Engineering topics

Rai-Choudhury, P.

Publications and source records attributed to Rai-Choudhury, P..

At least 19 records

NASA NTRS1986

Effect of low-energy hydrogen ion implantation on dendritic web silicon solar cells

The effect of a low-energy (0.4 keV), short-time (2-min), heavy-dose (10 to the 18th/sq cm) hydrogen ion implant on dendritic web silicon solar cells and material was investigated. Such an implant was observed to improve the cell open-circuit voltage and short-circuit current appreciably for a number of cells. In spite of the low implant energy, measurements of internal quantum efficiency indicate that it is the base of the cell, rather than the emitter, which benefits from the hydrogen implant. This is supported by the observation that the measured minority-carrier diffusion length in the base did not change when the emitter was removed. In some cases, a threefold increase of the base diffusion length was observed after implantation. The effects of the hydrogen implantation were not changed by a thermal stress test at 250 C for 111 h in nitrogen. It is speculated that hydrogen enters the bulk by traveling along dislocations, as proposed recently for edge-defined film-fed growth silicon ribbon.

Rohatgi, A.↗

NASA NTRS1986

Development of high-efficiency solar cells on silicon web

Work was performed to improve web base material with a goal of obtaining solar cell efficiencies in excess of 18% (AM1). Efforts in this program are directed toward identifying carrier loss mechanisms in web silicon, eliminating or reducing these mechanisms, designing a high efficiency cell structure with the aid of numerical models, and fabricating high efficiency web solar cells. Fabrication techniques must preserve or enhance carrier lifetime in the bulk of the cell and minimize recombination of carriers at the external surfaces. Three completed cells were viewed by cross-sectional transmission electron microscopy (TEM) in order to investigate further the relation between structural defects and electrical performance of web cells. Consistent with past TEM examinations, the cell with the highest efficiency (15.0%) had no dislocations but did have 11 twin planes.

Meier, D. L.↗

NASA NTRS1986

Development of high-efficiency solar cells on silicon web

Work is reported aimed at identifying and reducing sources of carrier recombination both in the starting web silicon material and in the processed cells. Cross-sectional transmission electron microscopy measurements of several web cells were made and analyzed. The effect of the heavily twinned region on cell efficiency was modeled, and the modeling results compared to measured values for processed cells. The effects of low energy, high dose hydrogen ion implantation on cell efficiency and diffusion length were examined. Cells were fabricated from web silicon known to have a high diffusion length, with a new double layer antireflection coating being applied to these cells. A new contact system, to be used with oxide passivated cells and which greatly reduces the area of contact between metal and silicon, was designed. The application of DLTS measurements to beveled samples was further investigated.

Meier, D. L.↗

NASA NTRS1986

High-efficiency silicon solar cells - Development, current issues and future directions

This paper provides a review of material, process, and design considerations that are essential for achieving high-efficiency silicon solar cells. It describes an approach that resulted in 18.0-18.5-percent-efficient cells on low-resistivity float-zone silicon material. It is shown experimentally that a combination of surface passivation and double-layer antireflection coating can provide 2-3 percent (absolute) improvement in cell efficiency provided the material, process and cell design are carefully selected.

Rohatgi, A.↗

NASA NTRS1985

Development of high-efficiency solar cells on silicon web

High-efficiency dendritic cells were discussed. The influence of twin planes and heat treatment on the location and effect of trace impurities was of particular interest. Proper heat treatment often increases efficiency by causing impurities to pile up at twin planes. Oxide passivation had a beneficial effect on efficiency. A very efficient antireflective (AR) coating of zinc selenide and magnesium fluoride was designed and fabricated. An aluminum back-surface reflector was also effective.

Rohatgi, A.↗

NASA NTRS1985

Process and design considerations for high-efficiency solar cells

This paper shows that oxide surface passivation coupled with optimum multilayer anti-reflective coating can provide approx. 3% (absolute) improvement in solar cell efficiency. Use of single-layer AR coating, without passivation, gives cell efficiencies in the range of 15 to 15.5% on high-quality, 4 ohm-cm as well as 0.1 to 0.2 ohm-cm float-zone silicon. Oxide surface passivation alone raises the cell efficiency to or = 17%. An optimum double-layer AR coating on oxide-passivated cells provides an additional approx. 5 to 10% improvement over a single-layer AR-coated cell, resulting in cell efficiencies in excess of 18%. Experimentally observed improvements are supported by model calculations and an approach to or = 20% efficient cells is discussed.

