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Comparison between the effects of CdCl2 heat treatment on CdTe films prepared by RF magnetron sputtering and close spaced sublimation methods
Abstract
CdTe films were prepared on Fluorine-doped tin oxide substrate by RF magnetron sputtering and close spaced sublimation (CSS) methods, respectively. These CdTe films were then treated with a wet CdCl2 heat process at different temperatures. The structural and optical properties of CdTe films were investigated by X-ray diffraction, scanning electron microscope and UV–Visible spectrophotometer. The results reveal that both types of CdTe films have a better crystalline and larger grain size after CdCl2 heat treatment. However, the (422) peak has a more preferential orientation than (511) peak after CdCl2 activation for CdTe films prepared by sputtering method, while these two peaks almost have the same intensity for CSS-prepared CdTe films. The transmittance of CdTe films prepared by CSS is apparently lower than sputtered CdTe films. Correspondingly, the efficiency of solar cells with CSS-prepared CdTe is 7.3, 2.6 % of sputtered CdTe films.
... *e-mail: chernandezex@gmail.com Additionally, a thermal annealing of CdTe thin films by CdCl 2 plays an important role in the photovoltaic process; it increases the CdTe grain size and reduce the grain boundary [14][15][16] . The record efficiency into CdS/CdTe solar cells are 21.3% at laboratory level and 16% at commercial solar panels 17 . ...
The polycrystalline CdS/CdTe thin film solar cell is one of the most important photovoltaic devices for cost-effective generation of solar electricity for terrestrial applications. A typical superstrate structure of CdTe solar cell has been studied through current-voltage (J-V) and secondary ion mass spectroscopy (SIMS) measurements. A close correlation between quality of interphases and its photovoltaic efficiency was determined. It was found an improvement of open circuit voltage (VOC) and efficiency associated to CdS and CdTe thermal treatments, and a reduction of diffusion of S and Cd into CdTe and CdS respectively. An efficiency of 12% has been reached on solar cells with a Te and ZnTe interlayer as part of the back contact. Low diffusion of Cu along absorbent material was observed when Te and ZnTe was used creating a stable back contact along the time. Diffusion and intermixing at each junction SnO2:F/CdS, CdS/CdTe and CdTe/Te or ZnTe/Cu/Au was found, establishing limit values of element diffusion along CdTe solar cells.
... The major focus among the chalcogenide materials is on Copper Indium Selenide (CuInSe 2 ), Copper indium gallium selenide (CIGS) and Cadmium Telluride (CdTe) materials systems. [2][3][4][5] However, the development of solar cells based on these materials seems to be saturating lately. This is owing to quite a few factors such as toxicity of cadmium in case of CdTe, scarcity and high cost of Indium and Gallium in case of CIS and CIGS and to certain extent, the saturation of efficiency for single junction devices. ...
Research in the field of thin film photovoltaic has been stimulated because of the low production cost and less material consumption and it has been accelerating after the exploration of chalcogenide materials viz. CIS, CIGS, CdTe and CZTS. But, toxicity of Cadmium, high cost of Indium and Gallium and plenty of barriers in improvement of efficiency in case of CZTS gave rise to research in similar and less explored chalcogenide materials and candidate material among them. Copper Manganese Tin Sulphide (CMTS) is one of the promising chalcogenide materials for development of low-cost, thin-film solar cells. However, being a new material, there are not many reports on the optimisation of device structure for this materials system. Particularly there is no systematic studies on the material-compatibility as far as the buffer layer is concerned for the CMTS based solar cell. In this work, we studied the effect of the buffer layer (CdS, Zn(O,S) and SnS2) on the performance of the CMTS based thin film solar cell device by using SCAPS 1-D simulation. We report that the device with SnS2 as a buffer layer shows comparatively high power conversion efficiency, that is 20.26%. Subsquently, the device structure of CMTS solar cell with SnS2 buffer layer was optimized for various physical parameters, viz. Thickness of the absorber and buffer layer, acceptor density of the absorber layer, donor density of the buffer (SnS2) layer, and the defect density. These results could be helpful in designing and experimentally implementing the CMTS based low-cost thin-film solar cell with Cd-free buffer layer.
