(1) The document discusses loss mechanisms in polymer-fullerene solar cells that limit performance. (2) With additive, nongeminate recombination occurs via free carrier and trap-assisted recombination. Reconstruction works well. (3) Without additive, incomplete reconstruction is due to voltage-dependent photogeneration from spatial trapping on fullerene islands.

1. Loss mechanisms
in Polymer-Fullerene Solar Cells
Carsten Deibel
Julius-Maximilians-University of Würzburg
223rd ECS meeting, Toronto
15th May 2013
deibel@disorderedmatter.eu

2. How Do Organic Solar Cells Work?
2
Step 1: Light Absorption
➟ Exciton Generation in Polymer
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
Voltage
Current

3. How Do Organic Solar Cells Work?
3
Step 2: Exciton Diffusion
➟ to Acceptor Interface
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
Voltage
Current
singlet losses

4. Step 3: Exciton Dissociation
➟ Polaron Pair Generation
How Do Organic Solar Cells Work?
4
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
charge transfer:
very fast and
very efficient
Voltage
Current
singlet losses

5. How Do Organic Solar Cells Work?
5
Step 4: Polaron Pair Dissociation
➟ Free Electron–Hole Pairs!
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
Voltage
Current
singlet losses
geminate losses

6. How Do Organic Solar Cells Work?
6
Step 5: Charge Transport
➟ Photocurrent
Fullerene
Aluminium Cathode
Transparent Anode
Polymer
Voltage
Current
singlet losses
geminate losses
nongeminate losses

7. for instance, PTB7:PC70BM 1:1.5
What are we looking at?
7
glass
PEDOT
V












additive DIO

8. for instance, PTB7:PC70BM 1:1.5
What are we looking at?
7
300
200
100
0
-100
currentdensity[A/m
2
]
0.80.60.40.20.0
voltage [V]
dark 1 sun
w/o add
with add
PCE [%] FF [%]
w/o add 3.8 51
with add 7.1 69
glass
PEDOT
V

9. PTB7:PC70BM 1:1.5 Morphology
8
phase
height
w/o additive, 3.8%
Alex Förtig
nm
nm

10. with additive, 7.1%
PTB7:PC70BM 1:1.5 Morphology
8
phase
height
w/o additive, 3.8%
Alex Förtig
nm
nm

11. Which processes are
limiting the
performance of these
organic solar cells?

12. Outline
10

13. Outline
10
conclusions
implications on organic
solar cell performance
nongeminate
recombination
with additive
geminate
recombination
without additive

14. Outline
10
conclusions
implications on organic
solar cell performance
nongeminate
recombination
with additive
geminate
recombination
without additive

15. j(V ) = e
Z
(G R) dx
⇡ jgen jloss(V )
jgen ⇡ jsc
Current–Voltage Reconstruction ...
11
From the continuity equation:
Voltage
Current
jloss(V ) /
n(V )
⌧(n)

16. 12
n(V) by charge extraction
act of Solvent Additive on PTB7:PC71BM Solar Cells 65
2
4
10
21
2
4
10
22
0.80.60.40.20.0
voltage [V]
10
21
2
4
10
22
2
4
chargecarrierdensity[m
-3
]
with add
w/o add
0.03 sun
1 sun
: Voltage dependent charge carrier density n(V ) from charge ex-
periments for PTB7:PC71BM devices with and without additive at
Alex Förtig
jloss(V ) /
n(V )
⌧(n)
Nongem. Loss Current

17. τ(n) by transient photovoltage
12
n(V) by charge extraction
act of Solvent Additive on PTB7:PC71BM Solar Cells 65
2
4
10
21
2
4
10
22
0.80.60.40.20.0
voltage [V]
10
21
2
4
10
22
2
4
chargecarrierdensity[m
-3
]
with add
w/o add
0.03 sun
1 sun
: Voltage dependent charge carrier density n(V ) from charge ex-
periments for PTB7:PC71BM devices with and without additive at
with add.
4
6
8
10
2
4
6
8
100
lifetime[µs]
3 4 5 6 7 8 9
10
22
2
charge carrier density [m
-3
]
1sun
1sun
w/out add.
Alex Förtig
jloss(V ) /
n(V )
⌧(n)
Nongem. Loss Current

18. reconstruction works well
... with Additive
Origin of nongeminate
recombination?
13
-150
-100
-50
0
50
CurrentDensity[A/m
2
]
0.60.40.20.0
Voltage [V]
meas. PL reconstr.
1 sun
0.32 sun
0.03 sun
Alex Förtig

19. 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
}

20. Expected:
Back to PTB7: Expected vs Observed
15Adv. Funct. Mater. 2, 1483 (2012)
3
4
5
6
7
8
9
10
-20
2
3
µ[Am]
4 6 8
10
21
2 4 6 8
10
22
2 4 6 8
10
23
charge carrier density [m
-3
]
10
-18
2
3
4
5
6
7
8
9
10
-17
k[m
3
s
-1
]
PTB7:PC!
with additive
71BM
~
T=300 K

21. Expected:
Back to PTB7: Expected vs Observed
15Adv. Funct. Mater. 2, 1483 (2012)
3
4
5
6
7
8
9
10
-20
2
3
µ[Am]
4 6 8
10
21
2 4 6 8
10
22
2 4 6 8
10
23
charge carrier density [m
-3
]
10
-18
2
3
4
5
6
7
8
9
10
-17
k[m
3
s
-1
]
PTB7:PC!
with additive
71BM
~
T=300 K

