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INVESTIGATION OF THIN FILM SOLAR CELLS ON CdTe BASE
G. S. Khrypunov, N. A. Kovtun, B. A. Novikov
National Technical University “Kharkov Polytechnic Institute”, firstname.lastname@example.org
In the photovoltaic material family, cadmium telluride is regarded as one of the most promising material for fabrication of high efficiency polycrystalline CdTe/CdS thin film solar cells. “CdCl2 heat treatment” is considered as a key step in CdTe/CdS device processing leading to interdiffusion between CdTe and CdS. It is usually carried out either by means of deposition of CdCl2 layer onto CdTe surface with following air annealing or annealing the structure in CdCl2 vapor. However in the former case the optimal cadmium chloride thickness value varies in the wide range depending on the base layer technology. To identify the structural mechanisms leading to the solar cell efficiency increase we have studied the effect of “CdCl2 treatment” on the output parameters of CdS/CdTe-based solar cells and crystal structure of the base layers deposited on glass substrates by different ways. In the first way both of CdS and CdTe layers were deposited by physical vapor deposition (PVD) method meanwhile in the second way the chemical bath deposition (CBD) and close-spaced sublimation (CSS) methods were used for CdS and CdTe films deposition, respectively.
The obtained heterostructures were then subjected to “CdCl2 treatment”. For the PVD structures, CdCl2 films of different thickness were deposited onto unheated CdTe surface by vacuum evaporation and then were air-annealed at 430 oС during 25 min. CdCl2 treatment results in the significant recrystallization of CdTe layer. The average grain size of the film increases from 1 µm to 4 µm due to the lattice strain caused by macrodeformations and stacking faults.
CBD/CSS samples were exposed to CdCl2 vapor at 400 oC for 5-7 min in vacuum chamber in the presence of 100 torr oxygen and 400 torr helium. “As-grown” CdTe films were characterized by clearly faceted surface morphology and an average grain size of about 3-4 µm. Unlike the thermally evaporated CdTe films, no considerable changes after CdCl2-treatment in the structural properties and surface morphology were observed for the CdTe films obtained by CSS method. The difference between the results on CdCl2-treatment can be explained by a lower concentration of defects and grain boundary area in CSS deposited CdTe films resulting in lower lattice strain energy which promote recrystallization and grain growth. As a result the CSS deposited CdTe films do not recrystallize at the temperatures and times used in the CdCl2 treatment.
The maximum efficiency (η=10.3%) of solar cells on the basis of cadmium telluride layers deposited by PVD method corresponds to 0,35 μm CdСl2 thickness at CdCl2 treatment The highest efficiency in the CBD/CSS fabricated solar cells was obtained with the parameters: Voc=773 mV, Jsc=23.92 mA/cm2, fill factor (FF)=68.7%, a total area conversion efficiency of 12.71%.
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