Master Thesis Presentation

Rony Pozner
Rony PoznerSoftware Engineer and Researcher
1
High Voltage Photovoltaic Cells
By
Roni Pozner
Carried out under the supervision of
Prof. Yossi Rosenwaks
Dep. of Physical Electronics
Tel-Aviv University
1. CPV operating conditions:
1. Solar concentrators in CPV systems.
2. Current mismatches in PV modules.
3. Electrical and thermal coupling in CPV systems.
2. Series resistance losses in CPV systems:
1. Effects of series resistance on cell efficiency.
2. Series resistance components in MJ cells.
3. VJ’s series resistance.
3. Vertical Multi Junction (VMJ)- Basic Concepts
4. VJ Vs. horizontal cells in CPV systems
5. Theory of Solar Cells
6. Advantages & Disadvantages
7. Simulation
8. Fabrication
Outline
15 x15 km
Israel consumes today
10 GW of electricity,
thus it needs only 200
km2 of solar panels to
supply most of its
electricity needs !
With Concentrated
PV Only 10x10 km2
are required
Theory Of Solar Cells
Illumination Absorption
Separation
Collection
    )
(
)
(
min 





 collection
separation
absorption
ation
illu
N
q
I 



Photovoltaic Cells – Basic
Concepts
 
sh
s
L
nKT
IR
V
q
ph
L
R
R
I
V
e
I
I
I
s














1
0
C
I
I ph
sc 

IN
M
M
IN
MAX
P
I
V
P
P



oc
sc
m
m
V
I
I
V
FF 
Vertical Junction (VJ) Cell
Vertical junction
No front grid, minimal inactive area
Decouple optics vs. electronics
High Voltage cell
Reduce series resistance effect
Parallel connection- reduce mismatch effect
Advantages & Disadvantages
Advantages - Cell
• Active Area Fraction - Less shading
“Back Contact” “Standard”
Decoupling Effects
Orthogonal carrier generation (optics) & carrier collection (electronics)
Collection efficiency independent of wavelength
High doping of N+,P+ layer, hence larger Voc
Record Devices-2008
• Lower resistance due to:
– Control over length and doping of N+,P+ layers.
– Photo conductivity phenomena.
Advantages - Concentration (cont.)
Solar concentrators in CPV systems
Fresnel Lenses Parabolic reflectors
Limitations of CPV technology
• Series resistance limits concentration
• Cell thickness: optics vs. electronics vs. mechanics
• Front grid, busbar: shading loss
• Illumination non-uniformity: mismatch loss
Series resistance
under light concentration
• Our cell efficiency still rises under 5000 suns. While ordinary Multijunction cells as
the highest efficiency under 500 suns and begin too decries under higher
concentrations. silicon solar cells as the highest efficiency under 200 suns.
• The reason for this phenomenon can be due to photoconductivity effects as well as
the low currents in the cell.
• This is because the mobility and the concentration of carriers are varying with
location and light concentration conditions.
• So in order to calculate the resistivity of the cell under concentration conditions we
need to take the mobility and the concentration of carriers as a function of their
location from the Sentaurus simulations.
MJ cell- Series Resistance
Components1
1. Electrode Resistance (RSE)
2. Contact Resistance
3. Lateral Resistance (RSL)
4. Layer (InGaP) Resistance
5. Tunnel Resistance (RT1)
6. Layer (InGaAs) Resistance
7. Tunnel Resistance (RT2)
8. Ge Substrate Resistance
Main Resistance Components:
RSE, RSL, RT1, RT2.
1K. Nishioka et al. / Solar Energy Materials & Solar Cells 90 (2006) 1308–1321
Vertical Diode- Series Resistance
Components
1. Electrode Resistance (RSE)
2. Contact Resistance (RSC)
3. p+ Layer Resistance (RSP)
4. Bulk Resistance (RSB)
5. n+ Layer Resistance (RSN)
Main Resistance Component: RSB.
p+ p n+
RSB
Series Resistance Losses
Neglecting the shunt resistance-
s
L
L
ph
R
I
I
I
I
q
nKT
V 


 )
1
ln(
*
0
s
L
L
ph
L
L R
I
I
I
I
q
nKT
I
P
2
0
)
1
ln(
* 



Rs
Rsh
Isc
Id
IL
V




2
L
Rs I
P 
Photoconductivity Effects
     
 
n
p n
n
p
p
e
y
x 

 




 0
0
,
 
C
f
RSB 
     
C
y
x
y
x
y
x ph ,
,
,
, 0 

 

Horizontal cells:
Vertical Diodes:
   
C
y
x
y
x ph ,
,
,
0 
 
σph Changes significantly with the
illumination
Const
RS 
Simulation Results
 
 



L
d
SB
dy
C
y
x
dx
w
C
R
0
0
,
,
1

R
SB
[Ω]
100
101
102
103
104
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Concentration
Vertical Junction Under
Concentration
2Slade et al. / Proceedings of the SPIE, Volume 5942, pp. 236-243
100
101
102
103
104
15
20
25
30
35
Concentration
Efficiency
[%]
Vertical Diode
Best Si cell2
Grid Pitch Dependence of the Series
Resistance and Shadow Loss1.
Grid Pitch
1K. Nishioka et al. / Solar Energy Materials & Solar Cells 90 (2006) 1308–1321
Efficiency for Concentration
(various cells)
Spectrolab- C1MJ2
2Kinsey et al. / Prog. Photovolt: Res. Appl. 2008; 16:503–508.
Best Silicon cells
4Mulligan et al. / Proceedings of the 28th IEEE Photovoltaics Specialists Conference.
3Slade et al. / Proceedings of the SPIE, Volume 5942, pp. 236-243 (2005).
10
1
10
2
22
23
24
25
26
27
Concentration
Efficiency
[%]
Best Si Cell3
(1.35 cm2
)
SunPow er Chipsize4
(0.0529 cm2
)
Nishioka et al., Solar Energy Mat. Solar Cells, 2006
Limit for High Concentration
High concentration
→ High current
→ High Rs loss
Reduce grid pitch
→ High shading loss
Spectrolab C1MJ
Kinsey et al., Prog. Photovolt: Res. Appl. 2008 0 100 200 300 400
23
24
25
26
27
28
Concentration
Efficiency
[%]
Measured
Model
SunPower Si cell
Mulligan et al., Proc. 28th IEEE PV Specialists Conf.
High Output Voltage Module
N cells
Series Connection of N Vertical Cells:
N Junctions
VMJ Cell
Junctions in series = Cell
• High voltage, low current
• External contacts at edges
Easy to convert to back contacts (external wrap-around)
Insulator Back contact
• Easy MIM connection (Monolithic process)
• High output voltage of each module.
• Small area of each module leads to uniform illumination on
each one of them.
Wire
Vertical Cell
Horizontal Cell
Advantages - Module (cont.)
Due to the relatively high output voltage of each module, it
can be connected in parallel to the other modules (instead
of series connection like in horizontal cells) this offers the
following advantages:
• Voltage coupled instead of current coupled response Better response to partly shading
conditions
• Less sensitive to non-uniform flux avoiding the use of homogenizer in concentrated PV
(Gideon’s Lecture…)
Advantages - Module (cont.)
VJ module under non-
homogenous illumination
V1
V2
VN
Vm
+ If the dimensions of the VJ module
are small comparing to the change
in illumination:
N
ph
ph
ph I
I
I ,
2
,
1
, .. 


4cm
4cm
Y
X
contact
P
N
contact
40 um
dX
1
2
N
1cm
Illumination input:
Average concentration=444
Maximum concentration=1148
1.66% spillage
VMJ Mismatch Loss
0 50 100 150 200
22
23
24
25
26
Number of junctions in cell (N)
Efficiency
[%]
VMJ cell array
Uniform illumination: 27.72%
Non-uniform illumination: 25.86% (N = 40)
→
Series Mismatch Loss (relative)= 6.7%
Conventional cell array
Series Mismatch Loss > 80%
No homogenizer !
Efficiency for concentration, Width=43.6μm, Depth=60
surface recombination =100
10
1
10
2
10
3
10
4
20
25
30
35
Concentration
Efficiency
[%]
Parallel illumination
Multidirectional Illumination
Multidirectional Illumination, no surface tex turing
• High output voltage of each module enable the use of a more
efficient DC/DC inverter.
Advantages - Module (cont.)
Disadvantages
• Fabrication difficulties
– The depth of the cell is relatively large (H)
– Metal thickness need to be as thin as possible (X)
(Karmiel’s Lecture…)
H
X
X
Semiconductor has low resistivity, hence the two cells above modeled as
connected in parallel by conducting wires.
Hole current from top to bottom reduces the voltage of 1 close to the voltage of 2
(circular currents)
Voc of the device will be close to the voltage at the deepest point.
Open Circuit Voltage
(1)
(2)
As H getting bigger, Voc decrease and Isc increases => so while increasing H
result in absorbing more light, it also reduces Voc, hence, the efficiency will
begin to drop at some point.
H
• Voc
Voc is proportional to )
1
log(
H
Disadvantages (cont.)
0.5um
Constant distance (between
junction and contact)
• Front Surface Recombination
Varying distance (between
junction and contact)
Most of the minority carriers in the vertical cell have a longer path compared
with the horizontal cell-more affected by surface recombination.
Most of the light is absorbed in the top 5 microns of the cell
back surface recombination is neglected in both the horizontal and
vertical cells due to very small carriers density generated there.
Vertical Cell
Horizontal Cell
Disadvantages (cont.)
• Contact Surface Recombination (Point Contacts):
Disadvantages (cont.)
Vertical Cell
Horizontal Cell
Disadvantages (cont.)
Simulations
Analysis
0
)
,
(
)
,
(
)
,
(
)
,
(
2
2
2
2






z
x
n
x
G
dz
z
x
n
d
D
dx
z
x
n
d
D n
n
Continuity equation:
)
)
(
exp(
)
(
)
(
)
,
( x
N
x
G 




