Plexcore^® Organic Conductive Inks

Plexcore^® OC Hole Injection Inks

Employing the regioselective polymerization techniques used to make P3HTs, Plextronics has developed a self-doping polymer poly(thiophene-3-[2[(2-methoxyethoxy)ethoxy]-2,5-diyl) (Figure 1), which is available as ready-to-use solutions for making hole injection layers (HIL) in OLED (organic light-emitting diode) devices (in combination with a matrix polymer and other additives, Figure 2).

Figure 1. Chemical structure of Plexcore^® OC Inks

Figure 2. Lab scale OLEDs made using Plexcore^® OC conductive inks as HIL.

Plexcore^® OC inks can be spin-coated onto an ITO-coated glass substrate (Product No. 703184) to fabricate an HIL of a hybrid phosphorescent organic light emitting diode device (Figure 3). Advantages of Plexcore^® OC materials include reduced acidity (for decreased electrode degradation), tunable properties for specific device optimization and versatility, and easy solution processing. Furthermore, the unique versatility of Plexcore^® OC can be utilized to match the work function of the hole injection layer to the rest of the device stack. This minimizes injection losses at interfaces, enables low voltage device operation, and extension of device lifetime. Plexcore^® OC materials are compatible with a wide variety of device architectures, including vapor deposited OLEDs and solution-processed polymer OLEDs with poly(phenylene vinyline) (PPV) and polyfluorene emitters.

Figure 3. OLED Device Stack

NPB: N,N’-Bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine (Product No. 556696)
CBP: 4,4’-N,N’-dicarbazole-biphenyl (Product No. 699195);
Ir(ppy)₃: tris-[2-phenylpyridinato-C2,N]iridium(III) (Product No. 694924);
BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine) (Product No. 699152);
Alq₃: tris-8-hydroxyquinoline aluminium (Product No. 697737);
Ca/Al: Calcium/Aluminium

Due to the tunable nature of Plexcore OC inks, the properties are easily manipulated to match the requirements of various OLED stack designs. Plextronics has developed an OC Ink Kit (Product No. 719102) which allows for the highly tunable development of devices. The kit features a selection of conductive inks with a range of electrical properties ideal for use as HILs in OLED devices. The properties of the kit components are outlined in Table 1.

Table 1. Plexcore^® OC Ink Kit (Product No. 719102)

Component	Resistivity (Ω-cm)	Work Function	Viscosity (cP)	pH	Surface Tension (dynes/cm)
RG-1110	10-300	-4.8	5-11	2.5-4.5	33-40
RG-1115	10-300	-5.25	5-11	2.5-4.5	33-40
RG-1150	300-1000	-4.8	5-11	5-11	33-40
RG-1155	300-1000	-5.25	5-11	5-11	33-40

Two standard Plexcore^® OC inks are also available. The properties of these inks are outlined in Table 2.

Table 2 Plexcore^® OC RG-1100 and Plexcore^® OC RG-1200

Product*	Work Function (eV)	Resistivity (W-cm)	Viscosity (cP)	pH	Surface Tension (dynes/cm)	Package Size	Product No.
Plexcore® OC RG-1200	5.1–5.2	500-3000	4-10	2.2–2.6	34-37	25 mL	699780
Plexcore® OC RG-1100	5.1–5.2	25-250	7-13	2.2–2.8	35–38	25 mL	699799

*Products of Plextronics, Inc., U.S. Pat. 6.166.172.

Commercial supply of Plexcore materials on multi-kilogram (Plexcore OS) and multi-liter (Plexcore^® OC) scale is available directly from Plextronics: www.plextronics.com

Figure 4 shows representative performance of green phosphorescent OLEDs made with Plexcore^® OC RG-1200 (Figure 4a) and Plexcore^® OC RG-1100 (Figure 4b). Turn on voltage of the devices is at the band gap of the emitter, indicating low interfacial energy barriers, which can be attributed to efficient hole injection. This leads to efficiencies of 14-15 cd/A, with modest efficiency drop-off at increasing current density.

Figure 4a. Plexcore^® OC RG-1200 (Product No. 699780) (Performance in phosphorescent OLED)

Figure 4b. Plexcore^® OC RG-1100 (Product No. 699799) (Performance in phosphorescent OLED)

Power efficiency of a phosphorescent OLED device utilizing Plexcore^® OC as the HIL is shown in Figure 5. The loss in power efficacy is minimized up to 1,000 units of brightness using the Plexcore^® OC technology.

Figure 5. Power Efficiency of a phosphorescent OLED Device with Plexcore^® OC HIL.

For information relating to processing of conducting inks for device fabrication, see Technical Bulletin AL-251.