In this paper, two novel deep-blue emitters, mm-BPPI and mm-CN-BPPI, were obtained by coupling two 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) blocks in a meta–meta pattern and using a central benzene moiety as the bridge. Owing to this meta–meta connection, the dimer conjugation degree of mm-BPPI was effectively limited, and thus mm-BPPI exhibited very similar photophysical and electrochemical properties as PPI. However, the thermal stability of mm-BPPI was improved successfully, overcoming the defects of PPI for practical applications. Non-doped optimized devices fabricated with mm-BPPI exhibited stable deep-blue emission with CIE coordinates of (0.156, 0.050). After inserting cyano groups, higher glass-transition temperature and a quasi-reversible redox process were observed in mm-CN-BPPI; moreover, the lower efficiency roll-off was realized in its non-doped devices. This study demonstrates a strategy towards stable deep blue emitters with higher stability, better efficiency and lower roll-off ratio by chemical modification (meta–meta coupling or cyano group insertion) in non-doped deep-blue-emitting devices based on aryl-substituted phenanthroimidazole blocks.

We have successfully demonstrated efficient single-layer organic light-emitting devices (OLEDs) with a current efficiency of 31.38 cd A−1. The efficiencies still remain as high as 31.36 cd A−1, 30.76 cd A−1 and 29.98 cd A−1 at the luminance of 1000 cd m−2, 5000 cd m−2 and 10000 cd m−2. The key feature of the device concept is uniformly doping iridium-bis-(4,6-difluorophenyl-pyridinato-N,C2)-picolinate (FIrPic) into a single organic layer to balance the transport of charge carriers. To better understand the mechanism of single-layer OLEDs, the position of the recombination zone and the influence of FIrPic on the transport properties are also studied in detail. Our work clearly reveals that the performance of the single-layer OLEDs can be dramatically improved by intentionally doping with a phosphorescent dye to balance the transport of charge carriers. This novel and versatile device concept provides a promising simple method to achieve high performance single-layer OLEDs.

•The key feature of the concept is to introduce double blue mixed-host EMLs with an orange ultrathin layer.•The white device without spacer or interlayer achieves superior color-stability.•The peak efficiencies of 40.8 lm/W and low turn-on voltage of 2.71 V are realized.•The double mixed-host EMLs concept proves to be quite useful in achieving excellent device performance.We demonstrate high-efficiency and superior color-stability white phosphorescent organic light-emitting diodes based on double blue mixed-host emission layers (EMLs) with different mixed ratios. The key feature of the concept is to introduce double blue mixed-host EMLs with an orange ultrathin layer sandwiched between them. The improved white device without spacer or interlayer achieves superior color-stability and reduced efficiency roll-off, which are consistent with the good ambipolar conductivity of the mixed-host layer. Moreover, peak efficiency of 40.8 lm/W and low turn-on voltage of 2.71 V are realized. The double mixed-host EMLs concept proves to be quite useful in achieving excellent device performance.

•We proposed an original device concept for simplified WOLEDs to achieve good color-stability.•The two-color and three-color WOLEDs achieve maximum power efficiencies of 45.5 lm/W and 32.8 lm/W.•The reason of the superior color-stability of the WOLEDs is discussed.•The mixed host structure plays key roles in achieving excellent color stability.We have demonstrated color-stable and highly efficient simplified white phosphorescent organic light-emitting diodes. The key feature is the use of a novel approach to confine the distribution of charge carriers and excitons across the whole blue emission layer. The resulting two-color white device has the maximum power efficiency and current efficiency of 45.5 lm/W and 43.5 cd/A with a very low color shift over a wide range of luminance. By systematically investigating the working mechanisms, we found that the ambipolar charge carrier transport ability of co-host layer which ensures the distribution of excitons to form in the whole blue emission layer was the critical factors for constructing color-stable white devices. Our results show that simplified white devices based on two organic materials achieving excellent color stability are possible.

