| 1 plastic solar cells m. a. loi zernike institute for advanced materials university of groningen,...
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| 1
Plastic solar cells
M. A. Loi
Zernike Institute for Advanced Materials
University of Groningen,
The Netherlands
e-mail [email protected]
| 2
Overview
1st hourSolar cells in general Solar Radiation p-n junction The organic version
2nd hour Improving plastic solar cells Low band-gap polymers Charge transfer states is detrimental?
| 3
Solar Cells I
› Long duration power supply
• Satellites
• Space vehicles
• Remote locations on earth
› Valid alternative to fossil fuels
› Pollution free
| 4
› Photovoltaic effect
• Becquerel (1839)
• Fritts {Selenium} (1883)
• Ohl {semiconductor junction solar cell}(1946)
• Chapin, Fuller, Person {Silicon p-n junction solar cells} (1954)
Solar Cells II
| 5
Motivations
› ENERGY Increasing energy need
Exhaustion of fossil fuels
Diversification of energy sources
Energy for all (2 billion people without electricity)
› ECOLOGY Pollution of environment
CO2 Responsible Climate
change
› ECONOMY Energetically independent
| 6
Solar Radiation› Every second in the sun
6 x 1011 kg H2 → He + 4 x 1020 J ☼
› At the average distance of the earth the solar radiation is 1353 W/m2
› The atmosphere attenuates the solar radiation
• Absorption water - IR
• Absorption Ozone – UV
• Scattering
Air Mass
| 7
Air Mass
› Air mass = the path length of the light from a celestial source relative to that at the zenith at sea level.
› increases as the angle between the source and the zenith increases (AM38 at the horizon).
› Out of the atmosphere AM0
› On earth surface with sun at the zenith AM1
› Average for terrestrial applications - 45˚ from the zenith AM1.5
AM= sec zenith angle
| 10
Solar cells – inorganic case
› Single bandgap material
• Photons with h<Eg lost energy
• Photons with h=Eg used energy
• Photons with h>Eg (h-Eg) lost
energy
Illuminated p-n junction
| 12
Ideal solar cell
IL current produced by solar radiation
Is diode saturation current
RL load resistance
LkTqV
s IeII )1( /
Shockley diode equation
kTE
n
n
Ap
p
DVCs
geD
N
D
NNAqNI /11
A device area
| 13
IV characteristics
oc
Lsc
V
II Short circuit current
Open circuit voltage
LkTqV
s IeI oc )1(0 /
s
L
s
Loc I
I
q
kT
I
I
q
kTV 1ln
| 15
IV characteristics-realistic
Shunt resistance – leakage current
SH
SSsL
SHDL
R
IRV
kT
IRVqIII
IIII
1)(
exp
Series resistanceJunction, impurity concentration
| 16
IV characteristics-realistic
SH
SSsL
DL
R
IRV
kT
IRVqIII
III
1)(
exp
The effect RSH is negligible
Rs in Si solar cells 0.7-0.4
| 17
Conversion efficiency
› FF; IL; Voc should be maximized for efficient solar cells!
ocL
mm
VI
VIFF Fill factor
Conversion efficiency
in
ocL
in
mm
P
VIFF
P
VI
EQE or IQE, quantum efficiency-percentage of photon converted in carriers (ISC)
| 21
Pro & conAdvantages
› tailoring of opto-electronic properties
› large areas
› low temperatures (RT)
› processing from solution
› roll to roll manufacturing
› light weight
› transparent
› low cost…….maybe…
Power paint?
| 22
Problems
› low ambient stability
› strongly bound excitons (Frenkel like)
› Exciton diffusion length rather short 5-20 nm.
› low mobility of charge carriers
•μn (c-Si) > 1000 cm2/Vs
•μh (polymer) ≈ 0.1 cm2/Vs
› difficult to obtain low band-gap materials
Pro & con
| 24
-
+
-
+
Frenkel exciton Stronghly bound (0.4 eV in PPV); radius 5 Å
Molecola-
+
Charge Transfer exciton
Polarons
Molecular semiconductorsMolecular semiconductors• coulomb interaction • elettron-phonon coupling
To start - photoexcitations
| 25
Triplet excitons
Frenkel excitons
Ground state
Flu
ores
cenc
e
Intermolecular excitons
non radiative states
Non
rad
iativ
e-em
issi
on
Phos
phor
esce
nce
| 26
The first examples
› Early works inspired by nature (photosynthesis)
› Porphyrins, phthalocyanines, perylenes (xerography), merocyanines
› Organic heterojunction devices: p-type / n-type organic semiconductors
› – 1970’s until 1995: organic heterojunction bilayers
› – 1985 Tang cell: PTCBI (45 nm) and CuPc (25 nm)
› 1% efficiency
| 28
The polymer approach!
