Kristopher O. Davis

High-Performance TiO2 -Based Electron-Selective Contacts for Crystalline Silicon Solar Cells.

Thin TiO2 films are demonstrated to be an excellent electron-selective contact for crystalline silicon solar cells. An efficiency of 21.6% is achieved for crystalline silicon solar cells featuring a full-area TiO2 -based electron-selective contact.

Investigation of the Internal Back Reflectance of Rear-Side Dielectric Stacks for c-Si Solar Cells

This paper addresses the calculation of internal back reflectance for various dielectrics that are used in rear-side passivated crystalline silicon solar cells. Optical modeling of various stack configurations is examined to explore the back-surface reflectance at the Si-dielectric interface for different film combinations and thicknesses as a function of wavelength and internal angle of incidence at the rear side. Specifically, configurations using aluminum oxide (AlOx), silicon nitride (SiNx), titanium dioxide (TiO2), and silicon dioxide (SiO2) were investigated with a focus on designing stack configurations that will also allow for high-quality passivation and are compatible with a high-volume manufacturing environment. In addition, samples were fabricated by plasma-enhanced and atmospheric pressure chemical vapor deposition of thin dielectric films onto polished and textured monocrystalline silicon wafers. Spectral reflectance curves of the samples are presented to supplement and validate the conclusions that are obtained from the optical modeling data.

Power line carrier permissive as a simple and safe method of enabling inverter ride-through operation of distributed grid-tied photovoltaic systems

Conventional anti-islanding techniques used in grid-tied photovoltaic (PV) systems pose many disadvantages at high levels of PV deployment. One such issue is the inability of these systems to ride-through grid disturbances. In this paper, the use of a Power Line Carrier Communications (PLCC) Permissive anti-islanding scheme is investigated as a means of safely enabling ride-through operation of grid-tied photovoltaic systems. Here potential fault scenarios are considered, along with performance, cost, and design considerations for the PLCC Permissive components, as well as potential system configurations and methods of implementation. While PV systems are the largest (and growing) form of distributed generation (DG) generating in parallel with utility feeders, it is important to note that this technique is effective for any DG technology, whether inverter-based or rotating, including wind, hydro and fossil fueled bio-gas machines.

Tailoring the Optical Properties of APCVD Titanium Oxide Films for All-Oxide Multilayer Antireflection Coatings

In this paper, the optical properties and microstructure of titanium oxide (TiOx) thin films deposited by in-line atmospheric pressure chemical vapor deposition (APCVD) are tailored to act as effective antireflection coatings (ARCs) in crystalline silicon solar cells. The ability to control the crystalline phase, microstructure, and optical properties of these TiOx films by varying the deposition conditions is demonstrated. Because the refractive index of TiOx can be widely varied by changing the deposition temperature, these films can be applied as single- or double-layer ARCs (DLARCs) in crystalline silicon solar cells featuring a thin front-side passivation layer (e.g., thermal silicon oxide, aluminum oxide) or as a rear-side capping layer in rear passivated cells. Reflectance measurements on oxide-based DLARCs deposited on anisotropically textured monocrystalline Si wafers are presented for unencapsulated samples, along with the modeled performance of similar structures encapsulated in ethylene-vinyl acetate under varying angles of incidence. In the experiments on unencapsulated samples, two of the oxide-based DLARCs outperform a standard SiNx ARC, and all four outperform the SiNx ARC when encapsulated.

New results for power line carrier-based islanding detection and an updated strengths and weaknesses discussion

As PV deployment levels increase, loss of mains detection, or islanding detection, has again arisen as a primary concern among the utility community. This is true especially in multi-inverter cases, cases with a mix of distributed resources, and on difficult feeders on which false tripping may be a disproportionately significant problem. Power line carrier communications can be effective in solving this problem for all types of distributed generation. This paper provides an update on laboratory and field testing of this technique; discusses some of its unique but lesser-known advantages; and examines some of its weaknesses.

Solar energy grid integration systems : final report of the Florida Solar Energy Center Team.

