The influence of a novel 1,2,4-linking hyperbranched poly(arylenevinylene) (1,2,4-hb-PAV) material, designed to feature intramolecular energy-funneling, on the transport and emission properties of organic light emitting diodes (OLEDs) has been studied. A comparison to conventional hyperbranched 1,3,5-linking polymers (1,3,5-hb-PAV), which do not exhibit this effect, has been made. For this purpose, single-layer organic light emitting diodes with a glass/indium–tin oxide/poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate)/active layer/Ca/Al structure and different active layer thicknesses have been fabricated and characterized, using either 1,2,4-hb-PAV or 1,3,5-hb-PAV as active layers. The current–voltage response has been interpreted in terms of a numerical model that includes a field-dependent carrier mobility, which allows us to estimate carrier mobility in the diodes. Stable green emission with Commission Internationale de L’Eclairage coordinates at (0.41,0.56) and a high luminous efficiency of 28 Cd/A has been obtained at very low driving currents (10 μA) for 1,2,4-hb-PAV material, versus 2.1 Cd/A for the conventional 1,3,5-linking material, in spite of the high photoluminescence quantum yield in thin film exhibited by both materials. The significant improvements in the performance of OLEDs based on 1,2,4-hb-PAV with respect to other conventional hyperbranched polymers are attributed to the inherent energy gradient from the shorter branches to the longer conjugated stem in this structure, which enables the characteristic funneling effect.
In general, the fabrications of the LEDs with mesa structure are performed grown by MOCVD method. In order to etch and separate each chips, the LEDs are passed the RIE and scribing processes. The RIE process using plasma dry etching occur some problems such as defects, dislocations and the formation of dangling bond in surface result in decline of device characteristic. The SAG method has attracted considerable interest for the growth of high quality GaN epi layer on the sapphire substrate. In this paper, the SAG method was introduced for simplification and fabrication of the high quality epi layer. And we report that the size of selective area do not affect the characteristics of original LED. The diameter of SAG circle patterns were choose as 2500, 1000, 350, and 200 µm. The SAG-LEDs were measured to obtain the device characteristics using by SEM, EL and I-V. The main emission peaks of 2500, 1000, 350, and 200 µm were 485, 480, 450, and 445 nm respectively. The chips of 350, 200 µm diameter were observed non-uniform surface and resistance was higher than original LED, however, the chips of 2500, 1000 µm diameter had uniform surface and current-voltage characteristics were better than small sizes. Therefore, we suggest that the suitable diameter which do not affect the characteristic of original LED is more than 1000 µm.
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