... This alloy broadening effect apparently overcomes any possible improvements through reduction of the bandgap offset at the HRT/CdSe 1-z S z interface. Although Ref. [49] has demonstrated that Urbach tail broadening can be reduced significantly by annealing the CdSe to the high temperatures of overlying CdTe deposition by close-space sublimation (CSS), evidence from studies of other II-VI compound materials suggest that the temperatures associated with the CdCl 2 treatment are not sufficient to generate the same effect as high temperature annealing [50][51][52][53]. ...
Thin film CdTe superstrate solar cells have been fabricated by sputtering starting from CdS/CdSe front layers deposited on transparent conductor coated glass. The performance of such devices is sensitive to the fabrication details including the temperature-time profile, which leads to CdSe/CdTe interdiffusion and formation of a CdTe 1-x Se x bandgap-graded absorber. Mapping spectroscopic ellipsometry (M-SE) has been applied to the CdS and CdSe thin films for process calibration, which involves determining the deposition rate in terms of effective thickness (volume/area) versus spatial position on the sample. The goal is to optimize the performance of the devices by correlating cell parameters with these two effective thicknesses. Intended variations in the thicknesses along with unintended spatial non-uniformities enable coarse and fine-scale optimization, respectively. Using these methods, the highest performance solar cells from the CdS/CdSe/CdTe structure are obtained with 13 nm CdS and 100 nm CdSe. An increase in the CdS thickness above 13 nm leads to a decrease in open-circuit voltage and fill-factor attributed to the formation of a CdSe 1-z S z interdiffusion region with z approaching 0.5, where the alloy electronic properties are likely to suffer. Our results demonstrate that M-SE, exploited in conjunction with deposition non-uniformities, serve as a viable approach for process optimization of complex solar cell structures.
... However, the exacts roles of chlorine diffusion on CdTe are still under debate as many explanation and mechanisms have been proposed. Researches in this topic have indicated that CdCl 2 treatment reduce the band gap of CdTe [5], enhances sulphur diffusion from CdS to CdTe forming a CdS x Te 1-x /CdTe y S 1-y interface increasing the atomic mobility of CdS or CdTe [6][7][8], and grains boundaries in CdTe are reduced leading less recombination centers for charge carries, also could separate the photogenerated electron-hole pair through local electric fields [9][10][11]. CdCl 2 thermal treatment promote chlorine segregation on CdTe surface reducing twin boundaries on this [11]. ...
... The chalcogenide based solar cells have been a main focus of research for the last two-three decades due to the early success of CuInSe 2 thin films as absorber in high efficiency solar cells [2]. Among the chalcogenides, Cu(In,Ga)Se 2 [1][2][3] and CdTe [4,5] light absorber materials have been dominated till now. The concerns of high toxicity of cadmium and scarcity of indium in those materials have driven the research to concentrate on other absorber materials made from earth abundant, environment benign precursors [6,7]. ...
The increasing energy demand and the limitations of the existing technologies due to the scarcity, cost and toxicity of the materials urge the researchers to hunt for efficient thin film solar cells based on earth-abundant, inexpensive and less toxic materials. For a decade, binary and ternary antimony based sulfides have gained attention due to their possible applications in solar cells. This interest is the basis of this review. In this review article, we describe basic properties of copper antimony sulfide (CuSbS2) thin films to investigate their photovoltaic applications. A detailed description of the preparation methods, studies on morphologies and optoelectronic properties based on published work, including our experience are presented. A systematic review is done to demonstrate emerging interest in the photovoltaic performance of this compound. This review gives an in depth discussion on the structure, morphology, optical and electrical properties of copper antimony sulfide thin films.
... The heat treatment of CdTe thin films in the presence of CdCl 2 is a key processing step for achieving high device performance. Although this treatment has been used over the past three decades [2][3][4], full understanding has not been achieved yet and this processing step requires careful research in some key areas. The defects signature in thin film CdTe has been identified as one of these key areas for careful investigation, in a recent comprehensive review [5], and this is the main subject of this publication. ...
