Cadmium Telluride Thin Films Research Articles

Stable Cu-Based Back Contacts for CdTe Thin Film Photovoltaic Devices

Cadmium telluride (CdTe) thin film photovoltaic devices fabricated in a-line process developed at Colorado State University (CSU) have shown stability during long-term (over a 5 year period) accelerated stress testing. These devices have a copper (Cu) containing back contact. The Cu profile as measured by secondary ion mass spectrometry characterization shows, for the maximum stressed device (23,399 h), that there is a significant (two times) change in the concentration of secondary Cu ions in the bulk of the material; however, the Cu concentration gradient at the back of the device has no significant change, and the CdS layer has no significant Cu concentration increase at open-circuit bias and 65°C temperature conditions. This indicates that with a proper CdCl2 treatment, Cu can be used to form the back contact for CdTe devices with acceptable stability. These devices have a projected field lifetime of greater than 60 years.

Comparative study of Hg xCd 1−xTe films grown on CdTe thin films previously deposited from two different techniques

High quality cadmium telluride (CdTe) thin films were grown on glass substrates with two different techniques, two evaporation source (TES) and closed space sublimation (CSS). Further to the above mercury telluride (HgTe) was then deposited by using single source on both CdTe thin films for obtaining Hg x Cd 1− x Te samples. The crystalline structure of the Hg x Cd 1− x Te sample grown from CSS-CdTe showed the preferential (1 1 1) orientation with smoother and larger grain size than those of TES-CdTe. The optical transmission for TES-CdTe sample was above 90% in the 1000–1500 nm range whereas it was significantly below 80% for CSS-CdTe sample. The optical transmission for TES-Hg x Cd 1− x Te and CSS-Hg x Cd 1− x Te was ∼60%. The resistivity at room temperature of TES-CdTe and CSS-CdTe was ∼3.33×10 9 Ω cm and ∼2.20×10 8 Ω cm, respectively, while the resistivity of TES-Hg x Cd 1− x Te and CSS-Hg x Cd 1− x Te samples was ∼1.73 Ω cm and ∼5.34×10 5 Ω cm, respectively. The comparative study of ternary compound prepared with the above techniques has been carried out for the first time.

Structural, dielectric, and electrical studies on thermally evaporated CdTe thin films

Sandwich structures of cadmium telluride (CdTe) thin films between Ag electrodes were prepared by thermal evaporation technique at a vacuum of ~2 × 10−5 torr. Structural characterization of these thin films was performed using X-ray diffraction (XRD) studies. The effect of temperature and frequency on the electrical and dielectric properties of these films was studied in detail and reported in this article. The experimental study indicates that for the CdTe thin film the dielectric constant and dielectric loss increases with temperature and decreases with frequency. However, A.C. conductivity increases both with temperature and frequency. The data of complex impedance measurements over the same range of temperature and frequency are used to describe the relaxation behavior of the CdTe film. Our results indicate that the transport behavior of carriers in CdTe thin films is consistent with the correlated barrier hopping (CBH) model.

CdCl2-treated CdTe thin films deposited by the close spaced sublimation technique

Cadmium telluride thin films were prepared by the close spaced sublimation (CSS) technique, using CdTe powder as evaporant and were submitted to a chemical treatment at 25°C in a saturated CdCl2 solution (2.08 g/100 mL methanol), followed by heat treatment under vacuum ~10−3 mbar at 400°C for 30 min. The effect of chemical and thermal treatments on the crystallographic, morphological, optical, and electrical properties were studied through XRD, scanning electron microscope (SEM), spectrophotometry, and dc electrical resistivity measurements, respectively. The studies revealed that the CdTe grows in face centered cubic phase and that post-deposition treatment affects the morphology as well as the crystallographic properties. The effect on the morphology is stronger. Increase of the grain size was observed in the samples treated thermally. The electrical resistivity drops by a factor of 20 in CdCl2-immersed and heated samples.

Study the effect of thickness and annealing temperature on the Electrical Properties of CdTe thin Films

The electrical properties of polycrystalline cadmium telluride thin films of different thickness (200,300,400)nm deposited by thermal evaporation onto glass substrates at room temperature and treated at different annealing temperature (373, 423, 473) K are reported. Conductivity measurements have been showed that the conductivity increases from 5.69X10-5 to 0.0011, 0.0001 (?.cm)-1 when the film thickness and annealing temperature increase respectively. This increasing in ?d.c due to increasing the carrier concentration which result from the excess free Te in these films.

