Singapore Berkeley Research Initiative for Sustainable Energy

Singapore, Singapore

Singapore Berkeley Research Initiative for Sustainable Energy

Singapore, Singapore
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Li Z.,Nanyang Technological University | Kulkarni S.A.,Nanyang Technological University | Boix P.P.,Nanyang Technological University | Shi E.,Peking University | And 9 more authors.
ACS Nano | Year: 2014

Organic-inorganic metal halide perovskite solar cells were fabricated by laminating films of a carbon nanotube (CNT) network onto a CH3NH 3PbI3 substrate as a hole collector, bypassing the energy-consuming vacuum process of metal deposition. In the absence of an organic hole-transporting material and metal contact, CH3NH 3PbI3 and CNTs formed a solar cell with an efficiency of up to 6.87%. The CH3NH3PbI3/CNTs solar cells were semitransparent and showed photovoltaic output with dual side illuminations due to the transparency of the CNT electrode. Adding spiro-OMeTAD to the CNT network forms a composite electrode that improved the efficiency to 9.90% due to the enhanced hole extraction and reduced recombination in solar cells. The interfacial charge transfer and transport in solar cells were investigated through photoluminescence and impedance measurements. The flexible and transparent CNT network film shows great potential for realizing flexible and semitransparent perovskite solar cells. © 2014 American Chemical Society.

Sum T.C.,Nanyang Technological University | Mathews N.,Nanyang Technological University | Mathews N.,Singapore Berkeley Research Initiative for Sustainable Energy
Energy and Environmental Science | Year: 2014

Solution-processed organic-inorganic perovskite solar cells are hailed as the recent major breakthrough in low-cost photovoltaics. Power conversion efficiencies approaching those of crystalline Si solar cells (exceeding 15%) have been reported. Remarkably, such phenomenal performances were achieved in a matter of 5 years-up from ∼3.8% back in 2009. Since then, the field has expanded exponentially. In this perspective, we review the basic working mechanisms of perovskite solar cells in relation to their intrinsic properties and fundamental photophysics. The current state-of-the-art and the open questions in this maturing field are also highlighted. This journal is © the Partner Organisations 2014.

Kumar M.H.,Nanyang Technological University | Yantara N.,Nanyang Technological University | Dharani S.,Nanyang Technological University | Graetzel M.,Ecole Polytechnique Federale de Lausanne | And 4 more authors.
Chemical Communications | Year: 2013

A ZnO compact layer formed by electrodeposition and ZnO nanorods grown by chemical bath deposition (CBD) allow the processing of low-temperature, solution based and flexible solid state perovskite CH3NH3PbI 3 solar cells. Conversion efficiencies of 8.90% were achieved on rigid substrates while the flexible ones yielded 2.62%. © 2013 The Royal Society of Chemistry.

Li G.,Nanyang Technological University | Xiong Q.,Nanyang Technological University | Xiong Q.,Singapore Berkeley Research Initiative for Sustainable Energy
Optics Express | Year: 2014

Semiconductor and metallic nanowires are attractive building blocks for a nanoscale integrated photonic platform. The scattering coefficients of the optical or plasmonic waveguide mode by 3-dimensional nanowire abrupt discontinuities including splices and endfaces are important figures of merit for realistic estimation of the coupling, lasing, or sensing performance. To tackle with such computationally challenging problems, we derive simple closed-form expressions based on linear equations and overlap integrals of normal modes to realize domain reduction and efficient analytical modeling. For the reflection coefficients at nanowire/waveguide endfaces, the analytical expressions incorporating all the bound modes and a few dozen leaky modes are highly accurate; whereas for the transmission coefficients at nanowire/waveguide splices, the model can be further simplified because only the input and the interested output bound modes need to be considered. Exhaustive validations using fully-vectorial simulation results as reference data show that the model is accurate and versatile for fundamental and high-order TE or TM modes, and for various architectures including high-index-contrast dielectric and plasmonic configurations, 3-D geometries or 2-D equivalents, and various operating wavelengths from ultraviolet to visible and the optical telecommunication bands in the infrared. Our model will facilitate the structure design and theoretical investigation of nanowire/waveguide photonic devices, especially lasers, resonators, sensors and couplers. © 2014 Optical Society of America.

