d. cahen, weizmann inst. 02/’12 how good can solar cells be? assessing possibilities for solar...
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![Page 1: D. Cahen, Weizmann Inst. 02/’12 How good can Solar Cells be? Assessing Possibilities for Solar Cells by Identifying](https://reader033.vdocuments.net/reader033/viewer/2022051516/56649e395503460f94b2ab0e/html5/thumbnails/1.jpg)
D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
How good can Solar Cells be?Assessing Possibilities for Solar Cells
by Identifying Basic Limitations to PV Performance
THANKS to many COLLEAGUES, esp.
Pabitra Nayak Juan Bisquert, Un. Jaume, Antoine Kahn, Princeton Un.
Lee Barnea, Ron Milo + other Weizmann Inst. colleagues Bolko von Roedern, David Ginley, Keith Emery, Rommel Noufi
NREL, US National Renewable Energy Lab
Bernard Kippelen, Georgia Tech, Arie Zaban, Bar Ilan U.;K.L. Narasimhan, TIFR, 3G Solar & many others!
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
on
e e
lect
ron
en
erg
yon
e e
lect
ron e
nerg
y
spacspacee
Generalized picture•Metastable high and low energy states
•Absorber transfers charges into high and low energy state
•Driving force brings charges to contacts
•Selective contacts
(1) cf. e.g., Green, M.A., (1) cf. e.g., Green, M.A., Photovoltaic principles.Photovoltaic principles. Physica E, 14 (2002) Physica E, 14 (2002) 11-1711-17
The Photovoltaic (PV) effect:
High High energyenergystatestate
Low Low energyenergystatestate
AbsorberAbsorber
e-
p+
conta
ct
conta
ct
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
Types of PV Cells
Elemental Semiconductors Single or multi-crystal Polycrystalline Amorphous thin film
Inorganic Compound Semiconductors Single crystal Polycrystalline thin film
Organic, Excitonic (molecules, polymer) Polycrystalline Interpenetrating network Nanocrystalline;
dye-sensitized
Primarily based on solid-state electronic material systems
Si,Ge
(Ga,In)(As,P)Cu(In,Ga)Se2
CdTe
Phenylene-vinylidene, PCBM++
Ru-dye+TiO2
(non)concentrator;single-& multi-
junction
homo- &hetero-junction;photo-electro-
chemical;MIS
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
A schematic of a Solar Cell
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
Solar Cell (r)evolutions
1st generation
Si
cm cheaper? simpler?
2nd generation
CdTe, CIGS
poly-crystalline
mmicro-crystalline & amorphous
nano-crystalline
~ 20 nm
3d generation
TiO2
organic (polymer/ small molecules
nano-crystalline
~ 20 nm
3d generation
TiO2
nano-crystalline
~ 20 nm
3d generation
TiO2
organic (polymer/ small molecules
single- & multi-crystalline
Quantum dot Cells
4 4.5 6 15 nm
next generations
work horse
stabilization self-healing
self-assembly
&nano !!
great science
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
OUTLINE• PV cell performances today
• Limits of PV solar energy conversion
• Empirical guides to limits or possibilities
• Losses in “excitonic” cells
• Summary & Future
![Page 7: D. Cahen, Weizmann Inst. 02/’12 How good can Solar Cells be? Assessing Possibilities for Solar Cells by Identifying](https://reader033.vdocuments.net/reader033/viewer/2022051516/56649e395503460f94b2ab0e/html5/thumbnails/7.jpg)
D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
IN
OUT
PowerRadiativeSolar
PowerElectrical %100
Definition of efficiency:
Lowest Loss Laboratory PV cells (1-4 cm2 except for tandems):
Data from Solar Cell Eff #39, Progr in PV 2012
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
12.3%ETA cells
/ WIS
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
OUTLINE• PV cell performances today
• Limits of PV solar energy conversion
• Empirical guides to limits or possibilities
• Losses in “excitonic” cells
• Summary & Future
![Page 10: D. Cahen, Weizmann Inst. 02/’12 How good can Solar Cells be? Assessing Possibilities for Solar Cells by Identifying](https://reader033.vdocuments.net/reader033/viewer/2022051516/56649e395503460f94b2ab0e/html5/thumbnails/10.jpg)
