Seth D. Shumatel,2 Douglas A. Hutchings2 , Hafeezuddin Mohammed2, Genevra Beilkel,2,. I'. I. 34 ... Solutions LLC Fayetteville, Arkansas, USA, 3Department of Electrical Engineering, University of ... for real B-H complex data on highly doped material [8]. ..... [4] R. Low, A Gupta, H.J. Gossmann, J. Mullin, V. Yelundur, B.
SELF ALIGNED HYDROGENATED SELECTIVE EMITTER FOR N-TYPE SOLAR CELLS l2 2 l2 2 Seth D. Shumate , Douglas A. Hutchings , Hafeezuddin Mohammed , Genevra Beilke , , I I' 34 3 3 Benjamin S. Newton , Matthew G. Young , Husam Abu-Safe' , Sh'1UYU S-Q , and Naseem HA I
2 Microelectronics-Photonics, University of Arkansas, Fayetteville, Arkansas, USA, Silicon Solar 3 Solutions LLC Fayetteville, Arkansas, USA, Department of Electrical Engineering, University of 4 Arkansas, Fay tteville, Arkansas, USA, Natural Science Division, Lebanese American University,
�
Byblos, Lebanon ABSTRACT
-
Selective emitter cell architectures are one
avenue for increasing industrial solar cell efficiency. N-type cell based technology is also gaining considerable attention for the same purpose. T his paper describes a novel, single step selective emitter
process
using
acceptor impurities.
atomic
hydrogen
to
passivate
boron
Grid lines act as a mask to hydrogenation,
experimental results on aluminum doped polycrystalline thin fIlms shows its effectiveness.
Cell fabrication is being
developed to test this process on real solar cells to verify experimental results.
Special processing considerations will
be discussed.
which lowers the surface concentration of electrically active boron between grid lines.
Using EDNA to model this complex
emitter, it was shown that Jsc can be increased in the emitter by 2 O.94mAlcm with a short, low temperature atomic hydrogen treatment.
A hydrogenation system has been developed, and
initial experimental results on aluminum doped polycrystalline thin films shows its effectiveness.
Cell fabrication is being
II. THEORY EDNA, a new emitter modeling software, was used to model the effect of hydrogen passivation of boron dopants [6]. The functionality of user-defmed dopant profIles made this
developed to test this process on fabricated solar cells to verify
work possible.
theoretical
are
velocity was not built into the program and was a user-defIned
SELECTIVE
been reported in the literature that for passivated surfaces,
results.
Special
processing
considerations
discussed.
INDEX
The treatment of surface recombination
parameter not linked to the dopant profile. TERMS
HYDROGENATION,
-
EMIT T ER, PHOTOVOLTAIC CELLS, N-TYPE SILICON.
However, it has
both n-type and p-type diffusions show surface recombination velocities (SRV) increasing with dopant concentration [7]. For the presented work, it was asswned that SRV was
I. INTRODUCTlON Selective
emitter
cell
architectures
are
an
interesting
proposition for the solar industry. Many processing schemes have been proposed for selective emitters: laser-based doping, emitter etch back techniques, and ion implantation to name a few [1]-[4]. Most work, naturally, has focused on selective n type emitters for p-type cells.
However, there is growing
interest in moving to n-type cells despite abandoning the elegance of the aluminum paste BSF/front side metallization fIre-through in order to reach effIciencies above 20%. Besides Sanyo and Sunpower, with unconventional, high-effIciency architectures, Yingli's Panda cells are some of the only commercially available n-type cells, which currently do not employ a selective emitter [5]. This paper describes a novel, one step selective emitter The idea is simple: a screen printed grid makes ohmic contact to a highly boron-doped p+ emitter. Gridlines act as a mask during an atomic hydrogenation step which lowers the sheet resistance between the gridlines by passivating boron. system
has
been
developed,
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
and
A
initial
by
boron
peak
dopant
density.
This
assumption has been shown experimentally for both boron and phosphorus emitters [7].