22. Expected:
Back to PTB7: Expected vs Observed
15Adv. Funct. Mater. 2, 1483 (2012)
3
4
5
6
7
8
9
10
-20
2
3
µ[Am]
4 6 8
10
21
2 4 6 8
10
22
2 4 6 8
10
23
charge carrier density [m
-3
]
10
-18
2
3
4
5
6
7
8
9
10
-17
k[m
3
s
-1
]
PTB7:PC!
with additive
71BM
~
T=300 K

23. 6/
Expected:
Back to PTB7: Expected vs Observed
15Adv. Funct. Mater. 2, 1483 (2012)
3
4
5
6
7
8
9
10
-20
2
3
µ[Am]
4 6 8
10
21
2 4 6 8
10
22
2 4 6 8
10
23
charge carrier density [m
-3
]
10
-18
2
3
4
5
6
7
8
9
10
-17
k[m
3
s
-1
]
PTB7:PC!
with additive
71BM
~
T=300 K

24. Trap Tail States by Thermally Stimulated Currents
Trapping is Important
Trap density = Lower Limit
Shape roughly exponential,
energy tail ~90 meV
16Julia Rauh
10
21
2
3
4
5
6
7
8
9
10
22
trapdensity[m
-3
]
0.300.250.200.150.100.050.00
Energy [eV]
PTB7:PC70BM
with DIO

25. Transient Absorption
Nongeminate Decay Dynamics
17
4
6
8
10
-5
2
4
6
8
10
-4
2
4
6
8
ΔOD[a.U]
10
-7
10
-6
10
-5
10
-4
10
-3
Time [s]
PTB7:PC71BM
with additive
300K 150K
4.5K
Clemens Grünewald, Julia Kern

26. Transient Absorption
Nongeminate Decay Dynamics
17
4
6
8
10
-5
2
4
6
8
10
-4
2
4
6
8
ΔOD[a.U]
10
-7
10
-6
10
-5
10
-4
10
-3
Time [s]
PTB7:PC71BM
with additive
300K 150K
4.5K
fast free–free
(Langevin type)
recombination
Clemens Grünewald, Julia Kern

27. Transient Absorption
Nongeminate Decay Dynamics
17
4
6
8
10
-5
2
4
6
8
10
-4
2
4
6
8
ΔOD[a.U]
10
-7
10
-6
10
-5
10
-4
10
-3
Time [s]
PTB7:PC71BM
with additive
300K 150K
4.5K
fast free–free
(Langevin type)
recombination
slow free–trapped
recombination
Clemens Grünewald, Julia Kern

28. Outline
18
conclusions
implications on organic
solar cell performance
nongeminate
recombination
with additive
geminate
recombination
without additive

29. 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
CurrentDensity[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

30. 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
CurrentDensity[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
measured/reconstructed
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:

31. Time Delayed Collection Field → P(V)
Main Reason: Photogeneration
w/out additive: voltage
dependent photogeneration
20Alex Förtig
5
6
7
8
9
1
Qtot/Q(-5V)
-5 -4 -3 -2 -1 0
prebias voltage [V]
w/o add
data
fit
with add
data origin unclear

32. j(V ) = e
Z
(G R) dx
⇡ jgen jloss(V )
jgen ⇡ jsc
Current–Voltage Reconstruction ...
21
From the continuity equation:
Voltage
Current
jloss(V ) /
n(V )
⌧(n)

33. j(V ) = e
Z
(G R) dx
⇡ jgen(V ) jloss(V )
Current–Voltage Reconstruction ...
22
From the continuity equation:
jloss(V ) /
n(V )
⌧(n)
Voltage
Current
jgen(V ) ⇡ jsc · P(V )

34. Time Delayed Collection Field
Reconstruction incl. Geminate Loss
23
5
6
7
8
9
1
Qtot/Q(-5V)
-5 -4 -3 -2 -1 0
prebias voltage [V]
w/o add
data
fit
with add
data

35. Time Delayed Collection Field
Reconstruction incl. Geminate Loss
23
5
6
7
8
9
1
Qtot/Q(-5V)
-5 -4 -3 -2 -1 0
prebias voltage [V]
w/o add
data
fit
with add
data
-80
-60
-40
-20
0
currentdensity[A/m
2
]
0.80.60.40.2
voltage [V]
0.18 sun
1 sun
w/o Add
measurement
reconstruction
j (V)gen

36. 2
4
10
21
2
4
10
22
0.80.60.40.20.0
voltage [V]
10
21
2
4
10
22
2
4
chargecarrierdensity[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

37. „Nanomorphology“ by PL
25Björn Gieseking
1.0
0.8
0.6
0.4
0.2
0.0
Photoluminescence(norm.)
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

38. „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
Photoluminescence(norm.)
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

39. ...on fullerene islands
Scenario: Spatial Trapping...
without additive
26
Fullerene
Polymer
Aluminium Cathode
Transparent Anode
Fullerene
Polymer
Aluminium Cathode
Transparent Anode
with additive

40. Conclusions
27
with additive, 7.1% w/o additive, 3.8%













41. Conclusions
27
with additive, 7.1% w/o additive, 3.8%












nongeminate recombination
free carrier and trap assisted
recombination

42. 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 trapping
on fullerene

43. Thanks to Prof.
Dyakonov and
Würzburg group!
Thank you!
deibel@disorderedmatter.eu
Bayerische
Akademie der Wissenschaften
EU, DBU, Elite network Bavaria