Optically Induced Capacitance Changes in Organic

Semiconductor Based Structures

J.W.E. Kneller and T. Kreouzis School of Physics and Astronomy,

Queen Mary University of London, UK

A.S. Andy, C.G. Parini, M. Pigeon and O. Sushko and R.S Donnan

School of Electronic Engineering and Computer Science, Queen Mary University of London, UK

Contents

• Brief introduction to organic electronics

• Dielectric constant change Δ𝜀𝑇ℎ𝑒𝑜𝑟𝑦

• Time of Flight • Steady State Photocurrent

•∆𝜀𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 = ∆𝜀(𝜐) • Impedance Spectroscopy • GHz - Quasi-Optical Free Space propagation

•∆𝜀𝑇ℎ𝑒𝑜𝑟𝑦 𝑣𝑠 ∆𝜀𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙

• Conclusions

⇒ ∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎 = ∆𝜀 𝜇,𝛥𝑛 ⇒ ∆𝜀𝑆𝑝𝑎𝑐𝑒charge = ∆𝜀 𝜇,𝐺 ⇒ ∆𝜀𝑒−ℎ = ∆𝜀 𝑟𝑐,𝛥𝑛

• Two semi-conducting organic materials used,

• P3HT and PCBM mixed in a 95:5 ratio respectively.

• When excited, one material donates an electron while the other accepts an electron.

Organic Materials

PCBM, phenyl-C61-butyric-acid methyl ester

P3HT, poly(3-hexylthiophene)

95:5 P3HT:PCBM Mixture

Donor

Acceptor

• The polymers are in contact with each other in a junction and in contact to electrodes

• Best way to achieve this is with a Bulk-Heterojunction which maximises dissociation at interface

• Devise: ITO:P3HT:PCBM:Al

Organic Electronics

Polymer B

Polymer A

Cathode

Anode

A simple bi-layer design A Bulk-Heterojunction

95:5 P3HT:PCBM device layout showing the layers of deposited material, the design and the finished device.

• LUMO P3HT = 3.7eV

• LUMO PCBM = 3.0eV

Organic Charge Transfer

Energy level diagram showing electron/hole transfer between the Highest Order Molecular Orbital (HOMO) and the Lowest Order Molecular Orbital (LUMO).

ℎ𝜐

To Anode

To Cathode

• A slab of conjugated polymer will have some dielectric properties:

𝜺 = 𝜺dark

• A slab of conjugated polymer under illumination will have different dielectric properties:

𝜺𝒊𝒍𝒍𝒖𝒎𝒊𝒏𝒂𝒕𝒆𝒅 = 𝜺𝒅𝒂𝒓𝒌 + ∆𝜺

Dielectric Constant

+

+

+

+ - -

- -

• Three different theoretical approaches:

• 𝛥𝜀𝑇ℎ𝑒𝑜𝑟𝑦 ⇒ ∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎 = ∆𝜀 𝜇,𝛥𝑛 ⇒ ∆𝜀𝑆𝑝𝑎𝑐𝑒cℎ𝑎𝑟𝑔𝑒 = ∆𝜀 𝜇,𝐺 ⇒ ∆𝜀𝑒−ℎ = ∆𝜀 𝑟𝑐,𝛥𝑛𝑒−ℎ

•∆𝜀𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 ⇒ ∆𝜀 𝜐 (DC and GHZ)

𝜟𝜺

P3HT:PCBM device in an open vacuum chamber

• Change in dielectric constant can be calculated according to established plasma theory: (for high mobility silicon)

• ∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎 is dependant on the density and mobility of the charge carriers

• These can be easily measured, thus ∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎 can be calculated.

•∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎 = − 𝑚𝑒𝜇𝑒2 +𝑚ℎ𝜇ℎ

2 ∆𝑛

𝜀0 ∈ ℝ

𝜟𝜺𝑻𝒉𝒆𝒐𝒓𝒚 ⇒ ∆𝜺𝑷𝒍𝒂𝒔𝒎𝒂 = ∆𝜺 𝝁,𝜟𝒏

Electron/Hole

Mobility

Dielectric

Constant

Change

Charge Carrier

Density M. El Khaldi,

F. Podevin, and

A. Vilcot,

Microwave and

Optical

Technology

Letters 47 (6),

570 (2005). ∆𝜺𝑷𝒍𝒂𝒔𝒎𝒂 ⇒ 𝜇 = 10-4cm2v-1s-1

⇒ ∆𝑛

• From Ohms Law: 𝐽 = ∆𝑛𝑒𝜇𝐸 • And Charge definition between two plates:

𝐸 =∆𝜙

𝑑=𝑉𝑒𝑓𝑓

𝑑=𝑉𝐵𝑖𝑎𝑠 − 𝑉𝐵𝑢𝑖𝑙𝑡−𝐼𝑁

𝑑

• We get ∆𝑛 as a function of photocurrent, mobility and voltage:

Δ𝑛 = 𝐼𝑑

𝐴𝑒𝜇𝑉𝑒𝑓𝑓

• So simply excite and measure photocurrent under a bias voltage.

Charge Carrier Concentration, Δn

Al

ITO

A

h𝜈

95%P3HT:5%PCBM

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0.00E+000

2.00E+021

4.00E+021

6.00E+021

8.00E+021

1.00E+022

Full Intensity

ND 0.1

ND 0.3

ND 0.5

ND 1

Voltage (V)

n

(m

-3)

Free Carrier Densityof 100nm, 95:5 P3HT:PCBM

• Measure photocurrent

• Calculate ∆𝑛

• Extrapolate to Zero Field, ∆𝑛𝐸=0

• Calculate ∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎

Charge Carrier Concentration, Δn

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

500

1000

1500

2000

2500

3000 Photocurrent of 100nm, 95:5 P3HT:PCBM

Full Intensity (µA)

ND 0.1 (µA)

ND 0.3 (µA)

ND 0.5 (µA)

ND 1 (µA)

Ph

oto

cu

rre

nt (

A)

Voltage (V)

Graphs showing Photocurrent 𝐼 (above) (quadratic as both V and 𝜇 dependant) and charge carrier concentration ∆𝑛 (left) against effective voltage 𝑉𝑒𝑓𝑓.

• Measure photocurrent

• Calculate ∆𝑛

• Extrapolate to Zero Field, ∆𝑛𝐸=0

• Calculate ∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎

Charge Carrier Concentration, Δn

0 20 40 60 80 100

0.00E+000

5.00E+020

1.00E+021

1.50E+021

2.00E+021

2.50E+021

3.00E+021

n

(E=

0) (m

-3)

Free Carrier Densityof 100nm, 95:5 P3HT:PCBM

Transmittance

Graphs showing zero field concentration ∆𝑛𝐸=0 (above) and change in dielectric constant ∆𝜀𝑇ℎ𝑒𝑜𝑟𝑦

(left) against % light transmittance. (100%=1234Wm-2)

0 20 40 60 80 100

0.00E+000

5.00E-014

1.00E-013

1.50E-013

2.00E-013

2.50E-013

3.00E-013

(T

he

ory

) (F

/m)

(Theory)

of 100nm, 95:5 P3HT:PCBM

Transmittance

• Can model photocapacitance as a spacecharge:

• Dependant on conductance G and mobility 𝜇

• As 𝑐 = 𝜀𝜀0𝐴

𝑑 and 𝐸0 =

𝑉

𝑑

• 𝑐 ∝ 𝑑3 𝑐 ≈ 10−38𝐹

• ∆𝜀𝑆𝑝𝑎𝑐𝑒 𝐶ℎ𝑎𝑟𝑔𝑒𝑠 = ∆𝜀 𝜇,𝐺 = 10−26

𝜟𝜺𝑻𝒉𝒆𝒐𝒓𝒚 ⇒ ∆𝜺𝑺𝒑𝒂𝒄𝒆𝒄𝒉𝒂𝒓𝒈𝒆 = ∆𝜺 𝝁,𝑮

Richard S.

Crandall,

Journal of

Applied

Physics 54 (12),

7176 (1983).

