Flexible, high-efficiency, low-cost solar cells can enable applications that take advantage of high specific power, flexible form factors, lower installation and transportation costs. Here, we report a certified record efficiency of 16.4% for a flexible CdTe solar cell that is a marked improvement over the previous standard (14.05%). The improvement was achieved by replacing chemical-bath-deposited CdS with sputtered CdS:O and also replacing the high-temperature sputtered ZnTe:Cu back contact layer with co-evaporated and rapidly annealed ZnTe:Cu. We use quantum efficiency and capacitance-voltage measurements combined with device simulations to identify the reasons for the increase in efficiency. Both device simulations and experimental results show that higher carrier density can quantitatively account for the increased open circuit voltage (VOC) and Fill Factor (FF), and likewise, the increase in short circuit current density (JSC) can be attributed to the more transparent CdS:O.
High-performance transparent conductive indium-tin-oxide (ITO) films on flexible glass have been flextested to 25–50k bend cycles without breakage, and with ∼0.1% change in sheet resistance. In contrast, commercial ITO/PET samples undergo ∼50–100% increase in sheet resistance in the same test, indicating that such coatings/substrates may not be acceptable for use in some products or fabrication procedures. The flexible glass substrate enables high-temperature processing, which facilitates the high performance of the coatings. Measurements of the volume resistivity and water vapor transmission rate (WVTR) indicate that Corning ® Willow ® Glass is suitable as a PV substrate material without need for barrier coatings or glass lamination.
Cadmium telluride (CdTe) solar cells are a commercially proven photovoltaic technology. Devices are composed of thin films which enable conversion of incident sunlight into electrical carriers and simultaneous transport of these carriers to an external load. At the front of the device, the layers should be highly transparent to minimize parasitic absorption while also supporting electron transport to the external load. In this thesis, two sputter-deposited front contact layers were investigated with the goal of improving both processing and materials science understanding. The first material investigated in this work was cadmium stannate (Cd2SnO4; CTO), a high-performance transparent conductive oxide (TCO). CTO, though used less frequently than on-line deposited fluorine-doped tin oxide, exhibits high transmission (>90%) and low resistivity ( 90% when oxygen was present in the sputtering ambient in the range 600–700 °C. Sputtered CTO and bilayer CTO/CdS films were annealed in contact with bare glass and in an uncovered configuration. The thin CdS layer in bilayer films was adequate to maintain or reduce resistivity when using the covered anneal, while it enabled improved mobility and transmittance for the uncovered anneal. Stoichiometry adjustment to a higher cadmium/tin ratio was found to be primarily responsible for increased carrier concentration, both through the proximity anneal and the covered anneal. Next, the CdS window layer was investigated. Important structural, optical, and electronic properties were altered by varying the sputtering ambient composition (oxygen/argon). Incorporation of oxygen in the films causes the films to lose crystallinity and increases the optical band gap through a shift in conduction band energy. CdS and oxygenated CdS (CdS:O) layers were incorporated in complete devices. Maximum efficiency >14% was achieved using a CdS:O layer containing ~40 atomic % oxygen and an optical band gap of 2.8 eV. Processing was scaled up to two higher throughput systems, in which different ambient compositions were required to achieve the optimal band gap. Similar device efficiencies of 13–15% were achieved by maintaining the window layer thickness at 100 nm and the optical band gap at 2.8 eV for each system/target combination. It is notoriously difficult to characterize the window layer and electrical junction in completed devices, because (1) they are buried between the glass on one side and a “thick” ~5-µm CdTe layer on the back, and (2) the CdS and CdTe layers are chemically similar. Two techniques were developed to enable characterization of the CdS…
Recent advances in the deposition of patterned thin film spectral filters have enabled a new class of radically miniaturized spectral sensors. This new technology enables numerically large arrays of spectral bandpass filters with unprecedented manufacturing economy. For example, a 64-channel array occupying two square millimeters and spanning 400-900 nm can be deposited with as few as eight coating steps. Mating this filter array to a photodiode array yields a tiny multispectral sensor with diverse applications. The bandpass filters are single-cavity Fabry-Perot designs with common top and bottom mirrors. The dielectric spacer layer between them determines the passband wavelength and is patterned to differing thicknesses using a binary scheme, i.e., each successive "sub-spacer" layer is half the thickness of the previous one. The technical challenge is uniformly patterning and depositing thinner and thinner sub-spacers, which can be only a few nanometers thick. We have demonstrated 64-channel arrays covering the spectral range of 400-900 nm and 775-1075 nm. These arrays have been mated to high-responsivity 2D silicon detectors, in much the same way that linear variable filters are mated to linear detector arrays. The resulting sensor is less than 3 x 3 x 1 mm in size and ideal for integration into mobile devices, wearable electronics, autonomous aerial vehicles, and countless industrial applications. Sensor performance is currently being evaluated for food quality and freshness measurement, drug identification, fuel quality measurement, explosives detection, colorimetry and illumination measurement, solar flux monitoring, remote sensing, and myriad other applications.
