High-Mobility and Low Carrier Concentration Transparent Conducting Oxide Rear Contact for Bifacial CIGS Solar Cells

Abstract

The power conversion efficiency of bifacial Cu (In, Ga)Se₂ (CIGS) solar cells under rear illumination is limited by low short circuit current density (Jsc) values. This study investigates the potential of high-mobility, low carrier concentration transparent conducting oxides (TCOs) as transparent rear contacts (TBCs) to enhance the performance of CIGS solar cells under rear illumination. We first show by optical simulations that TCO with high carrier mobility and low carrier concentration reduces parasitic absorption and improves light coupling into the CIGS absorber, respectively. Then, CIGS solar cells are realized by implementing In₂O₃:Sn (ITO) and the higher performing In₂O₃:H (IOH) and In₂O₃:Zr (InZrO) as TBC. These TBCs significantly improve the optical coupling of rear-side illumination into the CIGS absorber, improving the rear external quantum efficiency maximum value from about 50% to above 80%. The optical transparency of IOH and InZrO TBC remains relatively unaffected after the CIGS growth process, outperforming ITO on this aspect as well. The observed poor rear EQE at short wavelength is ascribed to a strong rear interface recombination. Finally, a prospective analysis of realistically achievable rear Jsc gains is provided when introducing a steeper Ga gradient at the rear interface and a passivated rear contact.