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NASA NTRS1985

Proceedings of the Flat-plate Solar Array Project Research Forum on High-efficiency Crystalline Silicon Solar Cells

The high-efficiency crystalline silicon solar cells research forum addressed high-efficiency concepts, surface-interface effects, bulk effects, modeling and device processing. The topics were arranged into six interactive sessions, which focused on the state-of-the-art of device structures, identification of barriers to achieve high-efficiency cells and potential ways to overcome these barriers.

Kachare, R.↗

NASA NTRS1985

High efficiency solar cell processing

At the time of writing, cells made by several groups are approaching 19% efficiency. General aspects of the processing required for such cells are discussed. Most processing used for high efficiency cells is derived from space-cell or concentrator cell technology, and recent advances have been obtained from improved techniques rather than from better understanding of the limiting mechanisms. Theory and modeling are fairly well developed, and adequate to guide further asymptotic increases in performance of near conventional cells. There are several competitive cell designs with promise of higher performance ( 20%) but for these designs further improvements are required. The available cell processing technology to fabricate high efficiency cells is examined.

Ho, F.↗

NASA NTRS1984

Summary of the high-efficiency crystalline solar cell research forum

An outline is given listing the speeches given in each session and the pertinent points is some speeches. There were six session, besides the overview session they were: (1) high efficiency concepts; (2) surface/interface effects; (3) bulk effects; (4) modeling; and (5) high efficiency device processing. Besides the emphasis placed on increasing efficiency of energy conversion, a great amount of discussion was spent on ways to reduce recombination and reducing surface defects.

Wolf, M.↗

NASA NTRS1984

Flat-plate module efficiency versus cost tradeoffs

The Five Year Research Plan energy cost methodology and in depth analyses based on the extensive data that are relevant to PV systems are used to facilitate the accomplishment of the $0.15/kWh energy cost goal. An equation is given for the five year energy cost methodology. The allocation guidelines are designed to be consistent with flat solar array (FSA) milestones for module cost, module efficiency, and the programmatic goal for energy cost. They are research targets that appear to be achievable, given prior accomplishments and planned activities in the areas of low cost silicon purification, low cost sheet material, high efficiency cell processing, low cost, long life encapsulants, and automated fabrication method. Extensive sensitivity analysis work has been performed that shows that these guidelines represent an efficient way to meet the intent of the DOE program.

Aster, R. W.↗

NASA NTRS1984

Sensitivity analysis of high-efficiency silicon solar-cell design parameters

Silicon solar cell design parameters were investigated to determine their bearing on cell efficiency. Among the parameters reviewed were: (1) bulk resistivity, (2) minority carrier lifetime cell thickness, (3) front junction depth, (4) front surface doping concentration, (5) front surface recombination velocity, and (6) back surface contact. The following were concluded: (1) there is good agreement between experimental and simulation results; (2) sheet material quality improvement is needed for high efficiency cells; (3) 20% cell of this design is feasible with 10 ms bulk lifetime material; and (4) for achieving efficiencies higher than 20% new cell designs including thin cells with light trapping and back surface field should be considered.

Mokashi, A. R.↗

NASA NTRS1984

Status of high-efficiency module design and fabrication

The fabrication of high efficiency solar energy conversion modules (13% efficiency at NOCT - Nominal Operating Cell Temperature) is discussed, with emphasis placed on reducing NOCT, since reduced operating temperature improves both efficiency and module lifetime.

Spitzer, M. B.↗

NASA NTRS1984

Extreme UV measurements of a varied line-space Hitachi reflection grating - Efficiency and scattering

The first extreme UV measurements of a grating mechanically ruled with smooth variations in the grating constant are presented. The grating characteristics are summarized and a schematic of the testing apparatus is shown and described. The procedure for obtaining the absolute grating efficiency is given. The results for that efficiency as a function of wavelengths of 68, 170, 256, 304, 584, and 1216 A are reported and discussed. Efficiency measurements were also made as a function of position across the ruled width at 256 A and 87 deg incident angle, and the results are discussed. Scattering measurements performed via computer acquisition of diffracted image profiles are reported.