D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
Efficiency(%) Manufacturer Technology (area, if < 600 cm2) BESTrated/minimum commercial
module /cell19.6 /?? SunPower Single-crystal Si non-standard jnctn
78% 17.1 / 16.3 Sanyo Single-crystal Si HIP jnctn 74% 15.2 / 14.6 Kyocera Multi crystal Si standard junction 75% 13.4 / 12.7 Evergreen EFG(ribbon) Si standard junction 77%
12.2 / 10.8 First Solar CdTe 71 % 13.1 / 11.2 Miasole CIGS 67 % 12.6 /11.4 Q-cells CIGS 64 %
6.3 / ?? Kaneka a-Si single junction * 66 %6.7 / 5.7 Uni-Solar a-Si, triple junction * 54 %
* stabilized values
5#,** 3GSolar dye (225) ~40 %3.9# Solarmer Organic polymer/ molecule (225)
~40 % # Pilot modules; few yrs stability; **not yet commercially available
Possibilities for Technological Progress
Nayak et al. Adv Mat. 2011 Module data from B v Roedern, NREL, 11/2011
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
Infra- visible ultra- -Red -violet
(IR) (UV)Solar Energy
Spectrum
Photovoltaic Conversion is a Quantum (threshold) Conversion
Process
WHY ?In Solar Cells Most Solar Energy
is “Lost” as Heat
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
e-
-voltage (qV)
Energy
e-
n-typep-type
h
h+
e-
useable photo-voltage (qV)
Energy
e-
n-typep-type
h
h+
Single p-n junction solar cell
O. Niitsoo
space
in Solar Cells Most Solar Energy is “Lost” as Heat
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
>Eg thermalized
< Eg not absorbed
Etendu; Photon entropy –TD~0.3eV @RT, lack of concentration
Carnot factor –TD
Emission loss- (current)
Electrical power out
Current – Voltage Characteristics
After Hirst & Ekins-DaukesProg.Photovolt:Res:Appl. (2010)
Losses in PV cell
0 1 2 3 40
10
20
30
40
50
60
70
80
Cur
rent
(m
A/c
m2)
Energy (eV)
Eg
Nayak et al. Energy Environ. Sci., 2012 (In Press)
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
Prince, JAP 26 (1955) 534Loferski, JAP 27 (1956) 777Shockley & Queisser JAP (1961)
Shockley-Queisser (SQ) Limit
photosynthesis
0.5 1.0 1.5 2.0 2.55
10
15
20
25
30
OPV
CIGS
c-Si
Eff
icie
ncy
(%
)
Band Gap (eV)
GaAs
InP
CdTe
DSCa-Si
SQ Limit
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
What can we do?
tandem (stacked)spectral splitting
(spatial)
Make better use of sunlight: “Photon management”
e.g., multi-junction photovoltaics
Bandgap (eV)
5 6 7 8 9 1 25 6 7 8 9 1 2
Four-junction device with bandgaps1.8 eV/1.4 eV/1.0 eV/0.7 eV
Theoretical efficiency > 52%
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
SPECTRAL SPLITTING
A two junction, four terminal photovoltaic device forenhanced light to electric power conversion usinga low-cost dichroic mirrorSven Ruhle, Akiba Segal, Ayelet Vilan, Sarah R. Kurtz, Larissa Grinis, Arie Zaban,Igor Lubomirsky, and David CahenJOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 1, 013106 2009
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
A two junction, four terminal photovoltaic device forenhanced light to electric power conversion usinga low-cost dichroic mirrorSven Ruhle, Akiba Segal, Ayelet Vilan, Sarah R. Kurtz, Larissa Grinis, Arie Zaban,Igor Lubomirsky, and David CahenJOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 1, 013106 2009
Electrical Performance with Dichroic Mirror
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
A two junction, four terminal photovoltaic device forenhanced light to electric power conversion usinga low-cost dichroic mirrorSven Ruhle, Akiba Segal, Ayelet Vilan, Sarah R. Kurtz, Larissa Grinis, Arie Zaban,Igor Lubomirsky, and David CahenJOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 1, 013106 2009
Electrical Performance with Dichroic Mirror
Needed:
cheap, h
igh E G +
high
V max
PV Cells
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
Improve performance using concentrated sunlight
but … diffuse (scattered)
radiation lost upon concentration
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
