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Semitransparent Organic Solar CellsTam Hoi Lam, Wu Zhenghui and Furong Zhu
Department of Physics and Institute of Advanced M t i lMaterials
Hong Kong Baptist University
UNSW, 30 Aug, 2012
Outline� A glimpse of photovoltaic technologies
Outlineg g
� ITO-based transparent cathode
� Optical enhancement in semitransparent
organic solar cells
� Applications of semitransparent OSCs
� Summary
2
Photovoltaic technologiesPhotovoltaic technologiesc-Si, poly c-SiIII-V semiconductor (GaAs InP
BulkBulkIII V semiconductor (GaAs, InP
Thin film c-Si, a-Si, hybrid SiII-VI semiconductor film (CdTe)
Solarcells
Thin FilmThin Film( )
CuInSe2, Cu(InGa)Se2
cellsOrganicOrganic
Solid dye-sensitized solar cellsOrganic/inorganic hybridOrganic (polymer & small molecule)
NanoNano--t t dt t d
Nano-particle, nano-wire & quantum dot-based solar cellsstructuredstructured based solar cells
3
Si & thin film solar cellsSi & thin film solar cells
4
Other PV technologiesg
TypeBenefit or intended benefit (all are light weight
d t ili )
Efficiency at which it is i bl
Challenges
compared to silicon) viableCIGS High efficiency at low cost, long life,
transparent , no disposal problems10-15% i.e. now Price of indium
F i l hi h ffi i (b l CIGS 10 15% i Cd i i
CdTe
Fairly high efficiency (below CIGS,above others). Well
10-15% i.e. now for buildings
Cd is a poison, controlled disposal only, so not allowed in consumer goods
t t h t i tetc, tough to print,supply of Te?
DSSCTolerant of polarized light, low level light, transparent, extreme angle of
10-15% i.e. now LiquidsPrice of ruthenium?DSSC
incoming light, choice of colors, no disposal problems
OSCLow cost, huge materials varieties, tunable electronic/optical properties
8-10% Cost, efficiency, stability narrow
5
OSC tunable electronic/optical properties stability, narrowspectrum
Limitations in OSCsNarrow absorption band
0.8
P3HT:PCBM (1:0 8)
Solar spectrum and available photocurrent
0.4
0.6
orpt
ion
(a.u
.)
P3HT:PCBM (1:0.8)
300 400 500 600 700 8000.0
0.2
Abs
o
• It limits the absorption and thus short circuit current density• Low carrier mobility ( 10-6 to 10-3 cm2/Vs)
Wavelength (nm)
• Low carrier mobility (~ 10-6 to 10-3 cm2/Vs)– Active layer cannot be too thick <300nm– Fill factor affected due to the large series resistance
6
Limitations in OSCsLimitations in OSCsInefficient utilization of the solar energy by current available materials.
4
6
Outx 10
-18 )
0.15
0.20
t
FF < 0.7 Voc < 0.65 VMe
O
O
2
4
Elec
trons
O
Phot
ons
In (x
0.05
0.10
Pow
er O
u
Pow
er In
14 3% ~ 5 %P3HT
PCBMS n
500 1000 1500 2000 2500 3000 35000
Wavenlength (nm)500 1000 1500 2000 2500 3000 3500
0.00
Wavelength (nm)
14.3% 5 %
Possible approaches:• Low band gap photoactive materials • Tandem cells
7
Tandem cells• Light in-coupling features , light harvesting
Advantages of OSCsAdvantages of OSCs
• Solution processable, lightweight, translucent & flexible, potential low cost Power generation windows
• Roll to roll coating technologies could realize scalable production processscalable production processwith high productivity
• Less energy consumptionArchitectural surfaces
• Less energy consumptionduring fabrication process, less CO2 emission.
