Biwu Ma

Management of singlet and triplet excitons for efficient white organic light-emitting devices

Lighting accounts for approximately 22 per cent of the electricity consumed in buildings in the United States, with 40 per cent of that amount consumed by inefficient (∼15 lm W-1) incandescent lamps. This has generated increased interest in the use of white electroluminescent organic light-emitting devices, owing to their potential for significantly improved efficiency over incandescent sources combined with low-cost, high-throughput manufacturability. The most impressive characteristics of such devices reported to date have been achieved in all-phosphor-doped devices, which have the potential for 100 per cent internal quantum efficiency: the phosphorescent molecules harness the triplet excitons that constitute three-quarters of the bound electron–hole pairs that form during charge injection, and which (unlike the remaining singlet excitons) would otherwise recombine non-radiatively. Here we introduce a different device concept that exploits a blue fluorescent molecule in exchange for a phosphorescent dopant, in combination with green and red phosphor dopants, to yield high power efficiency and stable colour balance, while maintaining the potential for unity internal quantum efficiency. Two distinct modes of energy transfer within this device serve to channel nearly all of the triplet energy to the phosphorescent dopants, retaining the singlet energy exclusively on the blue fluorescent dopant. Additionally, eliminating the exchange energy loss to the blue fluorophore allows for roughly 20 per cent increased power efficiency compared to a fully phosphorescent device. Our device challenges incandescent sources by exhibiting total external quantum and power efficiencies that peak at 18.7 ± 0.5 per cent and 37.6 ± 0.6 lm W-1, respectively, decreasing to 18.4 ± 0.5 per cent and 23.8 ± 0.5 lm W-1 at a high luminance of 500 cd m-2.

User-interactive electronic skin for instantaneous pressure visualization

Electronic skin (e-skin) presents a network of mechanically flexible sensors that can conformally wrap irregular surfaces and spatially map and quantify various stimuli. Previous works on e-skin have focused on the optimization of pressure sensors interfaced with an electronic readout, whereas user interfaces based on a human-readable output were not explored. Here, we report the first user-interactive e-skin that not only spatially maps the applied pressure but also provides an instantaneous visual response through a built-in active-matrix organic light-emitting diode display with red, green and blue pixels. In this system, organic light-emitting diodes (OLEDs) are turned on locally where the surface is touched, and the intensity of the emitted light quantifies the magnitude of the applied pressure. This work represents a system-on-plastic demonstration where three distinct electronic components--thin-film transistor, pressure sensor and OLED arrays--are monolithically integrated over large areas on a single plastic substrate. The reported e-skin may find a wide range of applications in interactive input/control devices, smart wallpapers, robotics and medical/health monitoring devices.

Flexible, highly efficient all-polymer solar cells.

All-polymer solar cells have shown great potential as flexible and portable power generators. These devices should offer good mechanical endurance with high power-conversion efficiency for viability in commercial applications. In this work, we develop highly efficient and mechanically robust all-polymer solar cells that are based on the PBDTTTPD polymer donor and the P(NDI2HD-T) polymer acceptor. These systems exhibit high power-conversion efficiency of 6.64%. Also, the proposed all-polymer solar cells have even better performance than the control polymer-fullerene devices with phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor (6.12%). More importantly, our all-polymer solar cells exhibit dramatically enhanced strength and flexibility compared with polymer/PCBM devices, with 60- and 470-fold improvements in elongation at break and toughness, respectively. The superior mechanical properties of all-polymer solar cells afford greater tolerance to severe deformations than conventional polymer-fullerene solar cells, making them much better candidates for applications in flexible and portable devices.

