Ga2O3 segregation in Cu(In, Ga)Se2/ZnO superstrate solar cells and its impact on their photovoltaic properties
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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2018, Solar Energy Materials and Solar CellsCitation Excerpt :During the deposition of CIGSe on ITO or on any other back contact, it is urged that formation of a secondary phase should be avoided except in the case of a Mo back contact in which MoSe2 IF phase has been found to be benign for the device operation [26]. When the chalcopyrite absorber is deposited on TCO at ~550 °C either in substrate or superstrate configuration, a GaOx phase usually forms between the chalcopyrite thin film and TCO contact [27,28]. The formation of this phase can be reduced by reducing (Ts)CIGSe to ~450 °C [27].
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2017, Thin Solid FilmsCitation Excerpt :An ohmic contact between the CIGS and SnO2:F has been previously reported [8–10]. However, a rectifying behavior of the CIGS/ZnO or ZnO:Al contact with the formation a resistive GaOx layer at the interface was commonly observed [8,11,12]. A Mo layer that reacts in MoSe2 during the CIGS process leads to a low resistance ohmic CIGS/ZnO:Al contact [13].
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2016, Solar Energy Materials and Solar CellsCitation Excerpt :The CIGS layers studied in this work were deposited by co-evaporation of Cu–In–Ga–Se, as this method allows a good controllability of the Ga-gradient and keeps the selenization of the underlying substrate to a minimum compared to selenization processes of metallic precursors. The deposition process for the CIGS absorbers followed a similar routine as the well-known 3-stage process [10], with the difference that Cu–Ga–Se is co-evaporated in the second stage at a substrate temperature of 520 °C; details are given in [7]. This leads to an inverted Ga gradient compared to the standard process, as required for the superstrate structure.