1 Ultrafast processes in molecules Mario Barbatti [email protected] Introduction.
Ultrafast processes in molecules
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description
Ultrafast processes in molecules. VII – Organic photovoltaics. Mario Barbatti [email protected]. Organic Photovoltaics (OPV). OPV advantages: Potential low cost Flexible, light, and thin Easy processing. Recent reviews: Mishra and Bäuerle , Angew Chem Int Ed 51 , 2020 (2012) - PowerPoint PPT Presentation
Transcript of Ultrafast processes in molecules
Organic Photovoltaics (OPV)
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OPV advantages:• Potential low cost• Flexible, light, and thin• Easy processing
Recent reviews:• Mishra and Bäuerle, Angew Chem Int Ed 51, 2020 (2012)• Shang, Li, Meng, Wang, Shuai, Theor Chem Acc 129, 291 (2011)• Carsten, Vladimir, Rep Prog Phys 73, 096401 (2010)• Brédas, Norton, Cornil, Coropceanu, Acc Chem Res 42, 1691 (2009)
Efficiency h
3• Green, Emery, Hishikawa, Warta, Dunlop, Prog Photovolt 19, 565 (2011)
Organic Photovoltaics (OPV)
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OPV advantages:• Potential low cost• Flexible, light, and thin• Easy processing
OPV drawbacks• Low efficiency (recombination, low charge mobility)• Short lifetime (oxidation, photochemical degradation)
OPV architecture
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Planar heterojunction (PHJ)
Bulk heterojunction (BHJ)D A PHJ (%) BHJ (%)
PA C60 1.1 a 1.6 b
6T C60 0.8 c 2.0 c
• a Mayer, Lloyd, Herman, Kasen, Malliaras, Appl Phys Lett 85, 6272 (2004)• b Sakai, Taima, Yamanari, Yoshida, Fujii, Ozaki, Jpn J Appl Phys 49, 032301 (2010)• c Sakai, Taima, Saito, Org Electron 9, 582 (2008)
OPV operation
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JSC
VOCVmp
Jmp
J
V
• OPVs are characterized in terms of macro-quantities like h, VOC, and JSC
• We do not expect to compute such quantities• But molecular computations may provide an indication of the adequacy of a D-A
D AD’ A’
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LE D’
1. Photoexcitation
LE D
2. Exciton diffusion
CT D→A+ -
3. Charge transfer
+ -CT D’→A’
4. Charge separation
5. R
ecom
bina
tion
O
PV p
hoto
phys
ics
Ideal electronic structure
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GS
CT D→A
LE Dbright
1. D should be photoexcited Large oscillator strength between
500 nm and 700 nm
2. CT D→A should be quickly populated with hot polarons LE D should be above and near CT D→A
D A
• To check these features, we need to classify the electronic states of the D-A complex
3. CT D→A should have a long lifetime
Large energy gap between CT D→A and states below
State classification
9• Crespo-Otero and Barbatti, Theor Chem Acc 131, 1237 (2012)
A B, ,
kA k k k kA A A B
c c S c c S
+
Using a Mulliken partition:
i
jj iA A
j iA A
+
-
Degree of delocalization over A and B
Amount of CT between A and B
II i j i j
i j
C
Considering a multielectronic wavefunction:
2 jI I ii jA A A
i jP C
- 2 jI I i
i jA A Ai j
P C
+
State classification
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0.0 0.5 1.0 1.5 2.0-1.0
-0.5
0.0
0.5
1.0
CT BA
CT AB
LOC(A)
PA
PA
DELOCLOC(B)
Benchmarking…
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Systematic investigation of the effects of:• D-A distance• D-A orientation• Oligomer size• Chemical environment• Chemical functionalization• Theoretical level
Computational details
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• wB97X-D• 6-31G(d)• S0 D-A optimization• TDDFT: 40-70 states• G09• State classification
• Example: P3HT-PCBM• 240 atoms• 2 days (Xeon 3.3 GHz
10 cores)
S
S 3
PC60BM
C6H13
C6H13
3
2.5
3.0
3.5
4.0
4.5
Ener
gy (e
V)
Dependence on functional
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• Extremely dependent on functional!• CTs are wrong without range-separation• C60 bands are blue shifted with range
separation.
