Photoluminescence characteristics of CdS layers deposited in a chemical bath and their correlation to CdS/CdTe solar cell performance

In this work, we study CdS films processed by chemical bath deposition (CBD) using different thiourea concentrations in the bath solution with post-thermal treatments using CdCl₂. We study the effects of the thiourea concentration on the photovoltaic performance of the CdS/CdTe solar cells, by the analysis of the I–V curve, for S/Cd ratios in the CBD solution from 3 to 8. In this range of S/Cd ratios the CdS/CdTe solar cells show variations of the open circuit voltage (V_oc), the short circuit current (J_sc) and the fill factor (FF). Other experimental data such as the optical transmittance and photoluminescence were obtained in order to correlate to the I–V characteristics of the solar cells. The best performance of CdS–CdTe solar cells made with CdS films obtained with a S/Cd ratio of 6 is explained in terms of the sulfur vacancies to sulfur interstitials ratio in the CBD–CdS layers.     

Physical properties of chemical bath deposited CdS thin films

Cadmium sulfide films of different thicknesses were deposited by chemical bath deposition (CBD) from a bath containing cadmium chloride, ammonium chloride, ammonium hydroxide and thiourea. The XRD patterns show that the films have a hexagonal phase with a preferential (0 0 2) orientation. The photoluminescence spectra show a defect structure, characteristics of the CdS films obtained by CBD. The electrical behavior in dark and under illumination, the optical properties and the band gap value reported in this work is in agreement with that reported in the literature.     

Spectral response of CdS/CdTe solar cells obtained with different S/Cd ratios for the CdS chemical bath

In this work, the influence of the variation of chemical bath thiourea concentration in the solution for depositing CdS layers upon the spectral response of chemical bath deposition (CBD)-CdS/CdTe solar cells is studied. Although changes in the short and long wavelength range for the spectral response of the cells were observed in dependence of the thiourea concentration, no significant changes were observed in the diffusion length of minority carriers in the CdTe layer, as determined from the constant photocurrent method, when the thiourea concentration is increased in the CdS deposition solution.     

The effect of heteropolyacids and isopolyacids on the properties of chemically bath deposited CdS thin films

The deposition of CdS films on ITO/glass substrates from a chemical bath containing cadmium acetate, ammonia, ammonium acetate and thiourea has been carried out with and without small amounts of heteropolyacids (HPA) (phosphotungstic acid (PTA): H₃[PW₁₂O₄₀], silicotungstic acid (STA): H₄[SiW₁₂O₄₀], phosphomolybdic acid (PMA): H₃[PMo₁₂O₄₀]) and isopolyacids (IPA) (tungstic acid (TA): H₂WO₄ and molybdic acid (MA): H₂MoO₄) for different deposition times. The chemical, morphological, structural and optical properties of the films have been determined. The composition in sulphur and in cadmium of the films’ surface and volume was determined for various HPA and IPA used in the deposition bath. The HPA and IPA which give the thickest film with the biggest grain size were deduced. The optical transmission at 400 nm of CdS films deposited with STA at short time (20 min) (50%) is higher than those of CdS deposited at longer time (6 h) (7%). The optical transmission of CdS deposited with STA at short time is higher (50%) than that of CdS deposited without STA (20%). The performances of heterojunctions CdS/CdTe solar cells fabricated from CdS films deposited with and without STA and CdTe films deposited without STA have been determined. It was shown that the CdS/CdTe heterojunction solar cells fabricated from CdS films deposited with STA exhibited better photon collection efficiency and solar cell efficiency (η=6%) than CdS/CdTe heterojunction solar cells fabricated from CdS films deposited without STA (η=3.3%).     

