Solution-processed nickel oxides (s-NiO x ) are used as hole injection and transport layers in solution-processed organic light-emitting diodes (OLEDs). By increasing the annealing temperature, the nickel acetate precursor fully decomposes and the s-NiO x film shows larger crystalline grain sizes, which lead to better hole injection and transport properties. UV−ozone treatment on the s-NiO x surface is carried out to further modify its surface chemistry, improving the hole injection efficiency. The introduction of more dipolar species of nickel oxyhydroxide (NiO(OH)) is evidenced after the treatment. Dark injection−space charge limited (DI−SCL) transient measurement was carried out to compare the hole injection efficiency of s-NiO x and poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) hole injection layers (HIL). The UV−ozone treated s-NiO x shows significantly better hole injection, with a high injection efficiency of 0.8. With a p-type thin film transistor (TFT) configuration, the high-temperature annealed s-NiO x film shows a hole mobility of 0.141 cm 2 V −1 s −1 , which is significantly higher compared to conventional organic hole transport layers (HTLs). Because of their improved hole injection and transport properties, the solution-processed phosphorescent green OLEDs with NiO x HIL/HTL show a maximum power efficiency of 75.5 ± 1.8 lm W −1 , which is 74.6 + 2.1% higher than the device with PEDOT:PSS HIL. The device with NiO x HIL/HTL also shows a better shelf stability than the device with PEDOT:PSS HIL. The NiO x HIL/HTL is further compared with PEDOT:PSS HIL/N,N′-Di(1-naphthyl)-N,N′diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) HTL in the thermal-evaporated OLEDs. The device with NiO x HIL/HTL shows a comparable efficiency at high electroluminescence (EL) intensities.

Organic light-emitting diodes (OLEDs) have become a promising candidate for lighting and display applications. High efficiency OLEDs require a multilayer device architecture to provide exciton confinement and balance charge transport. Conventional OLEDs are made by vacuum process, and the manufacturing cost can be reduced by solution processing. However, unlike vacuum-deposited OLEDs, solution-processed multilayer OLEDs are more challenging to make. The key for multilayer solution processing is to have the layer structure which can withstand solvents used in subsequent processing. We review the materials' strategies to make multilayer solution-processed OLEDs. Specifically, we will discuss the use of cross-linkable organic materials, metal oxides, and orthogonal solvent systems to deposit various functional layers in an OLED. Downloaded From: http://photonicsforenergy.spiedigitallibrary.org/ on 06/04/2015 Terms of Use: http://spiedl.org/terms Ho et al.: Review of recent progress in multilayer solution-processed organic light-emitting diodes Journal of Photonics for Energy 057611-2 Vol. 5, 2015 Downloaded From: http://photonicsforenergy.spiedigitallibrary.org/ on 06/04/2015 Terms of Use: http://spiedl.org/terms Ho et al.: Review of recent progress in multilayer solution-processed organic light-emitting diodes Journal of Photonics for Energy 057611-3 Vol. 5, 2015 Downloaded From: http://photonicsforenergy.spiedigitallibrary.org/ on 06/04/2015 Terms of Use: http://spiedl.org/terms Ho et al.: Review of recent progress in multilayer solution-processed organic light-emitting diodes Journal of Photonics for Energy 057611-4 Vol. 5, 2015 Downloaded From: http://photonicsforenergy.spiedigitallibrary.org/ on 06/04/2015 Terms of Use: http://spiedl.org/terms Ho et al.: Review of recent progress in multilayer solution-processed organic light-emitting diodes Journal of Photonics for Energy 057611-5 Vol. 5, 2015 Ho et al.: Review of recent progress in multilayer solution-processed organic light-emitting diodes Journal of Photonics for Energy 057611-6 Vol. 5, 2015 Downloaded From: http://photonicsforenergy.spiedigitallibrary.org/ on 06/04/2015 Terms of Use: http://spiedl.org/terms Ho et al.: Review of recent progress in multilayer solution-processed organic light-emitting diodes Journal of Photonics for Energy 057611-7 Vol. 5, 2015 Downloaded From: http://photonicsforenergy.spiedigitallibrary.org/ on 06/04/2015 Terms of Use: http://spiedl.org/terms Ho et al.: Review of recent progress in multilayer solution-processed organic light-emitting diodes Journal of Photonics for Energy 057611-8 Vol. 5, 2015 Downloaded From: http://photonicsforenergy.spiedigitallibrary.org/ on 06/04/2015 Terms of Use: http://spiedl.org/terms Ho et al.: Review of recent progress in multilayer solution-processed organic light-emitting diodes Journal of Photonics for Energy 057611-9 Vol. 5, 2015 Downloaded From: http://photonicsforenergy.spiedigitallibrary.org/ on 06/04/2015 Terms of Use: http://spiedl.org/terms Ho et al.: Review of recent progress in multil...

