Microscopic Analysis of Residuals on Polycrystalline CdTe Following Wet CdCl 2 Treatment
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Microscopic Analysis of Residuals on Polycrystalline CdTe Following Wet CdCl2 Treatment Timothy A. Gessert, Manuel J. Romero, Craig L. Perkins, Sally E. Asher, Rick Matson, Helio Moutinho, and Doug Rose1 National Renewable Energy Laboratory, Golden, CO 80401, U.S.A. 1 First Solar, LLC, Perrysburg, OH 43551, U.S.A. ABSTRACT In this study we report on the spatial distribution and composition of residuals on the CdTe surface following a typical wet CdCl2 treatment, and the effect that our ion-beam milling has on this residual-coated surface. Results show that residuals are spatially discrete, located primarily along grain boundaries, and are likely a cadmium oxychloride. Results also show that the residuals may penetrate deep into the CdTe surface such that typical ion-beam milling procedures do not produce complete residual removal. INTRODUCTION The demonstration of a manufacturable, stable, low-resistance, ohmic contact for p-CdTe polycrystalline photovoltaic devices is an important research goal of the CdTe community. We have demonstrated devices with fill factors approaching 77% by incorporating a Cu-doped ZnTe contact interface layer between the CdTe absorber and a Ti metallization [1]. This contacting process uses ion-beam milling instead of wet-chemical etching to prepare the CdTe surface. Because ion-beam milling is a vacuum-compatible process, and it produces less waste than wet chemical etching, this type of process may provide advantages for large-scale manufacturing. Most back-contact processes used for CdTe devices involve a wet-chemical pretreatment that completely dissolves the near-surface region, preferentially etches grain boundaries to a significant depth (>1 µm), and produces a Te-rich surface layer (~250 nm thick) [2]. This Terich layer is critical for low-resistance ohmic contact formation when chemical pretreatments are used [3]. Another benefit of wet pretreatments is that any unwanted CdCl2 surface residue dissolves into the etching solution. However, if dry etching is used for contact pretreatment (e.g., ion-beam milling, reactive-ion etching), less material is removed from the CdTe surface (~100 nm), and the condition of the surface prior to pretreatment may significantly affect the posttreatment condition of the CdTe surface, ultimately affecting device performance. We have shown that replacing wet-chemical surface treatments with ion-beam milling produces a CdTe/ZnTe contact interface with sufficiently low resistance to yield CdS/CdTe devices with high fill factors. Nevertheless, our understanding of the CdTe surface, both before and after the ion-beam precontact treatment, remains uncertain. Indeed, much of our contact optimization is guided by resultant device performance (i.e., fill factor) rather than understanding what may limit current transport at the CdTe/ZnTe interface. We have reported preliminary studies on these CdCl2-treated surfaces, as well as the effect of ion-beam milling [4,5,6]. However, these studies utilized relatively large-area probes and did not account for the possi
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