Step-by-Step Guide for Harnessing Organic Light Emitting Diodes by Solution Processed Device Fabrication of a TADF Emitter

We focus on Organic Light-Emitting Diodes, and these devices offers better contrast, wide viewing angle, faster response. As technology advances, the world is moving towards flexible devices. OLED is one of the best choice for flexible displays, and flexible OLEDs opens new avenues in foldable displays, smart textiles, and futuristic devices.

For making OLEDs, as of now, there are two different approaches:vacuum thermal deposition and solution-processed device fabrication. Vacuum thermal deposition has several limitations, including the need for high temperatures, increased energy consumption, and greater material requirements. Switching to solution-processed device fabrication is advantageous as it is simple, cost-effective, and compatible with flexible substrates.

Currently, iridium-based emitters are being utilized in this place and we are trying to replace these emitters with the pure organic emitters. Therefore, we are focusing on the development of thermally activated delayed fluorescence emitters, which are made of pure organic compounds, and the performances are similar to iridium-based emitters. To begin, design a donor-acceptor-based thermally activated delayed fluorescence emitter based on findings from a comprehensive literature survey.

Using a mixture of ethyl acetate and N-hexane as the eluent, purify the synthesized compound by column chromatography. To sublime the compound, use a temperature gradient high-vacuum thermal sublimation setup to obtain a high-purity product. For substrate cleaning, take patterned Indium Tin Oxide-coated glass substrates.

Then, place the substrates into a substrate holder for cleaning. Submerge the substrates into a beaker containing acetone. And sonicate them in an ultrasonic bath for 10 minutes.

Next, sequentially in separate beakers, take isopropyl alcohol, 1%Hellmanex III soap water, ultrapure water, acetone, and isopropyl alcohol. Place the substrate in the solution and sonicate for 10 minutes each. Then, take out the substrates.

And dry them using a nitrogen gun. Use a multimeter to identify the Indium Tin Oxide-coated side of the substrate by testing for conductivity. Using an ultraviolet ozone cleaner, clean the Indium Tin Oxide surface.

For spin coating, apply the whole injection layer on top of the Indium Tin Oxide surface. Use a 0.45-micrometer polytetrafluoroethylene syringe filter to filter the commercially available PEDOT:PSS, and cover the cleaned conductive top surface of the substrate. Then, place the coated substrate on a hot plate at 140 degrees Celsius for approximately 20 minutes for annealing.

Using a wet swab or lint-free tissue, gently wipe the sides and bottom of the substrate to remove residue. Inside the glove box, spin coat a 30 nanometer layer of PVK as the whole transport layer over the whole injection layer. Then, place the substrate back on the hot plate at 140 degrees Celsius for approximately 20 minutes to anneal the PVK layer.

Filter the emissive layer of solution through a 0.45-micrometer polyvinylidene fluoride syringe filter. Spin coat a 30-nanometer emissive layer for 60 seconds. After that, wipe the sides and bottom of the Indium Tin Oxide-patterned substrate with toluene.

Place the cleaned substrate onto a metallic substrate holder inside the thermal evaporator. Using vacuum thermal deposition, deposit the electron transport layer, electron injection layer, and cathode layer. After verifying the cathode and anode connections, keep the fabricated device on the measurement setup.

Now supply voltage to the device to obtain illumination. The current density increased sharply with applied voltage, reaching a maximum of 291 milliamperes per square centimeter at 20 volts. The luminance reached a maximum of 9, 050 candelas per square meter.

The external quantum efficiency showed a maximum of 7.5%and decreased gradually with increasing luminance. However, this can be improved by further optimization. The current efficiency reached a maximum of 23.09 candelas per ampere, and the power efficiency reached a maximum of 7.63 lumens per watt, with both decreasing as luminance increased.

The electroluminescent spectrum peaked at approximately 515 nanometers with a full width at half maximum of 94 nanometers at 12 volts. The CIE chromaticity coordinates derived from the electroluminescence spectrum were 0.28 and 0.53, indicating green emission.