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2011 NCN@Purdue-Intel Summer School Notes on Photovoltaic Devices
Lecture 14What is Different about Thin-Film PV
M. A. Alam and S. Dongaonkar
[email protected] and Computer Engineering
Purdue UniversityWest Lafayette, IN USA
NCN Summer School: July 2011 Notes on the Fundamental of Solar Cell
2
copyright 2011
This material is copyrighted by M. Alam under the following Creative Commons license:
Conditions for using these materials is described at
http://creativecommons.org/licenses/by-nc-sa/2.5/
Alam 2011
The lecture series on solar cells
Introduction to Solar cells Physics of Crystalline Solar cells Simulating Solar Cells What is different about thin film solar cell Organic photovoltaics
3Alam 2011
superposition and recombination
4
Outline of the lecture
4
1) Background information about thin film solar cells
2) Photo current from the transmission perspective
3) Dark current, shunt conduction, and weak diodes
4) Variability, reliability, and lifetime of solar cells
5) Conclusions
Alam 2011
Different types of solar cells
5
Crystalline Silicon
*Google Images
p-n
Amorphous silicon
p-i-n
Flexible organic
m-i-m
Alam 2011
Si too thick and expensive …
Economics of solar cells
6
C-Si CdTe a-Si CIGS OPV
Material/m2 207 50-60 64 100-125 37
Process/m2 123 86 73 130 23-37
Total/m2 350 130 138 230 50-80
Cost/W 1.75 0.94 -1.2 0.9-1.4 1.63 1-1.36
• All costs are approximate (J. Kalowekamo/E. Baker, Solar Energy, 2009. Goodrich, PVSC Tutorial, 2011.
c-Si installation, labor, etc. $3.75/WOthers … $1.00-1.50/W
…. but thin film solar cell has their own problems !
Features of thin film solar cells
7
(2) Thin doped region
Ala-Si:H
TCO
Laser Scribe
Glass
(1) Thin absorption layer
(4) Grain boundaries(3) Contact diffusion
(5) Series connection
Alam 2011
Equivalent circuit of thin film solar cells
Voc
Idark
max ph ocI FVP F= × ×
IT
Iph
Superposition does not hold …
RL
RL
phI
( )0
0
1
1
/( )
ln( )
Bph
ph
qV T
C
k
OV
I
IT q
I I e
k I
= −
⇒ = +
−SHIdIRI
8
IT
9
Outline of the lecture
9
1) Background information about thin film solar cells
2) Photo current from the transmission perspective
3) Dark current, shunt conduction, and weak diodes
4) Variability, reliability, and lifetime of solar cells
5) Conclusions
Alam 2011
10
Basics of current flow
10
Wrong contact loss +Recombination loss
0 03 2L Ln n pJ J J q qυ υ= − = −
0
0 0
4
4 2
Ln n
L L L Rn p n n
J J q
J J J J J
q q
υ
υ υ
≠ =
= − = −
= −
0 0
,,
, , , ,
4 26 6
6 6
L pL n
L n R n L p R p
q q
qG qG
υ υ
γγγ γ γ γ
= × − ×
= × − ×+ +
0 0
,,
, , , ,
3 26
6 6
L pL n
L n R n rec L p R p rec
q
qG
υ υ
γγγ γ γ γ γ γ
= × −
= × − + + + +
Alam 2011
11
Basics of transmission over a barrier
11
,B R Bk TR L Aeγ −← = E
LeftContact
RightContact
,B L Bk TL R Aeγ −← = E
LeftContact
RightContact
,B R Bk TR Aeγ −= E
Left Contact Right ContactDevice
,B L Bk TL Aeγ −= E
Alam 2011
12
Photocurrent without recombination
,,
0, , , ,
, ,
0, , , ,
Wph L pL n
L n R n L p R p
W L n R n
L n R n L n R n
Jdx
qG
dx
γγγ γ γ γ
γ γγ γ γ γ
= −
+ +
= − + +
∫
∫
( )/0 0
( )/ ( )/00 0 0 0
E W x kTW
E W x kT E W x kT
edx
e e
υ υυ υ υ υ
− −
− − − −
= −
+ + ∫
0Lγ υ=
( )/0
E W x kTR eγ υ − −=
RL2 2
log cosh coth2 2
D D
D D
L LW WW W
W L W L
= × ≅ −
D
kTL
E≡
‘Price length’ and point of no return …. Alam 2011
Properties of ‘Sokel’ photo-current
2log cosh
2
2coth
2
ph D
D
D
D
J L WW
qG W L
L WW
W L
= ×
≅ −
L Rγ γ
L Rγ γ=
L Rγ γ<
L Rγ γ=
VA=0
Vbi=VA>0
VA>Vbi=0
Vbi
VA
nJqG
Voltage dependent photocurrent, different from Si p-n junction …
Vbi
Sokel and Hughes, JAP, 53(11), 1982.
