Elsevier

Organic Electronics

Volume 13, Issue 10, October 2012, Pages 1853-1858
Organic Electronics

Molecular order, charge injection efficiency and the role of intramolecular polar bonds at organic/organic heterointerfaces

https://doi.org/10.1016/j.orgel.2012.05.038Get rights and content

Abstract

The effect of orientational changes in thin films of the non-crystalline hole transport material α-N-N′-diphenyl N-N″-bis(1 naphthayl)-1,1′-biphenyl-4,4′-diamine (α-NPD) on the energy level alignment and the film electronic structure has been investigated by angle-resolved ultraviolet photoelectron spectroscopy and related to the transport characteristics of hole-only devices. Changes in the anisotropic α-sexithiophene (α-6T) substrate from a “standing” to a “flat” molecular orientation induced by mechanical rubbing lead to molecular order and a preferential orientation in subsequently deposited thin α-NPD films and cause a reduction of the charge injection barrier at the organic/organic interface. The results show that the height of this barrier is determined by the surface dipoles of the individual organic films that relate to the orientation of intramolecular polar bonds at the interface.

Highlights

► The molecular orientation within non-crystalline transport materials can be controlled. ► This affects the charge-injection efficiency at organic–organic interfaces. ► The energy-level alignment is determined by the intramolecular polar bond orientation.

Introduction

Over the last decades, the mechanisms underlying the functioning of organic electronics devices have been the topic of intense research such that in particular organic light-emitting devices (OLEDs) do now rapidly mature into new applications and products [1], [2], [3]. In such OLEDs, amorphous or non-crystalline materials are commonly employed. It has been found that these amorphous films may exhibit a preferential orientation [4] or even a particular defined local packing. This affects charge carrier transport and light emission that depend strongly on the local molecular order [5], [6], [7], [8], [9], [10]. In a comprehensive study, Adachi and coworkers investigated the molecular orientation and electronic properties of a large number of organic amorphous thin film materials and found that linear or planar molecules show a tendency to orient flat with respect to the substrates [4]. Longer molecules are inclined to form films with a larger anisotropy [4]. It was also shown that the molecular orientation of linear-shaped 4,4′-bis[(N-carbazole)styryl]biphenyl molecules can be controlled by the substrate roughness and temperature [11]. Charge carrier mobilities have been observed to be higher for films with a horizontal molecular orientation as compared to others highlighting the significant role of the molecular orientation on the charge-transport characteristics of organic amorphous films. Introducing highly oriented p-sexiphenyl (6P) films as the emissive layer of light-emitting devices, Era et al. demonstrated polarized electroluminescence from oriented 6P films [7]. Yanagi and Okamoto prepared 6P films exhibiting either a “lying” or a “standing” orientation, denoted l- and s-6P, epitaxially on KCl (0 0 1) surfaces and constructed a multilayered electroluminescent device containing these 6P films using indium tin oxide (ITO) as electrode material and 2-(4-biphenylyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole (PBD) as the electron transport layer [9]. As compared to the s-6P devices, those employing an l-6P layer emitted a higher electroluminescence (EL) intensity within a narrower spectrum, even at a lower driving voltage. These examples demonstrate that optimizing the molecular packing and orientation in active layers is an efficient way to control the charge transport and optical properties in order to further enhance the performance of OLEDs.

As shown by Arai et al., amorphous but simultaneously uniaxially and horizontally oriented organic films can be obtained by mechanical brushing [12]. When used in OLEDs with polarized emission, the EL intensity was higher parallel to the brushing direction as compared to the perpendicular one. Nevertheless, simple and practical methods for the effective control of the molecular orientation and of the resulting electronic properties are still limited. In this context, recently, we reported that upon mechanical rubbing of thin α-sexithiophene (α-6T, the chemical structure shown in Fig. 1a) films on ITO substrates, using a Nylon cloth, the molecular orientation changes from the “standing” to a “lying” configuration [13]. Using the “flat” α-6T films as substrates for α-NPD (see chemical structure in Fig. 1a) films in hole-only devices, current densities are dramatically increased (42 times at a driving voltage of 1.0 V) with respect to devices with a “lying” α-6T film.

In the present work, angle-dependent ultraviolet photoelectron spectroscopy (UPS) has been used to investigate the molecular orientation and electronic structure of the α-6T and subsequently deposited α-NPD films providing information on the energy level alignment in the devices. The mechanism responsible for the enhanced hole injection at the organic/organic interface is clarified and linked to changes in the preferential molecular orientation within the α-NPD films caused by the rubbing of the α-6T substrate.

Section snippets

Experimental and calculations

ITO substrates have been cleaned by conventional ultra-sonication and ultraviolet–ozone treatment [13]. α-6T (Aldrich) films with a thickness of about 15 nm were then deposited under high-vacuum conditions (of 2.0 × 10−7 mbar) onto the cleaned ITO substrates, at the rate of about 1 Å/s. Some of the sample surfaces have been rubbed 15 times with a dust-free Nylon cloth inside a nitrogen-filled glove box. For the samples prepared for the UPS measurements, α-NPD (Nippon Steel Chemical) films with a

As-prepared and rubbed α-6T films on ITO

Fig. 1c shows the current density J as a function of the applied voltage V of the hole-only devices, either with as-deposited or with rubbed α-6T films. As discussed previously [13], the current densities J of the devices made with rubbed α-6T films are more than one order of magnitude higher than that for those made with as-deposited α-6T films. It is therefore important to understand how the charge injection barriers between ITO and the α-6T film, on one hand, and at the organic/organic

Conclusions

Upon mechanical rubbing of an α-6T film prepared on an ITO substrate, subsequently deposited non-crystalline films of the α-NPD are found to become oriented. These orientational changes improve the performance of hole-only devices based on these α-NPD/α-6T heterojunctions and have been linked to changes of the energy level alignment and to the spatial overlap of the π orbitals of α-6T and of the α-NPD phenyl and naphthyl side groups at the organic/organic heterointerface. In the case of vacuum

Acknowledgments

We acknowledge fruitful discussions with Yuanping Yi (School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, USA). This work has been supported by Grants-in-Aid for Scientific Research of Japan (Nos. 21760005, 20241034, and 20108012), by the MARUBUN Foundation and by the Japan Society for the Promotion of Science (JSPS) through its Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program).

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