Tay Q.,Nanyang Technological University | Liu X.,Nanyang Technological University | Tang Y.,Nanyang Technological University | Jiang Z.,Nanyang Technological University | And 2 more authors.
Journal of Physical Chemistry C | Year: 2013

Highly crystalline pure brookite and two-phase anatase/brookite TiO 2 nanostructures were synthesized via a simple hydrothermal method with titanium sulfide as the precursors in sodium hydroxide solutions. The control of the phase composition has been demonstrated via solution concentration and reaction time, and the phase transformation mechanism has been elucidated. Photocatalytic activities of the as-synthesized two-phase anatase/brookite TiO2, pure anatase nanoparticles, and pure brookite nanoplates were appraised via photocatalytic hydrogen evolution in aqueous methanol solution. Results have shown that the photocatalytic activity is higher for the two-phase anatase/brookite TiO2 and brookite nanoplates as compared to pure anatase nanoparticles despite the lower surface areas of the two-phase anatase/brookite TiO2 and brookite nanoplates. From the Mott-Schottky analysis, brookite phase is shown to have a more cathodic conduction band edge potential than anatase phase, which leads to more energetically favorable hydrogen reduction. Moreover, femtosecond transient absorption spectroscopy measurements suggests that the photoexcited electrons transfer from brookite to anatase phase - leading to further enhancement of the photocatalytic activity. In comparison with the highly active two-phase commercial benchmark P25, our synthesized two-phase anatase/brookite TiO 2 is 220% more active when measured by the H2 yield per unit area of the photocatalyst surface. © 2013 American Chemical Society.

Zhang Q.,Nanyang Technological University | Ha S.T.,Nanyang Technological University | Liu X.,Nanyang Technological University | Sum T.C.,Nanyang Technological University | And 3 more authors.
Nano Letters | Year: 2014

Near-infrared (NIR) solid-state micro/nanolasers are important building blocks for true integration of optoelectronic circuitry.1 Although significant progress has been made in III-V nanowire lasers with achieving NIR lasing at room temperature,2-4 challenges remain including low quantum efficiencies and high Auger losses. Importantly, the obstacles toward integrating one-dimensional nanowires on the planar ubiquitous Si platform need to be effectively tackled. Here we demonstrate a new family of planar room-temperature NIR nanolasers based on organic-inorganic perovskite CH3NH3PbI3-aXa (X = I, Br, Cl) nanoplatelets. Their large exciton binding energies, long diffusion lengths, and naturally formed high-quality planar whispering-gallery mode cavities ensure adequate gain and efficient optical feedback for low-threshold optically pumped in-plane lasing. We show that these remarkable wavelength tunable whispering-gallery nanolasers can be easily integrated onto conductive platforms (Si, Au, indium tin oxide, and so forth). Our findings open up a new class of wavelength tunable planar nanomaterials potentially suitable for on-chip integration. © 2014 American Chemical Society.

Xing G.,Nanyang Technological University | Mathews N.,Nanyang Technological University | Mathews N.,Singapore Berkeley Research Initiative for Sustainable Energy | Lim S.S.,Nanyang Technological University | And 7 more authors.
Nature Materials | Year: 2014

Low-temperature solution-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light sources. Organic semiconductors and colloidal quantum dots are considered the main candidates for this application. However, stumbling blocks in organic lasing include intrinsic losses from bimolecular annihilation and the conflicting requirements of high charge carrier mobility and large stimulated emission; whereas challenges pertaining to Auger losses and charge transport in quantum dots still remain. Herein, we reveal that solution-processed organic-inorganic halide perovskites (CH 3 NH 3 PbX 3 where X = Cl, Br, I), which demonstrated huge potential in photovoltaics, also have promising optical gain. Their ultra-stable amplified spontaneous emission at strikingly low thresholds stems from their large absorption coefficients, ultralow bulk defect densities and slow Auger recombination. Straightforward visible spectral tunability (390-790 nm) is demonstrated. Importantly, in view of their balanced ambipolar charge transport characteristics, these materials may show electrically driven lasing. © 2014 Macmillan Publishers Limited.

