Hybrid-PV Berlin, 05-2013
Inorganic, Organic and Hybrid Solar Cells:
How different are they?
with Pabitra K. Nayak
Work done in part in collaboration with A. Kahn and J. Bisquert
Cahen group WIS 2013
The Photovoltaic (PV) effect:
Generalized picture
• Metastable high and low
energy states
contact
econtact
one electron energy
Absorber
p+
space
• Absorber transfers charges
into high and low energy state
• Driving force brings charges to
contacts
• Selective contacts
 ~~ photosynthesis
(1) cf. e.g., Green, M.A., Photovoltaic principles. Physica E, 14 (2002) 11-17
Cahen group WIS 2013
Current Types of PV Cells
Primarily based on solid-state electronic material systems
• Elemental Semiconductors
(non)
concentrator;
single-& multijunction
– Single or multi-crystal
– Polycrystalline films
– Amorphous thin film
• Inorganic Compound
Semiconductors
homo- &
•
hetero-junction;
photoelectrochem;
MIS-inversion
Si
,Ge
(Ga,In)(As,P)
– Single crystal
– Polycrystalline thin film Cu(In,Ga)Se2
CdTe
Organic, Excitonic
(molecules, polymer;hybrid)
– Interpenetrating network P3HT, PCBM;
– Polycrystalline thin film
Porphyrin ++
– Nanocrystalline;
dye+TiO2
dye- or SC-sensitized
………………………………………………………
(ZnO)
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How efficient can they be?
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1954
2013
Chapin
Fuller
Pearson
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“THE”(?) LIMIT: Shockley-Queisser* (SQ)
detailed balance, photons-in = electrons-out + photons-out; on earth, @ RT,
for single absorber / junction;
cf. also Duysens (1958) “The path of light in photosynthesis” Brookhaven
Symp. Biol. 11, 18-25
photosynthesis
SQ Limit
30
GaAs
25
Prince, JAP 26 (1955) 534
Loferski, JAP 27 (1956) 777
Shockley & Queisser JAP (1961)
c-Si
Efficiency (%)
*
InP
20
CIGS
CdTe
15
DSC
a-Si
10
OPV
5
0.5
1.0
1.5
Band Gap (eV)
Cahen group WIS 2013
2.0
2.5
Losses in PV cell
Etendu; Photon entropy –TD
80
~0.3eV @RT, lack of concentration
Current (mA/cm2)
70
Carnot factor –TD
60
Eg
Emission loss- (current)
50
40
30
< Eg
not absorbed
Electrical power out
Current – Voltage
Characteristics
20
>Eg
thermalized
10
0
0
1
2
3
Energy (eV)
After Hirst & Ekins-Daukes
Prog.Photovolt:Res:Appl. (2010)
Nayak, ……, Cahen., Energy Environ. Sci., 2012 (
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4
3d generation PV: Present (?) Status
ETA cells
12.3%
10.8%
/ WIS
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3d generation PV: Present (?) Status
Cahen group WIS 2013
Maximal possible vs. experimental
photocurrents
50
Max
(Jsc
2
Current density(mA/cm )
DSSC
JMP
(a) = black dye, (b) = N719
Si
40
)
JSC
& (c) = YD2-O-C8+Y123
OPV (a) = Mitsubishi, (b) = Konarka
CIGS
30
InP
GaAs
CdTe
20
Natural PS
<~10-2
mA/cm2
DSSC
(a)
DSSC
OPV (a)
DSSC
(c)
(b)
a-Si
OPV (b)
10
1.0
PKN, JB, DC, 2011, AM
1.5
Absorption Edge (eV)
Cahen group WIS 2013
2.0
Thanx 2 Lee B
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 from 0 to 1 eV
small red rectangle gives new optimal energy value.
70
2
Current density(mA/cm )
60
50
40
30
\
20
10
0
0.0
After Ron Milo, WIS
PKN, JB, DC, 2011, AM
0.5
1.0
1.5
2.0
Absorption Edge (eV)
Cahen group WIS 2013
2.5
3.0
qVMP / EG
RT bandgap qVMP* Energy qVMP/EG
abs. edge[eV] [eV] loss
[%]
Cell type
(absorber)
sc-Si
1.12
0.61