For a theoretical comparison, a
boron dopant profile was modified from the one generated by EDNA to include the hydrogen passivated profile calculated for real B-H complex data on highly doped material [8]. These profIles were then entered into the "Measured Data" section of the program to determine the effects on the quality of the emitter.
Hydrogen passivation of boron has been
characterized by Herrero, et
al., and the concentration of B-H
complexes was found to reach 99% passivation near the surface after only 30 minutes of hydrogenation at Tsubstrate = 150°C [8].
The B-H complex data were digitized using
OriginPro 8.6.
using atomic hydrogen to passivate boron acceptor impurities.
hydrogenation
influenced solely
Figure 1 shows the original boron profIle as
well as the hydrogenated profIle based on the experimental data. EDNA was used to compare the emitter characteristics of the two doping profiles in Fig. 1. It should be noted that the program does not consider current generated or collected from the base. All simulated emitters were illwninated with AM1.5 global data built into the software.
001110
III. HYDROGENATION SYSTEM lE+21
A hydrogenation system with independent substrate heating -- Original Boron Profile
lE+20
! lE+19 .� � lE+18 .[ o Q
Hydrogen gas is catalytically cracked by a
heated tungsten filament.
m
Q bQ
was constructed.
I
�
- -
Hydrogenated Profile
The advantage of this system is an
absence of plasma damage.
The tungsten filament in our
system is 10 cm away from the substrate, minimizing heating
,
from the filament.
The substrate is heated by two 500 W Initial
halogen lamps directly above the substrate holder.
acceptor impurity passivation studies have been carried out by
lE+17
varying
substrate
temperature
and
Polycrystalline films of thickness 300
lE+16
nrn
gas
pressure.
were prepared on
glass by top-down aluminum induced crystallization [10]. Films were used to measure increase in film resistivity due to
lE+15 o
0.1
0.2
0.3
0.5
0.4
0.6
Depth h1m)
Samples were then thermally
hydrogenation treatments.
recovered to the original resistivity values. Figure 3 shows results for varying substrate temperature on
Figure 1 Simulated boron profile from EDNA and hydrogenated profile calculated with experimental data from Herrero,et at., [8].
increase in resistivity for samples hydrogenated for 30 minutes
It is possible that a hydrogenated emitter will not behave the
Original resistivity was just over 0.05Q-cm. Not surprisingly,
same way as a lightly doped emitter in terms of decreased
the optimal substrate temperature from this study was 150°C.
SRV.
It is also possible that high quality passivation could
prove difficult at temperatures below 200°C.
Therefore, the
at 1 Torr with a filament temperature of T fiI
=
I900°C.
These samples showed an average increase in resistivity of over 400%. At Tsub
=
I90oe, resistivity is less because of the
surface recombination velocity of both profiles was varied 6 from 250 cmls to 1x10 cm/s. The behavior of each emitter is
cooling time at the end of the process, where, for a period of
shown in Fig. 2. At low SRV, the hydrogenated emitter out
which B-H complexes begin to break.
several minutes, the sample was above the temperature at
performs the heavily doped emitter. Absolute increase in Jsc 2 from the hydrogenated emitter is as much as 0.94 mA/cm •
Resistivity vs. Temperature
This value is typical for both experimental and theoretical studies of selective emitters [1], [9].
0.3
However, as SRV
increases, the hydrogenated emitter experiences severe losses
0.25
due to surface SRH recombination according to the model.
"W
The
"/5
original
boron
profile
is
less
sensitive
to
surface
recombination.
;0.15 ·s '"
�
::R. 0
>. u c Q)
Ti �
01�------------------so
90
1SO
200
2SO
Figure 3 Resistivity versus substrate temperature for 30 minute hydrogenation treatment of p+ polysilicon thin films. The first data point was the average resistivity for all samples before hydrogenation.
80
c
70
Thermal recovery of the resistivity of these samples versus
Q)
'5
()
100
Substrate Temperature (0e)
UJ
.Q t5
0.1
o.OS
100
�
0.2
E
temperature is shown in Fig. 4.
