancing the conformality of the electron transport layer and reducing recombination losses. This leads to an increase in the steady-state open-circuit voltage by 0.1 V, while also enhancing overall efficiency, reproducibility, and stability, with a six-fold improvement in operational stability.

Chapter five details the building and optimisation of a customised thermal evaporator, critical for achieving high-quality, reproducible thin films. Several steps that the author took to build and fine-tune an evaporator are shown in the chapter.

Chapter six explains the way to optimise the co-evaporation process for PbSn perovskites. By systematically adjusting the evaporation rates of CsI, PbI₂, the Sn alloy, and FAI, a reproducible recipe for high-performance PbSn PSCs is successfully developed. Additionally, the new methodology of evaporation Sn content, using the alloy of Sn (the mixture of SnI₂ and SnF2) was developed by the author. The chapter also covers single SnI₂ evaporation without SnF₂, which showed promising results but posed reproducibility challenges.

Chapter seven offers a direct comparison between solution-processed and thermally evaporated PbSn perovskites. The evaporated films demonstrate smoother, more compact structures without typical SnF₂ aggregation on the surface from the solution processing. Devices fabricated using thermal evaporation show superior efficiency and stability, achieving a maximum-powerpoint efficiency of 17.01%, compared to 14.33% for solution-processed devices. The steady-state open-circuit voltage of thermally evaporated devices reaches approximately 0.8 V, surpassing the 0.74 V observed in solution-processed devices, along with a 0.04 increase in the quasi-fill factor. Stability tests under light and heat stress under open-circuit conditions reveal that evaporated PbSn devices have 4 times superior stability than their solution-processed counterparts.

Overall, this thesis concludes by highlighting the potential of vacuum evaporation as a scalable, stable, and high-efficiency fabrication technique for PbSn PSCs, recommending further exploration of all-perovskite tandem applications. The thesis also advances the understanding of PbSn perovskites, focusing on thermal evaporation, interface engineering, performance enhancement, and long-term stability.