“Etendue” VOC loss (“photon entropy”) ΔF = ΔH – TΔS
ΔqVOC = EG – kT ln W = EG – kTln 46,200 = EG – 10.7 kT =
280 meV @ 300K
Experiments on III-V alloys
yield
350 -550 mV for Eg-Voc
Hirst & Ekins-DaukesProc. 24th Eur. PV Solar En. Conf. Hamburg
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0.0
0.5
1.0
1.5
2.0
0.6 1 1.4 1.8 2.2 2.6 3Bandgap Eg (eV)
Eg
/q,
Vo
c, a
nd
(E
g/q
) -
Vo
c (
V)
0
100
200
300
400
500
600
700
800
Inte
ns
ity
pe
r U
nit
Ph
oto
n E
ne
rgy
(W
/(m
2 . e
V))
VocEg from EQE(Eg/q) - Vocradiative limitAM1.5D, low-AOD
Voc of solar cells with wide range of bandgaps and comparison to radiative limit
d-A
lGa
InP
Ga
As
1.4
- e
V G
aIn
As
o-G
aIn
P
AlG
aIn
As
d-A
lGa
InP
d-G
aIn
P
d-A
lGa
InP
0.9
7-e
V G
aIn
As
Ga
InN
As
1.1
0-e
V G
aIn
As
1.2
4-e
V G
aIn
As
1.3
0-e
V G
aIn
As
Ge
(i
nd
ire
ct g
ap
)
AlG
aIn
As
Richard King
“Etendue” VOC loss (“photon entropy”)
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
OUTLINE• PV cell performances today
• Limits of PV solar energy conversion
• Empirical guides to limits or possibilities
• Losses in “excitonic” cells
• Summary & Future
![Page 23: D. Cahen, Weizmann Inst. 02/’12 How good can Solar Cells be? Assessing Possibilities for Solar Cells by Identifying](https://reader033.vdocuments.net/reader033/viewer/2022051516/56649e395503460f94b2ab0e/html5/thumbnails/23.jpg)
D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
Empirical Guide 1Current
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0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00
20
40
60
80
100Maximum Current Density Available
in 1 Sun @ AM 1.5G
J
(m
A/c
m2 ),
EQ
E (
%)
Energy (eV)
J, 100%
J0-, EQE()
0
1
2
3
4
5
Sp
ect
ral P
ho
ton
Flu
x D
en
sity
( 1
014 p
ho
ton
s/se
c-cm
2 )
from B. Kippelen, Georgia Tech
Current Limitation
Harvest more photons
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JMP q d
(JMP JSCmax )
a From EQE * for best performing cell of given type
:Limit or Opportunity ?
Nayak et al. Adv. Mater., May 2011
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JMP q d
(JMP JSCmax )
a From EQE * for best performing cell of given type
:Limit or Opportunity ?
Nayak et al. Adv. Mater., May 2011
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Maximal possible vs. experimental photocurrents
Natural PS<~10-2
mA/cm2
Thanx 2 Lee B
1.0 1.5 2.0
10
20
30
40
50
DSSC(c)DSSC
(b)
OPV (a)
(Jsc
Max)
JMP
JSC
Curr
ent
dens
ity(
mA/c
m2 )
Absorption Edge (eV)
Si
CI GS
I nP GaAs
CdTe
DSSC(a)
OPV (b)
a-Si
DSSC
(a) = black dye, (b) = N719
& (c) = YD2-O-C8+Y123
OPV (a) = Mitsubishi, (b) = Konarka
PKN, JB, DC, 2011, AM
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Empirical Guide 2Voltage
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:Limit or Opportunity ?
JSC q d
(JSC JSCmax )
a From EQE * for best performing cell of given type
PKN, JB, DC, 2011, AM
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Voc / EG : Limit or Opportunity ?
a From EQE * for best performing cell of given type
PKN, JB, DC, 2011, AM
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Voc / EG : Limit or Opportunity ?
a From EQE * for best performing cell of given type
PKN, JB, DC, 2011, AM
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1000 800 600 400
1 1.5 2 2.5 30.3
0.4
0.5
0.61000 800 600 400
1 1.5 2 2.5 30.3
0.4
0.5
0.6
CO
ST:
qV
hν –
qV
OC
[eV
]
Absorption Edge Energy [eV]
Shockley-Queisser or detailed balance limit COST as function of minimal
excitation energy Wavelength [nm]
WRONG
S-Q based on R.Milo,WIS Thanx 2 Lee B
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0.0 0.5 1.0 1.5 2.0 2.5 3.00
10
20
30
40
50
60
70
Curr
ent
dens
ity(
mA/c
m2 )
Absorption Edge (eV)
Geometrical illustration of solar spectrum loss
due to “over-potential”Consider ~ 1 eV or 2 eV absorption edge “Assume 1 eV “overpotential” shifts reference energy 1 eV to right small purple rectangle gives new optimal energy value.
