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Peters I.M.,Solar Energy Research Institute of Singapore
Optics Express | Year: 2014

Fundamental limits for path lengths of light in isotropic absorbers are calculated. The method of calculation is based on accounting for occupied states in optical phase space. Light trapping techniques, such as scattering or diffraction, are represented by the way how the available states are occupied. One finding of the presented investigation is that the path length limit is independent of the light trapping mechanism and only depends on the conditions for light incidence to, and escape from the absorber. A further finding is that the maximum path length is obtained for every light trapping mechanisms which results in a complete filling of the available states in phase space. For stationary solar cells, the Yablonovitch limit of 4dn2, with n the refractive index of the absorber, is a very good approximation of this limit. © 2014 Optical Society of America.

Wan A.,National University of Singapore | Jiang L.,National University of Singapore | Sangeeth C.S.S.,National University of Singapore | Nijhuis C.A.,National University of Singapore | Nijhuis C.A.,Solar Energy Research Institute of Singapore
Advanced Functional Materials | Year: 2014

The reproducibility of the electrical characteristics of molecular junctions has been notoriously low. This paper describes a method to construct tunnel junctions based on self-assembled monolayers (SAMs) by forming reversible electrical contacts to SAMs using top-electrodes of a non-Newtonian liquid-metal (GaOx/EGaIn) stabilized in a microfluidic-based device. A single top-electrode can be used to form up to 15-25 junctions. This method generates SAM-based junctions with highly reproducible electrical characteristics in terms of precision (widths of distributions) and replicability (closeness to a reference value). The reason is that this method, unlike other approaches that rely on cross-bar or nano/micropore configurations, does not require patterning of the bottom-electrodes and is compatible with ultra-flat template-stripped (TS) surfaces. This compatibly with non-patterned electrodes is important for three reasons. i) No edges of the electrodes are present at which SAMs cannot pack well. ii) Patterning requires photoresist that may contaminate the electrode and complicate SAM formation. iii) TS-surfaces contain large grains, have low rms values, and can be obtained and used (in ordinary laboratory conditions) within a few seconds to minimize contamination. The junctions have very good electrical stability (2500 current-voltage cycles and retained currents for 27 h), and can be fabricated with good yields (≈78%). Molecular electronic devices normally require patterning of the bottom-electrodes resulting in electrode edges at which self-assembled monolayers (SAMs) cannot pack well. Here, a method that is compatible with ultra-flat template-stripped electrodes without the need for patterning of the bottom-electrode is reported. This method generates SAM-based junctions in good yields and reproducibility between top-electrodes and operators with good electrical stability. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Wong J.,Solar Energy Research Institute of Singapore
IEEE Transactions on Electron Devices | Year: 2013

Recently proposed perturbation analysis and reciprocity relations in photovoltaic conversion are demonstrated in 2-D simulations. The main parameters discussed are the current-conversion efficiency, which enables accurate calculation of the sensitivity of maximum power to different recombination parameters, and its derived local series resistance, whose weighted average is exactly equal to the series resistance obtained by the double illumination method. Both parameters can be displayed as spatial maps to visually aid multidimensional computer simulations. Finally, the suitability of mapping the local series resistance from luminescence images is discussed with the help of simulated examples. The experimental and computational procedures to generate these maps are among the easiest proposed so far in the literature and require no ad hoc assumptions. © 1963-2012 IEEE.

Wong J.,Solar Energy Research Institute of Singapore
Conference Record of the IEEE Photovoltaic Specialists Conference | Year: 2013

Solar cell designs with complex metallization geometries such as metal wrap through (MWT), interdigitated-back-contact (IBC) cells, and metal grids with non-ideal features like finger breaks, finger striations and non-uniform contact resistance, are not amenable to simple series resistance (Rs) analysis based on small unit cells. In order to accurately simulate these cells, we developed a program that captures the cell metallization geometry from rastered images/CAD files, and efficiently meshes the cell plane for finite element analysis, yielding standard data such as the I-V curve, voltage and Rs distribution. The program also features a powerful post processor that predicts the rate of change in efficiency with respect to incremental changes in the metallization pattern, opening up the possibility of intelligent computer aided design procedures. © 2013 IEEE.

Yang D.,National University of Singapore | Jirutitijaroen P.,National University of Singapore | Walsh W.M.,Solar Energy Research Institute of Singapore
Solar Energy | Year: 2012

We apply time series analysis to forecast next hour solar irradiance including cloud cover effects. Three forecasting methods are proposed using different types of meteorological data as input parameters, namely, global horizontal irradiance (GHI), diffuse horizontal irradiance (DHI), direct normal irradiance (DNI) and cloud cover. The first method directly uses GHI to forecast next hour GHI through additive seasonal decomposition followed by an Auto-Regressive Integrated Moving Average (ARIMA) model. The second method forecasts DHI and DNI separately using additive seasonal decomposition followed by an ARIMA model and then combines the two forecasts to predict GHI using an atmospheric model. The third method considers cloud cover effects. An ARIMA model is used to predict cloud transients. GHI at different zenith angles and under different cloud cover conditions is constructed using nonlinear regression, i.e., we create a look-up table of GHI regression models for different cloud cover conditions. All three methods are tested using data from two weather stations in the USA: Miami and Orlando. It is found that forecasts using cloud cover information can improve the forecast accuracy. © 2012 Elsevier Ltd.

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