Zheng-Hong Lu

Universal energy-level alignment of molecules on metal oxides

Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.

Efficient and stable solution-processed planar perovskite solar cells via contact passivation

Passivating traps in perovskites Low-temperature processing of planar organic-inorganic perovskite solar cells made through solution processing would allow for simpler manufacturing and the use of flexible substrates. However, materials currently in use form interfaces with charge carrier trap states that limit performance. Tan et al. used chlorine-capped TiO2 colloidal nanocrystal films as an electron-selective layer, which limited interface recombination in solution-processed solar cells. Such cells achieved certified efficiencies of 19.5% for active areas of 1.1 cm2. Science, this issue p. 722 Chlorine-capped TiO2 nanocrystal films processed below 150°C effectively passivate detrimental carrier trap states. Planar perovskite solar cells (PSCs) made entirely via solution processing at low temperatures (<150°C) offer promise for simple manufacturing, compatibility with flexible substrates, and perovskite-based tandem devices. However, these PSCs require an electron-selective layer that performs well with similar processing. We report a contact-passivation strategy using chlorine-capped TiO2 colloidal nanocrystal film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells. We fabricated solar cells with certified efficiencies of 20.1 and 19.5% for active areas of 0.049 and 1.1 square centimeters, respectively, achieved via low-temperature solution processing. Solar cells with efficiency greater than 20% retained 90% (97% after dark recovery) of their initial performance after 500 hours of continuous room-temperature operation at their maximum power point under 1-sun illumination (where 1 sun is defined as the standard illumination at AM1.5, or 1 kilowatt/square meter).

Chlorinated Indium Tin Oxide Electrodes with High Work Function for Organic Device Compatibility

Closer matching of the energy levels of transparent electrodes and active materials in organic light-emitting diodes improves efficiency. In organic light-emitting diodes (OLEDs), a stack of multiple organic layers facilitates charge flow from the low work function [~4.7 electron volts (eV)] of the transparent electrode (tin-doped indium oxide, ITO) to the deep energy levels (~6 eV) of the active light-emitting organic materials. We demonstrate a chlorinated ITO transparent electrode with a work function of >6.1 eV that provides a direct match to the energy levels of the active light-emitting materials in state-of-the art OLEDs. A highly simplified green OLED with a maximum external quantum efficiency (EQE) of 54% and power efficiency of 230 lumens per watt using outcoupling enhancement was demonstrated, as were EQE of 50% and power efficiency of 110 lumens per watt at 10,000 candelas per square meter.

Perovskite energy funnels for efficient light-emitting diodes

Organometal halide perovskites exhibit large bulk crystal domain sizes, rare traps, excellent mobilities and carriers that are free at room temperature-properties that support their excellent performance in charge-separating devices. In devices that rely on the forward injection of electrons and holes, such as light-emitting diodes (LEDs), excellent mobilities contribute to the efficient capture of non-equilibrium charge carriers by rare non-radiative centres. Moreover, the lack of bound excitons weakens the competition of desired radiative (over undesired non-radiative) recombination. Here we report a perovskite mixed material comprising a series of differently quantum-size-tuned grains that funnels photoexcitations to the lowest-bandgap light-emitter in the mixture. The materials function as charge carrier concentrators, ensuring that radiative recombination successfully outcompetes trapping and hence non-radiative recombination. We use the new material to build devices that exhibit an external quantum efficiency (EQE) of 8.8% and a radiance of 80 W sr-1 m-2. These represent the brightest and most efficient solution-processed near-infrared LEDs to date.

Surface electronic structure of plasma-treated indium tin oxides

X-ray photoelectron spectroscopy (XPS) has been used to study the electronic structures of indium tin oxide (ITO) surfaces treated by O+, Ar+, and NHx+ plasmas. The XPS data show that there is a significant change in core level energies (In 3d5/2 O 1s, and Sn 3d5/2), in donor concentration (Sn4+), in valence band maximums (VBM), and in work functions on ITO surfaces being treated by O+ and NHx+ plasmas, compared with that of virgin and Ar+ plasma treated surfaces. Based on these experimental data, a surface band-bending theory is proposed. The theory explains that when Fermi energy of the plasma-treated surface is shifted towards the middle of the band gap: core levels will shift their energies to lower binding energies, VBM will bend upward, and work function will increase, as observed.

