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.     

Physical basis for the design of CdS/CdTe thin film solar cells

Solar cells based on polycrystalline semiconductor thin films have great potential for decreasing the cost of photovoltaic energy. However, this kind of solar cells has characteristics very different from those fabricated on crystalline silicon for which the carrier-transport and behavior is clearly known. Instead, for hetero-junction solar cells made on less known polycrystalline materials the design is almost empirical. In this work, several physical aspects related to the behavior of polycrystalline thin film solar cells will be discussed, and some considerations for an adequate design of this kind of solar cells will be made. For example, the recombination at the grain boundaries and its influence on the short circuit current as a function of the crystallite sizes on the active material is considered. Based on this, the appropriate thickness of each layer and their resistivity will be discussed. As an example, these considerations will be applied to CdS/CdTe heterojunction solar cells, taking into account typical properties of CdTe thin films used for solar cells.     

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.     

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.     

Can we improve the record efficiency of CdS/CdTe solar cells?

Polycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaic systems. The two key properties of this material are its near ideal band gap for photovoltaic conversion efficiency of 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.In this paper we make a physical analysis of the typical CdS/CdTe superstrate solar cell, and we show that present record efficiencies are very close to the practical efficiency limit for a CdS/CdTe hetero-junction cell. We show that a current estimate for the maximum efficiency of hetero-junction CdS/CdTe solar cells is around 17.5%, in contrast to old theoretical predictions, which calculate about 30% efficiencies for ideal homo-junction CdTe solar cells. This analysis explains why the record efficiency for this kind of cells has been stable for the last 10 years, going up by less than 1% from 15.8% to only 16.5%.     

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%.     

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%.     

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%.     

Stability of CdTe/CdS thin-film solar cells

The recent literature regarding the stability of CdTe/CdS photovoltaic cells (as distinguished from modules) is reviewed. Particular emphasis is given to the role of Cu as a major factor that can limit the stability of these devices. Cu is often added to improve the ohmic contact to p-CdTe and the overall cell photovoltaic performance. This may be due to the formation of a Cu₂Te/CdTe back contact. Excess Cu also enhances the instability of devices when under stress. The Cu, as Cu⁺, from either Cu₂Te or other sources, diffuses via grain boundaries to the CdTe/CdS active junction. Recent experimental data indicate that Cu, Cl and other diffusing species reach (and accumulate at) the CdS layer, which may not be expected on the basis of bulk diffusion. These observations may be factors in cell behavior and degradation, for which new mechanisms are suggested and areas for future study are highlighted. Other possible Cu-related degradation mechanisms, as well as some non-Cu-related issues for cell stability are discussed.     

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).     

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.     

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.     

S and Te inter-diffusion in CdTe/CdS hetero junction

Effects of post formation thermal annealing of the CdTe–CdS device on the inter-diffusion of S and Te at the junction in a substrate configuration device have been studied by Auger electron spectroscopy. While the migration of S and Te atoms increases with annealing temperature, the extent of S diffusion is always higher than the diffusion of Te atoms. Inter-diffusion of S and Te causes the formation of CdTe_1-xS_x ternary compound at the CdTe–CdS interface.     

CdTe/CdS Solar cells on flexible molybdenum substrates

Development of CdTe/CdS solar cells on flexible metallic substrates is highly interesting due to the light weight and flexible nature of the solar modules. We have deposited CdTe films onto flexible molybdenum substrates using close-spaced sublimation technique and the CdTe/CdS junction was developed by depositing a thin layer of CdS onto the CdTe substrate from a chemical bath. The devices were characterized by Current–voltage (I–V) and photocurrent spectroscopy techniques. Prior to the deposition of the transparent conducting layer, the devices were annealed in air at different temperatures and found that the devices annealed at 400°C have better photovoltaic parameters. The efficiency of a typical device under 60 mW cm⁻² illumination was estimated as 3.5%.     

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.     

Optimized chemical bath deposited CdS layers for the improvement of CdTe solar cells

CdS layers grown by chemical bath deposition (CBD) are treated in different ways to improve the performance of CdS/CdTe solar cells. It has been found that the open circuit voltage of the CdS/CdTe solar cell increases when the CBD CdS is annealed with CdCl₂ before the deposition of CdTe by close spaced sublimation (CSS). A thin CBD CdS (∼80 nm) with bi-layer structure can significantly improve the short circuit current of the CdS/CdTe solar cells.     

High efficiency ultra-thin sputtered CdTe solar cells

Large scale manufacturing of CdTe PV modules at the GW/yr level may be constrained due to the limited availability of the relatively rare (Te) element and the volume of potentially hazardous (Cd) material being used in the typically 3–8 μm thick CdTe absorber layer. However, we find that it is possible to reduce the CdTe layer thickness without much compromise in efficiency. The CdS/CdTe solar cells were fabricated using magnetron sputtering with ultra-thin CdTe layers in the range of 0.5–1.28 μm. The ultra-thin films and cells were characterized using X-ray diffraction (XRD), optical transmission, scanning electron microscopy (SEM), current–voltage and quantum efficiency measurements. These results were compared with those of standard 2.3 μm thick CdTe sputtered cells. Different post-deposition processing parameters were required for cells with ultra-thin and standard CdTe thicknesses to achieve high efficiency. Ultra-thin CdTe cells showed crystallographic texture and CdTe_1−xS_x alloy formation after CdCl₂ treatment very similar to standard CdTe cells. Optimization of the post-deposition CdCl₂ treatment and back-contact processing yielded cells of 11.2% efficiency with 0.7 μm CdTe compared to 13.0% obtained with standard 2.3 μm CdTe cells.     

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.     

A study of vapor CdCl2 treatment by CSS in CdS/CdTe solar cells

We report the effect of CdCl₂ vapor treatment on the photovoltaic parameters of CdS/CdTe solar cells. Vapor treatment allows combining CdCl₂ exposure time and annealing in one step. In this alternative treatment, the CdS/CdTe substrates were treated with CdCl₂ vapor in a close spaced sublimation (CSS) configuration. The substrate temperature and CdCl₂ powder source temperature were 400 °C. The treatment was done by varying the treatment time (t) from 15 to 90 min. Such solar cells are examined by measuring their current density versus voltage (J-V) characteristics. The open-circuit voltage (V_oc), short circuit current density (J_sc) and fill factor (FF) of our best cell, fabricated and normalized to the area of 1 cm², were V_oc = 663 mV, J_sc = 18.5 mA/cm² and FF = 40%, respectively, corresponding to a total area conversion efficiency of η = 5%. In cells of minor area (0.1 cm²) efficiencies of 8% have been obtained.     

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.