Optimisation of rear reflectance in ultra-thin CIGS solar cells towards >20% efficiency

In order to decrease their cost and the use of rare metal elements, thin film solar cell thicknesses are continuously reduced at the expense of their efficiency, due to a lack of absorption for long wavelengths. Optimisation of cells rear reflectance (Rb) thus becomes meaningful to provide non-absorbed light a second chance to be harvested by the active cell layer. In this sense, we present a way to keep the rear reflectance in advanced Cu(In, Ga) Se2 (CIGS) cell as high as possible while keeping in mind the progress already done regarding the rear passivation techniques. We show that introducing a stack of thin Al2O3 and aluminium between the CIGS layer and the rear molybdenum electrode increases Rb up to 92% in the long wavelength 800-1100 nm range. Several other stacks, using MgF2, SiO2 or TiO2, are optimised in order to investigate the best trade-off between passivation, material consumption and performances, resulting in Rb ranging from 42% (moderate case) to 99% in the best case. Those CIGS rear interface reflectance optimisations were performed by using a standard transfer matrix method (TMM) in the long wavelength range. Seven interesting stacks are then analysed for solar cell performances using SCAPS simulation software to understand the impact of rear reflectance on short circuit current density (Jsc) and eventually on the cell efficiency (η), for ultra-thin CIGS absorber thicknesses (<1 μm). Based on these results, we propose Rb optimisation to achieve Jsc > 40 mA/cm2 and η > 20% with a 500 nm-thick CIGS absorber film using CIGS-Al2O3-Mo stack, where the Al2O3 thickness can be chosen in between 104 and 139 nm. This way, we can ensure good rear reflectance (Rb = 65%) and reduced interface recombination while being industrially feasible with present technologies.