Edelstein, J.↗

NASA NTRS1985

Efficient, frequency-stable laser-diode-pumped Nd:YAG laser

One of the main goals of the study was to demonstrate a low-power efficient Nd:YAG laser oscillator for applications in remote coherent Doppler anemometry. An electrical-to-optical slope efficiency of 6.5 percent has been achieved by using commercially available CW laser diodes of up to 100 mW to pump monolithic Nd:YAG rod lasers. The observed Nd:YAG oscillation threshold is at 2.3 mW of laser-diode output power, i.e., a small fraction of the rated output power. The highest Nd:YAG CW output power reached is 4.4 mW at an overall electrical-to-optical efficiency of 1.5 percent. The frequency jitter is less than 10 kHz in 0.3 s.

Zhou, B.↗

NASA NTRS1984

High-efficiency silicon solar cells

Silicon solar cells are described which operate at energy conversion efficiencies independently measured at 18.7 percent under standard terrestrial test conditions (AM1.5, 100 mW/sq cm, 28 C). These are apparently the most efficient silicon cells fabricated to date. The high-efficiency results from a combination of high open-circuit voltage due to the careful attention paid to the passivation of the top surface of the cell, high fill factor due to the high open-circuit voltage and low parasitic resistance losses, and high short-circuit current density due to the use of shallow diffusions, a low grid coverage, and an optimized double layer antireflection coating.

Green, M. A.↗

NASA NTRS1984

Status of high efficiency module design and fabrication

The status of an ongoing DOE program to develop an AM1 photoelectric module with 15 percent conversion efficiency at normal heating temperatures, is reviewed. Emphasis is given to the efforts of a private company to develop a high efficiency module which also has high durability in normal operating conditions. The main design options considered are: high efficiency modules; large area modules; and optimized module fabrication techniques. The design of an automatic system for encapsulating module stacks is described.

Nowlan, M. J.↗

NASA NTRS1985

Overview of SERI's high efficiency solar cell research

The bulk of the research efforts supported by the Solar Energy Research Institute (SERI) High Efficiency Concepts area has been directed towards establishing the feasibility of achieving very high efficiencies, 30% for concentrator and more than 20% for thin film flat plate, in solar cell designs which could possibly be produced competitively. The research has accomplished a great deal during the past two years. Even though the desired performance levels have not yet been demonstrated, based on the recent progress, a greater portion of the terrestrial photovoltaics community believes that these efficiencies are attainable. The program will now allocate a larger portion of resources to low cost, large area deposition technology. The program is currently shifting greater emphasis on to the study of crystal growth in order to provide the understanding and tools needed to design a large area process.

Benner, J. P.↗

NASA NTRS1986

Requirements for high-efficiency solar cells

Minimum recombination and low injection level are essential for high efficiency. Twenty percent AM1 efficiency requires a dark recombination current density of 2 x 10 to the minus 13th power A/sq cm and a recombination center density of less than 10 to the 10th power /cu cm. Recombination mechanisms at thirteen locations in a conventional single crystalline silicon cell design are reviewed. Three additional recombination locations are described at grain boundaries in polycrystalline cells. Material perfection and fabrication process optimization requirements for high efficiency are outlined. Innovative device designs to reduce recombination in the bulk and interfaces of single crystalline cells and in the grain boundary of polycrystalline cells are reviewed.

Sah, C. T.↗

NASA NTRS1985

High-efficiency module design

Progress on the development of a high efficiency module is described. The effort includes development of high efficiency cells using crystalline silicon wafers from float zone silicon. Module-size cells, 53 sq cm in area, were fabricated with efficiency of 18%. Operating temperature reduction is also studied.

