Progress towards high efficiency thin film CdTe solar cells

Roll-over behavior in current-voltage curve introduced by an energy barrier at the front contact in thin film CdTe solar cell

This report describes research to demonstrate (1) thin film cadmium telluride solar cells with a quantum efficiency of 75% or higher at 0. 44 {mu}m and a photovoltaic efficiency of 11.5% or greater, and (2) thin film zinc telluride and mercury zinc telluride solar cells with a transparency to sub-band-gap radiation of 65% and a photovoltaic conversion efficiency of 5% and 8%, respectively. Work was directed at (1) depositing transparent conducting semiconductor films by solution growth and metal-organic chemical vapor deposition (MOCVD) technique, (2) depositing CdTe films by close-spaced sublimation (CSS) and MOCVD techniques, (3) preparing and evaluating thin film CdTe solar cells, and (4) preparing and characterizing thin film ZnTe, CD{sub 1-x}Zn{sub 1-x}Te, and Hg{sub 1-x}Zn{sub x}Te solar cells. The deposition of CdS films from aqueous solutions was investigated in detail, and their crystallographic, optical, and electrical properties were characterized. CdTe films were deposited from DMCd and DIPTe at 400{degrees}C using TEGa and AsH{sub 3} as dopants. CdTe films deposited by CSS had significantly better microstructures than those deposited by MOCVD. Deep energy states in CdTe films deposited by CSS and MOCVD were investigated. Thin films of ZnTe, Cd{sub 1- x}Zn{sub x}Te, and Hg{sub 1-x}Zn{sub x}Te were deposited by MOCVD, and their crystallographic, optical, and electrical properties were characterized. 67 refs.

Vapor transport deposition is an excellent method for preparing large area CdTe thin films with high quality and uniformity. Polycrystalline CdTe thin films were deposited by home-made vapor transport deposition system (VTD). The effects of substrate temperature on the property of CdTe film and the performance of CdTe solar cell were investigated. CdTe thin films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Vis spectrometer, and Hall Effect system. The results show that the CdTe thin films deposited by vapor transport deposition are cubic phase with a preferred orientation in (111) direction. The average grain size increases from 2 m to 6 m and the carrier concentration increases from 1.93×10 cm to 2.36×10 cm when the substrate temperature increases from 520 °C to 620 °C. This suggests that high substrate temperature can increase the carrier density significantly due to the suppressed defect recombination. The performance of CdTe thin film solar cells deposited at different substrate temperatures demonstrates that high substrate temperature (610°C) can greatly improve the efficiency, open circuit voltage and fill factor of the solar cells. But the substrate temperature higher than 610°C will reduce the spectral 142 无 机 材 料 学 报 第 31卷 response of the cells in long wavelength region, which results in the degradation of solar cell performance. The small-area CdTe thin film solar cell without back contact layer deposited at substrate temperature of 610°C obtains the best conversion efficiency of 11.2%.

Thin film CdTe solar cells have been recognized as reliable alternative for the manufacture of cheap photovoltaic solar cells of the future, due to its excellent absorber characteristics and simple, low-cost manufacturability. However, for the attainment of higher cell performances, additional studies are needed to increase cell efficiency through further development of better quality films and new fabrication processes. In the substrate-structured CdTe thin film solar cells, the CdTe absorber layer is deposited directly onto the substrate or through a back contact layer. But the quality of deposited film is believed to depend on the type and smoothness of the substrate. In this work CdTe was deposited on different substrates by RF sputtering and the effects on the deposited films were studied in terms of their structural and morphological forms. The substrates used were: pure molybdenum sheets (Mo), molybdenum-sputtered on molybdenum (Mo/Mo), molybdenum-sputtered polyimide (PI/Mo) and molybdenum-sputtered glass (glass/Mo). The characterization tools used included XRD, SEM and AFM. The results showed that all surfaces produced uniform, compact and pinhole-free films; however, those on smoother surfaces produced larger as-deposited grain sizes of up to1.7μm as against 1.3 for rougher surfaces. Non-uniformities such as overgrowth and voids were observed, but only films on PI showed evidence of cracking and peel-offs.

CdTe thin film solar cells have been fabricated on a variety of glass substrates (borosilicate and soda lime). The CdS films were deposited to a thickness of 500–2000 Å by the chemical bath deposition (CBD), rf sputtering, or close spaced sublimation (CSS) processes. The CdTe films were deposited by CSS in the temperature range of 450–625 °C. The main objective of this work is to fabricate high efficiency solar cells using processes that can meet low cost manufacturing requirements. In an attempt to enhance the blue response of the CdTe cells, ZnS films have also been prepared (CBD, rf sputtering, CSS) as an alternative window layer to CdS. Device behavior has been found to be consistent with a recombination model.

