Development of reflective back contacts for high-efficiency ultrathin Cu(In,Ga)Se2 solar cells

Because of poor light absorption, Cu(In,Ga)Se2-based (CIGS) solar cells with an ultrathin absorber layer (<500 nm) require the development of reflective back contacts. To enhance rear reflectance in CIGS ultrathin devices, we investigate novel back contact architectures based on a silver metallic mirror covered with a thin layer of In2O3:Sn (ITO), which is fully compatible with nanopatterning for further light trapping improvements. First, numerical electromagnetic simulations of complete solar cells have been performed for a 490 nm thick CIGS absorber with various back contacts. We predict a short-circuit current density of JSC = 34.0 mA/cm2 for a 490 nm thick CIGS absorber with a silver nanostructured mirror. Second, we have fabricated and characterized 490 nm thick CIGS solar cells with transparent back contacts made of ITO, and reflective back contacts made of silver covered with ITO. Solar cells with a transparent ITO back contact exhibit an average efficiency of 10.0%, compared to 9.3% for standard molybdenum back contacts. A 5 nm thick Ga2O3 layer is revealed at the ITO/CIGS interface by transmission electron microscopy and energy dispersive X-ray spectroscopy. When silver is added, the reflective back mirror leads to a JSC improvement of 4.6 mA/cm2 (from 22.4 to 27.0 mA/cm2). These results pave the way for efficient ultrathin CIGS solar cells on reflective back contacts.