 

Generation equation:
x
z
Light Intensity
Absorption Coefficient
Matlab Simulation
A Matlab simulation of varying cell depths:
The result gave us an initial intuition regarding the optimization that should be
done.
L
H
Sentarus Simulation
Optimization Factors
Bulk Material – N / P
Lifetime – SC / MC
Length
Depth
SR – Front / Bottom
SR - Contacts
Front Pyramids
Front Anti Reflective Coating
Back Reflective Coating
Back Diffused Mirror
Bulk Doping
N+,P+ Doping
N+,P+ Length
Metal Contact (Shading)
Temperature
Sun Angle
Flux Non-Uniformity
• The thickness of the thin film (ARC and RC) is determined by:
Phase Changes between the mediums has great
significance .
• Si3N4 is a good ARC due to two reasons:
– Si3N4 has n~2 and the best ARC need refractive index of
– Si3N4 is also very good for the passivation.
• SiO2 is not the optimal material for RC but it is built in the SOI substrate so we can
use it for our needs
Optimization of ARC ,
RC and texture
2 2
1
( 0.5)
2 4
m
m
d
n n
 


 
1.87
ARC air si
n n n
 
Optimization of ARC ,
RC and texture
Figure 18 - Absorption
depth for silicon [3]
Figure 17 - Spectral Radiation at
the Earth's Surface [3]
 The optimal width of ARC is for the
wavelength of 0.6um.
 The optimal width of RC is for the
wavelength of 0.9um .
This is due to:
- Sunlight intensity .
- Absorption probability.
Silicon-
Handle wafer
light
T1
T2
R1 T3
R2
SiO2
Figure 15
Optimization of ARC ,
RC and texture
 The thickness of the thin film (ARC and RC) is determined by:
2 2
1
( 0.5)
(1)
2 4
m
m
d
n n
 


 
Figure 19 - Anti-reflective coating
 The difference between
ARC and RC is the number of
phase Changes between the
mediums.
Texturing
 Reduced reflection.
 Better "light trapping”
Master Thesis Presentation
Optimization of ARC ,RC and texture
Reflectivity without ARC layer is R=40% .
Reflectivity ARC (Si3N4 ) is R=15%.
Reflectivity with ARC (Si3N4 ) and RC is R=18%.
Transmitted light without RC is T=10%
Transmitted light with RC is T=6%. Relation between the refraction
index and the wavelength for a
silicon surface
Reflectivity for 40um depth cell with ARC & RC
coatings
0
0.2
0.4
0.6
0.8
1
0.3 0.5 0.7 0.9 1.1
Wavelength [um]
R(%)
Cell with ARC (Si3N4)
Cell with ARC (Si3N4) & RC
Cell without coatings
The high end of the
spectrum the graph is
not consistent due to
the reflectivity from the
second interface of air
and silicon at the
bottom of the cell.
Sentarus Simulation
12.5053 18.6185
18.2978 18.9897 21.3228
Bulk Material – N / P
Lifetime – SC / MC
Length
Depth
SR – Front / Bottom
SR - Contacts
Front Pyramids
Front Anti Reflective Coating
Back Reflective Coating
Back Diffused Mirror
Bulk Doping
N+,P+ Doping
N+,P+ Length
Metal Contact (Shading)
Temperature
Sun Angle
Flux Non-Uniformity
Sentarus Simulation
0
5
10
15
20
25
1.00E-02 1.00E-03 1.00E-04 1.00E-05 1.00E-06 1.00E-07
tau (Sec)
Efficency
(%)
L
H
Bulk Material – N / P
Lifetime – SC / MC
Length
Depth
SR – Front / Bottom
SR - Contacts
Front Pyramids
Front Anti Reflective Coating
Back Reflective Coating
Back Diffused Mirror
Bulk Doping
N+,P+ Doping
N+,P+ Length
Metal Contact (Shading)
Temperature
Sun Angle
Flux Non-Uniformity
Cell Optimization
12
14
16
16
18
18
20
20
20
22
22
2
2
22
22
22
2
2
24
24
2
4
2
4
2
4
2
5
Width [um]
Depth
[um]
50 100 150 200 250
20
40
60
80
100
120
140
Efficiency for Various cell sizes (1 sun)
Cell Optimization
30
30.5
30.5
31
31
31
31.5
31.5
31.5 31.5
32
3
2
32
32
3
2
.
5
32.5
3
2
.
5
33
3
3
Width [um]
Depth
[um]
30 40 50 60 70
40
60
80
100
120
140
Efficiency for Various cell sizes (1000
suns)
Sentarus Simulation
L
H
Bulk Material – N / P
Lifetime – SC / MC
Length
Depth
SR – Front / Bottom
SR - Contacts
Front Pyramids
Front Anti Reflective Coating
Back Reflective Coating
Back Diffused Mirror
Bulk Doping
N+,P+ Doping
N+,P+ Length
Metal Contact (Shading)
Temperature
Sun Angle
Flux Non-Uniformity
Sentarus Simulation
L
H
Bulk Material – N / P
Lifetime – SC / MC
Length
Depth
SR – Front / Bottom
SR - Contacts
Front Pyramids
Front Anti Reflective Coating
Back Reflective Coating
Back Diffused Mirror
Bulk Doping
N+,P+ Doping
N+,P+ Length
Metal Contact (Shading)
Temperature
Sun Angle
Flux Non-Uniformity
Sentarus Simulation
0
2
4
6
8
10
12
14
16
18
20
1
.
0
0
E
+
1
3
1
.
0
0
E
+
1
4
1
.
0
0
E
+
1
5
5
.
0
0
E
+
1
5
1
.
0
0
E
+
1
6
5
.
0
0
E
+
1
6
1
.
0
0
E
+
1
7
1
.
0
0
E
+
1
8
Na(Bulk) [cm-3]
Efficency
L
H
Na
Bulk Material – N / P
Lifetime – SC / MC
Length
Depth
SR – Front / Bottom
SR - Contacts
Front Pyramids
Front Anti Reflective Coating
Back Reflective Coating
Back Diffused Mirror
Bulk Doping
N+,P+ Doping
N+,P+ Length
Metal Contact (Shading)
Temperature
Sun Angle
Flux Non-Uniformity
Sentarus Simulation
17.3
17.4
17.5
17.6
17.7
17.8
17.9
18
18.1
18.2
18.3
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Nd+,Na+ [um]
Efficency
L
H
Bulk Material – N / P
Lifetime – SC / MC
Length
Depth
SR – Front / Bottom
SR - Contacts
Front Pyramids
Front Anti Reflective Coating
Back Reflective Coating
Back Diffused Mirror
Bulk Doping
N+,P+ Doping
N+,P+ Length
Metal Contact (Shading)
Temperature
Sun Angle
Flux Non-Uniformity
5
10
15
20
25
30
35
0.5 1.0 1.5 2.0 2.5
Absorber Energy Gap (eV)
Solar
Cell
Efficiency
(%)
Ge
Si
CISe
CIS
CIGSe
CIGSSe
CdTe
GaAs
InP
CGSe
Cu2S
CdS
Theoretical limit for
ground-based solar
cells.
Comparison of record performances with theoretical efficiency limits.
Single Junction
Solar Cells
a-Si
Si-based
III-V based
CuIn-chalcogenide
Si and GaAs are
near theoretical
limits already.
Single-crystal devices
III-V Multijunction Cell record: 32%
Photovoltaic Opportunities
Junction Optimization
Optimize:
Junction geometry
Depth, width
N+, P+ width
Metal contact width
Material properties
Doping
Hole/electron lifetime
Surface treatments
Passivation
Front AR
Back reflector
Front texturing 40m
P 1016
P+ 1018
Double AR
Coating
Pyramids
Reflective
Coating
Process
dependent
1ms
h e
 
