2019
In this study, we proposed and optically investigated a back-contacted design for ultra-thin CIGS solar cells. CIGS is a high-performing thin-film photovoltaic material, but reducing the absorber thickness is important for lower material use and cost. The problem is that a submicron absorber captures less light and becomes more vulnerable to losses in front layers and the front metal grid. We asked whether placing both electrical contacts at the rear could remove those losses while still collecting electrons and holes effectively.
In a conventional cell, light passes through front layers that absorb some of it, while the metal grid shades part of the active surface. Our interdigitated back-contacted design places alternating electron and hole contacts behind a 750-nanometre CIGS absorber. The front is freed from contact shading and parasitic absorption, leaving more high-energy photons for the absorber. But this also makes carrier collection demanding: charges must reach the correct rear contact without excessive recombination.
We studied carrier collection with band-diagram modelling and optimised the optical structure using rigorous three-dimensional simulations. The design uses passivating aluminium oxide, low-index magnesium fluoride for antireflection, gallium-doped zinc oxide for electron collection, molybdenum for hole collection and a rear silver reflector. We assessed high-aspect-ratio front textures and a double-layer antireflection coating that works with the natural CIGS morphology.
For a realistic textured front surface, the model predicted a photocurrent density of 38.84 mA/cm². That is more than a 38 percent improvement in optical performance over the reference front/back-contacted cell and within 7.7 percent of the Green absorption benchmark. Further texture optimisation produced even stronger results in the detailed analysis.
Moving contacts to the rear is not simply a geometric rearrangement. It asks for careful passivation, doping, contact spacing and material choices. We see this work as design guidance for a CIGS architecture that uses less absorber material while getting far more from the light that reaches it—an important step towards thinner, efficient and industrially viable thin-film solar cells.