Effects of metal-doped indium-tin-oxide buffer layers in organic light-emitting devices

Organic light-emitting devices were fabricated by using different metal (V, Zr, Hf)-doped indium-tin-oxide (ITO) buffer layers on an ITO anode. The metal-doped ITO buffer layers were 15 nm thick with different metal concentrations. Both resistivity and work function of the ITO buffer layer were manipulated by these metal dopants. Different effects on the devices, such as reduced turn-on voltage, improved luminance, and enhanced current efficiency, were investigated. A low turn-on voltage was observed for devices with small work function and resistivity. The lowest turn-on voltage (3 V) was found on a device with a V-doped ITO buffer layer. The devices usually have a similar current density (J)-voltage (V) characteristics, but not the luminance-J or the current efficiency-J characteristics when the ITO buffer layers have the same work function. The devices with the Hf-doped ITO buffer layers show the best luminance performance among those considered. At 100 mA/cm², a luminance of 15,000 cd/m², and a current efficiency of 15 cd/A have been achieved. The balance between the carrier concentration and the energy barrier for the hole injection is possibly responsible for such performance.