Perovskite ternary halides in solar cell applications

The global energy consumption increases annually. Although the current supply of fossil and nuclear fuels is enough to meet the growing energy demand it is more sustainable for the society to rely more on renewable energy sources because of the environmental concerns of fossil fuel combustion and nuclear fission. One of the promising renewable energy sources is solar power. Currently the competitiveness of solar power with the conventional energy sources is limited by the expensiveness of the solar cells. The most successful solar cell technologies require expensive fabrication methods and materials. However, this situation can drastically change with the emergence of perovskite ternary halide solar cells. Perovskite ternary halides possess optical and electronic properties required for efficient light absorption in solar cells. In addition, they consist of abundant inexpensive elements and can be deposited using easy and low-cost sol-gel methods. During the last four years, the efficiency of perovskite solar cells was improved by a factor of 4 and the record holding devices now reach 16 % efficiency. Due to the novelty of perovskite solar cells there are formidable gaps in the knowledge of perovskite ternary halide properties and mechanisms of perovskite solar cell operation. Literature review in this thesis focuses on classifying known perovskite ternary halides, their properties and deposition methods as well as highlighting discrepancies and most important knowledge gaps. Special attention is also devoted to different types of perovskite solar cells and their working principles. The literature review is complemented by calculations on the existence and the optical properties of not yet studied perovskite ternary halides, and on lattice matching of different known perovskite halides in the context of multijunction perovskite solar cells. Conventional perovskite halide deposition methods are inexpensive but also limited to small area substrates. In order to reach economic feasibility the perovskite deposition methods must be scalable to large area substrates. In the experimental part of this thesis, an attempt is made to solve the scalability challenge by developing perovskite ternary halide deposition processes for techniques that have scalability as an inherent property. These techniques are atomic layer deposition and electrodeposition. Although an electrodeposition process for cesium tin(II) iodide was developed, the morphology and phase of the deposits along with the stability of the electrodeposition solutions were difficult to control. For atomic layer deposition processes the volatility of the halides presents a challenge, however encouraging preliminary results were obtained in the cases of binary iodide deposition such as cesium iodide and copper(I)iodide. These results can serve as a basis for the future research of both binary and ternary halide atomic layer deposition.