This work is aimed at studying defect level distributions in the bandgap of CdTe thin films, used for solar cell development. In particular, the effects of CdCl2 treatment on the defect levels are the main objectives of this research. Four different nearly optimised CdTe thin films were electroplated using three different Cd-precursors (CdSO4, Cd(NO3)2 and CdCl2), and bulk CdTe wafers purchased from industry were studied using low temperature photoluminescence. The finger prints of defects, 0.55 eV below the conduction band down to the valence band edge were investigated. In all of the CdTe layers, four electron trap levels were observed with varying intensities but at very similar energy positions, indicating that the origin of these defects are mainly from native defects. CdCl2 treatment and annealing eliminates two defect levels completely and the mid-gap recombination centres are reduced drastically by this processing step. The optical bandgap of all four as-deposited CdTe layers is ~1.50 eV, and reduces to ~1.47 eV after CdCl2 treatment. The material grown using the CdCl2 precursor seems to produce CdTe material with the cleanest bandgap, most probably due to the built-in CdCl2 treatment while growing the material.
High-quality (111)-orientated CdTe thin films are grown epitaxially on SrTiO3 (001) substrates by molecular beam epitaxy (MBE). The evolution of epitaxial growth was in-situ monitored by reflection high energy electron diffraction. The surface morphologies of the CdTe epitaxial layers (CTELs) are characterized by atomic force microscope, and atomic flat surface is confirmed as the thickness of the CTELs is below ∼40 nm. Three types of x-ray diffraction, including single crystal ω-2θ scan, double crystal x-ray rocking curve and φ-scan, are performed to characterize the structural quality of the CTELs. A full width at half maximum of ∼110 arcsec is yielded when the thickness of the CTELs is beyond ∼100 nm supported by the X-ray rocking curve. And φ-scan illustrates the appearance of four types of domains in the CTELs. Finally, direct band transition is determined, and the temperature dependent transmittance spectra exhibit that the optical band gap decreases from 1.581 eV to 1.503 eV with the temperature from 10 K to 300 K because of the electron-phonon interaction and the lattice thermal expansion.
Over the past few years, the studies of CdS/CdTe solar cells have become the mainstream, which have achieved good performance of exceeding 20 %. Here, we prepare CdSe thin films by pulsed laser deposition (PLD) which is one of promising techniques to synthesize semiconductor thin films. In this paper, the optical and electrical properties as well as the crystal structure of CdSe thin films deposited under different temperatures are investigated. We find that the deposition rate adds firstly, and then decreases with the increase of substrate temperature. Transmittance spectrum suggests the absorption edges of CdSe thin films under the condition of high temperature move towards short wavelength. The band gaps of CdSe thin films are 1.948, 1.976, 2.013 and 1.978 eV at the deposition temperature of 150, 250, 350, 450 °C, respectively. X-ray diffraction analysis indicates the formation of cubic phase with a strong (111) preferential orientation. The surface morphology of CdSe thin film and the cross-sectional structure of CdSe/CdTe layer are also analyzed. Based on these studies, we further fabricate the CdSe (PLD)/CdTe solar cells that present excellent performance of response in the range of long wavelength over 900 nm and a good efficiency of near 10 % has been realized.
The "roll-over" phenomenon in current-voltage (J-V) curves of CdS/CdTe devices is usually recognized as a result of the formation of a higher back barrier. When Cu has not been intentionally added to the back contact, roll-over is understandable. However, the mechanism was unclear for forming J-V roll-over in a CdTe cell with a back contact containing Cu. We did extensive characterizations, including XRD, XPS, SIMS, TEM, and EDS, and "recontact" experiments to understand this phenomenon. The results show that me roll-over comes from the formation of Cu-related oxides at the back side of the device during processing, rather man the diffusion of Cu to the front side of me device. Discussions related to the J-V roll-over mechanisms will also be presented.
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.
