Black Photoresist for Patterning Pixel Define Layer of Organic Light Emitting Diode with Polyimide as Thermal Stabilizer

In order to improve the visibility for outside use of organic light emitting diodes (OLEDs), the polarized film and black matrix pattern have been used in the small and medium sized OLEDs; however, these cause problem of reducing the emission efficiency of OLED. Changing the color of pixel define layer (PDL) from brown to black is an important point for improving the efficiency and visibility of OLEDs. In this work we studied the photoresist material containing black pigment and the photolithographic process for patterning of black PDL on OLEDs. The black PDL patterns made with our synthesized polyimides as thermal stabilizer were found to give high thermal stability over 300 ̊C.


Introduction
Organic light emitting diodes (OLEDs) have extended their applications from small size mobile devices to large OLED TVs, flexible OLED, and lightings.The changes have become possible due to such merits of OLEDs as fast response time, low operating voltage, wide viewing angle as well as light weight and thin-film features [1] [2] [3] [4].The panel of small size OLED has separate subpixel structure of red (R), green (G), and blue (B) light emission from pixel define layer (PDL) which is fabricated by photolithographic process.Currently positive-tone photosensitive polyimide is used to make three dimensional pixel microstructures in the PDL.So that the color of the PDL is yellowish brown, the usual color of polyimide films.Since the yellowish brown polyimide layer of PDL can reflect the ambient light, the visibility of the OLED display is reduced, especially used outdoors.In order to improve the visibility of OLED display, black matrix patterns need to be fabricated on top of the pixel define layer in addition to the 1/4λ polarizing film on top of the black matrix pattern layer as shown in the cross-sectional view of OLED panel in Figure 1.
If the pixel define layer could be patterned with a black photoresist, the black matrix pattern and 1/4λ polarizing film may not be required for the improved visibility of OLED devices.However, the photosensitivity of the positive-tone polyimide currently used is not high enough to give black PDL patterns which have thickness of 1.0 -1.2 µm.So far to the best of our knowledge, thermally stable black PDL researches on OLED display have not been reported yet.
In this work, a negative-tone photoresist was investigated for the photolithographic patterning of black PDL of OLED, since the polymerization depth of UV light is deeper than that of positive-tone photosensitive polyimide.The components necessary for the black negative photoresist include photoinitiator, photosensitizer, binder polymer developable with aqueous alkaline solution, multifunctional monomer for crosslinking reaction, black millbase and polyimide [5] [6] to increase the thermal stability of patterned black PDL of OLED devices.
The last component of the black negative photoresist is very important since the process temperature of black PDL reaches close to 300˚C including post-curing step after photopatterning.This is the reason why photosensitive polyimide has been used in the normal (not black) pixel define layer patterning.Another important part of the black negative photoresist for patterning black PDL of OLED is the complete solubility of all the components of negative photoresist in the common organic solvent in this case propyleneglycol monomethylether acetate (PGMEA).If this condition is not met, the resulting black PDL pattern will have saw-toothed deformation due to the aggregates made by incomplete solubility of Figure

Synthesis of Polyimides by Solution Polymerization
Dianhydride (6FDA, 14.9 mmol) and monoanhydride (PAn, 2 mmol) in 30 wt% NMP solution was added into two different diamine (4,4-ODA, 15 mmol and AH6FP, 15 mmol) 30 wt% NMP solutions separately at 0˚C and stirred for 1 hour.Then the two reaction mixture were bought up to 45˚C and stirred for 23 hours and finally at 130˚C for 4 hr for thermal imidization as shown in Figure 2.
After cooling down the viscous reaction mixture was poured into excess amount of water and the precipitates were collected by centrifugation.The precipitates were redissolved in EtOAc and the organic layer was washed with brine twice.
The organic layer was dried with anhydrous Na 2 SO 4 and evaporated to dryness under vacuum to afford polyimides as red-orange powder (SPIO and SPIF).

Synthesis of Polyimides by Melt Polymerization
Dianhydride (6FDA or MCDA), diamine (AH6FP) and monoanhydride (PEPA or TMA) in powder form were well ground and mixed by using mortar.The powder mixture in a stainless container with metal cup was placed in the electric furnace and heated for 10 min at 240˚C under nitrogen atmosphere.After cooling down to room temperature, the resulting polyimides were collected and ground to fine powder as shown in Figure 3.

