Weili Yu

Ambipolar solution-processed hybrid perovskite phototransistors

Organolead halide perovskites have attracted substantial attention because of their excellent physical properties, which enable them to serve as the active material in emerging hybrid solid-state solar cells. Here we investigate the phototransistors based on hybrid perovskite films and provide direct evidence for their superior carrier transport property with ambipolar characteristics. The field-effect mobilities for triiodide perovskites at room temperature are measured as 0.18 (0.17) cm2 V−1 s−1 for holes (electrons), which increase to 1.24 (1.01) cm2 V−1 s−1 for mixed-halide perovskites. The photoresponsivity of our hybrid perovskite devices reaches 320 A W−1, which is among the largest values reported for phototransistors. Importantly, the phototransistors exhibit an ultrafast photoresponse speed of less than 10 μs. The solution-based process and excellent device performance strongly underscore hybrid perovskites as promising material candidates for photoelectronic applications.

CsPbxMn1–xCl3 Perovskite Quantum Dots with High Mn Substitution Ratio

CsPbX3 (X = Cl, Br, I) perovskite quantum dots (QDs) are potential emitting materials for illumination and display applications, but toxic Pb is not environment- and user-friendly. In this work, we demonstrate the partial replacement of Pb with Mn through phosphine-free hot-injection preparation of CsPbxMn1-xCl3 QDs in colloidal solution. The Mn substitution ratio is up to 46%, and the as-prepared QDs maintain the tetragonal crystalline structure of the CsPbCl3 host. Meaningfully, Mn substitution greatly enhances the photoluminescence quantum yields of CsPbCl3 from 5 to 54%. The enhanced emission is attributed to the energy transfer of photoinduced excitons from the CsPbCl3 host to the doped Mn, which facilitates exciton recombination via a radiative pathway. The intensity and position of this Mn-related emission are also tunable by altering the experimental parameters, such as reaction temperature and the Pb-to-Mn feed ratio. A light-emitting diode (LED) prototype is further fabricated by employing the as-prepared CsPbxMn1-xCl3 QDs as color conversion materials on a commercially available 365 nm GaN LED chip.

Aggregation emission properties and self-assembly of conjugated oligocarbazoles.

Conjugated oligocarbazoles with a 9,10-divinylanthracene core have been synthesized, and exhibit the transition from aggregation-induced emission (AIE) to aggregation-induced emission enhancement (AIEE) behaviour with extending conjugation length; self-assembly of the Cz4 molecule affords nanorings with high fluorescent efficiency.

Efficient polymer/nanocrystal hybrid solar cells fabricated from aqueous materials

Efficient, low-cost polymer/nanocrystal (NC) hybrid solar cells fabricated from aqueous materials, a PPV precursor and CdTe NCs, were demonstrated. The cells showed an efficiency of 2.14% under AM 1.5 (100 mW cm−2). This work provided a facile and feasible protocol to produce efficient solar cellsvia a cheaper and greener route.

Optically controlled electroresistance and electrically controlled photovoltage in ferroelectric tunnel junctions

Ferroelectric tunnel junctions (FTJs) have recently attracted considerable interest as a promising candidate for applications in the next-generation non-volatile memory technology. In this work, using an ultrathin (3 nm) ferroelectric Sm0.1Bi0.9FeO3 layer as the tunnelling barrier and a semiconducting Nb-doped SrTiO3 single crystal as the bottom electrode, we achieve a tunnelling electroresistance as large as 105. Furthermore, the FTJ memory states could be modulated by light illumination, which is accompanied by a hysteretic photovoltaic effect. These complimentary effects are attributed to the bias- and light-induced modulation of the tunnel barrier, both in height and width, at the semiconductor/ferroelectric interface. Overall, the highly tunable tunnelling electroresistance and the correlated photovoltaic functionalities provide a new route for producing and non-destructively sensing multiple non-volatile electronic states in such FTJs.

Generation of Multiple Excitons in Ag2S Quantum Dots: Single High-Energy versus Multiple-Photon Excitation

We explored biexciton generation via carrier multiplication (or multiple-exciton generation) by high-energy photons and by multiple-photon absorption in Ag2S quantum dots (QDs) using femtosecond broad-band transient absorption spectroscopy. Irrespective of the size of the QDs and how the multiple excitons are generated in the Ag2S QDs, two distinct characteristic time constants of 9.6-10.2 and 135-175 ps are obtained for the nonradiative Auger recombination of the multiple excitons, indicating the existence of two binding excitons, namely, tightly bound and weakly bound excitons. More importantly, the lifetimes of multiple excitons in Ag2S QDs were about 1 and 2 orders of magnitude longer than those of comparable size PbS QDs and single-walled carbon nanotubes, respectively. This result is significant because it suggests that by utilizing an appropriate electron acceptor, there is a higher possibility to extract multiple electron-hole pairs in Ag2S QDs, which should improve the performance of QD-based solar cell devices.

