A research team from the University of New South Wales (UNSW) has fabricated a wide-bandgap kesterite (CZTS) solar cell using a defect regulation strategy that reportedly reduces open-circuit voltage losses. Minimizing these losses is a key challenge for kesterite photovoltaics, which are based on earth-abundant elements but have historically suffered from relatively large voltage deficits.
“Voltage is a direct measure of the energy loss inside the solar cells. Every detrimental defect reduces the voltage. Defect control determines the final efficiency and how much energy can be converted from sunlight,” the scientists said in a statement. “Improving the open-circuit voltage of of CZTS beyond its current level is essential to unlock its full potential.”
The researchers explained that previous studies have shown that a copper (Cu)-poor and zinc (Zn)-rich chemical environment can promote the formation of beneficial defects in CZTS while suppressing defects that contribute to carrier recombination. However, the formation of Cu-rich and Zn-poor regions in the CZTS film can alter the local chemical environment and promote unfavorable defects and secondary phases.
To control this process, the team sought to strengthen Cu–S bonding in the precursor, thereby stabilizing Cu and limiting its out-diffusion. In an initial approach, the researchers replaced metallic Cu with copper(I) sulfide (Cu₂S) during the co-sputtering process. Raman and X-ray photoelectron spectroscopy (XPS) measurements indicated that the modified precursor had stronger Cu–S bonding, greater structural order and reduced cation disorder.
However, using Cu₂S throughout the deposition process also increased tin (Sn) loss during sulfurization. This resulted in defects and pinholes in the absorber, making it unsuitable for solar-cell fabrication. To address this issue, the researchers used metallic Cu during the initial stage of precursor deposition and replaced it with Cu₂S during the final minutes. The approach was designed to stabilize Cu near the surface while limiting Sn loss during subsequent sulfurization.
The optimized process significantly reduced Cu out-diffusion and produced a more uniform elemental distribution. Importantly, it helped maintain a Cu-poor and Zn-rich local environment during the early stages of CZTS crystallization, when the defect structure of the absorber is established.
Low-temperature cathodoluminescence measurements showed that the optimized CZTS absorber has lower non-radiative recombination and fewer deep localized defect states than the reference sample. The optimized absorber also exhibited a stronger contribution from transitions involving shallow defects, indicating improved defect quality.
The CZTS solar cell fabricated using the proposed defect-engineering approach achieved a power conversion efficiency of 12.6%, with 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 also recorded, with an open-circuit voltage of 846.7 mV measured using a 0.2021 cm² aperture area. The device was also found to maintain stable performance after 231 days of storage in a nitrogen-filled desiccator.
“Whereas reducing photovoltage loss in wide-bandgap semiconductors remains a major challenge, we demonstrate an open-circiut voltage corresponding to 65.3% of the Shockley–Queisser (SQ) limit in this work,” the researchers stated, adding that wide-bandgap kesterite devices have generally achieved open-circuit voltages of around 60% of the corresponding SQ limit.
“The defect control strategy we’ve developed is going to be really useful for designing and optimising other compound semiconductors,” said UNSW researcher Xiaojing Hao. “I hope our defect control technology and principle can really be used by other people who are designing more top-cell candidates for tandem solar cells. The design principle isn’t only about this material. It’s about not only looking at the final recipe, we need to design from when we’re mixing the ingredients and keep them uniformly distributed. The ingredients may change from one semiconductor to another, but the design principle is the same.”
The solar cell was described in “Early-stage local chemistry regulation enabling open-circuit voltage of 847 mV in wide-bandgap Cu2ZnSnS4 solar cells,” published in nature energy.
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