Surface analysis of CdTe thin film solar cells

Surface analysis of CdTe thin film solar cells

Thin Solid Films 387 Ž2001. 161᎐164 Surface analysis of CdTe thin film solar cells J. Fritsche a,U , S. Gunst a , E. Golusdaa , M.C. Lejard a , A. Th...

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Thin Solid Films 387 Ž2001. 161᎐164

Surface analysis of CdTe thin film solar cells J. Fritsche a,U , S. Gunst a , E. Golusdaa , M.C. Lejard a , A. Thißena , T. Mayer a , A. Klein a , R. Wendt b , R. Gegenwart b , D. Bonnet b , W. Jaegermann a a

Fachbereich Materialwissenschaften, Technische Uni¨ ersitat ¨ Darmstadt, Petersenstr. 23, D-64287 Darmstadt, Germany b ANTEC GmbH, Industriestr. 2-4, D-65779 Kelkheim, Germany

Abstract The surface properties of CdTe thin film solar cells prepared by ANTEC using the close-space sublimation ᎏ ŽCSS. ᎏ technique have been analyzed by X-ray diffraction ŽXRD., atomic force microscopy ŽAFM., photoelectron emission microscopy ŽPEEM., high-resolution scanning electron microscopy ŽHRSEM. and photoelectron spectroscopy ŽXPS. after different pretreatment conditions. Exposure of the CdTe films to air leads to surface oxidation with the formation of TeO 2 and CdO. The amount of surface oxides depends on the CdCl 2 activation process. Activated surfaces are less oxidized than non-activated surfaces. Due to that surface oxidation, the surface is more n-type, indicating the formation of a surface barrier. The surface oxide can be removed by mild sputtering. The results suggest that no extra surface defects are introduced by this procedure. Before sputtering, Cl is found on the surface of the activated material, although no such contamination is found in the stoichiometric bulk material using XPS. A variation in the Fermi level position is observed for the non-activated to the activated CdTe material from weakly to higher p-doped levels. This type of conversion is evidently restricted to the near surface area as further in the bulk, weakly p-doped CdTe is found again. The results indicate that, besides the surface composition, the electronic properties of the film also depend on the different pretreatment steps. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: CdTe; CdS; Close-space sublimation; Photoelectron spectroscopy; Surface analysis; Thin film solar cell

1. Introduction Polycrystalline thin film solar cells like CdTerCdScells have the advantage of reasonable conversion efficiencies ŽCdTe: f 10%. combined with a low-cost production process. For this reason, these cells have been intensively investigated during recent years and have reached the edge of pilot production w1᎐4x. However, despite this technological success, systematical investigations of the influences of the morphology of the polycrystalline film and the different electronic interface properties on conversion efficiency are still missing. Furthermore, the effect of the empirical ‘magic production step’, a thermal activation of the CdTe film

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Corresponding author. Tel.: q49-6151-166370; fax: q49-6151166308. E-mail address: [email protected] ŽJ. Fritsche..

using CdCl 2 , is not well understood. Without that activation step, high conversion efficiencies cannot be reached. For these reasons, we have started to systematically investigate the film and interface properties of CdTe thin film solar cells in order to understand the effect of empirically optimized preparation steps on device performance. We hope that this knowledge will contribute to the development of thin film solar cells with increased conversion efficiencies, and a robust and cheap preparation technology. 2. Results and discussion CdTe thin film solar cells as prepared by ANTEC are composed of the layers glassrITOŽs indium tin oxide s transparent conductive glass s TCO . rCdSr CdTermetallic back contact. CdS and CdTe are deposited onto glassrITO substrates by close-space sublimation w4,5x. High-resolution scanning electron micros-

0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 8 5 1 - 4

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copy ŽHRSEM. cross sections of solar cells with different conversion efficiencies show remarkable differences in film morphology ŽFig. 1.. A ‘good’ solar cell is characterized by a compact layer with large crystallites in the range of the film thickness. A ‘bad’ solar cell shows a large number of small crystallites with many holes, which is evidently due to insufficient control of nucleation and subsequent film growth at non-optimal substrate temperatures. The TCO and the extremely thin CdS heterojunction layer can be seen in Fig. 1. A comparison of a HRSEM top view picture and atomic force microscopy ŽAFM. topographic image ŽFig. 2. of the film indicates that even the ‘good’ solar cell does not possess an ideal morphology. Both pictures show an equivalent topography. The crystallites are separated by pinholes and craters. The photoelectron emission microscopy image ŽFig. 2. shows inhomogeneities of the surface potentials for, e.g. the work function. Because of the identical contrast variations in the topography and PEEM images, the evident differences in work function can be related to different doping of the crystallites andror to different ionization potentials. As a consequence, a lateral inhomogeneous solar cell results, which is composed of active and less active areas. In addition, the grain boundaries may act as recombination pathways. These effects evidently contribute to the significant reduction of the practically achieved conversion efficiencies from the theoretical limit.

