Charge transport and recombination in bulk-heterojunction solar - - PowerPoint PPT Presentation

Charge transport and recombination in bulk-heterojunction solar cells

Ronald Österbacka

Department of Physics and Center for Functional Materials Åbo Akademi University

Finnish-Japanese Workshop on Functional Materials, 25-26.05.2009 http://www.funmat.fi

FunMat Schematic

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Center for Functional Materials

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Our vision

We believe that new functional materials will enable the printed intelligence revolution

Our mission

To create a strong multidisciplinary research environment for the development of new materials and demonstrating new functionalities by printing

Key objectives

• Excellence and innovativeness in research
• Highly inter- and multidisciplinary approach to research
• Strong national and international networking
• To become the leader in paper based printed intelligence research

Outline

• Introduction to charge transport and recombination

–Effect of Langevin recombination

• Recombination studies with TOF

–In pure polymers –In annealed bulk-heterojunction solar cells

• Double Injection Transients (DoI)

–Effect of trapping

• Suggested model

–Nanomorphology important –2D delocalization of charge carriers

• Effect of reduced recombination on magnetortransport
• Summary

Bulk-Heterojunction Solar Cells

Glass ITO PEDOT:PSS Aluminum

I

Light

• G. Yu, et al. Science 270, 1789 (1995), J.J.M. Halls, et al., Nature 376, 498 (1995)

Mixture of electron accepting PCBM and electron donating polymer

OMe O

OMe O ( )n

PCE ~ 2.5%

Second generation BHSC

Xiaoniu Yang, et al., Nano Letters, 5, 579-583 (2005)

Uoc jsc FF=

light

• c

sc

P FF U j PCE

PCE ~ 5%

• Organic materials have low mobilities
• To have same efficiency we need higher carrier densities
• Higher density leads to lower carrier lifetime -> Lower current!
• and the second order recombination parameter

are important parameters for testing suitable materials.

• Main goal – to understand transport and recombination in

polymeric solar cells.

Why transport and recombination?

1

) ( n

E en j

n= carrier density = carrier mobility e= electron charge E=electric field

Efficiency proportional to the current

Langevin Recombination

Expected in low-mobility ( <1 cm2/Vs) materials Langevin recombination is determined by the probability for the charge carriers to meet in space, independent of the subsequent fate of the carriers Necessary condition: The carrier mean free path is much smaller than the Coulomb capture radius rc, i.e. a << rC. To reach a photocurrent density of 15mA/cm2 for d=300 nm and V

• c=0.5V:

L/ > 5 · 10-3 cm2/Vs.

nm 19 4

2

kT e r

c

) , ( ) ( T F e

f p n L

2

n np dt dn dt dp

L L

• P. Langevin, Ann. Chim. Phys. 28, 433 (1903).

Consequences of Langevin recombination

• Langevin recombination leads to low

photogeneration efficiency of Onsager type

–Field-dependent generation! –Lower the Fill-Factor

• 1

1 2 3 4

Untreated Treated Qext [Arb. Units] U0 [V]

• A. Pivrikas et al, unpublished

r

kT rc

t ttr

ne t ttr n ne t ttr 1 Consequences of Langevin recombination

• Langevin recombination leads to low

photogeneration efficiency of Onsager type

–Field-dependent generation! –Lower the Fill-Factor

• Efficiency will be limited

–Only ~CU can be extracted from the device

Photo-SCLC: Bimolecular lifetime Langevin!

Recombination measured using TOF

• 1. Small Charge Current (SCC) mode.
• 2. Space Charge Perturbed Current (SCPC).
• 3. Space Charge Limited Current (SCLC).

0.0 0.1 0.2 0.3 0.0 0.2 0.4 0.6

t [ms]

L = 300 J L = 100 J L = 30 J L = 10 J L = 3 J L = 1 J L = 0.3 J L = 0.1 J L = 0.03 J L = 10

• 3 J

j [mA]

te

• A. Pivrikas et al. PRB 71, 125205, (2005) A. Pivrikas, et. al., PRL 94, 176806 (2005).

Q = CU

Q <<CU

te tcusp

j/jSCLC t/ttr

1 0.5 0.0 1.5 1.0 2 4 3

1 2 3 4 5 10 15 20

t [ms]

110 J 71 J 40 J 13 J 4.7 J 1.6 J 0.3 J 0.07 J

j [ A/cm

2]

RRa-PHT P3HT/PCBM

Qext 30CU0

/

L=10-4

0.1 1 10 / L=0,0001 / L=0,01 / L=1

Qe/CU0

a) 0.1 1 10 100 1000 50 100 150 / L=0,0001 / L=0,01 / L=1

t1/2/ttr Q0/CU0

b)

Reduced Recombination in RRPHT/PCBM Solar cells

• A. Pivrikas, et. al., PRL. 94, 176806 (2005).