Rohati, A.↗

NASA NTRS1984

Development of high efficiency solar cells on silicon web

Web base material is being improved with a goal toward obtaining solar cell efficiencies in excess of 18% (AM1). Carrier loss mechanisms in web silicon was investigated, techniques were developed to reduce carrier recombination in the web, and web cells were fabricated using effective surface passivation. The effect of stress on web cell performance was also investigated.

Rohatgi, A.↗

NASA NTRS1984

Development of high-efficiency solar cells on silicon web

The development of high efficiency solar cells on a silicon web is discussed. Heat treatment effects on web quality; the influence of twin plane lamellae, trace impurities and stress on minority carrier lifetime; and the fabrication of cells are discussed.

Rohatgi, A.↗

NASA NTRS1982

Silicon dendritic web material

The development of a low cost and reliable contact system for solar cells and the fabrication of several solar cell modules using ultrasonic bonding for the interconnection of cells and ethylene vinyl acetate as the potting material for module encapsulation are examined. The cells in the modules were made from dendritic web silicon. To reduce cost, the electroplated layer of silver was replaced with an electroplated layer of copper. The modules that were fabricated used the evaporated Ti, Pd, Ag and electroplated Cu (TiPdAg/Cu) system. Adherence of Ni to Si is improved if a nickel silicide can be formed by heat treatment. The effectiveness of Ni as a diffusion barrier to Cu and the ease with which nickel silicide is formed is discussed. The fabrication of three modules using dendritic web silicon and employing ultrasonic bonding for interconnecting calls and ethylene vinyl acetate as the potting material is examined.

Meier, D. L.↗

NASA NTRS1982

Silicon materials task of the Low Cost Solar Array Project: Effect of impurities and processing on silicon solar cells

The effects of impurities and processing on the characteristics of silicon and terrestrial silicon solar cells were defined in order to develop cost benefit relationships for the use of cheaper, less pure solar grades of silicon. The amount of concentrations of commonly encountered impurities that can be tolerated in typical p or n base solar cells was established, then a preliminary analytical model from which the cell performance could be projected depending on the kinds and amounts of contaminants in the silicon base material was developed. The impurity data base was expanded to include construction materials, and the impurity performace model was refined to account for additional effects such as base resistivity, grain boundary interactions, thermal processing, synergic behavior, and nonuniform impurity distributions. A preliminary assessment of long term (aging) behavior of impurities was also undertaken.

Hopkins, R. H.↗

NASA NTRS1982

Solar cell contacts

Two experimental contact systems were examined and compared to a baseline contact system consisting of evaporated layers of titanium, palladium, and silver and an electroplated layer of copper. The first experimental contact system consisted of evaporated layers of titanium, nickel, and copper and an electroplated layer of copper. This system performed as well as the baseline system in all respects, including its response to temperature stress tests, to a humidity test, and to an accelerated aging test. In addition, the cost of this system is estimated to be only 43 percent of the cost of the baseline system at a production level of 25 MW/year. The second experimental contact system consisted of evaporated layers of nickel and copper and an electroplated layer of copper. Cells with this system show serious degradation in a temperature stress test at 350 C for 30 minutes. Auger electron spectroscopy was used to show that the evaporated nickel layer is not an adequate barrier to copper diffusion even at temperatures as low as 250 C. This fact brings into question the long-term reliability of this contact system.

Meier, D. L.↗

NASA NTRS1981

Silicon materials task of the low-cost solar array project. Phase 4: Effects of impurities and processing on silicon solar cells

The effects of impurities, various thermochemical processes, and any impurity-process interactions upon the performance of terrestrial solar cells are defined. The results form a basis for silicon producers, wafer manufacturers, and cell fabricators to develop appropriate cost benefit relationships for the use of less pure, less costly solar grade silicon.

Hopkins, R. H.↗

NASA NTRS1980

Impurities in silicon solar cells

The paper investigates the effects of metallic impurities on the performance of silicon solar cells. Czochralski and polycrystalline ingots were employed with boron and phosphorus as primary dopants and with controlled additions of secondary impurities. The data obtained from over 200 crystals indicate that impurity-induced performance loss is primarily due to a reduction of the base diffusion length. Based on this observation, a model is developed which predicts cell performance as a function of secondary impurity concentrations. The model calculations are in good agreement with experimental values except for Cu, Ni, Fe, and to a lesser degree, carbon, which at higher concentrations degrade the cell by junction defect mechanisms.

