Effects of Thermal Annealing on Performance of Organic Light-Emitting Diodes

Abstract

The effects of postfabrication thermal annealing on the performance and stability of organic light-emitting diodes ͑OLEDs͒ were systematically investigated in standard indium tin oxide/4,4Ј,4Љ-tris͑3-methylphenylphenylamino͒triphenylamine/N, NЈ-diphenyl-N, NЈ-bis͑l-naphthyl-phenyl͒-͑1,1Ј-biphenyl͒-4,4Ј-diamine Alq 3 devices. Electrical and optical performance, surface morphology of organic thin films, and operation stability of the devices with different thermal treatments were measured and monitored. It was found that devices that were thermally annealed at 70°C exhibited the best performance. Moreover, those devices had the best thin-film morphology and the lowest degradation of electroluminescent performance. The operating stability of the heat-treated device was improved by a factor of over 1.7 times than annealed devices and a half-brightness lifetime of over 7200 h was obtained. The 70°C thermally annealed devices have modified and enhanced the bonding at the interfaces of the organic layer, resulting in decreased turn-on voltage, less leakage current, higher luminescence, and longer lifetime.

Effect of the electric field during annealing of organic light emitting diodes for improving its on/off ratio

“…In some cases, an improved device performance upon annealing was attributed to a flatter and pinhole free morphology and more intimate contact between a m-MTDATA and the NPB interface. 15 No improvement in the present case suggests a good morphology and contact in the standard device itself, also corroborated by its high current efficiency. Thus, we also examined whether an applied reverse bias field can play a role in suppressing the reverse bias current and improve the on/off ratio.…”

“…14 It is also reported that the mild heating at 70 1C might have modified and enhanced the bonding at the interfaces of an organic layer, resulting in decreased turn-on voltage, less leakage current, higher luminescence, and longer lifetime in the case of an Alq 3 based OLED. 15 The improvement in a rubrene based device upon annealing is attributed to the change in the interface characteristics and carrier diffusivity. 16 Properties of polyfluorene OLEDs are also seen to be affected by annealing.…”

Determining the C60 molecular arrangement in thin films by means of X-ray diffraction

“…In particular, the quickly growing field of organic electronics, making possible devices such as organic solar cells (Hains et al, 2010;Riede et al, 2008) and organic light-emitting diodes (Reineke et al, 2009), depends on thin films with certain properties. In recent years, much effort has been invested into controlling the molecular arrangement and morphology of such organic films (Pfuetzner et al, 2011;Chen et al, 2007;Schü nemann et al, 2011;Gebeyehu et al, 2001). Unfortunately, it is difficult to obtain structural information on such films down to atomic distances, although most of the electronic properties of the layers are defined on such a sub-nanometre scale (Feng et al, 2009).…”

Temperature-dependent nanomorphology–performance relations in binary iridium complex blend films for organic light emitting diodes

“…This is due to a fact that the latter offers numerous feasible advantages such as inexpensive process, low material waste, and large-scale manufacturing for lightweight flexible devices [2][3][4]. To better understand such a mechanism, atomic force microscopy (AFM) has been widely performed to reveal the correlations between surface planarity and device performance with respect to thermal annealing treatment of OLEDs [17][18][19][20][21]. For instance, the chemical and electrical properties of polymer-based electroluminescent devices could be controlled by modifying the synthetic chemistry techniques of emitting materials.…”

“…To better understand such a mechanism, atomic force microscopy (AFM) has been widely performed to reveal the correlations between the surface planarity and the device performance with respect to the thermal annealing treatment of OLEDs. [17][18][19][20][21] Besides the surface planarity of the organic films, the distribution of guest and host components in the emitting layer during thermal annealing are also the governing microstructural components for developing efficient solution-processed OLEDs. Previous studies suggested that the aggregation of guest molecules yields the loss of luminescence efficiency due to their concentration quenching.…”