Synthesis of single-crystal-like nanoporous carbon membranes and their application in overall water splitting

Nanoporous graphitic carbon membranes with defined chemical composition and pore architecture are novel nanomaterials that are actively pursued. Compared with easy-to-make porous carbon powders that dominate the porous carbon research and applications in energy generation/conversion and environmental remediation, porous carbon membranes are synthetically more challenging though rather appealing from an application perspective due to their structural integrity, interconnectivity and purity. Here we report a simple bottom–up approach to fabricate large-size, freestanding and porous carbon membranes that feature an unusual single-crystal-like graphitic order and hierarchical pore architecture plus favourable nitrogen doping. When loaded with cobalt nanoparticles, such carbon membranes serve as high-performance carbon-based non-noble metal electrocatalyst for overall water splitting.

Aqueous-solution-processed hybrid solar cells from poly(1,4-naphthalenevinylene) and CdTe nanocrystals.

Poly(1,4-naphthalenevinylene), prepared from a water-soluble precursor, was used to fabricate hybrid solar cells by blending with water-soluble CdTe nanocrystals (NCs) to act as the photoactive layer. In composites with CdTe NCs as the electron acceptors in a bulk heterojunction configuration, the devices exhibited a short-circuit current density of -6.14 mA/cm(2), an open-circuit voltage of 0.44 V, a fill factor of 0.32, and a power conversion efficiency of 0.86% under AM1.5G conditions. Because the devices were fabricated from water-soluble materials, the procedure was generally simple and environmentally friendly in comparison to the conventional devices fabricated from oil-soluble materials.

Ultrahigh Carrier Mobility Achieved in Photoresponsive Hybrid Perovskite Films via Coupling with Single-Walled Carbon Nanotubes

Organolead trihalide perovskites have drawn substantial interest for photovoltaic and optoelectronic applications due to their remarkable physical properties and low processing cost. However, perovskite thin films suffer from low carrier mobility as a result of their structural imperfections such as grain boundaries and pinholes, limiting their device performance and application potential. Here we demonstrate a simple and straightforward synthetic strategy based on coupling perovskite films with embedded single-walled carbon nanotubes. We are able to significantly enhance the hole and electron mobilities of the perovskite film to record-high values of 595.3 and 108.7 cm2 V-1 s-1 , respectively. Such a synergistic effect can be harnessed to construct ambipolar phototransistors with an ultrahigh detectivity of 3.7 × 1014 Jones and a responsivity of 1 × 104 A W-1 , on a par with the best devices available to date. The perovskite/carbon nanotube hybrids should provide a platform that is highly desirable for fields as diverse as optoelectronics, solar energy conversion, and molecular sensing.

Photoelectrochemical and electrocatalytic properties of thermally oxidized copper oxide for efficient solar fuel production

We report the use of a facile and highly scalable synthesis process to control growth products of earth-abundant Cu-based oxides and their application in relevant photoelectrochemical and electrochemical solar fuel generation systems. Characterization of the synthesized Cu(I)/Cu(II) oxides indicates that their surface morphology and chemical composition can be simply tuned by varying two synthesis parameters (time and temperature). UV-Vis spectroscopy and impedance spectroscopy studies are performed to estimate the band structures and electronic properties of these p-type semiconductor materials. Photoelectrodes made of Cu oxides possess favorable energy band structures for production of hydrogen from water; the position of their conduction band is ≈1 V more negative than the water-reduction potential. High acceptor concentrations on the order of 1018–1019 cm−3 are obtained, producing large electric fields at the semiconductor–electrolyte interface and thereby enhancing charge separation. The highly crystalline pristine samples used as photocathodes in photoelectrochemical cells exhibit high photocurrents under AM 1.5G simulated illumination. When the samples are electrochemically reduced under galvanostatic conditions, the co-existence of the oxide with metallic Cu on the surface seems to function as an effective catalyst for the selective electrochemical reduction of CO2.