› Active layer:
bulk heterojunction - hole conducting material - electron conducting material
› Operation principle:
• Exciton photoexcitation
• Diffusion of the excitons towards the organic-organic interface
• Charge separation/electron transfer
• Transport of charge carriers towards the electrodes
| 29
Photoinduced Charge Generation
MDMO PPV 3,7 - dimethyloctyloxy methyloxy
PPV
PCBM1-(3-methoxycarbonyl) propyl-1-
phenyl [6,6]C61
O
O n
DONOR ACCEPTOR
N. S. Sariciftci et al., Science 258, 1474 (1992)
An ultra-fast e- transfer occurs between Conjugated Polymer / Fullerene composites upon illumination. The transition time is less than 40 fs.
exciton
Back transfer very slow! s - ms
| 30
The driving force!
› Electron affinity fullerene derivatives!
OMe
O
PolymerPCBM
-6 eV-5.2 eV
-4.2 eV
-3.5 eV
| 31
Bulk Heterojunctions
h MDMO-PPVPCBMPCBM
e-
ITO on Glass / Plastic
e-
P+
e-
e-
e-
e-
e-
P+
Al Electrode
Al Electrode
e-
| 32
P-Solar Cells - FILM PREPARATION
Spin Casting is a easy coating technique for small areas. Material loss is very high. Doctor Blade Technique
was developed for large area coating
Doctor Bladehas no material loss
| 34
Plastic Solar Cells - CONTACTING
The cathode electrode is
applied by evaporation.
Different electrodes are used
for different applications.
Sealing is absolutely necessary for anincreased life time of plastic solar cells.
| 36
Bulk Heterojunctions
h MDMO-PPVPCBMPCBM
e-
ITO on Glass / Plastic
e-
P+
e-
e-
e-
e-
e-
P+
Al Electrode
Al Electrode
e-
| 38
Now..
Organic solar cells performances depend on the material properties and microscopic structure of the bulk heterojunction!
P3HT
4,5-5.0 %
> 60 polymers checked last 5 years!
| 39
Optimization
eff = Isc * Voc * FF / Iinc
Isc Tuning of the Transport Properties - Mobility
Voc Tuning of the Electronic Levels of the Donor Acceptor
Systems
FF Tuning of the Contacts and Morphology
Iinc Tuning of the Spectral Absorbance/Absorbing more
light (low bandgap)
| 42
Organic Solar cells
Polymer(donor)
PCBM(acceptor)
Power conversion efficiency ~ 5 - 6%
bulk heterojunction
bulk heterojunction3D heterostructure
hole conducting material
+electron conducting material
| 43
Remember-Organic Solar Cells› Working mechanism-steps
• Excitons photoexcitation
• Diffusion of the excitons towards the interface
• Charge separation/electron transfer
• Transport of charge carriers towards the electrodes
› Organic solar cells performances depend on
• the material properties
• the microscopic structure of the bulk hetero-junction
| 44
The driving force!
› Donor and acceptor LUMO energy offset!
OMe
O
PolymerPCBM
-6 eV-5.2 eV
-4.2 eV
-3.5 eV
Ultrafast phenomena!
| 45
Enhancing devices efficiency
› Optimize the materials properties• Matching solar spectrum! NIR materials
• Relative position of the energy levels of the donor and acceptor
optimal offset between LUMO (D) – LUMO(A)
for electron transfer at least 0.3 – 0.5 eV
P3HT:PCBM: LUMO (D) – LUMO(A) ~1.1 eV
› Optimize the morpholog• microscopic phase separation
( exciton diffusion length ~ 5 – 7 nm )
• presence of a percolation pathway
| 46
Remember-Solar cells parameters
› JSC – short-circuit current
› Jph – photocurrent
› FF – fill factor:
› VOC – open circuit voltage
SCOC JV
JVFF
maxmax
light
SCOC
lightin
out
P
JVFF
P
P
P
P max
› power conversion efficiency
LUMO (A)3
4
5
6
Don
or
Acc
epto
r
En
ergy
(eV
)
Voc
HOMO (D)
LUMO (D)
| 47
En
ergy
(eV
)
P3HT:PCBM
The reduction of the LUMO offset
• power conversion efficiency ~ 3.8 %• LUMO offset ~ 1.1 eV• Voc~ 0.59 V
Voc
3
4
5
6
Don
or
Acc
epto
r
bisPCBM
• LUMO offset ~ 1.0 eV• Voc~ 0.73 V
Power conversion efficiency ~ 4.5 % !!!
