Muhammad A. Alam (2011), "Solar Cells Lecture 4: What is Different about Thin-Film Solar Cells?," http://nanohub.org/resources/11949.

1. 2011 NCN@Purdue-Intel Summer School
Notes Summer School: July 2011
NCN on Photovoltaic Devices
Notes on the Fundamental of Solar Cell
Lecture 14
What is Different about Thin-Film PV
M. A. Alam and S. Dongaonkar
alam@purdue.edu
Electrical and Computer Engineering
Purdue University
West Lafayette, IN USA

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/
2 Alam 2011

3. 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
superposition and recombination
Alam 2011
3

4. Outline of the lecture
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 4

5. Different types of solar cells
p-n p-i-n m-i-m
Crystalline Silicon Amorphous silicon Flexible organic
Si too thick and expensive …
Alam 2011 5
*Google Images

6. Economics of solar cells
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
c-Si installation, labor, etc. $3.75/W
Others … $1.00-1.50/W
…. but thin film solar cell
has their own problems !
• All costs are approximate
(J. Kalowekamo/E. Baker, Solar Energy, 2009. Goodrich, PVSC Tutorial, 2011. 6

7. Features of thin film solar cells
(2) Thin doped region
(1) Thin
absorption layer
(3) Contact diffusion (4) Grain boundaries
Laser Scribe
Al
a-Si:H
TCO
Glass
(5) Series connection
Alam 2011 7

8. Equivalent circuit of thin film solar cells
IT
Idark
IT
RL
Iph Voc
RL
Pmax = Iph × Voc × FF
Iph
IR Id ISH = Iph − I0 (e qV / kBT − 1)
I
⇒ VOC
= ( kT q ) ln(1 + I ph
I0 )
Superposition does not hold …
8

9. Outline of the lecture
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 9

10. Basics of current flow
Wrong contact loss +Recombination loss
4 qυ
Jn ≠ Jn = 0
L
Jn = Jn − J p = 3qυ0 − 2qυ0
L L
J= J −J = J −J
L
n
L
p
L
n
R
n
= 4 qυ0 − 2qυ0 3 2 
= ×  υ0 − υ0 
6q
6 6 
4 2
= 6q × υ0 − 6q × υ0  γ L ,n γ L ,p 
6 6 =
qG ×  − 
γ L ,n γ L ,p  γ L ,n + γ R ,n + γ rec γ L ,p + γ R ,p + γ rec 
 
= qG × − qG ×
γ L ,n + γ R ,n γ L ,p + γ R ,p
Alam 2011 10

11. Basics of transmission over a barrier
−EB ,L kBT
γ L←R = Ae
−EB ,R kBT
γ R←L = Ae
Right Right
Left Contact Left Contact
Contact Contact
−EB ,L kBT
γ L = Ae γ R = Ae
−EB ,R kBT
Left Contact Device Right Contact
Alam 2011
11

12. Photocurrent without recombination
J ph  γ L ,nW γ L ,p 
=
qG ∫0 dx  γ L ,n + γ R ,n − γ L ,p + γ R ,p 
 
 
γ R = υ0 e= W dx  γ L ,n
− E ( W − x )/ kT γ R ,n 
∫0  γ L ,n + γ R ,n − γ L ,n + γ R ,n 
 
γ L = υ0  
W  υ0 υ0 e − E(W − x )/ kT 
= ∫0
dx 
 υ0 + υ0 e − E ( W − x )/ kT
− 
υ0 + υ0 e − E(W − x )/ kT 
 
2L W  2L W 
RL = D log cosh
W× ≅ W  D − coth 
kT W 2 LD  W 2 LD 
LD ≡
E
‘Price length’ and point of no return ….
Alam 2011 12

13. Properties of ‘Sokel’ photo-current
Sokel and Hughes, JAP, 53(11), 1982.
J ph 2L W γL  γR
= W × D log cosh
qG W 2 LD
VA=0
 2 LD W 
≅W − coth 
 W 2 LD 
γL = γR
Jn
qG Vbi=VA>0
Vbi
VA γL = γR
Vbi γL < γR
VA>Vbi=0
Voltage dependent photocurrent, different from Si p-n junction …

