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By employing cesium carbonate doped tris-(8-hydroxyquinoline) aluminum/MoO3 as the effective charge generation layer(CGL),high efficiency two-unit series connected tandem organic light-emitting diodes were fabricated.It is found the performance of the two-unit tandem devices can be controlled by the thickness of the MoO3 layer.The 30 nm-thick MoO3 CGL tandem device exhibits the best device performance with the current efficiency of 14.5 cd/A,which is above 2.0 times than that of the control device over a wide range of current densities.It's suggested that the connecting layer broke down under a high current density,leading to the decrease of current efficiency.The improvement of tandem device performance is attributed to the efficient electron and hole injection from charge generation layer to two adjacent emission units.

Ultra-thin hydrogenated amorphous germanium films,with various thickness from 160 nm to 5 nm were grown by plasma enhanced chemical vapor deposition technique.The film structure was characterized by Raman spectroscopy,which exhibited a broad band centered around 280 cm-1 indicating their amorphous nature.The film thickness and optical properties were evaluated by ellipsometer spectroscopy.The measured thickness was well consistent with the pre-designed value and the optical band gap was about 1 eV which slightly increased with the decrease of film thickness.The temperature dependent conductivity of the films was measured.The electronic transport was believed to occur in the extended-states and the corresponding activation energy is about 0.3~0.4 eV.The light emission in infrared region from the films can be detected at low temperature.The sample for 160 nm had a broad luminescence band which can be divided into two sub-bands centered at 0.78 eV and 0.67 eV,respectively.By decreasing the films thickness less than 10 nm,the luminescence band beaome narrower and only a band at 0.8 eV was observed.It may be due to the relatively more hydrogen in the ultrathin films,which passivated the defect states and suppressed the defect-related luminescence.The non-radiative activation energy increased in ultrathin films suggesting the improved efficiency.