Cd-enriched cadmium telluride (CdTe) polycrystalline films were grown on corning glass substrates by close spaced sublimation
(CSS) technique. To our knowledge, Cd-enriched CdTe thin films by CSS have not been reported earlier. The structural investigations
performed by means of X-ray diffraction (XRD) technique, scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy
(EDX) showed that the deposited films exhibit a polycrystalline structure with 〈111〉 as preferred orientation. The structural,
optical, and electrical properties of these films were analyzed as a function of the Cd concentration. For the films having
an excess of Cd, the electrical resistivity dropped several orders of magnitude. The deposited films also showed that the
value of resistivity decreased with increasing temperature manifesting the semiconducting behavior of the films. The results
showed that using this deposition technique, n-type Cd-enriched CdTe polycrystalline film could be produced.
Processing of CdTe/CdS solar cells requires annealing of CdS and CdTe/CdS in different ambients. It has been proven that the application of a CdCl2 treatment (or its variant) is important for high efficiency solar cells. This treatment influences the structural and interface properties of the layers. We have grown CdS layers either by a chemical bath deposition (CBD) or a high vacuum evaporation (HVE) on different transparent conducting oxides (TCO): tin oxide doped with fluorine (FTO) and indium tin oxide (ITO) coated glass substrates. The CdTe layers have been grown by a HVE method. Effects of the CdCl2 treatment on the recrystallization of CdTe and CdS have been studied with X-ray diffraction and scanning electron microscopy. An increase in the grain size of CdTe from about 0.5 to 3–7 μm, along with the loss of the preferred (111) growth orientation has been observed. The strain and recrystallization of CdTe, and intermixing of the CdTe and CdS layers strongly depend on the deposition and annealing temperatures. An optimum treatment and a minimum thickness of CBD–CdS is required for high efficiency solar cells. CdS layers and the method of their deposition also have a strong influence on the microstructure of CdTe and photovoltaic properties. Solar cells with efficiency of 11.2 and 2.5% are obtained with HVE and CBD grown CdS window layers.
In this work, CdTe films were prepared by r.f. magnetron sputtering. Influence of dry CdCl2 treatments on the structural and optical properties of CdTe films was investigated by field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), and UV–Visible spectrophotometer. Also, CdS/CdTe thin film solar cells were fabricated by all sputtering process and the cell characteristics were studied. The as-deposited CdTe thin films had a zincblende structure highly oriented with the (111) direction parallel to the substrate surface. The CdTe films lose their preferred orientation and become polycrystalline by heat treatment with dry CdCl2 treatment. The wet CdCl2 treatment exhibited a similar trend, but is less effective for grain growth than dry CdCl2 treatment. The photovoltaic properties of the CdS/CdTe solar cell were considerably improved with the increase in the annealing temperature, which was caused by the increase of grain size in CdTe films. However, the wet CdCl2 process was not suitable for reproducible fabrication.
CdTe thin films were deposited by CSS at a pressure of 1mbar, under different mixtures of argon and oxygen and in vacuum, on glass substrates. The samples were prepared under three source–substrate temperature conditions. The films were characterized by X-ray diffraction, UV–vis spectroscopy, SEM and two-probe resistivity. XRD patterns indicated that the films grown at low oxygen content and source temperature of 550°C with substrate temperature of 400°C showed a high preferential orientation in the (111) plane. The crystalline grain size was between 31 and 40nm. The films had a direct band gap in the range 1.44–1.49eV. SEM images showed higher aggregate sizes for the films deposited at higher temperatures and grain size decreases as oxygen content increases. The resistivity of the films was around 1×106Ωcm.
The optical constants n(E) and k(E) of thin films lead-free vitreous coatings have been determined. The extinction coefficient k was measured in the VIS/UV range using absorption techniques and the real part of the complex index of refraction n was calculated by using the Kramers-Kronig analysis. The sample was prepared from recycled soda-lime glass and melting anhydrous boric oxide in varying proportions, and then deposited on (100) MgO substrates for optical measurements. These measurements shown an optical energy gap about 3.2 eV and high values of n and k in the ultraviolet region, as reported for vitreous materials doped with PbO
For the measurement of optical constants, the electrodeposited films of CdTe were lifted off their opaque substrates and transferred onto glass slides using a transparent liquid adhesive. This technique proved to give results more reliable than those obtained on samples in which CdTe is deposited on CdS-coated conducting glass. The measured optical dispersion in the photon energy range of E<1.5 eV is in excellent agreement with that for the single crystal. The optical absorption coefficient was determined in the E<3.5 eV range and was compared with that for the single crystal. The results revealed two direct allowed transitions at 1.50 eV [Γ8 valence band(VB)→Γ6 conduction band(CB)] and 2.43 eV [Γ7(VB)→Γ6(CB)] and three indirect allowed transitions at 1.27 eV [L4,5(VB)→Γd], 1.83 eV [L6(VB)→Γd], and 2.84 eV [Γ8(VB)→L6(CB)]. The 1.27 and the 1.83 eV transitions, which have not been reported previously and were not detected in single-crystal data, are attributed to the transitions to a grain-boundary-related defect energy band Γd, 0.65 eV above Γ8 (VB). The indirect transitions at 1.83 and 2.84 eV are assisted by phonons having energies of 80 and 84 meV, respectively. © 1997 American Institute of Physics.
