High Efficiency Mechanically Stacked Multi-Junction Solar Cells for Concentrator Photovoltaics

摘要

Multi-junction solar cells based on III-V materials have achieved the highest efficiencies of any present photovoltaic technology. Combined with their application in terrestrial concentration systems, multi-junction solar cells offer a promising way towards achieving very low cost per kilowatt hour.The objective of this PhD thesis is to study and develop novel high efficiency Mechanically Stacked Multi-Junction (MSMJ) solar cells. The sub-junctions in a mechanical stack are fabricated separately on their individual substrates and then combined into one single mechanically stacked multi-junction device. As a result, each junction has its own contacts and can be connected separately. In contrast, the Monolithically stacked Multi-Junction (MMJ) solar cells have different solar cell materials grown directly on a single substrate, and connected in series by tunnel diodes. Although currently being state-of-the-art, there are some inherent restrictions for the monolithic design, such as the requirements of current matching and lattice matching. However, mechanically stacked junctions do not suffer from these limitations, and offer a very generic way of realizing multi-junction solar cells. The strongly reduced sensitivity to spectral variations offered by MSMJ solar cells also results in a clear advantage in terms of energy production potential.The main topics of this PhD research include the development of thinned III-V top cells and high flux germanium bottom cells; the development of mechanical stacking technology of the sub-cells and their interconnections; understanding and predicting the performance of the MSMJ solar cells through numerical simulation of the optical, electrical and thermal characteristics of the stack.Very high performance thin film III-V solar cells have been demonstrated, with the best efficiencies of 24.7% for GaAs and 16.7% for InGaP respectively (under AM1.5g), which comes very close to the best standard 1-sun cells fabricated at imec. Ge bottom cells have adopted a deeper and more highly doped emitter profile, optimized for the long wavelength range above 900nm. After the demonstration of individual sub-cells, mechanically stacked GaAs-Ge dual-junction cells and InGaP-GaAs-Ge triple-junction cells are demonstrated as well. Efficiencies of 27.0% (24.7 + 2.3%) under AM1.5g and 31.3% (27.7 + 3.6%) at 29 suns are achieved for the best 2J mechanical stack, while 26.2% (16.5 + 8.40 + 1.28%) under AM1.5g, and 28.2% (17.0 + 9.0 + 2.2%) under 7 suns are realized for the best 3J mechanical stack. The reflection losses and series resistance losses are identified as the biggest losses limiting performance of the realized stacks. Further efficiency improvements are possible, such as using grid designs with finger width of less than 3 µm.In parallel to the practical work, a 2D numerical simulation model has been set up and calibrated with experimental results, in order to understand and predict the performance of the MSMJ solar cells. An efficiency of 32.18% (17.0 + 11.5 + 3.6%, under AM1.5g) is simulated for a mechanically stacked InGaP-GaAs-Ge solar cell. In addition, annual energy production utilizing multi-junction solar cells has been predicted. Detailed balance cell efficiencies are calculated under realistic spectral conditions. For the particular case studied, the MMJ solar cells underperform the MSMJ cells in term of energy production due to the influence of daily and yearly spectral variations.