Ultrathin Cu(In,Ga)Se2 Solar Cells with a Passivated Back Interface: A Comparative Study between Mo and In2O3:Sn Back Contacts

Point-contact passivation layers have been proven beneficial in most solar cells (SCs). However, the latest theoretical simulations suggested that a high back-contact recombination velocity S-b can also be beneficial in ultrathin CIGSe (Cu(In,Ga)Se-2) SCs if they have a relatively high back potential barrier height Eh. SCAPS simulations predicted that a high S-b will deteriorate the SC efficiency Eff when E-h is in the range of 0-0.17 eV (Ohmic contact). Yet, when E-h is greater than 0.17 eV (Schottky contact), a high S-b can also diminish the current limitation arising from the back Schottky diode since it has a reverse direction to the main p-n junction. Therefore, a high S-b can support the carriers in passing the Schottky barrier via recombination, thus enhancing the cell performance. This work aims to verify the simulation prediction in practical experiments. To achieve different S-b values, we fabricate SiO2 passivation layers with point contacts of various dimensions by nanosphere lithography. The passivation effects are studied comparatively on Mo and ITO (In2O3:Sn) back contacts. The emphasis is on Eh, which is marginal for Mo but acts Schottky-like on ITO. We show that for Mo-based solar cells, the E-h is trivial; hence, a high S-b (without SiO2 passivation) deteriorates the efficiency. In contrast, on ITO, the reference sample without SiO2 shows less current limitation than the passivated ones, implying that a high S-b improves the efficiency. Comparing the differences of SiO2 on Mo and ITO back contacts in experiments, with the contrasting behavior of S-b on Ohmic and Schottky contacts in simulation, we conclude that E-h decides about the role of S-b in ultrathin CIGSe SCs. These findings deepen the understanding of the Schottky back contact and pave the way for future optimization of bifacial semitransparent ultrathin CIGSe SCs.