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.     

Influence of CdS growth process on structural and photovoltaic properties of CdTe/CdS solar cells

The morphology of CdS layers grown by chemical bath deposition (CBD) and high vacuum evaporation (HVE) have been investigated. The grains of CBD-CdS are more compact and smooth than those of HVE-CdS. The annealing and CdCl₂ treatment cause grain growth, which is stronger for the CdCl₂ treated samples. The grain-size of the as-deposited CdTe on CBD-CdS is about 5 times larger than of those grown on HVE-CdS. The structural and electrical properties of CdTe/CdS solar cells are strongly dependent on the CdS. The grain size of CdCl₂ treated CdTe layers are similar, irrespective of the transparent conducting oxide substrate and CdS deposition method. The efficiency of solar cells on thin CBD-CdS is low (about 5.6%) because of pin-holes and a large intermixing of CdTe-CdS. The cells on HVE-CdS yield a higher current density despite thicker HVE-CdS as compared to CBD-CdS. The efficiency of solar cells on HVE-CdS is 12.3%.     

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%.     

CdTe solar cell degradation studies with the use of CdS as the window material

We present in this work the degradation effects with time in thin film CdTe/CdS solar cells, where the CdS and CdTe layers are deposited by chemical bath deposition (CBD) and close space vapor transport (CSVT), respectively. The CdS thin films were grown from different baths by varying the S/Cd ratio. The variation of the S/Cd ratio allowed us to control the morphology and the density of defects, thus giving rise to better quality CBD CdS films. Depending on the S/Cd ratio an improvement of the morphology and capacitance signal was observed, these factors have also an influence on the open-circuit voltage, short-circuit current density, fill factor and conversion efficiency of the solar cell. The variation with time of these parameters in our devices was tracked during a period of 3 years measured directly on the exposed back contact regions (CdTe/Cu/Au). A discussion on the deterioration of the photovoltaic (PV) performance of the solar cells is presented in correlation with the local environmental conditions. This particular environment has contamination, and represents another type of stress for standard PV operations. These conditions reduce the mean life time of solar cells beyond short periods; this can be of interest for PV community.     

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%).     

Recent technical advances in thin-film CdS/CdTe solar cells

CdS/CdTe solar cells have attracted attention recently for their potential as low-cost, high-efficiency solar cells of the future. It is because the CdTe layer (used for photoelectric conversion) has a bandgap energy of 1. 51 eV, which corresponds well to sunlight spectra, and the direct transition type energy band structure enables formation of thinner films.We have already industrialized CdS/CdTe solar cells in mass production stage using a printing-sintering process, as large-area modules for electric power generation(Higuchi , 1993, Omura , 1991), and as cells for indoor applications (primarily in calculators. Suyama , 1986). However, this solar cell has a conversion efficiency of approximately 6%.Recently, there has been considerable research into thin-film CdS/CdTe solar cells which have a thinner CdS film formed by CVD or CBD (Britt , 1993) process, and thus are photosensitive to light with wavelengths of 500 nm or less. At present stage of our art, in solar cells formed by the CSS with a CdTe film on CVD CdS, a conversion efficiency of 15. 05% has been obtained in cells with an area of 1 cm² (verified at JQA).     

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.     

Interfacial mixed-crystal layer in CdS/CdTe heterostructure elucidated by electroreflectance spectroscopy

Electroreflectance (ER) spectroscopy has been carried out on the CdS/CdTe thin-film solar cells to reveal the CdS/CdTe interfacial properties, which is believed to play a crucial role in determining the photovoltaic performances. The ER feature at around 1.46 eV is to be ascribed as due to the mixed-crystal CdTe_0.95S_0.05 layer unintentionally formed at the CdS/CdTe interface during CdTe deposition on CdS. Comparing the ER spectrum with the quantum efficiency (QE) response of the solar cell, the indication is that the n-p junction locates itself in the mixed-crystal layer, because no pronounced response due to the CdTe layer is observed in the ER spectrum whereas it is observed in the QE spectrum.     

Chemical bath deposition for the fabrication of antireflective coating of spherical silicon solar cells

A CdS film as an antireflective (AR) coating has been successfully deposited on spherical silicon solar cells by chemical bath deposition, which is a novel deposition method of AR coatings for spherical silicon solar cells. The CBD method is a growth method in an aqueous solution and enables film formation for electronic devices with arbitrary shapes. The solar cell performance of the cell with the CdS film showed a 16% increase in short circuit current compared to that without an ARC. The result confirms that the CBD method is useful for the ARC fabrication of spherical silicon solar cells.     

Effect of electron irradiation on the properties of CdTe/CdS solar cells

Polycrystalline CdTe/CdS solar cells are used in space, as well as terrestrial, applications. The results of the studies on the effect of 8 MeV electron irradiation on p-CdTe/n-CdS thin film solar cells prepared by radio frequency (RF) sputtering are presented in this article. Solar cell parameters like short circuit current (I_sc), open circuit voltage (V_oc), fill factor (FF), conversion efficiency (η), saturation current (I_s) and ideality factor (n) have been considered. CdTe thin film solar cells exhibit good stability against electron irradiation up to 100 kGy.     

