Self-primed ethylene-vinyl acetate (EVA) and non-EVA (PMG) encapsulant formulations were developed that have greater resistance to damp heat exposure at 85 deg C and 85% relative humidity (RH) (in terms of adhesion strength to glass substrates) than a commonly used commercial EVA product. The self-primed EVA formulations were developed on the basis of high-performing glass priming formulations that have previously proven to significantly enhance the adhesion strength of unprimed and primed EVA films on glass substrates during damp heat exposure. The PMG encapsulant formulations were based on an ethylene-methylacrylate copolymer containing glycidyl methacrylate.

A III-V on Si multijunction solar cell promises high efficiency at relatively low cost. The challenges to epitaxial growth of high-quality III-Vs on Si, though, are extensive. Lattice-matched (LM) dilute-nitride GaNPAs solar cells have been grown on Si, but their performance is limited by defects related to the nitrogen. Advances in the growth of lattice-mismatched (LMM) materials make more traditional III-Vs, such as GaInP and GaAsP, very attractive for use in multijunction solar cells on silicon.

The Solargenix Advanced Trough Development Project was initiated in the Year 2000 with the support of the DOE CSP Program and, more recently, with the added support of the Nevada Southwest Energy Partnership. Parabolic trough plants are the most mature solar power technology, but no large-scale plants have been built in over a decade. Given this lengthy lull in deployment, our first Project objective was development of improved trough technology for near-term deployment, closely patterned after the best of the prior-generation troughs. The second objective is to develop further improvements in next-generation trough technology that will lead to even larger reductions in the cost of the delivered energy. To date, this Project has successfully developed an advanced trough, which is being deployed on a 1-MW plant in Arizona and will soon be deployed in a 64-MW plant in Nevada. This advanced trough offers a 10% increase in performance and over an 20% decrease in cost, relative to prior-generation troughs.

We report the effect of the hydrogen (H) content (CH) on the crystallization kinetics, surface morphology and grain growth for Hot Wire CVD a-Si:H films containing 12.5 and 2.7 at.% H which are crystallized by rapid thermal anneal (RTA). For the high CH film we observe explosive H evolution, with a resultant destruction of the film for RTA temperatures >750 deg C. At RTA temperatures ~600 deg C, both films remain intact with similar morphologies. At this same lower RTA, the incubation and crystallization times decrease, and the grain size as measured by X-Ray Diffraction increases with decreasing film CH. SIMS measurements indicate that a similar film CH (<0.5 at.%) exists in both films when crystallization commences. The benefits of a two-step annealing process for the high CH film are documented.

Our analyses show that defect clusters can lower the efficiency of multicrystalline silicon (mc-Si) solar cells by 2 to 4 absolute percentage points. This large loss can be recovered if impurities precipitated at the defect cluster sites can be gettered. We describe a new technique for gettering precipitated impurities.

The National Renewable Energy Laboratory (NREL) Solar Radiometry and Metrology task provides traceable optical radiometric calibrations and measurements to photovoltaic (PV) researchers and the PV industry. Traceability of NREL solar radiometer calibrations to the World Radiometric Reference (WRR) was accomplished during Pyrheliometer Comparison at NREL in October 2004. Ten spectral and more than 200 broadband radiometers for solar measurements were calibrated this year. We measured detailed spectral distributions of the NREL and PV industry Pulsed Solar Simulators and are analyzing the influence of environmental variables on radiometer uncertainty. New systems for indoor and outdoor solar radiometer calibrations and ultraviolet (UV) spectral measurements and UV radiometer calibrations were purchased and tested. Optical metrology functions support the NREL Measurement and Characterization Task effort for ISO 17025 accreditation of NREL Solar Reference Cell Calibrations and have been integrated into the NREL quality system and audited for ISO17025 compliance.

Thin hydrogenated amorphous silicon (a-Si:H) layers deposited by hot-wire chemical vapor deposition (HWCVD) are used as both emitters and back contacts in silicon heterojunction solar cells. Low interface recombination velocity and high open-circuit voltage are achieved by a low substrate temperature (<150 deg C) intrinsic a-Si:H deposition which ensures immediate amorphous silicon deposition. This is followed by deposition of doped a-Si:H at a higher temperature (>200 deg C) which appears to improve dopant activation. With an i/n a-Si:H emitter, we obtain a confirmed efficiency of 17.1% on textured p-type float-zone (FZ) silicon with a screen-printed aluminum back-surface-field (Al-BSF) contact. Employing a-Si:H as both the front emitter and the back contact, we achieve a confirmed efficiency of 17.5%, the highest reported efficiency for a p-type c-Si based heterojunction solar cell.

This paper presents a brief overview of the status and accomplishments during fiscal year (FY) 2005 of the Photovoltaic (PV) Exploratory Reliability and Performance R&D Subtask, which is part of the PV Module Reliability R&D Project (a joint NREL-Sandia project).

