UNSW lab kesterite cell reaches 12.6% efficiency

Inside a laboratory at the University of New South Wales, a solar cell built from copper, zinc, tin and sulfur reached 12.6% power-conversion efficiency. The wide-bandgap kesterite device tackles a persistent problem: voltage losses have long held back this family of solar materials, despite its use of earth-abundant elements.

The UNSW team focused on the defects that form as the CZTS absorber crystallizes. A copper-poor, zinc-rich environment can favor beneficial defects and suppress the defects that make charge carriers recombine. But copper-rich and zinc-poor regions can appear during processing, creating unfavorable defects and secondary phases.

The researchers adjusted the recipe rather than only the final mixture. They deposited metallic copper first, then replaced it with copper(I) sulfide, or Cu₂S, during the final minutes. Using Cu₂S throughout the process increased tin loss and created defects and pinholes, so the timed change was designed to stabilize copper near the surface without causing that additional damage. Measurements found stronger copper-sulfur bonding, greater structural order and a more uniform elemental distribution.

The resulting cell delivered an open-circuit voltage of 852.8 mV, a short-circuit current density of 21.0 mA/cm² and a fill factor of 70.1%. A certified efficiency of 12.36% was recorded, with an open-circuit voltage of 846.7 mV measured through a 0.2021 cm² aperture area. Low-temperature cathodoluminescence also indicated lower non-radiative recombination and fewer deep localized defect states than in the reference sample.

So what changes in practice? Not a product on a roof today: this is a laboratory solar cell. But the team says its method could help researchers design other compound semiconductors, including candidates for the top cell in tandem solar devices. The device maintained stable performance after 231 days in a nitrogen-filled desiccator, while its open-circuit voltage reached 65.3% of the Shockley–Queisser limit, compared with around 60% generally achieved by wide-bandgap kesterite devices, according to the researchers.