Berlin 2018 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 17: Focussed Session: Atomic Scale Characterization

HL 17.5: Talk

Tuesday, March 13, 2018, 11:00–11:15, EW 202

Structural and electronic properties of AlkInSe2/CuInSe2 (Alk = K, Rb, Cs) interfaces from density functional theory calculations — •Maria Malitckaya, Hannu-Pekka Komsa, Ville Havu, and Martti Puska — Department of Applied Physics, Aalto University, P.O. Box 11000, Espoo, Finland

The efficiency of Cu(In, Ga)Se₂ (CIGS) - based solar cells has increased very fast, thanks to the alkali post deposition treatment (PDT). The latest record efficiency of CIGS solar cells has been achieved by using heavy alkali (K, Rb, and Cs) PDT. After K, Rb, and Cs PDT, chalcopyrite solar cells exhibit improved junction quality, open-circuit voltages and fill factors. However, the physical reasons behind these effects are still under discussion. At the microscopic level CIGS absorbers demonstrate changes in surface composition and morphology after Alk PDT. This effect has been associated with formation of Alk-In-Se secondary phases. Using first-principles calculations within the density functional theory, we have considered the role of alkali metal atoms in the efficiency enhancement. We have investigated the most important parameters of AlkInSe₂ phases, secondary phase formation during the PDT process and AlkInSe₂/CuInSe₂ interfaces.We have found that conduction band offsets are small for all orientations of the interfaces. The increase of the surface band gap seen in XPS measurements can be explained by the formation of AlkInSe₂ phases. In the case of K-PDT the measured band gap of 2.5 eV is consistent with the calculated value for monoclinic KInSe₂.