Atomic-scale structure and band-gap bowing in Cu(In,Ga)Se2

Abstract

Mixed systems such as the Cu(In,Ga)Se 2 chalcopyrite semiconductor consist of different local atomic arrangements, that is, of different combinations of first-nearest-neighbor cations surrounding the Se anions. The anion position of Cu-III-VI 2 compounds is predicted to strongly influence the material band gap. We therefore used extended x-ray absorption fine structure spectroscopy to study the atomic-scale structure of Cu(In,Ga)Se 2 as a function of composition. Based on these results, the anion position was modeled for all first-nearest-neighbor configurations using a valence force-field approach. We show that the atomic-scale structure strongly depends on the kind of first-nearest-neighbor atoms. Structural relaxation of the anion occurs with respect to both (i) Cu and group III atoms and (ii) In and Ga atoms. In both cases, the average anion displacement exhibits a nonlinear behavior with changing composition and thus results in two separate but significant contributions to the band gap bowing observed in Cu(In,Ga)Se 2 .

Local versus global electronic properties of chalcopyrite alloys: X-ray absorption spectroscopy and ab initio calculations

“…EX-AFS measurements of both CIGS and CIGSe have shown that the element-specific bond lengths are nearly constant over the whole compositional range despite the change in lattice constants. 5,12 This behavior closely resembles the findings for other mixed semiconductor systems as first reported for (In,Ga)As 27 and later confirmed for many other III-V and II-VI ternary compounds.…”

“…This means that although the band gap and thus the energy of the conduction band minimum changes, only the anion absorption edges exhibit a corresponding shift supplying relevant macroscopic electronic information whereas the cation absorption edges remain unchanged. X-ray absorption measurements of CuIn x Ga 1−x Se 2 (0 ≤ x ≤ 1) also show no appreciable shift in the Cu, Ga and In K-edge positions 5 suggesting that these findings represent general features of material systems with different cation species such as, but not limited to, chalcopyrites. Note that a similar behaviour was in fact observed in the completely different context of the investigation of magnetism and insulator/metal transitions as a function of doping for Sr doped cobaltites (La 1−x Sr x CoO 3 ).…”

“…CIGS alloys crystallize in the chalcopyrite structure 5 where each Ga, In or Cu cation is bonded to four S anions. If all atoms occupied ideal lattice sites, the Ga-S and In-S bond lengths would be identical and would change with alloy composition according to the change in lattice constants.…”

“…13 The SQS supercells contained 40, 54, or 64 atoms and they include all three possible environments for S: 2 Cu and 2 Ga neighbors, 2 Cu and 2 In neighbors and 2 Cu, 1 Ga and 1 In neighbors, as described in Ref. 5 . We did not consider defects (e.g.…”

Chalcopyrite compound semiconductors for thin film solar cells

“…The temperature dependence of the detrapping effect can be developed into a method to characterise the trap states themselves [95]. Besides affecting the doping density and the recombination behaviour, the electronic band structure is influenced by the details of the atomic structure: while the average bond lengths in Cu(In,Ga)Se 2 follow a linear Vegard's law, it was found by X-ray absorption measurements, that the In-Se and Ga-Se bond lengths remain essentially constant throughout the whole composition range between CuInSe 2 and CuGaSe 2 [96]. This leads to a non-linear displacement of the anions, which contributes to the bowing behaviour of the band gap.…”