Large‐Area Blade‐Coated Deep‐Blue Polymer Light‐Emitting Diodes with a Narrowband and Uniform Emission

Abstract Large‐area polymer light‐emitting diodes (PLEDs) manufactured by printing are required for flat‐panel lighting and displays. Nevertheless, it remains challenging to fabricate large‐area and stable deep‐blue PLEDs with narrowband emission due to the difficulties in precisely tuning film uniformity and obtaining single‐exciton emission. Herein, efficient and stable large‐area deep‐blue PLEDs with narrowband emission are prepared from encapsulated polydiarylfluorene. Encapsulated polydiarylfluorenes presented an efficient and stable deep‐blue emission (peak: 439 nm; full width at half maximum (FWHM): 39 nm) in the solid state due to their single‐chain emission behavior without inter‐backbone chain aggregation. Large‐area uniform blade‐coated films (16 cm2) are also fabricated with excellent smoothness and morphology. Benefitting from efficient emission and excellent printed capacity, the blade‐coated PLEDs with a device area of 9 mm2 realized uniform deep‐blue emission (FWHM: 38 nm; CIE: 0.153, 0.067), with a corresponding maximum external quantum efficiency and the brightness comparable to those of devices based on spin‐coated films. Finally, considering the essential role of deep‐blue LEDs, a preliminary patterned PLED array with a pixel size of 800 × 1000 µm2 and a monochrome display is fabricated, highlighting potential full‐color display applications.


Chemicals
All reagents from commercial sources were used without further purification, unless otherwise noted. Anhydrous THF was pre-dried over molecular sieves. All dry reactions were performed with glassware that was flamed under a high vacuum and backfilled with N 2 .
General Measurement and Characterization 1 H and 13 C NMR spectra were recorded on a Bruker Ultra Shield plus 400 MHz spectrometer.
The molecular weights were estimated by gel permeation chromatography (GPC) analysis using DMF as the eluent and linear polystyrene as the standard. The UV-visible absorption spectra were taken with a Shimadzu UV-1750 spectrometer at room temperature, and photoluminescence spectra were measured using the Hitachi F-4600. Thermogravimetric analysis (TGA) was acquired by TGA2 (Mettler Instruments). Differential scanning calorimetry (DSC) data was measured by DSC214 Polyma (NETZSCH Instruments) with the measured temperature from 30 to 320℃ at a rate of 10 ℃/min. The film morphologies of polymer films were measured with AFM in tapping mode (Bruker's Dimension Icon). The dynamic light scattering (DLS) measurements were carried out using an ALV/CGS-3. The All the thick film were spin-coated with a speed of 1500 r/min (60 s).

Fabrication and Characterization of PLEDs
The PLED devices were prepared and characterized following the process as follows. The ITO substrates were cleaned in an ultrasonic bath with detergent, deionized water, alcohol and acetone, dried in an oven at 120°C for 2 hours, blown surface by N 2 and treated with ultraviolet ozone for 15 min before spincoating. Firstly, a 40 nm thick PEDOT: PSS was spincoated with a speed of 1500 r/min (30 s) and then annealed at 120°C for 20 minutes. For the blade-coated device, a 150 nm thick PEDOT: PSS was blade-coated with a speed of 5 mm/s, gap was 50 μm and heated with 80°C, and then annealed at 120°C for 30 minutes. Then, the 3 emitting layer was spin-coated from toluene solution (10 mg/mL) with a speed of 1500 r/min (60 s) and annealed at 100°C for 15 minutes in nitrogen-filled glovebox. And the thickness of the polymer spin-coated film was estimated at about 40 nm. The emitting layer was bladecoated from toluene solution (10 mg/mL) with a speed of 15 mm/s, gap was 200 μm and heated with 50°C, and annealed at 100°C for 15 minutes in nitrogen-filled glovebox. And the thickness of polymer blade-coated film is estimated at about 40 nm too. Finally, the residue layers, such as 25 nm TPBi, 1 nm LiF, and 100 nm Al, were deposited by thermal evaporating at a pressure below 1 × 10 −5 mbar. The J-L-V curves were recorded using a combination of a Keithley source meter (model 2602) and a luminance meter. The EL spectra of the devices were measured using a PR-655 spectrophotometer. All the measurements were taken in ambient conditions at room temperature.
First, fully dried a Shrek flask (25 mL) cleaned in advance, then degassed with N 2 five times, when it had cooled to room temperature, wrapped it with tin foil and kept it away from light for standby. Then put bipyridine (0.5 g) into a Shrek flask quickly, add Ni(COD) 2 (0.5 g) in dark, degassed with N 2 for five times again. Next, add COD (0.3 mL) slowly, followed by adding the dry deaeration DMF (5 mL) into the reaction tube and activate it under the condition of 75 o C away from light for 30 min.
Under N 2 protection, the prepared PODPF-Cz (0.5 g) sample was dissolved in dry deaeration toluene (10 mL). Inject the sample solution into the Shrek flask and raise the temperature to 85℃ under N 2 atmosphere equipped with a magnetic stir bar at dark for 3 days. At the end of the polymerization, bromobenzene (0.3 mL) was added to the flask and refluxed for 1 day.
After the reaction, the solution was cooled to room temperature, filtered to remove the particle precipitate, then purified by a neutral alumina chromatography column, and column 4 chromatography was carried out with DCM as the eluent. The solution was concentrated to a viscous state, precipitated with methanol, and filtered to obtain the product. Finally, Soxhlet extraction was carried out with acetone, refluxed under heating and stirring at 90 o C for 3 days.
The product was vacuum dried to obtain a yellow powder solid. In this way, we prepared four polymers: PODPF-Cz, PDDPF-Cz, POBPF-Cz, and PDBPF-Cz.