ESRにESRによる高効率有機太陽電池のよる高効率有機太陽電池の評価と素子高効率有機太陽電池の評価と素子特性向上
評価と素子特性向上

筑波大数物、筑波大学学際センター
丸本一弘

数理物質融合科学センター（CiRfSE）第１回ワークショップ、筑波大学、つくば、2015年3月5日

Outline
1. Electron spin resonance (ESR)
New analytical method of organic solar cells
2. ESR study of organic solar cells
Direct observation of charge accumulation
during device operation
1) P3HT:PC61BM blend films
Power conversion efficiency 3.5%2) PTB7:PC71BM blend films
High power conversion efficiency 8.5%
Light-induced ESR and performance of devices⇒ analyze the internal states of solar cellsIntrinsic deterioration mechanism
3. Summary

Why ESR is useful for organic devices?
Traditional research methods for organic devices• Electrical measurements (current, voltage,…)• Scanning microscopes (AFM, STM,…), etc.
cannot directly observe internal states in organic devices.
Understanding of device operation
Improvement of device performance
Advantages of ESR for organic devices:• In situ direct observation of internal states in organic devices• High sensitive and high precision analysis at molecular level~1010 spins are enough for ESR measurement• Quantitative measurements of charges with spins in devices
First ESR application to organic devices was reported in 2004
J. Phys. Soc. Jpn. 73 (2004) 1673.

Principal of ESR and system
High sensitive and precision analytical method
Evaluation of organic devices at the molecular level
Principle of ESR

ESR system

Electron energy in magnetic field H
Spin Hamiltonian:

Microwave
generator

H = μBH⋅g⋅S = gμBHms (ms = ±1/2)0hν= gμBH00.03 meV (0.3 K)

ms = −0dIdH

HH0Hhν

H0 = gμ12Shν
ElectromagnetE1
ms =2Detector

SampleCavity resonator

Microwave resonant absorption inmagnetic field for charges withmagnetic moment, spin SB0H
∆HppResonant magnetic field: g value
A unique value for material

Organic solar cells
Features・Advantages: low cost, flexible, light weight・Disadvantages: low efficiency and durability
Recently, ∼12% efficiency has been reported
Sumitomo ChemicalProblems for practical application
・Further improvement of performance: more than 15%
・Elucidation of degradation mechanisms
Deterioration mechanism due to an intrinsic problem,
not due to extrinsic problems such as O2 and H2O・Charge accumulation in devices is reported
for polymer solar cells
Charge accumulation deteriorate device performanceDeterioration and recovery of performance
P3HT:PCBM solar-cell performance is deteriorated by irradiation
and recovered by thermal annealing under N2 condition• Reversible• No IPCE change
No molecular degradation• Intrinsic problem,
not extrinsic problems
Deterioration due tocharge accumulation

T. Yamanari et al., 2010 35th PVSC IEEE,(2010) 001628.

• Extrinsic problems can besolved by device sealing.
• However, the sealing cannot
solve intrinsic problem.

Which molecules charges accumulate? Where?

Charge accumulation in organic solar cells

AAl
LiF+- - - P3HT:PCBM+ + + +PEDOT:PSS
ITO
quartz substrate

For ideal solar cell, no charge
accumulation occurs
One photon creates hole and electron,
which are collected by electrodes.
However, in actual solar cells,
what happens?
…Charges accumulate.

Simulated solar irradiation

Microscopic analysis of organic solar cells
by detecting charge accumulation during device operation
The sites can be clarified by unique g values for materials

What happens from charge accumulation?
Distance of charge
carrier’s motion
longJsc

JscInteraction between charge carriers
and accumulated charges
Accumulated charges
cause carrier scattering,decreasing Jsc

ESR and Jsc

Voc

Voc

Accumulated charges
create additional potential,
decreasing Voc

ESR and Voc

shortshort

10 µs

1s

1hESR

Lifetime ofaccumulated
longcharge

Device structure of P3HT:PCBM cells for ESR
OMe
O3 mmQuartz
substrate

PCBM: n-type

P3HT: p-type
OOOO+SSSOITO

Pd/LiF/AlmO

PEDOT:PSS: hole buffer
Quartzglass tubeA

PEDOT:PSS

Al
Pd/LiF
P3HT:PCBMPEDOT:PSS
ITO
quartz substrate

・Pd/LiF/Al (1.2/0.6/50 nm)
・P3HT:PCBM (165 nm)
Bulk heterojunction
active layer
・PEDOT:PSS (40 nm)
・ITO: transparent electrode

P3HT:PCBM

Device was sealed in an
ESR sample tube undervacuum condition toavoid extrinsic problems

Light-induced ESR system
SignalAnalyzer

Phase
Detector
Microwave
BridgeKeithley 2612A
Source Meter
Unit

MagnetElectromagnet

Irradiation

SolarSimulator

JEOL FA200 X-band ESR spectrometer

Bunkoukeiki OTENTOSUN-150BMX
DeviceCavity resonator

TE011 cylindrical cavity
Optical window (φ = 8 mm)

Optical system: large irradiation
area more than φ = 10 mm

Simultaneous measurements of ESR and device performance at RT

ESR signals of ITO/P3HT:PCBM
Light-induced ESR (LESR):
difference between signals underirradiation and dark condition

ESR signal (IMn unit)

ITO/P3HT:PCBM
0.5

darkirrad.
LESR

0g-valueg = 2.00215-0.5 ∆Hpp = 0.204 mT
321

321.5

322

Magnetic Field (mT)
C6H13SSC6H13SSC6H13

nC6H13

yWatanabe et al., APL96 (2010) 173302.

zP3HT
radicalcation

PCBM
radicalanion

2.00152–
2.003101.998452.00058

O. G. Poluektov et al., J. Phys. Chem B 114(2010) 14426.

x= 2.00310
gx =gx2.00310= 2.00152
gy =gy2.00152
g = 2.00203
gz = z2.00203

Hole accumulation in P3HT
in the film by irradiation
Hole = Radical cation= Positive polaron

Absence of PCBM signal is due to the signal
broadening from fast spin-relaxation time at RT

Transient response of hole accumulation
ITO/P3HT:PCBM
Irradiation

ITO/PEDOT:PSS/P3HT:PCBM
Dark
Irradi.

