Multiscale approaches to high efficiency photovoltaics

While renewable energies are achieving parity around the globe, efforts to reach higher solar cell efficiencies becomes ever more difficult as they approach the limiting efficiency. The so-called third generation concepts attempt to break this limit through a combination of novel physical processes and new materials and concepts in organic and inorganic systems. Some examples of semi-empirical modelling in the field are reviewed, in particular for multispectral solar cells on silicon (french ANR project MULTISOLSI). Their achievements are outlined, and the limits of these approaches shown. This introduces the main topic of this contribution, which is the use of multiscale experimental and theoretical techniques to go beyond the semi-empirical understanding of these systems. This approach has already led to great advances at modelling which have led to modelling software which is widely known. Yet a survey of the topic reveals a fragmentation of efforts across disciplines, firstly, such as organic and inorganic fields, but also between the high efficiency concepts such as hot carrier cells and intermediate band concepts. We show how this obstacle to the resolution of practical research obstacles may be lifted by inter-disciplinary cooperation across length scales, and across experimental and theoretical fields, and finally across materials systems. We present a European COST Action MultiscaleSolar kicking off in early 2015 which brings together experimental and theoretical partners in order to develop multiscale research in organic and inorganic materials. The goal of this defragmentation and interdisciplinary collaboration is to develop understanding across length scales which will enable the full potential of third generation concepts to be evaluated in practise, for societal and industrial applications.Comment: Draft paper accompanying a plenary presentation to the World Renewable Energy Conference WREC 2015, June 2015, Bucharest. In press (IOP

Thermal Characterization and Lifetime Prediction of LED Boards for SSL Lamp

This work presents a detailed 3-D thermo-mechanical modelling of two LED board technologies to compare their performance. LED board are considered to be used in high power 800 lumen retrofit SSL (Solid State Lighting) lamp. Thermal, mechanical and life time properties are evaluated by numerical modelling. Experimental results measured on fabricated LED board samples are compared to calculated data. Main role of LED board in SSL lamp is to transport heat from LED die to a heat sink and keep the thermal stresses in all layers as low as possible. The work focuses on improving of new LED board thermal management. Moreover, reliability and lifetime of LED board has been inspected by numerical calculation and validated by experiment. Thermally induced stress has been studied for wide temperature range that can affect the LED boards (-40 to +125°C). Numerical modelling of thermal performance, thermal stress distribution and lifetime has been carried out with ANSYS structural analysis where temperature dependent stress-strain material properties have been taken into account. The objective of this study is to improve not only the thermal performance of new LED board, but also identification of potential problems from mechanical fatigue point of view. Accelerated lifetime testing (e.g., mechanical) is carried out in order to study the failure behaviour of current and newly developed LED board

Transport in tunnelling recombination junctions: a combined computer simulation study

The implementation of trap-assisted tunneling of charge carriers into numerical simulators ASPIN and D-AMPS is briefly described. Important modeling details are highlighted and compared. In spite of the considerable differences in both approaches, the problems encountered and their solutions are surprisingly similar. Simulation results obtained for several tunneling recombination junctions made of amorphous silicon (a-Si), amorphous silicon carbide (a-SiC), or microcrystalline silicon (µc-Si) are analyzed. Identical conclusions can be drawn using either of the simulators. Realistic performances of a-Si/a-Si tandem solar cells can be reproduced with simulation programs by assuming that extended-state mobility increases exponentially with the electric field. The same field-enhanced mobilities are needed in single tunneling recombination junctions in order to achieve measured current levels. Temperature dependence of the current-voltage characteristics indicates that the activation energy of enhanced mobilities should be determined. Apparent discrepancies between simulation results and measurements are explained and eliminated making use of Gill’s law. For application in tandem and triple solar cell structures, tunneling recombination junctions made of (µc-Si) are the most promising of all examined structures.Fil: Vukadinovic, M.. University of Ljubljana; EsloveniaFil: Smole, F.. University of Ljubljana; EsloveniaFil: Topič, M.. University of Ljubljana; EsloveniaFil: Schropp, R. E. .. Utrecht University; Países BajosFil: Rubinelli, Francisco Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentin

Pure Space-Charge-Limited Electron Current in Silicon

Phosphorus diffusion on π‐type silicon is used to fabricate n^+πn^+ structures of base widths between 3 μ and 60 μ with π‐type resistivities of 300 Ω⋅cm and 8 kΩ⋅cm. The V‐I characteristics of the structures are measured at room temperature and at liquid‐nitrogen temperature. The change in current for constant applied voltage is also observed in that temperature range. The results are interpreted in terms of simple models based on the assumption that pure space‐charge‐limited current of electrons is present. The models describe well the characteristics measured on 300‐Ω⋅cm samples, except for the range of small biases on the thinnest samples. It is concluded that the drift velocity of electrons at 78°K tends towards saturation at 1.0×10^7 cm∕sec ± 10%. The current observed at this temperature actually reaches this value. The critical electric field at 78°K is 10^3 V∕cm±30% but the meaning of this concept for electrons in silicon is vague. The temperature dependence of the current at fixed bias voltages is in general agreement with the variation of the low field mobility. Results obtained on 8‐kΩ⋅cm samples need clarification. Effects of breakdown and trapping are not observed

Post-test simulations for the NACIE-UP benchmark by STH codes

This paper illustrates the results obtained in the last phase of the NACIE-UP benchmark activity foreseen inside the EU SESAME Project. The purpose of this research activity, performed by system thermal–hydraulic (STH) codes, is finalized to the improvement, development and validation of existing STH codes for Heavy Liquid Metal (HLM) systems. All the participants improved their modelling of the NACIE-UP facility, respect to the initial blind simulation phase, adopting the actual experimental boundary conditions and reducing as much as possible sources of uncertainty in their numerical model. Four different STH codes were employed by the participants to the benchmark to model the NACIE-UP facility, namely: CATHARE for ENEA, ATHLET for GRS, RELAP5-3D© for the “Sapienza” University of Rome and RELAP5/Mod3.3(modified) for the University of Pisa. Three reference tests foreseen in the NACIE-UP benchmark and carried out at ENEA Brasimone Research Centre were analysed from four participants. The data from the post-test analyses, performed independently by the participant using different STH codes, were compared together and with the available experimental results and critically discussed

Fluctuation Dominated Josephson Tunneling with a Scanning Tunneling Microscope

We demonstrate Josephson tunneling in vacuum tunnel junctions formed between a superconducting scanning tunneling microscope tip and a Pb film, for junction resistances in the range 50-300 k$\Omega$. We show that the superconducting phase dynamics is dominated by thermal fluctuations, and that the Josephson current appears as a peak centered at small finite voltages. In the presence of microwave fields (f=15.0 GHz) the peak decreases in magnitude and shifts to higher voltages with increasing rf power, in agreement with theory.Comment: 4 pages, REVTeX, submitted to PR