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Title: Atomic Structure and Electronic Properties of c-Si/a-Si:H Interfaces in Si Heterojunction Solar Cells

Abstract

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.

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
882601
Report Number(s):
NREL/CP-520-38935
DOE Contract Number:
AC36-99-GO10337
Resource Type:
Conference
Resource Relation:
Related Information: Presented at the 2005 DOE Solar Energy Technologies Program Review Meeting held November 7-10, 2005 in Denver, Colorado. Also included in the proceedings available on CD-ROM (DOE/GO-1020060-2245; NREL/CD-520-38577)
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; PHOTOVOLTAICS; SOLAR; HETEROJUNCTION SOLAR CELLS; PV; NREL; Solar Energy - Photovoltaics; Silicon Materials and Devices

Citation Formats

Yan, Y., Page, M., Wang, Q., Branz, H. M., Wang, T. H., and Al-Jassim, M. M.. Atomic Structure and Electronic Properties of c-Si/a-Si:H Interfaces in Si Heterojunction Solar Cells. United States: N. p., 2005. Web.
Yan, Y., Page, M., Wang, Q., Branz, H. M., Wang, T. H., & Al-Jassim, M. M.. Atomic Structure and Electronic Properties of c-Si/a-Si:H Interfaces in Si Heterojunction Solar Cells. United States.
Yan, Y., Page, M., Wang, Q., Branz, H. M., Wang, T. H., and Al-Jassim, M. M.. Tue . "Atomic Structure and Electronic Properties of c-Si/a-Si:H Interfaces in Si Heterojunction Solar Cells". United States. doi:. https://www.osti.gov/servlets/purl/882601.
@article{osti_882601,
title = {Atomic Structure and Electronic Properties of c-Si/a-Si:H Interfaces in Si Heterojunction Solar Cells},
author = {Yan, Y. and Page, M. and Wang, Q. and Branz, H. M. and Wang, T. H. and Al-Jassim, M. M.},
abstractNote = {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.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Nov 01 00:00:00 EST 2005},
month = {Tue Nov 01 00:00:00 EST 2005}
}

Conference:
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  • We develop a fully analytical model in order to describe the temperature dependence of the low frequency capacitance of heterojunctions between hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). We demonstrate that the slope of the capacitance-temperature (C-T) curve is strongly enhanced if the c-Si surface is under strong inversion conditions compared to the usually assumed depletion layer capacitance. We have extended our analytical model to integrate a very thin undoped (i) a-Si:H layer at the interface and the finite thickness of the doped a-Si:H layer that are used in high efficiency solar cells for the passivation of interface defectsmore » and to limit short circuit current losses. Finally, using our calculations, we analyze experimental data on high efficiency silicon heterojunction solar cells. The transition from the strong inversion limited behavior to the depletion layer behavior is discussed in terms of band offsets, density of states in a-Si:H, and work function of the indium tin oxide (ITO) front electrode. In particular, it is evidenced that strong inversion conditions prevail at the c-Si surface at high temperatures down to 250 K, which can only be reproduced if the ITO work function is larger than 4.7 eV.« less
  • Despite energy conversion efficiencies exceeding 22%, current understanding of the physics behind heterojunction solar cells remains incomplete. The role of hydrogen and ion bombardment during the plasma deposition as well as the influence of an epitaxial layer remains a subject of debate. Our results suggest that hydrogen plays a key role in the fabrication of high efficiency heterojunction solar cells. We show that ion bombardment is not as detrimental as is often thought. Moreover we find that an epitaxial layer is not necessarily harmful to the cell's V{sub oc}. We propose a criterion linking the layer's epitaxy and the cells'more » performance. To further investigate the role of the H{sub 2} plasma, we carry out in situ ellipsometry measurements on various kinds of c-Si wafers. The effects of this H{sub 2} plasma strongly depend on the resistivity of the c-Si wafer, suggesting that plasma conditions must be tuned to optimize cell efficiency according to the c-Si resistivity.« less
  • In this work a-Si:H/c-Si heterostructures with good electronic properties of a-Si:H were prepared by 55 kHz Plasma Enhanced Chemical Vapor Deposition (PECVD). Current-voltage and capacitance-voltage characteristics of a-Si:H/c-Si heterostructures were measured to investigate the influence of low frequency plasma on the growing film and amorphous silicon/crystalline silicon boundary. It was established that the interface state density is low enough for device applications (<2{center_dot}10{sup 10} cm{sup {minus}2}). The current voltage measurements suggest that, when forward biased, space-charge-limited current determines the transport mechanism in a-Si:H/c-Si heterostructures, while reverse current is ascribed to the generation current in a-Si:H and c-Si depletion layers.
  • The researchers extensively studied the effects of annealing or thermal history of cell process on the minority carrier lifetimes of FZ n-type c-Si wafers with various i-layer thicknesses from 5 to 60 nm, substrate temperatures from 100 to 350 degrees C, doped layers both p- and n-types, and transparent conducting oxide (TCO).