 
0.5 m
25–50
m
N+ 1019
0.5 m
Current analysis for Si
0 0.2 0.4 0.6 0.8 1
0
0.5
1
1.5
2
x 10
-5
V [Volt]
I
[Amp] Auger coeff=6.7*10
-32
; Efficiency= 29.14%
Auger coeff=1.6*10
-30
; Efficiency= 29.00%
Auger coeff=1*10
-28
; Efficiency= 25.99%
43.6um*60um cell under x 1000 concentration for several Auger
coefficients. The line in blue is for the default Auger coefficient in
Sentaurus.
Scalloped sidewalls Simulations
Process Flow
61
• How to fabricate the deep vertical PN junction?
• How to fabricate high aspect ratio trenches with
steep(?) side walls?
• How to deposit the contacts in these high aspect
ratio trenches?
The main Challenges:
Other VMJ Cells
Multi-wafer process, Si
40 junctions/cell
25.5 V
19.2% at ×2,500
Sater & Sater, 29 IEEE PV Specialists Conference, 2002
Green Field Solar
Our approach
Monolithic production of junctions in a single wafer
Optimal junction width < wafer thickness
Higher efficiency, better use of materials
Cost
Other VMJ Cells-Sliver
Multi-wafer process, Si
40 junctions/cell
25.5 V
19.2% at ×2,500
Sater & Sater, 29 IEEE PV Specialists Conference, 2002
Green Field Solar
Our approach
Monolithic production of junctions in a single wafer
Optimal junction width < wafer thickness
Higher efficiency, better use of materials
Cost
Another High Voltage Cell
Monolithic Inline Module (MIM)
Horizontal junctions, GaAs
25 junctions/cell
25.4 V
22% at ×200 (peak)
Loeckenhoff et al., IEEE 4th World Conf. PV Energy Conversion, 2006
Fraunhofer ISE
• Series resistance
• Large gaps- inactive area
• Front grid- shading
• Complex manufacturing
Our approach
Monolithic+ Vertical Junctions
Proposed basic Process Flow
SiO2
Handle
wafer
1. SOI Substrate
2. Trench fabrication
using DRIE process
3. Ion implantation of the
n+ side of the trench
4. Ion implantation of the
p+ side of the trench
5. Contact fabrication
using deposition
~40[um]
~0.5[um
]
~0.5[um
]
~40[um]
Previous
Cell
Next Cell
Generic concept
Can be applied to III-V material.
We will send you soon alternative process steps
66
Process B (alternative)
SiO2
Active
surface
Handle
wafer
•
SOI
Substrate
•
trenches manufacture in
DRIE process
•
Doping by
diffusion p+ of
the left trench.
•
Doping by diffusion
n+ of the right
trench.
•
removal of the
barrier by DRIE
process
•
contact
manufacture by
deposition
Suggestions to solve the
difficulties in the PV junction
fabrication
In our original cell design there are several difficulties :
• The light that encounter the contacts was reflected and was lost.
• There is no proper way to overcome the surface recombination at the
contacts.
My suggestion:
Neighboring
cell
Bulk P
Contacts
ARC
p+ side n+ side
Neighboring
cell
SiO2-RC
silicon
The cell efficiency was
improved by 1% in comparison
to the original cell design with
2um metal contacts( without
surface recombination).
Proposed alternative
process step
localized contacts
Trade in between :
Higher passivated light absorption area (Green color)
And higher series resistance. Optimization will be done.
The advantages of this design over the original design are:
• Passivation layer on the sidewalls of the trench (ARC) to reduce recombination.
• Localized contacts at the bottom of the trench.
• Light can enter the cell from the trench sidewalls.
• Improved light trapping than the original cell design.
The disadvantages of this design are:
• The fabrication process is more complicated.
• The series resistance becomes higher than the original cell design and the portion that
becomes higher is the part that isn't effected with high light concentrations.
Sidewalls angle + Metals Simulations
Fabrication
The fabrication process
and the cell structure
• The new vertical cell design is a very
ambitious design. Almost all of the
fabrication steps are at the edge of the
technology abilities today.
p+ region 0.5um doping 1019
Contacts 1-3um
n+ region 0.5um doping 1019
p bulk region 39um doping 1016
Handle wafer
SiO2
Figure 5 – The vertical cell on a SOI substrate
Basic fabrication process
Active surface
SiO2
Silicon- Handle wafer
Active surface
SiO2
Silicon- Handle wafer
SiO2
Active surface
SiO2
Silicon- Handle wafer
SiO2
Photoresist
Active surface
SiO2
Silicon- Handle wafer
SiO2
Photoresist
Active surface
SiO2
Silicon- Handle wafer
SiO2
SiO2
Silicon- Handle wafer
SiO2
Silicon- Handle wafer
SiO2
Silicon- Handle wafer
A – SOI Substrate
B – Oxide growth
C – Covering with Photoresist
D – Exposure to light
E – Photoresist developing
F – DRIE (Deep Reactive Ion Etching)
G – Photoresist removal
SiO2
Silicon- Handle wafer
H - Implantations
J – Oxide removal
SiO2
Silicon- Handle wafer
I – Contacts fabrication
SiO2
SiO2
SiO2
SiO2
Mask
SiO2
SiO2
Figure 6 – The Basic fabrication process
74
Why High aspect ratio?
High AR (Aspect Ratio) trench
fabrication methods
 Our requirements are high AR (Aspect Ratio) trench >20:1 (not standard
process)
 literature review results:
- Laser grooving.
- mechanical grooving.
- DRIE (Deep Reactive Ion Etching) methods (Bosch and Cryo processes).
Suitable for
our cell
Suitable for
contacts
Step
sidewalls
Aspect
ratio
depth
Technique
X
√
√
About 2:1
About 50um
Laser grooving
X
√
√
About 2:1
>60um
Mechanical grooving
√
√
√
>20:1
>60um
Bosch process
√
√
√
>20:1
>60um
Cryogenic process
Table 1- Abilities of the different techniques for the fabrication of the trenches
• There are other methods for DRIE ,but these methods are not published in the
literature because of confidentiality reasons.
• The DRIE process in TOWER:
High AR (Aspect Ratio) trench
fabrication methods
Mag x 4000 Mag x 15000 Mag x 100000
Mag x 100000
Mag x 300000
Mag x 5000
Mag x 28000
Stage
pecimen
SEM
Electron beam
Figure 7- Test
utline
• The Bosch process is the only commercial method for making high aspect ratio trenches, it
is necessary to check the effects of this process on the cell efficiency.
• Scalloped sidewalls - The Bosch process as two working cycles etching and passivating.
Scalloped are formed an the sidewalls of the trench.
• The changes in the cell efficiency from a cell without scalloping were decrease of less than
0.3%
High AR (Aspect Ratio) trench
fabrication methods
characterization
Figure 1 – characteristic
scalloped [7]
scalloping dimensions [9]
structure in the simulation
• Leg effect – the leg effect caused when the etching process reaches the SiO2 but
the etching process continue. This usually occur when we have trenches with
different widths on the same wafer.
• The changes in
the cell efficiency
from a cell without
the leg effect were
decrease of
less than 0.25%.
• The leg dimensions are 2x2 um The leg effect on a trench from tower
the structure in the simulation
High AR (Aspect Ratio) trench
fabrication methods
characterization
The most suitable methods for metal
deposition are:
• Advanced PVD Sputtering methods like
Highly Ionized Sputter Process.
• Atomic layer deposition (ALD).
High cost, rare academic use.
Deposition methods
for high AR trenches
Ta layer deposited with HIS
on the bottom and sidewall
scallops of a trench with AR
of 30:1
[
10
.]
• Sidewalls angel – are a byproduct of the Bosch process that can be caused by
wrong calibration of the machine.
• The angels for the left side of the figure are 92.66 deg and for the right side 87.44
deg. Thus angle where chosen because they are more than the average deviation
of the machine which is ± 1 deg.
• The cell efficiency improved from a cell with 90 deg sidewalls in about 0.1%
High AR (Aspect Ratio) trench
fabrication methods
characterization
Our Mask
82
Work Environment
Testing and simulations were carried out using the
Synopsys TCAD Sentaurus Tool Suite:
1. Process Simulator (real physical models)
The creation of PN junction will be done using Ion Implantation:
Figure 11. Ion Implantation
Implantation Models
Monte Carlo
(atomistic)
Analytic
Based on point
response
distributions
83
Work Environment
(a,b)



gas
ds
s
b
s
a
y
x
F
N
y
x
C d ))
(
),
(
,
,
(
)
,
(
Analytic Model:
 An ion beam incident at point (a,b) is assumed to generate
a distribution function F(x,y,a,b).
 In order to calculate the concentration C(x,y) at point (x,y),
the superposition of all distribution functions F(x,y,a,b) of all
possible points of incidence needs to be computed:
Figure 12. Ion Implantation Model Sentaurus Process [14]
Where Nd- total dose per
exposed area
84
Work Environment
1. Process Simulator (real physical models)
2. Structure Editor (geometry based)
3. Device Simulator (electronic behavior)
4. Tecplot Viewer (result analyzer)
Important, saves money and time!
Structure
Editor
Process
Simulator
Device
Simulator
85
Fabrication Issues
Figure 13. Effect of various implantation energies [Tecplot viewer]
N-Type Implant
~1018[cm-3]
Phosphorus
N-Type Implant
~1018[cm-3]
Phosphorus
P-Type Bulk
~1015[cm-3]
Boron
P-Type Bulk
~1015[cm-3]
Boron
P-Type Bulk
~1015[cm-3]
Boron
There are some major issues with conventional Doping
Techniques:
• Ion Implantation and Diffusion are isotropic processes,
this is bad for the profile. The deeper, the wider.
• Penetration depths reach a few microns at most at high
energy.
86
There is no possible way to create a Vertical
PN Junction using conventional techniques!
Fabrication Issues
87
According to the literature review that I conducted, the best
method for creating the Vertical PN Junction, is by Implantation
through a Trench Sidewall, apparently this method is being
used in the fabrication of both High Power and High Mbit DRAM
devices [6,7]:
Vertical PN Junction Fabrication Method
Figure 14. Implantation in Trench Sidewall [6]
Trench
88
Trench
Trench
My Suggested Solution
[tecplot]
Figure 16. Ion Implantation through trench sidewall
Trench
89
Process Flow - Simulation Results
[tecplot]
Previous
Cell
Next Cell
90
Figure 18. Zoom in of Vertical Junction
0.5[um]
[tecplot]
Process Flow - Simulation Results
91
[tecplot]
0.5[um]
0.5[um]
Process Flow - Simulation Results
92
Verification
0.5[um]
Process
Simulator
Structure
Editor
Device
Simulator
The Process Simulator, which takes into account more
physical effects due to processing steps, verifies the
results yielded from the geometry based Structure Editor.
Device
Simulator
Same Results
93
Verification
0.5[um]
SIMS Characterization (Solid State Institute, Technion):
Samples from Tower
Semiconductor LTD.
94
Verification
0.5[um]
SIMS Characterization:
Micro – Cleaving from SELA
LTD.
95
Verification
0.5[um]
SIMS Characterization Issue:
Implanted Sidewall
Bottom of trench
96
• A new method for creating vertical PN junction cell arrays was
proposed.
• Fabrication Techniques required for the creation of a Vertical
PN Junction were introduced.
• A new simulation platform based on real physical models was
described.
• A novel solution for doping apposing trench sidewalls with
different dopants was tested for the very first time.
• This solution can be applied to other fields in the Micro
Electronic Industry, such as High MBIT DRAM devices and
lateral power devices.
Summary
97
Substrate
Substrate will be SOI wafer with an active surface
equal to the thickness of the cell it self 25-50um.
For example, Icemostech company Inventory sample100 mm
Diameter wafer of SOI with Device Thickness of 35 + 1 um cost
about 200$
SiO2
Active
surface
Handle
wafer
35 + 1 um
0.5 -3 Ohm cm
400+5um
Ohm cm
1-100
10+0.5u
m
Icemostech:
Samples with trenches in the
active surface. Minimum
opening 3 microns. Aspect
ratio up to 1:40 cost ?
•
Formation of a texture
surface underneath the active
surface in order to increase
light trapping in the cell
(diffusive reflection).
98
Initial Model-
Synopsys TCAD Sentaurus Process simulator

We used this concept to
implant both p+ and n+ doping
on each of the trench’s
apposing sidewalls, using the
Process Simulator.