M. Lenes et al, Adv. Mater. 2008, 20, 2116
| 48
PL of thermally annealed films
The devices performance:
P3HT:PCBM – 3% P3HT:bisPCBM – 3.6%
electron transfer is more efficient for P3HT:PCBM
P3HT:bisPCBM – PL ≈ 60 ps P3HT:PCBM – PL ≈ 41 ps
| 49
PL of solvent annealed films
The devices performance:
P3HT:PCBM – 3.8% P3HT:bisPCBM – 4.6%
electron transfer is more efficient for P3HT:PCBM
P3HT:bisPCBM – PL ≈ 38 ps P3HT:PCBM – PL ≈ 31 ps
| 50
AFM measurementsP3HT:PCBM P3HT:bisPCBM
spin coated
slow dried
3.9 nm
12.4 nm10.7 nm
4.6 nm
• surfaces is smoother for
samples prepared by
thermal annealing
• difference in RMS
roughness between
P3HT:PCBM and
P3HT:bisPCBM
10x10m
| 51
The blend in solution
P3HT:PCBM & P3HT:bisPCBM
PL ≈ 156 ps
→ the efficiency of the electron transfer is the same in both blends
| 52
Conclusion 1› Increasing power conversion efficiency by tailoring
the energy levels
• P3HT:bisPCBM – higher Voc
• P3HT:bisPCBM – higher power conversion efficiency
• P3HT:PCBM – faster PL decay in the thin film
• In solution – the same PL decay
› A small reduction of the LUMO offset does not have a significant influence on the electron transfer
› The P3HT:bisPCBM blend is limited by diffusion -the morphology can be still optimize
| 53
-
+
-
+
Frenkel exciton Stronghly bound (0.4 eV in PPV); radius 5 Å
Molecule-
+
Charge Transfer exciton
Polarons
Molecular semiconductorsMolecular semiconductors• coulombic interaction • elettron-phonon coupling
Remember-Photoexcitations
| 54
Intermediate state?
M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007)
CT like intermediate states
are considered for
modelling IV of solar cells
V.D. Mihailetchi et al., PRL (2004)
Recent reports consider the energy transfer from the polymer to the PCBM as the first step of the charge separation
OMe
ONN
S
R
R
*
*
n
*RR
S NSN
S*
n
| 55
OMe
O
PCBM
NN
S
R
R
*
*
n
F8BT
10-4
10-3
10-2
10-1
100
101
102
-0,4 -0,2 0 0,2 0,4 0,6 0,8 1
Ph
oto
curr
ent
(mA
/cm
2 )
Voltage (V)Very poor PV performances!!
Energy transfer?
| 56
450 500 550 600 650 700 750 800 850
PCBM PLcorr
P2+66%PCBM PLcorr
P2+33%PCBM PLcorr
P2+20%PCBM PLcorr
P2+0.5%PCBM PLcorr c
P2 PLcorr c
Pho
tolu
min
esc
en
ce
Wavelength (nm)
F8BT
0,5% PCBM
20% PCBMPCBM
33%PCBM
66%PCBM
M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007)
OMe
O
PCBM
NN
S
R
R
*
*
n
F8BT
Energy transfer?
400 500 600 700 800
Pho
tolu
min
esce
nce
Wavelength (nm)
F8BT
0,5% PCBM
20% PCBM
PCBM
33%PCBM 66%PCBM
| 57
0 500 1000 1500 2000
F8BT @ 530 nm
66%PCBM @ 520nm
PCBM @ 720nm
66%PCBM @ 720nm
Pho
tolu
min
esce
nce
Time (ps)
Energy Transfer?
