D.L. Morel

High efficiency CSS CdTe solar cells

Cadmium telluride (CdTe) has long been recognized as a strong candidate for thin film solar cell applications. It has a bandgap of 1.45 eV, which is nearly ideal for photovoltaic energy conversion. Due to its high optical absorption coefficient essentially all incident radiation with energy above its band-gap is absorbed within 1‐2 mm from the surface. Thin film CdTe solar cells are typically heterojunctions, with cadmium sulfide (CdS) being the n-type junction partner. Small area efficiencies have reached the 16.0% level and considerable efforts are underway to commercialize this technology. This paper will present work carried out at the University South Florida sponsored by the National Renewable Energy Laboratory of the United States Department of Energy, on CdTe/CdS solar cells fabricated using the close spaced sublimation (CSS) process. The CSS technology has attractive features for large area applications such as high deposition rates and efficient material utilization. The structural and optical properties of CSS CdTe and CdS films and junctions will be presented and the influence of some important CSS process parameters will be discussed. q 2000 Elsevier Science S.A. All rights reserved.

Reduction of recombination current in CdTe/CdS solar cells

A study of the current–voltage behavior of recombination current in CdTe/CdS solar cells has shown that the reverse saturation current, J0, and the diode quality factor, A, are correlated. Our better devices typically have low values of both A and J0. Spectral response data indicate that devices with a gradual decline in quantum efficiency beginning at about 600 nm show better performance than devices with a sharp drop at the CdS band gap of 510 nm, which is interpreted as an indication that mixing between CdS and CdTe during processing reduces the amount of recombination current at the interface.

The effect of the CdCl2 treatment on CdTe/CdS thin film solar cells studied using deep level transient spectroscopy

Abstract Thin film CdTe/CdS solar cells have been studied using deep level transient spectroscopy. The correlation-deep level transient spectroscopy (DLTS) technique was utilized as conventional analysis methods such as the boxcar-based approach were found to be inadequate under certain experimental conditions. The primary objective was to study the effect of a key processing step in the fabrication of thin film CdTe solar cells, namely the post-deposition heat treatment in the presence of CdCl2. The substrate temperature as well as the ambient used during this process were varied around predetermined conditions for optimum solar cell performance, in order to identify performance-limiting defects, and in general improve our understanding of thin film CdTe solar cells. Solar cells without the CdCl2 heat treatment were also fabricated. A series of electron and hole traps were found in the various devices studied, with electron traps being present primarily in solar cells with limited performance.

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

Abstract 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.

RF sputtered back contacts for CdTe/CdS thin film solar cells

One of the critical issues associated with the fabrication of thin film CdTe/CdS solar cells is the formation of a stable, low-resistance back contact. This paper reports on the formation of a back contact to CdTe cells using RF sputtering of a Cu/sub x/Te target. The cells used in this study are of the commonly used superstrate configuration glass/SnO/sub 2//CdS/CdTe. The back contact was formed by depositing a thin Cu/sub x/Te film by RF sputtering followed by the deposition of a metal. The devices were characterized using I-V, C-V, and spectral response measurements. Fill factors in excess of 70% have been obtained using the Cu/sub x/Te/metal contact. The best device measured at the National Renewable Energy Laboratory exhibited a conversion efficiency of 14.9%.

Introduction of Sb in CDTE and its effect on CDTE solar cells

One approach to further increase the performance of CdTe cells is by increasing their open-circuit voltage from its current state-of-the-art levels of 830–850 mV, which could be accomplished by increasing the net doping concentration in CdTe. Antimony (Sb) is a group V element and suitable p-type dopant for CdTe because of its low ionization energy, low diffusion coefficient, and relative ease of incorporation into CdTe. In this paper, the effects of Sb doping in CdS/CdTe solar cells have been studied. Antimony was introduced in CdTe by diffusion. The effects of the diffusion parameters, and the excess Sb on the CdTe surface were investigated. The best results were obtained when the Sb heat treatment was carried out at 430°C for 25 minutes. The results indicate that Sb incorporation into CdTe can yield higher net hole carrier concentration, however, the Voc values obtained to-date remain in the 800–830 mV range.

Vapor chloride treatment studies of CdTe/CdS solar cells

The most commonly used processing step for the fabrication of CdTe thin film solar cells is a heat treatment in the presence of CdCI/sub 2/. This paper discusses results on the effect of a vapor CdCI/sub 2/ heat treatment on the characteristics of thin film CdTe/CdS solar cells. The heat treatment was carried out in three different ambient environments, He, O/sub 2/, and H/sub 2/, and over a wide range of temperatures. Best solar cell performance to-date was achieved for cells heat-treated in the presence of O/sub 2/. Solar cells heat-treated in He and H/sub 2/ exhibited lower ffs and Vocs respectively. The process was optimized for high throughput and demonstrated state of the art Vocs and ffs for short annealing times.

CdS: characterization and recent advances in CdTe solar cell performance

Cadmium sulfide (CdS) films deposited by chemical bath deposition (CBD) have been used for the fabrication of high efficiency CdTe and CuIn/sub 1-x/Ga/sub x/Se/sub 2/ thin film solar cells. An attractive alternative deposition technology with manufacturing advantages over the CBD is the close spaced sublimation (CSS). In this work CdTe/CdS solar cells prepared entirely by the CSS exhibited 15.0% efficiencies under global conditions as verified at the National Renewable Energy Laboratory. This paper reports on studies carried out on as deposited and heat treated CSS CdS films and all CSS CdTe/CdS solar cells using photoluminescence, X-ray diffraction, and I-V-T measurements.

CdS films prepared by the close-spaced sublimation and their influence on CdTe/CdS solar cell performance

Cadmium sulfide (CdS) films have been deposited on SnO/sub 2//7059 glass substrates by the close-spaced sublimation technique (CSS), and subsequently used for the fabrication of CdTe/CdS solar cells. The effect of various deposition parameters-ambient and temperature profiles-on film properties and solar cell performance has been investigated. One of the main objectives of this work has been to deposit device quality CdS films of small thickness in order to fabricate CdTe solar cells with enhanced blue response. Solar cells have been characterized using PL, I-V, I-V-T, SR, and C-V measurements. A photovoltaic conversion efficiency of 14.2% under AM1.5 conditions has been measured at the National Renewable Energy Laboratory.

Effects of thermal stressing on CdTe/CdS solar cells

The effects of thermal stressing on CdTe/CdS thin film solar cells have been investigated. Cadmium telluride solar cells fabricated by close-spaced sublimation, and contacted with Cu-based back contacts have been subjected to temperatures ranging from 70 to 120/spl deg/C in inert ambient for over 3500 hours. The average starting V/sub oc/ and ff for all stressed devices were 840 mV and 72% respectively. The devices were periodically removed from the stress environment and their light and dark J-V characteristics were measured. It has been found that all cells exhibited some degree of degradation, which was accelerated with temperature. Changes in device characteristics appeared to be gradual for temperatures in the range of 70-90/spl deg/C. In all cases a significant portion of the observed degradation took place within the first 500-600 hours. SIMS analysis indicated that the stress process resulted in Cu-accumulation near the CdTe/CdS junction, suggesting that Cu is at least partially responsible for the observed junction degradation.