GoKawiilEnd-group deactivation of TADF polymers
The bromo end groups of ATRP-synthesized TADF polymers were removed via hydrogenation. In a typical procedure, G-ortho-P (1.0 equiv.) and copper(I) bromide (CuBr; 2 equiv.) were charged into a Schlenk flask and evacuated for 20 min, followed by 3 vacuum–argon cycles. Next, 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA; 2.0 equiv.), degassed via argon sparging for 10 min, was injected into the flask, and the resulting mixture was stirred at room temperature for 30 min under an argon atmosphere. Tributyltin hydride (2.0 equiv.) was then injected into the Schlenk flask under argon. The flask was placed in an oil bath at 60 °C for 2 h with stirring. After a specific time, the Schlenk flask was cooled to room temperature to stop the reaction. Then the viscous mixture was dissolved in tetrahydrofuran, filtered through a column filled with activated basic Al 2 O 3 , precipitated once in hexane and twice in methanol, and then dried under vacuum overnight. Final solid powders were obtained as the product and ready for further use after characterization with NMR, size-exclusion chromatography and MALDI-TOF.
Block copolymerization affording ELPRs
ELPRs were synthesized via ATRP using the self-hosted precursor polymers as macroinitiators. The crosslinker loading was modulated by varying the stoichiometric feed ratio of the crosslinkable monomer (MAc or MAl) to the macroinitiator. In a representative synthesis of G-ortho-ELPR, G-ortho-P (1.0 equiv.) and CuBr (1.5 equiv.) were charged into a flame-dried Schlenk flask and subjected to 3 vacuum–argon cycles. Separately, a solution of MAc monomer and PMDETA (1.5 equiv.) in toluene was degassed via argon sparging for 20 min. The degassed monomer/ligand solution was then transferred to the Schlenk flask via syringe under argon. The polymerization was conducted at 60 °C and subsequently stopped by cooling the flask to room temperature. The resulting viscous mixture was diluted with tetrahydrofuran and filtered through a column filled with activated basic Al 2 O 3 to remove the residue copper ions. The final block copolymer was isolated by precipitation once in hexane and twice in methanol, followed by drying under vacuum overnight to yield solid powders of G-ortho-ELPR.
Direct photolithography
In a typical direct photo-patterning process, a certain amount of ELPR polymers were dissolved in toluene and filtered through a 0.22-µm PTFE membrane, affording 10–25 mg ml−1 precursor solution. Then this ELPR precursor can be spin-coated either on silicon wafer for photo-pattern demonstration or on top of the HTL layer for OLED device fabrication. The thickness of spin-coated ELPR films can be tuned by changing spin-coating speed and/or the precursor concentration of the ELPR. The mask contact aligner exposure was performed on an ABM ultraviolet mask aligner through a 4-inch quartz photomask. The 405–365 nm exposure mirror was installed on an ABM aligner that allows i-line exposure. The typical dose range used for ELPR patterning is 6,000–8,500 mJ cm−2. The development was performed by rinsing the substrate with toluene to remove the un-crosslinked regions.
Direct e-beam lithography
The film deposition of ELPR polymers for direct EBL is the same process as that of direct ultraviolet photolithography. ELPR solution concentration can be varied between 10 mg ml−1 to 25 mg ml−1 if thickness tuning is needed. The e-beam exposure was performed on EBPG5200, Raith with a fixed 100 keV voltage from the Binnig and Rohrer Nanotechnology Center (BRNC) at the IBM research centre in Zurich. The optimal dose for each single-coloured ELPR is different, but typically ranges from 100 μC cm−2 to 500 μC cm−2. The development was performed by rinsing the substrate with toluene to remove the un-crosslinked regions. The aluminium nanodisks were patterned with a conventional poly(methyl methacrylate) and methacrylic acid (PMMA/MMA) bilayered e-beam lift-off process.
OLED fabrication
For pursuing the best OLED performance, all the synthesized polymers used as the EML underwent end-group deactivation before device integration. Indium-tin-oxide-coated glass substrates were cleaned and treated with oxygen plasma. For bulk devices, a PEDOT:PSS blend (Clevios AI 4083/CH 8000 mix) was spin-coated (4,000 rpm, 50 s) with adjustable relative ratio to a thickness range of 40–80 nm and annealed at 150 °C for 15 min on a laminar flow bench. Subsequently, the devices were transferred to a nitrogen-filled glovebox to deposit an optional HTL and polymer EML via spin-coating. For photo-patterned devices, a PTAA layer (5 mg ml−1 in toluene) was spin-coated (4,000 rpm, 50 s) and annealed at 120 °C for 10 min. To understand the role of the hole-injection layer and the HTLs on the operational stability of the OLED devices, we used a hole-injection material, nickel oxide (NiO x ), and two other hole-transport materials, N4,N4′-di(naphthalen-1-yl)-N4,N4′-bis(4-vinylphenyl)biphenyl-4,4′-diamine (v-NPB) and poly(9-vinylcarbazole (PVK). Subsequently, a 20-nm TADF polymer EML was spin-coated (2,000 rpm, 50 s). In the case of ELPRs, the EML was crosslinked via 365-nm ultraviolet exposure. For the multicolour patterning device (ETH logo), each ELPR layer was ultraviolet-irradiated through a photomask selectively and developed in toluene; this process was repeated for additional colours to achieve pixelated EMLs. Finally, the substrates were transferred into a physical vapour deposition chamber, and a 50-nm 1,3,5-tri(m-pyridin-3-ylphenyl)benzene (TmPyPB) or 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) electron-transport layer, a 1.5-nm 8-hydroxyquinolinolatolithium interface layer, and a 100-nm aluminium cathode were sequentially deposited via thermal evaporation through a shadow mask at a vacuum level of 7.5 × 10−8 torr.
... continue reading