Publication date: 10th April 2024
Photovoltaic (PV) cells are highly attractive for converting abundant solar radiation directly into electrical energy. However, common PV cells operate close to room temperature. It may be of interest to also have PV cells operating under harsh conditions, e.g. at temperatures above 150 °C. Photovoltaic cells based on heterojunctions between SrTiO3 (STO) and perovskite oxides such as La0.9Sr0.1CrO3-δ (LSCr) exhibit high photovoltages (>1 V at 350 °C) under UV illumination. Moreover, the photocurrents produced by such cells continually increases during operation, leading to a “self-enhancement” of the cells’ performance.
In this contribution, this self-enhancement effect is investigated and a mechanistic explanation is derived. Model cells were prepared by growing LSCr thin films on STO single crystal by pulsed laser deposition and characterized by electrochemical impedance spectroscopy (EIS) and DC measurements. We distinguish two effects governing the cells’ self-enhancement: A first process, independent of the surrounding atmosphere, is related to a stoichiometry polarization of the entire STO single crystal upon current flow. A second process is highly sensitive to contaminants in the surrounding atmosphere and is strongly accelerated in pure, synthetic atmospheres. We suggest that it is related to photo-oxidation of the STO single crystal. EIS revealed that both processes result in a decrease of both the electronic transport resistance through the STO single crystal and, to a lesser degree, the space charge resistance of the photo-active junction. Operando EIS under illumination and with DC bias was used to record the cells' photo-power characteristics and identify individual loss processes. Owing to the illumination induced strong increase of the STO single crystal conductivity, the photo-currents of such enhanced cells are limited by the space charge resistance.
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