Elsevier

Thin Solid Films

Volumes 403–404, 1 February 2002, Pages 212-215
Thin Solid Films

Ga2O3 segregation in Cu(In, Ga)Se2/ZnO superstrate solar cells and its impact on their photovoltaic properties

https://doi.org/10.1016/S0040-6090(01)01526-7Get rights and content

Abstract

Cu(In, Ga)Se2 (CIGS) layers were grown on ZnO for superstrate thin film solar cells. With analytical electron microscopy, a continuous Ga accumulation of 15–60 nm thickness was observed at the ZnO–CIGS interface. This accumulation was identified as a separate layer between ZnO and the CIGS absorber layer and coincides with an increase in the oxygen concentration. X-Ray photo-electron spectroscopy measurements at the ZnO–CIGS interface revealed a decrease in the binding energy of Ga with increasing depth away from the interface. This change in binding energy is attributed to a change in Ga from an oxidized state to the selenate state in the bulk absorber. A comparison of different substrates and Rutherford back scattering measurements on the ZnO layer strongly suggest loosely-bound oxygen in ZnO, leading to the oxidation of Ga, and hence, to the segregation of Ga2O3 at the ZnO–CIGS interface.

Introduction

Cu(In, Ga)Se2 (CIGS) thin film solar cells are among the most promising candidates for high-efficiency, low-cost terrestrial photovoltaics. When grown in superstrate configuration, the extra glass encapsulation that CIGS cells require in conventional substrate configuration is not necessary. Thus, a high potential for cost reduction, crucial for a large-scale distribution, is inherent to CIGS superstrate cells. Layers of good structural quality are grown at a superstrate temperature of 550°C. At these temperatures, the CdS buffer layer, which forms a heterojunction in substrate configuration, does not yield high efficiency cells [1], because of inter-diffusion of elements. Therefore, superstrate cells have been developed by replacing CdS with ZnO buffer layers [2], [3]. The CIGS growth process and the structural properties of the ZnO–CIGS interface influence the photovoltaic properties. In the present work, the influence of diffusion at the interface during the growth of CIGS layers is investigated. This work describes the chemical properties of the CIGS–ZnO interface.

Section snippets

Experimental details

The schematic cross section and an SEM image of a superstrate cell are shown in Fig. 1. The details of growth and properties of the layers are described elsewhere [2], [3]. Approximately 1-μm-thick layers of Al-doped ZnO were rf-sputtered onto soda lime glass (SLG), followed by the deposition of 200 nm of undoped ZnO.

The Cu(In, Ga)Se2 absorber layers were grown by co-evaporation of elements in an ultra-high vacuum system. During most of the growth process, the substrate temperature was kept

EDS and I–V measurements

EDS linescans were performed on the cross-sectional samples to determine compositional gradients perpendicular to the ZnO–CIGS interface. Special attention was paid to aligning the interface parallel to the electron beam. Fig. 2 shows the concentration of different elements near the ZnO–CIGS interface. All curves are normalized to the measured Se concentration. A 50-nm-wide region right at the interface shows a strong accumulation of Ga. It should be noticed that the layer was grown with

Conclusions

Using EDS line scans, a layer of increased Ga accumulation was detected at the CIGS–ZnO interface of CIGS superstrate solar cells. Two-dimensional maps show this layer to extend over the whole interface.

As the ratio of O to Zn increases where the Ga accumulation is found, Ga oxide was assumed to be present. To corroborate these assumptions, XPS measurements were performed. Through the observation of the shift in the binding energy of Ga during the change from Ga in the oxidized state to the

Acknowledgements

The authors are very grateful to Dr R. Hauert for the XPS analysis and to Dr M. Döbeli for the RBS and HFS measurements on ZnO.

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