## 3 Results and Discussion  
The _J-V-L_ features of device A and device B are displayed in Figure 3a. The turn-on voltage (V\({}_{\text{on}}\)) of device B (2.6 V) appeared to be lower than that of device A (2.8 V). This means that there is a best match in the energy level between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of every organic layer for device A. [22] Meanwhile, the current density as well as luminance of device B rose more sharply with the increase in voltage compared with device A. Device A exhibited the largest CE and PE, which were 29.4 cd A\({}^{-1}\) and 21.5 lm W\({}^{-1}\), respectively. Besides, the CE and PE were 6.1 cd A\({}^{-1}\) and 2.6 lm W\({}^{-1}\) for device B. We also fabricated multi-emissive layer WOLEDs (Device C), and Figure 4 displays the corresponding PE and CE. Device C reveals a high CE value of 5.3 cd A\({}^{-1}\) at the 100 cd m\({}^{-2}\) or of 5.4 cd A\({}^{-1}\) at the 1000 cd m\({}^{-2}\) or of 5.7 cd A\({}^{-1}\) at the 10,000 cd m\({}^{-2}\) and a turn-on voltage of 5.5 V compared with device A. Device C uses NPB as the HIL, but device A and device B use MoOx as the HIL. If NPB shows a deeper HOMO level (5.6 eV), it is more difficult to inject a hole in device C in comparison to device A and B. The weak hole injection is likely to cause both a high turn-on voltage and an imbalance between electron as well as hole carriers. [23] Simultaneously, the large number of multiple EMLs as well as the interlayer of the device C cause a stronger electric field, of which luminance is the same as that of other devices, which also leads to the lower efficiency of device C. In particular, these three devices all show a relatively low roll-off in CE with increasing luminance. The host materials, of which the triplet energy levels are higher compared with emitters, are used as the interlayers in different devices. These interlayers function as confining triplet excitons exhibited by dopants in emissive layers on the one hand and explore the recombined zone for electron and hole on the other hand. Therefore, the low efficiency roll-off characteristics of the devices are attributed to the introduction of interlayers [24]. AFM was measured to thoroughly study the surface morphology of a DBR layer with a tapping mode, and the result is shown in Figure 5. The DBR structure exhibited a root mean square (RMS) surface roughness of 2.31 nm, suggesting that the films have a lower peak defeat, which is beneficial for subsequent film preparation. Obviously, it seems that the peak defeat and high surface roughness can damage the emission layers. The uniform film of DBR layers with low surface roughness is also favorable for improving the device performance. Figure 3: (a) _J–V_ curves (solid symbols) together with _L–V_ curves (open symbols). (b) Current efficiency (CE) versus luminance (solid symbols), and power efficiency (PE) versus luminance (open symbols) for devices A and B. Squares show device A and circles show device B. The reflectance spectrum of the DBR ZrO\({}_{2}\)/zircone films is shown in Figure 6, together with the reference device without a DBR structure using the same configuration as Device A. Obviously, DBR layers exhibit very high reflectivity, especially for the green region, with a reflectivity of up to 90%. The DBR structure also has a stop band, which is broad enough to confine the whole EL emission in addition to the wavelength of cavity resonance. According to these results, it is convincing that the ALD method can effectively and precisely control the thickness to improve the reflectivity of DBRs in comparison with other film deposition methods, such as sputtering or electron beam evaporation. The normalized EL spectra of device A and device B with the DBR microcavity are displayed in Figure 7. The EL spectrum of the reference device without a DBR structure demonstrates a spectrally broad emission at the peak of 512 nm, and the full-width at half-maximum (FWHM) reaches 65 nm (Figure 5).