Thin Film Photovoltaic Cells (TFPV) with a reduced peak efficiency of 11%, represent nearly 8% world share of the actual solar cell market. This figure will be near too dramatic in the nearest future, because it seems it will reach the 20% of the market in the next few years, due to the scarcity of Si and the cost-cutting potential that many people perceive in TF technology [1]. For these reasons, a strong effort must be carried out in the research and development of TFPV experimental approaches which do not use Si as the matrix element.

    Very recently Schaffner et al. have demonstrated 12% efficiency in CdTe/CdS solar cells developed by using low temperature close space sublimation (CSS) [2]. Our group has also recently demonstrated the feasibility to produce CSS-grown CdTe and CdS films for solar cell applications by using resistive heaters [3].

    In this work we analyse both experimentally and numerically the growth of CdTe/CdS films by using Close Space Sublimation Transport (CSS) with resistive heaters. A photograph of two CdS and CdTe films are shown in Fig. 1.

    The structure and composition of the films are studied as a function of different growth parameters.  The properties of both CdTe and CdS films are studied by using High Resolution Secondary Electron Microscopy (HR-SEM), Energy Dispersive X-ray Analysis (EDX), optical absorption and X-Ray Diffraction. Numerical simulations have been performed in order to obtain information about the thermal field during the growth process for different boat materials and dimensions. These numerical results have been correlated to the structural properties of the films.

    The analysis from SEM-EDX show good stoichiometry of the films, both in the case of CdS (S: 49.48 % at., 50.52 % at.), and CdTe films (51.29 % at., 48.71 % at.). SEM images on both CdTe and CdS films prepared in this work are shown in Figure 2.

Figure 1. (a) Photograph of a final glass/FTO/CdS/CdTe solar cell fabricated by using Close Space Sublimation. (b) Picture of the CdS window film deposited underneath CdTe.

Figure 2. SEM images taken from (a) CdS and (b) CdTe films prepared in this work by Close Space Sublimation.

References

[1] J .Heup, New Energy 1 (2009) 72.

[2] J. Schaffner, M. Motzko, A. Tueschen, A. Swirschuk, H.J. Schimper, A. Klein, T. Modes, O. Zywitzki, and Wolfram Jaegermann, J. Appl. Phys. 110 (2011) 064508.

[3] J. L. Plaza, O. Martínez, S. Rubio, V. Hortelano and E. Diéguez, CrystEngComm, 2013, 15, 2314–2318.

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1. Influence of CdS deposition technique on CdS/CdTe thin films
2. Passivation effect of CdS thin films grown by Chemical Bath Deposition on gallium antimonide surfaces