Dark

2

Nspin (1012)

12Nspin (10 )Irradiation off

Irradiation off
121
Deeply trappedcomponent

010Time (h)Irradiation on

PEDOT:PSS insertion
Hole accumulation withslow rate is enhanced

2001020Time (h)3040

Irradiation on

Slow hole accumulation:
deep trappings in P3HT at
PEDOT:PSS/P3HT:PCBM interface
Fast component:bulk P3HT:PCBM materials

LESR signals under short-circuit conditions

irradiation timedark
0~1 h3~4 h
6~7 h
12~13 h18~19 h

316

318

320

LESR

Increase in LESR signals
LESR signal (IMn unit)

ESR signals under solar irradi.

3 Short-circuit
condition
21Vbias = 0 V
T = 290 K

0-1

g = 2.0022-2

H // substrate

-3

322

324

∆Hpp = 0.25 mT

320.5

Magnetic Field (mT)

Simulated solarirradiation
18~19 h12~13 h
6~7 h
3~4 h
0~1 h

321
321.5
Magnetic Field (mT)322

ESR parameters:
g = 2.0022, ΔHpp = 0.25 mT

Hole accumulation in P3HT
in the device by irradiation

Consistent with those ofholes in neat P3HT films

Hole = Radical cation= Positive polaronAdv. Mater. 25 (2013) 2362.

No electron accumulation in PCBM was confirmed
from low temperature ESR measurements

Hole accumulation and device performance
Short-circuit condition-1.9-2.2

Nspin
Jsc

20.2

20.3
1NspinVoc-2.3
010Time (h)

0.4

200

Irradiation on

Voc (V)-2.1

Nspin (1012)

42-2

Open-circuit
condition
Jsc (mA/cm )Short-circuit
6 condition
Nspin (1012)

Open-circuit condition

10Time (h)20Irradiation on

Nspin: number of accumulated
holes in P3HT
Increase in Nspin
Decrease in Jsc, Voc
Clear correlation
Adv. Mater. 25 (2013) 2362.

Hole accumulationin P3HT
Deterioration ofdevice performance

Intrinsic deterioration mechanism

Accumulation sites for fast and slow components-1.9-2.2

Nspin
Jsc

20.2

20.3
1NspinVoc-2.3
010Time (h)

0.4

200

Irradiation on

20Irradiation on

AFast component:hole accumulationin P3HT in bulkmaterials

10Time (h)AlLiF
P3HT:PCBM+ + + ++ + + +PEDOT:PSS
ITO
quartz substrate

Slow component:hole accumulation in
P3HT at organicinterfaces

Voc (V)-2.1

Nspin (1012)

42-2Open-circuit
condition
Jsc (mA/cm )Nspin (1012)

Short-circuit
6 condition

Mechanism of the decrease in Voc
Vacuum-level shift
HoleAccumulationin P3HT

Simulated
solar irradiation
P3HT
HOMO
PEDOT:
PSS

P3HT
HOMO
PEDOT:PSS~1 nm
Before irradiation

VOC = HOMOD − LUMOA

After irradiation+ BBD + BBA − ∆φ D − ∆φ A

Interfacial electric dipole layer
0.2∆Voc =ed

ε 0ε r S∆N spin

0.3
1Voc (V)

212Nspin (10 )Open-circuitcondition

Nspin
Voc0.40Irradiation on

10Time (h)20M. Brinkmann et al.,Adv. Mater. 18 (2006) 860.

Mechanism of the decrease in Jsc1

τ

=1

τ SC+

τ HA

1

τ HA+ Accumulated hole

+Accumulated holeSurface con.:~2×2 nm2(~4% per unit)+ Hole carrier-1.9-212τ : collision time of carrier scatteringτSC : w/o hole accumulation
τHA : w/ hole accumulation

Nspin (10 )Short-circuit
6 condition
4-2.1-2.2Nspin
Jsc2

Jsc (mA/cm2)

Matthiessen’s rule-2.3
010Time (h)20Irradiation onNspin=+µ µSC cµ HA
11∴ Jsc = neµE = ne

µ : mobility of deviceµSC : w/o hole accumulationµHA : w/ hole accumulation
Nspin : number of holes
K. Harada et al., APL 91 (2007) 092118.

µSC µHAEµHA + ( µSC / c ) Nspin

Jsc decreases as Nspin increases
because of carrier scatteringby accumulated holes

SummaryESR studies of internal states in organic thin-film solar cells
Charge accumulation duringdevice operation• P3HT:PC61BM polymer solar cells• High efficiency PTB7:PC71BM cells
Intrinsic deterioration mechanism
ESR analysis:
useful knowledge forunderstanding of device operation andimprovement of device performanceat the microscopic levelAcknowledgements

Collaborators:
T. Fujimori, M. Ito, T. Nagamori, M. Yabusaki, D. Liu,
T. Kubodera (U. Tsukuba)Prof. T. Mori (Aichi I. T.),
Dr. T. Yamanari, Dr. Y. Yoshida (AIST)
Dr. T. Yasuda, Dr. L. Han (NIMS)

Funds:
JST, PRESTO; JSPS;

Univ. of Tsukuba; TIMS, U. Tsukuba