Is the Cu/Zn Disorder the Main Culprit for the Voltage Deficit in Kesterite Solar Cells?

Photovoltaic thin film solar cells based on kesterite Cu₂ZnSn(S_x,Se_1–x)₄ compounds (CZTSSe) have reached >12% sunlight-to-electricity conversion efficiency. This is still far from the >20% record devices known in Cu(In_1–y,Ga_y)Se₂ and CdTe parent technologies. A selection of >9% CZTSSe devices reported in the literature is examined to review the progress achieved over the past few years. These devices suffer from a low open-circuit voltage (V_oc) never better than 60% of the V_{oc max}, which is expected from the Shockley-Queisser radiative limit (S-Q limit). The possible role of anionic (S/Se) distribution and of cationic (Cu/Zn) disorder on the V_oc deficit and on the ultimate photovoltaic performance of kesterite devices, are clarified here. While the S/Se anionic distribution is expected to be homogeneous for any ratio x, some grain-to-grain and other non-uniformity over larger area can be found, as quantified on our CZTSSe films. Nevertheless, these anionic distributions can be considered to have a negligible impact on the V_oc deficit. On the Cu/Zn order side, even though significant bandgap changes (>10%) can be observed, a similar conclusion is brought from experimental devices and from calculations, still within the radiative S-Q limit. The implications and future ways for improvement are discussed.