We demonstrated color stability improved white phosphorescent organic light-emitting diodes (WOLEDs) based on red, orange and blue emission layers. Iridium(III) Bis(3,5-diflouro)-2-(2-pyridyl)phenyl-(2-carboxypyridyl) was doped into red emission layer (R-EML) and orange emission layer (O-EML) to lower the electrons injection barrier and facilitate the ambipolar charge carriers balance. Consequently, the recombination region was extended to the R-EML and O-EML, leading to the excellently stable spectra and the reduction of triplet–triplet annihilation. Then the resulting device with a negligible Commission International de L'Eclairage coordinates shift of (0.003, 0.007) within a wide luminance range as well as a high color rendering index of 90 was gained, which was comparable to the profit caused by the conventional method of introducing the interlayer. And the emission mechanism of the WOLEDs was also discussed.

meta-Coupling isomers usually exhibit bluer emission than do the para-isomers, but the loss of efficiency with respect to photoluminescence (PL) and electroluminescence (EL) is an inevitable result in most cases, particularly for deep blue emitters. In this study, three blue emitting isomers, 4,4′-bis(1-phenyl-phenanthro[9,10-d]imidazol-2-yl)biphenyl (BPPI), 3,4′-bis(1-phenyl-phenanthro[9,10-d]-imidazol-2-yl)biphenyl (L-BPPI) and 3,3′-bis(1-phenyl-phenanthro[9,10-d]-imidazol-2-yl)biphenyl (Z-BPPI), were chosen as model compounds to investigate the essential reason behind the meta-coupling effect due to their different coupling forms, viz. para–para, para–meta, and meta–meta, respectively, in similar dimeric phenanthroimidazole frameworks. A combination of detailed photophysical data, device performance and DFT calculations for the excited state provided valuable information. In particular, the relationship between certain key parameters in calculations as well as PL or EL properties was confirmed, such as oscillator strength and quantum yield, among others, which could effectively reduce the issues related to synthesis and characterisation using prior computer simulations. Good agreement was observed in the results obtained from calculation and experiments, and it was concluded that meta-tuning barely realised improvement in EL, unless some special excited states formed or an exciton conversion channel appeared, as in the case of reverse intersystem crossing.

•Efficient simplified doping-free orange and white OLEDs are demonstrated.•White devices have stable spectra with CIE coordinates of (0.38, 0.44).•The maximum efficiency of white devices is 33.6 cd/A (30.1 lm/W).•The operational mechanism of orange and white OLEDs is discussed.We demonstrate simplified doping-free orange phosphorescent organic light-emitting diodes (OLEDs) based on ultrathin emission layer. The optimized orange device has the maximum current efficiency of 52.1 cd/A and power efficiency of 36.3 lm/W, respectively. Efficient simplified doping-free white OLEDs employing blue and orange ultrathin emission layers have excellent color stability, which is attributed to the avoidance of the movement of charges recombination zone and no differential color aging. One white device exhibits high efficiency of 33.6 cd/A (30.1 lm/W). Moreover, the emission mechanism of doping-free orange and white OLEDs is also discussed.The schematic architecture (a) and proposed energy-level of the orange devices (b).

•Efficient orange and white OLEDs with simplified structure are demonstrated.•The orange and white OLEDs have efficiencies of 51.5 cd/A and 41.6 cd/A.•The designed devices have lower efficiency roll-off.•The emission mechanism of the orange devices has been proposed.Efficient orange phosphorescent organic light-emitting devices based on simplified structure with maximum efficiencies of 46.5 lm/W and 51.5 cd/A were reported. One device had extremely low efficiency roll-off with efficiencies of 50.6 cd/A, 45.0 cd/A and 39.2 cd/A at 1000 cd/m2, 5000 cd/m2 and 10,000 cd/m2 respectively. The reduced efficiency roll-off was attributed to more balanced carrier injection and broader recombination zone. The designed simplified white device showed much lower efficiency roll-off than the control one based on multiple emitting layers. The efficiency of simplified white device was 40.8 cd/A at 1000 cd/m2 with Commission Internationale de I’Eclairage coordinates of (0.39, 0.46).