Highly Efficient Blue Phosphorescence from Triarylboron-Functionalized Platinum(II) Complexes of N-Heterocyclic Carbenes

The first examples of BMes(2)-functionalized NHC chelate ligands have been achieved. Their Pt(II) acetylacetonate complexes have been synthesized and fully characterized. These NHC-chelate Pt(II) compounds display highly efficient blue or blue-green phosphorescence in solution (Φ = 0.41-0.87) and the solid state (Φ = 0.86-0.90). Highly efficient electroluminescent devices based on these new Pt(II) compounds have also been fabricated.

Highly Efficient Perovskite-Quantum-Dot Light-Emitting Diodes by Surface Engineering

A two-step ligand-exchange strategy is developed, in which the long-carbon- chain ligands on all-inorganic perovskite (CsPbX3 , X = Br, Cl) quantum dots (QDs) are replaced with halide-ion-pair ligands. Green and blue light-emitting diodes made from the halide-ion-pair-capped quantum dots exhibit high external quantum efficiencies compared with the untreated QDs.

Aluminum doped zinc oxide for organic photovoltaics

Aluminum doped zinc oxide (AZO) was grown via magnetron sputtering as a low-cost alternative to indium tin oxide (ITO) for organic photovoltaics (OPVs). Postdeposition ozone treatment resulted in devices with lower series resistance, increased open-circuit voltage, and power conversion efficiency double that of devices fabricated on untreated AZO. Furthermore, cells fabricated using ozone treated AZO and standard ITO displayed comparable performance.

Visible Colloidal Nanocrystal Silicon Light-Emitting Diode

We herein demonstrate visible electroluminescence from colloidal silicon in the form of a hybrid silicon quantum dot-organic light emitting diode. The silicon quantum dot emission arises from quantum confinement, and thus nanocrystal size tunable visible electroluminescence from our devices is highlighted. An external quantum efficiency of 0.7% was obtained at a drive voltage where device electroluminescence is dominated by silicon quantum dot emission. The characteristics of our devices depend strongly on the organic transport layers employed as well as on the choice of solvent from which the Si quantum dots are cast.

(1-Naphthyl)phenylamino functionalized three-coordinate organoboron compounds: syntheses, structures, and applications in OLEDs

New three-coordinate organoboron compounds functionalized by a (1-naphthyl)phenylamino group, B(mes)2(dbp-NPB) (1), B(db-NPB)3 (2), and B(dbp-NPB)3 (3), have been synthesized. A variable temperature 1H NMR study showed that the aryl groups around the boron center in these compounds have a rotation barrier ∼70 kJ mol−1. The new boron compounds are amorphous solids with Tg being 110 °C, 171 °C and 173 °C, respectively. The electronic properties of the new boron compounds were investigated by cyclic voltammetry and UV–visible spectroscopy. All three boron compounds are blue emitters in the solid state. In solution the emission spectra of the boron compounds shift toward a longer wavelength with increasing solvent polarity. In CH2Cl2, the emission quantum efficiency of the three compounds was determined to be 0.22, 0.27 and 0.23, respectively. Several series of electroluminescent (EL) devices where compounds 1–3 are used as either an emitter/electron transport material, a hole transport material, or a hole injection material have been fabricated and their performance has been compared to the corresponding devices of BNPB, a previously investigated molecule, NPB, a commonly used hole transport material, and CuPc, a commonly used hole injection material. The EL results indicate that the new boron compounds are not suitable as emitters/electron transport materials, but they are promising as hole transport and hole injection materials in EL devices.