Initiated in 2008, the Solar Energy Grid Integration Systems (SEGIS) program is a partnership involving the U.S. DOE, Sandia National Laboratories, private sector companies, electric utilities, and universities. Projects supported under the program have focused on the complete-system development of solar technologies, with the dual goal of expanding utility-scale penetration and addressing new challenges of connecting large-scale solar installations in higher penetrations to the electric grid. The Florida Solar Energy Center (FSEC), its partners, and Sandia National Laboratories have successfully collaborated to complete the work under the third and final stage of the SEGIS initiative. The SEGIS program was a three-year, three-stage project that include conceptual design and market analysis in Stage 1, prototype development and testing in Stage 2, and moving toward commercialization in Stage 3. Under this program, the FSEC SEGIS team developed a comprehensive vision that has guided technology development that sets one methodology for merging photovoltaic (PV) and smart-grid technologies. The FSEC teams objective in the SEGIS project is to remove barriers to large-scale general integration of PV and to enhance the value proposition of photovoltaic energy by enabling PV to act as much as possible as if it were at the very least equivalent to a conventional utility power plant. It was immediately apparent that the advanced power electronics of these advanced inverters will go far beyond conventional power plants, making high penetrations of PV not just acceptable, but desirable. This report summarizes a three-year effort to develop, validate and commercialize Grid-Smart Inverters for wider photovoltaic utilization, particularly in the utility sector.

Transmission Electron Microscopy Studies of Electron-Selective Titanium Oxide Contacts in Silicon Solar Cells

In this study, the cross-section of electron-selective titanium oxide (TiO2) contacts for n-type crystalline silicon solar cells were investigated by transmission electron microscopy. It was revealed that the excellent cell efficiency of 21.6% obtained on n-type cells, featuring SiO2/TiO2/Al rear contacts and after forming gas annealing (FGA) at 350°C, is due to strong surface passivation of SiO2/TiO2 stack as well as low contact resistivity at the Si/SiO2/TiO2 heterojunction. This can be attributed to the transformation of amorphous TiO2 to a conducting TiO2-x phase. Conversely, the low efficiency (9.8%) obtained on cells featuring an a-Si:H/TiO2/Al rear contact is due to severe degradation of passivation of the a-Si:H upon FGA.

Effects of solar resource variability on the future Florida transmission and distribution system

A common critique of photovoltaic energy is the susceptibility of the systems to high variability- passing clouds can affect a sites day-to-day energy production substantially. This research developed a tool to simulate photovoltaic energy systems in several scenarios throughout the state of Florida and quantifies the hour-to-hour impact of these systems on the statewide generation mix using 11 years of historical weather data. While the hourly changes in aggregate system output for distributed PV systems was predictable between months, finer geographic granularity of irradiance data coupled with sub-hourly time intervals are required to further develop this model into one that is indispensable for utility system operators.

Effect of UV-Ozone Exposure on PCBM

The effect of ultraviolet (UV)-light ozone exposure on phenyl-C61-butyric acid methyl ester (PC61BM) is studied using solid-state modification of the material. In this study, solution processed PCBM films that have been deposited and dried over glass were exposed to UV ozone at ambient temperature and pressure from 0 to 120 min. Core-level and valence-band photoelectron spectroscopy studies indicate that the UV-ozone-induced oxidation results in C-O-C bonds in PCBM and a change in the highest occupied molecular orbital level, respectively. The modification of PCBM with C-O-C binding was observed from 5 to 60 min, but further exposure, typically beyond 120 min, was found to degrade the material. A solubility study also showed an exposure time-dependant solubility change of the material, in solvents like toluene and acetone, where it indicates the change in the chemistry of the material that can be related to the C-O-C binding of the material by the exposure. The method is an easily implementable approach for flexibility on choice of solvents for PCBM, and it has implications on the processing and stability of the material.

Silicon Heterojunction System Field Performance

A silicon heterostructure photovoltaic system fielded for 10 years has been investigated in detail. The system has shown degradation, but at a rate similar to an average Si system, and still within the module warranty level. The power decline is dominated by a nonlinear <italic>V</italic>_oc loss rather than more typical changes in <italic>I</italic>_sc or Fill Factor. Modules have been evaluated using multiple techniques including: dark and light <italic>I</italic>–<italic>V</italic> measurement, Suns-<italic>V</italic>_oc, thermal imaging, and quantitative electroluminescence. All techniques indicate that recombination and series resistance in the cells have increased along with a decrease of factor 2 in minority carrier lifetime. Performance changes are fairly uniform across the module, indicating changes occur primarily within the cells.