Post-Deposition Sulfur Incorporation into CuInSe 2 Thin Films
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Post-Deposition Sulfur Incorporation into CuInSe2 Thin Films Jochen Titus, Hans-Werner Schock1, Robert W. Birkmire, William N. Shafarman and Udai P. Singh Institute of Energy Conversion, University of Delaware, Newark, DE 19716 U.S.A. 1 Institut für Physikalische Elektronik, Universität Stuttgart, 70569 Stuttgart, Germany ABSTRACT The effect of initial film composition and substrate in the sulfurization of CuInSe2 was investigated. CuInSe2 films deposited on either soda-lime glass (SL) or Corning 7059® borosilicate glass (7059) substrates were reacted in flowing H2S for times from 1 to 8 hours. Films with Cu-rich composition, Cu/In > 1, reacted for 1 hour had nearly all the Se replaced by S. For Cu-poor films the incorporation of S was significantly reduced. In addition, in Cu-poor films on SL glass CuInS2 and NaInS2 were found at the film surface. These phases were not detected in films on 7059 substrates or in Cu-rich films. A phenomenological model is proposed to explain the formation of segregated surface phases in Cu-poor films on SL substrates. INTRODUCTION A graded band gap absorber layer could be used to increase the voltage obtained with Cu(In,Ga)Se2 based solar cells by reducing the recombination current in the space charge region while leaving carrier generation and collection relatively unaffected [1,2]. A Cu(In,Ga)(Se,S)2 film that has a higher S content near its surface is such a structure. The incorporation of S in a Cu(In,Ga)Se2 film has been shown to depend on the composition and structure of the film. With greater than 25% Cu in the film, the rate of S incorporation increases during co-evaporation of the elements [3] or post-deposition sulfurization of Cu(In,Ga)Se2 [4]. In addition, films with small grains take up S faster than films with large grains [4,5]. In Cu-rich CuInSe2 films on SL substrates, S incorporation has been quantitatively described as a combination of bulk and grain boundary diffusion [6]. On CuGaSe2 and Cu(In,Ga)Se2 films, post-deposition sulfurization produces a completely sulfurized surface layer that has been correlated with a structure visible in scanning electron microscope (SEM) cross-sectional images [7]. In the same reference, it was observed that in sulfurized CuInSe2 films a Na compound tends to segregate at the surface. In this paper, the effect of variations in the ratio of atomic concentrations of Cu and In (Cu/In) and substrate material on the sulfurization of CuInSe2 films is investigated. The results are explained on the basis of diffusion and reaction mechanisms. EXPERIMENT Films of CuInSe2 with an approximate thickness of 2 µm were made by co-evaporation of the elements in a bilayer process, with initially Cu-rich and subsequently Cu-poor film growth [8]. The substrates used were Mo covered SL and 7059 glass, and the substrate temperature during CuInSe2 deposition was 550ºC. Energy dispersive x-ray spectroscopy (EDS) and Auger H1.5.1
electron spectroscopy depth profiling confirmed good lateral and vertical uniformity of the composition, respectively. The films
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