•Nearly 20% EQE phosphorescent WOLEDs with an ultra-thin red and green co-doped layer and dual blue layers achieved.•Device working mechanism investigated in terms of exciton confinement and energy transfer.•Excited-state lifetime of FIrpic various in different hosts, which leads to different white emission spectra.Phosphorescent white organic light emitting diodes (WOLEDs) with a multi-layer emissive structure comprising two separate blue layers and an ultra-thin red and green co-doped layer sandwiched in between have been studied. With proper host and dopant compositions and optimized layer thicknesses, high-performance WOLEDs having a power efficiency over 40 lm/W at 1000 cd/m2 with a low efficiency roll-off have been produced. Through a systematic investigation of the exciton confinement and various pathways for energy transfer among the hosts and dopants, we have found that both the ultra-thin co-doped layer and two blue emitting layers play a vital role in achieving high device efficiency and controllable white emission.

•We evaluate blue PHOLED devices using Ir(iprpmi)3 as the emitting dopant.•The charge recombination zone is controlled by varying the Ir(iprpmi)3 concentration.•External quantum efficiencies (EQE) greater than 20% have been achieved.•Devices with Ir(iprpmi)3 have improved lifetimes over those using the classic light-blue dopant FIrpic.A blue phosphorescent emitter based on tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III), Ir(iprpmi)3, as the dopant and 3,3′-bis(N-carbazolyl)biphenyl, mCBP, as the host have been evaluated in OLED devices. By optimizing the dopant concentration and the materials for the electron and hole-transport layers, external quantum efficiencies greater than 20% have been achieved. Improved device lifetimes over those using the classic light-blue dopant FIrpic have also been achieved. These improvements can be attributed to the control of the electron-hole recombination and emission regions within the emitter layer as well as the choice of material for the transport layers.

•We investigate how hole transport in FIrpic doped mCBP impacts OLED device operational voltage and stability.•FIrpic is unstable for both hole transport and charge recombination processes.•The host mCBP is stable in supporting hole transport but unstable with respect to electron–hole recombination.•We conclude that FIrpic doped mCBP is unsuitable for practical use as an emissive layer in OLED devices.By incorporating prospective materials for blue PHOLED emissive layers into model OLEDs, we have investigated how hole transport in a prototypical blue phosphorescent emitter, FIrpic (bis(4,6-difluorophenyl-pyridinato-N,C2) picolinate Iridium) doped mCBP (4′-bis(3-methylcarbazol-9-yl)-2,2′-biphenyl), can impact PHOLED device operational stability. We found the host mCBP to be stable in supporting hole transport, but unstable with respect to electron–hole recombination. As a dopant, FIrpic was found to be unstable with respect to both hole transport and charge recombination processes. Our results indicate that FIrpic doped mCBP is unsuitable for use as an emissive layer in OLED devices and we provide a general strategy to screen materials and better understand their stability.Graphical abstract

High-temperature annealing (HTA), a process step prior to vapor cadmium chloride (VCC) treatment, has been found to be useful for improving the crystallinity of CdTe films and the efficiency of ultra-thin CdTe solar cells. Scanning electron microscopy, optical absorption, photoluminescence measurements and analyses on photoluminescence results using spectral deconvolution reveal that the additional HTA step produces substantial grain growth and reduces grain boundary defects. It also reduces excessive sulfur diffusion across the junction that can occur during the VCC treatment. The HTA step helps to produce pinhole-free CdTe films and reduce electrical shorts in ultra-thin CdTe solar cells. An efficiency of about 11.6% has been demonstrated for ultra-thin CdS/CdTe solar cells processed with HTA step.