Tel Aviv University
Ort Braude College
VERTICAL JUNCTION HIGH-EFFICIENCY
CONCENTRATOR PHOTOVOLTAIC CELLS
Researchers
TAU
OBC
Yossi Rosenwaks (EE)
yossir@eng.tau.ac.il
Avi Kribus (ME)
kribus@eng.tau.ac.il
Rona Sarfaty (EE)
rsarfaty@braude.ac.il
Students
TAU
R. Pozner, G. Segev
OBC
M. Jubran, L. Grinberg, V. Timofeyev, A. Gabai, N. Slame
Funding
Israel Ministry of National Infrastructures
IP
Provisional patent application.
Planar (horizontal junction) solar cell
under concentrated illumination
Cyrium Technologies Inc
III-V MHJ
28
Amonix
27
26
25
Efficiency [%]
its biggest
disadvantage
is the
phenomenon
of the decline
of efficiency at
concentrations
of a couple of
hundred suns.
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23
Sun Power
Chip size cell
22
21
20
2
1
10
100
Concentration
1000
10000
The Vertical Junction (VJ)
Our claim is that changing the geometry of the junction
will lead to improved cell performance.
A) Better Optimization
Capabilities
B) Ideal For High
Concentration
C) Avoiding the trade-off
between front contact grid
shading and series
resistance
3
Low Series Resistance
1. Components
Vertical
1.
2.
3.
4.
5.
Electrode Resistance (RSE)
Contact Resistance (RSC)
p+ Layer Resistance (RSP)
Bulk Resistance (RSB)
n+ Layer Resistance (RSN)
Main Resistance Component:
Bulk Resistance RSB.
4
Horizontal
Lower Series Resistance under illumination
2 Photoconductivity
 x, y    0 x, y    ph x, y, C 
  q n
Horizontal
Vertical
 0  x, y    ph  x, y, C 
5
Illumination contributes
significantly to conductivity
and decreases resistance
 0  x, y    ph  x, y, C 
σ hardly change
with illumination
Simulation Results
L
1
RSB C    d
w0
dx
  x, y, C dy
Resistivity [Ωcm]
0
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
1
6
10
100
Concentration
1000
High Voltage series connected VJ
High-voltage cells with low series resistance, and low
currents, and under illumination photoconductivity reduce
significantly the resistivity,
can be good candidates for concentrating PV (CPV)
systems.
7
7
Realization: Other VMJ Cells
Multi-wafer process, Si
Green Field Solar
High-voltage VMJ cells have
been produced by stacking
multiple wafers followed by
orthogonal cutting.
40 junctions/cell
25.5 V
19.2% at ×2,500Suns
Sater & Sater, 29 IEEE PV Specialists Conference, 2002
Our approach
Monolithic production of junctions in a single wafer
If the vertical junctions in a VMJ cell are manufactured monolithically on
a single wafer, rather than by stacking of multiple
wafers, then the width of each junction can be varied independently of the wafer thickness
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= better use of materials.
Assumed monolithic implementation
1.
Next Cell
SOI Substrate
2. Surface treatments:
Passivation, Front AR,
Back reflector, Front texturing
Previous
Cell
3. Trench fabrication using DRIE
process
5. Ion implantation of the p+ side
of the trench
6. Contact fabrication using
deposition/filling the trenches
with Metal
~40[um]
4. Ion implantation of the n+ side
of the trench
~40[um]
~0.5[um]
SiO2
Handle
wafer
Generic concept
Can be applied to III-V material.
We will send you soon alternative process steps
9
~0.5[um]
Junction Modeling and Optimization setup
Schematics of a Vertical Junction
10
We have used the Sentaurus
TCAD (Synopsys Inc.), device
software tools.
The device simulator solves
the coupled continuity and
Poisson equations, under
specific boundary
conditions.
It was assumed that the
device characteristics are
uniform in the third
dimension, L in Figure (a)
which permits a twodimensional analysis.
Tel Aviv University
School of Electrical Engineering
We insert material parameters of
realistic wafer quality and fabrication processes
The Shockley-Read-Hall (SRH) bulk lifetime - 1 ms for both
electrons and holes corresponding to Floating Zone (FZ) silicon
substrate .
The back surface recombination (BSR) (the silicon-insulator, 1 µm
SiO2, interface). No surface treatments are assumed in the back
surface region, leading to high BSR 1000 cm/s.
Recombination velocity at the side metal contacts was set to 106 cm/s
for both electrons and holes;
High value - for possible damage during the trench fabrication,
implantation (or diffusion) through the sidewalls and the
metallization interface.
Lower interface recombination velocities were found to increase
the efficiency by about 3%.
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Junction Optimization
Current analysis for Si
N+ 1019
P+ 1018
Pyramids
Depth, width,
widths of the N+,
P+ layers
And the Metal
contact.
 h   e  1 ms
25–50 m
Optimize:
Junction geometry
Double AR
Coating
P 1016
Reflective
Coating
0.5 m
0.5 m
40m
Process
dependent
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Representative graphical results
1.Effect of surface recombination
Efficiency vs. width and front surface recombination velocity for
BSR=1000 cm/s under 1sun (a) and 1000 sun (b),
Optimal width and depth ~80x50m are smaller
compared to the junctions presented in the past (~250X500m)
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2. Effect of bulk doping
resistivity vs. minority carrier lifetime and mobility
Efficiency vs. junction width and bulk doping, 1sun (a), 1000 suns (b);
dashed lines show the location of the optimal width for each doping level
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14
Tel Aviv University
School of Electrical Engineering
Effect of solar concentration
The efficiency
continues to
increase for
concentration
levels well
over a 1000.
15
Efficiency as a function of solar flux concentration for a VJ of width
43.6 µm and depth 60 µm, FSR=0, BSR=0 cm/s (a), depth of 60 µm,
FSR=10, BSR=1000 cm/s (b), depth of 200 µm, FSR=10, BSR=0 cm/s
(c), depth of 200 µm, FSR=10 cm/s , BSR=1000 cm/s (d).
VJ under Concentrations
32
30
28
Vertical
Junction
Efficiency [%]
Amonix
26
24
22
20
1
10
Sun Power
Chipsize
cell
100
Concentration
16
1000
10000
Summary
1. We presented an analysis and optimization of a
vertical junction Silicon solar cell under 1 to 2000
suns concentration, using a set of properties
representing realistic wafer quality and fabrication
processes, and a full 2-D model without major
simplifying assumptions.
2. The junction optimal design was found for different
values of fabrication characteristics such as FSR,
BSR and SRH lifetime.
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conclusions
1. The optimal junction dimensions are much
smaller than the dimensions used in previous
VMJ cell tests and analyses
2. Conversion efficiency under high concentration
can reach close to 30%, much higher than the
20% reported for past VMJ cells
3. In addition to the inherently low series resistance
effect, photoconductivity is responsible for the
outstanding performance of VMJ cells under high
concentration
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