W. S. Sampath

Sputtered, oxygenated CdS window layers for higher current in CdS/CdTe thin film solar cells

The efficiency of manufactured CdS/CdTe photovoltaic modules can be greatly improved through better collection of available current from the solar spectrum. Typically, only light absorbed in the CdTe absorber layer is collected by the device and light absorption in other layers is lost. A major loss occurs in the CdS window layer, which is strongly absorbing for photon energies above 2.4 ev. RF sputter deposition of CdS in the presence of oxygen has been shown to increase transmission in this spectral region, leading to higher device current. In this work films were examined for transmission and stability upon heating to process temperatures. Numerous devices were made from oxygenated CdS films which showed increased quantum efficiency at wavelengths below 500 nm.

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.

Effects of Ion Implantation Conditions on the Tribology of Ferrous Surfaces

The effects of implnted nitrogen ion dose and target surface temperature during implantation on the wear characteristics of iron (ferrite) and 304 stainless steel (austenite) have been studied systematically

Effect of back-contact copper concentration on CdTe cell operation

CdTe solar cells were fabricated with five different concentrations of copper, including zero, used in back-contact formation. Room-temperature J-V curves showed progressive deterioration in fill factor with reduced copper. J/sub SC/ and QE were similar for all Cu-levels. Capacitance measurement suggested enhanced intermixing at the back contact with copper present. Photocurrent mapping was much less uniform for reduced-Cu cells. Elevated-temperature stress induced very little change in J-V when sufficient Cu was used in the contact.

Single vacuum chamber with multiple close space sublimation sources to fabricate CdTe solar cells

Photovoltaic technologies have shown efficiencies of over 40%, however, manufacturing costs have prevented a more significant energy market penetration. To bridge the gap between the high efficiency technology and low cost manufacturing, a research and development tool and process was built and tested. This fully automated single vacuum photovoltaic manufacturing tool utilizes multiple inline close space sublimation (CSS) sources with automated substrate control. This maintains the proven scalability of the CSS technology and CSS source design but with the added versatility of independent substrate motion. This combination of a scalable deposition technology with increased cell fabrication flexibility has allowed for high efficiency cells to be manufactured and studied. The single vacuum system is capable of fabricating a 3.1 × 3.6 in. substrate every 45 min with a cell efficiency of 12% with a standard deviation of 0.6% as measured over 36 months. The substrate is generally scribed into 25 small area dev...

Characterization of Sulfur Bonding in CdS:O Buffer Layers for CdTe-based Thin-Film Solar Cells.

On the basis of a combination of X-ray photoelectron spectroscopy and synchrotron-based X-ray emission spectroscopy, we present a detailed characterization of the chemical structure of CdS:O thin films that can be employed as a substitute for CdS layers in thin-film solar cells. It is possible to analyze the local chemical environment of the probed elements, in particular sulfur, hence allowing insights into the species-specific composition of the films and their surfaces. A detailed quantification of the observed sulfur environments (i.e., sulfide, sulfate, and an intermediate oxide) as a function of oxygen content is presented, allowing a deliberate optimization of CdS:O thin films for their use as alternative buffer layers in thin-film photovoltaic devices.

Sublimation of Mg onto CdS/CdTe films fabricated by advanced deposition system

One way to improve the efficiency of CdS/CdTe solar cells is to incorporate complex CdTe alloys into the device architecture. While formation of wider bandgap Cd1−xMgxTe has been demonstrated with sputtering and co-evaporation, use of sublimation techniques for large areas is yet to be investigated. Using a sublimation technique suitable for large area processing, we deposited Mg thin films onto TEC10 and TEC10/CdS/CdTe structures. XPS depth profiling and ellipsometry show that the Mg and the underlying CdTe layer intermixed substantially, formed an alloyed film, and incorporated oxides into the film. For non-CdCl2 treated CdS/CdTe devices coated with Mg, a 120 mV gain in Voc was realized by utilizing a Br2/methanol etching process prior to metallization. Nonetheless, additional measures are needed to effectively implement this complex alloy into a high efficiency device structure. Use of statistical process optimization and high vacuum systems are suggested to improve the film quality.

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

Deposition and characterization of Cd1−xMgxTe thin films grown by a novel cosublimation method

Photovoltaic cells utilizing the CdS/CdTe structure have improved substantially in the past few years. Despite the recent advances, the efficiency of CdS/CdTe cells is still significantly below their Shockley–Queisser limit. CdTe based ternary alloy thin films, such as Cd1−xMgxTe (CMT), could be used to improve efficiency of CdS/CdTe photovoltaic cells. Higher band gap Cd1−xMgxTe films can be the absorber in top cells of a tandem structure or an electron reflector layer in CdS/CdTe cells. A novel cosublimation method to deposit CMT thin films has been developed. This method can deposit CMT films of band gaps ranging from 1.5 to 2.3 eV. The cosublimation method is fast, repeatable, and scalable for large areas, making it suitable for implementing into large-scale manufacturing. Characterization of as-deposited CMT films, with x varying from 0 to 0.35, reveals a linear relationship between Mg content measured by energy dispersive x-ray spectroscopy and the optical band gap. Glancing angle x-ray diffraction ...

Plasma cleaning of TCO surfaces prior to CdS/CdTe deposition

We describe a hollow cathode plasma formed within a heated pocket deposition (HPD) source covered with a transparent conductive oxide (TCO)-coated substrate. Super-hydrophilic behavior and little change in TCO transmission of TCO substrates exposed to the plasma are observed. The “plasma cleaner” source was placed in-line with the CSU advanced deposition system (ARDS), and CdS and CdTe films were deposited on both plasma and conventionally cleaned substrates. Plasma-cleaned substrates result in improved cadmium sulfide films without pinholes after plasma exposure times as short as 5 sec. Cells formed with plasma cleaned substrates display improved VOC, fill factor, and efficiency.