Polyimide Syntheses and Solubility in PGMEA Solvent
Usually solution polymerization of polyimides are carried out in two steps, that is, polyamic acid in the first step and second imidization step either by chemical

Synthesis and Properties of Polyimides by Melt Polymerization
As was noted in the solution polymerization session it took a long time to test the solubility of resulting polyimide samples in PGMEA common solvent.
Therefore a melt polymerization of polyimides was conducted to save the reaction time.Two typical examples were shown in Figure 3.
In Table 1

Photolithography and Thermal Stability of Pixel Define Layer Patterns
The black photoresist is composed of photoinitiator, photosensitizer, binder polymer, multifunctional monomer, black pigment dispersion in PGMEA solvent Another important major component of black photoresist is black pigment.In this experiment a commercial lactam black (LT-1, 23 wt% in PGMEA obtained from SKC htm Co., Korea) was used.The black pigment LT-1 is in a finely dispersed state (average particle size; 50 nm) in PGMEA solvent.The compatibility of all other components of the black photoresist with the black pigment is very important, otherwise micro-coagulums may be formed which will give saw-toothed or wavy patterns in the black pixel define layer.

Dianhydride Diamine Monoanhydride
The photolithographic process was as following.The black photoresist was spin coated on the wafer (Si/SiO 2 200 nm) with a stepwise speed of 300 rpm/5 sec, 500 rpm/5 sec, and 5000 rpm/50 sec.After soft bake at 120˚C for 100 sec, the dry black photoresist thin film was exposed to UV irradiation (120 mJ/cm 2 ) through the photomask.After hard bake at 120 º C for 100 sec, the black PDL thin film was developed with AZ-300 MIF developer (tetramethylammonium hydroxide 2 wt% in water, Bayer Co., Germany) for 120 sec followed by water rinsing and drying with N 2 gun.
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 3.The effect of polyimides in the black photoresist on the thermal stability of the resulting PDL patterns was investigated by TGA analysis.The black PDL patterns obtained on the silicon wafer was scratched off by using sharp razor blade and then the 1 wt% loss temperatures were checked.As shown in Figure 8 the black photoresist (Pt-5) exhibited higher 1 wt% loss temperatures than Pt-0 without polyimide as thermal stabilizer.

Conclusion
In this work, a negative-tone photoresist was investigated for the photolitho-

Figure 5 .
Figure 5. Structures, abbreviations and melting temperatures of monomers used in the melt polymerization of polyimides.
photolithographic process and post-cure treatment.That is main reason why polyimide is required as thermal stabilizer in the black photoresist formulation.
graphic patterning of PDL with black color since the penetration depth of UV light is deeper than that of positive-tone photoresist containing photosensitive polyimide.The typical formulation of a negative tone photoresist for the patterning of black PDL of OLEDs included photoinitiator, photosensitizer, dispersant, binder polymer, multifunctional monomer (DPHA, dipentaerythritol hexaacrylate), black millbase and polyimide which were dissolved completely in the G. Shi et al.DOI: 10.4236/msa.2018.96040563 Materials Sciences and Applications

Table 1 .
Monomer combination, melt polymerization condition and solubility of resulting polyimides.(a) Effect of combination of monomers on solubility.(b) Effect of melt polymerization condition on solubility.
are shown the relationship between the melt polymerization condition of polyimides and solubility in PGMEA, NMP and DMAC solvents.The chemical structures and melting points of dianhydrides, diamines and monoanhydrides are shown in Figure 5.As shown inTable 1(a), the combinations of presence of trifluoromethyl groups in 6FDA, AH6FP, BF6, TFDB monomers or alicyclic structure in MCDA monomer can prevent the charge transfer complex formation by the steric hinderance mechanism thus promote solubility in G. Shi et al.DOI: 10.4236/msa.2018.96040559 Materials Sciences and Applications The effects of melt polymerization temperature and reaction time on the solubility of resulting polyimide are shown in Table 1(b).When the reaction temperature exceed 240 º C or reaction time over 10 min, the resulting polyimides exhibited insolubility or partial solubility (turbidity) in PGMEA solvent.This could be resulted from the crosslinking reaction between phenolic −OH groups with removal of water molecules.homogeneous during melt polymerization.As the MCDA and TMA do not react with each other until the temperature reach to 240˚C melt polymerization temperature, they may give narrow distribution of Mw in MT series compared to the G. Shi et al.DOI: 10.4236/msa.2018.96040560 Materials Sciences and Applications

Table 3 .
Formulation of black photoresist and stability of black PDL pattern.