Yizheng Jin

Interfacial Control Toward Efficient and Low‐Voltage Perovskite Light‐Emitting Diodes

High-performance perovskite light-emitting diodes are achieved by an interfacial engineering approach, leading to the most efficient near-infrared devices produced using solution-processed emitters and efficient green devices at high brightness conditions.

High-performance planar heterojunction perovskite solar cells: preserving long charge carrier diffusion lengths and interfacial engineering

AbstractWe demonstrate that charge carrier diffusion lengths of two classes of perovskites, CH3NH3PbI3−xClx and CH3NH3PbI3, are both highly sensitive to film processing conditions and optimal processing procedures are critical to preserving the long carrier diffusion lengths of the perovskite films. This understanding, together with the improved cathode interface using bilayer-structured electron transporting interlayers of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/ZnO, leads to the successful fabrication of highly efficient, stable and reproducible planar heterojunction CH3NH3PbI3−xClx solar cells with impressive power-conversion efficiencies (PCEs) up to 15.9%. A 1-square-centimeter device yielding a PCE of 12.3% has been realized, demonstrating that this simple planar structure is promising for large-area devices.

Transparent and flexible thin films of ZnO-polystyrene nanocomposite for UV-shielding applications

A solution casting approach was developed to obtain flexible and self-supporting ZnO-polystyrene (PS) nanocomposite thin films (ca. 360 μm) which were highly transparent in the visible region and exhibited excellent UV-absorbing properties. The nanocomposite films were prepared from homogeneous solutions of ligand-modified ZnO nanocrystals and PS. UV-Vis spectra, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and photoluminescence (PL) characterization techniques were employed to study optical and structural properties, as well as thermal stabilities of the nanocomposite films. Results revealed the high UV-shielding efficiency of the composites: for a film containing 1.0 wt. % of ZnO nanocrystals, over 99% of UV light at wavelengths between 200 and 360 nm was absorbed while the optical transparency in the visible region was slightly below that of a neat PS film. Minute amounts of organic ligands minimized aggregation of the ZnO nanocrystals, leading to the homogeneous blend solutions and eventually the well dispersed ZnO-PS nanocomposite films with stable optical properties. The present work is of interest for developing transparent UV-shielding materials and should help in the understanding and design of inorganic-polymer nanocomposites with desired properties.

Facile and large-scale synthesis and characterization of carbon nanotube/silver nanocrystal nanohybrids

A facile and efficient aqueous phase-based strategy to synthesize carbon nanotube (CNT)/silver nanocrystal nanohybrids at room temperature is reported. In the presence of carboxyl group functionalized or poly(acrylic acid)-?(PAA-) grafted CNTs, silver nanoparticles were in situ generated from AgNO3 aqueous solution, without any additional reducing agent or irradiation treatment, and readily attached to the CNT convex surfaces, leading to the CNT/Ag nanohybrids. The produced silver nanoparticles were determined to be face-centred cubic silver nanocrystals by scanning transmission electron microscopy (STEM), electron diffraction (ED) and x-ray powder diffraction (XRD) analyses. Detailed experiments showed that this strategy can also be applied to different CNTs, including single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), multiwalled carbon nanotubes (MWNTs), and polymer-functionalized CNTs. The nanoparticle sizes can be controlled from 2?nm to 10?20?nm and the amount of metal deposited on CNT surfaces can be as high as 82 wt%. Furthermore, large-scale (10 g or more) CNT/Ag nanohybrids can be prepared via this approach without the decrease of efficiency and quality. This approach can also be extended to prepare Au single crystals by CNTs. The facile, efficient and large-scale availability of the nanohybrids makes their tremendous potential realizable and developable.

Structural and optoelectronic properties of C60 rods obtained via a rapid synthesis route

High purity single crystal C60 rods with uniform dimensions are synthesized by a rapid and facile approach which can be completed over a timescale of typically a few minutes. The morphology of the fullerene product has been characterized in detail by scanning electron microscopy, scanning transmission electron microscopy, and atomic force microscopy, demonstrating that the resulting materials are solid, hexagonal cross-sectioned rods with novel faceted tips. High resolution transmission electron microscopy investigations reveal that the rods are face-centered cubic packed single crystals. Vibrational and electronic spectroscopy studies provide compelling evidence that the rods are a van der Waals solid since the electronic structure of the component C60 molecules is largely preserved. The structures obtained are found to possess novel optoelectronic properties exhibiting low energy absorption not reported in related structures and materials to date. Furthermore significant room temperature photoluminescence is obtained from the C60 rods accompanied by a small blue shift of the spectra which is also observed for the first ‘allowed’ absorption transitions. Given their rapid synthesis, excellent purity, optical and charge transport properties these fullerene structures are expected to be a promising materials for nanoelectronic devices including thin film organic solar cells and photodetectors.