Thin film CdTe solar cells by close spaced sublimation: Recent results from pilot line

Magnesium-doped Zinc Oxide (MZO) films have recently been proposed as a transparent buffer layer for thin film CdTe solar cells. In this study, the band gap of MZO buffer layers was tuned for CdTe solar cells by increasing the substrate temperature during deposition. Films were deposited by radio-frequency magnetron sputtering. Devices incorporating an optimised MZO buffer layer deposited at 300 °C with a band gap of 3.70 eV yielded a mean efficiency of 12.5% and a highest efficiency of 13.3%. Transmission electron microscopy showed that MZO films are uniformly deposited on the transparent conductive oxide (TCO) layer surface. The favourable band alignment seems to positively counterbalance the low doping level of the MZO layer and its high lattice mismatch with CdTe. Titanium-doped indium oxide, tin-doped indium oxide and aluminium-doped zinc oxide TCOs were also used as alternatives to fluorine-doped tin oxide (FTO), in combination with MZO films. The use of titanium-doped indium oxide and tin-doped indium oxide TCOs did not improve the device efficiency achieved compared with FTO, however using aluminium-doped zinc oxide coupled with a boro-aluminosilicate glass substrate the mean and highest efficiencies were further improved to 12.6% and 13.4% respectively.

The formation of low resistance ohmic contacts on p-type CdTe is a major challenge for the superstrate solar cell the dominant device configuration for this technology. It is even a greater challenge for the substrate configuration, since the back contact material is deposited first and is therefore influenced by subsequent high temperature fabrication processing. In this paper, the structural and electrical properties of ZnTe and Sb 2 Te 3 films deposited by the close-spaced sublimation and evaporation respectively are presented. Thin film CdTe solar cells of the substrate configuration have been fabricated on flexible foil substrates using ZnTe and Sb 2 Te 3 as back contacts, and their performance has been evaluated.

Degradation of Mg-doped zinc oxide buffer layers in thin film CdTe solar cells

A new fabrication route based on a solution chemistry with metal alkoxide was developed for a compound semiconductor cadmium telluride (CdTe) film. CdTe-double-alkoxide solution was prepared by the reaction of cadmium acetate with Na-alkoxide in the presence of Te-alkoxide in ethanol. Considering the moisture sensitive properties of metal alkoxides, cadmium telluride films was produced by heat-treatment at 400°C in H2 of hydrolysis product films of the corresponding metal alkoxides, which were formed by screen printing and spin coating. Although the good CdTe film couldnot be obtained by screen printing, spin coating has yielded a dense CdTe film. A surface of the spin coating film was leveling by mechanically polishing. The produced films were characterized by X-ray diffraction, atomic force microscopy, optical absorption, and optical irradiation current-voltage measurements. Optical absorption spectra were observed the characteristic absorption in about 830nm. The optical bandgap of the films was estimated from the optical absorption versus a photon energy plots, and obtained values were 1.43±0.01eV. The measured electrical conductivities of the films varied from 10-8 to 10-5 (Ω·cm)-1 with optical intensity.

A possible way to reduce absorber layer thickness in thin film CdTe solar cells

The objectives of this contract are to investigate thin films of cadmium telluride on low cost substrates and to determine the feasibility of using these films for high efficiency solar cells. Efforts during this program have been directed to the construction of apparatus for the chemical vapor deposition of cadmium telluride films, the selection and preparation of substrates, the deposition and characterization of cadmium telluride films, and the fabrication and characterization of solar cells. Cadmium telluride films have been deposited on a number of substrates by the direct combination of cadmium and tellurium on the substrate surface at 500/sup 0/C or higher at rates of up to 0.6 ..mu..m/min. The structural, crystallographic, and electrical properties of cadmium telluride films deposited over a wide range of conditions have been evaluated. A series of doping experiments have been carried out using iodine and indium as the n-type dopant, and phosphorus, arsenic, and antimony as the p-type dopant. Low resistivity films have not been produced thus far. In/W/graphite substrates have been used for the deposition of n-type films with an ohmic interface. However, no suitable substrates have been found to form an ohmic interface with p-type films. Solar cells prepared from these films exhibit relatively good short-circuit current density, up to 15 mA/cm/sup 2/, but their conversion efficiencies are severely limited by the high series resistance of the devices.

Material utilisation when depositing CdTe layers by inline AP-MOCVD

Absorption in the CdS window layer limits the photocurrent obtained from thin film CdTe solar cells. These absorption losses can be avoided by tailoring the band gap of the CdS material to improve the light transmission at shorter wavelengths. This can be achieved by adding oxygen to the CdS to create Oxygenated Cadmium Sulphide films (CdS:O). In this paper we report on a new process in which oxygen is incorporated into the thin film CdS during deposition by pulsed DC magnetron sputtering. Spectrophotometric measurements of transmission show that the absorption edge is shifted from 500nm to 350nm by incorporating the oxygen into the deposited films. In this way, light absorption in the window layer of the solar spectrum utilised by the CdTe solar cells was significantly reduced. Ellipsometric measurements showed that the refractive index of the CdS decreased from 2.45 to 2.1 at λ=632.8nm and the band gap shifted from 2.38eV to 3.1eV as a result of incorporating the oxygen into the film. The refractive index dispersion profiles suggest that the oxygenated films are amorphous/semi-amorphous and this was confirmed by X-ray Diffraction measurements and TEM cross-section images.

Cadmium sulfide continues to be the most successful and widely used n-type heterojunction partner in thin film CdTe solar cells. In most cases solar cell performance is enhanced if the CdS films are heat treated prior to the deposition of the CdTe. This paper discusses the effect of H2 annealing on the resistivity of CSS-CdS films and the use of a mobility activation model to explain the observed changes in resistivity. Photoluminescence measurements of CSS CdS films heat-treated in He and in CdCl2 vapor have also been carried out. In all cases the heat treatments lead to an increase in the intensity of a photoluminescence band believed to be associated with sulfur vacancies.