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

Cu(In,Ga)Se2 is one of the most promising materials for thin film solar cells with record efficiencies of more than 20% on laboratory scale [1]. Nevertheless, many aspects of this complex multinary compound are still not fully understood. The ternary parent compounds CuInSe2 and CuGaSe2 crystallize in the chalcopyrite type crystal structure. Due to the different properties of the neighbouring cations (Cu or group III atoms), the Se anions are typically displaced from their ideal lattice sites. This subtle structural variation is predicted to have a strong influence on important material properties, most prominently the energy band gap [2]. The mixed compound Cu(In,Ga)Se2 also crystallizes in the chalcopyrite structure with the lattice constants changing linearly with composition between those of the ternary parent compounds. Locally, however, the Se anion is now surrounded by different first nearest neighbour (1st NN) configurations. Since each group III lattice site is occupied by either In or Ga, the Se 1st NN shell of defect-free material can consist of (i) two Cu and two In atoms, (ii) two Cu, one In and one Ga atom or (iii) two Cu and two Ga atoms. Given the remarkable influence of the anion position on the energy band gap, a detailed knowledge of the atomic-scale structure is therefore indispensable for a more comprehensive understanding of this highly relevant material system. Extended x-ray absorption fine structure spectroscopy (EXAFS) is a powerful technique for studying the local structural environment around the various atomic species in compound materials. EXAFS thus provides information complementary to that obtained by other techniques such as diffraction. Using EXAFS, we have studied the element-specific 1st NN distances of Cu(In,Ga)Se2 powder samples and polycrystalline thin films as a function of the In/(In+Ga) ratio [3]. The average In-Se, Ga-Se and Cu-Se bond lengths are strikingly different from each other and remain close to the values of the ternary parent compounds over the whole compositional range. This is in stark contrast to the changing lattice constants and shows that the short-range atomic arrangement of Cu(In,Ga)Se2 deviates significantly from the long-range crystallographic average. We have also simulated the structural parameters for each of the three 1st NN configurations using a valence force field approach. As expected, the anion position strongly depends on the kind of neighbouring cations and the Cu(In,Ga)Se2 system is characterized by the coexistence of different anion displacements within the material. Excellent agreement between the calculated element-specific average bond lengths and the experimental values is obtained for a slightly non-random distribution of the In and Ga cations where a mixed Cu-In-Ga configuration is preferred over the pure Cu-In or Cu-Ga 1st NN environments. Regarding the influence of the anion position on the energy band gap, two different effects have to be distinguished, namely relaxation of the Se anion with respect to (i) Cu and group III atoms and (ii) In and Ga atoms. While the former is already present in the ternary parent compounds, the latter is unique to the mixed 1st NN configuration and thus occurs only in the mixed Cu(In,Ga)Se2 system. In both cases, the average anion displacement exhibits a nonlinear behaviour with changing composition resulting in two separate yet significant contributions to the nonlinear change of the energy band gap known as band gap bowing. [1] P. Jackson et al., Prog. Photovolt.: Res. Appl. 19, 894 (2011). [2] J. Vidal et al., Phys. Rev. Lett. 104 056401 (2010). [3] C.S. Schnohr et al., Phys. Rev. B 85, 245204 (2012)