•We successfully demonstrated efficient non-doped OLEDs with simplified device structure.•We also discussed the working mechanism of the OLEDs based on ultrathin emissive layers.•The designed white device showed very stable spectra and higher power efficiency than the control one.•The effect of the ultrathin layer thickness on the performance of OLEDs was investigated.Efficient red, orange, green and blue monochrome phosphorescent organic light-emitting diodes (OLEDs) with simplified structure were fabricated based on ultrathin emissive layers. The maximum efficiencies of red, orange, green and blue OLEDs are 19.3 cd/A (17.3 lm/W), 45.7 cd/A (43.2 lm/W), 46.3 cd/A (41.6 lm/W) and 11.9 cd/A (9.2 lm/W). Moreover, efficient and color stable white OLEDs based on two complementary colors of orange/blue, three colors of red/orange/blue, and four colors of red/orange/green/blue were demonstrated. The two colors, three colors and four colors white OLEDs have maximum efficiencies of 30.9 cd/A (27.7 lm/W), 30.3 cd/A (27.2 lm/W) and 28.9 cd/A (26.0 lm/W), respectively. And we also discussed the emission mechanism of the designed monochrome and white devices.

Efficient white phosphorescent organic light-emitting diodes with excellent stable spectra based on an orange ultrathin non-doped layer and a blue doped emission layer are reported. The emission color can be changed by adjusting the thickness of the 4,4′-bis(carbazol-9-yl)biphenyl (CBP) layer or the mixed ratio of CBP and 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) in the mixed interlayer. On the basis of the mixed CBP:TPBi interlayer, we successfully fabricated a white device with superior stable spectra and lower turn-on voltage. Moreover, the white device with an optimum mixed ratio of 2:1 in the mixed interlayer has a power efficiency of 40.1 lm W−1, which is 2.1 times as high as that of the device with a pure CBP spacer.

meta-Coupling isomers usually exhibit bluer emission than do the para-isomers, but the loss of efficiency with respect to photoluminescence (PL) and electroluminescence (EL) is an inevitable result in most cases, particularly for deep blue emitters. In this study, three blue emitting isomers, 4,4′-bis(1-phenyl-phenanthro[9,10-d]imidazol-2-yl)biphenyl (BPPI), 3,4′-bis(1-phenyl-phenanthro[9,10-d]-imidazol-2-yl)biphenyl (L-BPPI) and 3,3′-bis(1-phenyl-phenanthro[9,10-d]-imidazol-2-yl)biphenyl (Z-BPPI), were chosen as model compounds to investigate the essential reason behind the meta-coupling effect due to their different coupling forms, viz. para–para, para–meta, and meta–meta, respectively, in similar dimeric phenanthroimidazole frameworks. A combination of detailed photophysical data, device performance and DFT calculations for the excited state provided valuable information. In particular, the relationship between certain key parameters in calculations as well as PL or EL properties was confirmed, such as oscillator strength and quantum yield, among others, which could effectively reduce the issues related to synthesis and characterisation using prior computer simulations. Good agreement was observed in the results obtained from calculation and experiments, and it was concluded that meta-tuning barely realised improvement in EL, unless some special excited states formed or an exciton conversion channel appeared, as in the case of reverse intersystem crossing.

In this paper, two novel deep-blue emitters, mm-BPPI and mm-CN-BPPI, were obtained by coupling two 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) blocks in a meta–meta pattern and using a central benzene moiety as the bridge. Owing to this meta–meta connection, the dimer conjugation degree of mm-BPPI was effectively limited, and thus mm-BPPI exhibited very similar photophysical and electrochemical properties as PPI. However, the thermal stability of mm-BPPI was improved successfully, overcoming the defects of PPI for practical applications. Non-doped optimized devices fabricated with mm-BPPI exhibited stable deep-blue emission with CIE coordinates of (0.156, 0.050). After inserting cyano groups, higher glass-transition temperature and a quasi-reversible redox process were observed in mm-CN-BPPI; moreover, the lower efficiency roll-off was realized in its non-doped devices. This study demonstrates a strategy towards stable deep blue emitters with higher stability, better efficiency and lower roll-off ratio by chemical modification (meta–meta coupling or cyano group insertion) in non-doped deep-blue-emitting devices based on aryl-substituted phenanthroimidazole blocks.