Next-generation wearable electronics calls for flexible non-volatile devices for ubiquitous data storage. Thus far, only organic ferroelectric materials have shown intrinsic flexibility and processibility on plastic substrates. Here, we discovered that by controlling the heating rate, ferroelectric hafnia films can be grown on plastic substrates. The resulting highly flexible capacitor with a film thickness of 30 nm yielded a remnant polarization of 10 μC/cm 2 . Bending test shows that the film ferroelectricity can be retained under a bending radius below 8 mm with bending cycle up to 1,000 times. The excellent flexibility is due to the extremely thin hafnia film thickness. Using This article is protected by copyright. All rights reserved. the ferroelectric film as a gate insulator, a low voltage non-volatile vertical organic transistor was demonstrated on a plastic substrate with an extrapolated date retention time up to 10 years.

use a low refractive index electron transport layer (ETL).To design a high-efficiency OLED, it is important to have a multilayer structure to control the radiative recombination such that excitons are confined to the emissive layer (EML) using high triplet charge blocking layers. In addition to exciton confinement, the electron and hole transport layers should be chosen in such a way to maintain the charge balance to avoid triplet-polaron quenching. In a multilayer OLED, the ETL also plays a key role in determining the light extraction efficiency since it is the intermediate layer between the EML and the metallic cathode, and a large portion of the dipole radiation from the EML is lost to the evanescent region where the in-plane wave-vector is larger than the total wave-vector, resulting in radiation that is coupled to the surface plasmon polariton (SPP) and the "lossy surface waves" on the metallic cathode. [4] The magnitude of the loss to the SPP mode is determined by the dielectric constants of the metallic cathode and the ETL, therefore the refractive index of the ETL can significantly impact the out-coupling efficiency of an OLED. There have been some reports demonstrating the effect of the ETL's refractive index on light extraction efficiency in OLEDs by simulation. [5,6] However, experimental study of the effect of ETL refractive index on device performance is rather limited. [7] In this work, we demonstrate the effect of ETL's refractive index on device efficiency using a solution processed OLED with a copper-based thermally activated delayed fluorescent (TADF) emitter, [(2-(Diphenylphosphino)-4-isobutylpyridine) (PPh3)2Cu2I2] (Cu(I)-iBuPyrPHOS). The emitter has a photoluminescence quantum yield (PLQY) of 70%. [8] Based on the PLQY and assuming an out-coupling efficiency of 20%, a maximum external quantum efficiency (EQE) of 14% is expected. Using 2,9-di(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (NBPhen) and 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) as ETLs, we showed that an optimized device has a maximum EQE of 12% which is close to the estimate of the maximum EQE of 14%. Surprisingly, using tris-[3-(3-pyridyl)mesityl] borane (3TPYMB) as the ETL we achieved a maximum EQE of 21%, showing nearly a 76% enhancement just by changing the ETL alone. Upon investigation of the origin of the efficiency enhancement, we found that the refractive index of 3TPYMB is 1.65, which is the lowest among all commonly used ETLs, resulting in a significant enhancement in light extraction efficiency. Our device data are also confirmed by the optical simulation results.A low refractive index electron transport layer (ETL) can be very effective in enhancing the out-coupling efficiency of an organic light-emitting diode (OLED). However, most organic films show a refractive index close to 1.8. 1.65 (at 550 nm), which is the lowest refractive index ETL among the commonly used ETLs up to date. Using 3TPYMB as an ETL, a solution processed OLED is demonstrated with nearly a 76% enhancement in external quantum efficiency (...

The exciton quenching properties of solutionprocessed nickel oxide (NiO x ) and vanadium oxide (VO x ) are studied by measuring the photoluminescence (PL) of a thin emitting layer (EML) deposited on top of the metal oxides. Strong exciton quenching is evidenced at the metal oxide/EML interface, which is proved to be detrimental to the performance of optoelectronic devices. With a thin polyvinylpyrrolidone (PVP) passivation polymer adsorbed on top of metal oxides, the PL quenching is found to be effectively suppressed. A short UV−O 3 treatment on top of the PVP-passivated metal oxides turns out to be a key procedure to trigger the chemical binding between the PVP passivation polymer and the metal oxide surface species, which turns out to be necessary for efficient hole injection and extraction for organic light emitting diodes (OLEDs) and solar cell devices, respectively. With the PVP passivation layer followed by UV−O 3 treatment, the OLEDs incorporating NiO x as a hole transport layer (HTL) shows a record current efficiency of 90.8 ± 2.1 Cd A −1 with significantly suppressed efficiency rolloff, the OLEDs incorporating VO x as a hole injection layer (HIL) also shows higher current efficiencies at higher luminescence. Both perovskite solar cells and polymer solar cells incorporating NiO x HTLs show a 60% enhancement in power conversion efficiency (PCE) with PVP passivation polymer. ■ INTRODUCTIONRecently, metal oxide semiconductors have been used as charge transport layers in organic light-emitting diodes (OLEDs), photovoltaic cells (OPVs), and quantum dot (QD) devices. 1−17 On the basis of their charge transport properties and energy band alignment, they can be used as electron transport layers (ETLs), electron injection layers (EILs), hole transport layers (HTLs), or hole injection layers (HILs). Compared to their organic counterparts, metal oxides have several advantages such as high carrier mobility, tunable energy level alignment, and good air-stability. Furthermore, as most metal oxides are insoluble in organic solvents, they are compatible with solution processing for multilayer devices. Several metal oxides have already been shown to be solution-processable, making them compatible with roll-to-roll processing for OLED and OPV fabrications. 1,5,7,11−17 However, for most of the metal oxides synthesized in air, their surfaces are known to be rich of hydroxyl species which act as exciton quenching sites affecting the device performance. 18−20 In our previous work, while efficient solution-processed OLEDs incorporating solutionprocessed NiO x can be demonstrated, 1 the efficiency roll-off is very strong when NiO x is used as an HTL. Contradictory to other reports that the efficiency roll-off is due to the poor charge balance, 21−23 we found that it is actually due to strong quenching at the NiO x HIL/HTL interface. To alleviate the quenching problem, strategies such as changing the carrier profile in the active layers by modifying the injection layers 19 and inserting an exciton blocking layer to spatially sepa...