Transparent back contacts for chalcopyrites: Temperature dependency of Cu diffusion into i-ZnO substrates

¹ Helmholtz-Zentrum Berlin für Materialien und Energie - Albert-Einstein-Str. 15, 12489 Berlin, Germany
² Freie Universität Berlin - Arnimallee 14, 14195 Berlin, Germany

Received: 3 November 2014
Accepted: 10 January 2015

Abstract

The interface between CuInSe₂ and i-ZnO as it is needed for new chalcopyrite concepts like superstrate and tandem solar cells is investigated with X-ray photoelectron spectroscopy. A specially developed preparation permits depth profiling of the underlying i-ZnO layer. Thereby, interfacial reactions are revealed by tracing the diffusion of Cu, In and Se through the i-ZnO. Formation of an In₂O₃ layer in between i-ZnO and CuInSe₂ is shown for deposition temperatures of $480\ ^{\circ}\text{C}$ and $520\ ^{\circ}\text{C}$ . The experiments and thermodynamical ChemSage simulations indicate that this In₂O₃ layer already forms at $330\ ^{\circ}\text{C}$ during the first physical vapor deposition step. So far In₂O₃ formation was believed only to occur at temperatures above $500\ ^{\circ}\text{C}$ . An increase of Cu diffusion into the i-ZnO with deposition temperatures above $500\ ^{\circ}\text{C}$ can be explained with additional formation of Zn_kIn₂O_k+3 (k = 5 or k = 7) phases.