Poly injection currents

Judging from the present OLED status, the most important research was Partridge s report on the EL utilized poly(vinyl carbazole) (PVCz) thin films in 1982.26-29 He used the 500-nm-thick PVCz thin films doped with fluorescent molecules as an emissive center, equipped with the efficient hole-injection electrode (SbCls/PVCz) and the electron-injection electrode (cesium) as a low-workfunction metal. Although no quantitative measurement of luminance was described, surprisingly the injection current density reached 1 mA/cm2. Nowadays, we can fabricate very similar OLED devices with superior EL performance. Thus, Partridge s device contributed to establishing the prototypes of present OLED devices. 

The pulse response of emission from a PAT, e.g., PODT, consists of two independent parts a fast and a slow transition part. The fast response corresponds to carrier transit between electrodes, and the anomalous slow response, which becomes significant at higher current, is explained by heating at the junction due to the injection current [717], The use of poly(thienylene vinylene) thin film as a buffer layer between ITO and poly(2,5-dialkyloxy-p-phenylene vinylene) results in increasing the breakdown voltage and increasing luminescence [718]. Further organic electroluminescence devices are described in literature [719,720]. 

The simple barrier model implies a strong dependence of the injection current on the position of the transport level in the organic layer. This has been verified with LED structures using ITO as the anode mataial in contact with poly-(phenyl phe-nylenevinylene) (PPPV), PPPV doped into polystyrene (PS), and trimethoxystilbene-... 

The main application of poly (vinyl formal) is as a wire enamel in conjunction with a phenolic resin. For this purpose, polymers with low hydroxyl (5-6%) and acetate (9.5-13%) content are used. Similar grades are used in structural adhesive (e.g. Redux) which are also used in conjunction with phenolic resin. Poly(vinyl formal) finds some use as a can coating and with wash primers. Injection mouldings have no commercial significance since they have no features justifying their use at current commercial prices. 

Currem field characteristics measured wiih conjugated polymers sandwiched between an indium-tin oxide (ITO) anode and an aluminum cathode are usually hole dominated and are, consequently, appropriate for testing injection/lransport models for the case of unipolar current How. Data shown in Figure 12-1 refer to injection-limited currents recorded on typically 100 nm thick spin-coated films of derivatives of poly(y d/"fi-phenylenevinylene) (PPV) and a planarized poly(/ /" -pheny-leue) employing a Keilhley source measure unit. The polymers were ... 

Figure 3. Current response of a poly(l,2-diaminobenzene)-coated electrode to hydrogen peroxide (a), ascorbic acid (b), uric acid (c), cysteine (d), and control human serum (e). Flow injection amperometric operation. (Reproduced from ref. 18. Copyright 1990 American Chemical Society.)...

Fig. 3.29(b) shows the published J-V curves of an Al/OCiCio/ITO diode by Jain et al. [40], [OCiCio is poly(2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene vinylene).] In this paper [40] Jain et al. attributed the extremely fast rise of the hole current at low voltages to Shockley like current due to the forward biased Al Schottky contact. In their later paper [56] Jain et al. showed that by including the PFE they can fit the theory of bulk limited trap controlled space charge currents with the same experimental results (see Fig. 3.29(b)). Jain et al. [40] suggested that PFE induces the high injection effect earlier, which presumably makes the Schottky barrier at the Al contact small. 

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