Hole selective MoOx contact for silicon solar cells

Using an ultrathin (∼ 15 nm in thickness) molybdenum oxide (MoOx, x < 3) layer as a transparent hole selective contact to n-type silicon, we demonstrate a room-temperature processed oxide/silicon solar cell with a power conversion efficiency of 14.3%. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap. Specifically, in the absence of carbon contamination, we measure a work function potential of ∼ 6.6 eV, which is significantly higher than that of all elemental metals. Our results on the archetypical semiconductor silicon demonstrate the use of nm-thick transition metal oxides as a simple and versatile pathway for dopant-free contacts to inorganic semiconductors. This work has important implications toward enabling a novel class of junctionless devices with applications for solar cells, light-emitting diodes, photodetectors, and transistors.

Bright Light‐Emitting Diodes Based on Organometal Halide Perovskite Nanoplatelets

Bright light-emitting diodes based on solution-processable organometal halide perovskite nanoplatelets are demonstrated. The nanoplatelets created using a facile one-pot synthesis exhibit narrow-band emissions at 529 nm and quantum yield up to 85%. Using these nanoparticles as emitters, efficient electroluminescence is achieved with a brightness of 10 590 cd m(-2) . These ligand-capped nanoplatelets appear to be quite stable in moisture, allowing out-of-glovebox device fabrication.

Bodipy-backboned polymers as electron donor in bulk heterojunction solar cells

Bodipy-based polymers, which possess a high absorption coefficient with a bandgap of approximately 1.6 eV, have been used as electron donor in solution-processed bulk heterojunction (BHJ) solar cells containing PCBM as acceptor. A power conversion efficiency (PCE) of approximately 2% has been achieved with V(oc) of approximately 0.8 eV and J(sc) of approximately 4.8 mA cm(-2).

Fully Printed Halide Perovskite Light-Emitting Diodes with Silver Nanowire Electrodes

Printed organometal halide perovskite light-emitting diodes (LEDs) are reported that have indium tin oxide (ITO) or carbon nanotubes (CNTs) as the transparent anode, a printed composite film consisting of methylammonium lead tribromide (Br-Pero) and poly(ethylene oxide) (PEO) as the emissive layer, and printed silver nanowires as the cathode. The fabrication can be carried out in ambient air without humidity control. The devices on ITO/glass have a low turn-on voltage of 2.6 V, a maximum luminance intensity of 21014 cd m(-2), and a maximum external quantum efficiency (EQE) of 1.1%, surpassing previous reported perovskite LEDs. The devices on CNTs/polymer were able to be strained to 5 mm radius of curvature without affecting device properties.

Enhanced Optical and Electrical Properties of Polymer‐Assisted All‐Inorganic Perovskites for Light‐Emitting Diodes

Highly bright light-emitting diodes based on solution-processed all-inorganic perovskite thin film are demonstrated. The cesium lead bromide (CsPbBr3 ) created using a new poly(ethylene oxide)-additive spin-coating method exhibits photoluminescence quantum yield up to 60% and excellent uniformity of electrical current distribution. Using the smooth CsPbBr3 films as emitting layers, green perovskite-based light-emitting diodes (PeLEDs) exhibit electroluminescent brightness and efficiency above 53 000 cd m-2 and 4%: a new benchmark of device performance for all-inorganic PeLEDs.

Site isolation in phosphorescent bichromophoric block copolymers designed for white electroluminescence.