Device performance characterization and junction mechanisms in CdTe/CdS solar cells

The performance of CdTe/CdS solar cells has been successfully characterized in terms of a device model based on Shockley–Read–Hall (SRH) recombination theory. The model has been applied to a large number of devices from our laboratory in the 10–15% efficiency range and is able to provide key insights into the diode properties of our devices and the fundamental mechanisms that determine performance. Methods for the reliable measurement of key device parameters are presented, and the results are verified by simulating the characterization data in a self-consistent manner. Crossover between the dark and light J–V curves has been identified as a front contact phenomenon arising from the presence of CdS. Junction mechanisms and an observed relationship between reverse saturation current and diode quality factor are discussed. Our techniques indicate that all values of diode quality factor are between 1 and 2 which is consistent with SRH recombination theory and explainable in terms of the location and lifetimes of the recombination centers. It is found that devices with large diode factors are dominated by midgap states. Reduction of midgap states results in a reduction of the diode factor and improved performance.     

Influence of growth and microstructure of chemical deposited CdS films on the electrical properties of CdS/CdTe solar cells

Thin film n-CdS/p-CdTe solar cells were prepared from chemical bath deposited CdS and electrodeposited CdTe layers. The microstructural and some electrical properties of these layers were studied and connection with photovoltaic performance of the cells was shown. Especially, adherence of the CdS films and the quality of heterojunction interface manifesting themselves in the value of the open-circuit voltage U_oc depend on the cadmium precursor used for CdS deposition and on whether pH buffered conditions were applied or not. The number of CdS layers in the cells needed to obtain U_oc of about 700 mV is connected with the CdS deposition conditions.     

15.1% Highly efficient thin film CdS/CdTe solar cell

High-efficiency CdS/CdTe solar cells with thin CdS film have recently been developed. Semiconductive layers of CdS via the CVD method and of CdTe via the CSS method were deposited on an ITO/#7059 substrate. Cell performance depends primarily on the thickness of CdS film, and the conversion efficiency is highest for a CdS film thickness of around 60 nm. Since the CdS film thickness decreases by about 30% during deposition of the CdTe layer, a thickness of 95 nm is required to obtain a 60 nm-thick CdS film after deposition of a CdTe layer. By observing the CdS film during the CdTe deposition process, a decrease was detected before CdTe layer completely covers the surface of the CdS film. By optimizing the thickness of CdS film, an efficiency of 15.12% for the best cell under AM 1.5 verified at JQA was obtained. This fabrication process has good reproducibility; 92.5% of 1 cm² solar cells fabricated under the same conditions have efficiencies above 14%.     

Recent progress on CdTe/CdS thin film solar cells

The CdTe/CdS thin film solar cell is the most suitable to be fabricated on the form of thin films. The processes used to make all the films, which compose the cell, are quite simple and fast. An efficiency of 16.5% has been reached on laboratory scale and modules of 0.6 × 1.2 m² with efficiency larger than 8% are now fabricated and commercialized. A strong contribution to the development of this type of solar cells has been given by the Parma University group with the discovery of a new ohmic back contact for CdTe which is very stable in respect to any other ohmic contact used for CdTe, and by the development of a new all dry process to make the cell. An efficiency of 15.8% has been recently obtained on a 10⁻⁴ m² soda-lime glass without using any copper or any other metal of the first group of the periodic table of the elements at the back contact.     

A suitable deposition method of CdS for high performance CdS-sensitized ZnO electrodes: Sequential chemical bath deposition

A suitable deposition method of CdS is necessary for the high performance CdS-sensitized ZnO electrodes. In this paper, chemical bath deposition (CBD) and sequential chemical bath deposition (S-CBD) methods were used to deposit CdS on ZnO mesoporous films for ZnO/CdS electrodes. The analysis results of XRD patterns and UV-vis spectroscopy indicated that CBD deposition method leaded to the dissolving of ZnO mesoporous films in deposition solution and thickness reduction of ZnO/CdS electrodes. Absorption in visible region by the ZnO/CdS electrodes with CdS deposition by S-CBD was enhanced as deposition cycles increased due to the stability of ZnO mesoporous films in the S-CBD deposition solutions. The results of photocurrent-voltage (I-V) measurement showed that the performance of ZnO/CdS electrodes with CdS deposition by CBD first increased and then decreased as deposition time increased, and the greatest short-circuit current (J_sc) was obtained at the deposition time of 4 min. The performance of ZnO/CdS electrodes with CdS deposition by S-CBD increased as deposition cycles increased, and both open-circuit voltage (V_oc) and J_sc were greater than those electrodes with CdS deposition by CBD when the deposition cycles of S-CBD were 10 or greater. These results indicated that S-CBD is a more suitable method for high performance ZnO/CdS electrodes.     

