Fabrication and Characterization of Functional ALD Metal Oxide Thin Films for Solar Applications

Motivated to revolutionize our today's fossil fuels based energy production, my work concentrated on the investigation of promising low-cost materials for photoelectrochemical hydrogen production and photovoltaic electricity generation. Hydrogen presents a fully scalable energy storage solution while photovoltaics have the biggest potential for clean electricity generation. Both are combined in the hydrogen-based economy that will be introduced in Chapter 1. A clear way to achieve this revolutionary technological and societal goal is through fundamental understanding of the complex electronic properties of the most promising low-cost semiconductors offering strong visible light absorption. The modern fields of photoelectrochemical (PEC) water splitting and photovoltaics have a lot in common: materials, scientific concepts and theoretical background. In other words, their complementarity was a strong motivation for the interdisciplinary work presented in this thesis. The main focus in this thesis is on hematite, which is a promising low-cost material offering visible light absorption and the chemical robustness for photoelectrochemical water oxidation. However, it has two major drawbacks: firstly, for a semiconductor, hematite has extremely low electron and hole mobilities. This makes it challenging to collect charges that are photo-generated deep within the hematite layer and far away from the surface. Secondly, water oxidation appears to be limited by trap states located in the mid band gap region. Chapter 3 addresses these drawbacks showing that doping of hematite from the underlayer, surface passivation from annealing treatments and/or overlayers are all key parameters to consider for the design of more efficient iron oxide electrodes. By better understanding the underlying principles of over- and underlayers, I was able to design multilayered hematite photoanodes comprised of functional thin films to obtain a significant reduction in the water oxidation overpotential. Whereas hematite thin film electrodes were fabricated by ultrasonic spray pyrolysis in Chapter 3, I introduce a new atomic layer deposition (ALD) route towards crystalline, highly photoactive, phase pure and impurity-free hematite films in Chapter 4. With this thin film model system I could precisely demonstrate that only the 10 nm thick space charge region of hematite is photoactive, which presents a major challenge when considering that around 60-70 nm are needed to achieve sufficient light absorption as shown in Chapter 5. In light of this charge transport limitation, I propose and demonstrate new host-guest electrode designs that would be indispensible for the realization of high performance ALD hematite photoanodes. To complement these studies, I demonstrate in Chapter 5 the basis for an optoelectronic modeling of the hematite PEC device that helps identifying and eliminating major optical and electronic losses in the PEC cell. In Chapter 6 my work on ALD SnO2 as an electro