14. Blocking layer and photocurrent
J ph W  γ L ,n γ R ,n  γR ~ 0
qG ∫ 0
dx 

− 
γ L ,n + γ R ,n γ L ,n + γ R ,n  VA=0
 
J ph = qGW
J ph
qG
Vbi
γR ~ 0
VA Vbi=VA>0
With blocking layer …
Blocking is essential for many types of thin film PV ….
14

15. Photocurrent with recombination
Crandall, JAP, 54(12), 19823
 n ≡ υ0 × τ n = µn × E × τ n
J ph W  γ L ,n γ L ,p  W
∫ dx  − = ∫ dx e − x /=  c 1 − e −W /  c 
 
c

  
qG  γ L ,n + γ R ,n + γ rec γ L ,p + γ R ,p + γ rec 
0 0
 
J ph
qG
{  }
= W − W −  n 1 − e −W /  n 

15

16. Photo-current in crystalline cells
with electron mirror without electron mirror
J ph J L ,n JL ,p
= − =  n 1 − e −W /  n  ~  n
   n ≡ Dnτ n  W
qG qG qG
Voltage independent photocurrent is unaffected by electron mirrors
Electron blocking layer does suppress dark current, increases Voc.
16

17. Numerical validation: Effect of blocking layer
With blocking Without blocking
Dτ  W Dτ  W
For low quality Si PV, blocking is not essential 17

18. Photocurrent with field/recombination
J ph W υ0 e − x /  n − υ0 e −(W − x )/  k e −(W − x )/  n 
= ∫ dx  
qG 0

 υ0 + υ0 e −(W − x )/  k 

W  e − x /  n − e −( W − x )/  k e −( W − x )/  n 
= ∫ dx  − ( W − x )/  k 
0
 1+ e 
 2 LD
J ph W 
≅W − coth 
qG  W 2 LD 
nph kT
LD ≡
E
Matches with numerical simulation very well …
Alam 2011 18

19. Outline of the lecture
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 19

20. Dark current without recombination
γL γR
Jn qnLυ0 − qnRυ0
γL + γR γL + γR
nL ,0 nR ,0 = γ R ,0 γ L ,0 ND NA
−E
= Ae − qVbi
k T
γ L ,0 Ae B ,0 B kB T
nR ,0 = ni2 N A
γ R ,0 = A ×1 ⇒ γ L ,0 γ R ,0 = e −qV bi kB T
nL ,0 = ND
nL ,0 = nR ,0 e + qVbi kB T
Vbi
ni2 − qVbi kB T
nR ,0 = ni2 N A nL ,0 = e
NA ER ←L = qVbi
Alam 2011
EL ←R = 0
20

21. Calculating dark current without recombination
γ L = Ae −q(V bi − V ) kB T
γL γR
Jn qnL ,0υ0 − qnR ,0υ0 γ R= A ×1
γL + γR γL + γR ni2 + qVbi kB T
nL ,0 = e
qυ0 NA
Jn
γL + γR
( nL ,0γ L − nR ,0γ R )
qυ0 ni2 qVbi
e kB T − q ( Vbi −V )/ kB T
e − 1
e − q ( Vbi −V )/ kB T
+ 1 NA   Vbi
ni2  µn (V − Vbi ) / d  qVb
 + q(V −Vbi ) / kBT  e − 1
kB T
q
NA e + 1  
 ni2 ni2   µn (V − Vbi ) / d  qV
  + q(V −Vbi )/ kBT  e − 1 ≡ I0 e qV − 1
kB T kB T
Jd = Jn + J p = q  +
 N A ND   e
  + 1    
21

22. Theory and practice of thin film dark IV
Theory Experiment
Id ≡ I0 e qV

kB T
− 1

Butterfly
wings!
A real solar cell IV seldom looks like textbook IV!
These wings helped create many complicated models.
22
Alam 2011

23. Contact diffusion and shunt conduction
(2) Thin doped region
(1) Thin
absorption layer
(4) Grain boundaries
(3) Contact diffusion
A real solar cell IV seldom looks like textbook IV 23