14
Blocking layer and photocurrent
14
~ 0Rγ
VA=0
Vbi=VA>0
Vbi
VA
phJ
qG
Blocking is essential for many types of thin film PV ….
, ,
0, , , ,
Wph L n R n
L n R n L n R n
Jdx
qG
γ γγ γ γ γ
= − + +
∫
With blocking layer …
phJ qGW=
~ 0Rγ
15
Photocurrent with recombination
0n n n nEυ τ µ τ≡ × = × ×
/ /
01 ec c
W x Wcdx e− − = = − ∫
{ }/1 e np Wn
hJ
qGW W − − −= −
Crandall, JAP, 54(12), 19823
,,
0, , , ,
Wph L pL n
L n R n rec L p R p rec
Jdx
qG
γγγ γ γ γ γ γ
= −
+ + + + ∫
16
Photo-current in crystalline cells
16
, /, 1 e ~nph L p WL nn n n n n
J JJD W
qG qG qGτ− = − = − ≡
Voltage independent photocurrent is unaffected by electron mirrors
Electron blocking layer does suppress dark current, increases Voc.
with electron mirror without electron mirror
D Wτ D Wτ
With blocking Without blocking
Numerical validation: Effect of blocking layer
For low quality Si PV, blocking is not essential 17
18
Photocurrent with field/recombination
18
/ ( )/ ( )/0 0
( )/00 0
/ ( )/ ( )/
( )/0 1
n k n
k
n k n
k
x W x W xWph
W x
x W x W xW
W x
J e e edx
qG e
e e edx
e
υ υυ υ
− − − − −
− −
− − − − −
− −
−=
+ −
= +
∫
∫
Alam 2011
2coth
2Dh
D
pJ WW
qGL
LW
≅ −
pD
h
E
n kTL ≡
Matches with numerical simulation very well …
19
Outline of the lecture
19
1) Background information about thin film solar
2) Photo current from transmission perspective
3) Dark current, shunt conduction, and weak diodes
4) Variability, reliability, and lifetime of solar cells
5) Conclusions
Alam 2011
Dark current without recombination
20
0 0L R
nL R
RR
LL
nJ qn qγ γ
υ υγ γ γ γ
= −+ +
, ,00 ,0,0 RL R Ln n γ γ=
,0
,0 ,0 0
,
,
0
1
B B bi B
bi B
E k T qV k T
qV kL
T
L
R R
Ae Ae
A eγ
γ
γ γ
− −
−
= =
= × ⇒ =
02
, iR Ann N=
,0,0bi B
Rk T
LqVenn +=
0
2
,bi BqV k Ti
AL en
nN
−=
ND NA
2,0R i An n N=
biV
,0L Dn N=
0R L bi
L R
qV←
←
==
EEAlam 2011
21
Calculating dark current without recombination
21
,, 0 000L R
nL R
L RL R
nJ q qnγ γ
υ υγ γ γ γ
= −+ +
( )
1
bi Bq V V k TL
R
eAAγ
γ
− −
= ×=
2
,0bi Bi
A
qV k TL
nn e
N+=
( )
( )/
0
2( )/
,0 ,0
0 11bi B
bi B bi B
n L RR
q
RL
qV k V V
L
q V V k TT k Ti
A
qJ
q ne
N
n n
ee
υγ γ
γυ
γ
−− −
−
= −+
= − +
( )/
2 2
0
( ) /1 1
1bi
B B
Bq V V k TqV k T qV k Tn bii i
d n pA D
V V dn nJ J J q e I e
N N e
µ+ −
− = + = + − ≡ − +
( )
2
/
( ) /1
1bi B
b B
q V V k TqV k Tn bii
A
V V dnq e
N e
µ+ −
− = − +
biV
Theory and practice of thin film dark IV
22
A real solar cell IV seldom looks like textbook IV!These wings helped create many complicated models.
0 1BqV k TdI I e ≡ −
Theory Experiment
Butterfly wings!