Study the effect of thickness and annealing temperature on the Electrical Properties of CdTe thin Films

The electrical properties of polycrystalline cadmium telluride thin films of different thickness (200,300,400)nm deposited by thermal evaporation onto glass substrates at room temperature and treated at different annealing temperature (373, 423, 473) K are reported. Conductivity measurements have been showed that the conductivity increases from 5.69X10-5 to 0.0011, 0.0001 (?.cm)-1 when the film thickness and annealing temperature increase respectively. This increasing in ?d.c due to increasing the carrier concentration which result from the excess free Te in these films.

Preparation and characterization of cadmium telluride thin films by vacuum evaporation

Preparation and characterization of cadmium telluride thin films by vacuum evaporation

Pulse plated cadmium telluride films and their characteristics

Cadmium telluride thin films were deposited on conducting glass and titanium substrates by the pulse plating technique at different duty cycles in the range 10–50%. The films were characterised by X-ray diffraction and were found to possess single phase cubic structure. Optical studies indicated a direct band gap of 1.45 eV. Surface morphology of the films indicated that the crystallite size increases with increase of duty cycle. X-ray photoelectron spectroscopy studies confirmed the formation of CdTe. Electron-dispersive X-ray studies were made to estimate the composition. Cross-plane resistivity measurements indicated that the resistivity decreases with increase of duty cycle.

Simulations of absorbance efficiency and power production of three dimensional tower arrays for use in photovoltaics

The production of cheap energy from the sun will be a major research objective in the coming years. Major strides must be made in solar cell efficiency, including increasing absorbance efficiency of a cell by etching or texturing. In order to increase the absorbance efficiency of solar cells, we have developed a three-dimensional solar cell structure by depositing a cadmium telluride thin film overtop carbon nanotube towers. These towers act as both a scaffolding and an electrical interconnect. Multiple photon interactions as they reflect between these towers increase the absorption efficiency. We have developed a theoretical model and computer simulation to maximize the number of photon interactions due to the geometrical characteristics of the system (aspect ratio, spacing, size, shape, etc.). Simulated modeling has shown that by optimization of parameters, a three-dimensional cell can obtain up to a 300% increase in power production over traditional cells.

Pulse Plated Cadmium Telluride Films and Their Characteristics

Cadmium Telluride thin films were deposited on conducting glass and titanium substrates by the pulse plating technique at different duty cycles in the range 10 - 50 %. The films were characterized by x-ray diffraction and they were found to possess single phase cubic structure. Optical studies indicated a direct band gap of 1.45 eV. Surface morphology of the films indicated that the crystallite size increases with increase of duty cycle. XPS studies confirmed the formation of CdTe. EDAX studies were made to estimate the composition. Cross plane resistivity measurements indicated that the resistivity decreases with increase of duty cycle.

Emissions from Photovoltaic Life Cycles

Photovoltaic (PV) technologies have shown remarkable progress recently in terms of annual production capacity and life cycle environmental performances, which necessitate timely updates of environmental indicators. Based on PV production data of 2004-2006, this study presents the life-cycle greenhouse gas emissions, criteria pollutant emissions, and heavy metal emissions from four types of major commercial PV systems: multicrystalline silicon, monocrystalline silicon, ribbon silicon, and thin-film cadmium telluride. Life-cycle emissions were determined by employing average electricity mixtures in Europe and the United States during the materials and module production for each PV system. Among the current vintage of PV technologies, thin-film cadmium telluride (CdTe) PV emits the least amount of harmful air emissions as it requires the least amount of energy during the module production. However, the differences in the emissions between different PV technologies are very small in comparison to the emissions from conventional energy technologies that PV could displace. As a part of prospective analysis, the effect of PV breeder was investigated. Overall, all PV technologies generate far less life-cycle air emissions per GWh than conventional fossil-fuel-based electricity generation technologies. At least 89% of air emissions associated with electricity generation could be prevented if electricity from photovoltaics displaces electricity from the grid.