60
Samples were sequentially
annealed for 30 minutes from 125°e to 325°e in steps of 500e with resistivity measured after each heating cycle until
50
the original resistivity was recovered. 102
103
10'
10S
106
Surface Recombination Velocity (cm/s)
This figure indicates
that B-H complexes are stable for more than an hour at temperatures above I75°e.
Figure 2 Emitter collection efficiency for both original profile and hydrogenated profiles shown in Fig. 1.
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
001111
samples
%Original Resistivity vs. Annealing Temperature
450 400 350 � �300 250 � 200 c 'i 150 '" � 100 .: 50 0 -50
To
-+-75C __ noc __ 150C
-190C
calculate
the
junction
depth,
software
tungsten from
the
It calculates
depth of 2.6 !Jm was calculated. This is much deeper than the ideal junction depth for solar cells. This depth of 2.6 !Jm was used to calculate the theoretical dopant profiles for modeling in EDNA. Figure 5 shows a theoretical 2.6 !Jm profile as well as a 30 minute hydrogenated profile. emitter
100
qJ
300
200
400
Annealing Temperature (DC)
Several
considerations
be
made
for
cell
processing
based
on
simulation
results
and
is
clear
that
to
As
(high SRV) hydrogenated cell shows a degradation compared to
the unpassivated (high SRV) non-hydrogenated cell. 2 EDNA predicted this degradation to be 1.23 mA/cm • Figure
lE+21
·DODD / ODDOOO�
-Original Boron Profile
DDOOOO� 0000-;
_
'"
6
lE+20
I=:
This is true of normal selective emitter
are more susceptible to SRH surface recombination [9]. must
be
deposited
at
·P.IE+19 0 0 I=:
...0 0
�
lE+18
temperatures below about 200°C or quickly to retain the desired profile. This is assuming heating and deposition times of less than an hour, which is dependent on deposition configuration and processing parameters. PlasmaTherm PECVD requires
only
system 5
we use
minutes
For example, the for
to
reach
substrate
The wafer used for the boron diffusion was single crystal, n 3 15 type, with a background doping of lxlO cm- • The
Using a PDS® P-type boron nitride source for the
boron diffusion, the pre-deposition soak was completed for 1 hour at 1l00°C.
- - 30 minutes H
,
r r
J
lE+17 0.0
0.5
Next, the drive in lasted for 30 minutes at
1100°C. Finally, the wafer underwent a boron deglaze in HF. An aluminum grid and back side metallization were added to complete the solar cell. One cell was hydrogenated for 30 minutes with a filament ° temperature of 1900 C and a hydrogen pressure of 1 Torr. Two 500 W halogen lamps heated the substrate holder and
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
1.5
1.0
Depth (�m)
2.0
2.5
Figure 5 Simulated 2.6 11m boron diffusion (blue solid line) and calculated 30 minute hydrogenation active dopant profile (red dashed line).
V. DISCUSSION
Before the boron pre-deposition
and diffusion, the wafer underwent a piranha clean and HF oxide etch.
I
anti-reflection
temperatures of 250°C.
thickness was 500-550 !Jm.
I I
�
eo
layer
both
observed after hydrogenation.
structures because, as shown in Fig. 2, lightly doped emitters passivation
for
7 shows the Light J-V curves for the cell before and after 2 hydrogenation. There was a 1 mA/cm degradation in Jsc
whether atomic hydrogen treatment can be used as a selective
any
efficiency
was the case with the 0.5 !Jm junction depth, the unpassivated
determine
emitter technology.
and
�.
passivation layer must be applied
hydrogenated
�-;
a
collection
Figure 6 shows the
original profiles versus surface recombination velocity.
... ...,.- .. DO
studies.
hydrogenation
coatings
heated
parameters. By entering the diffusion parameters and the SSL 3 20 of boron in silicon at 1100°C of 3.37xl0 cm- , a junction
IV. EXPERIMENTAL
Second,
The
junction depth of common dopants in silicon using user-input
Figure 4 Samples annealed at various temperatures until returned to original resistivity values.
it
° 150 C.
filament was used to catalytically crack hydrogen gas.
u
First,
approximately
University of Illinois named DifCad was used.
