Charge transport layers and passivation for improved efficiency and stability of n-i-p perovskite solar cells

Thesis

Perovskite solar cells have become a promising photovoltaic technology for harvesting energy from the sun. However, despite their low-cost processing and high performance, a few issues remain for their wider application. One of the main concerns is their stability. Commercializable solar photovoltaic technologies must remain stable under constant illumination and high-temperature not only for a few days as they would be tested in a lab, but for several years. In particular, low-cost, dopant-free, and stable hole-transporting materials need to be found to replace Spiro-OMeTAD, due to its instability issues. Furthermore, better passivation strategies need to be discovered and employed.

The primary aim of this project was to look for alternative charge-transfer materials to replace those typically used in a regular architecture solar cell device, and to make the perovskite absorber material more stable by additive engineering. To achieve this, various hole-conducting materials and passivation molecules were tested, and those which improved the film quality were further characterized and tested in devices for their performance and stability. The lessons learned from these studies led to the fabrication of a very stable solar cell architecture.

Chapter 4 focuses on a pair of alternative hole-conductors that are dopant-free and low in cost. They were found to perform just as well as spiro-OMeTAD whilst being more stable. Chapter 5 discusses an often-overlooked additive approach of applying aromatic amines to passivate the perovskite absorber. Our average benzylamine-modified perovskite devices maintained 80% of their initial efficiency over 2,400 hours in a 65°C 1 sun aging test. Meanwhile, the average control devices without additives degraded to 45% of their initial efficiency. Chapter 6 investigates the potential of applying a fullerene-based self-assembling monolayer to stabilize the electron-transporting layer/perovskite interface. However, the results of the fullerene self-assembling monolayer-modified interface were less stable. Despite that, the reasons for their instability were investigated and a future research direction is proposed to improve them.

The novel charge-transporting materials and bulk/interface modifications studied in this thesis are promising solutions for stabilizing the regular architecture of perovskite solar cells. Moreover, the investigation into why certain passivation methods work and others fail can help us design and select better materials and passivation methods for more durable solar cells.

Authors: Zhou, S

• DOI: 10.5287/ora-wvgk8pzze
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: charge-transport layers; perovskite passivation; solar cell stability; metal-halide perovskite; n-i-p perovskite solar cells
• Subjects: Photovoltaic effect; Photovoltaic power generation; Solid state physics; Methylammonium lead iodide; Perovskite solar cells; Photovoltaic cells