Annealing effect of Cu2Te---Au contact to evaporated CdTe film on photovoltaic properties of CdS/CdTe solar cell

The annealing effect of an evaporated Cu₂Te---Au contact to CdTe film on the photovoltaic properties of thin-film CdS/vacuum-evaporated CdTe solar cells has been investigated. V_oc and J_sc for the cells with Cu₂Te---Au contact increased greatly with increasing annealing temperature and showed a maximum value at around 250. Photovoltaic properties of the cell with Cu₂Te---Au contact were improved by annealing in a greater extent than those of the cell with Te---Au or Te---Cu contacts. The cells with Cu₂Te---Au contact exhibit a higher conversion efficiency, comparing with the cells with Te---Au or Te---Cu contacts. The cell with Cu₂Te---Au contact showed a conversion efficiency of 10%. Cu₂Te---Au contact acts as the best pseudoohmic contact on vacuum evaporated CdTe film.     

Chemically deposited CdS by an ammonia-free process for solar cells window layers

Chemically deposited CdS window layers were studied on two different transparent conductive substrates, namely indium tin oxide (ITO) and fluorine doped tin oxide (FTO), to determine the influence of their properties on CdS/CdTe solar cells performance. Three types of CdS films obtained from different chemical bath deposition (CBD) processes were studied. The three CBD processes employed sodium citrate as the complexing agent in partial or full substitution of ammonia. The CdS films were studied by X-ray diffraction, optical transmission spectroscopy and atomic force microscopy. CdS/CdTe devices were completed by depositing 3 μm thick CdTe absorbent layers by means of the close-spaced vapor transport technique (CSVT). Evaporated Cu-Au was used as the back contact in all the solar cells. Dark and under illumination J-V characteristic and quantum efficiency measurements were done on the CdS/CdTe devices to determine their conversion efficiency and spectral response. The efficiency of the cells depended on the window layer and on the transparent contact with values between 5.7% and 8.7%.     

Modification in the chemical bath deposition apparatus, growth and characterization of CdS semiconducting thin films for photovoltaic applications

In this paper, growth and characterization of CdS thin films by Chemical Bath Deposition (CBD) technique using the reaction between CdCl₂, (NH₂)₂CS and NH₃ in an aqueous solution has been reported. The parameters actively involved in the process of deposition have been identified. A commonly available CBD system has been sucessfully modified to obtain the precious control over the pH of the solution at 90°C during the deposition and studies have been made to understand the fundamental parameters like concentrations of the solution, pH and temperature of the solution involved in the chemical bath deposition of CdS. It is confirmed that the pH of the solution plays a vital role in the quality of the CBD–CdS films. Structural, optical and electrical properties have been analysed for the as-deposited and annealed films. XRD studies on the CBD–CdS films reveal that the change in Cadmium ion concentration in the bath results in the change in crystallization from cubic phase with (1 1 1) predominant orientation to a hexagonal phase with (0 0 2) predominant orientation. The structural changes due to varying cadmium ion concentration in the bath affects the optical and electrical properties. Optimum electrical resistivity, band gap and refractive index value are observed for the annealed films deposited from 0.8 M cadmium ion concentration. The films are suitable for solar cell fabrication. Further on, annealing the samples at 350°C in H₂ for 30 min resulted in an increased diffraction intensity as well as shifts in the peak towards lower scattering angles due to enlarged CdS unit cell. This in turn brought about an increase in the lattice parameters and narrowing in the band-gap values. The results are compared with the analysis of previous work.     

