Elsevier

Thin Solid Films

Volume 515, Issue 24, 15 October 2007, Pages 8733-8737
Thin Solid Films

Investigation of the properties of indium tin oxide-organic contacts for optoelectronic applications

https://doi.org/10.1016/j.tsf.2007.03.120Get rights and content

Abstract

This paper presents some investigations on the electrical transport properties of ITO/single (double) layer organic semiconductor (m-DNB, benzil, PTCDA, Alq3) contacts in SIS-like (ITO/organic/Si) and MIS-like (ITO/organic/metal) heterostructures. The IV characteristics have emphasised the injection properties of different contacts and the effect of space charge limited currents in correlation with the type and preparation conditions of the contacts. We have studied the influence of the type of contact (In/ITO; In/Al) on the electrical conduction in Alq3/PTCDA/Si/In heterostructure. In a planar grid contact configuration for In/Al/PTCDA/Al/In structure we have observed the effect of the low electric field on the shape of the IV characteristic.

Introduction

Organic semiconductors are a class of materials offering a great potential for electronic [1], [2], optoelectronic [3] and photovoltaic [4] applications because of their mechanical flexibility and low production cost compared to inorganic semiconductor. Organic light emitting devices (OLED) are interesting for applications in the flat panel displays for TV and monitor technology because they are characterized by high brightness and wide viewing angle. The OLED structure is delimitated by the metallic layer and the indium tin oxide (ITO)/glass interface. High efficiencies, low working voltages and high lifetime involve low electrical resistivity and good chemical stability of the organic semiconductor/metal contact to assure a good charge injection and transport.

The electrical properties of the OLED are controlled by the mobility of the charge carriers and the heights of the barriers, whereas the optical properties by the refractive index mismatches at the interfaces glass/air and organic layer/ITO that generate the trapping of a large fraction of the light by total internal reflection into glass and ITO [5].

The properties of the ITO thin films, usually used as transparent anode (thickness, resistance/square, substrate type), affect the properties of the organic devices whose efficiency remains limited by the small charge carrier mobility in the organic semiconductors and by the particularities of the carriers injection in the ITO/organic contact. The carrier injection mechanism at the interface between metals and organic molecular solids are more complex than that at the metal/inorganic semiconductor contacts because of the space charge limitation effects in the organic films that can obscure the contact properties. Many studies have been effectuated on the metal/organic interface [6], but very little information exists on the interface organic/oxide electrode. The hole/electron injecting contacts must be metals with high/low work function to reduce the height of the injection barriers [7]. As a consequence, it is difficult to inject holes in an electron-transporting layer. The efficient injection of the holes into a p-type conduction organic layer is possible from a semiconductor ITO layer with a work function situated between 4.6 eV and 5 eV [8]. If the effective electrode work function is higher than the ionisation potential of the organic semiconductor, the contact has an ohmic behaviour and the carriers are injected from the electrode to the organic. The main difficulty of this process is associated with the high ionisation potential of some organic semiconductor, as 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) (6.2 eV [9]; 6.8 eV [10]), tris (8-hydroxyquinoline) aluminium (Alq3) (5.7 eV [8]; 5.95 eV [10]).

We have investigated the injection properties through ITO (metal)/organic/metal (semiconductor) structures and the effect of the type, preparation conditions and succession order of the layers on the transport properties of the carriers.

Section snippets

Experimental

We have investigated the electrical performance of the contact between an ITO film and some organic semiconductor films, characterized by hole [PTCDA] or electron [Alq3] conduction. We have also studied the electrical properties of single and double organic layer structures between ITO, with resistance RITO = 40 Ω/sq, and silicon contacts (examples: glass/In/ITO/PTCDA/Si(p)/In, glass/In/ITO/Alq3/PTCDA/Si(p)/In) and between ITO and metallic contacts (examples: glass/In/ITO/PTCDA/Ga (In),

Results and discussion

Because the electronic transport in the organic thin films is limited by imperfections as grain boundaries, an increased ordering of the organic molecules is assured by the experimental thermal and vacuum conditions, substrate properties and organic molecule configuration, that lead to a sufficient overlap of the electronic conjugated systems corresponding to the adjacent molecules and generates an improvement of the carrier mobility. When an order is imposed by the substrate, the molecular

Conclusions

The transport properties of different organic semiconductors (m-DNB, benzil, PTCDA and Alq3) for different cathode (In/ITO, In/Al) and anode (Si/In, Cu/In(Ag), Al/In) contacts have been studied. [We name anode the positive biased electrode.] The highest charge carrier injection was obtained in In/ITO/PTCDA and In/Al/m-DNB structures with a Si(p)/In anode. With the same anode, charge carrier injection was not obtained in the benzil based structure for either the In/ITO or the In/Cu cathode. The

Acknowledgments

This investigation has been supported financially by Ministry of Education and Research through CEEX Programme, contract number 2CEX-11-1/25.07.2006.

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