Preparation and Characterization of CdTe for Solar Cells, Detectors, and Related Thin-Film Materials

Cadmium telluride (CdTe) thin films were prepared by the close-space sublimation (CSS) technique, using 99.99% pure CdTe powder as the evaporant. Films were then annealed at 400°C for 30 min and were later dipped in Cu(NO3)2-H2O solution at 80 ± 2°C. After immersion these films were again annealed at 400°C for 1 h to ensure the Cu diffusion into the films. X-ray diffraction (XRD) results confirmed the formation of a new compound copper telluride and a change in the morphology was observed by scanning electron microscopy (SEM). The DC electrical resistivity reduced from 106 Ω-cm for as-deposited to 10−3 Ω-cm for 15 h immersed film. As the wt.% of Cu increased, the mobility increased to some extent, while the carrier concentration showed a systematic increase. The film thickness and optical parameter such as refractive index, absorption coefficient, and the optical band gap were deduced by fitting the optical transmittance in the wavelength range 300 to 3000 nm. The transmission decreased with increasing immersion time of films in the␣solution. The Cu concentration was recorded as 0.9 wt.% for 3 min to 56.6 wt.% for 15 h immersed samples using an electron microprobe analyzer (EMPA). In the next step, ITO/CdS/CdTe heterojunctions with 10.9% solar cell efficiency were fabricated on glass slides.

Characterization of cadmium telluride thin films fabricated by two-source evaporation technique and Ag doping by ion exchange process

Cadmium telluride (CdTe) thin films were deposited onto scratch free transparent glass substrates by two-source evaporation technique, using Cd and Te as two different evaporants. In the next step films were heated under vacuum at 400°C for 1h and dipped in AgNO3–H2O solution at room temperature. These films were again heated under vacuum for 1h at 400°C to obtain maximum Ag diffusion. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), electrically i.e. DC electrical resistivity by van der Pauw method at room temperature, dark conductivity, activation energy analysis as a function of temperature by two-probe method under vacuum and optically by Lambda 900 UV/VIS/NIR spectrophotometer. The EDX results showed an increase of Ag content in the samples by increasing immersion time of the CdTe films in the solution.

Study of the physical properties of Bi doped CdTe thin films deposited by close space vapour transport

Bi doped cadmium telluride (CdTe:Bi) thin films were grown on glass-substrates by the close space vapour transport method. CdTe:Bi crystals grown by the vertical Bridgman method, varying the nominal Bi concentration in the range between 1 × 10 17 and 8 × 10 18 cm − 3 , were used in powder form for CdTe:Bi thin film deposition. Dark conductivity and photoconductivity measurements in the 90–300 K temperature range and determination by photoacoustic spectroscopy of the optical-absorption coefficient of the films in the 1.0 to 2.4 eV spectral region were carried out. The influence of Bi doping levels upon the intergrain barrier height and other associated grain boundary parameters of the polycrystalline CdTe:Bi thin films were determined from electrical, optical and morphological characterization.

Effect of swift heavy ion irradiation on spray deposited CdX (X = S, Te) thin films

Cadmium sulfide and cadmium telluride thin films are irradiated with high energy heavy ion beam to study the irradiation induced effects in these films. The polycrystalline thin film samples deposited by spray pyrolysis are irradiated with 60MeV Oxygen ions using tandem Pelletron accelerator. The X-ray diffraction patterns exhibit a reduction in peak intensities in both CdS and CdTe films. The grain size decrease with fluence is observed for both CdS and CdTe films, with more decrease for CdTe films. The AFM results support this observation. The films show opposite trend in the variation of electrical resistivity with irradiation fluence. A decrease in resistivity is observed for CdS films due to an increase of carrier concentration arising by the creation of sulfur vacancies during the irradiation. The creation of sulfur vacancies is confirmed by XPS studies. The stoichiometric changes seen from XPS studies support this observation. An enhancement of grain boundary scattering due to the reduction of grain size leads to the increase of electrical resistivity for CdTe films.

Materials Challenges for CdTe and CuInSe2Photovoltaics

AbstractThe record laboratory cell (∼1 cm2area) efficiency for thin-film cadmium telluride (CdTe) is 16.5%, and that for a copper indium diselenide (CuInSe2) thin-film alloy is 19.5%. Commercially produced CdTe and CuInSe2modules (0.5–1 m2area) have efficiencies in the 7–11% range. Research is needed both to increase laboratory cell efficiencies and to bring those small - area efficiencies to large-area production. Increases in laboratory CdTe cell efficiency will require increasing open-circuit voltage, which will allow cells to harvest more energy from each absorbed photon. This will require extending the minority carrier lifetime from its present τ ≤ 2 ns to τ ≥ 10 ns and increasing hole concentration in the CdTe beyond 1015cm2, which appears to be limited by compensating defects. Increasing laboratory CuInSe2-based cell efficiency significantly beyond 19.5% will also require increasing the open-circuit voltage, either by increasing the bandgap, the doping level, or the minority carrier lifetime. The photovoltaic cells in commercial modules occupy tens of square centimeters, and both models and experiments have shown that low-performing regions in small fractions of a cell can significantly reduce the overall cell per formance. Increases in commercial module efficiency will require control of materials properties across large deposition areas in a high-throughput environment to minimize such non-uniformities. This article discusses approaches used and research needed to increase the ultimate efficiencies of CdTe- and CuInSe2-based devices and translate these gains to commercial photovoltaic modules.