Arizona Public Service (APS) is currently installing new power facilities to generate a portion of its electricity from solar resources that will satisfy its obligation under the Arizona Environmental Portfolio Standard (EPS). During FY04, APS began construction on a 1-MWe parabolic trough concentrating solar power plant. This plant represents the first parabolic trough plant to begin construction since 1991. Site preparation and construction activities continued throughout much of FY05, and startup activities are planned for Fall 2005 (with completion early in FY06). The plant will be the first commercial deployment of the Solargenix parabolic trough collector technology developed under contract to the National Renewable Energy Laboratory. The plant will use an organic Rankine cycle (ORC) power plant, provided by Ormat. The ORC power plant is much simpler than the conventional steam Rankine cycle plant and allows unattended operation of the facility.

We illustrate the capabilities of the High Voltage Stress Test (HVST) which operates continuously in the array field east of the Outdoor Test Facility at the National Renewable Energy Laboratory. Because we know that photovoltaic (PV) modules generating electrical power in both residential and utility-scale array installations will develop high-voltage biases approaching 600 VDC and 1,000 VDC, respectively, we expect such high voltages will result in current leakage between cells and ground, typically through the frames or mounts. We know that inevitably such leakage currents are capable of producing electrochemical corrosion that adversely impacts long-term module performance. With the HVST, we stress or operate PV modules under high-voltage bias, to characterize their leakage currents under all prevailing ambient conditions and assess performance changes emanating from high-voltage stress. We perform this test both on single modules and an active array.

The objective of this research is to determine the operational characteristics key to efficient, low-cost, stable solar cells based on dye-sensitized mesoporous films (in collaboration with DOE's Office of Science Program). Toward this end, we have investigated the mechanism by which the adsorbent chenodeoxycholate, cografted with a sensitizer onto TiO2 nanocrystals, improves the open-circuit photovoltage (VOC) and short-circuit photocurrent density (JSC). We find that adding chenodeoxycholate not only shifts the TiO2 conduction-band edge to negative potentials but also accelerates the rate of recombination. The net effect of these opposing phenomena is to produce a higher photovoltage. It is also found that chenodeoxycholate reduces the dye loading significantly but has only a modest effect on JSC. Implications of these results to developing more efficient cells are discussed.

Crystallization of hydrogenated amorphous silicon (a-Si:H) films deposited on low-cost substrates shows potential for solar cell applications. Secondary ion mass spectrometry (SIMS) was used to study impurity incorporation, hydrogen evolution, and dopant diffusion during the crystallization process

As part of the work conducted in the PV Systems Reliability and Performance R&D Task, a 1.5-kWdc photovoltaic (PV) array consisting of 36 Solarex MST-43MV dual-junction a-Si modules was installed and its performance monitored for almost six years (September 1999 through May 2005) at the National Renewable Energy Laboratory (NREL) Outdoor Test Facility (OTF). This paper describes the system and its performance based on the PV for Utility-Scale Applications (PVUSA) power rating method.

We present a digest of our research on the thin-film material components that comprise the top and bottom cells of three different material systems and the tandem devices constructed from them.

Higher-efficiency solar cells improve the likelihood that concentrator photovoltaic systems will become cost effective. A four-junction GaAs- and Ge-based solar cell incorporating a 1-eV bandgap material has an ideal AM0 efficiency of ~40% and could also be used in a terrestrial concentrator module. The dilute-N GaAsN alloy's bandgap can be reduced to near 1 eV when the nitrogen content is 2% - 3%. Indium can also be added to the alloy to improve lattice matching to GaAs and Ge. We have used deep-level transient spectroscopy (DLTS) to characterize traps in both p-type and n-type GaAsN. For each type of material, the dominant DLTS signal corresponds to an electron trap having an activation energy of about 0.35 eV for p-type GaAsN and about 0.45 eV for n-type GaAsN. In both types of materials, the trap concentrations, modified by ..lambda..-effect factors, increase with both increasing N content and increased doping.

A family of prototype ..pi..-conjugated dendrimers has been synthesized and incorporated into solution-processable organic photovoltaic (OPV) devices. Bulk heterojunction devices were fabricated by blending the dendrimers with a solubilized fullerene. The best of these initial devices, which were not optimized for morphology, exhibited external quantum efficiencies of 22% at peak wavelength. It was found that 3-arm dendrimers, when sufficiently soluble, yielded higher photocurrents than their 4-arm counterparts. This was attributed to better planarity and intermolecular alignment of the 3-arm version. Device efficiency was seen to increase with increasing arm length. A reduced-band gap dendrimer was synthesized by attaching electron-withdrawing groups onto the core to yield an optical band gap of 1.82 eV.