Boix P.P.,Nanyang Technological University | Nonomura K.,Nanyang Technological University | Mathews N.,Nanyang Technological University | Mathews N.,Singapore Berkeley Research Initiative for Sustainable Energy | Mhaisalkar S.G.,Nanyang Technological University
Materials Today | Year: 2014

The recent emergence of efficient solar cells based on organic/inorganic lead halide perovskite absorbers promises to transform the fields of dye-sensitized, organic, and thin film solar cells. Solution processed photovoltaics incorporating perovskite absorbers have achieved efficiencies of 15% [1] in solid-state device configurations, superseding liquid dye sensitized solar cell (DSC), evaporated and tandem organic solar cells, as well as various thin film photovoltaics; thus establishing perovskite solar cells as a robust candidate for commercialization. Since the first reports in late 2012, interest has soared in the innovative device structures as well as new materials, promising further improvements. However, identifying the basic working mechanisms, which are still being debated, will be crucial to design the optimum device configuration and maximize solar cell efficiencies. Here we distill the current state-of-the-art and highlight the guidelines to ascertain the scientific challenges as well as the requisites to make this technology market-viable. © 2013 Elsevier Ltd.

Zhang Q.,Nanyang Technological University | Li G.,Nanyang Technological University | Liu X.,Nanyang Technological University | Qian F.,Lawrence Livermore National Laboratory | And 5 more authors.
Nature Communications | Year: 2014

Constrained by large ohmic and radiation losses, plasmonic nanolasers operated at visible regime are usually achieved either with a high threshold (10 2 -10 4 â €‰MWâ €‰cm â'2) or at cryogenic temperatures (4-120â €‰K). Particularly, the bending-back effect of surface plasmon (SP) dispersion at high energy makes the SP lasing below 450â €‰nm more challenging. Here we demonstrate the first strong room temperature ultraviolet (∼370â €‰nm) SP polariton laser with an extremely low threshold (∼3.5â €‰MWâ €‰cm â'2). We find that a closed-contact planar semiconductor-insulator-metal interface greatly lessens the scattering loss, and more importantly, efficiently promotes the exciton-SP energy transfer thus furnishes adequate optical gain to compensate the loss. An excitation polarization-dependent lasing action is observed and interpreted with a microscopic energy-transfer process from excitons to SPs. Our work advances the fundamental understanding of hybrid plasmonic waveguide laser and provides a solution of realizing room temperature UV nanolasers for biological applications and information technologies.

Kulkarni S.A.,Nanyang Technological University | Baikie T.,Nanyang Technological University | Boix P.P.,Nanyang Technological University | Yantara N.,Nanyang Technological University | And 3 more authors.
Journal of Materials Chemistry A | Year: 2014

Band-gap tuning of mixed anion lead halide perovskites (MAPb(I 1-xBrx)2 (0 ≤ x ≤ 1)) has been demonstrated by means of a sequential deposition process. The optical properties of perovskite hybrids can be flexibly modified by changing (mixing) the concentration of halogen precursors. The concentrations of precursor solution as well as the conversion time play an important role in determining the band-gap of perovskites. A systematic shift of the absorption band edge to shorter wavelengths is observed with increasing Br content in the perovskite films, which results in the decrement of the photocurrent. Nanorod like morphological features are also observed for perovskite films with an iodide to bromide molar ratio of <0.7. © 2014 the Partner Organisations.

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