Lone-pair effect on carrier capture in Cu₂ZnSnS₄solar cells

Abstract

The performance of kesterite thin-film solar cells is limited by a low open-circuit voltage due to defect-mediated electron-hole recombination. We calculate the non-radiative carrier-capture cross sections and Shockley-Read-Hall recombination coefficients of deep-level point defects in Cu 2 ZnSnS 4 (CZTS) from first-principles. While the oxidation state of Sn is +4 in stoichiometric CZTS, inert lone pair (5s 2 ) formation lowers the oxidation state to +2. The stability of the lone pair suppresses the ionization of certain point defects, inducing charge transition levels deep in the band gap. We find large lattice distortions associated with the lone-pair defect centers due to the difference in ionic radii between Sn(II) and Sn(IV). The combination of a deep trap level and large lattice distortion facilitates efficient non-radiative carrier capture, with capture cross-sections exceeding 10 −12 cm 2 .The results highlight a connection between redox active cations and 'killer' defect centres that form giant carrier traps. This lone pair effect will be relevant to other emerging photovoltaic materials containing ns 2 cations. arXiv:1810.11259v1 [cond-mat.mtrl-sci]

The electrical and optical properties of kesterites

“…Trapping is also observed in TRTS measurements [88,93]. The mechanism and defects involved with this minority carrier trapping is an active area of research [94][95][96][97]. Similar results can be expected from photoconductivity decay, photovoltage decay, and related measurements, which probe the decay of excess carriers.…”

“…Next, grain boundary recombination has been reported to contribute to bulk recombination of carriers [90]. Lastly, a more traditional non-radiative recombination mechanism through deep defects in the bulk are expected to play an important role in explaining the significant non-radiative losses measured in kesterites [90,95]. A relative comparison of the various loss mechanisms discussed here-in is illustrated in the energy band diagram shown in figure 4.…”

Doping and alloying of kesterites

“…Germanium (Ge) is probably one of the most suitable and interesting group IV elements for adding into the kesterite structure, in particular at the Sn-sites. Considering the most stable oxidation states of these two elements, +4 is more likely to occur with Ge than with Sn [103], thus avoiding presence of potentially harmful +2 oxidation states [104]. Additionally, a bandgap range from 1.0 to 2.25 eV is possible with the Cu 2 Zn(Sn 1-x Ge x )(S 1-y ,Se y ) 4 alloy [97,[105][106][107].…”

“…Research efforts to identify intrinsic defect(s) negatively affecting the minority carrier lifetime and diffusion length of the kesterite absorbers should be intensified. For instance, a recent theoretical study has suggested that the sulfur vacancy V S that is stabilized by the presence of Sn 2+ can act as a non-radiative site [104,177]. This should be verified experimentally, and if confirmed, efforts can be targeted to test doping/alloying strategies that compensate these defects and stabilize the normal-valence oxidation state of Sn 4+ .…”

Emerging inorganic solar cell efficiency tables (Version 1)

“…While in the past, only the position of the defect levels in the band gap were accessible, recent progress has made prediction of carrier capture and recombination rates possible from first-principles [73]. In this way the most detrimental defects can be identified, for example Sn Zn and V S are predicted to act as giant carrier traps in Cu 2 ZnSnS 4 [74]. However, such calculations are challenging in terms of human effort and calculation time so they have been performed for very few systems.…”