Improvement of efficiency in inverted bottom-emission white OLEDs by doping the hole transport layer

Recently, it is a critical issue to make larger display panels with low costs in organic light-emitting diodes (OLEDs). However, the uniformity and the cost of low temperature poly Si (LTPS) thin film transistors (TFTs) based backplanes for driving panels as well as the fine metal mask for pixel-patterning obstruct the realization of large-size OLED displays. To overcome these problems, using amorphous silicon (a-Si) TFTs which have high uniformity and cost-efficiency as the backplanes and white OLEDs (WOLEDs) which do not requiring any fine metal mask have been suggested previously. The inverted structure of OLEDs is much suitable rather than the conventional structure for a-Si TFTs because most a-Si TFTs have n-type channel. Here, we demonstrate highly efficient inverted bottom-emission WOLEDs by controlling the balance of electrons and holes injected from electrodes. The maximum external quantum efficiency (E.Q.E.) of inverted WOLEDs was increased from 6.4% to 8.6% (about 34% improvement). To the best of our knowledge, this value is the highest E.Q.E. without other optical light extraction techniques in inverted WOLEDs reported to date.