Abstract
Nonradiative electron–hole recombination is the bottleneck to efficient kesterite thin-film solar cells. We have performed a search for active point defect recombination centers using first-principles calculations. We show that the anion vacancy in Cu2ZnSnS4 (CZTS) is electrically benign without a donor level in the band gap. VS can still act as an efficient nonradiative site through the aid of an intermediate excited state involving electron capture by Sn. The bipolaron associated with Sn4+ to Sn2+ two-electron reduction stabilizes the neutral sulfur vacancy over the charged states; however, we demonstrate a mechanism whereby nonradiative recombination can occur via multiphonon emission. Our study highlights that defect-mediated recombination does not require a charge transition level deep in the band gap of a semiconductor. We further identify SnZn as the origin of persistent electron trapping/detrapping in kesterite photovoltaic devices, which is suppressed in the selenide compound.
Highlights
Thin-film solar cells require less materials and energy to produce compared to silicon-based technologies.[1,2] Kesterite-structured tetrahedral semiconductors such as Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), illustrated in Figure 1, have attracted much attention due to the earthabundance of Cu, Zn, and Sn.[3]
In the nonradiative recombination process described by Shockley−Read−Hall (SRH)[10,11] statistics, a deep level in the band gap of a semiconductor provides an intermediate state that facilitates the capture of both minority and majority carriers
There have been a number of density functional theory (DFT) studies of point defect formation in kesterite materials.[15−22] In CZTS and CZTSe, it has been shown that the defects with low formation energies (e.g., VCu and CuZn) have defect transition levels close to the band edges
Summary
The formation energy and electronic structure of a series of point defects were calculated from first-principles within the framework of DFT.[23,24] We employed the projector-augmented wave method[25] and the hybrid exchange−correlation functional of Heyd−Scuseria−Ernzerhof (HSE06),[26] as implemented in VASP.[27]. These orbital interactions and local relaxations are common for lone pair formation in ns[2] posttransition metal compounds.[30]. The band gap of the selenide is 0.5 eV smaller than that of the sulfide, which is due to a higher valence band edge associated with Se 3p.42 The spontaneous two-electron reduction of Sn(IV) to Sn(II) stabilizes the neutral charge state of the sulfur vacancy, without any thermal donor level in the band gap.
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