The role of cation disorder in the low conversion efficiency of Cu₂ZnSnS₄ kesterite solar absorber

The power conversion efficiency of kesterite solar cells remains at a low level compared to other thin-film technologies despite years of optimization. The low efficiency is often associated with the extensive cation disorder, but recent experiments show that a higher degree of ordering does not necessarily improve the open-circuit voltage, thereby questioning the actual role of cation disorder. Through a statistical treatment of disorder in the Cu2ZnSnS4 (CZTS) absorber, we show that extensive Cu--Zn disorder alone cannot be responsible for the large Urbach tails observed in many CZTS solar cells. While cation disorder reduces the band gap as a result of the Gaussian tails formed near the valence-band edge due to Cu clustering, band-gap fluctuations contribute only marginally to the open-circuit voltage deficit, excluding Cu--Zn disorder as the primary source of the low efficiency of CZTS devices. On the other hand, the extensive disorder stabilizes the formation of Sn on Zn antisite and its defect complexes, which as nonradiative recombination centers account for the large open-circuit voltage loss in CZTS. Our analysis suggests that a further improvement of the conversion efficiency for CZTS devices is challenging given the persistent presence of catio disorder.