Crystallization properties of IrQ(ppy) 2 organometallic complex films
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Taiju Tsuboi Kyoto Sangyo University, Kyoto 603-8555, Japan (Received 1 March 2017; accepted 4 April 2017)
Comparative studies between doped conducting polymers and electrochemical deposited organometallic compounds reveals the interplay between crystalline-amorphous phases with significant contributions to the internal quantum efficiency in the OLED devices. The coexistence of the amorphous and crystalline phase in the electrodeposited film is revealed by the minor microcrystal products which are present in the amorphous phase in thin films, while the many microcrystals are randomly distributed in the thick films. Concerning the doped conducting polymers, the level of doping induces crystalline effects as a result of the p–p stacking between molecules, due to the Forester energy transfer processes in which the transfer rate is increased with decreasing of the distances between neighboring molecules. The crystallization processes change the emission properties of the active layers both for the luminance level and all over color, ranging from yellow to red in the case of IrQ(ppy)2 compounds.
I. INTRODUCTION
Organic light emitting diodes (OLEDs) have been studied for many applications such as display and lighting since a pioneering work by Tang and Van Slyke.1 The OLED devices have structure of multilayer consisted of organic thin films and two electrodes. The films are currently formed by vacuum evaporation, spin-coating, or electrochemical deposition (ED). The evaporation method is the most popular, but requires expensive machines and more complex techniques to make uniform thin films. The spin coating method is difficult to control the thickness and uniformity of the film. Compared with these methods, the ED method is relatively simple and low cost and allows easy control of the film thickness and formation of large area film.2–4 Many papers have been published on the OLEDs with thin layer formed by the ED method.5–14 Several papers reported the external quantum, luminance, and power efficiencies. For example, Ostergard et al. reported the external quantum efficiency of about 0.35 103 at the maximum current density of 2.5 A/cm2 from OLED with the poly(para-phenylene) (PPV) emitting layer,8 Zhu et al. reported a luminance efficiency of 3.9 cd/A from the OLEDs with peripheral carbazole substituted ruthenium(II) complexes which was used as emitter,9 Li et al. reported a maximum luminescence of 3080 cd/m2 at 14 V and luminous efficiency of 0.35 cd/A at 14.5 V, and
Contributing Editor: Tao Xie a) Address all correspondence to this author. e-mail: silv@infim.ro DOI: 10.1557/jmr.2017.155
maximum external quantum efficiency of 0.6% from OLED with structure of ITO/TCPC (film thickness of 80–120 nm)/Ba/Al containing blue emitting peripheral carbazole substituted 9,9-spirobifluorene (TCPC) ED film.10 Li et al. reported a maximum external quantum efficiency of 0.72% and the maximum luminescence and luminous efficiency of 4224 cd/m2 and 0.47 cd/A, respectively, from ITO/TCPC (;100 nm)/Ba/Al,11,12 Gu et al. reported a maximum luminance of 3672
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