loss mechanisms in polymer-fullerene solar cells

Loss mechanisms in Polymer-Fullerene Solar CellsCarsten DeibelJulius-Maximilians-University of Würzburg

223rd ECS meeting, Toronto15th May 2013deibel@disorderedmatter.eu

How Do Organic Solar Cells Work?

2

Step 1: Light Absorption ➟ Exciton Generation in Polymer

Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

Voltage

Current

How Do Organic Solar Cells Work?

3

Step 2: Exciton Diffusion➟ to Acceptor Interface

Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

Voltage

Current

singlet losses

Step 3: Exciton Dissociation ➟ Polaron Pair Generation

How Do Organic Solar Cells Work?

4

Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

charge transfer: very fast and very efficient

Voltage

Current

singlet losses

How Do Organic Solar Cells Work?

5

Step 4: Polaron Pair Dissociation➟ Free Electron–Hole Pairs!

Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

Voltage

Current

singlet lossesgeminate losses

How Do Organic Solar Cells Work?

6

Step 5: Charge Transport ➟ Photocurrent

Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

Voltage

Current

singlet lossesgeminate losses

nongeminate losses

for instance, PTB7:PC70BM 1:1.5

What are we looking at?

7glass

PEDOT

V

additive DIO

for instance, PTB7:PC70BM 1:1.5

What are we looking at?

7

300

200

100

0

-100

curre

nt d

ensi

ty [

A/m

2 ]

0.80.60.40.20.0

voltage [V]

dark 1 sunw/o add with add

PCE [%] FF [%]

w/o add 3.8 51

with add 7.1 69glass

PEDOT

V

PTB7:PC70BM 1:1.5 Morphology

8

phas

ehe

ight

w/o additive, 3.8%

Alex Förtig

nm

nm

with additive, 7.1%

PTB7:PC70BM 1:1.5 Morphology

8

phas

ehe

ight

w/o additive, 3.8%

Alex Förtig

nm

nm

Which processes are limiting the

performance of these organic solar cells?

Outline

10

Outline

10

conclusions

implications on organicsolar cell performance

nongeminate recombination

with additive

geminate recombination

without additive

Outline

10

conclusions

implications on organicsolar cell performance

nongeminate recombination

with additive

geminate recombination

without additive

j(V ) = e

Z(G�R) dx

⇡ j

gen

� j

loss

(V )

jgen ⇡ jsc

Current–Voltage Reconstruction ...

11

From the continuity equation:

Voltage

Current

jloss

(V ) / n(V )

⌧(n)

12

n(V) by charge extraction5.2. Impact of Solvent Additive on PTB7:PC71BM Solar Cells 65

2

4

1021

2

4

1022

0.80.60.40.20.0

voltage [V]

1021

2

4

1022

2

4

char

ge c

arrie

r den

sity

[m-3

]

with add

w/o add

0.03 sun

1 sun

Figure 5.12: Voltage dependent charge carrier density n(V ) from charge ex-traction experiments for PTB7:PC71BM devices with and without additive atthree different light intensities.

range were performed, in analogy to the measurements at V

oc

described onpage 63. All voltages were corrected for the series resistance R

s

by calculatingV = V

app

� R

s

I. From the ohmic range of the dark j/V curve, the valuesR

s

⇡ 84 ⌦ for the device with additive and R

s

= 105 ⌦ for the one withoutadditive were derived. The voltage dependent charge carrier density for bothdevices is shown in Fig. 5.12 for three different light intensities.

The n(V ) relation and the dependence of ⌧ on n found under V

oc

conditions(Fig. 5.10) is used to calculate the charge carrier density dependent recom-bination rate R(n(V )) for the respective applied voltage by Eq. (2.4). Thisdata was fed into Eq. (4.14), which allowed to determine the nongeminaterecombination current j

loss

(n(V )).As the photogeneration of the sample with additive was voltage indepen-

dent, as shown in Fig. 5.11, the respective generation current j

gen

was assumedto be constant and set equal to the short circuit current density,

j

gen

⇡ j

sc

, (5.5)

similar to the case of P3HT:PC61BM (Sec. 5.1) and the approach in Ref. [58,94].

Instead, for the solar cell spin coated from pure CB solution, the voltagedependent polaron pair dissociation PP(V ) derived by TDCF is substantial

Alex Förtig

jloss

(V ) / n(V )

⌧(n)Nongem. Loss Current

τ(n) by transient photovoltage

12

n(V) by charge extraction5.2. Impact of Solvent Additive on PTB7:PC71BM Solar Cells 65

2

4

1021

2

4

1022

0.80.60.40.20.0

voltage [V]

1021

2

4

1022

2

4

char

ge c

arrie

r den

sity

[m-3

]

with add

w/o add

0.03 sun

1 sun

Figure 5.12: Voltage dependent charge carrier density n(V ) from charge ex-traction experiments for PTB7:PC71BM devices with and without additive atthree different light intensities.

range were performed, in analogy to the measurements at V

oc

described onpage 63. All voltages were corrected for the series resistance R

s

by calculatingV = V

app

� R

s

I. From the ohmic range of the dark j/V curve, the valuesR

s

⇡ 84 ⌦ for the device with additive and R

s

= 105 ⌦ for the one withoutadditive were derived. The voltage dependent charge carrier density for bothdevices is shown in Fig. 5.12 for three different light intensities.

