Towards Competitive Thin-Film Chalcogenides for Indoor Photovoltaics—Theoretical Insights and Experimental Innovations

Concomitant with the rise of the Internet of Things (IoT) and the increased demand for autonomous IoT nodes, Indoor Photovoltaics (IPV) has become one of the leading emerging applications of PV. In a market dominated by amorphous silicon and with a new generation of highly efficient metal-halide Perovskite and organic-based IPV cells, developing highly performing thin film for IPV application is a challenge for the community to stay relevant in the race. While inorganic thin films and particularly chalcogenide-based absorbers enjoy several advantages in terms of tuneability, flexibility, non-toxicity and being free of critical raw materials, the use of case of IPV with its high variability in illumination conditions brings new challenges, some of which are particularly specific to the thin film chalcogenide.
In this presentation, we will identify the three main challenges for the thin film community to solve, as well as secondary challenges, and we will explore pathways to address each of those using optoelectronic modelling. Firstly, the question of bandgap matching will be central and we will demonstrate how tuning the bandgap of thin film chalcogenide absorber can lead to vastly different performance. Additionally, we will demonstrate in a quantitative way how shunt resistance can impact performance under low light illumination by increasing the ratio between shunt current and photocurrent, and we will illustrate that point using a modified detailed balance model. Finally, the role of parasitic absorption by typical materials used for chalcogenide solar cells will be quantitatively assessed through transfer matrix optical modelling, showing that the choice in partner layer is essential and that the colour temperature of the light source can drastically change performance depending on which layer is part of the stack.
Those modelling results and conclusions are supported by experimental observations of state-of-the-art Kesterite, Sb₂S₃, and CdTe. We demonstrate that with the proper optimisations, experimental device efficiencies exceeding 18% in indoor conditions are possible for Kesterite and Sb₂S₃ while providing a clear pathway for efficiencies above 25%. The question of performance resilience is particularly addressed, as maintaining high performance even in low injection conditions is critical for IPV. Importantly, those experimental results are analysed in the context of their final application of power source of IoT nodes. Using an innovative approach developed by a consortium of more than ten international research centres, and incorporating variable lighting conditions representative of real-world environments, we illustrate in a simple and visual way how our experimental cells can power an array of real IoT devices by finding a common figure of merit between IoT and IPV. All the tools and computer resources used for this demonstration will be openly shared through the presentation.
This contribution, blending theoretical analysis, experimental demonstration and innovative methodologies, will provide important and straightforward guidelines for the thin film chalcogenide community to fabricate sustainable and competitive IPV cells possibly overcoming the most advanced technologies of the field.