In this paper, efficient phosphorescent white organic light-emitting diodes (WOLEDs) were fabricated based on ultrathin doping-free emissive layers and mixed bipolar interlayers. The energy transfer processes were proved via the research of WOLEDs with different interlayer thicknesses and transient photoluminescence lifetime. WOLEDs with optimized thickness of doping-free emissive layers show maximum current efficiency of 47.8 cd/A and 44.9 cd/A for three-colors and four-colors WOLEDs, respectively. The Commission Internationale de L’Eclairage coordinates shows a very slight variation of ( ± 0.02, ± 0.02) from 5793 cd/m2 to 11370 cd/m2 for three-colors WOLEDs and from 3038 cd/m2 to 13720 cd/m2 for four-colors WOLEDs, respectively. The stability of the spectra is attributed to the stable and sequential energy transfer among the various dyes. The color temperature of four-colors WOLEDs can be obtained from 2659 to 6636 by adjusting the thickness of ultrathin emissive layer.

Color-stable white organic light-emitting diode (WOLED) with bipolar mixed spacer based on ultrathin non-doped phosphorescent emitting layers was investigated. We find that the location and thickness of Bis(3,5-difluoro-2-(2-pyridyl) phenyl-(2-carboxypyridyl)iridium(III) (FIrPic) play a critical role on the spectral stability. Transient PL lifetime results show that the spectral variations with the driving voltage can be effectively controlled by enhanced energy transfer or weakened energy transfer respectively depending on different location of FIrPic. Meanwhile, green layer, located between blue layer and orange layer, acts an indispensable role in keeping the color stability by alleviating triplet–triplet annihilation and maintaining sequential energy transfer. The most stable spectrum of WOLED can be obtained with the change in Commission Internationale de L’Eclairage coordinates less than (0.001, 0.002) as the luminance increase from 697 cd m−2 to about 16 550 cd m−2 when FIrPic is on anode side. This discovery provides a significant method for achieving high-quality WOLEDs with superior color stability.

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Tetra-chromatic (Blue–Green–Red–Orange) white organic light-emitting diodes (WOLEDs) with superior color stability was demonstrated. Strong carrier trapping effects of red dyes cause significant color shift in multi-color WOLEDs. The high color stability here is attributed to the decrease of red dye trapping sites and enhancement of effective energy transfer to red dye while maintaining appropriate exciton concentration near red dye. As luminance increased from 1000 to 10 000 cd m−2, variations in Commission Internationale de L’Eclairage are merely (0.007, 0.007). Furthermore, this color-stable WOLED achieved a color rendering index close to 90 simultaneously. Our work shows the manipulation of red dye is crucial for achieving superior color stability in multi-color WOLEDs.

In this paper, efficient phosphorescent white organic light-emitting diodes (WOLEDs) with stable spectra are fabricated based on doping-free ultrathin emissive layers and mixed bipolar interlayers. To achieve WOLEDs, at least three kinds of light-emitting layers, i.e. blue, green and red, are needed. The traditional method to fabricate emissive layers is by co-evaporation, which can improve electroluminescent efficiency. However, the co-evaporation rate and dopant concentration are difficult to control, which leads to a bad reproducibility and thus goes against commercialization. In order to simplify the structures of WOLEDs and improve repeatability, several doping-free ultrathin emissive layers are used in this paper with 3 nm mixed bipolar interlayers separating them. The optimal ratio of bipolar hybrid material is determined by hole-only device, electron-only device and blue phosphorescent OLED. In addition, green, orange and red monochromatic OLED have also been fabricated separately, which are used to prove that mixed bipolar material is also suitable for the three phosphorescent emitting material. The WOLED with TCTA interlayers is fabricated to confirm that mixed bipolar material is beneficial to the characteristics of WOLEDs. The energy transfer process between different emitting materials is verified by studying the transient photoluminescence lifetime. The maximum efficiency of three-color and four-color doping-free WOLED are 52 cd/A (53.5 lm/W) and 13.8 cd/A (13.6 lm/W), respectively, and the maximum external quantum efficiency of three-color and four-color doping-free WOLED are 17.1% and 11.2%, respectively. Due to the sequential energy transfer structure between different emitting layers, the Commission Internationale de L'Eclairage coordinates shows a very slight variation of (0.005, 0.001) from 465 cd/m² to 15950 cd/m² for three-color WOLED. The Commission Internationale de L'Eclairage coordinates shows a variation of (0.023, 0.012) from 5077 cd/m² to 14390 cd/m² for four-color WOLED. The four-color WOLED shows a maximum color rendering index of 92.7 at 884 cd/m², and it reaches 88.5 at 14390 cd/m². In addition, the lifetime of phosphorescent OLED is usually poor due to the trap formed by triplet-polaron annihilation. The exciton distribution can be broadened and the exciton concentration can be reduced by using ultrathin light emitting layers (< 1 nm) and mixed bipolar interlayers. Therefore, triplet-polaron annihilation will be reduced, and the lifetime of OLEDs will be improved.