Fig. 2. Comparison of HRSEM Ža. AFM Žb. and PEEM Žc. images of a ‘good’ CdTe solar cell showing variations in crystallite properties.

Fig. 1. HRSEM cross section of CdTe solar cells Ža. ‘good’ cell, Žb. ‘bad’ cell..

In Fig. 3, three-dimensional-AFM topographic images for every relevant layer of a cell ŽITO, CdS, CdTe CdCl 2-activated and non-activated. are shown. No dramatic differences in surface morphology and roughness are visible. In particular, the activation step does not change the grain sizes. Only a slight surface

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Fig. 5. Core level and valence band spectra of CdTe films after different surface treatments. The position of EF in the bandgap is also shown. Fig. 3. Three-dimensional-AFM topography images of ITO Ža., CdS Žb., non-activated Žc. and activated CdTe Žd..

smoothening occurs during activation. This result supports that grain growth is observed only in small crystalline PVD layers, but not in CSS-CdTe, as studied in this work w6,7x. It is interesting to note that the grain size is smallest for the ITO layers and only slightly larger for the CdS film. It is not yet clear how the CdTe grain size develops from the CdS interface region to the larger crystallites seen on top. The crystallite distribution of different orientations can be investigated by X-ray diffraction ŽFig. 4.. The lower curve shows the diffraction pattern of a CdCl 2-activated and the upper curve of a non-activated CdTe surface. The main contributions belong to Ž100. and Ž111. oriented crystallites in roughly equal amounts. In accordance with earlier studies, w7x the activation step shows no influence on the crystallite distribution using XRD, although a modification of the CdSrCdTe-interface is presumed. The chemical composition of the layers and interfaces as well as their electronic properties can be investigated by photoelectron spectroscopy ŽXPS.. In the left diagram of Fig. 5, Te3d core level spectra for a non-activated and activated CdTe solar cell are shown both after exposure to air Ž‘as is’. and after Ar-sputtering in the vacuum chamber. Both ‘as is’ surfaces are severely oxidized with the formation of TeO 2 and CdO.

The oxidation is clearly indicated by the Te line shifting by approximately 4 eV to higher binding energies. The relative oxide intensities show a twice as strong oxidation tendency for the non-activated sample compared to the activated sample. The oxide film shifts the photoemission spectra to higher binding energies. Therefore, a surface barrier exist that forms a slightly n-doped surface. Sputtering removes the oxide completely. The surface becomes more p-type in character. After sputtering, it is obvious that the activated CdTe surface is more p-type than the non-activated with a difference in Fermi-level position of approximately 100 meV. Before sputtering, a Cl surface component can be observed for the activated sample. This contamination is no longer detectable after Ar-sputtering using XPS. No further influence of the sputtering process on the CdTe surfaces can be observed. The right diagram of Fig. 5 shows Cd4d and valence band spectra of an activated CdTe solar cell ‘as is’, after Ar-sputtering and after mechanical scratching, together with a schematic band line-up. After mild sputtering, a stoichiometric p-type CdTe surface appears, with the surface Fermi level near the valence band maximum. After the mechanical removal of a few micrometers of the surface, the surface Fermi level moves again to the center of the bandgap which indicates a restriction of the p-doped CdTe to a surface near layer. This unexpected doping variation in the CdTe adsorption layer of the thin film solar cell may have drastic consequences for the understanding and optimization of the cell performance. Instead of the assumed n-CdSrp-CdTe heterojunction there seems to be a n-CdSri-CdTerp-CdTe layer sequence in which the p-doped CdTe region probably extends far into the film at the grain boundaries. Acknowledgements

Fig. 4. Comparison of XRD-Patterns from an CdCl 2 -activated Župper curve. and non-activated Žlower curve. CdTe surface.

This work was supported by the Bundesministerium fur ¨ Bildung und Forschung ŽBMBF. Grant No. 0329857.

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