Implantation Parameters:
- Tilt: +1 degree
- Dose: 1X10^17[cm-2],
- Energy: 120[KeV]
Implantation Model: The
concentration behaves like a
Gaussian function, Peak near
the surface, while reducing
exponentially deeper into the
substrate.
99
P-Type Boron Concentration:
1X10^15[cm-3]
N+-Type Phosphorus Concentration:
~ 3X10^19[cm-3] (within 0.5[um] of surface)
~1.2[um]
~0.5[um]
~1.2[um]
~1.2[um]
~0.5[um]
Peak
0.7[um]
Implantation Parameters: Tilt: +1 degree,
Dose: 1X10^17[cm-2], Energy: 120[KeV]
Aluminu
m
Contact
Results:
P+-Type Phosphorus Concentration:
~ 3X10^19[cm-3] (within 0.5[um] of surface)
100
Alternative Approach :
high voltage/high current
P- Type Substrate
P
P P
P- Type Substrate (thin and cheap)
Metal
Contact
Trench
Dopin
g
Trench
n+
n+
Doping the walls of the trenches by diffusion of one type doping
101
Alternative Approach
(Cont.)
102
Process Flow (Cont.)
103
Connectivity Substrate
A
B E
D
C
A
B E
D
C
A. Top side, Pads for mating with Adhesive pads
B. Side View, Pads connected from top to bottom (vias)
C. Bottom Side, interconnection and Output pads
104
Connective
Adhesive
VMJ
Vertical Multi Junction (VMJ)
Two terminal VMJ:
Eg1>Eg2>Eg3
Eg1
Eg2
Eg3
hν
Voltage boosting
junctions
U3
U3
U3
U2
U3
U2
U1
Equivalent
circuit
Um1=2Um2=4Um3
Voltage matching is possible!!!
Vertical Multi Junction (VMJ)
Extra junctions hν
contact
s
p+ doping
n+ doping
Eg1
Eg2
contact
s
direct band gap material
short diffusion length
Multi terminal VMJ: Each Layer Has separated contacts
Neither voltage nor current matching is required!!!
Vertical Multi Junction (VMJ)
Two terminal VMJ:
Eg1>Eg2>Eg3
Eg1
Eg2
Eg3
hν
Insulato
r
n+ doping p+ doping
Contact
s
Equivalent
circuit
U3 U2 U1
Um1=Um2=Um3 the output voltage will be restricted by U3
Vertical Multi Junction (VMJ)
Multi terminal VMJ: Each Layer Has separated contacts
Eg1 Cross Section:
contacts
n+
doping
p+ doping
insulation
n+ doping
Eg1 layer top view
n+ doping
& contact
p+ doping
& contact
hν
Current flow
Vertical Multi Junction (VMJ)
Multi terminal VMJ: Each Layer Has separated contacts
Neither voltage nor current matching is required!!!
Eg1 layer top view Eg2 layer top view Eg3 layer top view
n+ doping
&
contact
n+ doping
&
contact
p+ doping
&
contact
p+ doping
&
contact
Vertical Multi Junction (VMJ)
Eg1 layer top view Eg2 layer top view Eg2 layer top view
Multi terminal
VMJ top view:
Cost
Cell manufacturing: rough estimate
~ $1/cm2 (SOI technology)
1,000 suns, 20% system efficiency →
Additional cost savings
Reduce or eliminate homogenizer
Reduce or eliminate protection diodes (parallel connection)
Higher voltage to inverter- increased efficiency
$0.05/We
Cost Estimates S. Kurtz, Tech. report NREL, july 2008
Assuming the following parameters :
Tandem : 70 mW/cm2 @ Efficiency 40 % gives 0.028 W/cm2
Our design : 70 mW/cm2 @ Efficiency 30 % gives 0.021 W/cm2
Using :
This gives : Tandem – 0. 35 $/W + 0. 23 $/W = 58 cent/W
Our design - 0. 48 $/W + 0.05 $/W = 53 cent /W
($/W)
Cost
Cell
)
(W/cm
power
Cell
)
($/cm
cost
Area
($/W)
cost
Cell 2
2


Conclusion: VJ in CPV systems
• Advantages:
o High efficiency under high flux concentration.
o Less sensitive to flux non-uniformities.
o High output Voltage.
• Disadvantages:
o Low efficiency comparing to MJ cells.
o Difficult implementation of by-pass diodes.
Summary
VMJ Cells with high voltage
Low series resistance- higher efficiency under concentration
Reduced Mismatch loss under non-uniform illumination
Savings in other system components
Project status
Investigating Si
Performing simulations, cell design optimization
Defining manufacturing processes
Seeking opportunities to manufacture and test VMJ cells
Future Directions
• VJ operation under High concentration.
• Temperature effects on VJs
• Effects of non-normal illumination
• Setup a PV characterization system.
• Test prototypes.
Si
Build, test, optimize VMJ cells
Build, test, optimize arrays
III-V materials
Ideas for tandem VMJ cells
Acknowledgments
• Prof. Yossi Rosenwaks (supervisor)
• Prof. Abraham Kribus (supervisor)
• Dr. Rona Sarfaty
• Gideon Segev
• Michel Jurban
• Liav Grinberg
• Vlad Timofeev
Funding
Israel Ministry of National Infrastructures
IP
Provisional patent application, TAU
Thank
you!
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Master Thesis Presentation

