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Probing Carrier Depletions on Grain Boundaries in Polycrystalline Si Thin Films by Scanning Capacitance Microscopy

Published online by Cambridge University Press:  01 February 2011

Chunsheng Jiang ,
H.R. Moutinho ,
B. To ,
P. Dippo ,
M.J. Romero  and
M.M. Al-Jassim
Chunsheng Jiang
Affiliation:
chun_sheng_jiang@nrel.gov, National Renewable Energy Laboratory, National Center for photovoltaic, 1617 Cole Blvd., Golden, CO, 80401, United States
H.R. Moutinho
Affiliation:
heli_moutinho@nrel.gov, National Renewable Energy Laboratory, Golden, CO, 80401, United States
B. To
Affiliation:
bobby_to@nrel.gov, National Renewable Energy Laboratory, Golden, CO, 80401, United States
P. Dippo
Affiliation:
pat_dippo@nrel.gov, National Renewable Energy Laboratory, Golden, CO, 80401, United States
M.J. Romero
Affiliation:
manuel_romero@nrel.gov, National Renewable Energy Laboratory, Golden, CO, 80401, United States
M.M. Al-Jassim
Affiliation:
mowafak_aljassim@nrel.gov, National Renewable Energy Laboratory, Golden, CO, 80401, United States
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Abstract

Grain boundaries (GBs) in polycrystalline Si thin-film solar cells are believed to limit the photovoltaic efficiencies. In this paper, we report on a nanometer-resolution measurement of the carrier depletion at the GBs, using scanning capacitance microscopy (SCM). The SCM images exhibit the following features: (1) Carrier concentrations are lower at locations around the GBs than on center regions of the grains; (2) The depletion width at the GBs varies considerably, between 0 and 100 nm, depending on individual GBs; (3) Intra-grain carrier depletion was also observed at point and line defects; and (4) The faceted features that were observed on the topography of the as-grown film surface appeared on the SCM images even after the film surface was polished flat. The direct measurement of the carrier depletion on the GBs demonstrates that the GBs in Si thin films indeed create charged gap states. The nonuniformity of the carrier depletions suggests that the gap states depend on specific GB structures, which should relate directly to the grain orientations and grain facets adjacent to the GB. The depletion around the intragrain defects indicates that the defects are charged and can be recombination centers, and thus, harmful to device performance. This paper reports the first step of our studies toward understanding the relationships between the electronic and structural properties on specific GBs.

Keywords

grain boundariesSiscanning probe microscopy (SPM)
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1066: Symposium A – Amorphous and Polycrystalline Thin-Film Silicon Science and Technology—2008 , 2008 , 1066-A04-02
Copyright
Copyright © Materials Research Society 2008

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