and Nanostructures in Optoelectronics and Photonics

11 downloads 0 Views 2MB Size Report
X-ray measurements are still needed for a more detailed understanding of ... [2] J. Eccher, A. C. B. Almeida, T. Cazati, H. von Seggern, H. Bock, I. H. Bechtold.
GRK1464: Graduate Program Micro- and Nanostructures in Optoelectronics and Photonics

Blends of Two Perylene Derivatives: Liquid Crystalline and Optoelectronic Properties J.

1 Vollbrecht ,

A.

1 Stepen ,

K.

1 Nolkemper ,

S.

1 Keuker-Baumann ,

H.

2 Bock ,

and H.

1 Kitzerow

1 Faculty of Science, University of Paderborn, Germany 2 Centre de Recherche Paul Pascal, Université Bordeaux, France

Motivation Organic semiconductors (OSC) are promising materials in electronics due to the potentially low production costs of devices, the applicability of OSCs on flexible substrates and as a means to develop areally emitting light sources. The investigation of new classes of OSCs is required to reach these goals. Discotic and calamitic liquid crystals with semiconducting properties exhibit high potential to enhance the performance of organic electronics because of the self-organizing properties of liquid crystals. In this study we explore how columnar mesophases can be induced by blending two perylene derivatives.[1-2] The first compound is a perylene tetraester (1) which can be easily synthesized and which has a columnar mesophase. The second compound is a dinaphthocoronene tetraester (2), which is not as easily available, shows no liquid crystalline mesophases, but distinguishes itself due to promising optoelectronic properties.[3-5]

Materials

Perylene Tetraester R1: n-pentyl

J Columnar mesophase J Easy to synthesize L Mediocre semiconductor

Combine Both Compounds! Good semiconductor J No columnar mesophase L Hard to synthesize L

1

2

Phase Transitions: crys → col : T = 120 °C col → iso : T = 204 °C Source: https://goo.gl/IxQQTj

crystalline

Source: http://goo.gl/4cGF9e

Phase Separation

Dinaphthocoronene Tetraester R2: 2-ethylhexyl

Phase Transitions: crys → iso : T = 227 °C

Source: http://goo.gl/kjnV6W

Liquid Crystalline Properties

columnar

2

1

x

n1 x n1  n2

0.0

1.0

Polarized Optical Microscopy: The interface of two nonmiscible phases is highlighted in blue and the columnar and crystalline (x = 0.0 - 0.15; x = 0.60 - 1.00) regions are also shown. The image was taken with slightly uncrossed polarizers at T = 188°C.

Differential Scanning Calorimetry: Pure compound 1 (x = 1.00) shows crystalline, columnar and isotropic phases. Pure compound 2 (x = 0.00) only shows crystalline and isotropic phases. The mixed phases show an increased number of phase transitions.

Phase Diagram: This diagram was obtained via POM- and DSC-measurements. In x = 0.1 – 0.5, two non-miscible phases coexist (highlighted in red). x = 0.5 – 1.0 are the most promising mixtures for OLED applications due to columnar mesophases (highlighted in green).

Optoelectronic Properties 140 nm PEDOT:PSS|15 nm TPD|15 nm 1 + 15 nm 2|90 nm Al

Pure 1 1+2 (x = 0.5) Pure 2 logarithmic plot

measured at U = 10 V

Scheme of an OLED.

x = 0.5 evaporated in a vacuum

Deposition techniques.

Substrate

h+-injector

Emitter

OLED Architecture: Glass | ITO | PEDOT:PSS | (TPD) | 1+2 | Al Anode

h+-transport

Cathode

OLED Fabrication: PEDOT:PSS was spin-coated, TPD only evaporated if 1+2 were evaporated, 1+2 evaporated or spin-coated and Al evaporated in a vacuum. Mixed thin films of 1+2 could be achieved via spin-coating of the respective CHCl3-solutions (easy) or via coevaporation of 1+2 (hard).

Electroluminescence of OLEDs with different emitters: pure 1 pure 2 1+2 (x = 0.5)

→ (lmax = 548 nm, FWHM = 75 nm) → (lmax = 538 nm, FWHM = 47 nm) → (lmax = 556 nm, FWHM = 66 nm)

Conclusion and Outlook Liquid Crystalline Properties:  Two non-miscible phases coexist in the region of x = 0.1 – 0.5, which is

disadvantageous for OLED applications.  A columnar mesophase can be observed in the region of x = 0.5 – 1.0.  X-ray measurements are still needed for a more detailed understanding of the mixed phases. Optoelectronic Properties:  Blends of 1+2 in the region of x = 0.5 – 1.0 work in principle as emitters in OLEDs.  No big changes can be observed in the electroluminescence spectra.  Better device performance with thermal evaporation than with spin-coating.  Influence of thermal treatment on device performance must still be studied.

Current-Voltage Characteristics: A typical diode behavior can be observed with a threshold voltage of Uth = 4.1 V. Luminance-Voltage Characteristics: A maximum luminance of LV,max = 9.6 cd/m² was achieved.

Acknowledgements We thank the German Research Foundation (DFG: GRK 1464) for financial support.

References [1] Y. Li, M.-G. Li, Y.-J. Su, J.-G. Liu, Y.-C. Han, S.-J. Zheng, L.-X. Wang. Chin. Chem. Lett. 2016, 27, 475-480. [2] J. Eccher, A. C. B. Almeida, T. Cazati, H. von Seggern, H. Bock, I. H. Bechtold. J. Lumin. 2016, 180, 31-37. [3] J. Vollbrecht, H. Bock, C. Wiebeler, S. Schumacher, H. Kitzerow. Chem. Eur. J. 2014, 20, 12026-12031 [4] J. Vollbrecht, C. Wiebeler, A. Neuba, H. Bock, S. Schumacher, H. Kitzerow. J. Phys. Chem. C. 2016, 120, 7839-7848. [5] J. Vollbrecht, C. Wiebeler, S. Schumacher, H. Bock, H. Kitzerow. Mol. Crys. Liq. Crys. 2016, accepted.

Center for Optoelectronics and Photonics Paderborn