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Organic Light Emitting Diodes (OLED) | Photonic Colloidal Crystals

Organic Light Emitting Diodes (OLED)

The main objective of this experiment is: (1) to synthesize molecules that exhibit electroluminescence and (2) to measure their properties in a sandwich device structure

BACKGROUND:

The basic organic light emitting diode (OLED) cell structure consists of a stack of thin organic layers sandwiched between a transparent anode and a metallic cathode. The organic layers consist of (i) a hole-injection layer, (ii) a hole-transport layer, (iii) an emissive layer, and (iv) an electron-transport layer. When a voltage is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light (electroluminescence). The structure of the organic layers and the choice of anode and cathode are designed to maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED device. OLEDs can be very thin (the active area producing the light is several hundred nm) and they have a wide viewing angle (up to 160 degrees). They have currently found use in portable devices such as cellular phones, digital video cameras, DVD players, and PDAs. OLED display technology can also be found in car audio components and cellular phones.

In this experiment, we are only looking at an organic molecule layer that can exhibit electroluminescence (there are no hole or electron transport layers). This layer is the emissive layer. Under an applied voltage, electrons are injected through the cathode (GaIn alloy) and holes are injected through the anode (indium tin oxide - ITO); charge recombination in the Ru complex results in light emission.

MATERIALS:

Synthesis:

• Tris-(2,2' bipyridyl)dichlororuthenium(II) hexahydrate
• Sodium tetrafluoroborate (Aldrich 20,221-5)
• Ethanol

OLEDs:

• Indium Tin Oxide (ITO) - coated slides
• Acetonitrile
• Cotton swab
• Liquid gallium indium alloy
• Sodium hydroxide
• Power supply
• Hersch Funnel
• Vacuum Filtration flask
  

Synthesis of Elecroluminescent Molecules

1. Prepare 45mM Tris-(2,2'-bipyridal)dichlororuthenium(II) hexahydrate. Dissolve 0.6030 g Tris-(2,2'-bipyridyl) dichlororuthenium(II) hexahydrate into 18 mL ddH2O in 30 mL beaker. Heat at 100°C.
2. Prepare 2M NaBF4. Dissolve 0.6686 g NaBF4 in 3 mL ddH2O. Heat to 100°C.
3. Prepare RuBPY solution. Add the NaBF4 solution dropwise over 1 to 2 minutes to the Ru(II) solution. Continue stirring for 5 min at 100°C. Turn off the hotplate and allow solution to slowly cool. RuBPY crystals will precipitate. Cool on an ice bath for 20 min and then vacuum filter. Wash three times with 1-mL portions of ice cold ethanol.
4. Dry crystals in oven at 105°C overnight.

PROCEDURE: (Click on pictures to view the videos)

	Identify the conducting side of the ITO glass by using a multimeter to measure resistance. The conducting side will have a resistance in the order of 20-50 Ohms. In this case, the display reads a voltage of 4.00 V and 0.08 Amp when in contact.
	If you are on the wrong side, it should read no current as seen in the display.
	Clean the ITO glass with ethanol and then dry with nitrogen.
	Bake it on a hot plate for 10 min. at 110 oC.
	Dissolve 10 mg [Ru(bpy)3](BF4)2 in 1 mL of acetonitrile. Drop a few drops of the RuBPY solution onto the conducting side of the ITO-coated glass slides. In a fume hood [or dessicator], evaporate the solvent. To get a more uniform coating, try controlling the airflow nitrogen. Place the slide on the hotplate and bake for 5 min.
	Use a syringe to deposit a small bead of liquid gallium-indium alloy on both the Ru-complex and a portion of the ITO-coated slide without the molecules. (This eutectic mixture of 75.5% gallium and 24.5% indium is a liquid above 16.5°C.)
	Touch the positive lead of a 6-volt power supply to the tin-oxide glass (not the [Ru(bpy)3](BF4)2 coating). Gently touch the negative lead to the gallium-indium alloy.

QUESTIONS:

1. Identify the conducting side of the ITO glass by using a multimeter to measure resistance. The conducting side will have a resistance of 20-30 Ohms.
2. What other types of molecules can be used for in OLEDs? Search the literature and list three others.
3. Why is a 6 V power supply needed? What would happen if the voltage were less than this value?