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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.

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 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.

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.

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