In-Device Assessment of the Nanometric i-ZnO Layer: A Case Study of CZTSe-Based Thin-Film Solar Cells

Nanometric zinc oxide (ZnO) layers are widely employed in optoelectronic devices due to their low cost, non-toxicity, and facile deposition by various methods. In Cu2ZnSn(S,Se)4 and Cu(In,Ga)Se2 thin-film photovoltaic (TF PV) technologies, this layer has been proposed to act as an electron transport layer protector, a shunt path blocker, and to facilitate the band alignment between the electron transport layer and the front transparent conductive oxide contact. However, its specific role in TF PV devices and its influence on technology scalability have not yet been fully clarified. This mainly arises from the difficulty in analyzing nanometric, wide band gap, non-conductive layers buried within multilayer structures. The present study develops a fast, non-destructive approach for the in-device characterization of the ZnO layer based on Raman spectroscopy, using kesterite-based devices as an example. The results reveal that the grain size of ZnO is a key factor in determining the maximum device performance, particularly the open-circuit voltage (with a 20 mV increase), and that even subtle fluctuations across the sample surface in ZnO grain size and crystalline quality induce variability in shunt resistance and short-circuit current, thereby defining overall solar cell performance. This study demonstrates the applicability of the proposed approach for the in-device analysis of ZnO properties and highlights the importance of their control for scaling up TF PV technologies.