For device C, the situation is similar to device B, as indicated in Figure 3c. However, there is a 0.3-eV barrier at the [LUMO]EML/[LUMO]BCP interface, and electrons are confined in the LUMO energy level of BCP. Meanwhile, the larger barrier of 0.7 eV at the interface of [HOMO]EML/[HOMO]BCP results in holes confined in the HOMO energy level of EML. Since electrons and holes are confined in different organic layers, which

increase the probability of excitons disassociation and decrease the Tanespimycin cost recombination efficiency of carriers [23], device C presents inferior EL performances. Therefore, the different level alignments both for [LUMO]EML/[LUMO]PBL and [HOMO]EML/[HOMO]PBL for devices A, B, and C lead to the different distributions and recombination efficiencies of carriers. That is also proven by their different EL spectra as shown in Figure 4. From the emission

spectra, we note that device A with type-I MQW structure offers a larger blue emission than the reference device www.selleckchem.com/products/MS-275.html which makes better CIE coordinates (see Table 1). For devices B and C with type-II MQW structure, there is a low possibility of carrier recombination due to the fact that only a single carrier could be confined in the EML, while another carrier is confine in PBL, which results in poor EL performances. It is a fact that strong blue emission and week red emission present in device C resulted from the accumulation of holes at the interface of [HOMO]blue-EML/[HOMO]BCP and that there are less holes in potential wells of green EML and red EML, especially in potential wells of red EML. Conclusions In conclusion, WOLEDs with type-I MQW structure offer higher EL performances

in contrast with the reference device with traditional three-layer structure. WOLEDs with TPBi as PBL exhibits a peak current efficiency and a power efficiency of 16.4 cd/A and 8.3 lm/W at about 1,000 cd/m2, which increase by 53.3% and 50.9% over the reference device, GPX6 respectively; meanwhile, a maximum luminance of 17,700 cd/m2 is achieved, which keeps a similar luminance with the reference device. The achievement of high EL performance with type-I MQW structure WOLEDs would be attributed to the uniform distribution and rigorous confinement of carriers and excitons within EMLs. However, when Bphen or BCP acts as PBL instead of TPBi, low EL performances (especially for BCP) are obtained, which are attributed to poor level alignment at the interface of EML/PBL for type-II MQW structure; thus, incomplete confinement and low recombination efficiency of carriers occur. In terms of the results, we find that type-I MQW is a promising structure design for improving white EL performance by choosing the suitable PBL.