Influence of Trapping on the Recombination Dynamics in

Disordered Organic Semiconductors

C. Deibel, J. Gorenflot, J. Lorrmann, A. Baumann, A. Wagenpfahl, J. Schafferhans, V. Dyakonov

Julius-Maximilians-University of Würzburg, Germany

E-MRS, 12th May 2011 in Nice

Nongeminate Recombination is Major Loss Mechanism

Motivation

2

glass

PEDOT

V

(1)

(2)

(1)

• 2nd order recombination?• Langevin? reduced?• influence of traps?

RLangevin = γ np

γ =q

�(µe + µh)

Photo-CELIV

and transient absorption for comparison

Experimental Method

3

‣ ns laser pulse

‣ delay time / recombination

‣ charge extraction

1.2

1.0

0.8

0.6

0.4

0.2

j [x

10-3

A/c

m2 ]

0.80.40.0

t [x10-3 s]

Photo-CELIV delay dependent @ T=150 K P3HT:PCBM 1:0.8

delay time tdelay betweenlaser and voltage pulse

shortdelay

long delay

‣ mobility and‣ carrier concentration

simultaneously

P3HT:PCBM (annealed) measured by photo-CELIV

Bulk Recombination

4

‣ Langevin recombination prefactor

1020

1021

1022

n [m

-3]

10-7 10-6 10-5 10-4 10-3 10-2

tdelay [s]

125 K

300 K

P3HT:PCBM 1:0.8annealed

Andreas Baumann

‣ temperature dependencetypical for Langevin recombination

Analysis: Fitting to the Continuity Equation

1021

2

3

456

1022

2

3

45

next [

m-3

]

10-7

10-6

10-5

10-4

10-3

10-2

tdelay [s]

experiment

T=150K

P3HT:PCBM 1:0.8

5

P3HT:PCBM (annealed) measured by photo-CELIV

Analysis: Fitting to the Continuity Equationdn

dt= −n

τ

1021

2

3

456

1022

2

3

45

next [

m-3

]

10-7

10-6

10-5

10-4

10-3

10-2

tdelay [s]

experiment

MR: ! = 6.6·10-4

s

T=150K

P3HT:PCBM 1:0.8

Monomolecular Recombination?

Langevin Recombination?

Analysis: Fitting to the Continuity Equation

1021

2

3

456

1022

2

3

45

next [

m-3

]

10-7

10-6

10-5

10-4

10-3

10-2

tdelay [s]

experiment

MR: ! = 6.6·10-4

s

Langevin

T=150K

P3HT:PCBM 1:0.8

7

dn

dt= − q

�r�0· µ

� �� �·n2

γL

Reduced Langevin Recombination!

Analysis: Fitting to the Continuity Equation

1021

2

3

456

1022

2

3

45

next [

m-3

]

10-7

10-6

10-5

10-4

10-3

10-2

tdelay [s]

experiment

MR: ! = 6.6·10-4

s

Langevin

red. Langevin: " = 0.057

T=150K

P3HT:PCBM 1:0.8

8

dn

dt= −ζ · q

�r�0· µ

� �� �·n2

γL

Analysis: Fitting to the Continuity Equation

1021

2

3

456

1022

2

3

45

next [

m-3

]

10-7

10-6

10-5

10-4

10-3

10-2

tdelay [s]

experiment

MR: ! = 6.6·10-4

s

Langevin

red. Langevin: " = 0.057

k#+1n#+1

with #+1=2.41

T=150K

P3HT:PCBM 1:0.8

9

Recombination Order >2? dn

dt= −kλ+1 · nλ+1

Nongeminate Recombination

10

3.4

3.2

3.0

2.8

2.6

2.4

2.2

2.0

reco

mbi

natio

n or

der

300250200150

T [K]

P3HT:PCBM 1:0.8

pristine annealed

BR

P3HT:PCBM (annealed) measured by photo-CELIVOrder of Decay

Andreas Baumann

carrier concentration dependent mobility Shuttle et al, Adv. Funct. Mater 20, 698 (2010)

2-dimensional recombination Juska et al, APL 95, 013303 (2009)

influence of trapping Zaban et al, Chem. Phys. Chem. 4, 859 (2003) Nelson, PRB 67, 155209 (2003)

influence of phase separation idea, but without change of order: Koster et al, APL 88, 052104 (2006)

qualitatively: Baumann et al, Adv. Funct. Mater. 21, 1687 (2011)

Why the High Recombination Order?

