High-efficiency fluorescent white organic light-emitting diodes (OLED) were fabricated by using double holetransporting-
layers (HTLs), poly(3,4-ethylene- dioxythiophene)-poly-(styrenesulfonate) (PEDOT) and N,N'-bis-(1-
naphthyl)-N,N'-diphenyl-1,10-biphenyl-4-4'-diamine (NPB). The diodes were composed of a single emissive-layer
(EML), with 0.5 wt% red 4-(dicyanomethylene)-2-tbutyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran doped in a
mixed-host of 25% trans-1,2-bis(6-(N,N-di-p-tolylamino)-Naphthalene-2-yl)ethene and 75% 1-butyl-9,10-naphthaleneanthracene.
The device structure comprised a 125 nm anode layer of indium tin oxide, a 25 nm first HTL of PEDOT, a 0
to 10 nm second HTL of NPB, a 30 nm EML, a 40 nm electron-transporting-layer of 2,2',2"-(1,3,5-benzenetriyl)-tris(1-
phenyl-1-H-benzimidazole), a 1 nm electron-injection-layer of lithium fluoride and a 150 nm cathode layer of aluminum.
With the addition of a 7.5 nm second HTL (NPB), the resultant power-efficiency at 100 cd/m2, for example, was
increased from 11.9 to 18.9 lm/W, an improvement of 59%. The improvement was even more marked at 1,000 cd/m2,
i.e. that the power-efficiency was increased from 9.1 to 16.5 lm/W, an improvement of 81%. The marked efficiency
improvement may be attributed to a better balance of carrier-injection in the desired emissive zone since the addition of
the NPB layer in between the first HTL and the EML may have effectively reduced the injection of excessive holes into
the EML due to the relatively high energy-barrier to hole, which was 0.5 eV, at the interface of the two HTLs. The
resultant hole-blocking function was plausibly more effective at higher voltage so that comparatively much less holes
would be injected into the EML, leading to a much better balanced carrier-injection and consequently a higher
efficiency-improvement at the higher brightness.