Cu(In,Ga)Se₂ thin films processed by co-evaporation and their application into solar cells

J. Sastré-Hernándezª, M.E. Calixto^a,c, M. Tufiño-Velázquezª, G. Contreras-Puenteª, A. Morales-Acevedo^b, G. Casados-Cruz^b, M.A. Hernández-Pérez^d, M.L. Albor-Aguileraª, and R. Mendoza-Pérez^e

ª Escuela Superior de Física y Matemáticas, Instituto Politécnico Nacional, México, D.F., 07738 México.

^b Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Departamento de Ingeniería Eléctrica, Av. IPN No. 2508, México, D.F., 07360 México.

^c Instituto de Física, Benemérita Universidad Autónoma de Puebla, Apartado Postal J-48, Puebla, Puebla, 72570, México.

^e Universidad Autónoma de la Ciudad de México, Av. Prolongación San Isidro 151; Col. San Lorenzo Tezonco, México, D.F. 09790, México.

Recibido el 3 de mayo de 2011;
aceptado el 8 de agosto de 2011

Abstract

Polycrystalline Cu(In,Ga)Se₂ (CIGS) solar cells are attractive because low cost techniques can be used to obtain high efficiency thin film photovoltaic devices. Several research groups around the world have developed CIGS/CdS solar cells with efficiencies larger than 15% [1] using evaporation, making it an attractive and reliable technique for thin film deposition. Our PVD system is provided with MBE-type Knudsen cells to deposit CIGS thin films on glass/Molibdenum (Mo) substrates. The deposition conditions for each metal source have been established by doing a deposition profile of temperature data vs. growth rate by co-evaporation to obtain CIGS thin film for solar cells. Characterization of the co-evaporated CIGS thin films was performed by X-ray diffraction (X-RD), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) techniques. Good quality polycrystalline films were obtained as shown by X-RD patterns. SEM micrographs show films having a very uniform appearance with large grain sizes (~1 µm). Photoluminescence (PL) studies on CIGS samples with different Ga and Cu concentrations (Ga/Ga+In) = 0.25 and 0.34 and (Cu/In+Ga) = 0.83, 0.88 and 0.94) have been performed. The EDS results have shown that is possible to control very precisely the CIGS thin film composition using these Knudsen cells. Film thicknesses of ~3-4 µm, were measured with an Ambios profilemeter XP 100 stylus type. A conversion efficiency of 10.9 % has been achieved for solar cells made from the co-evaporated absorbers.

Keywords: Cu(In; Ga)Se₂; Co-evaporation; thin films; solar cells.

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Acknowledgements

This work was partially supported by CONACYT under grant number 47587 and ICyT-DF under grant PICS08-54. The authors are grateful to Dr. Alvin D. Compaan, Department of Physics and Astronomy, The University of Toledo, and Dr WilliamN. Shafarman, Institute Of Energy Conversion, University of Delaware, for their invaluable contributions.

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