Electrical Properties of p-Type and n-Type Doped Inverse Silicon Opals - Towards
Optically Amplified Silicon Solar Cells
T. Suezaki,1,2,3 J. I. L. Chen,1 T. Hatayama,3 T. Fuyuki,3 G. A. Ozin1
1 Materials Chemistry Research Group, Department of Chemistry, University of Toronto
80 St. George Street, Toronto, ON M5S 3H6 (Canada)
2 Solar Energy Division, Kaneka Corporation
2-1-1, Hieitsuji, Otsu, Shiga 520-0104 (Japan)
3 Microelectronic Device Science Laboratory, Graduate School of Materials Science, Nara
Institute of Science and Technology
8916-5 Takayama, Ikoma, Nara 630-0192 (Japan)
Details for preparation of inverse silicon opals
We fabricated i-Si-o by infiltrating the voids of the silica opal template with Si using
chemical vapor deposition (CVD) with disilane and subsequently etching the silica spheres
with hydrofluoric acid (HF) [1]. Silica spheres with diameter 460 nm were synthesized by a
modified Stöber process involving the base catalyzed hydrolytic poly-condensation of
tetraethylorthosilicate. We chose silica opals with 460 nm sphere diameter for templates
because our previous work showed little dependence of σd on the photonic crystal lattice
constant of i-Si-o. In order to form the silica opal templates, spheres were dispersed in ethanol
for several hours and then crystallized as silica opal film by evaporation induced
self-assembly (EISA) method on sapphire substrates. These substrates were cleaned by
piranha before silica opal film formation. The template opal deposition conditions, such as
concentration of spheres in ethanol (2.5 wt. % sphere) and rate of evaporation (0.4 ml/Hour)
on sapphire substrates were optimized in order to achieve well-ordered opal film. Infiltration
of amorphous silicon into the interstitial voids of silica opals was subsequently performed
using a CVD apparatus and disilane as a silicon precursor. For the CVD we used a disilane
pressure of 1.0 torr and deposition temperature of 480 ˚C. Then reactive ion etching was
performed to remove silicon over-layers that were deposited during the silicon infiltration
process. The as-deposited Si in the interstitial voids of silica opals by CVD is intrinsically
amorphous. To obtain inverse structure, we etched silica opal with 2 wt. % HF. We prepared
all samples on sapphire substrates instead of glasses due to their high stability against HF
etching and that made it much easier to form intact electrodes for accurate measurements of
the dc electrical dark conductivities (σd) for the i-Si-o films. [2]
Measurements details
Reflectance spectra of each sample at every stage of processing were measured using a Perkin
Elmer UV/VIS/near-IR spectrometer Lambda 900. The probe light diameter was about 2 mm.
Scanning Electron Microscopy (SEM) images were obtained using Hitachi, S-5200.
To measure the dc electrical dark conductivity (σd), the current was recorded while the bias
across the electrode pair was increased from -10 V to +10 V in increments of 1 V. The σd was
determined from the resulting I-V curves, which were linear implying a good ohmic contact
between the aluminum and the i-Si-o, with the equation σd =Il/VS, where l is the gap length
between two electrodes and S is the cross sectional area of the films. The area includes not
only the area of the i-Si-o film but also the void spaces. Low temperature conductivity
measurements were performed with a low temperature measurement system (Nagase Co.) and
the activation energy (Ea) was determined from a plot of measured σd as function of reciprocal
temperature using the same electrode configuration under a bias of 10 V. A Keithley 6517A
High Resistance Meter was used for all electrical measurements.
The current-voltage characteristics of the pSi/i-cSi-o/nSi solar cell were measured under the
radiation of solar simulator (Wacom) and the quantum efficiency was measured by a spectral
response measurement system.
Comparison of optical property of i-cSi-o from theoretical calculation and experimental
result
Figure S1. Comparison of optical photonic properties of i-cSi-o from theoretical calculation
and experimental result. The left side is a photonic band diagram for i-cSi-o fabricated by
silica spheres with 460 nm for diameter from theoretical calculation. The right side is the
corresponding experimental reflectance spectrum of i-cSi-o obtained from 460 nm spheres.
The Г-L direction; corresponding to the [1 1 1] direction for a face centered cubic (fcc) lattice;
has three photonic band gaps (depicted with shading) from 600 nm to 2000 nm for the
examined structure in a theoretical calculation[3, 4]. The light with wavelengths at photonic
band gaps could not penetrate into the photonic crystal lattice and therefore those wavelengths
should be reflected. The experimental reflection spectrum of i-cSi-o is well matched to the
theoretical calculation in spite of the finite lattice and surface morphology that should cause a
deviation of intensity from perfect reflection.
Preparation of Solar Cell structure
On a borosilicate glass substrate, a n-type micro-crystalline Si (n:μc-Si) film of 100 nm was
deposited by plasma enhanced chemical vapour deposition (PECVD), on top of which an
i-type i-cSi-o film with 5 μm thickness was fabricated followed by HPP. Then p-type
micro-crystalline Si (p:μc-Si) film and Indium Tin Oxide film was deposited by PECVD and
sputtering respectively.
Solar Cell performance
Figure S2. Current-voltage characteristics for a c-Si thin film based p-i-n junction solar cell
under dark condition (black) and under air mass 1.5 irradiation (red)
Figure 3a shows the current-voltage characteristic of the structure with i-cSi-o under the air
mass 1.5 radiation; the behaviour typical of a photovoltaic device is observed. In contrast, the
current-voltage characteristic of the structure with c-Si thin film does not show any
photovoltaic behavior (Figure S2) due to its quite high resistance. Its performance and high
possibility of a SPC solar cell has been investigated previously[5, 6] where researchers studied
several special techniques such as crystallization, passivation, doping layer activation,
interface treatment and electrical contact to realize high performance. Note that our SPC
techniques are still very preliminary and n: c-Si is not in contact with any other high
conductive electrode like metals or conductive oxides. The bottom electrode was not
incorporated in the device structure due to the HF etching process we employed. Hence in our
structure, the n-layer itself must also posses high conductivity in the plane of the device,
which may explain the differences in solar cell performance between that of i-cSi-o and c-Si.
For i-cSi-o solar cell, the high porosity of i-cSi-o allowed hydrogen radicals to reach the
n-layer and consequently HPP efficiently enhanced the conductivity of n-layer to a level that
yielded overall solar cell behaviour. On the other hand, the dense 1μm thick i-type c-Si thin
film in the reference cell prevented the hydrogen radicals from reaching the n-type layer;
hence the resistivity was not improved and no photovoltaic behaviour could be observed.
References
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