Plasma assisted synthesis of silicon nanocrystals
for thin film solar cells
David J. Rowe, Rebecca J. Anthony, Uwe R. Kortshagen
Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota USA
Abstract: Theoretical efficiency limits and high production costs imposed by
single crystal materials may ultimately restrict the use of photovoltaics for energy
generation. Therefore, decreasing cell fabrication cost and increasing cell
efficiency should be targeted simultaneously to make photovoltaics more
accessible to the general public. One way to achieve such reductions is to use
silicon nanocrystals synthesized from a nonthermal plasma process to replace one
or more layers in a p-n (or p-i-n) junction solar cell. In this study, a nonthermal
argon-silane plasma doped with phosphine is employed in a flow-through reactor
design to generate phosphorus-doped silicon nanocrystals. In an elegant
approach, the doped nanocrystals are impacted to form films directly from the
gas phase. Solar cells are made from films deposited on silicon wafers,
exhibiting power conversion efficiency up to 3%.
Keywords: silicon nanocrystals, nonthermal plasma, impaction, photovoltaics
1. Introduction
In recent years, silicon nanocrystals (SiNCs) have
become an attractive material for opto-electronic
devices due to their unique properties. Phenomena
such as NC-size-dependent band gaps1,2, tunable
emission3,4, and the hotly debated multiple-exciton
generation process5 are some of the material
properties which are often proposed as new routes
for increasing Si solar cell efficiency and reducing
the cost of photovoltaics.
However, the
aforementioned nano-scale technologies are not the
only way to achieve low cost Si solar cells. Overall
cell cost reductions can be found by applying new
technologies to existing systems. For instance, the
emitter layer in a single crystal silicon (c-Si) solar
cell is traditionally formed through a thin film
technique usually requiring high temperature
budgets or slow growth rates, such as dopant
diffusion6, ion-implantation7, chemical vapor
deposition (CVD)8, or liquid phase epitaxy9. On the
other hand, researchers have recently developed
innovative methods for producing SiNCs that could
be used to make doped Si films for solar cells with
nano- or micro-sized grain sizes, e.g. hot wire
CVD10, pyrolysis11, laser ablation12, and cosputtering13. Unfortunately, the aforementioned
SiNC
production
methods
have
similar
disadvantages, such as large energy inputs, high
temperature budgets, and large particle size
distributions due to agglomeration of the particles
during synthesis. In 2005, our group demonstrated
that SiNCs could be produced in large yield and with
small size distribution using a low-pressure,
nonthermal rf plasma14. Then in 2010, Holman and
Kortshagen also demonstrated that undoped
germanium NCs could be accelerated through a
nozzle to form dense impacted films15. In this study,
we show that gas-phase impaction of plasmasynthesized doped SiNCs has potential for
efficiently depositing the n-type emitter layer for np
junction solar cells. We show that cells made with a
SiNC/c-Si heterojunction exhibit photovoltaic effect
and study the effect of SiNC layer thickness on
device performance.
2. Experimental Methods
Silicon nanocrystal/crystalline silicon (SiNCs/c-Si)
heterojunction solar cells were fabricated by
impacting phosphorus doped SiNCs onto boron
doped single crystal silicon wafer substrates (ρ = 110 ohm-cm). The SiNCs were produced in a flowthrough reactor (1 inch diameter Pyrex tube), using a
150W rf capacitively-coupled non-thermal plasma to
dissociate silane in the presence of argon 14.
Phosphorus doping was accomplished by
introducing a mixture of argon and 15% phosphine
balanced with hydrogen into the precursor stream16.
Figure 1 outlines the gas flow scheme, rates, and
operating pressures. These synthesis conditions
produced SiNCs with a mean diameter or 19 nm +/1.9 nm as measured by a Bruker-AXS
microdiffractometer with a 2.2 kW sealed Cu x-ray
source and then calculated from the Scherrer
equation. The atomic concentration by flow rate for
phosphorus to silicon is defined as the nominal
doping. As the process gases flow from the high
pressure SiNC synthesis region (15-19 Torr) to a
lower pressure deposition chamber (0.5 Torr), they
are accelerated through a rectangular nozzle
measuring 0.34mm x 10mm at the nozzle exit. The
substrates are placed on a feed-through pushrod and
manually swept through the impinging NC curtain
for 15s, 30s, 60s or 90s resulting in films of varying
thickness which form the emitter layer of the solar
cell. Two samples of each thickness were made
simultaneously during each deposition time.