Double Injection (DoI) Currents

• Apply a voltage pulse
• Record the transient

current

• First the RC-current is
• bserved
• Then the build-up of a

carrier density is observed

• The saturation curent is

limited by recombination

• At switch-off, a reservoir

extraction is observed

R.H. Dean. J. Appl. Phys. 40, 585 (1969) Mark & Lampert, Current Injection in Solids, Academic Press NY (1970)

2 4 6 2 4 6 8

t/ta j/j ; dj/dt

t1/2

tm

j=jS - jm

j(t)

dj/dt (a)

0.0 0.2 0.4 40 80 120

t [ms] j [mA/cm

2]

10 V 7V 5V 3V

(b)

tm

Simulated current transients

• G. Juska et al., JAP 101, 114305 (2007)

0.1 1 10 100 1 10

j/jSCLC t/ttr

1 0.1 0.01 /

L=0.001

1 10 100 0.1 1 10

Qex/CU tp/ttr tcusp tslow dQ/dt Q Qs /

L=10

• 3

As a function of As a function of pulse width

Effect of trapping

0.1 1 10 100 1000 0.1 1 10 slow-carrier trapping j / jSCLC t /ttr

n/ p=100

without trapping faster carrier trapping

Trapping in real solar cells

10

• 7

10

• 6

10

• 5

10

• 4

10

• 3

10

• 2

10

• 4

10

• 3

single shot current density (A/cm

2)

time (s) a-Si:H thickness 6 m transit time delay between pulses 0.1 s

RR-P3HT/PCBM a-Si:H

Note the absence of trapping in annealed RRPHT/PCBM BHSC!

Juska et al., in press

Why is there reduced recombination?

Xiaoniu Yang, et al., Nano Letters, 5, 579-583 (2005) Before annealing After annealing

A possible model for reduced recombination in RR-PHT/PCBM

• V. Arkhipov et al., APL 82, 4605 (2003); Osikowicz et al., Advanced Materials 19, 4213 (2007)
• Increased carrier generation / reduced

recombination due to an effective energy barrier for geminate pair recombination at the interface ( ~0.5 eV Osikowicz et al.)

• To reach the chain nearest the interface

the hole on the polymer has to overcome the same barrier

• Probability for carriers to meet in space

is reduced!

• Requires lamellar structures at the

interface

Lamellar structures important

Sirringhaus et al., Nature 401, 685 (1999); Österbacka et al., Science 287, 839 (2000) Head-to-tail = 100% Head-to-tail ~ 80%

Langevin recombination in MDMO- PPV/PCBM systems

10

• 8

10

• 6

10

• 4

10

• 2

10 0.01 0.1 1 10

Q

e/CU0

I/I0

0.0 4.0x10

• 5

8.0x10

• 5

1 2 3 4

t [s] j [mA/cm

2]

Increasing Intensity

2 1

L

Pivrikas et al., Nonlinear Optics, Quantum Optics: Concepts in Modern Optics, 37, 169-177 (2007)

Role of e-h recombination on OMAR

• 300 -200 -100

100 200 300 10

• 4

10

• 3

10

• 2

10

• 1

10 10

1

P3HT Diode

L~1

MDMO-PPV/PCBM

L~0.5

Un-treated

L~10

• 1

Treated

L~10

• 3

% MR B (mT)

• S. Majumdar et al., Physical Review B 79, 201202R (2009).

Correlation with magnetic behavior

• 200
• 100

100 200

• 2x10
• 8
• 1x10
• 8

1x10

• 8

2x10

• 8

Magnetization (Am2) B (mT)

P3HT

• 200
• 100

100 200

• 2x10
• 7
• 1x10
• 7

1x10

• 7

2x10

• 7

5 K 100 K Magnetization B (mT)

P3HT/PCBM

• 300 -200 -100

100 200 300 10

• 2

10

• 1

10 10

1

• 1

• 3

| % MR | B (mT)

• S. Majumdar et al., submitted.

V.I. Krinichnyi, Solar cells and Solar energy Materials 92, 942 (2008).

Summary

• Bimolecular recombination is important for characterization of

solar cell materials

• Langevin recombination:

–Probability for charge carriers to meet in space –Field dependent generation of Onsager type –Lower extraction efficiency

• Treated RR-PHT/PCBM blends shows greatly reduced

recombination compared to Langevin ~

L ~10-3

• Untreated RR-PHT/PCBM blends show ~

L ~ 0.1

• MDMO-PPV/PCBM blends show ~

L ~ 0.5

• Nanomorphology important ->delocalization in lamellas important!
• DoI is an extremely useful measurement technique for materials

with reduced recombination

• Take-home message: The probability to form electron – hole pairs

is crucial for the magnetotransport response.

The people!

• S. Majumdar, H. Majumdar, T. Mäkelä, L. Jian, R. Mohan, ÅA
• H. Aarnio, G. Sliauzys, J.K. Baral, N. Kaihovirta, D. Tobjörk, F.

Jansson, N. Björklund, M. Nyman, S. Sanden, F. Petterson, ÅA

• H. Stubb (emeritus), K.-M. Källman (lab engineer), ÅA
• Left the group: A. Pivrikas (Linz), T. Bäcklund (UPM), H. Sandberg

(VTT), M. Westerling (Perkin-Elmer)

• G. Juška, K. Arlauskas, K. Genevicius/Vilnius Univ
• R. Laiho/University of Turku
• N.S. Sariciftci, LIOS
• G. Dennler and M. Scharber, Konarka Austria
• D. Vanderzande, Universiteit Hasselt, Belgium
• Financial Support by the Academy of Finland, Technology

Development Center TEKES, and Magnus Ehrnrooth foundation

Organic Electronics group 2008

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Thank you!