Bryan E. Koene

Thermally Stable Hole-Transporting Materials Based upon a Fluorene Core

We report a new class of diamine hole-transporting materials (HTMs) based upon a fluorene core. Using a fluorene core, rather than a biphenyl group, leads to enhanced thermal stability, as evidenced by glass-transition (Tg) temperatures as high as 161 °C for N,N′-iminostilbenyl-4,4′-fluorene (ISF). The fluorene-based HTMs have lower ionization potentials (Ip) than their biphenyl analogs, which leads to more efficient injection of holes from the indium tin oxide (ITO) anode, and higher quantum efficiencies. Devices prepared with fluorene-based HTMs were operated under thermal stress. The failure of an organic light-emitting diode (OLED) under thermal stress has a direct correlation with the thermal stability of the HTM that is in contact with the ITO anode. OLEDs based on ISF are stable to over 140 °C.

LOW PRESSURE ORGANIC VAPOR PHASE DEPOSITION OF SMALL MOLECULAR WEIGHT ORGANIC LIGHT EMITTING DEVICE STRUCTURES

A new technique for the deposition of amorphous organic thin films, low pressure organic vapor phase deposition (LP-OVPD), was used to fabricate organic light emitting devices (OLEDs) consisting of a film of aluminum tris-(8 hydroxyquinoline) (Alq3) grown on the surface of a film of N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′biphenyl-4-4′diamine. The resulting heterojunction OLED was found to have a performance similar to conventional, small molecular weight OLEDs grown using thermal evaporation in vacuum. The LP-OVPD grown device has an external quantum efficiency of 0.40±0.05% and a turn-on voltage of approximately 6 V. The rapid throughput demonstrated with LP-OVPD has the potential to facilitate low cost mass production of conventional small molecule based OLEDs, and its use of low vacuum in a horizontal reactor lends itself to roll-to-roll deposition of organic films for many photonic device applications.

Asymmetric diamines as thermally stable hole transporters in organic light emitting devices, synthesis and characterization

One of the major causes of device degradation in organic thin film devices is the thermal instability of the hole transporting layer. In vacuum deposited organic light emitting devices (OLEDs) this layer usually consists of a biphenyl diamine such as TPD, which is a biphenyl that has a phenyl tolyl amine moiety at both ends. TPD has a glass transition temperature (Tg) of about 60 C. Devices made with TPD show catastrophic failure when the device is heated to near the Tg. In our efforts to design thermally stable hole transporters, as well as to understand how chemical structure relates to Tg, we have synthesized a variety of asymmetric biphenyl amines. These amines consist of a triaryl amine attached at one end of a biphenyl along with a different triaryl amine at the other end of the biphenyl.