2020
In this article, we proposed an interdigitated back-contacted, or IBC, structure for ultra-thin CIGS solar cells. CIGS is a strong thin-film photovoltaic absorber, but making it thinner than one micrometre—important for material use and cost—also makes high efficiency harder to maintain. Conventional front/back-contacted cells lose light in front layers and, particularly on flexible devices, behind a front metal grid. Those losses matter much more when the absorber is already thin.
Our approach moves both electrical contacts to the rear. Electron and hole contacts sit in an interdigitated pattern below the CIGS layer, leaving the light-facing surface free of conventional window layers and front-grid shading. That creates a clear optical benefit, but it also raises a demanding electrical question: how can charge carriers move laterally to the right contacts without excessive recombination? We therefore used calibrated opto-electrical simulations, not light-absorption calculations alone.
We compared the IBC structure with a front/back-contacted reference cell with similar absorber thickness and a measured efficiency of 11.9 percent. The model was calibrated against fabricated Solliance cells, matching simulated and measured current-voltage behaviour. The proposed design combines aluminium oxide passivation, magnesium fluoride for separation, gallium-doped zinc oxide as the electron contact, molybdenum as the hole contact and a rear silver reflector.
We then varied contact geometry, the gap between contacts, absorber thickness, defect density and gallium bandgap grading. Each choice comes with a trade-off. Wider electron contacts can reduce electrical shading, yet carriers have farther to travel to reach hole contacts. A thicker absorber catches more light but can increase bulk recombination. The useful configurations are the ones that balance these effects rather than maximising one parameter.
With optimised geometry and bandgap engineering, the IBC design reached a simulated efficiency of 17 percent. At reasonably low defect density, the model projected up to 19.7 percent efficiency with an open-circuit voltage comparable to record CIGS devices. We see this as more than an optical improvement: coordinated contact design, passivation and absorber engineering can give ultra-thin CIGS cells a credible route to high performance with less material.