We investigated the effect of Na incorporation on the electronic properties of polycrystalline CuIn0.7Ga0.3Se2 thin films using scanning tunneling microscopy and spectroscopy. The tunneling spectra indicate a reduced in-gap density of states at grain boundaries and reveal a downward band-bending in Na-rich grain boundaries with respect to the adjacent grains, in agreement with our conductive atomic force microscopy data. It thus appears that Na passivates deep-level defects at grain boundaries and induces a downward band-bending there. Moreover, we provide evidence that Na passivates mainly Cu vacancy related defects. We suggest that the grain-boundary passivation, which reduces the recombination rate of photogenerated carriers, is at least of major importance in the well known Na-induced improvement in the efficiency of the corresponding solar cells.
Bibliographical noteFunding Information:
The authors are grateful to Thorsten Rissom, HZB, for providing the CIGSe solar cells, to Bianca Bunn, Carola Ferber, Michael Kirsch, Tim Münchenberg, and Jan Schniebs for their help with the solar-cell production. This work was supported in part by the Israel Science Foundation, the Harry de Jur Chair in Applied Science (O.M.) and the Enrique Berman Chair in Solar Energy Research (I.B.).
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- electronic properties
- grain boundaries
- scanning tunneling spectroscopy