Microstructural Analysis of CuInSe2 Thin Films by Grazing Incidence X-Ray Diffraction
L. Leppin, R. Gunder, H. Stange, S. Schorr
Helmholtz Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
Chalcopyrite-type Cu(In,Ga)Se2 (CIGSe) polycrystalline thin films have been shown to be suitable candidates for absorber layers in a highly efficient conversion of solar energy into electric current. Today thin film solar cells with CIGSe compounds as absorber layer have the highest efficiency among all thin film solar cell technologies with record efficiencies above 21% . The presence of sodium within the absorber has been found to beneficially influence the efficiency of CISe solar cells . Therefore a detailed understanding of the influence of sodium on the microstructure of the CISe absorber, which affects the charge carrier mobility and recombination, is desirable.
As an attempt to achieve this understanding twenty-one CISe thin film samples were prepared by a 3-stage thermal co-evaporation process. By interrupting the process at different points in time three thin film series were produced having a Cu-poor, Cu- rich and slightly Cu- rich composition, respectively. Each series consists of seven samples with a distinct sodium content, which was controlled by varying the thickness of a NaF-precursor layer between the Mo back contact and the absorber. For a depth resolved investigation of the microstructure (i.e. micro-strain and domain size) grazing incidence X-ray diffraction was performed. In addition, a possible preferred orientation of the grains within the absorber was semi-quantitatively determined by X-ray diffraction using symmetric Bragg-Brentano geometry and applying the Lotgering method .
An increase of the sodium content was observed to cause an increased micro-strain. The highest micro-strain values were found at the surface of the absorber, which were attributed to segregated secondary phases, and at the absorber-back contact interface, which were attributed to a distortional lattice mismatch. The micro-strain was also found to correlate with the chemical composition, showing lowest values for samples being closest to stoichiometry.
 Jackson, P., Hariskos, D., Wuerz, R., Kiowski, O., Bauer, A., Friedlmeier, T. M. and Powalla, M. (2015), Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%. Phys. Status Solidi RRL, 9: 28–31. doi: 10.1002/pssr.201409520
 Chirila, P. Reinhard, F. Pianezzi, P. Bloesch, A. R. Uhl, C. Fella, L. Kranz, D. Keller, C. Gretener, H. Hagendorfer, D. Jaeger, R. Erni, S. Nishiwaki, S. Buecheler, and A. Tiwari, Nature Materials 12, 1107 (2013)
 Lotgering, F., Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structures—I. Journal of Inorganic and Nuclear Chemistry, 1959. 9(2): p. 113-123