2.0
2.5
3.0
3.5
4.0
4.5
wB97X-D(6-311G(df,2p))
M06-HFB3LYP PBE0 wB97X-D(6-31G(d))
M06-2X CAM-B3LYP
LC-BLYP( = 0.47)
LC-BLYP( = 0.29)
Ener
gy (e
V)
LC-BLYP( = 0.2)
f > 0.2
LOC(A)LOC(D)CT DACT ADDELOC
Double excitations…
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2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.52.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
21Gg
21B
6T C60
TDDF
T En
ergy
(eV)
DFT/MRCI Energy (eV)
30.0037.5045.0052.5060.0067.5075.0082.5090.00
% Singles
21A
• Constant blue shift between TDDFT and DFT/MRCI
• Exceptions: states with large multiple excitation character
Photophysics of D-A junctions
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2.5
3.0
3.5
4.0
4.5 T1uGgHu
HuGuT2u
Hg
Gg
C60
Ener
gy (e
V)
6T 6T-C60
T1g,T2g
B
A
DOS PA f Singlet Triplet
f > 0.2
LOC(A)LOC(D)CT DACT ADDELOC
GS
CT D→A
LE Dbright
• How much does the electronic structure of the D-A complexes resemble the ”Ideal Electronic Structure”?
Dependence on D-A distance
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• LOC does not show strong dependence on D-A distance
• CT is stabilized at short distances (Coulomb r -1 is expected)
0.00.22.5
3.0
3.5
4.0
4.5
2.8 3.0 3.2 3.4 3.6 3.8 4.02.5
3.0
3.5
4.0
4.5
f > 0.2
Ener
gy (e
V)
LOC(A)LOC(D)CT DACT ADDELOC
R0
CS
E (e
V)
D-A distance (Å)
Dependence on D-A orientation
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LOC(A)LOC(D)DELOC
CT A→DCT D→A2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
PA : C60 (perp)
Ener
gy (e
V)
QP : C60
Ener
gy (e
V)
1.9
TBP : C60
Ener
gy (e
V)
0.91.0
5T : C60
Ener
gy (e
V)
1.3
DCV5T : C60
Ener
gy (e
V)2.5
PA : C60 (stack)
Ener
gy (e
V)
BCBP : C60
Ener
gy (e
V)
1.2
4T : C60
Ener
gy (e
V) 1.0
5T : PC60BM
Ener
gy (e
V) 1.0
BTDA : C60
Ener
gy (e
V)
1.2
0.8
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
2.0
2.4
2.8
3.2
3.6
4.0
PA : C60 (perp)
Ener
gy (e
V)
QP : C60
Ener
gy (e
V)
1.9
TBP : C60
Ener
gy (e
V)
0.91.0
5T : C60
Ener
gy (e
V)
1.3
DCV5T : C60
Ener
gy (e
V)
2.5
PA : C60 (stack)
Ener
gy (e
V)
BCBP : C60
Ener
gy (e
V)
1.2
4T : C60
Ener
gy (e
V) 1.0
5T : PC60BM
Ener
gy (e
V) 1.0
BTDA : C60
Ener
gy (e
V)
1.2
0.8
• LOCs are not affected• CTs are strongly affected• CTs are stabilized by stacking
Dependence on D size
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• LOC(D)s are affected, but not LOC(A)s
• CTs are affected
S
S n/2
C60
2 4 6 8 10 12 142.5
3.0
3.5
4.0
4.5
f > 0.2
Ener
gy (e
V)
Number of monomers
LOC(A)LOC(D)CT DACT ADDELOC
Functionalization
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2.5
3.0
3.5
4.0
4.5
6T-C60
Ener
gy (e
V)
P3HT-C60 P3HT-PCBMDOS PA f
f > 0.2
LOC(A)LOC(D)CT DACT ADDELOC
Environment
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1 2 3 4 5 6 7 8 162.5
3.0
3.5
4.0
4.5En
ergy
(eV)
Dielectric Constant
f > 0.2
LOC(A)LOC(D)CT DACT ADDELOC
