Alain Dollet

Approaching the Shockley-Queisser limit: General assessment of the main limiting mechanisms in photovoltaic cells

In principle, the upper efficiency limit of any solar cell technology can be determined using the detailed-balance limit formalism. However, “real” solar cells show efficiencies which are always below this theoretical value due to several limiting mechanisms. We study the ability of a solar cell architecture to approach its own theoretical limit, using a novel index introduced in this work, and the amplitude with which the different limiting mechanisms affect the cell efficiency is scrutinized as a function of the electronic gap and the illumination level to which the cell is submitted. The implications for future generations of solar cells aiming at an improved conversion of the solar spectrum are also addressed.

Rate constants of reactions involving SiH4 as an association product from quantum Rice–Ramsperger–Kassel calculations

Quantum Rice Ramsperger Kassel (QRRK) calculations are carried out to calculate the pressure-dependent rate constants of reactions involving SiH4 as an association product. It is shown that good estimates of pressure-dependent rate constants can be obtained from QRRK in spite of the very poor treatment of the activated complex in this simple theory. However, possible errors arising from energy truncation must be avoided and reliable kinetic data must be fitted in order to select the final value of the adducts mean vibrational frequency. The results obtained in this work are satisfactorily compared with the experimental data available and with previous RRKM calculations. Parameterized forms of the pressure-dependent rate constants are derived for several bath gases, which can be used over the whole pressure and temperature range of interest for chemical vapour deposition (CVD) and related processes.

Determining passive cooling limits in CPV using an analytical thermal model

We propose an original thermal analytical model aiming to predict the practical limits of passive cooling systems for high concentration photovoltaic modules. The analytical model is described and validated by comparison with a commercial 3D finite element model. The limiting performances of flat plate cooling systems in natural convection are then derived and discussed.

Assessing the Efficiency of Advanced Multijunction Solar Cells in Real Working Conditions: A Theoretical Analysis

Multijunction (MJ) solar cells are currently seen as one of the most promising options toward achieving solar energy to electricity conversion efficiencies exceeding 50%. The detailed balance theory provides a practical tool for tailoring MJ solar cells, taking several strong assumptions regarding the spectral input or the cell temperature. However, solar cells in real working operation may meet conditions that differ significantly from the ideal conditions for which they were designed. Assessing the extent to which advanced MJ solar cell architectures are able to withstand changes in the operating conditions is thus crucial toward ensuring an efficient use of MJ solar cells in real working operation. In this paper, the sensitivity of current-constrained MJ solar cells to operating conditions was studied by quantifying the decrease in the conversion efficiency associated with a change in the spectral input, the illumination level, and the cell working temperature. The sensitivity of MJ solar cells to the spectral content of the light was shown to be significant for cell architecture involving four subcells and more. On the contrary, the gain in efficiency achieved by tailoring the combination of bandgap to a specific value of the working temperature or the illumination level was demonstrated to be low. The implications for future generations of MJ solar cells comprising up to ten subcells are also addressed.

Uniform temperature profile for a dense array CPV receiver under non uniform illumination profile

Previous experimental and numerical studies of hybrid cooling devices for CPV receivers were developed under uniform illumination profile conditions; but literature review shows that this uniformity assumption is difficult to satisfy in real conditions. This investigation presents the design and the validation of a hybrid cooling device able to tailor its local heat extraction capacity to 2D illumination profiles in order to provide a uniform temperature profile of the PV receiver as well as a low global thermal resistance coefficient. The inputs of the design procedure are the solar concentration, the coolant flow rate and its inlet temperature. As the illumination profile is 2D dependent, a matrix of pin fins is implemented and a hybrid Jet Impingement /Matrix of Pin Fins cooling device is experimentally tested and compared to a hybrid Jet Impingement / Microchannels cooling device developed previously. The results demonstrate similar performances for both designs. Furthermore, in contrast to the cooling scheme using longitudinal fins, the distribution of the pin fins can be tailored, in two dimensions, to the local need of heat extraction capacity.

Outdoor Characterization and Performance Evaluation of Integra‐Sun Prototype CPV Module

A prototype of concentrating photovoltaic (CPV) module developed by Integra‐Sun was characterized in this work. Characterizations were mostly performed outdoor, in real sunlight conditions. The characterization procedure is described in details, and various results are presented for the overall efficiency of the module as well as for the efficiency and quality of its main components: primary concentrator, secondary optics, solar cell cooling device. The performances of the module were assessed and compared to the target performances. The maximum overall DC efficiency was only 23%, instead of the expected 28% efficiency. Our analysis showed that all components meet the requirements except the Fresnel lens. Re‐machining the lens should allow the module to reach its target efficiency, but other criteria will be also considered for the development of an improved version of the module.

Cu(In,Ga)Se2 photovoltaic microcells for high efficiency with reduced material usage

Cu(In,Ga)Se2 microcells are photovoltaic devices of increased efficiency and low semiconductor consumption. They show an increase in efficiency due to concentrated illumination up to more than ×100, which is a breakthrough as thin films were previously limited to low concentration applications (about 10 suns). New measurements, made under concentrated natural solar illumination are presented, which confirm the conclusions of laser experiments. We also extend our approach to an other direction, that of using thin Cu(In,Ga)Se2 layers. This reduces further the volume of the solar cells and gives an insight in the effect of thickness as a key parameter controlling the performances of thin film microcells. On thinner microcells, optimum efficiencies are reached at illumination intensities over ×400. Due to their favorable architecture, microcells present efficient resistive and thermal management, leading to gains in efficiency and material usage.

Gas-phase kinetics analysis and reduction of large reaction mechanisms exemplified in the case of SiC deposition

Abstract Chemical vapor deposition (CVD) is commonly used to grow epitaxial SiC layers at relatively high temperature from SiH 4 –C 3 H 8 –H 2 mixtures. In order to model the deposition process, the kinetics of the gas-phase reactions involved must be known accurately and the main reaction pathways must be identified. For this purpose, we have selected the most recent sets of reactions and related kinetic data available for the C–H and Si–H chemical systems. However, the full mechanism so obtained is too large to be included in a CVD reactor model, because of the computational time required for the calculations and numerical convergence problems. As a consequence, the full mechanism has been reduced using the so-called ‘redundant species and reactions’ method due to Turanyi. Since reactions between Si–H species and C–H species have not been considered, the C–H and Si–H mechanisms have been reduced separately. As a result, a smaller subset of reactions and species describing accurately the gas-phase reactivity in the Si–C–H system has been obtained.

Design and characterization of a curved linear Fresnel lens concentrating photovoltaic and thermal system

The performances of a concentrating photovoltaic and thermal module developed by the French company Idhelio were assessed in this study. The characterization of the curved linear Fresnel lens-based module was performed in real sunlight conditions using a 2-axis solar tracker. The behavior of the different components was evaluated, as well as the overall electric and thermal efficiencies of the module which were finally compared to the target performances.

Assessment of a Phase Change Material (PCM) System for Moderating Temperature Rise of Solar Cells under Concentrated Sunlight

Experiments and simulations were carried out to assess a passive device for cooling photovoltaic cells under concentrated sunlight. The cooling device was made of a Graphite- Phase Change Material (PCM) composite inserted in an aluminum enclosure. The PCM considered in this work was selected among several commercially available materials. Experimental plots of material temperature versus time were recorded for various incident solar powers and compared to 3D-thermal simulation predictions. Theoretical cell temperature profiles obtained using the PCM-based device were compared to those obtained without PCM, that is, using bulk (PCM-free) aluminum heat sink. The interest of using PCM cooling systems in CPV applications was finally discussed.