The substrates, i.e. base layer, were fabricated from
single-side polished wafers, with a 300nm oxide
grown on the polished side and a 15nm-Al/85nm-Au
back contact deposited on the unpolished side via
electron beam metal evaporation. Each sample
1 sccm
23 sccm
80 sccm
Plasma
SiH4
MFC
500 sccm
dil gas
MFC
10 sccm
Ar
MFC
Quartz
tube
dil. Ar
MFC
15%
PH3/H 2
MFC
Pushrod
Nozzle
P1
substrate measured approximately 2 cm2. A 300um
x 300um window was etched through the oxide layer
using 10:1 buffered oxide etch (BOE) solution to
form the active area of the solar cell. The back
contact was annealed for 60s at 800°C in forming
gas to form an ohmic contact.
Preliminary
experiments showed that annealing the back contact
increased the open circuit voltage by a factor of two
and the short circuit current by a factor of five.
After the rapid thermal anneal, a mask of polyimide
tape was used to prevent subsequent film depositions
from forming conductive pathways around the edge
of the substrates. Immediately before the cell
substrates were loaded into the SiNC deposition
chamber, they were again dipped in 10:1 BOE
solution for 10 s to remove any native oxidation in
the active area that formed during preparation, rinsed
with de-ionized water, and blown dry with
compressed nitrogen.
After the SiNC layer was deposited, the cells were
transferred air-free to a nitrogen purged glovebox for
a low temperature anneal at 400°C for 10 minutes.
Then, the cells were again transferred air-free to an
AJA rf/dc sputtering system where 60nm of ITO
was sputtered on the SiNC layer as a top contact.
The samples saw minimal air exposure during the
transfer into the sputtering system. After the ITO
deposition, the cells were immediately measured, in
air, by a simple two-probe measurement system
comprised of a Keithley 2611A SourceMeter and a
solar simulating lamp. Electrical contact was made
on the ITO layer by probing the area around the
active area, and directly to the back contact. The
solar simulator provided an AM1.5 solar spectrum at
100 mW/cm2 local intensity.
A JEOL 6500
scanning electron microscope (SEM) was used for
cross-sectional SEM to measure film thickness and
estimate deposition rates.
3. Results and Discussions
RF power
source
Ring
electrodes
Si
ncs
P2
Substrate
Figure 1. Plasma reactor for SiNC synthesis and typical flow rates.
Upstream synthesis pressure (P1 = 15-19 Torr) and downstream
deposition pressure (P2 = 0.5 Torr).
By SEM, the measured thicknesses of the films for
15 s, 30 s, 60 s and 90 s is 174 nm, 348 nm, 696 nm
and 1044 nm, respectively. A cross section of the
cell at the boundary of the active area is shown in
Figure 2a. The current-voltage (I-V) curve for a
representative cell under dark and illuminated
ITO
80
SiO2
c-Si wafer
40
0
-40
-80
FF (%)
Current (uA)
doped Si ncs
The effect of the nominal doping level was also
investigated by making cells where the SiNC emitter
layer was undoped and nominally doped at 0.1%.
This was done by reducing the flows at the
phosphine MFC and diluted gas MFC, and
compensating the reduced flow with additional
argon from the main carrier gas MFC to maintain
104 sccm total flow and similar upstream and
downstream pressures. The performance parameters
for the three different doping levels are plotted
together in Figure 3b. The main difference in
reducing the doping level is exhibited in the shortcircuit current of the cells, which is the dominant
factor in the PCE. This is expected as lower doped
samples should have reduced conductivity, thereby
creating a large series resistance in the cell and
reducing the Jsc. It is not clear at this point if the
phosphorus added into the plasma is incorporated
into the SiNCs as electrically active donors or if the
phosphorus sits at the surface of the SiNC and
passivates dangling bonds that act as charge carrier
traps states. Both of these phenomena would
dark
light
b
-1.0 -0.5 0.0 0.5 1.0
Bias (V)
Figure 3. (a) Cross-sectional SEM and (b) dark and light current for a
phosphorus doped SiNC/c-Si heterojunction solar cell.
