models for organic solar cell and impedance spectroscopy results

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:


The flatbands model


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


Acceptor HOMO

Donor HOMO

EF

Cathod

e

w

V < Vfb

Acceptor LUMO

Donor LUMO

-


Acceptor HOMO

Donor HOMO

EF

Cathod

e

V = Vfb

Acceptor LUMO

Donor LUMO


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


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


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



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

mailto:[email protected]

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

models for organic solar cell and impedance spectroscopy results

Technology

pcbm donor acceptor

germa garciabelmonte

electrical field model

organic electronics2008

cathode workfunction

negative voltage

drift field

bhj blend