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The gallium gradient in Cu(In,Ga)Se2 (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co‐evaporation processes, plays a key role in the device performance of CIGS thin‐film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X‐ray measurements. In addition, the gallium grading of a CIGS layer grown with an in‐line co‐evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth‐dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co‐evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi‐Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross‐sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper‐vacancy‐mediated indium and gallium diffusion in CuInSe2 and CuGaSe2 were calculated using density functional theory. The migration barrier for the InCu antisite in CuGaSe2 is significantly lower compared with the GaCu antisite in CuInSe2, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co‐evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd.

Formation enthalpies and migration barriers of the copper vacancy in CuInSe2, CuGaSe22, CuInS2, and CuGaS2 are calculated by means of density functional theory with a screened-exchange hybrid functional of the Heyd–Scuseria–Ernzerhof type. The band gaps of all chalcopyrite phases are very well described by the hybrid functional using a single value for the Hartree–Fock screening parameter. The defect formation enthalpies of the copper vacancy in CuInS2 and CuGaS2 are around 0.8 eV higher than in CuInSe2 and CuGaSe2. This results in the absence of Fermi-level pinning for CuInS2 and explains a reduced tendency of CuInS2 and CuGaS2 to form ordered defect compounds. The calculated migration barrier of the copper vacancy in CuInSe2 is 1.26 eV and of comparable magnitude for CuGaSe2, CuInS2, and CuGaS2. From this data we estimate a diffusion coefficient for CuInSe2 and show that it is in agreement with measurements of diffusion in stoichiometric single crystalline samples when direct experimental methods are used.

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Formation enthalpies and migration barriers of copper interstitials and Frenkel pairs in CuInSe 2 (CIS) are determined by first-principles calculations within density functional theory using the nonlocal screened exchange Heyd-Scuseria-Ernzerhof (HSE06) functional. Interstitials occur on four symmetrically inequivalent sites with formation enthalpies of 0.17-0.38 eV, which are much lower than previously reported values based on local approximations. A direct interstitial and indirect interstitialcy diffusion mechanism with migration barriers as low as 0.22 and 0.34 eV are identified. The results provide evidence that the fast interstitial diffusion of copper is important for understanding metastabilities, Fermi-level pinning at interfaces, electric-field-induced creation of p-n junctions, and widely varying experimentally measured diffusion coefficients in CIS devices.