Organic light emitting diodes （OLEDs） incorporating an n-doping transport layer comprised of 8-hydroxy-quin- olinato lithium （Liq） doped into 4＇ 7- diphyenyl-1,10-phenanthroline （BPhen） as ETL and a p-doping transport layer that includes tetrafluro-tetracyano-quinodimethane （F4- TCNQ） doped into 4,4′, 4″-tris （3-methylphenylphenylamono） triphe- nylamine （m-MTDATA） are demonstrated. In order to examine the improvement in the conductivity of transport layers, hole-only and electron-only devices are fabricated. The current and power efficiency Of organic light-emitting diodes are improved significantly after introducing an n-doping （BPhen：33wt% Liq） layer as an electron transport layer （ETL） and a p-doping layer composed of m-MTDATA and F4- TCNQ as a hole transport layer （HTL）. Compared with the control device （without doping） , the current efficiency and power efficiency of the most efficient device （device C） are enhanced by approximately 51% and 89% ,respectively, while driving voltage is reduced by 29%. This improvement is attributed to the improved conductivity of the transport layers that leads to efficient charge balance in the emission zone.
The electron mobilities of 4, 7-diphenyl-1, 10-phenanthroline （BPhen） doped 8-hydroxyquinolinatolithium （Liq） at various thicknesses （50-300 nm） have been estimated by using space-charge-limited current measurements. It is observed that the electron mobility of 33 wt% Liq doped BPhen approaches its true value when the thickness is more than 200 rim. The electron mobility of 33 wt% Liq doped BPhen at 300 nm is found to be -5.2 × 10^-3 cm^2/（V.s） （at 0.3 MV/cm） with weak dependence on electric field, which is about one order of magnitude higher than that of pristine BPhen （3.4 × 10^-4 cm^2/（V.s）） measured by SCLC. For the typical thickness of organic light-emitting devices, the electron mobility of doped BPhen is also investigated.