The n(V ) relation and the dependence of ⌧ on n found under V

oc

conditions(Fig. 5.10) is used to calculate the charge carrier density dependent recom-bination rate R(n(V )) for the respective applied voltage by Eq. (2.4). Thisdata was fed into Eq. (4.14), which allowed to determine the nongeminaterecombination current j

loss

(n(V )).As the photogeneration of the sample with additive was voltage indepen-

dent, as shown in Fig. 5.11, the respective generation current j

gen

was assumedto be constant and set equal to the short circuit current density,

j

gen

⇡ j

sc

, (5.5)

similar to the case of P3HT:PC61BM (Sec. 5.1) and the approach in Ref. [58,94].

Instead, for the solar cell spin coated from pure CB solution, the voltagedependent polaron pair dissociation PP(V ) derived by TDCF is substantial

with add.

4

68

10

2

4

68

100

lifet

ime

[µs]

3 4 5 6 7 8 9

1022

2

charge carrier density [m-3

]

1sun

1sun

w/out add.

Alex Förtig

jloss

(V ) / n(V )

⌧(n)Nongem. Loss Current

reconstruction works well

... with Additive

Origin of nongeminate recombination?

13

-150

-100

-50

0

50C

urre

nt D

ensi

ty [A

/m2 ]

0.60.40.20.0

Voltage [V]

meas. PL reconstr. 1 sun 0.32 sun 0.03 sun

Alex Förtig

LUMO

HOMO

(1)

(2)

(1)

expected in nongeminate loss in low mobility materials

Langevin Recombination

(1) finding of charge carriers → mobility μ(2) recombination event (faster than (1))

14

R(n) / µ(n)n2}

Expected:

Back to PTB7: Expected vs Observed

15Adv. Funct. Mater. 2, 1483 (2012)

3

4

5

6789

10-20

2

3

µ [A

m]

4 6 81021

2 4 6 81022

2 4 6 81023

charge carrier density [m-3]

10-18

2

3

4

5

678910-17

k [m3s

-1]

PTB7:PC!with additive

71BM

~

T=300 K

Expected:

Back to PTB7: Expected vs Observed

15Adv. Funct. Mater. 2, 1483 (2012)

3

4

5

6789

10-20

2

3

µ [A

m]

4 6 81021

2 4 6 81022

2 4 6 81023

charge carrier density [m-3]

10-18

2

3

4

5

678910-17

k [m3s

-1]

PTB7:PC!with additive

71BM

~

T=300 K

Expected:

Back to PTB7: Expected vs Observed

15Adv. Funct. Mater. 2, 1483 (2012)

3

4

5

6789

10-20

2

3

µ [A

m]

4 6 81021

2 4 6 81022

2 4 6 81023

charge carrier density [m-3]

10-18

2

3

4

5

678910-17

k [m3s

-1]

PTB7:PC!with additive

71BM

~

T=300 K

6/

Expected:

Back to PTB7: Expected vs Observed

15Adv. Funct. Mater. 2, 1483 (2012)

3

4

5

6789

10-20

2

3

µ [A

m]

4 6 81021

2 4 6 81022

2 4 6 81023

charge carrier density [m-3]

10-18

2

3

4

5

678910-17

k [m3s

-1]

PTB7:PC!with additive

71BM

~

T=300 K

Trap Tail States by Thermally Stimulated Currents

Trapping is Important

Trap density = Lower Limit

Shape roughly exponential, energy tail ~90 meV

16Julia Rauh

1021

2

3

4

5

6789

1022

trap

dens

ity [m

-3]

0.300.250.200.150.100.050.00

Energy [eV]

PTB7:PC70BMwith DIO

Transient Absorption

Nongeminate Decay Dynamics

17

46810-5

2

46810-4

2

468

ΔO

D [a

.U]

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

Time [s]

PTB7:PC71BMwith additive

300K 150K

4.5K

Clemens Grünewald, Julia Kern

Transient Absorption

Nongeminate Decay Dynamics

17

46810-5

2

46810-4

2

468

ΔO

D [a

.U]

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

Time [s]

PTB7:PC71BMwith additive

300K 150K

4.5K

fast free–free (Langevin type) recombination

Clemens Grünewald, Julia Kern

Transient Absorption

Nongeminate Decay Dynamics

17

46810-5

2

46810-4

2

468

ΔO

D [a

.U]

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

Time [s]

PTB7:PC71BMwith additive

300K 150K

4.5K

fast free–free (Langevin type) recombination

slow free–trappedrecombination

Clemens Grünewald, Julia Kern

Outline

18

conclusions

implications on organicsolar cell performance

nongeminate recombination

with additive

geminate recombination

without additive

PTB7:PC70BM 1:1.5 w/o additive

I–V Reconstruction

Why?

