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Title: Research on the stability, electronic properties, and structure of a-Si:H and its alloys. Final subcontract report, 1 June 1991--31 May 1994

Abstract

The authors research has focused on defect metastability and a-Si:C:H alloys. A new aspect of the metastability is the growing interest in the defect recovery process. They have continued to explore the role of hydrogen in the metastability and other properties of a-Si:H. This has led them to perform first principles calculations of Si-H bonding configurations. Another new feature of the metastability work is the study of the effects in hydrogenated poly-silicon. They have grown and studied the properties of a-Si:C:H alloys, particularly to observe the effects of hydrogen dilution. Finally they have also studied the recent defect relaxation phenomenon, and concluded that the effects arise from contact effects and are not an intrinsic effect in a-Si:H. Section A presents some recent models of metastability. Section B discusses the metastability and equilibration effects in hydrogenated polysilicon, studied because of it`s close similarity to a-Si:H. Section C describes results on a-Si:C:H alloys. Section D contains first principle LDA calculations of Si-H bonds and relates these results to the a-Si:H diffusion and metastability properties. In section E the authors report capacitance measurements aimed at exploring the recent results by Cohen et al who find an anomalous relaxation process from the trap fillingmore » kinetics of a DLTS experiment.« less

Authors:
; ; ; ; ; ; ; ;  [1]
  1. Xerox Palo Alto Research Center, CA (United States)
Publication Date:
Research Org.:
National Renewable Energy Lab., Golden, CO (United States); Xerox Palo Alto Research Center, CA (United States)
Sponsoring Org.:
USDOE, Washington, DC (United States)
OSTI Identifier:
86992
Report Number(s):
NREL/TP-451-8101
ON: DE95009238; TRN: AHC29521%%48
DOE Contract Number:
AC36-83CH10093
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: Jul 1995
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; SILICON SOLAR CELLS; MATERIALS; SILICON; METASTABLE STATES; DEFECTS; ELECTRONIC STRUCTURE; PROGRESS REPORT; AMORPHOUS STATE; HYDROGEN; DIFFUSION; SILANES; ELECTRIC CONDUCTIVITY; MATHEMATICAL MODELS; POLYCRYSTALS; CAPACITANCE; EXPERIMENTAL DATA

Citation Formats

Jackson, W.B., Johnson, N., Nickel, N., Schumm, G., Street, R.A., Thompson, R., Tsai, C.C., Van de Walle, C., and Walker, J. Research on the stability, electronic properties, and structure of a-Si:H and its alloys. Final subcontract report, 1 June 1991--31 May 1994. United States: N. p., 1995. Web. doi:10.2172/86992.
Jackson, W.B., Johnson, N., Nickel, N., Schumm, G., Street, R.A., Thompson, R., Tsai, C.C., Van de Walle, C., & Walker, J. Research on the stability, electronic properties, and structure of a-Si:H and its alloys. Final subcontract report, 1 June 1991--31 May 1994. United States. doi:10.2172/86992.
Jackson, W.B., Johnson, N., Nickel, N., Schumm, G., Street, R.A., Thompson, R., Tsai, C.C., Van de Walle, C., and Walker, J. Sat . "Research on the stability, electronic properties, and structure of a-Si:H and its alloys. Final subcontract report, 1 June 1991--31 May 1994". United States. doi:10.2172/86992. https://www.osti.gov/servlets/purl/86992.
@article{osti_86992,
title = {Research on the stability, electronic properties, and structure of a-Si:H and its alloys. Final subcontract report, 1 June 1991--31 May 1994},
author = {Jackson, W.B. and Johnson, N. and Nickel, N. and Schumm, G. and Street, R.A. and Thompson, R. and Tsai, C.C. and Van de Walle, C. and Walker, J.},
abstractNote = {The authors research has focused on defect metastability and a-Si:C:H alloys. A new aspect of the metastability is the growing interest in the defect recovery process. They have continued to explore the role of hydrogen in the metastability and other properties of a-Si:H. This has led them to perform first principles calculations of Si-H bonding configurations. Another new feature of the metastability work is the study of the effects in hydrogenated poly-silicon. They have grown and studied the properties of a-Si:C:H alloys, particularly to observe the effects of hydrogen dilution. Finally they have also studied the recent defect relaxation phenomenon, and concluded that the effects arise from contact effects and are not an intrinsic effect in a-Si:H. Section A presents some recent models of metastability. Section B discusses the metastability and equilibration effects in hydrogenated polysilicon, studied because of it`s close similarity to a-Si:H. Section C describes results on a-Si:C:H alloys. Section D contains first principle LDA calculations of Si-H bonds and relates these results to the a-Si:H diffusion and metastability properties. In section E the authors report capacitance measurements aimed at exploring the recent results by Cohen et al who find an anomalous relaxation process from the trap filling kinetics of a DLTS experiment.},
doi = {10.2172/86992},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sat Jul 01 00:00:00 EDT 1995},
month = {Sat Jul 01 00:00:00 EDT 1995}
}

