Photoluminescence and electroluminescence properties of a new PPV derivative containing oxadiazole segments
Introduction
Since the discovery of polymeric electroluminescence (PEL) by a Cambridge group [1], an increasing amount of research has been devoted towards the understanding of the electroluminescence mechanism, the performance and the efficiency of the PEL devices. Single layer PPV (poly-phenylene vinylene) devices have relatively low efficiency, because the injection and transport of holes is preferential and recombination occurs between imbalanced carriers. Electron injection may be enhanced by the use of cathode with low work function [2] or by choosing polymer with large electron affinity [3]. An alternative method is inserting an electron transport layer [4] but in multilayer devices, the interface between electron transport and hole transport layers will influence the device performance. Recently, attempts have been made to combine two or three functional groups into a single molecule or polymer to form the combined electron transport segment, hole transport segment and emitter so that the system will have balanced charge transport properties between electrons and holes [5], [6], [7]. To improve the balance of hole and electron transport in a single molecule, we introduced the oxadiazole molecule into the PPV backbone, and synthesized poly[(2,5–diphenylene–1,3,4–oxadiazole)–4–4′–vinylene] (O-PPV) (Fig. 1) oligomers. From the EL characteristics of double-layer devices consisting of O-PPV and hole or electron transport layer respectively, the O-PPV shows both hole and electron transport properties. The electron transporting characteristics in O-PPV is attributed to the formation of excimers.
Section snippets
Experiment
We synthesized soluble O-PPV oligomers according to the HORNER reaction. The molecular structure of O-PPV is investigated by means of IR spectrum and NMR with a Nicolet 50× FTIR spectrometer and a Varian Unity Plus-400 spectrometer. The molecular weight was determined in THF by WATERS 510 gel permeation chromatography to be Mw=1000 and Mn=855. This result indicated that O-PPV oligomer had roughly four repeating units [8]. PPV thin film was fabricated by a standard precursor method [1]. The
Results and discussion
The optical absorption spectra of O-PPV in dilute solution and solid-state film are shown in Fig. 2. We noted that there are some vibronic structures in dilute solution spectrum with the peak at 350 nm and the absorption edge locating at 420 nm. For solid-state film, the absorption peak is at 300 nm and its absorption edge is at 460 nm. The PL spectra of dilute solution and solid-state film O-PPV are shown in Fig. 3. It is noted that their PL spectra are different. The solution spectrum has
Conclusion
In summary, we have synthesized an oxadiazole-containing PPV oligomer. From the EL properties of the double-layer devices of ITO/PPV/O-PPV/Al and ITO/O-PPV/Tb(AcAc)3Phen/Al, we confirm that O-PPV material has both electron and hole transporting properties. The electron mobility originates from oxdiazole segments in O-PPV backbone, and further is a result of interchain charge-transfer interactions in O-PPV. Compared with the single layer EL device of PPV, the EL efficiency of O-PPV is higher due
Acknowledgements
Thank Dr Sh.G. Yin for providing O-PPV materials. This project is supported by National Natural Science Foundation of China (NSFC), Contract No. 29992530. Acknowledge assistance of Laboratory of Organic Solid, Institute of Chemistry, Chinese Academy of Sciences (Beijing) and ‘the Climbing Program of Northern Jiaotong University’
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