8 Flexible, transparent & decorative
Motivation and challenges• Perceived benefits of OSC technologies
L hi h l fl ibili li h i h
ot at o a d c a e ges
– Lower cost, higher volumes, greater flexibility, light weight,
size and shape
– New device concepts inverted translucent– New device concepts, inverted, translucent
– The best research OSC efficiency approaching 10%
• Technical challengesTechnical challenges– New semiconducting and conducting materials with tunable
electronic and optical properties
– Efficient charge collection properties at organic/electrode interface
– light harvesting solution processing stability thin filmlight harvesting, solution processing stability, thin film encapsulation…
9
Conversion stepsConversion step Incident photons Loss mechanism
- Reflection, Transmission- Spectral mismatch
- Exciton recombination
Light absorption
E citon generation Exciton recombination
- Excition transfer with recombinations - No charge separation and subsequent
bi ti f it
Exciton generation
Exciton diffusionrecombinations of excitons
- Recombination of charges- Limited mobility of charges
Charge separationy g
- Recombination near electrodes- Energy barrier at electrodes
Charge transport
Separated charges at electrodes10
OSC is a thin film systemOSC is a thin film systemJ=2
N2=n2-ik2E1+
E +
J=3N3=n3-ik3
E +
J=M-1NM-1=nM-1-ikM-1
E + E +
J=2N2=n2-ik2E1+
E +
J=3N3=n3-ik3
E +
J=M-1NM-1=nM-1-ikM-1
E + E +Incident
E1-E2+
E2-
E3+
E3-
E3+
E3-
EM+
EM-E1-E2+
E2-
E3+
E3-
E3+
E3-
EM+
EM-
light
J=1N1=n1-ik1
OPV
J=MN=n0
J=1N1=n1-ik1
N=n0
P3HT:PCBM
Cathode (Ca/Ag)
A
V
PEDOT:PSS
P3HT:PCBM
Cathode (Ca/Ag)
A
V
PEDOT:PSS HT
T:PS
S
O
Cat
hode
(Ca/
Ag)
4.2eV
3.7eV
3.0eVe-
HT
T:PS
S
O
Cat
hode
(Ca/
Ag)
4.2eV
3.7eV
3.0eVe-
Glass substrate AirAir
Schematic representation of a thin film OSC
GlassITO
Light
GlassITO
Light4.8eV
P3H
PCB
M
PED
OT
ITO
5.2eV4.9eV
4.2eV
6.1eV
P3H
PCB
M
PED
OT
ITO
5.2eV4.9eV
4.2eV
6.1eV
OCH3
e-
S n
3
O
P3HT PCBM
11
Optical constants
1.55
1.60
0.08
0.10E
xtincex, n
n_PEDOT k_PEDOT
Optical constantsAirAir
1.45
1.50
0 02
0.04
0.06
ction coefficienRef
ract
ive
ind
300 400 500 600 700 800 900 10001.40 0.00
0.02 nt, k
Wavelength (nm)
1 0 1 0
SubstrateSubstrate
2.0
2.2
0.6
0.8
1.0
Extinctionin
dex,
n
n_P3HT:PCBMk_P3HT:PCBM
2.4
2.8
0.6
0.8
1.0
Extinction
e in
dex,
n
n_CuPc:C60 k_CuPc:C60
1.6
1.8
0.2
0.4n coefficient, k
Ref
ract
ive
i
1.6
2.0
0 0
0.2
0.4
n coefficient, k
Ref
ract
ive
12
300 400 500 600 700 800 900 10000.0
k
Wavelength (nm)
300 400 500 600 700 800 900 10000.0
Wavelength (nm)
Interlayer:Interlayer:1) Thin metal layer Catho
deEBL
• Discontinuous and high reflectivity in NIR
2) Oxide/organic interlayer
Anode contact
SC Blend
) O de/o ga c e aye
• Limited lateral conductivity
3) TCO electrode is suited for application in Transparent SubITO
PedotSC Blend
RZ
3) TCO electrode is suited for application in
• Organic, inorganic and hybrid solar cells
• Simple, effective and low cost technology
• Greater fabrication flexibility
13
High performance TCO (ITO)• Low resistance, high transparency with smooth surface
C ibili f d i f b i i i h d h
High performance TCO (ITO)
• Compatibility of device fabrication with no damage to the
underlying functional organic layers
• Excellent delineation and adhesion capabilities
• Size, shape and substrate flexibility
• Lower cost and higher volumes
• Emerging new applications:Emerging new applications:
Flexible electronics including FOLED displays, lighting,
OSC organic sensorsOSC, organic sensors …
14
Low processing temperature ITOp g p
5
6
6
7 Sheet resistivity Mobility Carrier concertration 70
3)
3
4
4
5
resi
stiv
ity(E
-04O
.cm
)
40
50
60
Mobility(cm
2/V.son
certr
atio
n(E+
20/c
m
1
2
0 1 2 3 4 52
3
hydrogen partial pressure(E 3Pa)
shee
t r
20
30
s) c
arrie
r co
hydrogen partial pressure(E-3Pa)
80
100
(%)
Cut off has a blue shift
20
40
60
Tran
smia
ttanc
e
H2 0 Pa (Eg: 3.87eV)
H2 1.1 x10-3Pa (Eg:3.92eV)
H 2 0 x10-3 Pa (E : 4 01eV)
Cut-off has a blue shift
15
200 400 600 800 1000
0
W avelength (nm)
H2 2.0 x10 Pa (Eg: 4.01eV)
H2 2.6 x10-3 Pa (Eg: 4.01eV)
Low processing temperature ITO• Deposition at a temp < 60OC
Low processing temperature ITO
• TCO (ITO, AZO, IZO)/PC & PET etc.