19

reconstruction incomplete

j(V ) = jsc

� jloss

(n(V ))

-50

-40

-30

-20

-10

0

10

Cur

rent

Den

sity

[A/m

2 ]

0.80.60.40.2

Voltage [V]

meas. PL reconstr. 0.56 sun 0.32 sun 0.18 sun 0.03 sun

PTB7:PC70BM 1:1.5 w/o additive

I–V Reconstruction

Why?

19

reconstruction incomplete

j(V ) = jsc

� jloss

(n(V ))

-50

-40

-30

-20

-10

0

10

Cur

rent

Den

sity

[A/m

2 ]

0.80.60.40.2

Voltage [V]

meas. PL reconstr. 0.56 sun 0.32 sun 0.18 sun 0.03 sun

1.0

0.8

0.6

0.4

0.2

mea

sure

d/re

cons

truct

ed

0.80.60.40.2

Voltage [V]

ratio PL Voc 0.03 sun 0.18 sun 0.32 sun 0.56 sun 1 sun

First try:

Time Delayed Collection Field → P(V)

Main Reason: Photogeneration

w/out additive: voltagedependent photogeneration

20Alex Förtig

5

6

7

8

9

1

Qto

t / Q

(-5

V)

-5 -4 -3 -2 -1 0prebias voltage [V]

w/o add data fit

with add data origin unclear

j(V ) = e

Z(G�R) dx

⇡ j

gen

� j

loss

(V )

jgen ⇡ jsc

Current–Voltage Reconstruction ...

21

From the continuity equation:

Voltage

Current

jloss

(V ) / n(V )

⌧(n)

j(V ) = e

Z(G�R) dx

⇡ j

gen

(V )� j

loss

(V )

Current–Voltage Reconstruction ...

22

From the continuity equation:

jloss

(V ) / n(V )

⌧(n)

Voltage

Current

jgen(V ) ⇡ jsc · P (V )

Time Delayed Collection Field

Reconstruction incl. Geminate Loss

23

5

6

7

8

9

1

Qto

t / Q

(-5

V)

-5 -4 -3 -2 -1 0prebias voltage [V]

w/o add data fit

with add data

Time Delayed Collection Field

Reconstruction incl. Geminate Loss

23

5

6

7

8

9

1

Qto

t / Q

(-5

V)

-5 -4 -3 -2 -1 0prebias voltage [V]

w/o add data fit

with add data

-80

-60

-40

-20

0

curr

ent d

ensi

ty [A

/m2 ]

0.80.60.40.2

voltage [V]

0.18 sun

1 sun

w/o Add

measurement reconstruction

j (V)gen

2

4

1021

2

4

1022

0.80.60.40.20.0

voltage [V]

1021

2

4

1022

2

4

char

ge c

arrie

r den

sity

[m-3

]

with add

w/o add

0.03 sun

1 sun

What if...

reverse reconstruction: n(V) from I(V)

24Alex Förtig

incomplete extraction

„Nanomorphology“ by PL

25Björn Gieseking

1.0

0.8

0.6

0.4

0.2

0.0

Pho

tolu

min

esce

nce

(nor

m.)

2.01.81.61.41.2

Energy / eV

1.0

0.5

0.0

1100 1000 900 800 750 700 650

Wavelength / nm

w/o add. 3 % DIO

PTB7 PC71BM

„Nanomorphology“ by PL

additive: relative decrease of fullerene PL→ smaller fullerene domains

25Björn Gieseking

1.0

0.8

0.6

0.4

0.2

0.0

Pho

tolu

min

esce

nce

(nor

m.)

2.01.81.61.41.2

Energy / eV

1.0

0.5

0.0

1100 1000 900 800 750 700 650

Wavelength / nm

w/o add. 3 % DIO

PTB7 PC71BM

...on fullerene islands

Scenario: Spatial Trapping...

without additive

26

Fulle

rene

Polymer

Aluminium Cathode

Transparent Anode

Fulle

rene

Polymer

Aluminium Cathode

Transparent Anode

with additive

Conclusions

27

with additive, 7.1% w/o additive, 3.8%

Conclusions

27

with additive, 7.1% w/o additive, 3.8%

nongeminate recombination

free carrier and trap assisted recombination

Conclusions

27

with additive, 7.1% w/o additive, 3.8%

nongeminate recombination

free carrier and trap assisted recombination

geminate & nongeminate

field dependent photogeneration

spatial trappingon fullerene

Thanks to Prof. Dyakonov and Würzburg group!

Thank you! deibel@disorderedmatter.eu

Bayerische Akademie der Wissenschaften

EU, DBU, Elite network Bavaria