�
must
to
It is interesting to compare the polycrystalline TAlC films with the solar cells.
10 films were hydrogenated with the
same system and parameters, and they showed a 175%-250% increase in resistivity (see Table I). After a one hour anneal at 350°C, the seven films that were annealed had a decrease in resistivity indicating the hydrogen had left. Before the 5/2012 measurement,
films
3,
4,
and
5
underwent
another
hydrogenation step at 250°C, 1 torr, for 30 minutes. Then, a RTA at 500°C for 10 seconds was performed. The fact that samples 8, 9, and 10, which were not annealed, had consistent
001112
resistivity
measurements
over
a
long
period
of
time
hydrogenations of less than 10 minutes only marginally
demonstrates that hydrogen did not leave the TAlC samples.
increase
The performance of the hydrogenated cell, however, nearly
processes.
returned to its original value after a few months at room
enable better surface passivation, and enable higher Voc and
sheet
resistance
unlike
other
selective
emitter
This would enable low-shadowing grid designs,
temperature. There are two possible explanations for this: the
Jsc values without sacrificing the fill factor. More work will
boron-hydrogen passivation for this cell was not stable under
be done to determine the effectiveness of this treatment with
room temperature conditions or the native oxide growth over
excellent surface passivation and whether or not hydrogen can
that time passivated the cell well enough to lower the surface
passivate
recombination velocity to below lO,OOO cm/s.
reflection/passivation coatings.
There was
the
boron
diffused
emitter
through
the
anti
insufficient evidence for either case and this will be an area of further investigation.
Initially, however, the degradation due
predicted by EDNA. one
of
these
cells
Another batch of cells was made and was
hydrogenated.
It
also
showed
degradation in Jsc after hydrogenation and also showed a recovery over time.
The grid from this cell was etched and
had only a 20% increase in resistivity compared to the other cells from the same batch.
Table 2 compares the sheet
-0.003 e u
--
< --- -0.005 >-
.... .... rrJ
s::
III
0
corresponding hydrogenation times. The measured increase in
s::
of
various
dopant
passivation
profiles
resistivity would point to an effective hydrogenation time of between 5 and 10 minutes.
....
-0.007
III
-0.009
= U
-0.011
t::
-0.013
70 -2.6 micron Boron Profile ---30
- - Post H
---
N
with
resistance
-PreH
-0.001
to hydrogenation seemed to be consistent with the degradation
minutes H
---
-
----
- -----
-0.015 0
0.2
0.6
0.4
Voltage (Volts) Figure 7 Light J-V curves before (blue solid line) and after (red dashed line) hydrogenation. TABLE I TAlC FILM RESISTIVITY MEASUREMENTS 0
45
p
3.E+02
3.E+03
3.E+04
(post-H)
O-cm
O-cm
1
0.052
0.144
0.052
0.052
2
0.057
0.190
0.057
0.057
3
0.058
0.189
0.053
0.069
4
0.074
0.259
0.066
0.074
5
0.055
0.167
0.053
0.060
6
0.056
0.187
0.059
0.061
7
0.058
0.194
0.058
0.057
8
0.053
0.148
No anneal
0.148
9
0.054
0.144
No anneal
0.143
Surface Recombination Velocity (cm/s)
VI. CONCLUSIONS A study of hydrogen passivation of boron acceptor impurities in n-type solar cells has been carried out.
It has been
concluded that for boron doped solar cells, a hydrogenation treatment initially passivates boron impurities, as indicated by a decrease in Jsc as predicted by EDNA. The simplicity of this method
to
potentially
form
selective
emitters
on
fully
fabricated cells clearly sets it apart from more complicated etch-back
or
laser
doping
steps.
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
Likewise,
short
P
(pre-H)
(350 C anneal 1 hr.)
