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

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)(Alq3), 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.

Organic and polymer light-emitting diodes (OLEDs and PLEDs [1], respectively) have attracted a great deal of interest for application in emissive flat panel displays and as new light sources. One reason is that the OLEDs and PLEDs are composed by several layers on the substrate; thickness of these layers is less than 200 nm. Therefore, these devices are lightweight and thin, and can be fabricated on flexible substrate. Now many efforts have been made to improve their luminance and lifetime. It has been shown, for example, that the luminance of an OLED can be improved by producing a bilayer (or multilayer) structure by successive sublimation steps. However, for PLEDs, it is usually difficult to fabricate a bi- [2] or multilayer [3] structure with common processes due to the dissolution between polymer layers.
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]     Adding an electron transport material to the hole transport material is one way to achieve better electron injection and to balance the carriers, and thus higher luminance can be expected. In this study, the electron transport material (Alq 3 ) was added into the hole transport (P3HT) material and the results are shown in Fig. 2. In the case of PLEDs with uniformly blended layers, both the current and the luminance of PLEDs prepared with the spray layer were higher than those of PLEDs prepared with the spin-coated layer. This may be because solvent residue is left in the spin-coated layer and/or that Alq 3 aggregates easily in layers prepared by the spin coating process due to the solvent residue. With the VS method, where almost no solvent residue is left in the layer, Alq 3 molecules did not migrate after reaching the substrate, which markedly impeded the aggregation.
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. This low V on is an advantage of the VS method for PLED applications. Unfortunately, the highest measured luminance (L max ) of the PLEDs with a blend layer was lower than the L max of PLEDs with pure structures. In the blend structure case, the efficiency was even lower than that of the PLEDs with a pure P3HT active layer.
To improve the luminance of PLEDs, the blended layer was exchanged for a graded doped layer where the structure consisted of a P3HT matrix in which the concentration of Alq 3 was increased linearly from 0 to 20%. Thus, while there is no Alq 3 in the polymer near the interface with the PEDOT:PSS layer, the concentration of Alq 3 increases linearly in the polymer until about 20% in the region near the interface with the cathode. We have reported the structure of these graded layers prepared by VS method analyzed by scanning transmission electron microscopy (STEM) [14,15]. Although in the present study the polymer film was thinner, the graded structure is expected to be similar to that reported previously.
The graded structure device has a turn-on voltage of 9 V, which is about the same as that of the pure and blend structure devices (Tab. 1), however, it emits much brighter light than the pure and graded structure devices (Fig. 2b). As the heterojunction interfaces are virtually eliminated in the graded structure, this increase in brightness was probably because there is a better balance of charge carrier in such a way that there is a higher recombination probability of holes and electrons [5], leading to a large increase in PLED performance. In the present study, the optimized highest ratio of Alq 3 to P3HT was about 1:4, but suitable ratios have still to be determined for other combinations of electron and hole transport materials.
All PLEDs showed very similar EL spectra to that of the graded structured PLED shown in the insert of Fig. 2b. In addition, all PLEDs emitted yellow-orange light and the preparation method and structure of active layers did not influence the color. Thus, only P3HT emit light, while Alq 3 acted only as an electron transport material without emitting light.
In summary, this report described the successful fabrication of a PLED in which the polymer active layer had graded doping with electron transport molecules by the VS profiles can also be prepared. The conversion efficiency of not only PLEDs but also polymer solar cells can be improved by using a graded structure. Therefore, the VS method is very promising for the fabrication of polymer optoelectronic devices, not only PLEDs but also polymer solar cells.