Elsevier

Thin Solid Films

Volume 520, Issue 6, 1 January 2012, Pages 2283-2288
Thin Solid Films

Optical, morphological, structural, electrical, molecular orientation, and electroluminescence characteristics of organic semiconductor films prepared at various deposition rates

https://doi.org/10.1016/j.tsf.2011.09.060Get rights and content

Abstract

Extremely high deposition rates of ≈ 7200 nm s 1 for N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) and of ≈ 1700 nm s 1 for tris(8-hydroxyquinoline)aluminum (Alq3) are found to be possible by controlling source–substrate distances and crucible temperatures. Shapes of ultraviolet–visible absorption spectra and photoluminescence (PL) spectra, atomic force microscope images, X-ray diffraction patterns, PL quantum yields, PL lifetimes, and PL radiative decay rates of the films remain independent of the deposition rates ranging from 0.01 to 1000 nm s 1. On the other hand, hole currents of hole-only α-NPD devices increase ≈ 3 times while electron currents of electron-only Alq3 devices decrease by ≈ 1/60 as the deposition rates are increased from 0.01 to 10 nm s 1. The increase in hole current is confirmed to arise from an increase in hole mobility of α-NPD measured using a time-of-flight technique. The increase in hole mobility is probably due to a parallel orientation of an electronic transition moment of α-NPD at the higher deposition rates. Moreover, the three orders of magnitude increase in deposition rate from 0.01 to 10 nm s 1 of α-NPD and Alq3 results in a relatively small increase in voltage of ≈ 15% and a decrease in external quantum efficiency of ≈ 30% in organic light-emitting diodes (OLEDs). The reduction of the OLED performance is attributable to the marked decrease in electron current relative to the slight increase in hole current, indicating a decrease in charge balance factor at the higher deposition rates.

Introduction

A vacuum thermal deposition technique used to manufacture multilayer organic light-emitting diodes (OLEDs) based on small molecules [1] is inherently time-consuming, resulting in the overall cost of manufacturing OLEDs greater than that of manufacturing widely commercialized liquid crystal displays and plasma panel displays. Constructing the multilayer OLED structure at a high deposition rate is suggested to be an alternative way to reduce the tact time [2]. The deposition rates of organic layers embedded into OLEDs are typically less than ≈ 0.1 nm s 1 and are considered one of the key factors affecting performance of OLEDs. Indeed, Chen and coworkers have shown that electron mobilities of tris(8-hydroxyquinoline)aluminum (Alq3) decrease and power consumption of OLEDs increases by a small increase in deposition rate of Alq3 from 0.2 to 0.7 nm s 1 [3]. Lie and coworkers have demonstrated that electroluminescence (EL) efficiencies and operational lifetimes of OLEDs are improved with increasing deposition rates of bis(10-hydroxybenzo[h]qinolinato)beryllium (Bebq2) from 0.03 to 1.3 nm s 1 [4]. The variation range of the deposition rates previously used to investigate device performance is less than two orders of magnitude [3], [4], [5], [6], [7]. There has been still a lack of understanding on how optical and electrical characteristics of organic films as well as performance of OLEDs are affected when the deposition rates are widely changed by several orders of magnitude. In this study, we investigate the wide dependence of the deposition rates of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) and Alq3 on their optical, morphological, structural, electrical, molecular orientation, and EL characteristics.

Section snippets

Experimental details

Glass substrates coated with a 150 nm layer of indium tin oxide (ITO) and fused silica substrates were cleaned using conventional ultrasonication, followed by ultraviolet–ozone treatment. Organic films were vacuum-deposited on the substrates under a pressure of ≈ 10 4 Pa using fused silica or carbon crucibles as deposition sources. In our deposition setup, temperatures of the crucibles and distances between the crucible and the substrate were controllable ranging in temperature from room

Results and discussion

The deposition rate–crucible temperature characteristics of α-NPD and Alq3 with the different source–substrate distances are shown in Fig. 2(a) and (b), respectively. The extremely high deposition rates of ≈ 7200 nm s 1 for α-NPD (at a temperature of 400 °C and a distance of 1 cm) and of ≈ 1700 nm s 1 for Alq3 (at a temperature of 400 °C and a distance of 0.5 cm) are found to be possible. The temperature of the substrate surface during the α-NPD deposition is recorded using a tiny thermocouple with a

Summary

Optical, morphological, structural, electrical, molecular orientation, and EL characteristics of organic films depending on organic deposition rates are investigated. Shapes of UV–vis absorption spectra and PL spectra, AFM images, XRD patterns, PL quantum yields, PL lifetimes, and PL radiative decay rates of films of α-NPD and Alq3 remain independent of the deposition rates ranging from 0.01 to 1000 nm s 1. On the other hand, hole currents of hole-only α-NPD devices increase ≈ 3 times while

Acknowledgment

The authors are grateful to Prof. Hiroyuki Okada (University of Toyama) for useful discussion and Dr. Hidetoshi Fujimura (FUJIFILM CO.) for measurements of mass spectrometry and liquid chromatography. This work is supported by Grants-in-Aid for Scientific Research (Grant Nos. , , ). Part of this work is based on “Development of the next generation large-scale organic EL display basic technology (Green IT Project)” with New Energy and Industrial Technology Development Organization (NEDO).

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