Loss mechanisms in Polymer-Fullerene Solar Cells
43
Loss mechanisms in Polymer-Fullerene Solar Cells Carsten Deibel Julius-Maximilians-University of Würzburg 223 rd ECS meeting, Toronto 15 th May 2013 [email protected]
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A combination of transient photovoltage (TPV), voltage dependent charge extraction (CE) and time delayed collection field (TDCF) measurements is applied to poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b:4,5-b’]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]- phenyl-C71-butyric acid (PC71BM) bulk heterojunction solar cells to analyze the limitations of photovoltaic performance. Devices are processed from pure chlorobenzene (CB) solution and a subset was optimized with 1,8-diiodooctane (DIO) as co-solvent. The dramatic changes in device performance are discussed with respect to the dominating loss processes. While in the devices processed from CB solution, severe geminate and nongem- inate recombination is observed, the use of DIO facilitates efficient polaron pair dissociation and minimizes geminate recombination. Thus, from the determined charge carrier decay rate under open circuit conditions and the voltage dependent charge carrier densities n(V), the nongeminate loss current jloss of the samples with DIO alone enables us to reconstruct the current/voltage (j/V) characteristics across the whole operational voltage range. Geminate and nongeminate losses are considered to describe the j/V response of cells prepared without additive, but lead to a clearly overestimated device performance. We attribute the deviation between measured and reconstructed j/V characteristics to trapped charges in isolated domains of pure fullerene phases.
Transcript of Loss mechanisms in Polymer-Fullerene Solar Cells
Loss mechanisms in Polymer-Fullerene Solar CellsCarsten DeibelJulius-Maximilians-University of Würzburg
223rd ECS meeting, Toronto15th May [email protected]
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! [email protected]
Bayerische Akademie der Wissenschaften
EU, DBU, Elite network Bavaria
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