  • 1. 1 High Voltage Photovoltaic Cells By Roni Pozner Carried out under the supervision of Prof. Yossi Rosenwaks Dep. of Physical Electronics Tel-Aviv University
  • 2. 1. CPV operating conditions: 1. Solar concentrators in CPV systems. 2. Current mismatches in PV modules. 3. Electrical and thermal coupling in CPV systems. 2. Series resistance losses in CPV systems: 1. Effects of series resistance on cell efficiency. 2. Series resistance components in MJ cells. 3. VJ’s series resistance. 3. Vertical Multi Junction (VMJ)- Basic Concepts 4. VJ Vs. horizontal cells in CPV systems 5. Theory of Solar Cells 6. Advantages & Disadvantages 7. Simulation 8. Fabrication Outline
  • 3. 15 x15 km Israel consumes today 10 GW of electricity, thus it needs only 200 km2 of solar panels to supply most of its electricity needs ! With Concentrated PV Only 10x10 km2 are required
  • 4. Theory Of Solar Cells Illumination Absorption Separation Collection     ) ( ) ( min        collection separation absorption ation illu N q I    
  • 5. Photovoltaic Cells – Basic Concepts   sh s L nKT IR V q ph L R R I V e I I I s               1 0 C I I ph sc   IN M M IN MAX P I V P P    oc sc m m V I I V FF 
  • 6. Vertical Junction (VJ) Cell Vertical junction No front grid, minimal inactive area Decouple optics vs. electronics High Voltage cell Reduce series resistance effect Parallel connection- reduce mismatch effect
  • 8. Advantages - Cell • Active Area Fraction - Less shading “Back Contact” “Standard”
  • 9. Decoupling Effects Orthogonal carrier generation (optics) & carrier collection (electronics) Collection efficiency independent of wavelength High doping of N+,P+ layer, hence larger Voc
  • 11. • Lower resistance due to: – Control over length and doping of N+,P+ layers. – Photo conductivity phenomena. Advantages - Concentration (cont.)
  • 12. Solar concentrators in CPV systems Fresnel Lenses Parabolic reflectors
  • 13. Limitations of CPV technology • Series resistance limits concentration • Cell thickness: optics vs. electronics vs. mechanics • Front grid, busbar: shading loss • Illumination non-uniformity: mismatch loss
  • 14. Series resistance under light concentration • Our cell efficiency still rises under 5000 suns. While ordinary Multijunction cells as the highest efficiency under 500 suns and begin too decries under higher concentrations. silicon solar cells as the highest efficiency under 200 suns. • The reason for this phenomenon can be due to photoconductivity effects as well as the low currents in the cell. • This is because the mobility and the concentration of carriers are varying with location and light concentration conditions. • So in order to calculate the resistivity of the cell under concentration conditions we need to take the mobility and the concentration of carriers as a function of their location from the Sentaurus simulations.
  • 15. MJ cell- Series Resistance Components1 1. Electrode Resistance (RSE) 2. Contact Resistance 3. Lateral Resistance (RSL) 4. Layer (InGaP) Resistance 5. Tunnel Resistance (RT1) 6. Layer (InGaAs) Resistance 7. Tunnel Resistance (RT2) 8. Ge Substrate Resistance Main Resistance Components: RSE, RSL, RT1, RT2. 1K. Nishioka et al. / Solar Energy Materials & Solar Cells 90 (2006) 1308–1321
  • 16. Vertical Diode- Series Resistance Components 1. Electrode Resistance (RSE) 2. Contact Resistance (RSC) 3. p+ Layer Resistance (RSP) 4. Bulk Resistance (RSB) 5. n+ Layer Resistance (RSN) Main Resistance Component: RSB. p+ p n+ RSB
  • 17. Series Resistance Losses Neglecting the shunt resistance- s L L ph R I I I I q nKT V     ) 1 ln( * 0 s L L ph L L R I I I I q nKT I P 2 0 ) 1 ln( *     Rs Rsh Isc Id IL V     2 L Rs I P 
  • 18. Photoconductivity Effects         n p n n p p e y x          0 0 ,   C f RSB        C y x y x y x ph , , , , 0      Horizontal cells: Vertical Diodes:     C y x y x ph , , , 0    σph Changes significantly with the illumination Const RS 
  • 19. Simulation Results        L d SB dy C y x dx w C R 0 0 , , 1  R SB [Ω] 100 101 102 103 104 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Concentration
  • 20. Vertical Junction Under Concentration 2Slade et al. / Proceedings of the SPIE, Volume 5942, pp. 236-243 100 101 102 103 104 15 20 25 30 35 Concentration Efficiency [%] Vertical Diode Best Si cell2
  • 21. Grid Pitch Dependence of the Series Resistance and Shadow Loss1. Grid Pitch 1K. Nishioka et al. / Solar Energy Materials & Solar Cells 90 (2006) 1308–1321
  • 22. Efficiency for Concentration (various cells) Spectrolab- C1MJ2 2Kinsey et al. / Prog. Photovolt: Res. Appl. 2008; 16:503–508. Best Silicon cells 4Mulligan et al. / Proceedings of the 28th IEEE Photovoltaics Specialists Conference. 3Slade et al. / Proceedings of the SPIE, Volume 5942, pp. 236-243 (2005). 10 1 10 2 22 23 24 25 26 27 Concentration Efficiency [%] Best Si Cell3 (1.35 cm2 ) SunPow er Chipsize4 (0.0529 cm2 )
  • 23. Nishioka et al., Solar Energy Mat. Solar Cells, 2006 Limit for High Concentration High concentration → High current → High Rs loss Reduce grid pitch → High shading loss Spectrolab C1MJ Kinsey et al., Prog. Photovolt: Res. Appl. 2008 0 100 200 300 400 23 24 25 26 27 28 Concentration Efficiency [%] Measured Model SunPower Si cell Mulligan et al., Proc. 28th IEEE PV Specialists Conf.
  • 24. High Output Voltage Module N cells Series Connection of N Vertical Cells:
  • 25. N Junctions VMJ Cell Junctions in series = Cell • High voltage, low current • External contacts at edges Easy to convert to back contacts (external wrap-around) Insulator Back contact
  • 26. • Easy MIM connection (Monolithic process) • High output voltage of each module. • Small area of each module leads to uniform illumination on each one of them. Wire Vertical Cell Horizontal Cell Advantages - Module (cont.)
  • 27. Due to the relatively high output voltage of each module, it can be connected in parallel to the other modules (instead of series connection like in horizontal cells) this offers the following advantages: • Voltage coupled instead of current coupled response Better response to partly shading conditions • Less sensitive to non-uniform flux avoiding the use of homogenizer in concentrated PV (Gideon’s Lecture…) Advantages - Module (cont.)
  • 28. VJ module under non- homogenous illumination V1 V2 VN Vm + If the dimensions of the VJ module are small comparing to the change in illumination: N ph ph ph I I I , 2 , 1 , ..    4cm 4cm Y X contact P N contact 40 um dX 1 2 N 1cm
  • 29. Illumination input: Average concentration=444 Maximum concentration=1148 1.66% spillage
  • 30. VMJ Mismatch Loss 0 50 100 150 200 22 23 24 25 26 Number of junctions in cell (N) Efficiency [%] VMJ cell array Uniform illumination: 27.72% Non-uniform illumination: 25.86% (N = 40) → Series Mismatch Loss (relative)= 6.7% Conventional cell array Series Mismatch Loss > 80% No homogenizer !
  • 31. Efficiency for concentration, Width=43.6μm, Depth=60 surface recombination =100 10 1 10 2 10 3 10 4 20 25 30 35 Concentration Efficiency [%] Parallel illumination Multidirectional Illumination Multidirectional Illumination, no surface tex turing
  • 32. • High output voltage of each module enable the use of a more efficient DC/DC inverter. Advantages - Module (cont.)
  • 33. Disadvantages • Fabrication difficulties – The depth of the cell is relatively large (H) – Metal thickness need to be as thin as possible (X) (Karmiel’s Lecture…) H X X
  • 34. Semiconductor has low resistivity, hence the two cells above modeled as connected in parallel by conducting wires. Hole current from top to bottom reduces the voltage of 1 close to the voltage of 2 (circular currents) Voc of the device will be close to the voltage at the deepest point. Open Circuit Voltage (1) (2)
  • 35. As H getting bigger, Voc decrease and Isc increases => so while increasing H result in absorbing more light, it also reduces Voc, hence, the efficiency will begin to drop at some point. H • Voc Voc is proportional to ) 1 log( H Disadvantages (cont.)
  • 36. 0.5um Constant distance (between junction and contact) • Front Surface Recombination Varying distance (between junction and contact) Most of the minority carriers in the vertical cell have a longer path compared with the horizontal cell-more affected by surface recombination. Most of the light is absorbed in the top 5 microns of the cell back surface recombination is neglected in both the horizontal and vertical cells due to very small carriers density generated there. Vertical Cell Horizontal Cell Disadvantages (cont.)
  • 37. • Contact Surface Recombination (Point Contacts): Disadvantages (cont.) Vertical Cell Horizontal Cell Disadvantages (cont.)
  • 39. Analysis 0 ) , ( ) , ( ) , ( ) , ( 2 2 2 2       z x n x G dz z x n d D dx z x n d D n n Continuity equation: ) ) ( exp( ) ( ) ( ) , ( x N x G         Generation equation: x z Light Intensity Absorption Coefficient
  • 40. Matlab Simulation A Matlab simulation of varying cell depths: The result gave us an initial intuition regarding the optimization that should be done. L H
  • 42. Optimization Factors Bulk Material – N / P Lifetime – SC / MC Length Depth SR – Front / Bottom SR - Contacts Front Pyramids Front Anti Reflective Coating Back Reflective Coating Back Diffused Mirror Bulk Doping N+,P+ Doping N+,P+ Length Metal Contact (Shading) Temperature Sun Angle Flux Non-Uniformity
  • 43. • The thickness of the thin film (ARC and RC) is determined by: Phase Changes between the mediums has great significance . • Si3N4 is a good ARC due to two reasons: – Si3N4 has n~2 and the best ARC need refractive index of – Si3N4 is also very good for the passivation. • SiO2 is not the optimal material for RC but it is built in the SOI substrate so we can use it for our needs Optimization of ARC , RC and texture 2 2 1 ( 0.5) 2 4 m m d n n       1.87 ARC air si n n n  
  • 44. Optimization of ARC , RC and texture Figure 18 - Absorption depth for silicon [3] Figure 17 - Spectral Radiation at the Earth's Surface [3]  The optimal width of ARC is for the wavelength of 0.6um.  The optimal width of RC is for the wavelength of 0.9um . This is due to: - Sunlight intensity . - Absorption probability. Silicon- Handle wafer light T1 T2 R1 T3 R2 SiO2 Figure 15
