Photovoltaic thin film solar cells based on kesterite Cu2ZnSn(Sx,Se1–x)4 compounds (CZTSSe) have reached >12% sunlight-to-electricity conversion efficiency. This is still far from the >20% record devices known in Cu(In1–y,Gay)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 (Voc) never better than 60% of the Voc 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 Voc 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 Voc 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.
Bibliographical noteFunding Information:
G.D., G.L and S.B. conceived and led the project, S.B. monitored the literature, G.L. supervised most of the experimental work, B.D., C.M., C.C and A.J. carried out the fabrication of the CZTSSe samples and devices. The I(V) and IQE data were collected by C.C., C.M. and G.L. The optical measurements to derive Eg and photoluminescence were performed by G.R and S.S. The C(V) measurements were performed by G.R. The STEM analysis was conducted by B.D. The XRD refinement analysis was conducted by A.L. The CL measurements were performed by F.D., S.B. and G.D wrote the manuscript, with contributions and feedback from all co-authors; In particular, S.S. wrote the review section, and A.W. reviewed the section. The work at Bath was supported by the EPSRC (Grant No. EP/K016288/1 and EP/L017792/1) and the ERC (Starting Grant 27757).
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- solar cells
- voltage deficit