0.51
54
GaAs
1.42
1.00
0.42
70
InP
1.28
0.75
0.53
59
1.45
0.71
0.74
49
Cu(In~0.7Ga~0.3)Se
~1.15a
0.60
~0.55
52
a-Si:H I
SC
IMP
DSSC (black dye)
~1.75
0.70
~1.05
~40
~0.75
~1.3
0.53
~0.9
PMAX
~1.6
0.69
~1.0
~1.75
0.73
~1.65 a
0.67
~0.98
~1.65 a
0.68 b
~0.97
V
~1.8
1.05
VMP V(0.75)
OC
~41
~43
~42
~41
~41
CdTe
I
(red N719)
Ru-free porphyrin
OPV Mitsubishi
Konarka ( Belectric)
Photosynthesis fuel
a
From EQE b Calculated
* for best performing cell of given type
Nayak et al. Adv. Mater., May 2011, updated
Cahen group WIS 2013
(42)
Theoretical (Shockley-Queisser) LOSS as
function of minimal excitation energy
So, what is “the” problem?
[eV]
qVoperational
qVqVhνhv–- qV
(= MP)[eV]
operation(=MP)
SQ- Limit Loss
a-Si
OPV Mitsubishi
OPV Konarka
1.0
PS
DSC-latest
CuGaSe2
DSC-N719
0.8
CdTe
DSC-Black
(GaIn)P
0.6
CIGS
C -Si
InP
...............
Includes basic add’l
loss of disordered
material
GaAs
0.4
PS: natural
photsynthesis
0.2
1.0
S-Q from R.Milo,WIS
1.2
1.4
1.6
Absorption Edge [eV]
PKN, JB, DC, 2011, AM
Cahen group WIS 2013
1.8
2.0
p/n vs. excitonic solar cells
ORGANIC
INORGANIC
HIGH
LOW
Exciton binding energy ~ 10 meV
• high dielectric constant
• minority carrier device
EF
~ 0.1-0.3 eV
n
• exciton splitting
• includes jiggling & wiggling
m*e4
EB  
(4 0 )2 2 2 2
 dielectric constant
from B. Kippelen, Georgia Tech
• low dielectric constant
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Electron-hole pair: inorganic vs organic PV
Organic
Inorganic
Exciton
binding
energy >> kT
Exciton binding
energy < kT
→ dissociation
by E-field in the
space charge region
Binding energy
~ 10 meV
Binding energy
~ 100 - 300 meV
Energy (eV)
CBM
VBM
P
n
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→ requires
donor/
acceptor,
(D/A) type
structure
Solar Cell (r)evolutions
1st
generation
Si
2nd generation
CdTe, CIGS
Single- crystalline poly-crystalline
cm
m
3d generation
Organic (polymer/ small molecule)
TiO2
nano crystalline
~ 20 nm
grain size 
cheaper and simpler ?
DISORDER 
HYBRID PV
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amorphous
(a-Si:H;polymers)
Why go HYBRID ?
Best or worst
of two worlds?
Exchange between
George Bernard Shaw
or Anatole France (?)
and
dancer Isadora Duncan
Cahen group WIS 2013
Estimated Energy Payback Times for Solar Cells
Organic PV (OPV)
Proc. IEEE PVSC 2010
A. Anctil et al.,
RIT, NREL
courtesy David Ginley, NREL
Cahen group WIS 2013
Bulk Organic Heterojunction PV Cells
Metal contact
Limited control
over morphology
Active Layer
Hole-transport
Transparent contact
layer Transparent
Substrate
P3HT
PCBM
HYBRID Organic- Inorganic Heterojunction
Metal oxides:
• good electron acceptors cf. to polymers
• good carrier mobility
• solution-processable
• chemically stable
• mechanically robust
• continuous networks can be deposited
Cahen group WIS 2013
Dye-Sensitized
Solar Cell
© Frey lab, IIT-Technion 2013
OPV Materials of Interest
5.3
5.7
A. Kahn, Princeton U.; after Ratcliff et al. JPCL 2011
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20
THX TO ARTEM BAKULIN
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Hybrid PV Cells; decreasing disorder
Bulk heterojunction cell
Dye-sensitized / ETA
Cathode
-
D
D
A
Anode
Substrate
Light
OM Perovskite
Cahen group WIS 2013
Si / Organic Hybrid Cells
5
0
J (mA/cm2)
-5
-10
-15