  • 45. Optimization of ARC , RC and texture  The thickness of the thin film (ARC and RC) is determined by: 2 2 1 ( 0.5) (1) 2 4 m m d n n       Figure 19 - Anti-reflective coating  The difference between ARC and RC is the number of phase Changes between the mediums. Texturing  Reduced reflection.  Better "light trapping”
  • 47. Optimization of ARC ,RC and texture Reflectivity without ARC layer is R=40% . Reflectivity ARC (Si3N4 ) is R=15%. Reflectivity with ARC (Si3N4 ) and RC is R=18%. Transmitted light without RC is T=10% Transmitted light with RC is T=6%. Relation between the refraction index and the wavelength for a silicon surface Reflectivity for 40um depth cell with ARC & RC coatings 0 0.2 0.4 0.6 0.8 1 0.3 0.5 0.7 0.9 1.1 Wavelength [um] R(%) Cell with ARC (Si3N4) Cell with ARC (Si3N4) & RC Cell without coatings The high end of the spectrum the graph is not consistent due to the reflectivity from the second interface of air and silicon at the bottom of the cell.
  • 48. Sentarus Simulation 12.5053 18.6185 18.2978 18.9897 21.3228 Bulk Material – N / P Lifetime – SC / MC Length Depth SR – Front / Bottom SR - Contacts Front Pyramids Front Anti Reflective Coating Back Reflective Coating Back Diffused Mirror Bulk Doping N+,P+ Doping N+,P+ Length Metal Contact (Shading) Temperature Sun Angle Flux Non-Uniformity
  • 49. Sentarus Simulation 0 5 10 15 20 25 1.00E-02 1.00E-03 1.00E-04 1.00E-05 1.00E-06 1.00E-07 tau (Sec) Efficency (%) L H Bulk Material – N / P Lifetime – SC / MC Length Depth SR – Front / Bottom SR - Contacts Front Pyramids Front Anti Reflective Coating Back Reflective Coating Back Diffused Mirror Bulk Doping N+,P+ Doping N+,P+ Length Metal Contact (Shading) Temperature Sun Angle Flux Non-Uniformity
  • 50. Cell Optimization 12 14 16 16 18 18 20 20 20 22 22 2 2 22 22 22 2 2 24 24 2 4 2 4 2 4 2 5 Width [um] Depth [um] 50 100 150 200 250 20 40 60 80 100 120 140 Efficiency for Various cell sizes (1 sun)
  • 51. Cell Optimization 30 30.5 30.5 31 31 31 31.5 31.5 31.5 31.5 32 3 2 32 32 3 2 . 5 32.5 3 2 . 5 33 3 3 Width [um] Depth [um] 30 40 50 60 70 40 60 80 100 120 140 Efficiency for Various cell sizes (1000 suns)
  • 52. Sentarus Simulation L H Bulk Material – N / P Lifetime – SC / MC Length Depth SR – Front / Bottom SR - Contacts Front Pyramids Front Anti Reflective Coating Back Reflective Coating Back Diffused Mirror Bulk Doping N+,P+ Doping N+,P+ Length Metal Contact (Shading) Temperature Sun Angle Flux Non-Uniformity
  • 53. Sentarus Simulation L H Bulk Material – N / P Lifetime – SC / MC Length Depth SR – Front / Bottom SR - Contacts Front Pyramids Front Anti Reflective Coating Back Reflective Coating Back Diffused Mirror Bulk Doping N+,P+ Doping N+,P+ Length Metal Contact (Shading) Temperature Sun Angle Flux Non-Uniformity
  • 54. Sentarus Simulation 0 2 4 6 8 10 12 14 16 18 20 1 . 0 0 E + 1 3 1 . 0 0 E + 1 4 1 . 0 0 E + 1 5 5 . 0 0 E + 1 5 1 . 0 0 E + 1 6 5 . 0 0 E + 1 6 1 . 0 0 E + 1 7 1 . 0 0 E + 1 8 Na(Bulk) [cm-3] Efficency L H Na Bulk Material – N / P Lifetime – SC / MC Length Depth SR – Front / Bottom SR - Contacts Front Pyramids Front Anti Reflective Coating Back Reflective Coating Back Diffused Mirror Bulk Doping N+,P+ Doping N+,P+ Length Metal Contact (Shading) Temperature Sun Angle Flux Non-Uniformity
  • 55. Sentarus Simulation 17.3 17.4 17.5 17.6 17.7 17.8 17.9 18 18.1 18.2 18.3 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Nd+,Na+ [um] Efficency L H Bulk Material – N / P Lifetime – SC / MC Length Depth SR – Front / Bottom SR - Contacts Front Pyramids Front Anti Reflective Coating Back Reflective Coating Back Diffused Mirror Bulk Doping N+,P+ Doping N+,P+ Length Metal Contact (Shading) Temperature Sun Angle Flux Non-Uniformity
  • 56. 5 10 15 20 25 30 35 0.5 1.0 1.5 2.0 2.5 Absorber Energy Gap (eV) Solar Cell Efficiency (%) Ge Si CISe CIS CIGSe CIGSSe CdTe GaAs InP CGSe Cu2S CdS Theoretical limit for ground-based solar cells. Comparison of record performances with theoretical efficiency limits. Single Junction Solar Cells a-Si Si-based III-V based CuIn-chalcogenide Si and GaAs are near theoretical limits already. Single-crystal devices III-V Multijunction Cell record: 32% Photovoltaic Opportunities
  • 57. Junction Optimization Optimize: Junction geometry Depth, width N+, P+ width Metal contact width Material properties Doping Hole/electron lifetime Surface treatments Passivation Front AR Back reflector Front texturing 40m P 1016 P+ 1018 Double AR Coating Pyramids Reflective Coating Process dependent 1ms h e     0.5 m 25–50 m N+ 1019 0.5 m Current analysis for Si
  • 58. 0 0.2 0.4 0.6 0.8 1 0 0.5 1 1.5 2 x 10 -5 V [Volt] I [Amp] Auger coeff=6.7*10 -32 ; Efficiency= 29.14% Auger coeff=1.6*10 -30 ; Efficiency= 29.00% Auger coeff=1*10 -28 ; Efficiency= 25.99% 43.6um*60um cell under x 1000 concentration for several Auger coefficients. The line in blue is for the default Auger coefficient in Sentaurus.
  • 61. 61 • How to fabricate the deep vertical PN junction? • How to fabricate high aspect ratio trenches with steep(?) side walls? • How to deposit the contacts in these high aspect ratio trenches? The main Challenges:
  • 62. Other VMJ Cells Multi-wafer process, Si 40 junctions/cell 25.5 V 19.2% at ×2,500 Sater & Sater, 29 IEEE PV Specialists Conference, 2002 Green Field Solar Our approach Monolithic production of junctions in a single wafer Optimal junction width < wafer thickness Higher efficiency, better use of materials Cost
  • 63. Other VMJ Cells-Sliver Multi-wafer process, Si 40 junctions/cell 25.5 V 19.2% at ×2,500 Sater & Sater, 29 IEEE PV Specialists Conference, 2002 Green Field Solar Our approach Monolithic production of junctions in a single wafer Optimal junction width < wafer thickness Higher efficiency, better use of materials Cost
  • 64. Another High Voltage Cell Monolithic Inline Module (MIM) Horizontal junctions, GaAs 25 junctions/cell 25.4 V 22% at ×200 (peak) Loeckenhoff et al., IEEE 4th World Conf. PV Energy Conversion, 2006 Fraunhofer ISE • Series resistance • Large gaps- inactive area • Front grid- shading • Complex manufacturing Our approach Monolithic+ Vertical Junctions
  • 65. Proposed basic Process Flow SiO2 Handle wafer 1. SOI Substrate 2. Trench fabrication using DRIE process 3. Ion implantation of the n+ side of the trench 4. Ion implantation of the p+ side of the trench 5. Contact fabrication using deposition ~40[um] ~0.5[um ] ~0.5[um ] ~40[um] Previous Cell Next Cell Generic concept Can be applied to III-V material. We will send you soon alternative process steps
  • 66. 66 Process B (alternative) SiO2 Active surface Handle wafer • SOI Substrate • trenches manufacture in DRIE process • Doping by diffusion p+ of the left trench. • Doping by diffusion n+ of the right trench. • removal of the barrier by DRIE process • contact manufacture by deposition Suggestions to solve the difficulties in the PV junction fabrication
  • 67. In our original cell design there are several difficulties : • The light that encounter the contacts was reflected and was lost. • There is no proper way to overcome the surface recombination at the contacts. My suggestion: Neighboring cell Bulk P Contacts ARC p+ side n+ side Neighboring cell SiO2-RC silicon The cell efficiency was improved by 1% in comparison to the original cell design with 2um metal contacts( without surface recombination). Proposed alternative process step localized contacts Trade in between : Higher passivated light absorption area (Green color) And higher series resistance. Optimization will be done.
  • 68. The advantages of this design over the original design are: • Passivation layer on the sidewalls of the trench (ARC) to reduce recombination. • Localized contacts at the bottom of the trench. • Light can enter the cell from the trench sidewalls. • Improved light trapping than the original cell design. The disadvantages of this design are: • The fabrication process is more complicated. • The series resistance becomes higher than the original cell design and the portion that becomes higher is the part that isn't effected with high light concentrations. Sidewalls angle + Metals Simulations
  • 70. The fabrication process and the cell structure • The new vertical cell design is a very ambitious design. Almost all of the fabrication steps are at the edge of the technology abilities today. p+ region 0.5um doping 1019 Contacts 1-3um n+ region 0.5um doping 1019 p bulk region 39um doping 1016 Handle wafer SiO2 Figure 5 – The vertical cell on a SOI substrate
  • 71. Basic fabrication process Active surface SiO2 Silicon- Handle wafer Active surface SiO2 Silicon- Handle wafer SiO2 Active surface SiO2 Silicon- Handle wafer SiO2 Photoresist Active surface SiO2 Silicon- Handle wafer SiO2 Photoresist Active surface SiO2 Silicon- Handle wafer SiO2 SiO2 Silicon- Handle wafer SiO2 Silicon- Handle wafer SiO2 Silicon- Handle wafer A – SOI Substrate B – Oxide growth C – Covering with Photoresist D – Exposure to light E – Photoresist developing F – DRIE (Deep Reactive Ion Etching) G – Photoresist removal SiO2 Silicon- Handle wafer H - Implantations J – Oxide removal SiO2 Silicon- Handle wafer I – Contacts fabrication SiO2 SiO2 SiO2 SiO2 Mask SiO2 SiO2 Figure 6 – The Basic fabrication process
  • 73. High AR (Aspect Ratio) trench fabrication methods  Our requirements are high AR (Aspect Ratio) trench >20:1 (not standard process)  literature review results: - Laser grooving. - mechanical grooving. - DRIE (Deep Reactive Ion Etching) methods (Bosch and Cryo processes). Suitable for our cell Suitable for contacts Step sidewalls Aspect ratio depth Technique X √ √ About 2:1 About 50um Laser grooving X √ √ About 2:1 >60um Mechanical grooving √ √ √ >20:1 >60um Bosch process √ √ √ >20:1 >60um Cryogenic process Table 1- Abilities of the different techniques for the fabrication of the trenches
  • 74. • There are other methods for DRIE ,but these methods are not published in the literature because of confidentiality reasons. • The DRIE process in TOWER: High AR (Aspect Ratio) trench fabrication methods Mag x 4000 Mag x 15000 Mag x 100000 Mag x 100000 Mag x 300000 Mag x 5000 Mag x 28000 Stage pecimen SEM Electron beam Figure 7- Test utline
