Modeling of optical losses in graphene contacted thin film solar cells

For the first time, we model the optical loss of short-circuit current density, J_sc, and conversion efficiency, η, of thin film solar cells made of CdS/CdTe and CIGS/CdS chalcogenide materials having a graphene nanolayer electrode. The model is based on known materials optical constants e.g. refractive index and extinction coefficient reported in literature. Graphene has a superior transmission, conductivity and thermal stability than conventional transparent conductive oxide (TCO) and metallic back contacts. We compare our modeling results with experimental data reported in the literature and re-optimize the thickness of device components. The interface reflections result in ΔJ_sc=-8.5% whereas absorption in graphene, ZnO, CdS layers (with typical thicknesses) results in ΔJ_sc=-25% for glass/graphene/ZnO/CdS/CdTe, and about 24% for glass/MO/CIGS/CdS/ZnO/graphene solar cells. CIGS materials with higher bandgap (1.38 eV vs. 1.17 eV) cause a higher ΔJ_sc. Graphene/ZnO contacted devices show lower efficiency loss with respect to graphene monolayer without ZnO layer. ZnO has a middle value of optical properties which balances the light refraction at the interface of graphene and chalcogenide materials.