Yee-Fun Lim

SnSe Nanocrystals: Synthesis, Structure, Optical Properties, and Surface Chemistry

The colloidal synthesis of SnSe nanoparticles is accomplished through the injection of bis[bis(trimethylsilyl)amino]tin(II) into hot trioctylphosphine:selenium in the presence of oleylamine. Through the manipulation of reaction temperature particles are grown with the average diameter reliably tuned to 4-10 nm. Quantum confinement is examined by establishing a relationship between particle size and band gap while the in depth growth dynamics are illuminated through UV-vis-NIR spectroscopy. Surface chemistry effects are explored, including the demonstration of useful ligand exchanges and the development of routes toward anisotropic particle growth. Finally, transient current-voltage properties of SnSe nanocrystal films in the dark and light are examined.

Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies

Internanocrystal coupling induced excitons dissociation in lead salt nanocrystal assemblies is investigated. By combining transient photoluminescence spectroscopy, grazing incidence small-angle X-ray scattering, and time-resolved electric force microscopy, we show that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy. Our results have important implications for the design of nanocrystal-based optoelectronic devices.

Solution-processed nanocrystal quantum dot tandem solar cells.

J.J.C., W.N.W., and R.S.H. contributed equally to this work. This work was supported in part by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). J.J.C. acknowledges support from the NSF IGERT fellowship. W.N.W. and R.S.H. acknowledge support from undergraduate research fellowships from the Cornell Engineering Learning Initiative and the KAUST-CU center. Y.F.L. acknowledges fellowship from Agency of Science, Technology and Research (A*STAR), Singapore. J.L.L. and J.A.M. acknowledge support from the U.S. National Science Foundation through grants DMR-1006633 and DMR-0706508. J.J. acknowledges funding through the Flexible Electronics Theme of the CSIRO Future Manufacturing Flagship, aCSIRO Office of Chief Executive Postdoctoral Fellowship and the Australian Department of Innovation, Industry, Science and Research, International Science Linkage Grant, CG100059. The authors thank Prof. Rene A. J. Janssen and David Moore for a critical reading of the manuscript and useful discussions.

A survey of electron-deficient pentacenes as acceptors in polymer bulk heterojunction solar cells

We have prepared, characterized and surveyed device performance for a series of electron deficient pentacenes for use as acceptors in polymer bulk heterojunction solar cells, using P3HT as the donor material. All of the materials reported here behaved as acceptors, and variations in the position and nature of the electron-withdrawing group on the pentacene core allowed tuning of device open-circuit voltage. Photocurrent was strongly correlated with the pentacene crystal packing motif; materials with 2D π-stacking interactions performed poorly compared with materials exhibiting 1D π-stacking interactions. The best pentacene acceptors gave repeatable device efficiency in excess of 1.2%, compared with 3.5% exhibited for PCBM-based devices.

Spray-deposited poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) top electrode for organic solar cells

Spray deposition is emerging as an attractive low-cost and high throughput method for organic solar cell (OSC) fabrication. In this letter, we demonstrate that a highly conductive formulation of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) can be spray deposited to form the top electrode for an OSC. An inverted solar cell fabricated in this way with a blend of poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester as the active layer achieved a power conversion efficiency of 2.0% under AM 1.5 100 mW/cm2 illumination.

Soluble n-type pentacene derivatives as novel acceptors for organic solar cells

6,13-Bis(triisopropylsilylethynyl) (TIPS)-pentacene has proven to be a promising soluble p-type material for organic thin film transistors as well as for photovoltaics. In this work, we show that adding electron-withdrawing nitrile functional groups to TIPS-pentacene turns it into an n-type material, which can be used as an acceptor for organic solar cells. Several new cyanopentacenes with different trialkylsilyl functional groups have been synthesized. The HOMO–LUMO energy levels can be tuned by varying the number of nitrile groups, while the trialkylsilyl groups control crystal packing and film morphology. Solar cells were fabricated from a blend of poly(3-hexylthiophene) (P3HT) as the donor and the cyanopentacenes as acceptors, and we found that the acceptors that stack in a 1D “sandwich-herringbone” exhibited the best performance of derivatives in this study. A solar cell fabricated from a blend of P3HT and 2,3-dicyano-6,13-bis-(tricyclopentylsilylethynyl)pentacene (2,3-CN2-TCPS-Pn) exhibited a power conversion efficiency of 0.43% under 100 mW cm−2AM 1.5 illumination.

Epitaxial ferroelectric BiFeO3 thin films for unassisted photocatalytic water splitting

Considering energy band alignment and polarization effect, ferroelectric BiFeO3 thin films are proposed as the photoanode in a monolithic cell to achieve unassisted photocatalytic water splitting. Significant anodic photocurrent was observed in our epitaxial ferroelectric BiFeO3 films prepared from sputter deposition. Both negative polarization charges and thinner films were found to promote the anodic photocatalytic reaction. Ultraviolet photoelectron spectroscopy proved that the conduction and valence band edges of BiFeO3 straddle the water redox levels. Theoretical analyses show that the large switchable polarization can modify the surface properties to promote the hydrogen and oxygen evolutions on the surfaces with positive and negative polarization charges, respectively.

Vegetable-extracted carbon dots and their nanocomposites for enhanced photocatalytic H2 production

In this work, we describe a facile construction of eco-friendly and effective nanocomposites from carbon dots (CDs), a novel type of carbon nanomaterial, for photocatalytic purposes. A series of benign CDs with favourable photoluminescence (PL) features were initially obtained from the hydrothermal treatment of natural vegetables including guava, red pepper, peas and spinach. The spinach-extracted CDs, with the highest PL emission intensity, were integrated with versatile TiO2 ensembles, i.e. nanoparticles (NPs) and nanotubes (NTs), to form desirable nanocomposites by further hydrothermal synthesis. TEM images, coupled with XPS spectra, confirm that the CDs are well decorated on the surfaces of TiO2. Due to the favourable electron transfer property of CDs, the H2 generation rates are enhanced to 75.5 and 246.1 μmol g−1 h−1 for TiO2 NP/CD and NT/CD nanocomposites, which are 21.6 and 3.3 times the rates of bare NPs and NTs, respectively. The strategy reported here may contribute to the fabrication of green and efficient nanocomposites for diverse applications.

Facile synthesis of colloidal CuO nanocrystals for light-harvesting applications

CuO is an earth-abundant, nontoxic, and low band-gap material; hence it is an attractive candidate for application in solar cells. In this paper, a synthesis of CuO nanocrystals by a facile alcohothermal route is reported. The nanocrystals are dispersible in a solvent mixture of methanol and chloroform, thus enabling the processing of CuO by solution. A bilayer solar cell comprising of CuO nanocrystals and phenyl-C61-butyric acid methyl ester (PCBM) achieved a power conversion efficiency of 0.04%, indicating the potential of this material for light-harvesting applications.

Random Telegraph Signal Noise in Gate-All-Around Si-FinFET With Ultranarrow Body

For the first time, the random telegraph signal (RTS) and its corresponding flicker noise (1/f) were investigated in gate-all-around p-type Si-FinFETs. For a device with gate width of ~ 100 nm (fin height) and length of ~ 200 nm, the typical RTS capture/emission time constants were ~ 0.1-1 ms. Very large RTS amplitudes (DeltaId/Id up to 25%) were observed, which is an effect attributable to the extreme device scaling and/or interface quality of FinFETs. The estimated scattering coefficients (alpha~10-12-10-13 ) are found to be higher than typical values obtained from MOSFETs. These findings demonstrate the relevance of RTS for FinFET operation