Investigation and feasibility study of SrTiO3 thin films in high temperature solid oxide solar cells

Abstract

Providing and storing clean energy is getting more and more important, fighting global problems like climate change or air pollution. One way to do so is combining a high temperature solid oxide solar cell with a solid oxide electrolyser cell gaining a photoelectrochemical cell, which can be used for small-scale energy harvesting applications. Such micro-harvestorers could for example replace batteries in unwired minimised wireless sensor nodes (WSN), needed for the Internet of Things (IoT), which has been identified as one of the most important technologies in the near future. As small batteries also have a limited lifetime, the replacement would have environmental and economic benefits.Knowing that SrTiO3 (STO) as a single crystal has the potential of being used in high temperature solar cell applications when combining it, e.g., with a LSCr top electrode, the question arose, if also STO thin films can be used as such, making the final product even smaller and thus more economic to produce.Therefore, high temperature STO thin film based solid oxide solar cell samples were prepared and investigated to generally observe, whether STO thin films can work in such micro-solar cell applications and further to get a better understanding of electrical and defect chemical processes under operation. The preparation was done with pulsed laser deposition (PLD). The solar cell is ablated onto a 10 x 10 mm2 SrTiO3 single crystal in three steps, growing layers of (La,Sr)CoO3 (LSC) (counter electrode), SrTiO3 (absorber layer) and (La,Sr)CrO3 (LSCr) (working electrode). To analyse the samples, different methods are applied, such as electrochemical impedance spectroscopy (EIS), voltage and current measurements under UV light, transmission electron microscopy (TEM) and mass spectrometry (Online-LASIL-ICP-MS, LA-ICP-MS). Varying certain parameters like the thickness, the geometry of the STO layer as well as the laser fluence revealed that STO thin film solar cell samples are feasible and that the best results can be achieved with a STO7 % (7 % Sr excess in the target) layer with a thickness of about 840 nm using a fluence of around 1 J/cm2. Open circuit voltages up to 830 mV and short circuit currents up to 0.193 mA/cm2 were reached under ambient air conditions and 300°C set temperature. Open circuit voltages up to 870 mV and short circuit currents up 0.25 mA/cm2 are gained under synthetic air conditions at 300°C. It was found that the deposition of a dense and at the same time stoichiometric STO thin film seems to be the key for photovoltaically-active cells.However, the feasibility of reproducing the samples during preparation still poses problems. This could be due to various reasons, including the Sr excess in the STO target, the interaction between the target and the laser pulse or laser fluence variations over longer deposition times.