  • 75. • The Bosch process is the only commercial method for making high aspect ratio trenches, it is necessary to check the effects of this process on the cell efficiency. • Scalloped sidewalls - The Bosch process as two working cycles etching and passivating. Scalloped are formed an the sidewalls of the trench. • The changes in the cell efficiency from a cell without scalloping were decrease of less than 0.3% High AR (Aspect Ratio) trench fabrication methods characterization Figure 1 – characteristic scalloped [7] scalloping dimensions [9] structure in the simulation
  • 76. • Leg effect – the leg effect caused when the etching process reaches the SiO2 but the etching process continue. This usually occur when we have trenches with different widths on the same wafer. • The changes in the cell efficiency from a cell without the leg effect were decrease of less than 0.25%. • The leg dimensions are 2x2 um The leg effect on a trench from tower the structure in the simulation High AR (Aspect Ratio) trench fabrication methods characterization
  • 77. The most suitable methods for metal deposition are: • Advanced PVD Sputtering methods like Highly Ionized Sputter Process. • Atomic layer deposition (ALD). High cost, rare academic use. Deposition methods for high AR trenches Ta layer deposited with HIS on the bottom and sidewall scallops of a trench with AR of 30:1 [ 10 .]
  • 78. • Sidewalls angel – are a byproduct of the Bosch process that can be caused by wrong calibration of the machine. • The angels for the left side of the figure are 92.66 deg and for the right side 87.44 deg. Thus angle where chosen because they are more than the average deviation of the machine which is ± 1 deg. • The cell efficiency improved from a cell with 90 deg sidewalls in about 0.1% High AR (Aspect Ratio) trench fabrication methods characterization
  • 80. 82 Work Environment Testing and simulations were carried out using the Synopsys TCAD Sentaurus Tool Suite: 1. Process Simulator (real physical models) The creation of PN junction will be done using Ion Implantation: Figure 11. Ion Implantation Implantation Models Monte Carlo (atomistic) Analytic Based on point response distributions
  • 81. 83 Work Environment (a,b)    gas ds s b s a y x F N y x C d )) ( ), ( , , ( ) , ( Analytic Model:  An ion beam incident at point (a,b) is assumed to generate a distribution function F(x,y,a,b).  In order to calculate the concentration C(x,y) at point (x,y), the superposition of all distribution functions F(x,y,a,b) of all possible points of incidence needs to be computed: Figure 12. Ion Implantation Model Sentaurus Process [14] Where Nd- total dose per exposed area
  • 82. 84 Work Environment 1. Process Simulator (real physical models) 2. Structure Editor (geometry based) 3. Device Simulator (electronic behavior) 4. Tecplot Viewer (result analyzer) Important, saves money and time! Structure Editor Process Simulator Device Simulator
  • 83. 85 Fabrication Issues Figure 13. Effect of various implantation energies [Tecplot viewer] N-Type Implant ~1018[cm-3] Phosphorus N-Type Implant ~1018[cm-3] Phosphorus P-Type Bulk ~1015[cm-3] Boron P-Type Bulk ~1015[cm-3] Boron P-Type Bulk ~1015[cm-3] Boron There are some major issues with conventional Doping Techniques: • Ion Implantation and Diffusion are isotropic processes, this is bad for the profile. The deeper, the wider. • Penetration depths reach a few microns at most at high energy.
  • 84. 86 There is no possible way to create a Vertical PN Junction using conventional techniques! Fabrication Issues
  • 85. 87 According to the literature review that I conducted, the best method for creating the Vertical PN Junction, is by Implantation through a Trench Sidewall, apparently this method is being used in the fabrication of both High Power and High Mbit DRAM devices [6,7]: Vertical PN Junction Fabrication Method Figure 14. Implantation in Trench Sidewall [6] Trench
  • 86. 88 Trench Trench My Suggested Solution [tecplot] Figure 16. Ion Implantation through trench sidewall Trench
  • 87. 89 Process Flow - Simulation Results [tecplot] Previous Cell Next Cell
  • 88. 90 Figure 18. Zoom in of Vertical Junction 0.5[um] [tecplot] Process Flow - Simulation Results
  • 90. 92 Verification 0.5[um] Process Simulator Structure Editor Device Simulator The Process Simulator, which takes into account more physical effects due to processing steps, verifies the results yielded from the geometry based Structure Editor. Device Simulator Same Results
  • 91. 93 Verification 0.5[um] SIMS Characterization (Solid State Institute, Technion): Samples from Tower Semiconductor LTD.
  • 94. 96 • A new method for creating vertical PN junction cell arrays was proposed. • Fabrication Techniques required for the creation of a Vertical PN Junction were introduced. • A new simulation platform based on real physical models was described. • A novel solution for doping apposing trench sidewalls with different dopants was tested for the very first time. • This solution can be applied to other fields in the Micro Electronic Industry, such as High MBIT DRAM devices and lateral power devices. Summary
  • 95. 97 Substrate Substrate will be SOI wafer with an active surface equal to the thickness of the cell it self 25-50um. For example, Icemostech company Inventory sample100 mm Diameter wafer of SOI with Device Thickness of 35 + 1 um cost about 200$ SiO2 Active surface Handle wafer 35 + 1 um 0.5 -3 Ohm cm 400+5um Ohm cm 1-100 10+0.5u m Icemostech: Samples with trenches in the active surface. Minimum opening 3 microns. Aspect ratio up to 1:40 cost ? • Formation of a texture surface underneath the active surface in order to increase light trapping in the cell (diffusive reflection).
  • 96. 98 Initial Model- Synopsys TCAD Sentaurus Process simulator  We used this concept to implant both p+ and n+ doping on each of the trench’s apposing sidewalls, using the Process Simulator.  Implantation Parameters: - Tilt: +1 degree - Dose: 1X10^17[cm-2], - Energy: 120[KeV] Implantation Model: The concentration behaves like a Gaussian function, Peak near the surface, while reducing exponentially deeper into the substrate.
  • 97. 99 P-Type Boron Concentration: 1X10^15[cm-3] N+-Type Phosphorus Concentration: ~ 3X10^19[cm-3] (within 0.5[um] of surface) ~1.2[um] ~0.5[um] ~1.2[um] ~1.2[um] ~0.5[um] Peak 0.7[um] Implantation Parameters: Tilt: +1 degree, Dose: 1X10^17[cm-2], Energy: 120[KeV] Aluminu m Contact Results: P+-Type Phosphorus Concentration: ~ 3X10^19[cm-3] (within 0.5[um] of surface)
  • 98. 100 Alternative Approach : high voltage/high current P- Type Substrate P P P P- Type Substrate (thin and cheap) Metal Contact Trench Dopin g Trench n+ n+ Doping the walls of the trenches by diffusion of one type doping
  • 101. 103 Connectivity Substrate A B E D C A B E D C A. Top side, Pads for mating with Adhesive pads B. Side View, Pads connected from top to bottom (vias) C. Bottom Side, interconnection and Output pads
  • 103. VMJ
  • 104. Vertical Multi Junction (VMJ) Two terminal VMJ: Eg1>Eg2>Eg3 Eg1 Eg2 Eg3 hν Voltage boosting junctions U3 U3 U3 U2 U3 U2 U1 Equivalent circuit Um1=2Um2=4Um3 Voltage matching is possible!!!
  • 105. Vertical Multi Junction (VMJ) Extra junctions hν contact s p+ doping n+ doping Eg1 Eg2 contact s direct band gap material short diffusion length Multi terminal VMJ: Each Layer Has separated contacts Neither voltage nor current matching is required!!!
  • 106. Vertical Multi Junction (VMJ) Two terminal VMJ: Eg1>Eg2>Eg3 Eg1 Eg2 Eg3 hν Insulato r n+ doping p+ doping Contact s Equivalent circuit U3 U2 U1 Um1=Um2=Um3 the output voltage will be restricted by U3
  • 107. Vertical Multi Junction (VMJ) Multi terminal VMJ: Each Layer Has separated contacts Eg1 Cross Section: contacts n+ doping p+ doping insulation n+ doping Eg1 layer top view n+ doping & contact p+ doping & contact hν Current flow
  • 108. Vertical Multi Junction (VMJ) Multi terminal VMJ: Each Layer Has separated contacts Neither voltage nor current matching is required!!! Eg1 layer top view Eg2 layer top view Eg3 layer top view n+ doping & contact n+ doping & contact p+ doping & contact p+ doping & contact
  • 109. Vertical Multi Junction (VMJ) Eg1 layer top view Eg2 layer top view Eg2 layer top view Multi terminal VMJ top view:
  • 110. Cost Cell manufacturing: rough estimate ~ $1/cm2 (SOI technology) 1,000 suns, 20% system efficiency → Additional cost savings Reduce or eliminate homogenizer Reduce or eliminate protection diodes (parallel connection) Higher voltage to inverter- increased efficiency $0.05/We
  • 111. Cost Estimates S. Kurtz, Tech. report NREL, july 2008 Assuming the following parameters : Tandem : 70 mW/cm2 @ Efficiency 40 % gives 0.028 W/cm2 Our design : 70 mW/cm2 @ Efficiency 30 % gives 0.021 W/cm2 Using : This gives : Tandem – 0. 35 $/W + 0. 23 $/W = 58 cent/W Our design - 0. 48 $/W + 0.05 $/W = 53 cent /W ($/W) Cost Cell ) (W/cm power Cell ) ($/cm cost Area ($/W) cost Cell 2 2  
  • 112. Conclusion: VJ in CPV systems • Advantages: o High efficiency under high flux concentration. o Less sensitive to flux non-uniformities. o High output Voltage. • Disadvantages: o Low efficiency comparing to MJ cells. o Difficult implementation of by-pass diodes.
  • 113. Summary VMJ Cells with high voltage Low series resistance- higher efficiency under concentration Reduced Mismatch loss under non-uniform illumination Savings in other system components Project status Investigating Si Performing simulations, cell design optimization Defining manufacturing processes Seeking opportunities to manufacture and test VMJ cells
  • 114. Future Directions • VJ operation under High concentration. • Temperature effects on VJs • Effects of non-normal illumination • Setup a PV characterization system. • Test prototypes. Si Build, test, optimize VMJ cells Build, test, optimize arrays III-V materials Ideas for tandem VMJ cells
  • 115. Acknowledgments • Prof. Yossi Rosenwaks (supervisor) • Prof. Abraham Kribus (supervisor) • Dr. Rona Sarfaty • Gideon Segev • Michel Jurban • Liav Grinberg • Vlad Timofeev Funding Israel Ministry of National Infrastructures IP Provisional patent application, TAU

