Chalcopyrite-based devices show highest conversion efficiencies among present thin film architectures with values of 20% at laboratory scale. This outstanding performance has been achieved for quaternary Cu(Inx,Ga1-x)Se2 (x˜0.7) compound material. However, a strong correlation between the performance and the gallium content or, in other words, low versus high bandgap materials has been recognized. One critical issue in this discussion is the formation of a copper-depleted near-surface phase with 1:3:5 and 1:5:8 stoichiometries. In earlier reports, surface phases with corresponding compositions have been found on CuInSe2, CuGaSe2 and Cu(Inx,Ga1-x)Se2 thin films. These near-surface phases show a positive influence on the performance of cells based on low bandgap Cu(Inx,Ga1-x)Se2 material due to n-type inversion and band gap widening compared to bulk properties. A tendency towards a neutral or even a negative impact of the near-surface phase on wide band gap material (high gallium content) has recently been reported . Nevertheless, the structural models of copper-poor chalcopyrite-related compounds have been controversially discussed in literature but a stannite-type structural model is most suitable as will be presented. In any case, the relation of the structural properties between chalcopyrite and 1:3:5 phases is crucial for the performance of related devices.
In this contribution we will report about the structural analysis of the Cu(Inx,Ga1-x)3Se5 solid solution series by means of anomalous x-ray scattering using synchrotron radiation, powder and single crystal neutron diffraction. Contributions of the isoelectronic species Cu+ and Ga3+ could be separated by these experiments. Bulk samples synthesized from the elements and heat treated at 650°C after the main reaction step - the latter in order to allow equilibrium structure formation - were investigated. Structural data like lattice parameters, tetragonal distortion and cation distribution were obtained for the complete Cu(Inx,Ga1-x)3Se5 solid solution series. The stannite-type structural model was assigned to all members of the investigated 1:3:5s which will be strengthened by simulations. We observed that the tetragonal distortion vanishes for compositions close to a gallium content as used for highest efficiency Cu(Inx,Ga1-x)Se2 devices. However, the tetragonal distortion depends critically on the cation distribution which is in turn controlled by the thermal history of the sample, as we have recently reported for pure CuGaSe2 . This means that we can plot a direct correlation for the misfit between chalcopyrite and 1:3:5 phases depending on the gallium content and the thermal treatment of the considered thin films. These results will widen the understanding of the chalcopyrite-based thin film photovoltaic devices.
 S. Lehmann et al., Phys. Stat. Sol. A (in press)