Defect Engineering in Earth‐Abundant Cu₂ZnSn(S,Se)₄ Photovoltaic Materials via Ga³⁺‐Doping for over 12% Efficient Solar Cells

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Abstract

<jats:title>Abstract</jats:title><jats:p>The efficiency of earth‐abundant Cu<jats:sub>2</jats:sub>ZnSn(S,Se)<jats:sub>4</jats:sub> (CZTSSe) solar cells is considerably lower than the Shockley–Queisser limit. One of the main reasons for this is the presence of deleterious cation disordering caused by Sn<jats:sub>Zn</jats:sub> antisite and 2Cu<jats:sub>Zn</jats:sub>+Sn<jats:sub>Zn</jats:sub> defect clusters, resulting in a short minority carrier lifetime and significant band tailing, leading to a large open‐circuit voltage deficit, and hence, low efficiency. In this study, Ga‐doping is used to increase the CZTSSe solar cell efficiency to as high as 12.3%, one of the highest for this type of cells. First‐principles calculations show that the preference of Ga<jats:sup>3+</jats:sup> occupying Zn and Sn sites has a benign effect on suppressing the formation of the Sn<jats:sub>Zn</jats:sub> deep donor defects by upwardly shifting the Fermi level, which is further confirmed by deep‐level transient spectroscopy characterization. Besides, the Ga dopants can also form defect‐dopant clusters, such as Ga<jats:sub>Zn</jats:sub>+Cu<jats:sub>Zn</jats:sub> and Ga<jats:sub>Zn</jats:sub>+Ga<jats:sub>Sn</jats:sub>, which also have positive effects on suppressing the band‐tailing states. The defect engineering via Ga<jats:sup>3+</jats:sup>‐doping may suppress the band‐tailing defect with a decreased Urbach energy, elevate the minority carrier lifetime, and in the end, enhance the <jats:italic>V</jats:italic><jats:sub>OC</jats:sub> from 473 to 515 mV. These results provide a new route to further increase CZTSSe‐based solar cell efficiency by defect engineering.</jats:p>