Longfeng Lv

Photocatalytic synthesis and photovoltaic application of Ag-TiO2 nanorod composites.

A photocatalytic strategy has been developed to synthesize colloidal Ag-TiO2 nanorod composites in which each TiO2 nanorod contains a single Ag nanoparticle on its surface. In this rational synthesis, photoexcitation of TiO2 nanorods under UV illumination produces electrons that reduce Ag(I) precursor and deposit multiple small Ag nanoparticles on the surface of TiO2 nanorods. Prolonged UV irradiation induces an interesting ripening process, which dissolves the smaller nanoparticles by photogenerated oxidative species and then redeposits Ag onto one larger and more stable particle attached to each TiO2 nanorod through the reduction of photoexcited electrons. The size of the Ag nanoparticles can be precisely controlled by varying the irradiation time and the amount of alcohol additive. The Ag-TiO2 nanorod composites were used as electron transport layers in the fabrication of organic solar cells and showed notable enhancement in power conversion efficiency (6.92%) than pure TiO2 nanorods (5.81%), as well as higher external quantum efficiency due to improved charge separation and transfer by the presence of Ag nanoparticles.

Active terahertz device based on optically controlled organometal halide perovskite

An active all-optical high-efficiency broadband terahertz device based on an organometal halide perovskite (CH₃NH₃PM₃, MAPbI₃)/inorganic (Si) structure is investigated. Spectrally broadband modulation of the THz transmission is obtained in the frequency range from 0.2 to 2.6 THz, and a modulation depth of nearly 100% can be achieved with a low-level photoexcitation power (~0.4 W/cm²). Both THz transmission and reflection were suppressed in the MAPbI₃/Si structure by an external continuous-wave (CW) laser. Enhancement of the charge carrier density at the MAPbI₃/Si interface is crucial for photo-induced absorption. The results show that the proposed high-efficiency broadband optically-controlled terahertz device based on the MAPbI₃/Si structure has been realized.

Controlled synthesis and defect dependent upconversion luminescence of Y2O3: Yb, Er nanoparticles

Er3+ and Yb3+ co-doped Y2O3 nanoparticles have been prepared by using a coprecipitation method followed by a post-thermal-treatment, in which a surfactant (cetyltrimethylammonium bromide, CTAB) plays an important role in the size-controlling and upconversion luminescence tuning. The green (2H11/2, 4S3/2 → 4I15/2) and red emission (4F9/2 → 4I15/2) intensity can be effectively tuned by varying the surfactant concentration, which can induce the defects in the as-obtained products. The probability of quenching and nonradiative relaxation from 4F7/2, 2H11/2, and 4S3/2 to 4F9/2 could be increased as the number of defects introduced by the surfactant increases, and thus the ratio of red to green emission is also changed. The upconversion mechanism has been analyzed and discussed, which may be a new complement for upconversion luminescence.

Conjugated polymer based active electric-controlled terahertz device

A modulation of terahertz response in a highly efficient, electric-controlled conjugated polymer-silicon hybrid device with low photo-excitation was investigated. The polymer-silicon forms a hybrid structure, where the active depletion region modifies the semiconductor conductivity in real time by applying an external bias voltage. The THz transmission was efficiently modulated by effective controlling. In a THz-TDS system, the modulation depth reached nearly 100% when the applied voltage was 3.8 V at an external laser intensity of 0.3 W/cm2. The saturation voltage decreased with increasing photo-excited intensity. In a THz-CW system, a significant decline in THz transmission was also observed with increasing applied bias voltage. This reduction in THz transmission is induced by the enhancement of carrier density.

Negative differential resistance and carrier transport of electrically bistable devices based on poly(N-vinylcarbazole)-silver sulfide composites

An electrically bistable device has been fabricated based on poly(N-vinylcarbazole) (PVK)-silver sulfide (Ag2S) composite films using a simple spin-coating method. Current–voltage (I-V) characteristics of the as-fabricated devices exhibit a typical electrical bistability and negative differential resistance (NDR) effect. The NDR effect can be tuned by varying the positive charging voltage and the charging time. The maximum current ratio between the high-conducting state (ON state) and low-conducting state (OFF state) can reach up to 104. The carrier transport mechanisms in the OFF and ON states are described by using different models on the basis of the experimental result.

