Optimized Optical Field Profile in Resonant-Cavity-Enhanced a-Ge:H Nanoabsorber Solar Cells for Tandem Cell Application

Amorphous germanium-based solar cells exploiting second-order Fabry–Perot resonances can reach strong infrared absorption with an absorber thickness of either 25 nm or less. Hence, they are a promising candidate for the replacement of micrometer-thick μc-Si:H bottom cells in a-Si:H/μc-Si:H tandem configurations. Here, we present a detailed experimental and simulation-aided analysis of the optical properties of such devices, with a particular focus on their potential application in the tandem cells. A clear relation of the results to the field profile inside the cavity was found, and guiding rules for optimization are deduced. By modeling the transmission behavior of an a-Si:H top cell, we evaluate the potential of the fabricated devices for tandem cell application, allowing a benchmark against μc-Si:H solar cells. The best sample was found to enable a tandem cell performance equivalent to a μc-Si:H bottom device reaching 5% single cell efficiency.