Jason M. Kephart

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.

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.

Reduction of window layer optical losses in CdS/CdTe solar cells using a float-line manufacturable HRT layer

Buffer, or high-resistance transparent (HRT) layers have been shown to increase the efficiency of CdS/CdTe solar cells. CdS/CdTe cells were fabricated on numerous float-line-manufacturable TCO and TCO/HRT-coated substrates. The behavior of the best-performing buffer layer was examined over a range of CdS thickness, and the device performance data show gradual dependence of open-circuit voltage and fill factor on CdS thickness below a critical value. The effect of fractional pinhole area is modeled, and it is proposed that pinholes do not dominate device behavior with thinner CdS, and instead band alignment effects explain the observed phenomena. Modeling indicates that the ohmic behavior of the buffer has a relatively small effect on devices with pinholes; instead, improving the diode quality of areas with thin or no CdS has a dominant effect.

Effect of the cadmium chloride treatment on RF sputtered Cd0.6Zn0.4Te films for application in multijunction solar cells

Single phase Cd0.6Zn0.4Te (CdZnTe) films of 1 μm thickness were deposited by radio frequency planar magnetron sputter deposition on commercial soda lime glass samples coated with fluorine-doped tin oxide and cadmium sulphide (CdS). The stack was then treated with cadmium chloride (CdCl2) at different temperatures using a constant treatment time. The effect of the CdCl2 treatment was studied using optical, materials, and electrical characterization of the samples and compared with the as-deposited CdZnTe film with the same stack configuration. The band gap deduced from Tauc plots on the as-deposited CdZnTe thin film was 1.72 eV. The deposited film had good crystalline quality with a preferred orientation along the {111} plane. After the CdCl2 treatment, the absorption edge shifted toward longer wavelength region and new peaks corresponding to cadmium telluride (CdTe) emerged in the x-ray diffraction pattern. This suggested loss of zinc after the CdCl2 treatment. The cross sectional transmission electron mi...

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.

Polycrystalline CdSeTe/CdTe Absorber Cells With 28 mA/cm 2 Short-Circuit Current

An 800 nm CdSeTe layer was added to the CdTe absorber used in high-efficiency CdTe cells to increase the current and produce an increase in efficiency. The CdSeTe layer employed had a band-gap near 1.41 eV, compared with 1.5 eV for CdTe. This lower band-gap enabled a current density increase from approximately 26 to over 28 mA/cm2. The open-circuit voltage obtained in the high-efficiency CdTe-only device was maintained and the fill-factor remained close to 80%. Improving the short-circuit current density and maintaining the open-circuit voltage lead to device efficiency over 19%. External quantum efficiency implied that about half the current was generated in the CdSeTe layer and half in the CdTe. Cross-sectional STEM and EDS showed good grain structure throughout. Diffusion of Se into the CdTe layer was observed. This is the highest efficiency polycrystalline CdTe photovoltaic device demonstrated by a university or national laboratory.

Progress and challenges with CdTe cell efficiency

CdTe solar-cell efficiency at Colorado State has now been independently verified above 18% on commercial glass. The parameters for the highest-efficiency cell are 863 mV for VOC, 26.8 mA/cm2 for JSC, and 79.2% for fill-factor, combining for 18.3% efficiency. The cell features that allowed the increases include higher-temperature pre-heating before CdTe deposition, a Te layer to facilitate the back contact, MgZnO for the buffer layer, and an anti-reflective coating. The current and fill-factor have achieved large fractions of their ideal values for the CdTe band gap, but as is typical for CdTe cells, the voltage has not. Two general strategies to address the voltage deficit are described. One of these, electron reflection with fully-depleted CdTe, has achieved voltage increases of 60 mV with 1-μm CdTe and higher with sub-micron absorbers.

Sputter-Deposited Oxides for Interface Passivation of CdTe Photovoltaics

Commercial CdTe PV modules have polycrystalline thin films deposited on glass, and devices made in this format have exceeded 22% efficiency. Devices made by the authors with a magnesium zinc oxide window layer and tellurium back contact have achieved efficiency over 18%, but these cells still suffer from an open-circuit voltage far below ideal values. Oxide passivation layers made by sputter deposition have the potential to increase voltage by reducing interface recombination. CdTe devices with these passivation layers were studied with photoluminescence (PL) emission spectroscopy and time-resolved photoluminescence (TRPL) to detect an increase in minority carrier lifetime. Because these oxide materials exhibit barriers to carrier collection, micropatterning was used to expose small point contacts while still allowing interface passivation. TRPL decay lifetimes have been greatly enhanced for thin polycrystalline absorber films with interface passivation. Device performance was measured and current collection was mapped spatially by light-beam-induced current.