Technical Report:

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  • Objective is to obtain a comprehensive understanding of structure and electronic properties of a-Si:H as they apply to solar cells. First observations were of light enhancement and field suppression of H diffusion in a-Si:H. Theoretical studies were made of hydrogen density of states distribution and its relation to defect metastability. Reduced density of light induced defect is observed in a-Si:H deposited in a remote hydrogen plasma reactor at 400 C. Kinetics of metastable defect creation using forward bias in a p-i-n diode to induce defects were studied and compared to light-induced defect creation in the same devices. Studies were mademore » of transport at high electric field and low temperature. Detailed studies were made of kinetics of dopant metastability in n-type and p-type a-Si:H.« less
  • This report describes work focusing on the defect and transport properties of a-Si:H with particular emphasis on defect metastability. Light-induced defects remain the major impediment to higher stabilized solar cell efficiencies. The many years of research have shown that this is a difficult problem to solve, and we take the view that a solution can be found only with a deep understanding of the mechanism; this has been our main goal. The metastability is closely related to the intrinsic defect properties, so these studies have also led to a greatly improved model of all the electronic properties of a-Si:H. Sectionmore » A of the report discusses a theoretical analysis of the defect pool model for the equilibrium dark defect density, with particular reference to whether there are significant charged defects. Section B discusses the use of an electronic transport model to analyze forward and reverse currents and extends it to the calculation of field dependences of bulk and contact currents, which give more precision to the analysis of defects from reverse bias thermal generation currents. Electronic transport is discussed in Section C. Work done on H transport and bonding is described in Section D.« less
  • This report describes work on the growth of a-Si:H and a-(Si,Ge):H materials and devices using well-controlled growth techniques. The a-Si:H materials were grown at higher temperatures (300{degrees}-375{degrees}C) using electron-cyclotron-resonance (ECR) plasma techniques with a remote H beam. These films have excellent electronic quality and show significant improvements in stability compared with glow-discharge-produced a-Si:H materials. Several problems were encountered during the fabrication of devices in these materials, and we were able to overcome them by a systematic work on buffer layers in these cells. We also studied alternative designs for improving the stability of a-Si:H cells and produced graded-gap a-Si:H cellsmore » using glow-discharge that are more stable than comparable standard, ungraded glow discharge devices. Finally, systematic work was done to produce good-quality a-(Si,Ge):H films, using triode radio frequency (RF) glow-discharge with ion bombardment during growth. Diagnostic devices were made using these films, and the properties of the material, such as Urbach energies and hole mobility-lifetime products, were measured in these devices. We found a systematic increase in the Urbach energies, and a corresponding decrease in the hole and electron {mu}{tau} products, as the Ge content of the alloys increases.« less
  • This report describes work to identify materials parameters that can quantitatively describe the solar cell performance correctly in the initial and stabilized states and are consistent with a microscopic model of the metastable defect site. The objective is to be accomplished by applying results of in-situ analyses of a-Si:H surfaces and the transparent conducting oxide (TCO)/p/i interfaces to complement the present understanding of the electronic properties of materials and devices. A second objective of the program is to demonstrate, characterize, and understand improved doped and undoped ``wide-gap`` materials for use in achieving 15% stabilized photovoltaic modules (``wide-gap`` materials are definedmore » as those materials with a band gap of at least 1.9 eV).« less
  • This research supported by NREL Subcontract XG-1-10063-1 over the past three years has involved, first of all, a fairly complete characterization of a two series of a-Si{sub 1-x}Ge{sub x}:H samples: a series of 9 films grown at the University of Delaware by the photo-CVD method (for 0.29 {<=} {times} {<=} 0.62) and series of 6 films grown at U.S.S.C. by the glow discharge method (for 0.20 {<=} {times} {<=} 0.50). Both these series of samples seem to represent what is close to the {open_quotes}state-of-the-art{close_quotes} in current a-Si,Ge:H alloys. The authors detailed comparison of the properties of the glow discharge materialmore » with the photo-CVD samples show remarkable similarities rather than significant differences. In particular, measurements of these samples: (1) allowed the assignment of defect energy levels from a detailed analysis of transient sub-band-gap photocapacitance and photocurrent spectra. (2) The authors found the density of deep defects to increase exponentially with the germanium content. (3) The authors found that the trapping lifetimes related {mu}{tau} products for holes decrease in direct proportion to the density of midgap defects in these samples, at least up to Ge fractions of 50at.%. (4) The authors have also made significant progress toward identifying both the optical and thermal defect transitions in the a-Si,Ge:H alloys.« less