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