Control of efficiency characteristics in green phosphorescent organic light-emitting devices

The efficiency characteristics in green phosphorescent devices at low luminance as well as high luminance were investigated by using single, host-mixed and multiple emitting layer (EML) structure. 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) was used as a host suppressing electron injection and transport, whilst a host (GH) with a spirobifluorene-type backbone was used as a host enhancing electron injection and transport. When two hosts were optimally mixed (CBP:GH = 3:1) or a triple EML structure with GH–EML sandwiched between two CBP–EMLs were used, the improved current efficiency at high luminance was obtained and at the same time, the current efficiency at low luminance was maintained low.