Organic light emitting diodes using fluorine doped tin oxide thin films, deposited by chemical spray pyrolysis, as anode
Introduction
Organic light emitting diodes (OLEDs), which are now on the market, are based on opposite charges injection from the electrodes to the organic materials where they interact and form excitons which can emit light through a transition from the excited to smaller energy state [1]. OLEDs can be obtained by vacuum deposition of small-molecule organic semiconductors onto a wide variety of substrates. This freedom of choice of materials may lead to the manufacture of inexpensive large area displays [2]. In OLEDs the anode, which allows holes injections, is usually a transparent conductive oxide (TCO), while the cathode, which allows electrons injection is based on aluminium.
Successful OLEDs need the use of tin doped indium oxide (ITO) coated substrates as TCO anode for hole injection. However, ITO is not ideal because it is scarce and therefore expensive. Indeed, to day, indium is a key metal component in manufacturing flat panels. About 45% of all indium is used in ITO. Demand for indium is expected to outstrip supply these years as major electronic companies’ ramp up production of flat screen televisions. Therefore, it is of interest trying to use another transparent conductive oxide (TCO) in OLEDs.
The use of another transparent conductive oxide as anode in OLEDs is usually quite disappointing even if, in the case of SnO2, some interesting results have been obtained [3], [4], [5]. Indeed, SnO2 is more stable than ITO, therefore when a polymer such as the poly(p-phenylenevinylene (PPV) is used as organic material, which means that a precursor has to be annealed after deposition to be converted to PPV, metal ion diffusion from the TCO to the organic material can be minimized. However if the work function of SnO2 (1 0 1) single crystals varies from 4.7 eV to 5.7 eV [6], [7] it is only 4.3–4.4 eV in the case of thin films [3], [4], which could minimize hole injection.
In the present work it is shown that the deposition of an ultra-thin gold film at the interface SnO2:F/organic material allows to improve significantly the OLEDs performances.
Section snippets
Experimental
The conductive SnO2:F films were deposited by chemical spray pyrolysis.
The substrates used were bare glasses. Before deposition, the substrates were cleaned by acetone for eliminating any greasy track. Then, they were cleaned with soap and abundantly rinsed with distilled water. Finally they were dried by a nitrogen flow. The substrate temperature during the deposition was around 510 °C. This temperature has been shown to be efficient to achieve performing SnO2:F films [8]. Tin chloride
Charaterization of the spray pyrolysed SnO2:F films
The XRD diagram of Fig. 3 shows that the SnO2:F films are crystallized in the expected tetragonal structure (JCPDM 77-0452). The peaks corresponding to (1 1 0), (1 0 1), (2 0 0), (2 1 1) and (3 1 0) directions are visible when 2θ varies from 10° to 65°. In randomly oriented powder, the XRD peak the most intense is the peak corresponding to the (1 1 0) direction. It can be seen in the present diffractogram, the peak along the (2 0 0) direction is the strongest. It can be concluded that crystallites are mainly
Conclusion
SnO2:F films deposited by chemical spray pyrolysis have been characterized by XRD, SEM, optical transmission and electrical measurements. The SnO2:F films exhibit optical and electrical properties required for their use as conducting electrode. These films have been used as anode in OLEDs, which typical structure are based on a simple hole transporting layer (HTL)/electron transporting layer (ETL) couple. The HTL is 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP). The ETL, which is also the emitting
Acknowledgements
This work has been financially supported by the France-Algérie contract CMEP 06 MDV 681, Y. Berredjem was supported financially by the CNOUS-France (Bourse de co-tutelle Franco-Algérienne) and S. Ouro Djobo was supported financially by a postdoc scholarship of the “Agence de la francophonie (AUF)”.
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