• Energy Research

International audience

International audience ; Micro-Concentrator photovoltaics modules promise to overcome the limitations of CPV such as thermal losses or resistive losses. Miniaturization involves new challenges in the field of cells fabrication, particularly the management of perimeter recombinations. In this paper, sub-millimetric InGaP/InGaAs/Ge solar cells with high performances are fabricated. We report record open circuit voltage of 2.39 V and 2.28 V for cells with mesa area of 0.25 mm 2 and 0.04 mm 2 respectively, indicating excellent sidewall passivation. Individual assessment of sub-cells non-radiative losses indicates that the top cell is the most impacted by perimeter recombinations.

Micro-structured anti reflective coatings (ARC) have been identified as a promising solution to reduce optical losses in Concentrator Photovoltaics modules (CPV). We fabricated and tested in field a CPV modules made of 4 sub-modules with a concentration factor of 250x, that embed either solar cells with micro-structured encapsulating ARC or solar cells with multilayer ARC as a reference. The micro-structured encapsulating ARC was made of semi-buried silica beads in polydimethylsiloxane (PDMS). The module was in operation for 1 year in the severe climatic conditions of Sherbrooke, Quebec, Canada, before extracting the sub-modules performance under Concentrator Standard Operating Condition (CSOC). An acceptance angle of +/-0.78 degree was determined for all sub-modules, demonstrating that improving angular collection at the cell level has no significant impact on the angle of acceptance at the module level. We report an increase of 12 to 14% of the short-circuit current and of 15 to 19% of maximum power at CSOC for solar cells with a micro structured encapsulating ARC compared to the reference. Despite a sub-optimal module design, we report a sub-module efficiency of 29.7% at CSOC for a cell with micro-structured encapsulating ARC. This proves the potential of micro-structured encapsulating ARC to improve CPV system performance and shows promise of reliability for sumi-buried microbeads in PDMS as encapsulating ARC. 6 pages 6 figures 3 tables

AbstractA new strategy for the development of III-V/Si tandem solar cells has recently been proposed consisting in low temperature PECVD epitaxy of silicon or silicon-germanium on gallium-arsenide. This paper thus gives first insights about theoretical but realistic maximum performance of such tandem cells by means of full numerical simulations considering perfect layers and interfaces. The consequences of using a thin epi-Si bottom cell instead of a thick silicon substrate are investigated. In case no light trapping scheme is considered, a minimum epi-layer thickness of 20μm is mandatory for the tandem to exhibit higher conversion efficiencies than a single GaAs solar cell. The epi-Si can yet be advantageously replaced by an epitaxial silicon-germanium alloy to increase the bottom cell optical absorption and thus decrease the minimum required thickness by a factor of ∼4 (∼5μm). Finally, simulations show that over 33% efficiency can be obtained for AlxGa1-xAs/epi-Si0.63Ge0.27, which confirms that this is a promising new concept.

Concentrator photovoltaic (CPV) technologies provide the highest photovoltaic conversion efficiency but remain too expensive for very large scale development. Reduction of the dimension (micro-CPV) is a promising approach towards cost reduction but necessitates sub-millimeter-scale high efficiency solar cells. In this paper, we review the challenges faced by sub-millimeter-scale solar cells for application in micro-CPV. We show that plasma etching processes are necessary to fabricate sub-millimeter-scale high-efficiency solar cells to avoid a waste of material in the isolation and dicing lines. We also show that despite the cell performance is known to degrade when the dimension of the cell is downscaled, this degradation can be negligible when optimized etching and passivation processes are used and when the cell operates under high concentration (<500x). The through-cell via contact architecture is a promising approach to avoid bus bars on the front side and therefore optimize the wafer usage and minimize dark current. Combining all these solutions, we claim that sub-millimeter-scale high efficiency solar cells as small as 0.01 mm2 can be fabricated with more than 90% of wafer material used for photovoltaic conversion and without performance degradation when operating under 1,000x concentration compared to 1 mm2 solar cells operating under 500x concentration. Challenges on characterization and in-line metrology remain to be solved and manufacturing lines need now to be adapted to provide commercial solutions for micro-CPV.

International audience ; Local climate and environmental conditions can impact the performance of concentrator photovoltaic (CPV) systems. There is a lack of experimental performance analysis of CPV systems, especially in the region with high snowfall and very low temperature in winters. In this paper, we present first a CPV system performance in humid continental climate and identify snow and frost as sources of losses that are not considered in conventional predictive models. We propose then a method to account for the negative effect of snow and frost on the system, by adding monthly soiling factors in the predictive model. The monthly soiling factors are modeled based on average monthly snow fall and ambient temperature. Applying this method, decrease in Root Mean Square Error (RMSE) between predicted and actual energy production from 24.51 to 5.07 % validates our model in humid continental climate for CPV systems.

Improvement of triple-junction (3J) III-V/Ge solar cells efficiency is hindered by the low current produced by the top and middle cells relative to the bottom cell (Ge). This can be explained by the difficulty of characterizing, on an individual basis, the subcells. We investigate the fabrication process of multi-terminal multi-junction solar cells (MTMJSC) and its potential as a promising architecture to independently characterize subcells of multi-junction solar cells. Here, we study monolithic triple-junction solar cells, with an InGaP top cell, an InGaAs middle cell and a Ge bottom cell interconnected by tunnel junctions. We demonstrate a fabrication process for MTMJSC on commercial wafers for characterization applications purposes. I-V measurements, under illumination, of two-terminals and MTMJSC were compared to validate that the MTMJSC fabrication process does not degrade the cells’ performance. The dark current of each subcell was also measured and an ideal-diode model used to determine the subcells electrical parameters. The results suggest a method to measure the relative absorption and the opto-electrical couplings between the subcells unambiguously, through EQE and electroluminescence measurements, based on basic micro-fabrication processes.