ESE206ORGANIC / POLYMERIC LIGHT EMITTERS (MSE 250 Experiment II)

OBJECTIVES

Make and characterize an organic LED from A to Z: fabricate diode structure; measure I-V characteristics, observe light output.

Measure I-V turn-on voltages for a series of inorganic LED’s spanning red to blue/UV emission

SUGGESTED READING

Light-emitting diodes:

http://micro.magnet.fsu.edu/primer/lightandcolor/ledsintro.html

BACKGROUND

An organic light-emitting diode (OLED) is a thin-film LED in which the emissive material is an organic compound. OLED technology is especially well suited for picture elements in display devices. These devices promise to be more versatile and less costly to fabricate than traditional LCD displays. OLED materials can be printed onto any suitable substrate using inkjet printer technology, and prototypes have been made of flexible displays. One of the great benefits of an OLED display over the traditional LCD displays is that OLEDs do not require a backlight to function; this means that they draw far less power. OLEDs may also be used as solid state lighting devices. OLEDs are available as distributed sources while the inorganic LEDs are point sources of light.

Currently, the biggest drawback to using OLEDs in devices is the limited lifetime of the organic materials, particularly those in blue OLEDs. Also, the intrusion of water can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.

An OLED works on the principle of electroluminescence. The key to the operation of an OLED is an organic luminophore. An exciton, which consists of a bound, excited electron and hole pair, is generated inside the emissive layer. When the exciton's electron and hole combine, a photon can be emitted. A major engineering challenge is tuning the OLED device such that an equal number of holes and electrons meet in the emissive layer. This is difficult because, in an organic compound, the mobility of an electron is much lower than that of a hole.

To create the excitons, a thin film of the luminophore is sandwiched between electrodes of differing work functions (the minimum energy needed to remove an electron from a solid). Electrons are injected into one side from a metal cathode, while holes are injected in the other from an anode. The electron and hole move into the emissive layer and can meet to form an exciton. Derivatives of PPV, poly(p-phenylene vinylene) and poly(fluorene), are commonly used as polymer luminophores in OLEDs. Indium tin oxide is a common transparent anode, while aluminium or calcium are common cathode materials. Other materials are often added between the emissive layer and the cathode or the anode to facilitate or hinder hole or electron injection, thereby enhancing the OLED efficiency.

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The OLEDs you will make in this experiment are slightly different than those described above using derivatives of PPV. The polymer that will be used in this experiment is polyvinyl alcohol (PVA), a nonconductive polymer that will be doped with light emitting, conductive molecules. With this PVA method, low cost processing techniques can be used to deposit organic films that would otherwise need to be vacuum evaporated onto substrates (like the expensive processes used to make semiconducting LEDs). The small molecules, [Ru(bpy)3](BF4)2 will be supported in the long-chain PVA matrix and can be induced to emit light by passing an electric current through the film. The resulting electroluminescence involves electrons hopping to transfer charge across the film, and light emission by phosphorescence from the excited state created in a combination reaction.

EXPERIMENTAL

Summary of Tasks: Fabrication of device, electrical and optical characterization.

Apply Ga-In alloy top contact; attach leads to Ga-In cathode and ITO anode.

Connect DC power supply, verify continuity, and light emission.

Measure I-V characteristics of the OLED. Record turn-on voltage of the device.

Measure the I-V curves for inorganic LEDs.

Spin coating thin layers onto Indium-Tin Oxide (ITO) slides

The ITO slides have been pre-cleaned and placed in a covered petri dish; the quality of the film can be greatly affected by dust particles.

Check the surface of the ITO slide for continuity using a multimeter. Carefully affix the slide (conductive side facing up) to the center of the spin coater.. Without introducing air bubbles, draw up the 1:5 solution of Ru(bpy)3](BF4)2 in a 1mL syringe

or pipette. Transfer solution onto the ITO surface of the slide until the solution nearly covers the slide (see

figure 1 below, left). Empty remaining solution back into the vial. Place the lid on the spin coater container. Plug in the transformer and attach the fan leads to the

alligator clips to power up the spin-coater. Allow the slide to spin for about 10 seconds, and then disconnect the leads. Remove the slide

and place it on a clean sheet of paper. Optional : Dip a cotton swab into distilled water and squeeze out the excess moisture. Use the

swab to wipe off a strip (about ¼” wide) of the Ru film along one edge of the slide to expose the ITO underneath (see figure 1 below, right).

Dry the film with the hot air gun.

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Figure 1.Inspect thin layer visually and under UV light

Inspect the slide visually to assess the quality and thickness of the film.

Fabrication of OLED devices

Figure 2.

Affix a 3-4” length of 1” wide double-stick tape to a transparency film and cut around it to remove from the sheet. Cut a ¾” piece (it must be small enough to leave the exposed portion of ITO uncovered) from the now self-adhesive transparent strip. This will be used as a mask for applying the liquid metal cathode.Holding the piece of transparency adhesive-side down over the OLED film (do not stick it to the surface yet), punch a hole in the location of the most uniform, defect free area of the film. This hole will be used as a mask to deposit the liquid metal cathode. Placing the top electrode over a pin-hole defect (such as those caused by dust particles on the ITO glass prior to spin coating) will short circuit the device and it will not illuminate.

Figure 3.

Affix the mask to the OLED film and press it down to remove any air bubbles. Being careful not to damage the film through the hole, apply pressure as shown in the above center figure to ensure intimate contact around the hole’s

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circumference. Place a small piece of transparent tape over and around the edge opposite the ¼” strip of exposed ITO glass. This will insulate the top electrode wire from the OLED film and ITO substrate.

Figure 4.

Dip a glass stirring rod or cotton swab into the Ga-In eutectic alloy and transfer a small drop onto the OLED film through the mask. Use only enough to just fill the volume of the hole (taking into account the thickness of the mask). The Ga-In is a hazardous material, and skin contact shoud be avoided. If any liquid spills, notify the lab assistant immediately. Place a length of thin wire across the surface, bridging the hole in the mask. Cut another ¾” piece of adhesive backed transparency and place it over the mask. When pressing the cover on, do not apply excessive force directly over the cathode or the film may be damaged. Seal around the wire being careful not to squeeze out excess Ga-In.

Figure 5.

If Ga-In escapes from underneath the cover slip the device should be discarded because it is unsafe to handle. Test the OLED using the 6VDC power supply; connect the positive (red) lead to the exposed ITO glass and the negative (black) lead to the cathode wire.

This lab originally written by Steve T. S Szewczyk, Lab Coordinator, Material Science and Engineering, SEAS. Edited by Sid Deliwala 26-Mar-2013. ESE is thankful to Hitesh Sahoo, M.S, Nano’13 for his contribution in planning and setting up the lab apparatus.

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Avoid placing too much Ga-In

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