24. Parasitic shunt leakage
@ low voltage
shunt
@ high voltage
intrinsic
Dongaonkar and Alam JAP, 2010 24

25. Interpretation of ‘shunt’ leakage
J n = qnµn E Va =
2 2J
L
3
2
3 εµn
dE qn
= 9εµn
dx κε 0 J ( Va ) = Va2
8L3
V δ +1 * EA
Ishunt = Aµ 2δ +1 γ=
kT
L
* G. Paasch et al., JAP 106, 084502 (2009) 25

26. Features of shunt leakage
V δ +1 Symmetry Exponents
Ishunt = Aµ 2δ +1
L
L-3
Dongaonkar and Alam, EDL, 2010 26

27. Outline of the lecture
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 27

28. Contact diffusion and shunt conduction
(2) Thin doped region
(1) Thin
absorption layer
(3) Grain boundaries
(4) Contact diffusion
28
Alam 2011

29. L I
Variability and weak diodes
I0 ≈ Iph (Voc ) e − qVoc / kT
= Iph (Voc )(e ( oc ) − 1)
q V −V / kT
I
l 2 × n × Iph (Voc1 )(e ( oc1 ) − 1)
q V −V / kT
= Iph (Voc 2 )(e ( oc 2 ) − 1)
q V −V / kT
= L2 l 2 ≈ e ( oc1 oc 2 ) !
q V −V / kT
n
I I0 (e qV / kT − 1) − Iph (V )
I0 (e qVoc / kT − 1) = (Voc )
Iph Like an impact crater, a single um-sized
weak diode can drain away 1-10 mm region !!
Voc = kBT × ln(1 + Iph I0 )
(karpov, PRB 69, 045325, 2004.) 29

30. (5) Series connection, shadow degradation,
and a very weak diode
30
Alam 2011

31. Being in shadow stresses the device
Shaded
Load line panel
Shaded cell operating
operating point
point
Initial cell Initial panel
operating point operating point
Shaded cells can get reverse biased!
31

32. Shadow degradation
Dongaonkar et al.
IRPS 2011
Dark current
Normal time
operation
Shadow Operating
degradation voltage
32

33. Light induced degradation
Y. Nakata, et al., JJAP, 1992
Slope – t1/3
T. Shimizu, JJAP, 2004
D. L. Staebler, et al., APL, 1977.
Ongoing discussion about
exponent n~1/3
33
M. Stutzmann, et al., PRB, 1985

34. Reaction-Diffusion Model for LID
G
H2
Reaction: 1011
Density of DBs [cm-3]
dNDB
= kF N0 G - kR NDB NH ~0 1010
dt
Free H Generation: Slope n ~0.3
109
dNH dNDB
= - kH NH 2
dt dt
108
2 /3
 kf N0G  10-4 10-2 100 102
NDB ∝ ( 3kH )
1/ 3 1/ 3
 t
 k   LS Time [sec] 34
 r 

35. Light induced degradation in crystalline PV
metal finger
n+ emitter ~ 100 nm
p absorber ~ 100 µm
p+ electron mirror ~ 100 nm
back reflector
Boron-doped Czochralski (Cz) crystalline PV equally
susceptible to LID. Float-zone and/or Ga doping better.
Alam 2011 35

36. Extrinsic and Intrinsic Solar Cell Reliability
• Electrochemical
Intrinsic
• Light Induced
Extrinsic
corrosion
Degradation • Moisture Ingress
• Hot spot • Glass fracture
breakdown
• Inverters reliability
• Shunt leakage
• Delamination
• Shadow degradation
• Improper insulation
• Weak diodes
• Bypass diode failure
Source - www.nrel.gov/pv/performance_reliability/pdfs/failure_references.pdf 36

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 37

38. Reference for images
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.html
http://www.solarthinfilms.com/active/en/home/photovoltaics/photovoltaics_and_thinfilms/thinfilm_photovoltaics.html
http://gotpowered.com/2011/ge-develops-thin-film-photovoltaic-panels/
http://kypros.physics.uoc.gr/images/web2.gif
38