3.E+05
Figure 6 EDNA results for collection efficiency versus surface recombination velocity for the original 2.6 11m doping profile (blue solid line) and the calculated 30 minute hydrogenation profile (red dashed line).
p
P
Meas. 5/2012
001113
10
No anneal
0.147
0.050
[9] M. Zanuccoli, P.F. bresciani, M. Frei, H.W. Guo, H. Fang, M. Agrawal, C. Feigna, E. Sangiorgi, "2-D Numerical Simulation and Modeling of Monocrystalline Selective Emitter Solar Cells," Thirty Fifth IEEE PVSC, 2010. [10] S. D. Shumate, M. K. Hafeezuddin, H. A Naseem, and D. A Hutchings, "Microstructural Influence of Hydrogenated Amorphous Silicon on Polycrystalline Emitter Solar Cells Prepared by Top-down Aluminum Induced Crystallization," Thirty-Seventh IEEE PVSC, 2011.
0.127
TABLE II SHEET RESISTANCE VERSUS HYDROGENATION TIME
Hydrogenation Time (minutes)
Sheet Resistance (DID)
0
33.53
5
38.63
10
49.77
20
60.39
30
97.67
ACKNOWLEDGEMENTS
This material was based on work supported Silicon Solar Solutions LLC and the National Science Foundation under EPS-lO03970. Altermatt
for
Thanks
to
creating
Keith
McIntosh
and
sharing
and
Pietro
EDNA
at
(http: //www.pvlighthouse.com.au/simulation/ hosted/EDNA/EDNA.aspx).
REFERENCES
[I] T.C. Roder,S.l Eisele,P. Grabitz,C. Wagner,G. Kulushich,lR. Kohler, and lH. Werner, "Add-On Laser Tailored Selective Emitter Solar cells," Progress in Photovoltaics: Research and Applications, 18,2010,pp. 505-510. [2] D.S. Ruby,P. Yang,M. Roy,and S. Narayanan,"Recent Progress on the Self-Aligned, Selective-Emitter Silicon Solar Cell," Twenty Sixth IEEE PVSC, 1997. [3] D. Rudoph, K. Peter, A Meijer, O. Doll, and I. Kohler, "Etch Back Selective Emitter Process with Single POCL3 Diffusion," Twenty-Sixth European Photovoltaic Solar Energy Conference and Exhibition,
2011. [4] R. Low, A Gupta, H.J. Gossmann, J. Mullin, V. Yelundur, B. Damiani, V.Chandrasekaran, D. Meier, B. McPherson, and A Rohatgi, "High Efficiency Selective Emitter Enabled Through Patterned Ion Implantation," Thirty-Seventh IEEE PVSC, 2011. [5] AR. Burgers,R.C.G. Naber,AJ. Carr,P.c. Barton,L.J. Geerligs, X. Jingfeng, L. Gaofer, S. Weipeng, A Haijiao, H. Zhiyan, P.R. Venema, and AH.G. Vlooswijk, "19% Efficient N-Type Si Solar Cells Made in Pilot Production," Twenty-Fifth European
Photovoltaic Solar Energy Conference and Exhibition, Fifth World Conference on Photovoltaic Energy Conversion,
2010. [6] K.R. Mcintosh and P. Altermatt, "A Freeware ID Emitter Model for Silicon Solar Cells," Thirty-Fifth iEEE PVSC, 2010. [7] P. Altermatt, H. Plagwitz, R. Bock, 1 Schmidt, R. Brendel, M.l Kerr, and A Cuevas, "The Surface Recombination Velocity at Boron-Doped Emitters: Comparison Between Various Passivation Techniques," Twenty-First European Photovoltaic Solar Energy Conference, 2006. [8] c.P. Herrero, M. Stutzmann, and A Breitschwerdt, "Boron Hydrogen Complexes in Crystalline Silicon," Physical Review B, 43, 2,1991,pp. 1555-1575.
978-1-4673-0066-7/12/$26.00 ©2011 IEEE
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