• T(� ) over 85%
R ~ 25 / ~130 f ITO• Rs~ 25 �/sq, ~130nm, for ITO
• Smooth surface, rms < 1.0 nm
• Low stress• Low stress
• High etching rate
• No damage to the underlying organic/polymerNo damage to the underlying organic/polymer materials
• Applications: top-emitting OLEDs, FOLED, t d & it tOSC i t t d
16
tandem & semitransparent OSCs, integrated org electronic devices …
St k d OSC 1 8 2 0Stacked OSCs1.01.21.41.61.8
1 0
1.5
2.0
coef
ficie
nt (a
.u.)
nce
(Wm
-2nm
-1)
AM1.5 Global P3HT:PCBM CuPc:C60Top Cell
Bottom Cell
Top cell
Cathode
0 00.20.40.60.8
0 0
0.5
1.0
rmal
ized
abs
orpt
ion
Spec
tral I
rradi
an
Bottom Cell
Interlayer300 400 500 600 700 800 90010001100
0.0 0.0
No
W avelength (nm)
SubstrateITO
� An increased Voc, photocurrent matching required
� Thinner active layer for better charge transport
E i l
17
� Extensive spectral response
Integrated absorptancein
nm
e in n
m
e in n
m
e in n
m
e
Absorptance contour mapping
60e (%
)
3%
ITO/PEDOT:PSS/P3HT:PCBM/Ca/Ag
teg ated abso pta ceer
thi
ckne
ss
Abs
orpt
anc
er t
hick
ness
Abs
orpt
anc
er t
hick
ness
Abs
orpt
anc
er t
hick
ness
Abs
orpt
anc
40
50
abso
rpta
nce
Ca/AgCa/Ag40%
53%
Laye
Laye
Laye
Laye
Wavelength (nm)
10
20
30
grat
ed li
ght P3HT:PCBM
PEDOT:PSS
ITO
Glass
P3HT:PCBM
PEDOT:PSS
ITO
GlassIntegrated absorptance of P3HT:PCBM layer:
0 50 100 150 200 250 300
10
Inte
g
P3HT:PCBM thickness (nm)���
��
����
dF
dFAA PCBMHTP
PCBMHTP)(
)()()( :3
:3
• Maximum absorption for P3HT:PCBM based OSCs occurs at 75nm, 200nm and 250nm.
• Oscillation absorption behavior � interference effect• Oscillation absorption behavior � interference effect.
18
Light distribution1.4
PSS
rAlBM
Os(%)
0.8
1.0
1.2
PED
OT:
P
AiA
P3H
T:PC
B
ITO
Gla
ss
nsity
, |E
|2
Interference40
50
60ab
sorp
tanc
e 53%
40% Cat
hode
0 2
0.4
0.6
Opt
ical
den
Interferenceeffect dominates
10
20
30
egra
ted
light
1.41.6
Air
Gla
ss Al
CB
M
T:PS
S
ITO 1.6
1.8
AirAl
CB
M
T:PS
S
ITO
Gla
ss
-100 0 100 200 300 400 500 6000.0
0.2O
75nm0 50 100 150 200 250 300Inte
P3HT:PCBM thickness (nm)
0.81.01.21.4 G
P3H
T:PC
PED
OT
tens
ity, |
E|2
0 81.01.21.4
P3H
T:PC
PED
OTG
200250nm
Cat
hode
Cat
hode
0.20.40.6
Opt
ical
int
0.20.40.60.8 200nm250nm
19 Appl. Phys. Lett. 90 (2007) 103505.
-100 0 100 200 300 400 500 6000.0
Position, x (nm)-100 0 100 200 300 400 500 600
0.0
Semitransparent OSCsSemitransparent OSCs- two competing performance indexes: T(�) & A(�)
Trans 50 50
In
P3HT:PCBM
interlayerITO
�40
45
50
40
45
50 tegrated lignce
(%)
Total transmittance Integrated absorptance
Glass
ITOPEDOT:PSS
P3HT:PCBM �P3HT:PCBM
30
35
30
35
ght absorptTran
smitt
a
0 20 40 60 80 100 120 140 16020
25
20
25
tance (%)
T
ITO cathode thickness (nm)
• ITO acts as refractive index matching layer to enhance the T(�)
• ITO improves the lateral conductivity of the top cathode
ITO cathode thickness (nm)
20
Appl. Phys. Lett. 90 (2007) 103505.