\
After Ron Milo, WIS PKN, JB, DC, 2011, AM
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0.0 0.5 1.0 1.5 2.0 2.5 3.00
10
20
30
40
50
60
70
Curr
ent
dens
ity(
mA/c
m2 )
Absorption Edge (eV)
Geometrical illustration of solar spectrum loss
due to “over-potential”Consider ~ 1 eV or 2 eV absorption edge “Assume 1 eV “overpotential” shifts reference energy 1 eV to right small purple rectangle gives new optimal energy value.
After Ron Milo, WIS PKN, JB, DC, 2011, AM
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VMP / EG : Limit or Opportunity?
a From EQE * for best performing cell of given type Nayak et al. Adv. Mater., May 2011
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VMP / EG : Limit or Opportunity?
a From EQE * for best performing cell of given type Nayak et al. Adv. Mater., May 2011
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Absorption Edge Energy [eV]
Shockley-Queisser or detailed balance limit LOSS as function of minimal
excitation energy
S-Q based on R.Milo,WISPKN, JB, DC, 2011, AM Thanx 2 Lee B
1000 800 600 400
1 1.5 2 2.5 3
0.3
0.4
0.5
0.6
qV
hν –
qV
op
era
tion
(=M
P) [
eV
]1000 800 600 400
1 1.5 2 2.5 3
0.3
0.4
0.5
0.6
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
OUTLINE• PV cell performances today
• Limits of PV solar energy conversion
• Empirical guides to limits or possibilities
• Losses in “excitonic” cells
• Summary & Future
![Page 39: D. Cahen, Weizmann Inst. 02/’12 How good can Solar Cells be? Assessing Possibilities for Solar Cells by Identifying](https://reader033.vdocuments.net/reader033/viewer/2022051516/56649e395503460f94b2ab0e/html5/thumbnails/39.jpg)
D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
...............
Includes basic add’l loss of amorphous/disordered material
1.0 1.2 1.4 1.6 1.8 2.0
0.2
0.4
0.6
0.8
1.0 DSC-latestOPV Mitsubishi
GaAs
OPV Konarka
a-Si PS
CuGaSe2
Absorption Edge [eV]
qVhv
- qV
oper
atio
nal (
= M
P) [e
V]
SQ- Limit Loss
(GaI n)P
DSC-N719
I nPCI GS
C -Si
DSC-Black CdTe
PS: natural photsynthesis
Shockley-Queisser or detailed balance limit LOSS as function of minimal
excitation energy
S-Q from R.Milo,WIS PKN, JB, DC, 2011, AM
qV
hν –
qV
op
era
tion
(=M
P) [
eV
]
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Why / how do we loose
with the excitonic cells?
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D
-
Cathode
Anode
SubstrateLight
DA
Bulk heterojunction cell
Paul W. M. Blom et al.
Static Disorder
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D+A-
D*A
DA
Nuclear co-ordinate
En
erg
y (
eV
)
Eg
λ
ΔG*
ΔG0
Vibronic relaxation after electron transfer
A-
λrel (1)
A
Nuclear co-ordinate
Ener
gy (e
V)
λrel (2)
ΔG0rec
Loss = λrel (hole) + λrel
(electron)
λrel (hole) = ~150 meV (UPS)
λrel (electron) = ~150 meV (DFT)
λ = λrel (1) + λrel (2)
Consider electron transfer & Vibronic relaxation
Nayak et al., EES, in press
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
Static and Dynamic disorder
Tail states
After Kera, Yamane and Ueno Progress in Surface Science, 2009
Nayak et al., EES, in press
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En
erg
y (
eV
)
CBM
VBM
Exciton binding energy < kT
→ dissociation by space charge region E-field
p n
EFp
Inorganic
Wannier exciton
LUMO (D)
HOMO (D)
LUMO (A)
HOMO (A)
12 3
3
ΔD/AEn
erg
y (
eV
)
Donor Acceptor
Exciton binding energy >> kT
→ requires donor/acceptor, (D/A) type structure
Organic
Frenkel exciton
p/n vs. excitonic solar cells
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Inorganic semiconductor
Exciton binding energy
< kT
→ dissociation by space charge
region E-field
Electron-hole pair:Organic vs. Inorganic PV
cells
from A. Kahn, Princeton U
Organic semiconductor
MOLECULAR PICTURE
Exciton binding
energy >> kT
→ requires donor/
acceptor, (D/A) type structure
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p/n vs. excitonic solar cellsINORGANICINORGANIC
• high dielectric constant
• minority carrier device
ORGANIORGANICC
• low dielectric
constant
• exciton splitting
• includes jiggling & wigglingfrom B. Kippelen, Georgia
Tech
Exciton binding energy ~ 10 meV ~ 0.1-0.3 eV
EF
n
* 4
2 2 20(4 ) 2B
m eE
dielectric constant
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{LUMO(A) – HOMO(D) gap} Voc correlation
Rand et al., Phys. Rev. B 75,
(2007)Δ= IE(D) – EA(A) – EBA/2 (?)