Numerical modeling of CdS/CdTe and CdS/CdTe/ZnTe solar cells as a function of CdTe thickness

CdTe-based solar cells have long been of interest for terrestrial usage because of their high potential conversion efficiency (in the range of 18–24%) with low-cost manufacturability and concern over environmental effects. In order to conserve material and address environmental pollution concerns as well as to reduce carrier recombination loss throughout the absorber layer, efforts have been carried out to decrease the thickness of the CdTe absorption layer to 1 μm. As a result, to date, the experimental part of this study has realized cell efficiencies of 15.3% and 11.5% with 7 and 1.2-μm-thick CdTe layers, grown by close-spaced sublimation (CSS) [N. Amin, T. Isaka, T. Okamoto, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 38 (8) (1999) 4666; N. Amin, T. Isaka, A. Yamada, M. Konagai, Sol. Energy Matter. Sol. Cells 67 (2001) 195]. Since some problems remain with such thin 1 μm CdTe layers, possible methods to realize higher efficiency have been investigated using novel solar cell structures, with the help of numerical analyses tools. In the theory part of this study, numerical analysis with a 1-D simulation program named NSSP (Numerical Solar Cell Simulation Program) has been used to simulate these structures. We investigated the viability of CdTe thickness reduction to 1 μm together with the insertion of higher band-gap materials (i.e., ZnTe) at the back contacts to reduce carrier recombination loss there. The study shows potential results of the thickness reduction of CdTe absorption layer for a conventional CdS/CdTe/Cu-doped C structure with around 16% efficiency for cells below 3  μm CdTe. Decreases were found in spectral response that suggest from minority carrier recombination loss at the back contact interface. A higher band-gap material like ZnTe has been inserted to produce a back surface field (BSF) to inhibit the minority carrier loss at the back contact. An increase in the efficiency to about 20% has been found for a 1 μm-thin CdTe cell, which can be attributed to the increased BSF effect at the back contact of thinner CdTe-based cells.     

Chemically-deposited Te layers improving the parameters of back contacts for CdTe solar cells

A chemical bath process was carried out for the deposition of Te layers on CdTe films grown by the close-space vapor transport technique (CSVT) on conducting SnO₂:F substrates. The Te layers were chemically-deposited on as-grown CdTe films and on previously CdCl₂-treated CdTe ones. After Te deposition, the CdTe films were annealed at temperatures from 200 to 400 °C. The Te layer on top of the CdTe films was studied by Raman Spectroscopy and by Scanning Electron Microscopy. The electrical resistivity of the annealed CdTe films was determined from current versus voltage measurements in a sandwich configuration, employing gold contacts on top of the CdTe modified surface. The results show that the combined effect of the Te layer on CdTe together with previous CdCl₂ treatment improves the electrical properties of CSVT-CdTe films. These results are quite promising for increasing performance of CdS/CdTe solar cells.     

Thin film solar cells of CdS/PbS chemically deposited by an ammonia-free process

A new type of solar cell with structure glass/ITO/CdS/PbS/conductive graphite was constructed and studied. Both window (CdS) and absorption (PbS) layers were deposited by means of the chemical bath deposition (CBD) technique. The maximum temperature employed during the solar cell processing was 70 °C and it did not include any post-treatment. In case of the CdS window layer, complexing agents alternative to ammonia were employed in the CBD process and their effects on the CdS films properties were studied. The solar cells are photosensitive in a large spectral range (all visible and near infrared regions); the cell with the area of 0.16 cm² without any special treatment has shown the values of open-circuit voltage V_oc of 290 mV and short circuit current J_sc of 14 mA/cm² with the efficiency η=1.63% (fill factor FF is 0.36) under illumination intensity of 900 W/m². It was found that the CBD-made PbS layer has a certain degree of porosity, which favorably affects its applicability in solar cell construction. The possible ways of device optimization, and in particular, the effect of the PbS grain size on its performance are discussed.     

