Vapor Deposition of Perovskite Solar Cells

Thanks to the continuous improvement of crystalline silicon (c-Si) solar cells, largely dominating the market, photovoltaic electricity is nowadays the cheapest source of energy on the market. Yet, solar energy is far from being completely harvested, as there is a fundamental limit to how much a single absorber can generate electrical power with solar irradiation. This conversion efficiency limit for one absorber is around 30%. One of the most promising ways to overcome this theoretical limit is to combine two light-absorbing materials, fabricating so-called 'tandem' solar cells.

The most realistic entry on the highly competitive photovoltaic market for a new technology is to take advantage of the already existing c-Si production lines and upgrade them to produce tandem devices. Among the best candidates to be paired with c-Si solar cells are halide perovskites, referring to the general ABX3 crystal structure (with A an organic cation, B a bivalent metal and X a halide). Hybrid organic/inorganic lead halide perovskites are extremely attractive thanks to bandgap tunability, exceptional optoelectronic properties and potential to be processed at low costs.

The objective of this work is to develop a vapor deposition process of perovskite thin films. Vapor based processes allow upscalable deposition in a controlled and solvent-free environment of thin films in a conformal manner. However, most existing vapor deposition processes are not well adapted to the organic precursors of perovskites, resulting in unreliable depositions with standard vapor processes.

After an introduction to perovskite vapor deposition, the first part of this thesis covers the basics of perovskite photovoltaics theory. The following chapter is composed of an analytical review of the existing perovskite vapor phase processes, deepened by a discussion of the linkage between theoretical considerations of thin film growth and experimental observations. The last chapter presents the experimental work achieved during this thesis.

The first part of this chapter discusses a study on the single source evaporation of metal halide precursors. Co-evaporated CsBr/PbI2 (1 : 6 molar ratio) metal halide films are compared to ball milled powders with varying relative ratios (CsBr/PbI2) (1 : 8 - 1 : 4). The powders are studied before and during the evaporation process via in situ GIWAXS, revealing no new phases in the powder mixes and thermally assisted halide exchange during the evaporation process. All deposited thin films exhibit stoichiometries different from the evaporated reference films. Yet, the 1 : 5 films enable 15% efficient solar cells, with 1.7eV perovskite absorbers.

The second part of this chapter demonstrates a simple, versatile organohalide vapor deposition process that is flexible, robust, low cost, upscalable and easy to reproduce. The growth of perovskite films based on CsBr/PbI2 templates is followed for different conditions using in situ photoluminescence and GIWAXS