PCE (%)
120
a
suggesting that the SiNC layer may have another
effect on the cell, such as improving light scattering
or light trapping for more efficient carrier
generation.
Jsc (mA/cm2) Voc (mV)
conditions is shown in Figure 2b. Figure 3a shows
the open circuit voltage (Voc in mV), short circuit
current density (Jsc in mA/cm2), fill factor (FF), and
power conversion efficiency (PCE) for each of the
eight cells fabricated. These SiNC layers were
nominally doped at 9% with phosphorus. In general,
there is an increase in the PCE for all cells with
increasing SiNC layer thickness, with the exception
of one of the 60 s deposition cells which we
measured a PCE of 6.5%. The reason for the high
PCE is not immediately apparent. This cell and the
other 60 s deposition cell, appear to have optimal V oc
and Jsc for the sample set, but poor FF. Since the
SiNCs size is large enough that quantum
confinement effects are no longer likely1, we assume
that the SiNCs have similar absorption properties to
that of bulk Si. Additionally, if we assume the
nominal doping level applies in the depletion width
approximation, the junction should be one-sided and
therefore, it is likely that the majority of the light
absorption is occurring in the c-Si wafer and the
SiNC layer acts simply as a doped emitter layer. If
this is indeed the case, it would be expected that
most of the SiNC film would not be necessary for
creating the depletion region, and therefore thicker
films of SiNCs should actually decrease device
performance due to increasing resistance of the
SiNC layer. However, our experiments show that
thicker films in fact increase device performance
400
a
300
260
220
200
200
40
12
8
4
0
20
0
36
32
28
24
20
6
26
24
22
20
1.2
0.8
4
2
0
b
240
0.4
250 500 750 1000
0.0
SiNC Film Thickness (nm)
int
0.1%
9%
SiNC Doping Level
Figure 2. Solar cell performance parameters for cells of varying SiNC
layer thickness (a) and varying doping levels (b). The black squares and
the red circles indicate the parameters for the cell with highest and
lowest PCE for each thickness, respectively.
manifest as increasing J sc with increasing doping
level. It should be noted that the Voc drops by
almost 40mV in the doped samples. This could also
be an indicator of the surface state being modified
by the inclusion of phosphorus, as interface changes
are often associated with changes in the V oc.
Unfortunately, this would not explain the large
variation of the Voc for the samples of different
thicknesses, as these SiNCs should not have had
different surfaces. More experiments are necessary
to elucidate the discrepancy between these results.
SEM of doped films show no major structural
differences in the films and in previous studies TEM
indicates that doping makes no major changes to the
morphology of the SiNCs. We were not able to
investigate doping levels higher than 9% due to
practical limitations imposed upon the system.
4. Conclusions
Doped SiNC/c-Si solar cells have been shown to
produce a moderate photovoltaic effect when
fabricated via inertial impaction. Power conversion
efficiencies have been shown to range from 0.3% to
3% by increasing the thickness of the doped SiNC
layer. Furthermore, a significant amount of dopant
must be introduced into the plasma to create SiNCs
that performs favorably as the n-type doped emitter
layer in a pn junction solar cell. The change in Voc
of the cells with different doping levels suggests that
the phosphorus doping is affecting the surface states
of the SiNCs. However, further testing is needed to
fully identify the role of dopant atoms in the SiNC
films.
5. Acknowledgments
The authors would like to thank Jihua Yang and
Prof. Steven Campbell for their invaluable
experience with solar cells and device physics, and
Gary Olin for his vital technical expertise. This
work was supported partially by 3M Science and
Technology Fellowship and the MRSEC Program of
the National Science Foundation under Award
Number DMR-0819885. Part of this work was
carried out in the College of Science and
Engineering Nanofabrication Center, University of
Minnesota, which receives partial support from NSF
through the NNIN program. Part of this work was
carried out in the College of Science and
Engineering Characterization Facility, University of
Minnesota, which has received capital equipment
funding from the NSF through the MRSEC.
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