Junbo Han

Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology

Organometal halide perovskites are inexpensive materials with desirable characteristics of color-tunable and narrow-band emissions for lighting and display technology, but they suffer from low photoluminescence quantum yields at low excitation fluencies. Here we developed a ligand-assisted reprecipitation strategy to fabricate brightly luminescent and color-tunable colloidal CH3NH3PbX3 (X = Br, I, Cl) quantum dots with absolute quantum yield up to 70% at room temperature and low excitation fluencies. To illustrate the photoluminescence enhancements in these quantum dots, we conducted comprehensive composition and surface characterizations and determined the time- and temperature-dependent photoluminescence spectra. Comparisons between small-sized CH3NH3PbBr3 quantum dots (average diameter 3.3 nm) and corresponding micrometer-sized bulk particles (2-8 μm) suggest that the intense increased photoluminescence quantum yield originates from the increase of exciton binding energy due to size reduction as well as proper chemical passivations of the Br-rich surface. We further demonstrated wide-color gamut white-light-emitting diodes using green emissive CH3NH3PbBr3 quantum dots and red emissive K2SiF6:Mn(4+) as color converters, providing enhanced color quality for display technology. Moreover, colloidal CH3NH3PbX3 quantum dots are expected to exhibit interesting nanoscale excitonic properties and also have other potential applications in lasers, electroluminescence devices, and optical sensors.

Broadband Black-Phosphorus Photodetectors with High Responsivity

An array of black-phosphorus photodetectors with channel lengths down to 100 nm is fabricated, and temperature-dependent photodetection measurements from 300 K down to 20 K are carried out. The devices show high photoresponse in a broadband spectral range with a record-high photoresponsivity of 4.3 × 10(6) A W(-1) at 300 K for the 100 nm device.

New generation perovskite solar cells with solution-processed amino-substituted perylene diimide derivative as electron-transport layer

In this study, for the first time, we introduced amino-substituted perylene diimide derivative (N-PDI) as an alternative electron transport layer (ETL) to replace the commonly used TiO2 in planar heterojunction perovskite solar cells. Two types of device structures, i.e., glass/FTO/N-PDI/CH3NH3PbI3−xClx/spiro-MeOTAD/Au and polyethylene terephthalate PET/ITO/N-PDI/CH3NH3PbI3−xClx/spiro-MeOTAD/Au, were fabricated on both rigid and flexible substrates using room-temperature solution processing technique. Based on the proposed device structures, power conversion efficiency (PCE) of 17.66% was obtained based on glass/FTO rigid substrates, and a PCE of 14.32% was achieved based on PET/ITO flexible substrates. The results showed that the terminal amino group in N-PDI enhanced the wetting capability of the surfaces to perovskite, reduced the surface work function of the FTO substrate and passivated the surface trap states of the perovskite films. These results confirm that small molecule semiconductor N-PDI can serve as an effective electron-transport material for achieving high-performance perovskite solar cells and draw molecular design guidelines for electron-selective contacts with perovskite.

Carbon Quantum Dots/TiOx Electron Transport Layer Boosts Efficiency of Planar Heterojunction Perovskite Solar Cells to 19%

In planar n-i-p heterojunction perovskite solar cells, the electron transport layer (ETL) plays important roles in charge extraction and determine the morphology of the perovskite film. Here, we report a solution-processed carbon quantum dots (CQDs)/TiO2 composite that has negligible absorption in the visible spectral range, a very attractive feature for perovskite solar cells. Using this novel CQDs/TiO2 ETL in conjunction with a planar n-i-p heterojunction, we achieved an unprecedented efficiency of ∼19% under standard illumination test conditions. It was found that a CQDs/TiO2 combination increases both the open circuit voltage and short-circuits current density as compared to using TiO2 alone. Various advanced spectroscopic characterizations including ultrafast spectroscopy, ultraviolet photoelectron spectroscopy, and electronic impedance spectroscopy elucidate that the CQDs increases the electronic coupling between the CH3NH3PbI3-xClx and TiO2 ETL interface as well as energy levers that contribute to electron extraction.

