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Characteristics of Organic Bulk Heterojunction Solar Cells Investigated by Impedance Spectroscopy.
Juan Bisquert and Germà Garcia-Belmonte
Photovoltaic and Optoelectronic Devices GroupUniversitat Jaume I
12071 CastellóSpain
Boston 30-11-2011
Cell mechanisms
h+
e-
Poly(3-hexylthiophene), P3HT
Fullerene derivative, PCBM
Donor
Acceptor
Acceptor HOMO
Acceptor LUMO
Donor HOMO
Donor LUMO-
-
+
EFp
(i)
(ii) (iii)(iv)
(v) Cathod
e
(vi)
-
+
(vi) EFn
VOC
(i) Exciton generation from absorved photon
(ii) Exciton disociation in the donor-acceptor interface
(iii) Electron transport (in the acceptor LUMO)
(iv) Electron extraction
(v) Hole transport (in the donorHOMO)
(vi) Hole Extraction
(vii) Bimolecular recombination
Cell mechanisms
Charge separation by electrical field: a very popular idea
P. Blom et al. PHYSICAL REVIEW B 72, 085205 (2005)
Deibel and Dyakonov
Rep. Prog. Phys. 73 (2010) 096401
Two different pictures
On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar CellsJuan Bisquert and Germa Garcia-Belmonte, J. Phys. Chem. Lett. 2011, 2, 1950–1964
dxFdrdr
Electrical field model
This model requires that (this is not usually remarked):The carrier density is fixed at the contacts
ImpliesThe voltage changes the bandbending
appca
bidr VV ,
dxFdrdr
)0()( FpFn ELEqV
ButThe voltage is not directly related to the drift field:
On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar CellsJuan Bisquert and Germa Garcia-Belmonte, J. Phys. Chem. Lett. 2011, 2, 1950–1964
The flatbands model
On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar CellsJuan Bisquert and Germa Garcia-Belmonte, J. Phys. Chem. Lett. 2011, 2, 1950–1964
The carrier density is variable at the contacts
The variation of potential can be absorbed at the interface
The Fermi level goes up flat,
the critical question is how the cathode works
I ask myself the questions
Am I totally sure that the BHJ blend is an insulator, that there is a built in potential in the dark that goes from one contact to the other?That Debye screening length is long (100 nm)?That the fill factor is caused by decreased photocurrent by less exciton dissociationSo it’s all about the electrical field
OR
Maybe I have some doubts and I am inclined to believe that there could be some background doping density, that there are plenty of free carriers in the blend, that they shield and absorb electrical fields…
Ask yourself the question
The blend is an
INSULATOR (no carriers at all, charges at the electrodes, field across layer)
OR
SEMICONDUCTOR (some carriers around, field at one contact)
Band bending at the cathode
Now assume that the blend is a p-doped semiconductorThe equilibration occurs at the cathode
We proposed this in Organic Electronics 2008The the built in voltage, is the difference between Fermi level of the blend
and cathode workfunction, about 0.4 eV.Garcia-Belmonte, G.; Munar, A.; Barea, E. M.; Bisquert, J.; Ugarte, I.; Pacios, R. "Charge carrier mobility and lifetime of organic bulk heterojunctions analyzed by impedance spectroscopy". Organic Electronics 2008, 9, 847-851
Band bending and Fermi levels: what happens when negative voltage is applied at the cathode
Acceptor HOMO
Acceptor LUMO
Donor HOMO
Donor LUMO
EF
Cathod
e
--
- - - --
w
V = 0
Acceptor HOMO
Donor HOMO
EF
-- - C
athode
w
V < Vfb
Acceptor LUMO
Donor LUMO
-
Band bending and Fermi levels
Acceptor HOMO
Donor HOMO
EF
-- -
w
V < Vfb
Acceptor LUMO
Donor LUMOC
athode
Cathod
e
Band bending and Fermi levels
Acceptor HOMO
Donor HOMO
EF
Cathod
e
w
V < Vfb
Acceptor LUMO
Donor LUMO
-
Band bending and Fermi levels
Acceptor HOMO
Donor HOMO
EF
Cathod
e
V = Vfb
Acceptor LUMO
Donor LUMO
Band bending and Fermi levels
Using MS analysis we can determine the built in voltage (at the cathode) and the amount of doping.
Depletion region – Mott Schottky plots
Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopywFrancisco Fabregat-Santiago, Germa Garcia-Belmonte, Ivan Mora-Sero´ and Juan BisquertPhys. Chem. Chem. Phys., 2011, 13, 9083–9118
Depletion region: examples
-P. P. Boix, G. Garcia-Belmonte, U. Muñecas, M.Neophytou, C. Waldauf, R. Pacios,
Appl Phys. Lett 95, 1, (2009)
Depletion region: examples
The bands and the Fermi levels, again negative voltage (or photogeneration)
The chemical capacitanceDielectric (depletion) capacitance
Measured DOS of organic BHJ
PCBM-P3HT solar cell
Measurement of the DOS
The chemical capacitance
Germà Garcia-Belmonte, Pablo P. Boix, Juan Bisquert, Michele Sessolo, and Henk J. Bolink
Solar Energy Materials and Solar Cells, 94, 366 (2010)
)()( 2Fnn
Fn
L EqgE
nqEC
Recombination in organic BHJ
How does organic BHJ work
With good selective contactsAnd charge separation and mutual shielding
We have flat Fermi levelsAnd flat bands
No electrical fields! Do not worry so much about drift-diffusion models, the critical question:
The current-potential curve, and consequently photovoltage, is entirely determined by
recombination
How is Voc limited (1)?
