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Bilayer and Graded Doped Structures in Polymer Light-emitting Diodes
TIME:2012-7-23 15:43:00

Bilayer and Graded Doped Structures in Polymer Light-emitting Diodes

Xiaoliang Mo (Ī) a)Guorong Chen (¹)


Materials Science Department, Fudan University, 220 Handan Road, Shanghai 200433, China

Toshiko Mizokuro (\־), Claire Heck, Nobutaka Tanigaki (ԫТ)

Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan

Takashi Hiraga (ƽR¡)

Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan

a)Electronic mail: xlmo@fudan.edu.cn

Homogeneous, bilayer, and graded doped structures in polymer light-emitting diodes (PLEDs) were fabricated by a vacuum spray method with the same solvent. PLEDs showed increased luminance in the bilayer and graded doping structures. The small electron transport molecule, Tris(8-hydroxyquinolinato)aluminum(III) (Alq3), was dispersed uniformly or with a graded structure along the film in the direction of growth in the hole transport polymer poly(3-hexylthiophene-2,5-diyl) (P3HT, regiorandom) layer of the PLEDs. The PLEDs composed of bilayers and graded structures emitted brighter light than those composed of pure polymer or blends of active layers prepared by spin coating and/or vacuum spray methods.

I. INTRODUCTION
Organic and polymer light-emitting diodes (OLEDs and PLEDs,1 respectively) have attracted a great deal of attention for application in emissive flat panel displays and as new light sources, and efforts have been made to improve their luminance and lifetime. For fabrication of PLEDs, the semiconductor polymer film is usually prepared by methods using solvents, such as spin coating, ink-jet printing, and spray methods. These traditional methods are relatively simple and can be processed in the atmosphere. However, it is difficult to avoid residuals, especially gas and solvent, in the polymer film, which may reduce the efficiency of PLEDs.

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. It has been reported that the luminance of an OLED can be improved by producing a bilayer (or multilayer) structure by successive sublimation steps. The bilayer structure confines charges at the heterojunction of two semiconductor layers. However, for PLEDs, it is usually difficult to fabricate a bilayer2 or multilayer3 structures with common processes due to the dissolution between polymer layers.

Another way to achieve this balance would 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 processes4C7 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 several attempts to produce polymer graded structures, such as molecular-scale interface engineering,9 self-organization,10 and the thermal transfer process11; however, the processes involved are not simple.

We have developed a vacuum spray (VS) method12C17 that allows fabrication of polymer films with a controllable dye distribution along the film direction of growth, by which bilayer and/or graded structures can be prepared. Moreover, films are prepared under vacuum; this can avoid the residuals in films, and is compatible with other vacuum film preparation methods that are used for preparation of other layers in the PLEDs. There are some reports about OLEDs18 and PLEDs19 prepared by conventional spray methods in single layer structure. This report describes the production of PLEDs based on films with homogenous, bilayer, and graded structures prepared using this VS method. For reference studies, devices based on spin-coated films were also investigated.

II. EXPERIMENTAL
A. Materials

In this study, the hole transport polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT) was used as the matrix to be dispersed by Tris(8-hydroxyquinolinato) aluminum(III) (Alq3) molecules, which is an electron transport material. Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) was used as the hole injection material.

B. Vacuum spray system
Figure 1 shows a schematic illustration of the vacuum spray method. The mixed solution is sprayed into a vacuum chamber at high pressure through a pinhole nozzle. The generated solution mists fly to the rotated substrate at very high speed. As the procedure is performed in a vacuum under heating with halogen lamps, the solvent in the solution mists evaporates very rapidly. Solvent in the mists evaporates thoroughly when they reach the substrate.

Fig. 1. Illustration of vacuum spray method


C. Thin film preparation by vacuum spray method
Due to the high speed of solvent evaporation in the vacuum chamber, the mists that reach the substrate contain only polymer and small molecules. Therefore, the concentration distribution of small molecules in the polymer matrix in the direction of film growth can be determined. Figure 2 shows a schematic illustration of the preparation of a polymer film with a graded structure by the vacuum spray method. During the total film preparation process, the small molecule to polymer ratio in solution is controlled from low to high. As a result, the small molecule concentration in the polymer film also varies from low to high along the direction of film growth.

Fig. 2. Schematic illustration of the preparation of polymer film with graded structure.

Four types of active layer were prepared by the vacuum spray method: a pure P3HT layer was prepared with P3HT/chloroform solution, a blend P3HT:Alq3 (4:1) layer was prepared with P3HT:Alq3 (4:1)/chloroform solution, and P3HT and Alq3 layers with bilayer and graded structures were prepared with both P3HT/chloroform solution and Alq3/chloroform solution. During the spray process, these two solutions were mixed at precise ratios by the HPLC pump and sprayed onto the substrate. In the case of the bilayer structure, only the P3HT/chloroform solution was sprayed initially. Subsequently, two solutions were mixed by the HPLC pump, the Alq3:P3HT ratio was fixed at 1:4, and sprayed. The spray times of the two steps were the same. In the case of the graded structure, in the total spray process, the Alq3:P3HT ratio increased linearly from 0 to 1:4. As references, the P3HT and P3HT:Alq3 blend films were also prepared by spin coating. In film preparation by the vacuum spray method, the substrate temperature (Ts) was controlled at 353 K.