Thin film CdS/CdTe solar cells: Research perspectives

Polycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaics. The two key properties of this material are its band gap (1.5 eV), close to the ideal for photovoltaic conversion efficiency (1.45 eV), and its high optical absorption coefficient. Thin film CdTe solar cells are typically hetero-junctions with CdS being the n-type partner, or window layer. Efficiencies as high as 16.5% have been achieved, but still there is some potential for increasing them.We make an analysis of the typical CdS/CdTe superstrate solar cell, and from it we establish critical issues and different lines of research in order to improve the current efficiencies. We also show that present record efficiencies are very close to the practical efficiency limit for a CdS/CdTe hetero-junction cell.     

15% Efficiency CdS/CdTe thin film solar cells using CdS layers doped with metal organic compounds

For improving the photovoltaic performance of CdS/CdTe thin film solar cells, the CdS window layer is one of the most crucial factors. Here we demonstrate the photovoltaic performances of the low-environmental-load CdS/CdTe solar cell employing the CdS layer doped with various metal organic (MO) compounds, i.e., (CH₃)₂SnCl₂, (C₆H₅)₃GeCl, (CH₃CO₂)₃In, [(C₂H₅)₂NCS₂]₂Zn. Due to the MO doping, the degree of (1 1 1) preferential orientation of CdTe on the CdS layer is improved remarkably, influencing the increases in V_oc and F.F. Being almost independent of the kind of the MO compounds, the short circuit current increases due to increasing optical transmittance of the MO-doped CdS layers. As a result, utilizing MO-doped CdS, we have achieved the conversion efficiency of 15.1%.     

Transformations of the wavelength of the light incident upon CdS/CdTe solar cells

A method for the improvement of the spectral response of the CdS/CdTe solar cell was proposed. The coatings of fluorescent coloring agent (FCA) on the cell made the cell sensitive to light at wavelengths below 510 nm, transforming the wavelength of the incident light from non-incentive region (below 510 nm) to incentive region (above 510 nm). The FCA coatings showed about 8% and 14% increases in the maximum power of the solar cell under the radiations of a white and day-light fluorescent lamp, respectively. Possible maximum output powers were predicted by using a simple model for the external quantum efficiency of the cell.     

Grain size dependence of the bandgap in chemical bath deposited CdS thin films

CdS thin films were deposited by chemical bath deposition onto glass substrates from chemical bath containing cadmium sulfate, thiourea and ammonia at pH=10.5. The temperature of the bath was maintained at either 75°C or 85°C and under mill stirring. After that the samples were annealed in air at 450°C. Analysis of the as-deposited thin films by energy dispersive X-ray analysis showed that almost all samples have a stoichiometric composition. The morphology of CdS films has been investigated by atomic force microscopy. The structural properties were determined by XRD and a cubic zincblende phase was present in all of the as-grown samples. Evidence of a wurtzite phase appeared after annealing. Grain sizes between 85 and 205 Å were determined from the XRD diffraction peak broadening. The sizes increase with both bath temperature and annealing. The optical properties were studied measuring the transmittance spectra. The room-temperature bandgap energies for each sample were determined from the transmittance by two different methods: extrapolating absorption coefficient and first derivative peak position. The bandgap energy varies from 2.48 to 2.35 eV following closely the quantum confinement dependence of energy against crystallite radius. This shows that the absorption edges of these samples are determined primarily by the grain sizes.     

Influence of ITO surface modification on the growth of CdS and on the performance of CdS/CdTe solar cells

We treated the surface of indium–tin oxide (ITO) substrates in two ways, (i) coating of thin insulating ITO layer or (ii) irradiation of the surface with accelerated ions, and investigated the change in sheet resistance (R_sh) and the water-contact angle (WCA). R_sh increased with the thickness of the insulating ITO layer or with the ion dose. WCA dropped as a result of the surface treatment to <15°. The microstructure, the surface morphology, the optical transmittance, and the stoichiometry of CdS improved with the surface treatment. CdS/CdTe solar cells showed a better performance as a result of ITO surface treatment.