Spitzer, M. B.↗

NASA NTRS1986

Potential high efficiency solar cells: Applications from space photovoltaic research

NASA involvement in photovoltaic energy conversion research development and applications spans over two decades of continuous progress. Solar cell research and development programs conducted by the Lewis Research Center's Photovoltaic Branch have produced a sound technology base not only for the space program, but for terrestrial applications as well. The fundamental goals which have guided the NASA photovoltaic program are to improve the efficiency and lifetime, and to reduce the mass and cost of photovoltaic energy conversion devices and arrays for use in space. The major efforts in the current Lewis program are on high efficiency, single crystal GaAs planar and concentrator cells, radiation hard InP cells, and superlattice solar cells. A brief historical perspective of accomplishments in high efficiency space solar cells will be given, and current work in all of the above categories will be described. The applicability of space cell research and technology to terrestrial photovoltaics will be discussed.

Flood, D. J.↗

NASA NTRS1985

Variation in efficiency of parallel algorithms

The present study has the objective to investigate some iterative parallel-processor linear equation solving algorithms with respect to efficiency for analyses of typical linear engineering systems. Attention is given to a set of n linear equations, Ku = p, where K = an n x n positive definite, sparsely populated, symmetric matrix, u = an n x 1 vector of unknown responses, and p = an n x 1 vector of prescribed constants. This study is concerned with a hybrid method in which iteration is used to solve the problem, while a direct method is used on the local processor level. Variations in the efficiency of parallel algorithms are explored. Measures of the efficiency are based on computer experiments regarding the algorithms. For all the algorithms, the wall clock time is found to decrease as the number of processors increases.

Hayashi, A.↗

NASA NTRS1986

Impurity effects in silicon for high efficiency solar cells

Model analyses indicate that sophisticated solar cell designs including, e.g., back surface fields, optical reflectors, surface passivation, and double layer antireflective coatings can produce devices with conversion efficiencies above 20 percent (AM1). To realize this potential, the quality of the silicon from which the cells are made must be improved; and these excellent electrical properties must be maintained during device processing. As the cell efficiency rises, the sensitivity to trace contaminants also increases. For example, the threshold Ti impurity concentration at which cell performance degrades is more than an order of magnitude lower for an 18-percent cell. Similar behavior occurs for numerous other metal species which introduce deep level traps that stimulate the recombination of photogenerated carriers in silicon. Purification via crystal growth in conjunction with gettering steps to preserve the large diffusion length of the as-grown material can lead to the production of devices with efficiencies aboved 18 percent, as has been verified experimentally.

Hopkins, R. H.↗

NASA NTRS1986

A review of high-efficiency silicon solar cells

Various parameters that affect solar cell efficiency were discussed. It is not understood why solar cells produced from less expensive Czochralski (Cz) silicon are less efficient than cells fabricated from more expensive float-zone (Fz) silicon. Performance characteristics were presented for recently produced, high-efficient solar cells fabricated by Westinghouse Electric Corp., Spire Corp., University of New South Wales, and Stanford University.

Rohatgi, A.↗

NASA NTRS1986

Development of high-efficiency solar cells on silicon web

Achievement of higher efficiency cells by directing efforts toward identifying carrier loss mechanisms; design of cell structures; and development of processing techniques are described. Use of techniques such as deep-level transient spectroscopy (DLTS), laser-beam-induced current (LBIC), and transmission electron microscopy (TEM) indicated that dislocations in web material rather than twin planes were primarily responsible for limiting diffusion lengths in the web. Lifetimes and cell efficiencies can be improved from 19 to 120 microns, and 8 to 10.3% (no AR), respectively, by implanting hydrogen at 1500 eV and a beam current density of 2.0 mA/sq cm. Some of the processing improvements included use of a double-layer AR coating (ZnS and MgF2) and an addition of an aluminum back surface reflectors. Cells of more than 16% efficiency were achieved.

Meier, D. L.↗