D. PLED fabrication and measurements
All the PLEDs reported here had the same structure: ITO/PEDOT:PSS/active layer (110 nm)/Mg:Ag (50 nm). The PEDOT:PSS layer was prepared by spin coating on the substrate. In addition, the PEDOT:PSS layer was annealed in vacuum at 373 K for 1 hour before the next process. The active layer was then prepared on it by the spin coating or vacuum spray method. Finally, a Mg:Ag (10:1) alloy layer was deposited on top as the cathode by thermal evaporation. The size of each PLED was 2 mm 2 mm.

For evaluation of their electroluminescence properties, a DC voltage was applied to the PLEDs, and the current flow and the luminance of light emitted from the devices were measured simultaneously. Finally, the current density-voltage (J-V) and luminance-voltage (L-V) characteristics were measured.

III. RESULTS AND DISCUSSION
A. Preparation of P3HT film with in-depth structures by the VS method

The VS method was confirmed to be capable of preparing P3HT films with pure P3HT, uniformly blended Alq3, bilayer and graded doped structures. All the films were transparent and uniform on the substrate. The Ts was set at 353 K during the spray process. This was just above the glass transition temperature (Tg) of P3HT, which is about 343 K. Ts above the Tg of a polymer can improve the smoothness of the film. The roughness data of films are shown in Table 1; the roughness ranged from 3.0 to 11.1 nm.

Table 1. Roughness of the films with in-depth structures, turn-on voltage (Von, at the minimum luminance of 0.1 cd/m2),  highest luminance (Lmax) and highest external quantum efficiency (EQEmax) of PLEDs with those structures.

B.Electrical and optical properties of PLEDs
All PLEDs, regardless of whether they have active layers prepared by spin coating or the VS method, showed very similar EL spectra. In addition, all PLEDs emitted yellow C orange light and the preparation method and structure of active layers did not influence the color. The photoluminance (PL) spectra of pure P3HT films prepared by spin coating and the VS method were measured, and the PL spectra are almost same. Thus, only P3HT emitted light, while Alq3 acted only as an electron transport material without emitting light.

1. PLEDs with pure P3HT layer
Figure 3 shows the electroluminescence properties of the PLEDs prepared by spin coating and VS methods. While the J-V characteristics (Fig. 3(a)) did not differ with production method for PLEDs composed of a pure P3HT layer, the L-V and the current efficiency (CE)-J characteristics (Fig. 3(b, c)) showed that PLEDs prepared by the VS method emitted brighter light and has higher CE in high J. Perlich et al.found that the spin-coated polymer film contain the solvent residue.20 The solvent residue in film was probably acted as traps, which impeded carriers flow through the film.  The L-V characteristics of these PLEDs  may have been because there was less solvent residue in the polymer layer when prepared by the VS method.

Fig. 3. Electroluminescence properties of PLEDs composed of a pure P3HT layer


(a) J-V characteristics of PLEDs. The insert shows the P3HT concentrations (dotted lines) in solution during the spray process and in thin film along the direction of growth.

(b) L-V characteristics of PLEDs. The insert shows the EL spectra.

(c) CE-J characteristics of PLEDs.

2. PLEDs with blend P3HT:Alq3 layer
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 achieve higher luminance. In the present study, the electron transport material (Alq3) was added into the hole transport (P3HT) material. In the case of PLEDs with uniformly blended layers (Fig. 4), both the current density and luminance of PLEDs prepared with the spray layer were higher than those of PLEDs prepared with the spin-coated layer. This may have been because of solvent residue left in the spin-coated layer and/or because Alq3 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, Alq3 molecules did not migrate after reaching the substrate, which markedly impeded aggregation.

Fig. 4. Electroluminescence properties of PLEDs composed of a P3HT:Alq3 (4:1) blend layer


(a) J-V characteristics of PLEDs. The insert shows P3HT concentrations (dotted lines) and Alq3 concentrations (solid lines) in solution during the spray process and in thin film along the direction of growth.

(b) L-V characteristics of PLEDs. The insert shows the EL spectra.

(c) CE-J characteristics of PLEDs.