High performance near-infrared photodetectors based on ultrathin SnS nanobelts grown via physical vapor deposition

Ultrathin SnS nanobelts were synthesized via physical vapor deposition (PVD). Furthermore, high performance near-infrared (NIR) photodetectors based on SnS nanobelts showed an excellent photoresponsivity of 300 A W−1 under 800 nm light illumination with an external quantum efficiency of 4.65 × 104% and a detectivity of 6 × 109 Jones, as well as fast rise and decay times (τrise = 36 ms and τdecay = 7 ms), suggesting a promising future for the utilization of the SnS nanobelts in practical NIR light sensors.

Highly Efficient Second-Harmonic Generation in Monolithic Matching Layer Enhanced Al

In this letter, we demonstrate significant improvement in second-harmonic (SH) generation using matching layer enhanced AlxGa1-xAs Bragg reflection waveguide in type-I and type-II phase-matching schemes. For a 1.8-ps pulsed pump around 1550 nm with an average power of 3.3 mW, peak SH powers of 28 and 60 muW were measured in type-I and type-II interactions, respectively. The associated normalized conversion efficiencies were estimated to be 5.30 times 103% and 1.14 times 104% W-1 middot cm-2 with SH process bandwidths of 1.7- and 1.8-nm full-width at half-maximum. Waveguides with various ridge widths ranging between 2.8 and 4.8 mum with 2.17-mm length were characterized.

_x

Sb2Se3 is a very promising photovoltaic material because of its attractive material, optical and electrical properties. Very recently, we reported a superstrate CdS/Sb2Se3 solar cell with 5.6% certified efficiency. In this letter, we focused on the optical properties of amorphous and polycrystalline Sb2Se3 thin films prepared by thermal evaporation. Using temperature dependent transmission spectrum and temperature dependent photoluminescence, the indirect optical transition nature and bandgap values as functions of temperature were acquired. Using ellipsometry measurements and Swanepoels envelope method, the refractive indices as well as the dielectric constant in a wide wavelength range of 193–2615 nm were obtained. These works would lay the foundation for the further development of Sb2Se3 thin film solar cells.

Ga

A series of Ag/titanium oxide composite films with Ag atom concentration of 28%–82% were prepared by cosputtering technique. The scanning electron microscopy and the x-ray photoelectron spectroscopy studies reveal that metallic Ag nanoparticles with a size of 20–50nm disperse in titanium oxide. The plasmon resonance absorption peak of the embedded Ag nanoparticles shifts to about 860nm at Ag atom concentration of 42%, the nonlinear absorption coefficient βeff and the nonlinear refractive index γ are −1.4×105cm∕GW and 3.8cm2∕GW, respectively, which were measured by the Z-scan technique at 790nm with a peak irradiance of 1.8GW∕cm2. The corresponding third-order susceptibility χ(3) is about 4.7×10−7esu.

_{1-x}

This work reports on incorporation of spectrally tuned gold/silica (Au/SiO2) core/shell nanospheres and nanorods into the inverted perovskite solar cells (PVSC). The band gap of hybrid lead halide iodide (CH3NH3PbI3) can be gradually increased by replacing iodide with increasing amounts of bromide, which can not only offer an appreciate solar radiation window for the surface plasmon resonance effect utilization, but also potentially result in a large open circuit voltage. The introduction of localized surface plasmons in CH3NH3PbI2.85Br0.15‐based photovoltaic system, which occur in response to electromagnetic radiation, has shown dramatic enhancement of exciton dissociation. The synchronized improvement in photovoltage and photocurrent leads to an inverted CH3NH3PbI2.85Br0.15 planar PVSC device with power conversion efficiency of 13.7%. The spectral response characterization, time resolved photoluminescence, and transient photovoltage decay measurements highlight the efficient and simple method for perovskite devices.

As Bragg Reflection Waveguides

Type-II difference-frequency generation with a bandwidth exceeding 40 nm was obtained in AlGaAs Bragg reflection waveguide. The external normalized conversion efficiency of 2.5 × 10^-2 %W^-1cm^-2 was obtained in a 1.5 mm long device.