White organic light-emitting diodes (WOLEDs) have attracted great attention for their potential use in full color displays and solid-state lighting applications due to several advantages, such as low cost and flexibility. To date, the most efficient WOLEDs have used small phosphorescent molecules in multilayer structured devices prepared by high vacuum vapor deposition. The key issue in these systems is that the phosphorescent emission produced by each individual metal complex Ir(III) or Pt(II), is narrow, thus requiring simultaneous emission from more than one color phosphor to illuminate across the visible region. Typically this is achieved through a combination of either three different chromophores emitting blue, green, and red, or of two different ones emitting green/blue and orange/red. If more than one phosphorescent emitter is present in a device, the electroluminescent color may be affected by the energy transfer (both Förster and Dexter) between emitters. Vapor deposition enables isolation of the various emitters to minimize the energy transfer and achieve the desired goal of multiple emission using techniques such as patterning, stacking, layered isolation, and exciton management. Because polymeric materials can be solution-processed, they constitute an interesting option for application in OLEDs due to their potential to reduce cost and increase scalability. Another advantage is that a single polymer chain can bear multiple functional groups, each contributing to the tuning of properties. For example, successful demonstrations of polymer WOLEDs have been based on blends of fluorescent polymers, polymers incorporating multiple fluorescent emitters in their side chains or their backbone and fluorescent polymers doped with small molecule phosphorescent emitters. However, these devices are generally fluorescent systems with limited internal quantum efficiencies or doped phosphorescent systems with poor stability. Furthermore, the occurrence of energy transfer limits the amount of low energy dopant that can be incorporated into these polymers, which affects their intrinsic efficiency. There have been some efforts to suppress this energy transfer using dendrimers for site isolation, but ultimately multilayer structures that can isolate phosphorescent emitters are needed. Unfortunately, this is extremely difficult to achieve with solution processing as the deposition of a layer must not affect any previously deposited layers. Block copolymers allow hierarchical supramolecular control over the spatial location of their functional component blocks as well as various nanoscale objects. This design flexibility has been exploited in the efficient fabrication of novel functional materials, such as nanostructured solar cells, photonic bandgap materials, highly efficient catalysts, and high-density magneticstorage media. Therefore, block copolymers have the unique potential to spontaneously achieve phosphorescent emitter isolation through self-assembly. Herein, we have explored their use as active materials for WOLEDs in which phosphorescent emitter isolation can be achieved. We have exploited the use of triarylamine (TPA) oxadiazole (OXA) diblock copolymers (TPA-b-OXA), which have been used as host materials due to their high triplet energy and charge-transport properties enabling a balance of holes and electrons. These coil–coil type TPA-b-OXA diblocks can produce various morphologies with controlled domain spacings ranging from 10–50 nm. By incorporating two different colored phosphorescent Ir(III) emitters (green–blue and orange–red emissive pendant styryl heteroleptic Ir(III) complexes) randomly into each different block, we have been able to produce a block-copolymer system, (TPA-r-Blue)-b-(OXA-r-Red), which can deliver site isolation of the two emitters. As a result of site isolation these diblock copolymers can be targeted to suppress energy transfer from high to lower energy emitters, which generally occurs at distances below 10 nm. With these block copolymers, we demonstrate a seld-assembled single layer solution processed WOLED that provides improved white color balance, and efficiency. Furthermore, by varying the molecular weight (MW) of (TPA-r-Blue)-b-(OXA-r-Red) and the ratio of blue to red emitters, we have investigated the effect of domain spacing on the electroluminescence spectrum and device performance. Polymers containing heavy metal complexes have been demonstrated previously for similar Ir(III) complexes through incorporation of ancillary ligand then post polymerization complex formation, or through the post polymerization attachment of preformed Ir(III) complexes. Unfortunately, these strategies are unsuitable since they do not allow incorporation of

One-dimensional organic lead halide perovskites with efficient bluish white-light emission.

Organic-inorganic hybrid metal halide perovskites, an emerging class of solution processable photoactive materials, welcome a new member with a one-dimensional structure. Herein we report the synthesis, crystal structure and photophysical properties of one-dimensional organic lead bromide perovskites, C4N2H14PbBr4, in which the edge sharing octahedral lead bromide chains [PbBr4 2−]∞ are surrounded by the organic cations C4N2H14 2+ to form the bulk assembly of core-shell quantum wires. This unique one-dimensional structure enables strong quantum confinement with the formation of self-trapped excited states that give efficient bluish white-light emissions with photoluminescence quantum efficiencies of approximately 20% for the bulk single crystals and 12% for the microscale crystals. This work verifies once again that one-dimensional systems are favourable for exciton self-trapping to produce highly efficient below-gap broadband luminescence, and opens up a new route towards superior light emitters based on bulk quantum materials.