Density of states characterization of TiO₂ films deposited by pulsed laser deposition for heterojunction solar cells

The application of titanium dioxide (TiO₂) in the photovoltaic (PV) field is gaining traction as this material can be deployed in doping-free heterojunction solar cells with the role of electron selective contact. For modeling-based optimization of such contact, knowledge of the titanium oxide defect density of states (DOS) is crucial. In this paper, we report a method to extract the defect density through nondestructive optical measures, including the contribution given by small polaron optical transitions. The presence of both related to oxygen-vacancy defects and polarons is supported by the results of optical characterizations and the evaluation of previous observations resulting in a defect band fixed at 1 eV below the conduction band edge of the oxide. Solar cells employing pulsed laser deposited-TiO₂ electron selective contacts were fabricated and characterized. The J-V curve of these cells showed, however, an S-shape, then a detailed analysis of the reasons for such behavior was carried out. We use a model involving the series of a standard cell equivalent circuit with a Schottky junction in order to explain these atypical performances. A good matching between the experimental measurements and the adopted theoretical model was obtained. The extracted parameters are listed and analyzed to shed light on the reasons behind the low-performance cells. [Figure not available: see fulltext.]