Inverted organic solar cells based on aqueous processed ZnO interlayers at low temperature

A facile solution processable and low temperature (≤150 °C) approach was developed to deposit ZnO electron transport interlayers for inverted organic solar cells. The ZnO thin films were fabricated from the stable and non-toxic aqueous precursor solutions of ammine-hydroxo zinc complex, [Zn(NH3)x](OH)2. The resulting inverted poly (3-hexylthiophene): [6-6]-phenyl C61 butryric acid methyl ester solar cells exhibited power conversion efficiency of 4.17% as well as decent stability. We demonstrate that the work function of the ZnO electron transport interlayers was critical in terms of governing the photovoltaic performance of the inverted devices.

Low-voltage zinc oxide thin-film transistors with solution-processed channel and dielectric layers below 150 °C

In this letter, solution processed low voltage (<3 V) zinc oxide (ZnO) thin-film transistors with the maximum process temperature not exceeding 150 °C were achieved. In the devices, an ultra-thin zirconium oxide layer was formed as the gate dielectric via ultraviolet irradiation assisted sol-gel processes, and the ZnO channel was processed from an aqueous precursor of ammine-hydroxo zinc complex. The devices can be operated under a voltage of 3 V, and show decent device performance with the field effect mobility of 0.45 cm2/V · s and an ON/OFF current ratio of 105.

Inverted all-polymer solar cells based on a quinoxaline–thiophene/naphthalene-diimide polymer blend improved by annealing

We have investigated the effect of thermal annealing on the photovoltaic parameters of all-polymer solar cells based on a quinoxaline-thiophene donor polymer (TQ1) and a naphthalene diimide acceptor polymer (N2200). The annealed devices show a doubled power conversion efficiency compared to nonannealed devices, due to the higher short-circuit current (J(sc)) and fill factor (FF), but with a lower open circuit voltage (V-oc). On the basis of the morphology-mobility examination by several scanning force microscopy techniques, and by grazing-incidence wide-angle X-ray scattering, we conclude that better charge transport is achieved by higher order and better interconnected networks of the bulk heterojunction in the annealed active layers. The annealing improves charge transport and extends the conjugation length of the polymers, which do help in charge generation and meanwhile reduce recombination. Photoluminescence, electroluminescence, and light intensity dependence measurements reveal how this morphological change affects charge generation and recombination. As a result, the J(sc) and FF are significantly improved. However, the smaller band gap and the higher HOMO level of TQ1 upon annealing causes a lower V-oc. The blend of an amorphous polymer TQ1, and a semi-crystalline polymer N2200, can thus be modified by thermal annealing to double the power conversion efficiency.

Solution-Processed Zinc Oxide Thin-Film Transistors With a Low-Temperature Polymer Passivation Layer

Bottom-gate top-contact zinc oxide (ZnO) thin-film transistors were fabricated with a low annealing temperature (150 °C) using ammine-hydroxo zinc precursors. The unpassivated devices present profound hysteresis in the measured current-voltage characteristics and a negative output conductance, which were attributed to the interaction of back surface with the oxygen molecules in the ambient atmosphere. To suppress the ambient influence without impacting the devices intrinsic performance, a simple low-temperature (75 °C ) solution-based passivation approach with polydimethylsiloxane was developed. The passivated devices present typical field-effect transistor behaviors of greatly improved device performance.

Morphology control of perovskite light-emitting diodes by using amino acid self-assembled monolayers

Amino acid self-assembled monolayers are used in the fabrication of light-emitting diodes based on organic-inorganic halide perovskites. The monolayers of amino acids provide modified interfaces by anchoring to the surfaces of ZnO charge-transporting layers using carboxyl groups, leaving the amino groups to facilitate the nucleation of MAPbBr3 perovskite films. This surface-modification strategy, together with chlorobenzene-assisted fast crystallization method, results in good surface coverage and reduced defect density of the perovskite films. These efforts lead to green perovskite light emitting diodes with a low turn-on voltage of 2 V and an external quantum efficiency of 0.43% at a brightness of ∼5000 cd m−2.