Robert A. Enzenroth

Advances in continuous, in-line processing of stable CdS/CdTe devices

A continuous, in-line process suitable for high throughput manufacturing of CdS/CdTe photovoltaic devices has been demonstrated. Utilizing this process, devices with efficiencies of 13% has been fabricated with a low iron soda lime glass (3″×3″) with ant-reflection coatings. The process has been extended to large area devices (16″ ×16″ substrate size). After CdCl<inf>2</inf> treatment, devices showed V<inf>oc</inf> ≫ 700 mV and J<inf>sc</inf> ≫ 20 mA/cm². This performance is similar to the performance of small area devices which showed good stability. Also we have employed Spectroscopic Ellipsometry (SE) as a nondestructive tool to characterize CdS/CdTe heterojunction specifically studying the effects of chemical treatment on the optical properties of the thin-film layers.

Transient Ion Drift Measurements of Polycrystalline CdTe PV Devices

The well known transient ion drift (TID) method is used to quantify the density of mobile Cu interstitial ions in polycrystalline CdTe PV cells. Average Cu_i ⁺ ion densities in optimally processed cells are 20% of the background ionized acceptor doping level. A preliminary estimate of the diffusion coefficient for Cu _i ⁺ ions in the polycrystalline CdTe absorber is D(Cu_i)=1.3E-6 [cm²/sec] times exp[-0.29 eV/(KB*T)] in the temperature range of 25degC to 55degC from TID measurements. Aspects of the TID method as pertains to practical thin film polycrystalline devices are discussed

Stable Cu-Based Back Contacts for CdTe Thin Film Photovoltaic Devices

Cadmium telluride (CdTe) thin film photovoltaic devices fabricated in a-line process developed at Colorado State University (CSU) have shown stability during long-term (over a 5 year period) accelerated stress testing. These devices have a copper (Cu) containing back contact. The Cu profile as measured by secondary ion mass spectrometry characterization shows, for the maximum stressed device (23,399 h), that there is a significant (two times) change in the concentration of secondary Cu ions in the bulk of the material; however, the Cu concentration gradient at the back of the device has no significant change, and the CdS layer has no significant Cu concentration increase at open-circuit bias and 65°C temperature conditions. This indicates that with a proper CdCl 2 treatment, Cu can be used to form the back contact for CdTe devices with acceptable stability. These devices have a projected field lifetime of greater than 60 years.

Consistent processing and long term stability of CdTe devices

A technology for processing of thin film CdS/CdTe devices has been developed in our laboratory. This in-line, continuous, pilot system enables unique processing steps and conditions not available with batch processing and allows the fabrication of a large number of devices. Results from the pilot scale system are applicable to systems processing larger areas. Utilizing the pilot system, significant progress has been made towards demonstrating consistent stability (resistance to degradation) for thin film CdTe photovoltaics. We have repeatedly shown that devices with good stability can be produced if processed at the optimum set of conditions. Small changes in processes can lead to significant differences in device stability. Among the processing steps for fabrication of CdTe devices, the CdCl/sub 2/ treatment has a significant effect on performance and stability. A metric has been developed to predict the stability of devices at the time of device fabrication. Accelerated stress testing is ongoing. Extremely long duration stress testing (65/spl deg/C, open circuit conditions for /spl sim/30,000 hours with 5 hours of illumination out of 8 hour cycle) has demonstrated that the rate of efficiency loss levels out with final efficiencies in the range of 8.5% /spl sim/ 9.5%. A production prototype system for processing nominally 2 MW/yr. is currently under construction. This system utilizes the process definition developed in the pilot system.

Effect of Chemical Treatment on the Optical Properties of a Cadmium Telluride Photovoltaic Device Investigated by Spectroscopic Ellipsometry

Spectroscopic ellipsometry has been successfully used to characterize the CdS/CdTe heterojunction solar cell deposited on TEC15 glass. The effects of copper treatment on the optical properties of a cadmium chloride treated photovoltaic device were investigated using ellipsometry. No changes in either the band gaps or critical points of CdTe layer were noticed as a result of copper treatment. The copper treated CdTe layer exhibited a higher refractive index in the visible and longer wavelengths (≤3 eV), as compared with the untreated layer. This was attributed to the increased disorder in the case of copper treated layers.

Measurement of cooling rates of a superstrate cooling apparatus for an integrated in-line manufacturing process for thin-film photovoltaic devices

A pilot-scale semiconductor deposition system for the continuous production of CdTe based solar cells has been developed. During initial process optimization it was found that glass superstrates needed to be cooled before the back contact application. In order to shorten the cycle time for cooling glass superstrates, a close contact cooling apparatus with conduction through the gas phase for use in a vacuum chamber is being developed. Initial measurements of the cooling rates for a test cooling apparatus indicate that 76×76×2mm3 superstrates can be cooled from 170to30°C in a period of 4min.

Performance of In-Line Manufactured CdTe Thin Film Photovoltaic Devices

In-line manufactured CdTe thin film photovoltaic devices were evaluated for their performance with respect to power conversion efficiency and reliability (stability). A National Renewable Energy Laboratory (NREL) certified power conversion efficiency (η) of 12.44% was measured for one CdTe device. A mean efficiency of 11.4% for 53 devices is shown. Results of reliability studies give a conservative estimate of -1% relative degradation per year in efficiency over a 20-year lifetime for these CdTe devices.

Effect of chemical treatment on the optical properties of a cdte photovoltaic device investigated by spectroscopic ellipsometry

Variable angle Spectroscopic Ellipsometry (SE) was used to study CdTe PV devices at multiple points in the production process, with specific attention to the effect of applying of CdCl2 and Cu treatment. Results suggest the potential value of this characterization technique.

Effect of microstructure on the performance of stressed cadmium telluride photovoltaic (PV) devices

Our group has demonstrated a commercially viable, continuous, in-line process to fabricate PV devices with efficiencies of 13% on a low iron soda-lime glass (3„×3„) with anti-reflection coatings. The process has been extended to large area devices (16„×16„ substrate size). In this paper we present our results on the microstructural features of PV devices subjected to stress conditions. The results help to gain insight into the stability and lifetime of CdTe PV modules.

Effect of Hail Impact on Thermally Tempered Glass Substrates Used for Processing CdTe PV Modules

The IEEE 1262 standard provides qualification tests and specification to evaluate photovoltaic (PV) modules intended for power generation applications. The hail impact test is one of the tests. It is conducted to verify that the module will withstand a simulated hail impact without damage. The modules are tested with 1-in. diameter ice balls traveling at a velocity of 52 miles/h. Based on the results, the use of 3-mm tempered glass substrate for processing is recommended to withstand the hail impact test. The stresses developed by the samples during the hail impact test have been estimated.