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 Sb can also be beneficial in ultrathin CIGSe (Cu(In,Ga)Se2) SCs if they have a relatively high back potential barrier height Eh. SCAPS simulations predicted that a high Sb will deteriorate the SC efficiency Eff when Eh is in the range of 0–0.17 eV (Ohmic contact). Yet, when Eh is greater than 0.17 eV (Schottky contact), a high Sb 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 Sb 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 Sb 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 Eh is trivial; hence, a high Sb (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 Sb improves the efficiency. Comparing the differences of SiO2 on Mo and ITO back contacts in experiments, with the contrasting behavior of Sb on Ohmic and Schottky contacts in simulation, we conclude that Eh decides about the role of Sb 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.