Notas do Editor

  1. ההרצאה בנוייה ממספר חלקים: מערכות לריכוז קרינה- מבנה בסיסי, התפלגות קרינה על תא במערכות לריכוז קרינה, צימוד בין מודל תרמי לחשמלי במערכות לריכוז קרינה. התנגדות טורית במערכות לריכוז קרינה: השפעות של התנגדות טורית על נצילות תאים, הרכב ההתנגדות הטורית בתאים רגילים ורב צמתיים, הרכב ההתנגדות הטורית ב- VJ. VJ מול תאים רגילים במערכות לריכוז קרינה. 4. VMJ
  2. מונחים בסיסיים בתאי PV... חשוב, זרם הקצר שווה בקירוב לזרם הפוטוני. מקובל כי הזרם הפוטוני פורפוציוני לעוצמת ההארה
  3. רואים כאן מראה שתי מערכות אחת מבוססת על מראות פרבוליות והשנייה על עדשות פרנל. אנחנו נתעסק בעיקר עם מראות פרבוליות.
  4. Amonix~ 400 sq. m gives 53 KW peak~150,000 KWhr/year
  5. המרכיבים השונים של ההתנגדות הטורית: ראשית נתבונן במסלול של נושאי המטען בין האלקטרודות: תנועת נושאי המטען תהיה : 3 השכבות של הצמתים , 2 דיודות מנהרה ה- emitter (תנועה לטרלית), להגעה לאלקטרודה העליונה. ההתנגדות הטורית תהיה בנויה ממספר התנגדויות המחוברות ביניהן בטור. 3 ההתנגדויות של הצמתים, דיודות המנהור, ההתנגדות השכבתית (sheet resistance) של שכבת ה- emitter, התנגדות המגע מתכת מל"מ והתנגדות האלקטרודה העליונה. המרכיבים המשמעותיים הם התנגדות האלקטרודות, ההתנגדות השכבתית של ה- emitter, וההתנגדויות של דיודות המנהרה.
  6. לכן, המרכיבים העיקריים של של ההתנגדות הטורית בצומת כזו יהיו- התנדות ה- bulk, התנגדות שכבות ה- n+,p+, התנגדות מגע המתכת מל"מ והתנגדות המגעים. מאחר ונושאי המטען חוצים את שכבות ה- n+,p+, המגעים שהן שכבות רחבות ודקות כך שההתנגדות שלהן נמוכה יחסית לזו של ה- bulk.
  7. כעת נתבונן בהשפעת ההתנגדות הטורית במערכות ריכוז קרינה. ם ניתן להזניח את השפעת ההתנגדות המקבילית. למשוואה פשוטה שמתארת את קשרי המתח זרם בתא +פונקציית ההספק לזרם בתא. פונקציית ההספק מתחלקת לאיבר שמתאים לייצור הספק ולאיבר ההספק הנצרך ע"י ההתנגדות הטורית. האיבר האחרון תלוי בזרם בריבוע בשעה שלראשון תלות נמוכה יותר בזרם. ככל שהזרמים בתא גדולים יותר, יותר הפסק יצרך ע"י ההתנגדות הטורית. לעובדה זו חשיבות רבה שכן במערכות CPV עובר זרם גבוה ביותר דרך מספר קטן של תאים. מסיבה זו בתאים המיועדים לריכוז קרינה ניתן דגש מיוחד על הקטנת ההתנגדות הטורית.
  8. המוליכות תלויה בריכוז נושאי המטען. את המוליכות נחלק לרכיב בסיס ולרכיב הפוטוקונדקטיביות. הסימום ב- bulk נמוך תהיה להארה השפעה משמעותית על מוליכות ה- bulk. לעומת זאת, בתא רגיל, שכבת ה- emitter מסוממת מאוד ולכן תוספת הפוטוקונדקטיביטיות תהיה זניחה.
  9. חישבנו את התנגדות ה- bulk כתלות בריכוז הקרינה. ניתן לראות כי ההתנגדות יורדת בצורה מאוד משמעותית עם הריכוז (גרף ההתנגדות קרוב מאוד ל- 1/C).
  10. כאן אנו רואים את הנצילות המחושבת כתלות בריכוז הקרינה. ניתן לראות שכתוצאה מהירידה בהתנגדות הטורית, הVJ- עמיד הרבה יותר בריכוז קרינה יחסית לתאי סיליקון רגילים ואפילו בהשוואה לתאים רב צמתיים (מקסימום הנצילות מתקבל בריכוז של כ- 2500).
  11. על מנת להקטין את ההתנגדות הטורית יהיה נכון לקצר את המסלול הלטרלי של נושאי המטען הקטנת המרווח בין האלקטרודות (grid pitch). trade off בין הורדת ההתנגדות הטורית לבין הצללה על התא. בצד ימין: לראות את הגדלים של רכיבי ההתנגדות הטורית כתלות ב grid pitch. בנוסף רואים את הפסדי ההצללה. בעיגול מסומנים הערכים האופטימאליים.
  12. בגרפים אלו אנו רואים את התוצאה הישירה של מה שראינו קודם. בגרפים אלו אנו רואים את הנצילות של התאים כתלות בריכוז הקרינה עליהם. כאמור, הזרמים בתאים פחות או יותר פרופורציונאליים לריכוז. ולכן עבור ריכוז קרינה גבוהה זרמים גבוהים יותר והפסדים גבוהים יותר כתוצאה מההתנגדות הטורית. מסיבה זו יש לכל התאים ריכוז אופטימאלי מסוים, עבור ריכוז קרינה גבוהה יותר ההפסדים יגדלו והנצילות תרד. בצד שמאל אנו רואים את הנצילות של תא 3 צמתים המתקדם ביותר. רואים שתא זה תוכנן לנצילות אופטימאלית בריכוז של 500 עד 1000 (תלוי בטמפרטורה). בצד ימין רואים את תאי הסיליקון המתקדמים ביותר, שם הנצילות המקסימאלית מתקבלת בריכוז של בין 100 ל- 200.
  13. כעת נבחן את ההתנהגות של מודול של VJ תחת הארה משתנה. נניח כי מודול כזה מורכב ממספר תאים מונוליתיים אשר מחוברים ביניהם במקביל. כל תא מונוליתי מורכב מ- N VJ במחוברות בטור. נניח כי רוחב VJ (רוחב של תא מונוליתי) הוא 1 ס"מ וכי גודל המודול כולו הוא 4cm*4cm. מאחר ואורכה של צומת וורטיקאלית (כ- 40 מיקרון) קטן מאוד יחסית לשאר הגדלים במערכת ניתן להניח כי עבור N מספיק קטן ההארה על כל הצמתים בתא מונוליתי תהיה אחידה.
  14. זוהי מפת ההארה בה נשתמש עבור החישוב. מפה זו היא למעשה החלקה של תוצאת ה- ray tracing שראינו מוקדם יותר. כאן חשוב לציין כי העובדה כי הריכוז נמוך מאוד בקצוות הופכת את המקרה הזה למקרה מאוד קיצוני. לכן, במערכות עם מרכזים פילוג ההארה יהיה תמיד יותר אחיד מזה המוצג כאן (בדרך כלל יהיה homogenizer אשר יהפוך את ההארה לאחידה יותר). 1.66%מהאנרגיה שיוצאת מהמרכז כלל לא תגיע למודול (זה ערך מתאים בסדר גודל למערכות אמיתיות).
  15. נצילות כתלות בגיאומטריות שונות ללא ריכוז קרניים מקבילות.
  16. נצילות כתלות בגיאומטריות שונות ריכוז- פי 1000 קרניים במפתח של 45 מעלות. אופטימום זז שמאלה--> מוריד את ההתנגדות הטורית של ה- bulk
  17. Dr Safaty Rona, Electrical Engineering, Ort Braude, 2009
  18. Dr Safaty Rona, Electrical Engineering, Ort Braude, 2009
  19. לסיכום- ראינו את היתרונות הברורים של ה- VJ תחת ריכוז קרינה גבוה, כמו כן ראינו כי מערכת המבוססת על VJ תהיה עמידה יותר בהארה לא אחידה (כאשר הגודל של מפת ההארה גדולים מאוד מגודל התא המונוליתי). לא דיברנו על כך אבל ישנה משמעות חשובה מאוד לכך שבמודול כזה יש מספר גדול של צמתים המחוברים בטור. מתח המוצא של מודול כזה יהיה גבוהה יחסית למודולים אחרים באותו הגודל וזהו יתרון גדול. שני חסרונות משמעותיים הם שהנצילות של ה- VJ עדיין נמוכה יחסית לזו של תאים רב צמתיים. עניין זה משמעותי כאשר נבצע חישוב כולל של $/W למערכת. בעייה נוספת היא שקיים קושי משמעותי בחיבור של דיודות מעקף למערכות כאלו עניין שיכול לגרום לבעיות בריכוז קרינה לא אחיד.
  20. בשלב זה אנו עובדים על מידול מדויק יותר ואופטימיזציה של פעולת ה- VJ תחת ריכוז קרינה גבוה. כמו כן בקרוב נתחיל ללמוד כיצד VJ פועלת בטמפרטורות שונות וכאשר ההארה מגיעה מזוויות שונות. במקביל אנו עובדים על הקמת מערך למדידת מערכות PV בו נוכל למדוד את ביצועי האבטיפוסים אשר אנו עתידים לבנות.