Alam 2011
Contact diffusion and shunt conduction
23
(2) Thin doped region(1) Thin absorption layer
(4) Grain boundaries
(3) Contact diffusion
A real solar cell IV seldom looks like textbook IV
Parasitic shunt leakage
24
shunt
@ low voltage
@ high voltageintrinsic
Dongaonkar and Alam JAP, 2010
Interpretation of ‘shunt’ leakage
1
2 1shunt
VI A
L
δ
δµ+
+= = AE
kTγ
* G. Paasch et al., JAP 106, 084502 (2009)
0
n nJ qnd qndx
µ
κε
=
=
EE
( )
32
32
2 23
98
an
na a
JV L
J V VL
εµεµ
=
=
*
25
Features of shunt leakageSymmetry Exponents
1
2 1shunt
VI A
L
δ
δµ+
+=
Dongaonkar and Alam, EDL, 2010 26
L-3
27
Outline of the lecture
27
1) Background information about thin film solar
2) Photo current from transmission perspective
3) Dark current, shunt conduction, and weak diodes
4) Variability, reliability, and lifetime of solar cells
5) Conclusions
Alam 2011
Contact diffusion and shunt conduction
28
(2) Thin doped region(1) Thin absorption layer
(3) Grain boundaries
(4) Contact diffusion
Alam 2011
Variability and weak diodes
29
/0
/0
0
(e 1) ( )
(e 1) ( )
ln(1 )
oc
qV kTph
qV kTph oc
oc B ph
I I I V
I I V
V k T I I
= − −
− =
= × +
L I
( )
/0
/
( )e
( )(e 1)
oc
oc
qV kTph oc
q V V kTph oc
I I V
I I V
−
−
≈
= −
( )
( )2
1 /
/2
12 ( )(e 1
(
)
)(e 1)
o
c
c
o
q V V kTph oc
q V V kTph oc
l Vn
I V
I−
−× −
= −
×
Like an impact crater, a single um-sized weak diode can drain away 1-10 mm region !!
( )1 2 /2 2 e !oc ocq V V kTn L l −= ≈
(karpov, PRB 69, 045325, 2004.)
(5) Series connection, shadow degradation, and a very weak diode
30Alam 2011
Being in shadow stresses the device
Initial panel operating point
Load lineShaded panel operating point
Initial cell operating point
Shaded cell operating point
Shaded cells can get reverse biased!31
32
Shadow degradation
32
timeNormal operation
Shadow degradation
Dark current
Operating voltage
Dongaonkar et al.IRPS 2011
Light induced degradation
33
D. L. Staebler, et al., APL, 1977.
Y. Nakata, et al., JJAP, 1992
T. Shimizu, JJAP, 2004
Slope – t1/3
M. Stutzmann, et al., PRB, 1985
Ongoing discussion about exponent n~1/3
34
Reaction-Diffusion Model for LID
G
RDB
F 0 DB H= GdN
k N - k N Ndt
2H DBH H
dN dNk N
dt dt= -
Free H Generation:
Reaction:
~ 0
( )2 3
1 3 0 1 33 fDB H
r
k N GN k t
k
∝
// /
H2
10-4 10-2 100 102108
109
1010
1011
Dens
ity o
f DBs
[cm
-3]
LS Time [sec]
Slope n ~0.3
Boron-doped Czochralski (Cz) crystalline PV equally susceptible to LID. Float-zone and/or Ga doping better.
Light induced degradation in crystalline PV
metal fingern+ emitter ~ 100 nm
p absorber ~ 100 µm
p+ electron mirror ~ 100 nmback reflector
35Alam 2011
Extrinsic and Intrinsic Solar Cell Reliability
Extr
insic
• Electrochemical corrosion
• Moisture Ingress• Glass fracture• Inverters reliability• Delamination• Improper insulation• Bypass diode failure
Intr
insi
c • Light Induced Degradation
• Hot spot breakdown
• Shunt leakage• Shadow degradation• Weak diodes
36Source - www.nrel.gov/pv/performance_reliability/pdfs/failure_references.pdf
37
conclusions
Economic incentive to develop thin film solar cell.
The unique features of thin film PV make photo-current voltage dependent, increases probability of formation of weak diodes and shunts.
In addition to the extrinsic reliability issues, we need to worry about shadow degradation, light induced degradation, etc.
The reliability/variability are key concerns – making modules less efficient than individual cells.
Alam 2011
38
Reference for images
38
http://www.viraload.com/2008/08/28/avasolar-raises-104m-for-thin-film-solar/
http://www.solarserver.com/solar-magazine/solar-news/current/kw42/nanosolar-to-expand-thin-film-cigs-solar-cell-manufacturing-to-115mw.htmlhttp://www.solarthinfilms.com/active/en/home/photovoltaics/photovoltaics_and_thinfilms/thinfilm_photovoltaics.htmlhttp://gotpowered.com/2011/ge-develops-thin-film-photovoltaic-panels/
http://kypros.physics.uoc.gr/images/web2.gif