Voc is given by the difference of Fermi levels. So the Schottky barrier measured by MS does not pose a
limitation to Voc (in fact the flatband of the barrier is about 0.4 V, and photovoltage can be much higher!)
This is because the cathode is not a pn junction, it is a heterojunction, it allows the Fermi level to rise past the flatband value. See again pages 11-15.
How is Voc limited (2)?
Voc is given by the difference of Fermi levels.
Photovoltage, is set by recombination, that establishes the number of carriers in each separate material.
Important for the final Voc value are two factors1. The relative energetics of the materials. If the LUMO of
the fullerene is high, Voc will tend to be higher2. The distribution of states at the Fermi levels. If there are
few trap states, the Fermi level will rise higher
Current-Voltage Characteristics of Bulk Heterojunction Organic Solar Cells: Connection Between Light and Dark CurvesPablo P. Boix, Antonio Guerrero, Luís F. Marchesi, Germà Garcia-Belmonte and Juan BisquertAdvabced Energy Materials 2011
Comparison of different fullereneswith similar reduction potential but different DOS
PCBM
DPM6
G. Gar
Germa Garcia-Belmonte Pablo P. Boix, Juan Bisquert, Martijn Lenes, Henk J. Bolink,Andrea La Rosa, Salvatore Filippone,and Nazario Martín§,J. Phys. Chem. Lett. 1 (2010) 2566-2571
DPM6 has a lower density of states in the region where the Fermi level moves, so higher Voc
The diode equation for a solar cell
0)0( jjr At V = 0 equilibrium of generation and recombination
Equilibrium of generation and recombination
Rise of the Fermi level enhances recombinationTmkqV
rBejVj /
0)(
Dark
SunlightTmkqV
scBejjjj /
00
Can be measured directly by recombination resistanceAlso can be expressed in terms
of carrier density (Durrant)
Recombination in organic BHJ
Here Durrant group reconstructs Voc from carrier density using the simple diode model. Works very well.
C. G. Shuttle, R. Hamilton, B. C. O’Regan, J. Nelson, and J. R. Durrant
16448–16452 PNAS, 2010 vol. 107 ∣ ∣
3 selected topics for investigation
assumingflat Fermi levelsflat bands, homogeneous carrier distribution!The current-potential curve determined by recombination
1. What is the role of disorder, especially on Voc?2. Does voltage fix carrier density (Rau reciprocity)3. How does recombination correlate with fullerene energetics
The Fermi levels lie below LUMO and above HOMO
1. Why is Voc less than HOMO-LUMO difference?This is explained by disorder
FpFn EEqV oc
Tkpn
B
22
Energy loss related to disorder
G. Garcia-Belmonte, J. Bisquert,
Appl. Phys. Lett. 96 (2010) 113301
2. Reconstruction of current voltage curves from recombination resistance data
Current-Voltage Characteristics of Bulk Heterojunction Organic Solar Cells: Connection Between Light and Dark CurvesPablo P. Boix, Antonio Guerrero, Luís F. Marchesi, Germà Garcia-Belmonte and Juan BisquertAdvabced Energy Materials 2011
T. Kirchartz, U. Rau Detailed balance and reciprocity in solar cells physica status solidi (a). 2008, 205, 2737-2751.
Here you have low mobilities, or poor morphology, you may worry about transport
Transport is fast, each phase is well interconnected, Fermi levels flat: recombination is the key issue
2. Reconstruction of current voltage curves from recombination resistance data
Thuc-Quyen Nguyen et al., Advanced Energy Materials 2011
a) DPP(TBFu) 2 :PC 60 BM, and,b) P3HT:PC 70 BM solar cells under different illumination intensities.
Same Voc at all light intensities: the simple diode model donm’t work
2. Reconstruction of current voltage curves from recombination resistance data
Current-Voltage Characteristics of Bulk Heterojunction Organic Solar Cells: Connection Between Light and Dark CurvesPablo P. Boix, Antonio Guerrero, Luís F. Marchesi, Germà Garcia-Belmonte and Juan BisquertAdvabced Energy Materials 2011
1rec
rec d
d
FV
jLR )(
)(ocrec
Bocrecsc VqR
TkLVjj
Here we show, that recombination resistance is independent of illumination, satisfying reciprocity. See that series resistance is variable!Further, from recombination, we can construct the phtocurrent at open circuit, and we see it is the same as that at short circuit.
3. Energetics of recombination
Recombination in Organic Bulk Heterojunction Solar Cells: Small Dependence of Interfacial Charge Transfer Kinetics on Fullerene Affinity Antonio Guerrero, Luis F. Marchesi, Pablo P. Boix, Juan Bisquert and Germa Garcia-Belmonte, submitted
Acknowledgments
Antonio Guerrero Pablo P. Boix
Homepage: www.elp.uji.es/jb.htmE-mail: [email protected]
www.hopv.org
You are invited to participate in the 4th international Conference on Hybrid and Organic Photovoltaics, from 16 to 19 May 2012, Uppsala, Sweden.
www.nanoge.org