IE EA
Evac
Afer A. Kahn, Princeton U
EF
HOMO
LUMO
EFDAΔ
VOC = IE(D) – EA(A) –(≥0.3) ?
Origin of VVOCOC
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G0 Gibbs free energy reorganization energy, relaxation due to vibronic modes Vif electronic coupling
0 00 exp BDA
B
J J e Nk T
k
k :rate of charge transfer (s-1) e: electron charge (C) NDA: surface density of DA complexes[cm-2]
212
02( )
exp4if
B
GV
kk
T
0
lnOCSCB JT
eV
kn
J
Origin of VVOCOC
after B. Kippelen, Georgia Tech
Voc as J0 & J00
Nayak et al., EES, in press
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D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
OUTLINE• PV cell performances today
• Limits of PV solar energy conversion
• Empirical guides to limits or possibilities
• Losses in “excitonic” cells
• Summary & Future
![Page 50: D. Cahen, Weizmann Inst. 02/’12 How good can Solar Cells be? Assessing Possibilities for Solar Cells by Identifying](https://reader033.vdocuments.net/reader033/viewer/2022051516/56649e395503460f94b2ab0e/html5/thumbnails/50.jpg)
D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html
SummaryThere are limits, beyond Shockley-Queisser for photo-conversion with organics (OPV, DSSC, PS, AP)
• static disorder ~ 0.2-0.3 eV
• dynamic disorder
vibronic coupling ~ 0.2-0.3 eV
• low dielectric constant~ 0.1 -0.3 eV Σ = 0.5 - 0.9 eV; cf. 1.05 eV !
Nayak et al. Adv. Mater., May 2011 PKN, JB, DC -TBP
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1.0 1.5 2.0 2.5 3.00
5
10
15
20
25
30
35
Cal
cula
ted
Eff
icie
ncy
(%)
Band gap (eV)
S-Q Limit
Shockley-Queisser (SQ) Limit
0.5 1.0 1.5 2.0 2.55
10
15
20
25
30
OPV
CIGS
c-Si
Effi
cien
cy (
%)
Band Gap (eV)
GaAs
InP
CdTe
DSC a-Si
SQ Limit
Nayak et al., EES, in press
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1.0 1.5 2.0 2.5 3.00
5
10
15
20
25
30
35
Cal
cula
ted
Eff
icie
ncy
(%)
Band gap (eV)
+ 80% EQE + Fill factor loss (n=2) + Tail state loss = 0.2eV + Vibronic loss = 0.25eV + Dielectric loss = 0.2eV + (Dielectric + vibronic) = 0.3eV
S-Q Limit
Extra Losses in Molecular Cells
0.5 1.0 1.5 2.0 2.55
10
15
20
25
30
OPV
CIGS
c-Si
Effi
cien
cy (
%)
Band Gap (eV)
GaAs
InP
CdTe
DSC a-Si
SQ Limit
Nayak et al., EES, in press
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SummaryThere are limits, beyond Shockley-Queisser for photo-conversion with organics (OPV, DSSC, PS, AP)
• static disorder ~ 0.2-0.3 eV
• dynamic disordervibronic coupling ~ 0.2-0.3 eV
• low dielectric constant~ 0.1 -0.3 eV Σ = 0.5 - 0.9 eV; cf. 1.05 eV !
High(er) optical absorption edge systems
Ways to beat those limits in artificial systems * composite materials? *smarter photon management• ………………………………….
Nayak et al. Adv. Mater., May 2011, EES, in press
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0.2 GWp ( ~40 MWc) plant at Golmud, PRC
World’s Largest Solar-Electric Plant (2009)
30 TWp (~ 6 TWC)requires 1 such plant, every HOUR, for the next~ 20 years (+ a bit of
storage…)
Solar Cell Power Stations TODAY
01/’12 Global installed PV power
~0.067 TWp
PRC goal >2011≥ 0.002 TWp/yr
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Israel’s electricity generation capacity ~11 GW ~ 0.011 TW (1.6 kW/capita)
China’s electricity growth plan: 0.1 TW/year ……
One such plant, every day,
for the next … 11 years
10 TW electricity from COAL ?
אם תרצואין זו אגדה
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Thin Film Solar Cells: Present Status
Data from B v Roedern, NREL, 2011
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Energy Pay-back Time for PV Cells