Fabrication of flexible CdTe solar modules with monolithic cell interconnection

CdTe solar cells and modules have been manufactured on polyimide (PI) substrates. Aluminum doped zinc oxide (ZnO:Al) was used as a transparent conductive oxide (TCO) front contact, while a thin high resistive transparent layer of intrinsic zinc oxide (i-ZnO) was used between the front contact and the CdS layer. The CdS and CdTe layers were evaporated onto the ZnO:Al/i-ZnO coated PI films in a high vacuum evaporation system followed by a CdCl₂ activation treatment and a Cu–Au electrical back contact deposition. In some cases prior to the cell deposition, the PI film was coated with MgF₂ on the light facing side and the effects on the optical and electrical properties of TCO and solar cells were investigated. The limitations on current density of solar cells due to optical losses in the PI substrate were estimated and compared to the experimentally achieved values. Flexible CdTe solar cells of highest efficiencies of 12.4% and 12.7% were achieved with and without anti-reflection MgF₂ coating, respectively.Laser scribing was used for patterning of layers and monolithically interconnected flexible solar modules exhibiting 8.0% total area efficiency on 31.9 cm² were developed by interconnection of 11 solar cells in series.     

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%.     

EFFECTS OF PROCESSING ON CdTe/CdS MATERIALS AND DEVICES

By analyzing CdTe/CdS devices fabricated by vacuum evaporation, a self consistent picture of the effects of processing on the evolution of CdTe cells is developed which can be applied to other fabrication methods. In fabricating CdTe/CdS solar cells by evaporation, a 400°C CdCI₂ heat treatment is used which recrystallizes the CdTe and interdiffuses the CdS and CdTe layers. The interdiffuson can change the bandgap of both the CdTe and CdS which modifies the spectral response of the solar cell. After this heat treatment a contacting/doping procedure is used which converts the CdTe conductivity to p-type by diffusion from Cu from the contact. Finally, the cell is treated with Br₂CH₃,OH which improves both Voc and FF. Analogous process steps are used in most fabrication processes for CdTe/CdS solar cells.     

Highly efficient large area (10.5%, 1376 cm) thin-film CdS/CdTe solar cell

Highly efficient large area (10.5%, 1376 cm²) thin-film CdS/CdTe solar cell sub-module has been fabricated. Very recently we also have fabricated a very large area sub-module of aperture area 5413 cm² exhibiting a conversion efficiency of 8.4%. Such a high efficiency has been achieved by depositing all the constituent films such as SnO₂: F, CdS and CdTe having greater uniformity and better quality under atmospheric pressure conditions. A post deposition treatment of CdTe surface with CdCl₂ has been optimized to improve the overall solar cell output performance significantly.     

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%.     

Photovoltaic properties of CdTe solar cells fabricated by close spaced sublimation with screen printed CdTe sources

Screen printed CdTe layer was employed as a source for close spaced sublimation process. By controlling the initial mixture of screen printed source, the Cd content in the CdTe film varied from 49.5% to 48%. The solar efficiency of CBD CdS/CdTe cell strongly depended on the stoichiometry of CdTe films due the bulk resistivity change ranging from 2 × 10⁶ to 3 × 10⁴Ω cm. By comparing with the dark forward current of screen printed CdS/CdTe cell, it is suggested that in the CBD CdS/CdTe cell the grain size of CBD US layer should be increased to reduce the interface leakage current at low forward bias.     

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.     

HIGH EFFICIENCY THIN FILM CdS/CdTe SOLAR CELLS†

Cadmium telluride is a most promising thin film photovoltaic material as shown by the higher than 10% efficient CdS/CdTe heterojunction solar cells. In this work, thin film CdS/CdTe solar cells have been prepared using CdS films grown from an aqueous solution and p-CdTe films deposited by metalorganic chemical vapor deposition (MOCVD) and close-spaced sublimation (CSS). The solution-grown CdS films, with high photoconductivity ratios and good optical transmission, are well-suited for optoelectronic devices. The CdTe films deposited by CSS show considerably better microstructure than those by MOCVD because of the higher substrate temperature used in the CSS process. The properties of CdS/CdTe heterojunctions have been studied. Solar cells of 1 cm2 with an AM 1.5 efficiency of 14.6%, verified at the National Renewable Energy Laboratory, have been CdTe films     

Chemically and electrochemically deposited thin films for solar energy materials

Direct energy gap materials, e.g. CdTe, CuInSe₂, CuInGaSe₂, CdSe, ZnP₂ and Zn₃P₂, are the most interesting for thin-film solar cell applications. Among the various methods of preparation of these films, chemical bath deposition and electrodeposition deserve special attention because they have been shown to be inexpensive, low-temperature and non-polluting methods. Based on Pourbaix diagrams of CdS, CdTe, CuInSe₂, CdSe, etc., drawn from basic considerations, the best parameters for their electrodeposition are deduced. Theoretical considerations on the chemical-bath deposition of CdS, CdSe and Sb₂S₃ are also indicated. In particular, the role of the complexing agent and of the ligands in chemical bath deposition quality is discussed, as are the uniformity and stability of the films. The photoelectrochemical, Schottky barrier and heterojunction solar cell properties based on chemically and electrochemically deposited thin films with heteropolyacids are shown. Future trends for chemically and electrochemically deposited polycrystalline thin films are addressed. Results from very recent work done in the improvement of chemically and electrochemically deposited thin films are presented. Significant results obtained on advanced CdS/CdTe, CdS/CIS and CdS/CIGS solar cells developed by industry and by laboratory groups worldwide are indicated. Emerging low cost materials or/and less environmental hazards materials which may introduce solar cells into worldwide market are considered in the conclusion.