Highly efficient hybrid tandem white organic light-emitting diodes with two different color-emitting units

White-color organic light-emitting diodes (WOLEDs) have aroused wide interests for future lighting because of low energy consumption in principle, while still suffer from the degradation by Joule heating and polaron-induced quenching under high luminance caused by high current in practical. To obtain high current efficiency, which means high luminance at low current density, tandem WOLEDs with multiple electroluminescence (EL) units connected in series with charge generation layers (CGLs) have been developed. Here we report the improved hybrid tandem WOLED with two EL units, EL1 and EL2, where EL1 and EL2 generate fluorescent blue and phosphorescent yellow emissions from a metal-free material BCzVBi and a Ir-complex (fbi)2Ir(acac), respectively, with CGL composed of a HATCN/NPB bilayer. The white-light emitting device shows the maximum current efficiency and power efficiency of 60.2 cd/A and 29.6 lm/W, respectively, without out-coupling structure. A low driving voltage of 6.5 V at 1,000 cd/m2 is realized, which is an important value because such a luminance is required for actually employed lighting device. The blue emission appears from 5.4 V, and the light color changes from yellowish to bluish white during 5.4 V to 13.4 V, which could be explained by the energy level structures of the device and proved by the J-V characteristics. Additionally, this tandem WOLED also shows photovoltaic effect. This work provides a design of room light source whose color temperature could be tuned through applied voltage when needed, and a display system for low-current operated amorphous Si-TFT due to its high current efficiency.