FF
Voc
Jsc
(100),
with MeOH ML
In-Ga back contact
PEDOT
by LOFO !
-20
-25
-30
-0.1
1015 cm-3,
= 10.8%
= 0.70
= 0.56 V
= 27.5 mA/cm2
0.0
0.1
0.2
0.3
0.4
V (V)
Cahen group WIS 2013
0.5
0.6
Ann Erickson, TBP
Origin
VOC … ordered
The Importance
ofofbeing
Voc  as J0 & J00 
VOC
kBT  J SC 
n
ln 

e
 J0 
 k :rate of charge transfer (s-1)
 B 
J 0  J 00 exp 
  e k N DA  e: electron charge (C)
 k BT 
 NDA: surface density of DA complexes[cm-2]
k  Vif
2
2


(

G


)
0
2
 exp  

4 kBT 

1
 G0 Gibbs free energy
  reorganization energy, relaxation due to vibronic modes
 Vif electronic coupling
after B. Kippelen, Georgia Tech
Cahen group WIS 2013
Nayak et al., EES, 2012
Dynamic Disorder
electron transfer & vibronic relaxation
D+AEnergy (eV)
D*A
λ
Energy (eV)
ΔG*
DA
A-
λ = λrel (1) + λrel (2)
λrel (2)
Vibronic
Relaxation
after
electron
transfer
ΔG0
λrel (1)
A
Nuclear co-ordinate
Loss = λrel (hole) + λrel (electron)
Eg
λrel (hole)
ΔG0rec
= ~150meV (UPS)
λrel (electron) = ~150meV (DFT)
ket = koexp -[(ΔG0+λ)2/4λkBT]
Nuclear co-ordinate
Cahen group WIS 2013
Nayak et al., EES, 2012
Nayak et al., EES, 2012
Static and Dynamic disorder
After Kera, Yamane and Ueno
Progress in Surface Science, 2009
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Tail states
Voc: crystalline vs. non-crystalline
Amorphous
CB Edge
EFn- EFp
VB Edge
Energy (eV)
Energy (eV)
Crystalline
Density of States
CB Edge
EFnEFp
VB Edge
Density of States
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a-Si tail state and Voc
In case of a-Si:H,the carriers
thermalize to the band tails
after photogeneration.
Energy (eV)
An extra loss of ~300 mV
occurs due to tail states.
CB Edge
0.33eV
EFn- EFp
0.44eV
VB Edge
Density of States
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T. Tiedje , APL 1982
Shockley-Queisser (SQ) Limit
35
S-Q Limit
25
SQ Limit
30
GaAs
25
c-Si
20
Efficiency (%)
Calculated Efficiency (%)
30
15
InP
20
CIGS
CdTe
15
10
a-Si
DSC
OPV
10
5
0.5
1.0
1.5
Band Gap (eV)
5
0
1.0
Nayak et al., EES, 2012
1.5
2.0
Band gap (eV)
Cahen group WIS 2013
2.5
3.0
2.0
2.5
Extra Losses in Molecular Cells
35
+ 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
25
SQ Limit
30
GaAs
25
c-Si
20
Efficiency (%)
Calculated Efficiency (%)
30
15
InP
20
CIGS
CdTe
15
10
a-Si
DSC
OPV
10
5
0.5
1.0
1.5
Band Gap (eV)
5
0
1.0
Nayak et al., EES, 2012
1.5
2.0
Band gap (eV)
Cahen group WIS 2013
2.5
3.0
2.0
2.5
Summary
• Oxides may not oxidize, but their surfaces can
…. and
•
inorganic surfaces are nastier than organic ones
…. and
• interfaces is where it is at
….. but
• most inorganic semiconductors are more ordered
than organic ones
• which leaves the question if “ordnung muss
sein”?
Cahen group WIS 2013