Oxide based intermediate band materials

In this project, we developed new materials for so-called intermediate band solar cells, in a new way. The materials were studied both theoretically and experimentally. The project was based on existing theoretical results which showed that a intermediate-band solar cell based on titanium dioxide (TiO2) doped with chromium (Cr) and nitrogen (N) could achieve an efficiency of 52%, if the solar cell was illuminated with maximum concentrated sunlight. In comparison, current solar cell technology can achieve 41%, under the same conditions. During the project, we made our own calculations of the efficiency, for normal, unconcentrated sunlight, and found that for TiO2 the maximum efficiency will be approximately 44%, if the doping leads to the desired change in the material properties, compared to 33% for current technology for unconcentrated light. For the intermediate band solar cells to be more efficient, the new material must absorb a larger part of the solar spectrum. For doped TiO2, we therefore want the doping with Cr and N to give increased absorption of light. Calculations have shown that ideally there should be equal amounts of chromium and nitrogen. In addition to doping TiO2 with other element, the desired optical properties can potentially be achieved by removing oxygen from the material, to create so-called sub-stoichiometric TiO2. Visually, the increased absorption of light can be seen by the material going from being completely transparent for pure TiO2, to becoming darker when increased amounts of doping are added or increased amounts of oxygen are removed. In the project, a postdoctoral fellow carried out new theoretical calculations of the properties of Cr and N doped TiO2, and the calculations showed that the doping could also lead to unwanted effects. The material could acquire properties that made them less suitable for use in intermediate band solar cells, despite increased absorption. An exception was if the Cr and N atoms are placed close to each other, and in layers separated by TiO2 without doping. The experimental work in the project concerned fabrication and characterization of mainly undoped and doped TiO2, and sub-stoichiometric TiO2. Other materials were also characterized (sub-stoichiometric MoO3, and doped Cu2O and ZnS). The first PhD student defended her thesis in 2019 and the second in 2023. The first did advanced, X-ray-based, characterization of the new materials, both doped and sub-stoichiometric oxides. She studied how both doping and deviations from stoichiometry can lead to the formation of electronic states in the band gap of the materials. Such electronic states are the core of intermediate band materials, and it is important to be able to detect these states in a robust way. The second PhD student investigated the materials' optical properties using spectroscopic ellipsometry. He also participated in improvement of a method for fabricating thin films of TiO2 where the amount of doping varied laterally in the film. Normally, one must make a new film for each amount of doping, so the new method speeds up the research. With the improved technique, the materials could be made as thin layers of doping separated by layers of TiO2. Incorporating similar amounts of chromium and nitrogen proved to be a challenge, but after developing the new method further, we succeeded in solving this challenge. We were able to incorporate larger amounts of chromium and nitrogen than others have achieved using pulsed laser deposition. When we studied the optical properties of the films with varying doping, we found increased absorption in parts of the solar spectrum in accordance with theoretical calculations, but only for those samples with corresponding amounts of chromium and nitrogen. We have obtained promising results for several of the materials that have been studied, although much work remains before it can be concluded whether we have managed to create intermediate band materials with all the desired properties. In addition to the postdoctoral fellow and the two PhD students, 17 master's students have been involved in the project, in the period 2016 - 2023.

Både de teoretiske og eksperimentelle resultatene har stor nytteverdi for videre utvikling av mellombåndmaterialer, både lokalt og globalt. Forståelsen av hvor viktig det er å kunne kontrollere plasseringen av dopeatomene er av stor betydning, for hvordan man velger å framstille slike materialer. Vi har etablert bruken av nye teknikker i forskningsgruppa i løpet av prosjektet, som gjør oss bedre i stand til å fortsette forskningen. Dette gjelder både innen de avanserte karakteriseringsteknikkene, men ikke minst innen framstilling av lagdelte materialer og bruk av ulike bakgrunnsgasser. Dette gir oss et fortrinn sammenlignet med andre grupper som ikke har de samme mulighetene og kunnskapen. Resultatene forventes å være viktige ikke bare for de som utvikler oksid-baserte mellombånd-materialer, men også for de som forsker på andre anvendelser av krom- og nitrogen-dopet TiO2, som for eksempel foto-katalyse (brukt for rensing av luft og vann) og foto-elektrokjemiske celler (brukt for hydrogenproduksjon).

The project aims to develop materials for so-called intermediate band solar cells (IBSCs) for renewable electricity generation. Many other solar cell improvements, such as light conversion and light trapping, give only incremental increases in efficiencies (i.e. from say 20% to 22%), while intermediate band solar cells potentially increases the efficiency with 50-100% (from 20% to 30-40%, or even more for concentrated light). The research includes a new approach to the design of new intermediate band materials, where defect-engineering will be used to optimize the filling of the intermediate band, as well as the electronic properties of the material. We will utilize the huge global research effort that has been undertaken to develop materials based on titanium-dioxide, TiO2, for photo-catalytic applications, and will apply the findings, with suitable adaptations, on intermediate band solar cells. In addition, we expand this knowledge by including point defects in the TiO2, when the electronic and optical properties are calculated using a density functional approach for co-doped TiO2. Finally, we will contribute with new knowledge on the optical properties of these materials, as well as knowledge on the electronic states within the TiO2 bandgap. The material properties will be obtained by using national infrastructures for advanced materials characterization.