Thin Film Deposition Processes for Halide Perovskites

I believe that replacing fossil fuels with renewables is vital for preserving a habitable Earth and promoting peace. An obsession with globalization and lack of commitment to the preservation of our planet have backfired. The crises of the last decade have taught us that we cannot carelessly rely on burning fossil fuels while disregarding the changes we are causing to the planet. Nor can we bathe in ignorance anymore, thinking that relying on corrupt authoritarian regimes to provide us with these fossil fuels will not have consequences. Russia’s invasion of Ukraine showed that the money paid for these fossil fuels is used to fund wars and to obstruct, sabotage and blackmail our countries.

Step away from fossil fuels

There is no single panacea solution that could free us from reliance on fossil fuels. Industry, electricity, and transportation account for most fossil fuels used. Alternative fuels for industry and transportation, such as hydrogen, can be produced using renewable electricity. While smaller-scale renewable electricity sources, such as wind and hydropower, enable grid flexibility, solar power remains the most realistic source for providing the bulk of the required electricity. Consequently, the global production capacity of solar cells has exponentially increased over the last few decades. To support this growth, attract investments, and promote the transition to solar power, the photovoltaic industry has dedicated significant research and development efforts to improve solar cell efficiency and reduce costs.

New generation of solar panels

We need to develop new solar cell materials to maintain the increase in solar cell production capacity and accelerate the implementation of solar power. Silicon, the mainstay of the photovoltaic industry, is expected to reach its efficiency improvement capacity within the next decade. Further advancements necessitate a transition to tandem solar cells, which combine silicon with a second solar cell absorber material, resulting in a higher maximum efficiency ceiling than single absorber solar cells. Expanding solar power into new application areas requires physical flexibility that brittle silicon cannot provide, creating opportunities for new solar cell materials.

Over the last decade, halide perovskites have developed into leading candidate materials for tandem and flexible solar cells. However, the implementation of perovskite photovoltaics requires addressing the challenges of their toxicity, stability, and scalability. These challenges also hinder the use of perovskites in applications beyond photovoltaics, such as microelectronic components and sensors, where they could offer lowered power consumption. Decreasing energy consumption is another way to decrease fossil fuel usage. 

The literature part of my thesis examines the current state of the perovskite field. The first section discusses the fundamental properties of perovskites and their connection to the unprecedently rapid development of efficient perovskite devices. The second section explores the current status of perovskite applications. The final section focuses on the challenges faced by perovskites and the progress made in addressing them.

In the experimental part of the thesis, I describe how we developed deposition processes for perovskite thin films that tackle the scalability challenge. Years of work culminated into electrodeposition and atomic layer deposition (ALD) processes for CH3NH3PbI3 and CsPbI3 perovskites as well as processes for binary compounds like CuI, CsI, PbCl2, PbBr2, PbI2, and PbS. These binary compounds are needed to tune perovskite properties or as auxiliary materials in perovskite devices. In the end I give my view on how viable the developed processes are and what we should focus on in future for implementing them in practice.

Pioneering work in the field of ALD

Prior to my work, there were no known ALD processes for metal bromides or iodides. Devising the chemistry capable of delivering these materials and simultaneously unlocking these huge compound groups for the whole ALD community was no easy feat. It required a lot of hard work, patience, help and support. I would not be able to pull this off without continuous backup from my supervisors: Docent Marianna Kemell, Professor Mikko Ritala and Professor Emeritus Markku Leskelä.

I am also grateful to all my resourceful collaborators at Helsinki Accelerator Laboratory, Department of Food and Nutrition of the University of Helsinki, Aalto University, VTT Oulu, Saint Mary's University and Carleton University as well as to our amazing HelsinkiALD research group. I am also thankful for the financial support from the Doctoral Programme in Materials Research and Nanoscience (MATRENA), the Faculty of Science of the Univerisity of Helsinki, Research Council of Finland, Otto A. Malm Foundation, Emil Aaltonen Foundation, Walter Ahlström Foundation and the Finnish Foundation for Technology Promotion.