•∆𝜀 = ∆𝜀𝑠𝑡𝑎𝑡𝑖𝑐 𝑐ℎ𝑎𝑟𝑔𝑒𝑠 + ∆𝜀𝑓𝑟𝑒𝑒 𝑐ℎ𝑎𝑟𝑔𝑒𝑠

𝜟𝜺𝑻𝒉𝒆𝒐𝒓𝒚 ⇒ ∆𝜺𝒆−𝒉 = ∆𝜺 𝒓𝒄,𝛥𝑛𝑒−ℎ

Coulombically bound e-h pairs • rotating with the electronic field • Traditionally assumed to be

negligible • Derived via:

Richard Friend New QMUL Model

Free electrons and holes • ∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎 • ∆𝜀𝑆𝑝𝑎𝑐𝑒𝑐ℎ𝑎𝑟𝑔𝑒

𝜟𝜺𝑻𝒉𝒆𝒐𝒓𝒚 ⇒ ∆𝜺𝒆−𝒉 = ∆𝜺 𝒓𝒄,𝛥𝑛𝑒−ℎ

∆𝜀𝑠𝑡𝑎𝑡𝑖𝑐 𝑐ℎ𝑎𝑟𝑔𝑒𝑠 = ?

• Can define polarizability per unit dipole as:

•∆𝑃 = 𝜀0 ∆𝜀 − 1 𝐸 •∆𝑃 = 𝑝∥ ∆𝑛

• Dipole moment parallel to Field:

•𝑝∥ ≤ 𝑝𝑒−ℎ ≤ 𝑒𝑟𝑐

• e-h pair density:

•∆𝑛𝑒−ℎ ≥𝜀0 ∆𝜀−1 𝐸

𝑒𝑟𝑐

• Using ∆𝜀 of order unity

•∆𝑛𝑒−ℎ ≥ 1022 m−3 • which is smaller than the maximum pair density of

1024 m-3

• Assuming a cube of coulomb radius completely filled with e-h dipoles:

• ∆𝜀𝑒−ℎ ≤𝑒𝑟𝑐∆𝑛𝑒−ℎV

𝑑

• ∆𝜀𝑒−ℎ ≤ 105

A. D.

Chepelianskii,

J. Wang, and

R. H. Friend,

Phys Rev Lett

112 (12),

126802 (2014).

• From capacitance definition: 𝑐 = 𝜀𝜀0𝐴

𝑑

• And RC circuits: 𝜔𝑅𝑒𝑠𝑜𝑛𝑎𝑛𝑡 =1

𝑅𝐶

• We get: Δ𝜀

𝜀=

Δ𝜐

𝜐

• Any change in 𝜐 comes from change in C from change in ε

• Measurable via Impedance Spectroscopy.

•Δ𝜐 = Δ𝜐𝐿𝑖𝑔ℎ𝑡 − Δ𝜐𝐷𝑎𝑟𝑘

∆𝜺𝑬𝒙𝒑𝒆𝒓𝒊𝒎𝒆𝒏𝒕𝒂𝒍 = ∆𝜺 𝝊

Al

ITO

h𝜈

R

~

Impedance Spectrometer

𝑐 = 1𝑛𝐹

𝑅 = 1𝑘Ω

0 50000 100000 150000 200000

250

300

350

400

450

500

Impedance Spectroscopy of 100nm 95:5 P3HT:PCBM

Frequency Resonance:

Dark Conditions

Full Intensity

Z''

()

(hz)

Impedance Spectroscopy

• Sweeps over a range of frequencies

• Finds Imaginary Impedance 𝑍′′ =1

𝑐𝜔𝑖

as a function of frequency.

• Can find resonance frequency and hence ∆𝜀𝐸𝑥𝑝.

∆𝜀𝐸𝑥𝑝 vs % of light (100% = 1234Wm-2)

Impedance Spectroscopy data showing highly reproducible frequency shifts between active and dark states.

0 25 50 75 100

0

1

2

3

(E

xp

erim

en

tal)

(

)

Transmittance

0 200 400 600 800 1000 1200

/ of 100nm 95:5 P3HT:PCBM - Sample 613

Power (W/m2)

∆𝜺 = 𝟎. 𝟏 (unsaturated)

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600

-Z''/

f/kHz

Dark 1

Light 1

Dark 2

Light 2

Dark 3

Light 3

Impedance Spectroscopy data showing highly reproducible frequency shifts between active and dark states.

Impedance Spectroscopy –REPRODUCIBLE!

0 10 20 30 40 50 60 70 80 90 100

0 10 20 30 40 50 60 70 80 90 100

0

50000

100000

150000

200000

250000

300000

Temperature

Experimental

Transmittance

(

Hz) E

xp

erim

en

tal

Temperature (OC)

• Mobility is highly dependant on temperature, T:

• 𝜇(𝑇,𝐸) = 𝜇(𝑇,𝐸=0)𝑒𝛾(𝑇) 𝐸

• From Graph: • As T increases, 𝜐 increases. • As Light increases, 𝜐 decreases.

• So heating of device not affecting results.

Temperature Dependence

T. Kreouzis

et. al.

Physical

Review B 73

235201

(2006) Left-bottom axis – Frequency resonance against % light (100% = 1234Wm-2) Left-top axis – Frequency resonance against temperature.

• Can measure at GHz frequencies.

• Uses free space propagation on quasi-optical setup • mm-wave transition

• Vector Network Analyser.

∆𝜀𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 ⇒ ∆𝜀𝐺𝐻𝑧

A.S. Andy

School of

Electronic

Engineering

and Computer

Science

Queen Mary,

University of

London

220 225 230 235 240 245 250

2.6

2.8

3.0

3.2

Real Dark

Real Active R

eal (

F/m

)

(Ghz)

High Frequency readings of Real from 95:5 P3HT:PCBM device

=0.1

∆𝜺 = 𝟎. 𝟏

• 𝛥𝜀𝑇ℎ𝑒𝑜𝑟𝑦

⇒ ∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎 = ∆𝜀 𝜇,𝛥𝑛 ≈ −10−9

⇒ ∆𝜀𝑆𝑝𝑎𝑐𝑒 𝐶ℎ𝑎𝑟𝑔𝑒𝑠 = ∆𝜀 𝜇,𝐺 ≈ 10−26

⇒ ∆𝜀𝑒−ℎ = ∆𝜀 𝑟𝑐,𝛥𝑛𝑒−ℎ ≤ +10+5

• ∆𝜀𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙

⇒ ∆𝜀 𝜐=100𝑘𝐻𝑧 𝐿𝑜𝑤 𝑙𝑖𝑔ℎ𝑡 𝐼𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 ≈ −0.1

⇒ ∆𝜀 𝜐=100𝑘𝐻𝑧 𝐻𝑖𝑔ℎ 𝑙𝑖𝑔ℎ𝑡 𝐼𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 ≈ +1

⇒ ∆𝜀 𝜐=100𝐺𝐻𝑧 𝐻𝑖𝑔ℎ 𝑙𝑖𝑔ℎ𝑡 𝐼𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 ≈ −0.1

⇒ |∆𝜀| 𝜐=100𝐺𝐻𝑧 𝐻𝑖𝑔ℎ 𝑙𝑖𝑔ℎ𝑡 𝐼𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 ≈ 0.1

(from phase shift)

∆𝜺𝑻𝒉𝒆𝒐𝒓𝒚 𝒗𝒔 ∆𝜺𝑬𝒙𝒑𝒆𝒓𝒊𝒎𝒆𝒏𝒕𝒂𝒍

Conclusions

• Can optically induce dielectric changes in P3HT:PCBM

• Can measure said changes:

• ∆𝜀𝐸𝑥𝑝 = ∆𝜀(𝜐)

•𝛥𝜀𝑇ℎ𝑒𝑜𝑟𝑦

• Only theoretical model that can explain dielectric change is that of static charges ⇒ e-h pairs

An ITO:P3HT:PCBM:Al device

⇒ ∆𝜀𝑃𝑙𝑎𝑠𝑚𝑎 = ∆𝜀 𝜇,𝛥𝑛 ⇒ ∆𝜀𝑆𝑝𝑎𝑐𝑒 𝐶ℎ𝑎𝑟𝑔𝑒𝑠 = ∆𝜀 𝜇,𝐺 ⇒ ∆𝜀𝑒−ℎ = ∆𝜀 𝑟𝑐,𝛥𝑛

x x √