Davis, J. R., Jr.↗

NASA NTRS1980

Effect of impurities and processing on silicon solar cells. Volume 1: Characterization methods for impurities in silicon and impurity effects data base

Two major topics are treated: methods to measure and evaluate impurity effects in silicon and comprehensive tabulations of data derived during the study. Discussions of deep level spectroscopy, detailed dark I-V measurements, recombination lifetime determination, scanned laser photo-response, conventional solar cell I-V techniques, and descriptions of silicon chemical analysis are presented and discussed. The tabulated data include lists of impurity segregation coefficients, ingot impurity analyses and estimated concentrations, typical deep level impurity spectra, photoconductive and open circuit decay lifetimes for individual metal-doped ingots, and a complete tabulation of the cell I-V characteristics of nearly 200 ingots.

Hopkins, R. H.↗

NASA NTRS1980

Solar cells and modules from dentritic web silicon

Some of the noteworthy features of the processes developed in the fabrication of solar cell modules are the handling of long lengths of web, the use of cost effective dip coating of photoresist and antireflection coatings, selective electroplating of the grid pattern and ultrasonic bonding of the cell interconnect. Data on the cells is obtained by means of dark I-V analysis and deep level transient spectroscopy. A histogram of over 100 dentritic web solar cells fabricated in a number of runs using different web crystals shows an average efficiency of over 13%, with some efficiencies running above 15%. Lower cell efficiency is generally associated with low minority carrier time due to recombination centers sometimes present in the bulk silicon. A cost analysis of the process sequence using a 25 MW production line indicates a selling price of $0.75/peak watt in 1986. It is concluded that the efficiency of dentritic web cells approaches that of float zone silicon cells, reduced somewhat by the lower bulk lifetime of the former.

Campbell, R. B.↗

NASA NTRS1979

Silicon Materials Task of the Low Cost Solar Array Project, Phase 3. Effect of Impurities and Processing on Silicon Solar Cells

The effects of impurities, various thermochemical processes, and any impurity process interactions on the performance of terrestrial silicon solar cells are defined. Determinations of the segregation coefficients of tungsten, tantalum, and cobalt for the Czochralski pulling of silicon single crystals are reported. Sensitive neutron activation analysis was used to determine the metal impurity content of the silicon while atomic absorption was used to measure the metal content of the residual liquid from which the doped crystals were grown. Gettering of Ti doped silicon wafers improved cell performance by one to two percent for the highest temperatures and longest times. The HCl is more effective than POCl3 treatments for deactivating Ti but POCl3 and HCl produced essentially identical results for Mo or Fe.

Hopkins, R. H.↗

NASA NTRS1979

Array Automated Assembly Task for the Low Cost Solar Array Project, Phase 2

Using silk screened evaporated and sputtered Al as the metal source, the formation of Al back surface fields was studied. The most satisfactory results were those obtained with the sputtered A1 and in which open circuit voltages (V sub oc) of 0.585 v (12 ohm cm FZ silicon) were achieved. The ultrasonic interconnect process is discussed. The process is shown to be satisfactory, but increased pull-strength may be obtained if some form of sintering is carried out on the metallized contacts. Plasma etching is shown to be feasible as a replacement for wet chemical cleaning prior to diffusion. Initial results on cells prepared by using electroless Pd/Ni plus either electroplated Ag or Cu show slightly poor performance than cells with the baseline evaporated Ti/Pd/Ag system. A mask designed for the 1.6 x 7.0 cm and 2.0 x 7.0 cm cells is described. This mask has a lower area coverage and total lower resistive loss than the previous mask design. It is also shown that the cell width should not exceed 2.0 - 3.0 cm for optimum efficiency.

Campbell, R. B.↗

NASA NTRS1979

Effects of Impurities and Processing on Silicon Solar Cells, Phase 3

Results of the 14th quarterly report are presented for a program designed to assess the effects of impurities, thermochemical processes and any impurity process interactions on the performance of terrestrial silicon solar cells. The Phase 3 effort encompasses: (1) potential interactions between impurities and thermochemical processing of silicon; (2) impurity-cell performance relationships in n-base silicon; (3) effect of contaminants introduced during silicon production, refining or crystal growth on cell performance; (4) effects of nonuniform impurity distributions in large area silicon wafers; and (5) a preliminary study of the permanence of impurity effects in silicon solar cells.

Hopkins, R. H.↗