The effect of high-frequency sonication on charge carrier transport in LPE and MBE HgCdTe layers

A systematic study of mercury cadmium telluride thin films subjected to high-frequency sonication was carried out. Photoconductivity spectroscopy and the Hall effect technique were used. The charge carrier transport parameters were determined from the Hall coefficient and conductivity measurements. The best agreement between experiment and calculation was obtained assuming that the layer with low-mobility electrons was formed as a consequence of the sonication. It was also determined that the parameters of HgCdTe thin films grown by MBE on GaAs substrates are stable to ultrasonic influence, whereas for HgCdTe thin films grown by LPE on CdZnTe substrates the conductivity-type conversion stimulated by sonication takes place. Substrate properties have been observed to play a significant role for the stability of the HgCdTe epilayers. The phenomenon of the negative differential resistance was detected during the sonication of the n-type HgCdTe bulk crystal.

Impact of Evaporation Rates of Cd and Te on Structural, Morphological, Optical, and Electrical Properties of CdTe Thin Films Deposited by a Two-Sourced Evaporation Technique

A two-sourced evaporation technique was used for deposition of cadmium telluride thin films onto scratch-free transparent glass substrates, using Cd and Te as two different evaporants. Nine samples were deposited at three Te evaporation rates, that is, 6.5, 4.5, and 2.5 nm/s as a function of three substrate temperatures at 400, 300, and 200 °C respectively, keeping the Cd evaporation rate fixed at 2.7 nm/s. All the samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), optically by Lambda 900 UV/vis/NIR spectrophotometer and electrically, that is, DC electrical resistivity, by the van der Pauw method at room temperature. Content composition was investigated by energy-dispersive X-ray analysis. Strong mutually supporting effects on structure, morphology, optical transmission, reflection, and electrical resistivity were observed.

Investigation of Cu-containing low resistivity CdTe thin films deposited by the two-source evaporation technique

Cadmium telluride (CdTe) thin films were deposited onto scratch-free transparent glass substrates by the two-source evaporation technique, using Cd and Te as two different evaporants. In the next step, films were heated under vacuum at 500 °C for 1 h and dipped in Cu(NO3)2–H2O solution at room temperature. These films were again heated under vacuum for 1 h at 500 °C to obtain maximum Cu diffusion. The samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), optically by a Lambda 900 UV/VIS/NIR spectrophotometer and electrically, i.e. dc electrical resistivity, by the van der Pauw method at room temperature, dark conductivity, and activation energy analysis as a function of temperature by the two-probe method under vacuum. The EDX results showed an increase of Cu content in the samples by increasing the immersion time of the CdTe films in the solution. The layer thickness of diffused Cu atoms in CdTe is determined by comparing the difference of absorption between as-deposited and immersed films with the absorption graph of Cu films of varying thicknesses.

Characterization of nanocrystalline cadmium telluride thin films grown by successive ionic layer adsorption and reaction (SILAR) method

Structural, electrical and optical characteristics of CdTe thin films prepared by a chemical deposition method, successive ionic layer adsorption and reaction (SILAR), are described. For deposition of CdTe thin films, cadmium acetate was used as cationic and sodium tellurite as anionic precursor in aqueous medium. In this process hydrazine hydrate is used as reducing agent and NH4OH as the catalytic for the decomposition of hydrazine. By conducting several trials optimization of the adsorption, reaction and rinsing time duration for CdTe thin film deposition was done. In this paper the structural, optical and electrical properties of CdTe film are reported. The XRD pattern shows that films are nanocrystalline in nature. The resistivity is found to be of the order of 411 × 103 Ω-cm at 523 K temperature with an activation energy of ∼ 0.2 eV. The optical absorption studies show that films have direct band gap (1.41 eV).