Back surface reflectors for III-V tandem solar cells: A polymer-based approach and thermal stress analysis

The integration of photovoltaic (PV) systems into aircraft, drones, and vehicles requires solar cells that combine high efficiency, low mass, and mechanical flexibility. Thin-film III–V semiconductor devices on foil substrates meet these criteria; however, they are prone to heat-induced stress that can lead to cracking of the epitaxial layers, compromising device integrity. A precise optimization of layer thickness and material composition is therefore essential to ensure thermal and mechanical stability during processing and operation. In this work, we investigate the impact of heat-induced stress in III-V thin film solar cells and identify the rear side metal stabilization as a major factor for stress generation due to mismatched thermal expansion coefficients. Two dual-junction GaInP/GaAs devices stabilized by an electroplated silver layer were fabricated and characterized. The microfabricated devices include a polymer buffer that planarizes the Ag mirror and enhances photon recycling. The champion device, with a total thickness of only 15 μm (excluding front contacts), achieves a calibrated efficiency of 30.9% and a power-to-mass ratio of 2.3 W g−1, demonstrating the potential of this approach for lightweight, high-performance photovoltaic applications.