Graded doped structure fabricated by vacuum spray method to improve the luminance of polymer light-emitting diodes

Abstract

An increase in luminance of a polymer light-emitting diode (PLED) was obtained by fabricating a graded doping structure using a vacuum spray method. The small electron transport molecule, Tris(8-hydroxyquinolinato) aluminum(III)(Alq 3 ), was graded dispersed along the film in the direction of growth in the hole transport polymer poly(3-hexylthiophene-2,5-diyl) (P3HT, regiorandom) layer of the PLED, despite being dissolved in the same organic solvent as the polymer. The PLED reported here, which is composed of a graded structure, emitted brighter light than PLEDs composed of pure polymer or of a blend of active layers prepared by spin coating and/or vacuum spray methods. Good balance between electron and hole injection is very important for the functioning of both OLEDs and PLEDs. In PLEDs, the most common way to achieve this balance is by blending the electron and hole transporting materials. The electroluminescent (EL) efficiency is higher when the electron and hole transport materials are separated and attached to the cathode and anode layers, respectively. Another way to achieve this balance could be by producing a structure in which the two materials are graded along the film direction of growth. As there is no interface between the electron and hole transport materials in this structure, it is expected that there will be an increase in the lifetime of the devices. For small organic molecules, graded structures can be fabricated by codeposition processes [4][5][6][7] and annealing [8], but for polymers it is difficult to control the gradient profile in the active layer due to dissolution by the solvent. There have been attempts to achieve polymer graded structures, such as molecular-scale interface engineering [9], self-organization [10], and thermal transfer process [11]; however, the processes involved are not simple.We have developed a vacuum spray (VS) method [12][13][14][15][16]

Graded doped structure fabricated by vacuum spray method to improve the luminance of polymer light-emitting diodes

“…As a general trend, PLEDs with a blend structure have higher working voltage [17], and in this study the turn-on voltage (V on , at the minimum luminance of 0.1 cd/m 2 ) of the PLEDs with a spin coating layer (see Table 1) confirmed this trend. However, the V on of the PLEDs with a blend layer prepared by VS method was about the same as that of PLEDs composed of pure structures.…”

Effects of graded distribution of scattering centers on ballistic transport

“…For example, the development of a depletion region in the vicinity of grain boundaries leads to a simultaneous increase in both Seebeck coefficient and electrical conductivity in two-phase polycrystalline materials with graded dopant concentration, 1 whereas the luminance increases in graded doped polymer light-emitting diodes due to an optimum balance between electron and hole injection. 2 Moreover, linear doping of the drift region of a double-diffused metal-oxide-semiconductor transistor assures high breakdown voltage and minimum on-resistance, 3 while a non-uniform base doping influences both the current gain 4 and the base transit time 4,5 of heterojunction bipolar transistors (HBTs). In particular, in InP-InGaAs HBTs, linear doping generates a built-in field, whereas an approximately Gaussian doping assures simultaneously reduced junction capacitance, base resistance, and tunneling leakage current, conditions otherwise difficult to meet in uniform doped structures.…”