N. Spalatu

The performance of thin film solar cells employing photovoltaic ZnSe/CdTe, CdS/CdTe and ZnTe/CdTe heterojunctions

This paper focuses on the photovoltaic parameters of ZnSe/CdTe, CdS/CdTe, and ZnTe/CdTe thin film heterojunction solar cells. ZnSe/CdTe, CdS/CdTe, and ZnTe/CdTe thin film heterojunction solar cells were fabricated by Close Space Sublimation (CSS) on TCO-coated glass substrates. All types of solar cells were fabricated in a superstrate configuration. The thickness of ZnSe and ZnTe layers was varied in order to adjust the solar cell performance. A similar cadmium chloride solution for the treatment of a CdTe layer with an elevated temperature air annealing of the completed devices before the back contact deposition was applied to ZnSe/CdTe and CdS/CdTe thin film heterojunctions solar cells with exception of ZnTe/CdTe. All cells were characterized through light and dark current density-voltage (J-V) measurements and quantum efficiency (QE) measurements. The saturation current, ideality factor and photovoltaic parameters for all thin film heterojunction solar cells are presented. The investigation at the room temperature under illumination of 100 mW/cm2 through the wide gap components of ZnSe/CdTe, CdS/CdTe, and ZnTe/CdTe heterojunctions showed a value of conversion efficiency (η) of solar energy to electric energy about 4.7%, 9.9%, and 1.3%, respectively. The incorporation of Zn at the ZnSe and CdTe interface doubles the short circuit current density and improves the performance of ZnSe/CdTe thin film heterojunction solar cells.

ZnSe films prepared by the close-spaced sublimation and their influence on ZnSe/CdTe solar cell performance

This paper focuses on physical properties of ZnSe thin films and photovoltaic parameters of ZnSe/CdTe thin film solar cells. X-ray analysis and SEM images show that ZnSe films are polycrystalline and exhibit wurtzite-zinc-blende structure. ZnSe/CdTe solar cells show an efficiency of about 4.7%. The external quantum efficiency (EQE) for these cells shows that the shape of these characteristics depends on ZnSe layer thickness. The incorporation of Zn at the ZnSe and CdTe interface, for the first time, doubles the short circuit current density and improves the performance of ZnSe/CdTe thin film solar cells.

Optical properties of CdS thin films deposited by close space sublimation

Highly transparent cadmium sulphide thin films were fabricated at relatively low temperatures by employing the close space sublimation technique. The maximum optical transmittance in the visible region for CdS layer with the ∼ 0.48 µm thickness reached 80%. The optical band gap varies slowly with thickness in the range of 2.42 –2.44 eV. The lowest direct absorption edge produces a reflection peak at 2.42 or 2.43 eV at room temperature depending on the thicknesses. We report curves for refractive, extinction coefficients and optical conductivity for CdS with different thicknesses.

Electrical properties of thermally annealed CdS thin films obtained by chemical bath deposition

Electrical conductivity and the Hall-effect are investigated in the temperature interval (80–400) K on thermally annealed in H2 CdS thin films obtained by chemical bath deposition. Different characters of the temperature dependence of conductibility are observed in the CdS films annealed at different temperatures. The Hall measurements allow calculating the values of the NA, ND, nex and ED. According to Hall measurements the CdS films show several donor levels at different energetic depths in dependence of the annealing temperature. The sample annealed at high temperatures than 350°C proves to be compensated with a sharply decreasing electrical conductivity with the temperature decrease.

The caracterization of the CdS-based solar cell heterojunctions

The CdS-based solar cell heterojunctions (HJ) have been obtained by growth of CdS at relatively lower temperature using the close space sublimation method (CSS). Investigation of the photovoltaic characteristics shows an efficiency of about 12 % for InP/CdS and 9.6 % for CdS/CdTe solar cell HJ. The analysis of the forward dark current-voltage and the capacitance-voltage characteristics indicate a tunnelling recombination current which flows through states near or at the interfaces of a thermal energy of about 0.62 eV for CdS/CdTe and 0.42 eV for InP/CdS solar cell heterojunctions. The solar energy conversion efficiency is influenced by the interface states through the open circuit voltage and the fill factor.

The fabrication and physics of cadmium-telluride thin-film solar cells

Conditions for efficiency improvement and optimization in CdS/CdTe solar cells prepared by hot wall technique (HWT) are discussed in this paper. CdS/CdTe solar cells with and without incorporated back layer contact (BC) have been studied by current-voltage and capacitance-voltage measurements. Analysis reveals the cells without back layer contact seriously suffer in fill factor (FF) due to J-V rollover in dark and illumination. Also, the FF is limited by a light-dependent shunt and series resistances. The efficiency of both types of cells is limited by a light-dependent shunt/series resistances and compensation mechanism of CdTe layer.

Influence of substrate temperature on photovoltaic parameters of CdS/DdTe/Te solar cells fabricated by Close Space Sublimation

Thin film CdS/CdTe solar cells were fabricated by close space sublimation at the substrate temperature ranging from 300degC plusmn 5degC to 340 plusmn 5degCdegC. The best photovoltaic parameters were achieved at substrate temperature 320degC and source temperature 610degC. The open circuit voltage and current density changes significantly with the substrate temperature and depends on the substrate temperature. The open circuit voltage and current density achieves 0, 81 V and 22, 75 mA/cm2, respectively. CdS/CdTe solar cells with an efficiency of 9, 56% were obtained.

Capacitance-voltage characteristics of the CdS/CdTe/Te heterojunctions

CdS/CdTe/Te heterojunction have been studied by capacitance-voltage measurements over the temperature range 293 K-393 K. The potential of diffusion, profile of the ionized-charge concentration in the space charge region of heterojunction and its thickness are derived from the heterojunction capacitance measured at different bias voltages. The charge carrier concentrations calculated from (S/C)2 = f(U) for low and high voltage ranges, respectively are (1-3)ldr1014 cm-3 and it is associated with the volume concentration of CdTe and (2-5) 1013 cm-3 which corresponds to the region of interface of the CdS/CdTe/.