Is a passivated back contact always beneficial for Cu (In,Ga)Se2 solar cells?

Abstract Ultrathin CIGSe solar cells on TCOs (transparent conductive oxides) have many promising potential applications due to their transparency. However, their performance is strongly constrained due to the back contact, often referred to acting as a Schottky junction. In this work, we show that the current limitation due to a back Schottky junction can be responsible for the apparent photocurrent loss and suppression of the forward current, which deteriorates cell performance and leads to the S‐shaped J ‐ V characteristics, respectively. However, due to the non‐negligible photocurrent in the back Schottky diode and the current amplification effect of a transistor, cells with a reduced absorber thickness overall exhibit a relieved current suppression, resulting in a light J ‐ V curve closer to the one with an Ohmic back contact. Notably, a higher back interface recombination velocity is capable of increasing the forward current of the Schottky diode by recombination, which mitigates the limitation of current flow through the back Schottky junction and thus improves the cell performance. The results show strong implications that introducing high density defects at the back interface is favorable for CIGSe solar cells on TCOs, rather than employing point‐contact structures for interface passivation. This suggests a different path to improve ultrathin CIGSe solar cells on TCOs compared to the devices on Mo with an Ohmic back contact.