The structural changes in CdS-CdTe thin films due to annealing

Abstract

Thin film solar cells based upon CdS-CdTe heterojunctions have become an important alternative to silicon based devices. The film structures formed during fabrication are critical to cell efficiency and thus their study is fundamental to improving device performance. We have used synchrotron x-ray diffraction to investigate the effect of a post deposition anneal upon the film structures and, in particular, have examined the dynamic formation of intermixed regions adjacent to the original, metallurgical interface. Our results have enabled us to produce a dynamic model for the structural changes which includes the extent of interdiffusion. We show that, for a 400 nm CdTe film in the presence of chlorine, the original CdS and CdTe layers are completely transformed into layers with average compositions CdS 0.93 Te 0.07 and CdTe 0.94 S 0.06 , respectively. We present evidence that the interdiffusion occurs during or following a recrystallization and that, to a limited extent, these changes also occur without chlorine.

The effect of annealing on the structure of CdTe films electro-deposited on metallic substrates

“…For the virgin samples, the value of a as-deposited ¼ 6:485 ( A which is larger than the powder (free of stress) sample a powder ¼ 6:481 ( A, and this suggests that the film is submitted to a compressive stress in the plane parallel to the substrate surface. In the case of annealed sample, the lattice parameter a treated ¼ 6:479 ( A, and similar results are reported in the literature [2][3][4][5][6]. The error in our estimation of the lattice parameter is 70.003 ( A.…”

“…Even though there are changes in the intensity of the various peaks due to annealing, the samples continue to show a preference for the (1 1 1) direction in each stage of the annealing process. Many authors have been reported changes in XRD patterns due to annealing in different environments [2][3][4][5][6].…”

“…Knowledge of the principal structural changes of the film during post-deposition treatments is very important in understanding and optimizing the performance of the photovoltaic devices. Many research groups have investigated the effect of the post-deposition heat treatments on the structural changes of CdTe thin films and its influence on the performance of CdTe/CdS solar cells [2][3][4][5][6]. But there are discrepancies in the results and conclusions which call for more investigations on the post-deposition heat treatments of the CdTe films.…”

“…Further it was observed that the re-crystallization has influenced the preferred orientation of the crystallites also. Rogers et al [3] investigated the effect of chlorine on the structural changes in CdTe films and concluded that the CdCl 2 treatment and the annealing have a substantive influence upon the structural changes. Nevertheless the significant increase in crystallinity at the early stages of the anneal process was independent of the presence of chlorine.…”

Ellipsometric investigation of S–Te inter-diffusion and its effect on quantum efficiency of CdS/CdTe thin films solar cell

“…concluded that the S-Te inter-diffusion occurred during the higher temperature CdTe deposition rather than lower temperature CdCl 2 processing. Rogers et al (1999) suggested that sulfur diffusion into CdTe occurs in a region delimited by a change in CdTe microstructure. According to our experimental data and the VASE analysis, we propose that a large amount of S-Te inter-diffusion occurred during high temperature process of CdTe deposition.…”