Due to its basic optical, electronic, and chemical properties, CdTe can become the base material for high-efficiency, low-cost thin film solar cells using robust, high-throughput manufacturing techniques. CdTe films suited for photovoltaic energy conversion have been produced by nine different processes. Using n-type CdS as a window-partner, solar cells of up to 16% efficiency have been made in the laboratory. Presently five industrial enterprises are striving to master low cost production processes and integrated modules have been delivered in sizes up to 60 × 120 cm2, showing efficiencies up to 9%. Stability, health, and environmental issues will not limit the commercial potential of the final product. The technology shows high promise for achieving cost levels of $0.5/Wp at 15% efficiency. In order to achieve this goal, scientists will have to develop a more detailed understanding of defect chemistry and device operation of cells, and engineers will have to develop methods for high-throughput manufacturing.
CdTe solar cells were fabricated by depositing a Au/Cu contact with Cu thickness in the range of 50 to 150Å on polycrystalline
CdTe/CdS/SnO2/glass structures. The increase in Cu thickness improves ohmic contact and reduces series resistance (Rs), but the excess Cu tends to diffuse into CdTe and lower shunt resistance (Rsh) and cell performance. Light I-V and secondary ion mass spectros-copy (SIMS) measurements were performed to understand the
correlations between the Cu contact thickness, the extent of Cu incorporation in the CdTe cells, and its impact on the cell
performance. The CdTe/CdS/SnO2/glass, CdTe/ CdS/GaAs, and CdTe/GaAs structures were prepared in an attempt to achieve CdTe films with different degrees
of crystallinity and grain size. A large grain polycrystalline CdTe thin film solar cell was obtained for the first time by
selective etching the GaAs substrate coupled with the film transfer onto a glass substrate. SIMS measurement showed that poor
crystallinity and smaller grain size of the CdTe film promotes Cu diffusion and decreases the cell performance. Therefore,
grain boundaries are the main conduits for Cu migration and larger CdTe grain size or alternate method of contact formation
can mitigate the adverse effect of Cu and improve the cell performance.
To make CdTe/CdS solar cells highly efficient, a Cu containing back contact (BC) is generally used. These cells degrade due to Cu diffusion to the front contact which causes shunting; this is shown with secondary ion mass spectroscopy (SIMS) depth profiling. To get a stable but still highly efficient cell, different BC materials and etching treatments were investigated. Chemical etching creates a back surface field (BSF) due to a p+-doped Te-rich CdTe surface. To overcome the naturally existing Schottky barrier between p-CdTe and any metal, a thin buffer layer was evaporated prior to the metallization. Amongst the many investigated BC materials, the most suitable are Sb or Sb2Te3 as a buffer and Mo for metallization. These cells showed high stability under accelerated tests corresponding to 70 years.
The authors use X-ray photoelectron spectroscopy to investigate the chemical reactivity of CdTe films exposed to a solution of CdCl2 dissolved in methanol. They show that annealing in vacuum does not instigate a chemical reaction between CdCl2 and CdTe, but that annealing in either He:O2 or pure He leads to the formation of a surface Cl residue comprising Cd, Te, Cl, and O in the form of oxides and oxychlorides. From detailed analysis of X-ray photoelectron data, they propose that CdO, TeO2, TeCl2O are building blocks for the surface Cl residue.