11

7

6

5

4

3

2

1

Ord

er o

f Dec

ay

30025020015010050

Temperature [K]

10-15

10-14

10-13

k br

[arb

.u.]

60x10-6402001/T2 [1/K2]

P3HT P3HT:PCBM

P3HT Fit

Phase Separation?

12

P3HT:PCBM (annealed) measured by transient absorption

‣ P3HT:PCBM, similar results as compared to photo-CELIV

‣ P3HT >140K due to polarons; second order recombination!

‣ difference P3HT vs blend: no phase separation in the formerJulien Gorenflot

13

Concentration Dependent Mobility?

56

10-10

2

3456

10-9

2

34

µ [m

2 /Vs]

1020 1021 1022

n [m-3]

125 K150 K175 K200 K300 K

P3HT:PCBM 1:0.8annealed

‣ at least, not for annealed P3HT:PCBM solar cells

P3HT:PCBM (annealed) measured by photo-CELIV

Andreas Baumann

Side Note: still „Reduced Recombination“

14

P3HT:PCBM (annealed) measured by photo-CELIV

10-20

10-19

10-18

10-17

k BR [m

3 /s]

1020 1021 1022

n [m-3]

P3HT:PCBM 1:0.8annealed fit Langevin125 K 175 K 300 K

‣ fit:

‣ Langevin:

n and μ(n) from CELIVλ and kλ from fitting CELIV

Andreas Baumann

Trapping?

Trap density (Lower Limit)P3HT:PCBM: 6-8⋅1022 m-3

P3HT: 1⋅1022 m-3

15

APL 93, 093303 (2008), Org. Electron. 11, 1693 (2010),Adv. Ener. Mater. accepted (2011)

P3HT:PCBM (annealed) by Thermally Stimulated Currents

6x1021

5

4

3

2

1

0trap

dens

ity (l

ower

lim

it) [

m-3

]

400300200100

activation energy [meV]

P3HT:PC61BM PC61BM P3HT

T3

T2

T1

‣ trapping in extrinsic traps does occur

‣ generally: in a hopping system, trapping also within intrinsic density of states

Julia Schafferhans

Delayed Bimolecular Recombination

16

Scenario Modelling

solving the continuity equation• trapping and release• exponential DOS (intrinsic)

=> recombination order >2• here: R ! nfree pfree

due to phase separation

rec order=2

Poly

mer

Fulle

rene

et

but !cannot be extracted

et

direct recombinationof free polarons

rec order >2

Poly

mer

Fulle

rene

et

but !cannot be extracted

et

delayed recombinationdue to emission from trap

Recombination Order λ+1

17

3

Abbildung 3: log(p)� aufgetragen gegen die Zeit für verschiedene relative energetische Unordnungen σ̂

Abbildung 4: λ+1 aufgetragen gegen die Zeit für verschiedene relative energetische Unordnungen σ̂

log(E0/kT)

disordered

ordered order 2: no delay

order >2: delayed recombination

Jens Lorrmann

carrier concentration dependent mobility - CELIV: order > 2 also for cases with μ(n) = const

2-dimensional recombination - CELIV/TA: recombination order dep. on temperature

influence of trapping - TSC experiment: extrinsic traps present multiple-trapping-and-release: intrinsic traps sufficient

influence of phase separation - 2nd order recombination in neat polymer indicates: phase separation may play role in blendsʻ high order

order > 2 from delayed recombination due to trapping

Conclusions: Recombination Order >2

18

Thank You!

Acknowledgments

19

EP VI

deibel@physik.uni-wuerzburg.dewww.disorderedmatter.eu

Bayerische Akademie der Wissenschaften

And thanks for the Total Young

Investigators Award!