hui liu, jinjun liu, department of chemistry, hemant m. shah and bruce w. alphenaar, department of...
DESCRIPTION
Organic Solar Cell he LUMO HOMO Anode Cathode ITO electron donor PC 60 BM electron acceptor hνhν Light absorption and formation of an exciton (electron-hole pair); Electron promoted to LUMO of electron donor (e.g., ITO) Indium Tin Oxide Electron transfer from electron donor to accepter (e.g., PC 60 BM) [6,6]-phenyl-C 61 -butyric acid methyl ester Subsequently reach to electrode. How does Organic Solar Cell work? [1] e eTRANSCRIPT
OBSERVATION OF SINGLET-TRIPLET TRANSITIONS IN CAPACITIVE PHOTOCURRENT
SPECTROSCOPY OF ORGANIC SOLAR CELLS
HUI LIU, JINJUN LIU, Department of Chemistry,
HEMANT M. SHAH and BRUCE W. ALPHENAAR, Department of Electrical & Computer Engineering,
University of Louisville
Outline
• Introduction– Organic solar cell: PCBM /ITO– Capacitive photocurrent (CPC) spectroscopy
• Experimental Results• Quantum chemistry calculations and interpretation of
experimental results– Geometry optimization– Excitation energies– Electron collision induced singlet-to-triplet transitions
• Summary • Future Work
Organic Solar Cell
he
LUMO
HOMO
Anode
Cathode
ITOelectron
donorPC60BMelectronacceptor
hν
Light absorption and formation of an exciton (electron-hole pair);
Electron promoted to LUMO of electron donor (e.g., ITO)
Indium Tin Oxide
Electron transfer from electron donor to accepter (e.g., PC60BM)
[6,6]-phenyl-C61-butyric acid methyl ester
Subsequently reach to electrode.
How does Organic Solar Cell work?
[1] http://www.worldcommunitygrid.org/research/cep1/faq.do
ee
PCBM conformers
Cis (0°)Gauche (60°)Trans (180°)
PC60BM PC70BMα isomer[2]
85%
[2] Wienk, M.; Kroon, J. et al. Angew. Chem. 115, 3493, (2003)
G+G-
Capacitive Photocurrent Spectroscopy
Spectra Physics OPA, 130 fs pulsed, Ti:Sapphire laser, rep rate: 1 KHz
Vout
Quartz
PC60BMAg
ITO
Cu
[3] Tanesh Bansal; Bruce W. Alphenaar. et al. Carbon, 50(3), 808, (2012)
Spectra of PCBM/ITO
Singlet
Transition at ~ 800 nm; Singlet state of PC60BM
Low optical absorbance, high dissociation in CPC
New feature observed at 1250nm and 1800 nm
New features
PC70BMPC60BMHigher absorbance in PC70BM
due to relaxed symmetry compared to PC60BM
Previously unseen features observed in the CPC
spectrum
Absorption features
New features
947 nm and 1667 nm
Quantum Chemistry Calculation
Relative energy (cm-1)Cis Trans(TS)
PC60BM 1481 0 2815
HF/3-21G 1755 0 3402
G-
PC70BM 1459 0 12 2782
HF/3-21 G 1767 0 8 3372
Gauche
B3LYP/3-21G
B3LYP/3-21G
G+
Singlet—Triplet states
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
wavelength (nm)
(c) triplet transitions
400 600 800 1000 1200 1400 1600 1800 2000 2200 24000.00
0.01
0.02
abso
rban
ce (A
.U) (b) singlet transitions
400 600 800 1000 1200 1400 1600 1800 2000 2200 24000
2
4
cap
aciti
ve
phot
ocur
rent
(p
A)
(a) CPC spectra
(a) CPC spectra.(b) Calculated
transitions to singlet states.
(c) Calculated transitions to triplet states.
PC60BM by ZINDO/SScaled factor: 0.71 Triplet
Singlet—Triplet states
(a) CPC spectra.(b) Calculated
transitions to singlet states.
(c) Calculated transitions to triplet states.
PC70BM by ZINDO/S
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
wavelength (nm)
(c) triplet transitions
400 600 800 1000 1200 1400 1600 1800 2000 2200 24000.00
0.05
0.10
abso
rban
ce (A
.U) (b) singlet transitions
400 600 800 1000 1200 1400 1600 1800 2000 2200 240002468
10
cap
aciti
ve
phot
ocur
rent
(p
A)
(a) CPC spectraScaled factor: 0.94
Triplet
Proposed mechanism
1.Free carrier excitation
2. Electron collision
3. Transition to T states
• New peaks observed in the CPC spectra of PC60BM/ITO & PC70BM/ITO junction that are absent in the absorption spectroscopy
• Comparison of experimental results & quantum chemistry calculations suggest singlet-triplet transitions
• ZINDO/S calculations, when scaled according to the S1S0 excitation energies, well reproduces the CPC spectra
• A model based on electron collision induced transitions have been proposed to explain the new features in the CPC spectra
Summary
Future Work
(a) (b)
Ultrafast Transient Absorption Spectroscopy
Transient spectroscopy to study triplet state dynamics
Extend to other absorption organic solar cell
Acknowledgement
Professors:Dr. Jinjun LiuDr. Bruce Alphenaar
Friends:Samantha StromDustin CumminsMalika RizmanovaBokhodir Mamedov
Technical support:Cardinal Research Cluster
Financial support:NSFU.S. Department of Energy