Eliminating S-Kink To Maximize the Performance of MgZnO/CdTe Solar Cells

Comparing to the traditional CdS buffer layer, zinc magnesium oxide (ZMO) offers the following advantages for CdTe-based thin-film solar cells: it introduces a spike to conduction band offset, which reduces interface recombination that is beneficial for increasing open-circuit voltage (VOC) and decreases parasitic optical absorption of the buffer layer that is favorable for enhancing short-circuit current (JSC). However, ZMO/CdTe thin-film solar cells often show the so-called S-kink behavior in their current–voltage curves, making it difficult to reproduce the expected benefits. Here, we show that S-kink can be successfully eliminated, and improved VOC and JSC can be simultaneously achieved if the CdCl2 treatment process is conducted in oxygen-free atmosphere. As a result, the device efficiencies increased from 9.2% to 16.1%. Our device characterizations and simulations reveal that a sufficiently high electron density of the ZMO buffer layer is critical to eliminate the S-kink, which is achievable through an oxygen-free CdCl2 treatment.