Two optical sub-bandgap transitions in CdTe thin-film solar cells have been identified using detailed transient photocapacitance and transient photocurrent spectroscopy measurements. A broad response centered at EV + 0.9 eV directly correlates with the quantity of Cu present in the absorber layer, while a second response at EV + 1.2 eV does not depend on Cu or Zn and may be an intrinsic defect. These results demonstrate the influence of Cu on the sub-bandgap density of states of CdTe, and they are critical to understanding, modeling, and improving its optoelectronic properties.
Abstract To feed a growing population, alternative sources of animal feed (e.g., lignocellulose) are needed to replace grains (e.g., corn). Oxidative lime pretreatment (OLP) increases lignocellulose digestibility by removing lignin and hemicellulose acetyl content. Adding a mechanical pretreatment (e.g., ball milling) further improves digestibility. This study determines the effectiveness of OLP and ball milling to enhance the ruminant digestibility of lignocellulose. For forage sorghum, the 48-h in vitro TDN were 40, 64, and 84 g nutrients digested/100 g organic matter (OM) for raw, short-term OLP, and short-term OLP + ball milling, respectively. In terms of compositional changes, OLP increases NDF and decreases non-fiber carbohydrate (NFC) and crude protein (CP), all of which would normally be associated with a decrease in digestibility. However, because OLP and ball milling beneficially change composition (lignin removal) and structural features (reduced crystallinity), digestibility actually increases. Although ball milling increases digestibility according to standard laboratory assays, it reduces particle size possibly allowing fine particles to escape from the rumen before they are digested, thus limiting its practical application. Nonetheless, this study indicates that mechanical pretreatment greatly increases digestibility, and therefore it is desirable to identify an effective mechanical treatment that retains fiber integrity.
Flexible glass enables high-temperature, roll-to-roll processing of superstrate devices with higher photocurrents than flexible polymer foils because of its higher optical transmission. Using flexible glass in our high-temperature CdTe process, we achieved a certified record conversion efficiency of 14.05% for a flexible CdTe solar cell. Little has been reported on the flexibility of CdTe devices, so we investigated the effects of three different static bending conditions on device performance. We observed a consistent trend of increased short-circuit current and fill factor, whereas the open-circuit voltage consistently dropped. The quantum efficiency under the same static bend condition showed no change in the response. After storage in a flexed state for 24 h, there was very little change in device efficiency relative to its unflexed state. This indicates that flexible glass is a suitable replacement for rigid glass substrates, and that CdTe solar cells can tolerate bending without a decrease in device performance.
The composition and structure of the CdS/CdTe heterojunction is critical to device performance. However, it is difficult to access this region in devices employing the conventional superstrate configuration. In this work, we report on the development of two techniques for exposing the CdS window layer in completed CdTe solar cells. First, we report on a chemical etch that selectively removes CdTe and exposes the CdS back surface. In addition, we demonstrate a thermo-mechanical lift-off technique that allows clean separation at the TCO/CdS interface. These techniques enable simple, quick sample preparation for characterization of the heterojunction region of completed devices.
Flexible, superstrate CdTe devices combine the advantages of a commercially demonstrated, low-cost manufacturing process with a lightweight, flexible form factor. Here, we present data on cell efficiencies greater than 16%, and the critical processing changes that have enabled recent efficiency increases. The devices in this study were made on Corning® Willow® Glass, which is a highly transparent, flexible, hermetic, and dimensionally stable substrate that can withstand high processing temperatures. To date, we have produced devices with several different combinations of front and back contacts on this glass and have found that it is compatible with most of our standard processing steps. One of our best devices to date has a certified efficiency of 16.2%, with a short-circuit current density (J sc ) of 25.6 mA/cm 2 , an open-circuit voltage of 820 mV, and a fill factor (FF) of 77.3%. The increased J sc in this cell is due to an improved sputtered CdS:O deposition process, and the high FF is due to a co-evaporated ZnTe:Cu back contact.
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