As a general trend, PLEDs with a blend structure have higher working voltage,21 and in this study the turn-on voltage (Von, at the minimum luminance of 0.1 cd/m2) of the PLEDs with a spin coating layer (see Table 1) confirmed this trend. However, the Von of the PLEDs with a blend layer prepared by the VS method was about the same as that of PLEDs composed of pure structures. This low Von is an advantage of the VS method for PLED applications. Unfortunately, the highest measured luminance (Lmax) of the PLEDs with a blend layer was lower than that of PLEDs with pure structures. The blend film here was not smooth enough (Table 1), which may explain the decrease in luminance of the PLEDs.

3. PLED with P3HT/P3HT:Alq3 bilayer
To improve the luminance of PLEDs, the blended layer was exchanged for a bilayer, where the structure consisted of half P3HT and half P3HT:Alq3 (4:1), prepared by the VS method. These two parts have the same thickness, and contact the PEDOT:PSS layer and Mg:Ag layer, respectively. Figure 5 shows the EL properties of the PLED with a bilayer structure. The bilayer structure PLED had a Von of 7 V, which was lower than those of the pure or blend structure devices. Moreover, the Lmax and highest external quantum efficiency (EQEmax) of this PLED was much higher than that of these other devices (Table 1). Thus, the bilayer structure can improve the EL efficiency of PLEDs. The PLED with a bilayer structure had advantages over blend and pure P3HT PLEDs because the bilayer structure confines charges at the heterojunction of the two layers.2,3,22 The graded structure would be the best choice to further improve the luminance and EQEmax of PLEDs composed of P3HT and Alq3.

Fig. 5. Electroluminescence properties of PLEDs composed of a P3HT/P3HT:Alq3 (4:1) bilayer


(a) J-V characteristics of PLEDs. The insert shows P3HT concentrations (dotted lines) and Alq3 concentrations (solid lines) in solution during the spray process and in thin films along the direction of growth.

(b) L-V characteristics of PLEDs. The insert shows the EL spectra.

(c) CE-J characteristics of PLEDs.

4. PLED with P3HT:Alq3 graded layer
One of the merits of the VS method is the convenient control of in-depth doped polymer thin films. A graded doped layer was fabricated by the VS method, where the structure consisted of a P3HT matrix in which the concentration of Alq3 was increased linearly from 0 to 20%. Thus, while there was no Alq3 in the polymer near the interface with the PEDOT:PSS layer, the concentration of Alq3 increased linearly in the polymer until about 20% in the region near the interface with the cathode.

The graded structure device had a turn-on voltage of 9 V, which was about the same as that of the pure and blend structure devices (Table 1), but it emitted much brighter light than the pure, blend, and bilayer structure devices (Fig. 6 (b)), and has much high highest external quantum efficiency (EQEmax) (Table 1). As the heterojunction interfaces were virtually eliminated in the graded structure, this increase in brightness was probably because of the improved balance of charge carriers in such a way that there was a higher recombination probability of holes and electrons,5 leading to a large increase in PLED performance. As mentioned above, it is even more difficult to prepare the required graded doping profile than the bilayer profile using other methods, and therefore the VS method is a good choice for fabrication of polymer devices with a graded structure. In the present study, the optimized highest ratio of Alq3 to P3HT was about 1:4, but suitable ratios have still to be determined for other combinations of electron and hole transport materials.

Fig. 6. Electroluminescence properties of PLEDs composed of a P3HT:Alq3 (4:1) graded layer


(a) J-V characteristics of PLEDs. The insert shows P3HT concentrations (dotted lines) and Alq3 concentrations (solid lines) in solution during the spray process and in thin film along the direction of growth.

(b) L-V characteristics of PLEDs. The insert shows the EL spectra.

(c) CE-J characteristics of PLEDs.


IV. CONCLUSION
In summary, this report described the successful fabrication of PLEDs in which the polymer active layer had homogeneous, bilayer, or graded structures doped with electron transport molecules by the VS method. The PLEDs with pure P3HT layer and P3HT:Alq3 blend layer prepared by VS showed better characteristics than those with polymer layers prepared by spin coating. This was because the polymer layer prepared by the VS method has no solvent residue and better dispersion of Alq3. The PLEDs with bilayers emitted brighter light than those produced with a homogenous layer (pure polymer and blend layer), regardless of whether the layer was prepared by the spin coating or VS method. The bilayer structure confines the charges near the heterojunction, which improves the charge balance of the PLED. The best PLEDs in this study were those with a graded structure. The graded structure has much better charge injection balance and elimination of the interface. Due to the excellent controllability of the doping process of the VS method, other structures such as multilayer or more complicated profiles can also be prepared. The conversion efficiency of not only PLEDs but also polymer solar cells can be improved by using a bilayer or graded structure. Therefore, the VS method is very promising for the fabrication of polymer optoelectronic devices, not only PLEDs but also polymer solar cells.

ACKNOWLEDGMENTS
This work was supported by the Industrial Technology Research Grant Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors would like to thank Technical Support Co, Ltd. Japan for manufacturing the vacuum spray apparatus.

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