Ligand-free rutile and anatase TiO2 nanocrystals as electron extraction layers for high performance inverted polymer solar cells

Ligand-free rutile and anatase TiO2 nanocrystals have been synthesized through a hydrolytic sol–gel reaction. The morphology, crystal structure, elemental composition and band structure of the obtained nanocrystals are characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and UV-visible absorption spectroscopy. These two kinds of nanocrystals could serve as electron extraction layers for improving the performance in inverted polymer solar cells. Compared with the device fabricated by using amorphous TiO2 (6.11%) and rutile TiO2 (6.93%), the device based on anatase TiO2 shows a significant enhancement in power conversion efficiency (7.85%). Meanwhile, the ideal current–voltage model for a single heterojunction solar cell is applied to clarify the junction property of the cell. The model demonstrates that the device based on anatase TiO2 has effective electron extraction and hole-blocking properties.

Investigation on Thermal Degradation Process of Polymer Solar Cells Based on Blend of PBDTTT-C and BM

The effects of thermal treatment on the photovoltaic performance of conventional and inverted polymer solar cells (PSCs) based on the combination of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thie-no[3,4-b]thiophene))-2,6-diyl] (PBDTTT-C) and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) are investigated. The transient photoconductivity, the absorption spectra, and the transmission electron microscopy (TEM) images have been employed to study the thermal degradation of the inverted PSCs. The degradation is attributed to the inefficient charge generation and imbalance in charge-carrier transport, which is closely associated with the morphological evolution of the active layer with prolonged heating time.

High sensitivity, fast response and low operating voltage organic photodetectors by incorporating a water/alcohol soluble conjugated polymer anode buffer layer

Low dark current density plays a key role in determining the overall performance of organic photodetectors (OPDs). However, both the donor domains and acceptor domains in the bulk heterojunction, which has high exciton dissociation efficiency, are in contact with the two electrodes. Therefore, the undesirable charge injection from the electrodes to the active layer is hard to avoid, leading to a high dark current density in most OPDs. In this work, we fabricate the OPDs based on a conventional poly(3-hexylthiophene) (P3HT)/(phenyl-C61-butyric-acid-methyl-ester) (PC61BM) bulk heterojunction. By incorporating a water/alcohol soluble conjugated polymer (WSCP), poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), interlayer between the anode and the active layer, the dark current density is effectively reduced from 0.07 mA cm−2 to 1.92 × 10−5 mA cm−2 under a −0.5 V bias. The resulting OPDs show a 1.93 × 105 signal-to-noise ratio (SNR), a 10 MHz bandwidth, and a 9.10 × 1012 Jones detectivity at a low reverse bias of −0.5 V (at 550 nm). Our research provides a promising way for high performance OPDs.

Monolayer graphene based organic optical terahertz modulator

We investigate a high-efficiency broadband terahertz wave modulator with structures made from the conjugated polymer [2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylennevinylene], graphene, and Si, irradiated with an external excitation laser. We demonstrate a strategy that can alleviate the tradeoff between the requirements of modulation depth and modulation speed in polymer/silicon terahertz wave modulators. Using terahertz time-domain and continuous-wave systems, we measured both the terahertz transmission modulation properties and the time responses of the modulator structures. The conjugated polymer/graphene/silicon structure achieved a high modulation factor of 93% for transmission as well as improved the modulation speed of the devices based on polymer/silicon. The high modulation efficiency of the polymer/graphene/silicon structure was induced by the enhancement in carrier density and the extremely high carrier mobility of graphene, respectively.

Enhanced amplified spontaneous emission from morphology-controlled organic–inorganic halide perovskite films

Organic–inorganic tri-halide perovskites (MAPbX3, where MA = CH3NH3, and X = Cl, Br, I) have shown promise as laser gain media. The laser characteristics of perovskite films are susceptible to their crystallinity and morphology. Herein, we demonstrate the morphology and crystallinity of perovskite films could be well-controlled in a modified sequential deposition process by using binary solvent mixtures involving N,N-dimethylmethanamide (DMF) and dimethylsulfoxide (DMSO). The highly crystalline and notably smooth MAPbI3 films on glass substrates yield outstanding amplified spontaneous emission (ASE) performance. The binary solvent engineering approach opens up new opportunities for the development of low-cost and high-efficient ASE devices based on perovskite materials.