7/30/2019 Potlog Frankfurt 2012

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NEW TEHNIQUES FOR IMPROVING THE EFFICIENCY OF SOLAR CELLS

BASED ON CdTe THIN FILMS

T. Potlog1, V. Fedorov1, P. Dumitriu1

J. Hiie2, V. Mikli2,,

V. Lughi3

, V. Sergo3

1Physics Department, Moldova State University, A. Mateevici str. 60, MD 2009 Chisinau

Moldova2Department of Materials Science, Tallinn University of Technology, Ehitajate tee 5,

19086 Tallinn Estonia3Department of Materials and Natural Resources, University of Trieste, 6a, via Valerio,

34127, Trieste, Italy

Current effort to improve the efficiency of thin film solar cells based on CdTe is

to use wider band gap buffer layers in order to allow more light to reach the junction,

consequently improving the short wavelength response of cells and to exclude formation

of solid solution CdTe1-xSx near CdS-CdTe interface with high resistivity and smallerband gap than that for CdS. In the present paper the study of structural, substructural and

photoelectrical properties of glass/SnO2CdS/ZnSe/CdTe, glass/SnO2/ZnSe/CdTe andglass/SnO2/ZnSe:Zn/CdTe heterosystems will be investigated. ZnSe films were deposited

by CSS. The deposition reactor of the films is identical to those used for CdTe and CdS.

The source material is 99.999% pure ZnSe powder. The ZnSe films were deposited

without any inert ambient gas, the source and substrate temperature ranges are 700-720C

and 360-420C respectively. The thickness of the ZnSe was varied, since this layer was

found to be uniform within 2-3% over the entire substrate area. Preliminary works

indicate that the difference in the electron CdTe and ZnSe affinities leads to adiscontinuity in the conduction band which impedes the transport of photo-generated

electrons from the CdTe. It has also been suggested that inter-diffusion at themetallurgical interface of a heterojunction can decrease this discontinuity. Some authorshave used ZnSe (Eg = 2.67 eV) and produced ZnSe/CdTe devices with efficiencies >

11%. In the present paper the three series of the samples were obtained:

glass/SnO2/CdS/ZnSe/CdTe, glass/SnO2/ZnSe/CdTe and glass/SnO2/ZnSe:Zn/CdTe byclose space sublimation method. The investigation of structural properties allowed us to

compare growth peculiarities of solar cells with ZnSe layer deposited on glass, CdS and

on glass/SnO2 substrates. It was determined that at high substrate temperatures (>360oC)

films evaporated on sublayer have better structural properties than films deposited on

glass substrate at the same growth conditions. Particularly, solid solutions on CdS/ZnSe

and ZnSe:Zn/CdTe interfaces were detected. The incorporation of Zn at the interface

between ZnSe and CdTe doubled short circuit current density. The saturation current,ideality factor, potential barrier height value and photovoltaic parameters will be

determined by the analysis of the dark and illuminated current-voltage and capacitance-

voltage characteristics.