The peak-watt rating is a primary indicator of PV performance. The peak power rating is the maximum electrical power that is produced when the PV device is continuously illuminated at 1000 Wm/sup-2/ total irradiance under International Electrotechnical Commission Standard 60904-2 reference spectrum, and 25 deg C cell temperature. Most manufacturers trace their peak-watt rating through calibrations performed at recognized terrestrial calibration facilities. Manufacturers typically perform intercomparisons among a set of their modules internally with other plants and among. Sometimes they have the same module measured at different calibration facilities to determine the differences in calibration. This intercomparison was to mimic this procedure and supply new thin film samples along with samples that could pose other problems. These intercomparisons sample the laboratories' everyday procedures better than a formal intercomparison where the laboratories' best procedures and data scrutiny are used.

While the purpose of geologic storage in deep saline formations is to trap greenhouse gases underground, the potential exists for CO{sub 2} to escape from the target reservoir, migrate upward along permeable pathways, and discharge at the land surface. In this paper, we evaluate the potential for such CO{sub 2} discharges based on the analysis of natural analogs, where large releases of gas have been observed. We are particularly interested in circumstances that could generate sudden, possibly self-enhancing release events. The probability for such events may be low, but the circumstances under which they occur and the potential consequences need to be evaluated in order to design appropriate site-selection and risk-management strategies. Numerical modeling of hypothetical test cases is suggested to determine critical conditions for large CO{sub 2} releases, to evaluate whether such conditions may be possible at designated storage sites, and, if applicable, to evaluate the potential impacts of such events as well as design appropriate mitigation strategies.

High-throughput combinatorial approaches have been used for the discovery and optimization of transparent conducting oxide (TCO) materials for PV applications. We report on current investigations in In-Zn-O, In-Ti-O and In-Mo-O systems. The InZnO system is shown to be amorphous in the best conducting range with a conductivity of ~ 3000 &#937;-cm-1 for 50%-70% In/Zn. The amorphous InZnO films are very smooth (2..ANG.. rms). In-Ti-O is found to be an excellent high-mobility TCO with mobilities of greater than 80 cm2/v-sec and conductivities of more than 6000 &#937;-cm-1 for sputtered thin film materials.

The atomic structure and electronic properties of crystalline silicon/hydrogenated amorphous silicon (c-Si/a-Si:H) interfaces in silicon heterojunction (SHJ) solar cells are investigated by high-resolution transmission electron microscopy, atomic-resolution Z-contrast imaging, and electron energy loss spectroscopy. We find that all high-performance SHJ solar cells exhibit atomically abrupt and flat c-Si/a-Si:H interfaces and high disorder of the a-Si:H layers. These atomically abrupt and flat c-Si/a-Si:H interfaces can be realized by direct deposition of a-Si:H on c-Si substrates at a substrate temperature below 150 deg C by hot-wire chemical vapor deposition from pure silane.

The federal government is an active participant in promotingsustainable design, construction and operations and in the use of USGBC'sLeadership in Energy and Environmental Design (LEED) Green BuildingRating System. This paper presents an overview of sustainableconstruction activities in the federal sector in 2005.

The PV Systems Performance and Reliability R & D group currently has seven grid-tied 1-2 kilowatt PV systems deployed at NREL's Outdoor Test Facility (OTF) and two 6 kilowatt systems mounted on the roof of NREL's Solar Energy Research Facility (SERF). The systems, which employ several PV module technologies including crystalline silicon (c-Si), amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium diselenide (CIS), are being monitored to determine the long-term performance and reliability of the modules and arrays under actual field conditions. The length of observation ranges from 2 months for our newest system to 11 years for our oldest systems. The annual degradation and seasonal fluctuation of the systems' power output are calculated using the PV for Utility-Scale Applications (PVUSA) power rating regression model.

The 1000 MW CSP project was initiated in FY02 based on a Congressional request of the DOE to investigate the feasibility of 1000 MW of Concentrating Solar Power in the Southwest by 2006. The original charge has grown and involved a number of activities including: outreach to the SW states, support of state-level activities in NM, CA, and CO, and analysis in support of the Western Governors' Association (WGA) 30 GW Clean Energy Initiative.

The goal of the Solar Communications Team is to get the right information to the right people at the right time in the right form at the right cost, and to measure the effectiveness of projects and our strategic communications plan. Our communications efforts in FY 2005 emphasized the following: 1) Reaching the Buildings and Consumer audiences (e.g., Solar Decathlon, International Builders' Show). 2) Developing and distributing critical program documents to key stakeholders (e.g., Solar Program Review Meeting Proceedings, Industry Roadmap, second Multi-Year Program Plan). 3) Conducting a gap analysis of communications products and evaluating their effectiveness. 4) Working with our program management to streamline business processes and improve communications of management expectations. 5) Developing and maintaining content for all Solar Program Web sites that reflect research and program accomplishments. 6) Representing the interests of the Solar Program at strategic events (technical conferences, meetings, workshops, community events).