| dc.description.abstract | Intermediate band solar cells (IBSCs) have a theoretically conversion efficiency that is approximately 50 % higher than for the solar cell technologies dominating today. IBSCs are based on a new type of materials, where an additional, intermediate, energy band is present between the valence band and conduction band.
Chromium and nitrogen (Cr + N) co-doped titanium dioxide (TiO2) have been proposed in literature as a candidate for such an intermediate band material. However, the ideal amount of doping for use in an IBSC is unknown.
The aim of this PhD project has therefore been to utilize combinatorial pulsed laser deposition (PLD) to deposit (Cr + N) co-doped TiO2 with a continuous compositional spread (CCS). The films were deposited on 50.8 mm diameter silicon substrates, and the amount of doping was controlled by separate ablation of a TiO2 target and a CrN target.
An improved method for the design of films made using combinatorial PLD was developed. Both CCS films and films with a uniform composition and thickness were designed and deposited. A high agreement between the designed/simulated and measured thickness, of both uniform (undoped) TiO2 films and (Cr + N) co-doped TiO2 CCS films was obtained.
For the CCS films, it was challenging to obtain the desired equal amounts of Cr and N doping. Initially, the CCS films were grown in an oxygen background gas, and we found that these films had a considerably lower nitrogen content than intended.
Consequently, we developed a deposition routine to improve nitrogen incorporation by ablating the CrN target in nitrogen background gas and capping the deposited material with TiO2 also ablated in nitrogen background gas.
We found that the films with the capping contained both chromium and nitrogen at much higher concentration levels, than for the films deposited in oxygen.
The characterization of the optical properties of the CCS films showed a sub-bandgap optical absorption at energies relevant for a TiO2 based intermediate band material, and in good agreement with what has been reported in literature, from density functional theory calculations.
The characterization of the micro-structure of the films revealed that the sub-bandgap absorption correlated with the local crystal quality of the film and could be associated with a crystalline phase present for high Cr and N doping concentrations.
At the same time the capping deposition routine had negative effects on the general crystalline quality. Further optimization of the nc-PLD process is needed to obtain higher crystalline quality. | en_US |