The polymer PL
decay becomes very
fast upon PCBM
blending
Energy transfer?
| 58
-10 0 10 20 30 40 50
PCBM-TR1 Nlaser N66%F8BTP
hoto
lum
ines
cenc
eTime (ps)
No clear evidence!
Long rise-time also in pristine PCBM
M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007)
Energy transfer to the PCBM singlet state then transferred to the triplet state.
S. Cook et al., APL (2006)
| 59
Electron transfer
*RR
S NSN
S*
n
OMe
O
F8DTBTPCBM
~4%
Polyfluorene copolymers
promising low-band gap
materials for PV
applications 10-4
10-3
10-2
10-1
100
101
102
-0,4 -0,2 0 0,2 0,4 0,6 0,8 1
Ph
otoc
urr
ent
(m
A/c
m2 )
Voltage (V)
M. Svensson et al., Adv. Mat. 15, 988 (2003);Q. Zhou et al., Appl. Phys. Lett. 84, 1653 (2004);F. Zhang et al., Adv. Funct. Mater. 16, 667 (2006)
| 60
OMe
O
PCBM 550 600 650 700 750 800 850
Pho
tolu
min
esce
nce
Wavelegth (nm)
F8DTBT
0.5% PCBM
10%PCBM20% PCBM
66% PCBM
*RR
S NSN
S*
nF8DTBT
| 61
Charge transfer excitons
550 600 650 700 750 800 850
Pho
tolu
min
esce
nce
Wavelegth (nm)
F8DTBT
5% PCBM
20% PCBM
PCBM 33%PCBM 66%PCBM
Red-shift with the increasing of the average dielectric constant
ε(PCBM) ~ 3.9;ε(Polymer) ~ 2.5-3.0
222
4
2 n
eEE gn
Rydberg-like transitions
| 62
PCBM1 = 320 ps; 2 = 3.1 ns
PCBM1 = 350ps; 2 = 1.0 ns
M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007)
0 500 1000 1500 2000
F8DTBT @ 630nm
66%PCBM @ 630nm
F8DTBT @ 730nm
66%PCBM @ 730nm
PCBM @ 820nm
66%PCBM @ 820nm
Pho
tolu
min
esce
nce
Time (ps)
| 63
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
300 400 500 600 700 800
PCBM
F8DTBT
66%F8DTBT+33%PCBM
33%F8DTBT+66%PCBM
66%F8DTBT+33%PCBM CALC
33%F8DTBT+66%PCBM CALC
Abs
orpt
ion
Coe
ffici
ent (
105 c
m-1
)
Wavelength (nm)
OMe
O
PCBM
*RR
S NSN
S*
n
F8DTBT
Ground state interaction?
| 64
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
300 350 400 450 500 550 600
PCBMF8BT66%F8BT+33%PCBM33%F8BT+66%PCBM66%F8BT+33%PCBM CALC33%F8BT+66%PCBM CALC
Wavelength (nm)
Abso
rptio
n C
oeffic
ien
t (1
05 c
m-1
)OMe
O
PCBM
NN
S
R
R
*
*
n
F8BT
| 65
Charge separation
There is an intermediate state between the Frenkel exciton and the free charge!
| 66
Is it general?Are more systems showing this phenomena?Typical of narrow band gap polymers?
D. Muehlbacher et al. Adv. Mater. 2006
Solar cells efficiency: =3.2 %
FIRST narrow band-gap O-semiconductor
PCPDTBT
nSS
NS
N
*
*
| 68
Concentration dependence
0 PCBM
1/11 PCBM1/3 PCBM
1/2 PCBM
Charge Transfer Exciton
(1,0)
(10,1) (2,1)(1,1)
| 69
New excited state in the blend with long decay time!
0 PCBM= 100ps
1/11 PCBM1 = 50ps2 = 1190ps
1/3 PCBM = 510ps
1/2 PCBM = 478ps
1/2 PCBM= 5ps
(ns)
0 PCBM= 150ps
Exciton CT Exciton
| 71
Conclusions II
Evidences of an excited state intermediate between
the exciton and the free carriers in heterojunctions
containing narrow band gap polymers – present also in
working devices!
Additives can reduce the CTE component acting on
the microstructure of the blend.
The suppression of the intermediate states in bulk
hetero-junctions is extremely important for the
optimization of organic solar cells. PCPDTBT/PCBM 3.8%
→5.5%