The Energy Level Alignment at the Buffer/Cu(In,Ga)Se2 Thin‐Film Solar Cell Interface for CdS and GaOx

Author:

Valenta Donald1ORCID,Yetkin Hasan Arif2ORCID,Kodalle Tim2ORCID,Bombsch Jakob1ORCID,Garcia‐Diez Raul1ORCID,Hartmann Claudia1ORCID,Ueda Shigenori34ORCID,Félix Roberto15,Frisch Johannes15ORCID,Bodenstein‐Dresler Lucas1ORCID,Wilks Regan G.15ORCID,Kaufmann Christian A.2,Bär Marcus1567ORCID

Affiliation:

1. Interface Design Helmholtz‐Zentrum Berlin für Materialien und Energie GmbH (HZB) Berlin Germany

2. PVcomB HZB Berlin Germany

3. Synchrotron X‐ray Station at SPring‐8 National Institute for Materials Science (NIMS) Kouto, Sayo Hyogo Japan

4. Research Center for Electronic and Optical Materials NIMS Tsukuba Ibaraki Japan

5. Energy Materials In‐Situ Laboratory Berlin (EMIL) HZB Berlin Germany

6. Helmholtz‐Institute Erlangen‐Nürnberg for Renewable Energy (HIERN) Berlin Germany

7. Department of Chemistry and Pharmacy Friedrich‐Alexander‐Universität Erlangen‐Nürnberg (FAU) Erlangen Germany

Abstract

AbstractSputter‐deposited GaOx (i.e., oxygen‐deficient gallium oxide) films are evaluated as a potential replacement for the standard CdS buffer layers in Cu(In,Ga)Se2 (CIGSe) based thin‐film photovoltaics. The energy level alignment at the GaOx/CIGSe and CdS/CIGSe interfaces are compared by means of direct and inverse photoemission. For the GaOx/CIGSe a (0.04 ± 0.07) eV (i.e., a small spike‐like) conduction band offset (CBO) and a (−3.21 ± 0.19) eV (i.e., a large cliff‐like) valence band offset (VBO) are found, which suggests a nearly ideal charge‐selective contact. The derived GaOx band gap of (4.80 ± 0.25) eV confirms its utility as a highly transparent buffer layer. However, the GaOx (with x derived to be 1.1 ± 0.1) exhibits considerable (presumably) defect‐related occupied states above the valence band maximum. It is proposed that these states may increase charge carrier recombination and decrease open circuit voltage in respective devices; also explaining why solar cells with standard CdS buffer outperform devices with GaOx buffer, despite less ideal electronic interface properties (CBO: (−0.18 ± 0.07) eV, VBO: (−0.98 ± 0.15) eV) and the smaller CdS band gap of (2.35 ± 0.22) eV.

Publisher

Wiley

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