Semitransparent OSCsDevice characteristics
15
cm2 ) 200nm P3HT:PCBM (A)
75nm P3HT:PCBM (B)Semitransparent OPV (C)
60
7063%
- Device characteristics
10
ensi
ty (m
A/c Semitransparent OPV (C)
30
40
50
40%
59%
PC
E %
0
5
Cur
rent
De
0
10
20
30
200nm P3HT:PCBM (A)75nm P3HT:PCBM (B)Semitransparent OPV (C)
IP
0.0 0.1 0.2 0.3 0.4 0.5 0.60
Voltage (V)
Devices V (V) J (mA/cm2) FF% PCE%
300 400 500 600 7000
Wavelength (nm)
Devices Voc (V) Jsc (mA/cm2) FF% PCE%A 0.57 14.77 34 2.8B 0.57 8.22 43 2.0C 0 54 6 67 51 1 83C 0.54 6.67 51 1.83
21
Semitransparent OSCs: ITO/Z P C /C /BPh /A /ITO(40 )ITO/ZnPc:C60/C60/BPhen/Ag/ITO(40nm)
(b)
10
20 Dark Semitransparent cell Control cell
cm2 )
50
60
70%
)
Calculated Measured
(a)
IEEE J, Selected Topics in Quantum 10
0
10
t den
sity
(mA
/c
20
30
40
50
ansm
ittan
ce (%
Electronics, 16 (2010) 1685.
LiF
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-20
-10
Cur
rent
Voltage (V)300 400 500 600 700 8000
10
Tra
Wavelength (nm)
Device�VOC��
(V)
FF��
(%)
JSC
(mA/cm2)
��
(%) 1.0
1.5LiFBphen
(V)� (%)� (mA/cm ) (%)
Control�device� 0.56� 60�±1� 11.8 ±�0.3 4.0�±�0.2
Semitransparent�
photovoltaic�cell�0.55� 57�±�1� 9.6�±�0.2� 3.0�±�0.1�
0.5
TOc:C
60
|E|2
220 50 100 150 200 250 300
0.0 Top
I
Ag
C60
ZnP
c
ITO
Position (nm)
Semitransparent OSCs: integrated OSC/OLEDSemitransparent OSCs: integrated OSC/OLED
Transparent cathode
TOPDOLED
TOLED
transparent cathode
TOPD
Reflective cathode
Transparent cathode
interlayerOLED
OSC
Al
0 8
1.0
unit) OPV IPCE
MOLED ELa)
60
80ITO anode
Transparent substrate
ITO anodeTransparent substrate
ITO anode
Type 1 Type 2
ITO/glassOSC
0.2
0.4
0.6
0.8 IPC
E (%
)
D in
tens
ity (a
rb.
20
40
60Type-1 Type-2
TOLED
Vs
OPD
Vs
OSCTOLED
ZnPc:C60-OSC/interlayer/Alq3:C545-OLED
300 400 500 600 700
0.0OLE
DWavelength (nm)
0
TOLED
RVIn
OPD
TR
VInT
TOLED
60 y
23
Org. Electron. 12 (2011) 1429
ITO/OSC-ZnPc:C60/interlayer/MOLED-Alq3:C545ITO/OSC ZnPc:C60/interlayer/MOLED Alq3:C545
Device EL peak position (nm)
FWHM(nm)
Luminous efficiency (cd/A)
Control OLED 526 32 10.5 (@ 4.5V)
OSC/OLED 521 33 11 5 (@ 4 5V)OSC/OLED 521 33 11.5 (@ 4.5V)
Device VOC (V) FF (%) JSC (mA/cm2) PCE (%)
Control OSC 0.56 58.9 11.4 3.76
OSC/OLED 0 55 57 2 10 5 3 30
Org. Electron. 12 (2011) 1429
24
OSC/OLED 0.55 57.2 10.5 3.30
SummarySummary• Semitransparent OSCs
– Optical admittance analysis
– Transparent electrode and optical coupling
M l i j i OSC WOLED li h i– Multi-junction OSCs, WOLED lighting,
optical sensors etc.
A li ti• Applications– Power generation windows
A hit t l f– Architectural surfaces
– Car windshield
Tension membrane
25
– Tension membrane
Thank you
26
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