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 (T_g) temperatures as high as 161 °C for N,N′‐iminostilbenyl‐4,4′‐fluorene (ISF). The fluorene‐based HTMs have lower ionization potentials (I_p) 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.