White organic light-emitting diodes (WOLEDs) have been of much interest because of their great potential to increase energy efficiency when serving as the backlight source in highly efficient flat-panel displays and because they are promising candidates for next-generation, large-area solid-state lighting.[1, 2] White light can be realized by mixing red, green, and blue (RGB; the three primary colors) light [1] or by mixing two complementary colors, such as sky blue and yellow lights.[3] Because of the ideal characteristics of the phosphors, in particular their ability to harvest triplet excitons and potential 100% internal quantum efficiency,[4] electrophosporescent emitters are widely used in WOLEDs to improve device performance.[5] After decades of research and development, WOLEDs with power efficiency (PE) beyond 100 lm W− 1 with outcoupling technology and 30 lm W− 1 without outcoupling technology have been achieved.[6] Nevertheless, high-efficiency phosphorescent WOLEDs (PHWOLEDs) with low operating voltages are still a great challenge.[7–10] For practical use, the low operating voltage is of great importance for improving power efficiency and realizing portable applications. However, the exothermic energy transfer from hosts to blue-phosphorescent dopants requires very high first triplet energy levels (T1) of the hosts (approaching 3.0 eV). The use of such host materials often considerably increases the transport barrier and therefore the operating voltages.[4b, 6, 11–15] As an alternative, blue fluorescent emitters are often used to complete the white-light emission at low operating voltages.[5a, 16–20] However, the utilization of fluorescent emitters remarkably reduces the quantum efficiency of the devices. Therefore, the contraction between high efficiency and low operating voltage is one of the main factors that constrains the practical applications of WOLEDs. To solve this problem, Leo et al. have reported devices with much more complicated configurations,[6] in which two or more hosts with the suitable energy levels of the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are utilized for RGB emitters so as to facilitate barrier-free carrier transport. Low operating voltages,≈ 3 V at 1000 cd m− 2 and≈ 4 V at 10 000 cd m− 2, were realized. Obviously, compared with multihost systems, the simpler devices based on a single host are superior in repeatability and at avoiding carrier accumulation at the interfaces in step-like energy levels of multi-emitter structures.[21] However, there are very few reports about the high-performance single-host WOLEDs,[15] since they require host materials with comprehensive properties including i) efficient energy transfer to the dopants whose triplet energy levels (T1) range from 2.0–2.7 eV; ii) excellent bipolar carrier-injection/transporting ability to reduce the operating voltages; and iii) wide and stable recombination zones for white-light emission. The widely used host materials with high T1, including tris (4-(9H-carbazol-9-yl) phenyl) amine (TCTA), N, N-dicarbazoly-3, 5-benzene (mCP), and 1, 4-bis (triphenylsilyl) benzene (UGH2), can not thoroughly meet these requirements.

Recently, aryl phosphine oxide (APO) hosts for electrophosphorescence have attracted intensive interest.[22] The phoshpine oxide moieties can efficiently polarize the molecules without extending the conjugated length of the active chromophore.[22b] Such a characteristic shows the potential of APOs as single hosts for WOLEDs. Nevertheless, only a few blue or white PHOLEDs based on APO hosts exhibit low driving voltage (< 5 V) and low efficiency roll-off (< 30%) at a luminance of 1000 cd …