Abstract
We investigated emission characteristics of tandem organic light emitting devices (OLEDs) with p-type materials as charge generation layer. The tandem OLEDs were fabricated by using <TEX>$MoO_x$</TEX>, <TEX>$WO_x$</TEX>, C60 and HATCN as p-type material or not using p-type material for charge generation. When HATCN was used as p-type material, it showed high current density at low applied voltage, but increase of efficiency was small because of charge unbalance in emitting layer. In case of tandem OLED not using p-type material, applied voltage increased remarkably because of difficulty of hole injection. In case of <TEX>$MoO_x$</TEX>, <TEX>$WO_x$</TEX> or C60 as p-type material, current emission efficiency increased greatly. In particular, current emission efficiency of tandem OLED using <TEX>$MoO_x$</TEX> as p-type material increased up to 3 times than current emission efficiency of single OLED. The Commission Internationale de l'Eclairage (CIE) 1931 color coordinates were changed by overlapping of 504 nm emission wavelength. As a result, emission efficiency of tandem OLED improved compared with single OLED, but driving voltage also increased by increase of organic layer thickness.
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