Elsevier

Optical Materials

Volume 91, May 2019, Pages 93-100
Optical Materials

The origin of changes in electrical properties of organic films fabricated at various vacuum-deposition rates

https://doi.org/10.1016/j.optmat.2019.03.012Get rights and content

Highlights

  • The current density increased for HODs and decreased for EODs by deposition rate.

  • The change in current densities was not related to charge trap characteristics.

  • The average surface roughness of neat films increased by deposition rate.

  • α-NPD molecules oriented more horizontally at higher vacuum-deposition rates.

  • HODs and EODs with the best electrical properties had the best stability.

Abstract

We investigated the dependence of electrical properties on vacuum-deposition rate for films of N,N՛-diphenyl-N,N՛-bis(1-naphthyl)-1,1՛-biphenyl-4,4՛-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminium (Alq3), hole- and electron-transport materials widely used in organic light-emitting diodes (OLEDs), respectively. Hole-only devices (HODs) of α-NPD showed an increase of hole current when α-NPD layers were fabricated at high deposition rates, while the tendency was opposite for electron-only devices (EODs) of Alq3. We found that the increased hole current at high deposition rates for HODs was caused by a horizontal orientation of α-NPD molecules relative to a substrate plane which facilitates hole transport through the films. On the other hand, the decreased electron current at high deposition rates for EODs could be ascribed to the increased surface roughness of Alq3 films which decreases electron injection. Additionally, we demonstrated that long-term operational stability was enhanced for HODs fabricated at high deposition rates and operated at a constant current. Use of a high deposition rate means a reduction of deposition time which is helpful in OLED manufacturing. The improved device performance when high deposition rates were used and the detailed understanding of its origins as we demonstrated here would lead to fabrication of high-performance OLEDs at a low cost in the future.

Introduction

Organic optoelectronic technology has drawn a huge potential, during last decades, in both science and industry by taking advantage of unlimited organic molecular design, low-cost processing, flexibility, and lightweight [[1], [2], [3]]. A comprehensive understanding of the electrical properties of organic thin films can be helpful for establishing fundamental science and leads to fabricate high performance devices [4]. The electrical properties of organic thin films can be improved by controlling their molecular orientation and aggregation structures through optimization of deposition conditions such as substrate temperature and deposition rate during film fabrication. The effects of deposition rates on electrical and optical characteristics of organic thin films were investigated by many researchers [1,[5], [6], [7], [8], [9]]. For example, it has been demonstrated that hole and electron carrier mobilities of organic thin films are changed by deposition rates [6,[10], [11], [12]]. However, the physical origins of these effects are still not fully understood.

In the present paper, the physical origins of electrical properties changed by vacuum deposition rates for films of N,N՛-diphenyl-N,N՛-bis(1-naphthyl)-1,1՛-biphenyl-4,4՛-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminium (Alq3) were investigated. Currents of hole-only devices (HODs) were clearly enhanced by increasing deposition rates of α-NPD. In sharp contrast to HODs, currents of electron-only devices (EODs) decreased when an Alq3 layer was fabricated at a high deposition rate. To make this opposite behavior clear, we studied charge carrier trap densities and depths, surface morphologies, molecular orientations, and densities of α-NPD and Alq3 films fabricated at different deposition rates. We found that the increased and decreased electrical properties are probably because of a horizontal orientation of α-NPD molecules relative to a substrate plane and increased surface roughness of Alq3 films, respectively. Finally, we demonstrated an enhancement of the operational stability when HODs were fabricated at a high deposition rate.

Section snippets

Experimental details

Glass substrates coated with an indium tin oxide (ITO) layer (100 nm) and silicon substrates were cleaned using ultrasonic bath in pure water, acetone, and isopropanol, respectively. To complete the cleaning process, ultraviolet-ozone treatment was carried out on the substrates.

For measuring the electrical properties and stability, HODs containing an α-NPD layer and EODs containing an Alq3 layer were fabricated. The structures of the HOD and EOD samples were glass substrate/ITO anode (100 nm)/

Results and discussion

The J-V characteristics of the α-NPD HODs and Alq3 EODs are shown in Fig. 1(a) and b. No electroluminescence was observed from these devices during the J-V measurements, indicating completely unipolar current flow. When the deposition rates were increased from 0.01 to 1nms1, the current densities of HODs increased while the current densities of EODs decreased, which is in agreement with the results reported in Refs. [6,23]. In addition, in order to examine the reproducibility of our

Conclusion

We found that the electrical properties of α-NPD and Alq3 films are influenced by their molecular orientation and surface roughness. By measuring the electrical, physical and structural properties of films deposited at different vacuum-deposition rates, the electrical properties of α-NPD HODs were enhanced at higher deposition rates due to the horizontal orientation of α-NPD molecules which can improve the overlap of π orbitals between neighbouring molecules. In the case of Alq3, high surface

Acknowledgement

Authors would like to thank Ministry of Science, Research and Technology of IR of Iran and Shahid Behehsti University for providing facilities associated with this research. Also supports from Kyushu University of Japan and OPERA members is appreciated.

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