Electrical response of a columnar liquid crystal applied

in a diode structure Juliana Eccher1, Gregório C. Faria2, Harald Bock3, Heinz von Seggern4,

Wojciech Pisula5, Wojciech Zajaczkowski5, Ivan H. Bechtold1.

UNIVERSIDADE FEDERAL DE SANTA CATARINA

XIII Brazilian Materials Research Society Meeting

1Universidade Federal de Santa Catarina, Departamento de Física, SC, Brasil. 2Universidade de São Paulo, Instituto de Física de São Carlos, SP, Brasil. 3University of Bordeaux, Centre de Recherche Paul-Pascal, CNRS , France. 4 Technical University of Darmstadt, Department of Electronic Materials,

Germany. 5Max-Planck-Institut for Polymer Research, Mainz, Germany.

João Pessoa, September, 2014.

Laboratório de Optoeletrônica Orgânica

e Sistemas Anisotrópicos e-mail: juh_19@yahoo.com.br

Liquid Crystals (LCs)

molecular order + fluidity (charge transport) (easy processability)

anisometric organic molecules rod-shape disk-shape

Crystal

Smetic A

Smetic C

Nematic Isotropic

Liquid

Temperature

Liquid-crystalline mesophases

2

Discotic Liquid Crystals

Hexagonal columnar mesophase (Colh):

Synthesis: Dr. Harald Bock, CRPP, France. Colh at room temperature. n-type semiconductor. µ > 1 cm2/Vs.

Material:

benzo[ghi]perylene-diimidodiester

3

Published in: J. Eccher et al. ACS Appl. Mater. Interfaces, 5,

11935-11943 (2013).

4

Orientation of the Columnar LCs

Edge-on Orientation (Planar Alignment)

Face-on Orientation (Homeotropic Alignment)

Transistor Diode

Advantage:

Modify the molecular orientation to control and improve the electro-optical properties of the devices (thermal annealing, surface treatments, electric and magnetic fields);

The mobility (µ) is highly dependent on the intracolunar alignment.

Results:

-60 -30 0 30 60 90 120 150 180

H = 9.1 kJ/mol

H = 8.8 kJ/mol

148.2 0C

Colh Iso

Iso

heating

Colh

He

at F

low

Temperature (oC)

cooling

153.4 0C

120 ºC

25 ºC

Polarizing optical microscopy

cooling: Iso 148.2ºC Colh

5

DSC (Differential Scanning Calorimetry)

Published in: J. Eccher et al. ACS Appl. Mater.

Interfaces, 5, 11935-11943 (2013).

6

Spin-coating thin film:

Thermal annealing:

device at 120 ºC

Polarizing optical microscopy:

Homeotropic alignment: face-on

(b

)

(c) (d)

(a) (b)

(e) (f)

face-on

3h

0h 0h

1h

3h

1h (c)

Published in: J. Eccher et al. ACS Appl.

Mater. Interfaces, 5, 11935-11943

(2013).

Charge Carrier Mobility from J-V Curves

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: Poole-Frenkel coefficient Boundary condition: E (0) = 0

Published in: J. Eccher et al. ACS Appl. Mater.

Interfaces, 5, 11935-11943 (2013).

8

J-V and Luminance-Voltage Characteristics

spin-coating film

Published in: J. Eccher et al. ACS Appl. Mater. Interfaces, 5, 11935-11943 (2013).

9

Charge Carrier Mobility from J-V Curves

Parameter Before During After

µ0 (cm2/Vs) 9.86 × 10-10 9.12 × 10-10 9.03 × 10-10

(cm/V) 1/2 0.44 × 10-2 1.98 × 10-2 2.11 × 10-2

µ at +15 V (cm2/Vs)

8.8 × 10-8 4.5 × 10-4 8.5 × 10-3

Mobility 5 orders of magnitude higher after alignment.

0 10 20 30 40 50 6010

-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Applied Voltage (V)

After

During

Before

Mo

bili

ty (

cm

2/V

s)

Published in: J. Eccher et al. ACS

Appl. Mater. Interfaces, 5, 11935-

11943 (2013).

CONCLUSIONS

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Spin-coating film: The annealing process resulted in the homeotropic alignment. A gain of 4 orders of magnitude in current and 5 orders of magnitude in charge carrier mobility was surprisingly obtained. The electroluminescence was also improved dramatically.

Evaporated film: Homeotropic alignment was not observed. A high rectification degree was obtained even before annealing, indicating that the molecular packing in the evaporated film is more ordered and compact than in the spin-coated film before annealing.

10th Ibero-American Workshop on

Complex Fluids

FLORIANÓPOLIS - SANTA CATARINA - BRAZIL

25-29 October 2015

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Chairman:

Prof. Ivan H. Bechtold

Prof. Hugo Gallardo

Website:

http://10ibero.paginas.ufsc.br/

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Thanks for attention!!!

Financial Supports:

Acknowledgments: