Synthesis of polyimide, binder polymer, and multifunctional monomers for patterning black pixel-defined layers of organic light-emitting diodes. Synthesis of polyimide, binder polymer and multifunctional monomers to pattern black pixel define layers of organic light. If a negative-tone black photoresist could be used in the pattern of PDL in OLED panels, the outdoor visibility could not be reduced, and also the process of black matrix formation and ¼ λ polarizing film could be eliminated.
The negative-tone black photoresist for patterning black PDL on OLED panel consists of photoinitiator, photosensitizer, binder polymer, multifunctional monomers, black grinding base and polyimide as thermal stabilizer. The last component polyimide is not directly involved in the photo pattern of black PDL in OLED panel. In this work, the synthesis of polyimides that are completely soluble in common organic solvents, especially in propylene glycol monomethyl ether acetate (PGMEA), was first performed and applied in the formulation of a negative-tone black photoresist.
Third, some new multifunctional monomers were synthesized and tested in the formulation of negative-tone black photoresist for the patterning of black PDL of OLED panels. 3-1.1 Synthesis of polyimides by melt polymerization and properties 3-1.2 Photolithography and thermal stability of pixels define layer patterns 3-2 Side chain polyimides as binder polymer.
Introduction
The latter component is required in the black photoresist to increase the thermal stability of the resulting thin PDL layer above 300℃, which is the high limit of the process temperature for OLED panel fabrication. Therefore, photosensitive polyimide is used in the positive tone photoresist in the OLED panel manufacturing process. Binder polymers are acrylate or cardo polymers with a high acid value above 120 that can be developed with an aqueous solution of tetramethyl ammonium hydroxide (TMAH).
The binder polymer should have acrylate groups along the polymer chain that can participate in the photo-crosslinking together with the multifunctional monomers, photoinitiator and photosensitizer when irradiated by the UV light in the photolithographic process.[8][9]. In this work, the synthesis of three component black photoresist such as polyimide[5][6] thermal stabilizer, binder polymer and multifunctional monomer was carried out from the viewpoint of high thermal stability of the resulting black PDL pattern and high definition of PDL pattern. After synthesis of the three important ingredients, the formulation of black photoresist[7][8][9] and photolithographic process to obtain fine PDL pattern were also investigated.
Experimental Methods
The powder mixture in a stainless container was located in the center of an electric oven and heated at 240 ºC for 10 minutes under N2 atmosphere. The binder polymer is not only one of the most important components in the black photoresist in terms of weight, but also a significant component that affects the shape of PDL patterns during the development step in the photolithographic process. New binder polymers with thermally stable imide bonds in the side chain were synthesized and applied to the black photoresist formulation to pattern black PDL on OLED panel.
First, PSMA (20 mmol based on maleic anhydride units) was dissolved in 20 mL of DMAc, and then AS, AIPA, and 6FAL were added sequentially once an hour and stirred at room temperature as shown in Figure 4. After formation of amide acid bonds between maleic anhydride units of PSMA and three aromatic amines (AS, AIPA, 6FAL), the free radical polymerization inhibitor (BHT) and chemical imidizing agent (acetic anhydride, 10 mL) were added and the reaction mixture was reacted for 4.5 h at 100°C to give side chain polyimides (SI-x). [12] After cooling, the reaction mixture was poured into excess water and the precipitates were collected by centrifugation. The solid product was washed with diethyl ether several times to give the SI series side chain polyimide as a brown powder.
The starting material PSMA (20 mmol based on maleic anhydride units) was dissolved in 20 ml DMAc solvent. The mixture solution of acrylate monomer 2-HEA (10 mmol) and catalyst pyridine (1 mmol) was added to the PSMA solution and the reaction mixture was stirred at room temperature for 16 hours. Third, acetic anhydride (10 mL) was added and the mixture was stirred at 100 °C for 4.5 h for the imidization reaction (Figure 5).
After the reaction, the mixture was poured into an excess of water and further work-up was the same as for the SI series in Section 2-2.2. PSMA (20 mmol based on maleic anhydride unit) was dissolved in 20 mL of DMAc and AIPA (6 mmol) and 6FAL (14 mmol) were added sequentially once per hour and stirred at room temperature. The reaction mixture was poured into water and further work-up was the same as SI series to obtain SEA-1.
For SEA-2 synthesis, PSMA was dissolved in PGMEA (net. 30 wt%) and the reaction proceeded as above without workup. However, the thermal stability of PETA is not high enough to withstand a temperature of up to 300ºC[8][9] which occurs in the photolithographic patterning process of OLED. In addition, common acrylates are not soluble in water-based developer such as tetramethylammonium hydroxide (TMAH), resulting in difficulties in the development step.
To solve this problem, we designed and synthesized new multifunctional acidic monomers by esterification reaction and tested them in the photolithography process for black PDL patterns on OLED panel to achieve better development and higher thermal stability. In FT-IR analysis, the sample solution was first added to hexane and the precipitating oil was mixed with dry KBr powder for subsequent pellet preparation.
Results and Discussions
This is the main reason why polyimide is required as a thermal stabilizer in the black photoresist formulation. The compatibility of all other components of the black photoresist with the black pigment is very important, otherwise micro-coagulum may form which will give sawtooth or wavy type patterns in the black pixel defining layer. Of the polyimide thermal stabilizers in Table 1 and Table 2, six samples soluble in PGMEA solvent were used in the black photoresist formulation as shown in Table 4.
Black photoresist samples (Pt-1, Pt-3, and Pt-4) failed to give good patterns in the black pixel definition layer, while other black photoresists exhibited excellent PDL patterns. Although all six polyimides used in the black photoresist were completely soluble in PGMEA solvent, there were differences in the diamine monomers in D6MP-4 (BF6) and D6MP-5 (TFDB) polyimides as shown in Figure 13 and Figure 3. The effect of of polyimides in the black photoresist on the thermal stability of the resulting PDL patterns was investigated by TGA analysis.
In this experiment, instead of SR-6300 shown in Table 4, the fluorene-based cardo-type linker polymer, which has both carboxylic group and acrylate group in the main chain, was used. to make black color photoresist due to the limited penetration depth of UV light in the photolithographic pattern of the defined black pixel layer in the OLED panel. The bonding polymer in the negative-tone black photoresist for PDL patterning in the OLED panel must meet the following specifications.
The other is high thermal stability of binder polymer, which is similar to the light-sensitive polyimide in the positive-tone photoresist. In the synthesis of the SI series binder polymers, three aromatic amines were reacted with the maleic anhydride units of PSMA as shown in Figure 4. The SI product was also recovered by precipitation in excess water to remove excess acetic anhydride followed by filtration, drying and redissolution in PGMEA, to be tested in the black photoresist.
In the SEI series, 2-HEA was first reacted with maleic anhydride units of PSMA as shown in Figure 5 to generate both carboxyl groups and double bonds. Since acetic anhydride was used for imidization, SEI must be worked up in the same way as SI series. However, the SEA-2 product was obtained in one-pot solution method without workup to be directly tested as binder polymer in the photolithographic pattern of black PDL in OLED panel.
The thermal stability of the reference black photoresist (PR-0) was not high enough to withstand PDL photolithographic patterning and subsequent post-curing processing (up to 300 °C), as shown in Table 7. In the photoresist solution, one of the main components is black mill base (LT-2, 23 wt.% in PGMEA), which is responsible for the high optical density.
Conclusion
Of this SEA series, the binder polymer was found to give a fine pattern of black PDL pattern with thermal stability above 300℃. Third, five multifunctional monomers were synthesized with different combinations of carboxyl groups and acrylate groups. Of these PM6, the multifunctional monomer with 6 acrylates and 2 carboxyl groups showed optimal properties of high definition black PDL pattern and high black mill base content up to 52 wt%.
Acknowledgement
Kang, “Analysis of light leakage between the adjacent pixels in a stacked white OLED display with color filters” Displays., Vol 45, 2016, p.
