As
single-junction Si solar cells approach their practical efficiency
limits, a new pathway is necessary to increase efficiency in order
to realize more cost-effective photovoltaics. Integrating III–V
cells onto Si in a multijunction architecture is a promising approach
that can achieve high efficiency while leveraging the infrastructure
already in place for Si and III–V technology. In this Letter,
we demonstrate a record 15.3%-efficient 1.7 eV GaAsP top cell on GaP/Si,
enabled by recent advances in material quality in conjunction with
an improved device design and a high-performance antireflection coating.
We further present a separate Si bottom cell with a 1.7 eV GaAsP optical
filter to absorb most of the visible light with an efficiency of 6.3%,
showing the feasibility of monolithic III–V/Si tandems with
>20% efficiency. Through spectral efficiency analysis, we compare
our results to previously published GaAsP and Si devices, projecting
tandem GaAsP/Si efficiencies of up to 25.6% based on current state-of-the-art
individual subcells. With the aid of modeling, we further illustrate
a realistic path toward 30% GaAsP/Si tandems for high-efficiency,
monolithically integrated photovoltaics.
An approximate expression proposed by Green predicts the maximum obtainable fill factor (FF) of a solar cell from its open-circuit voltage (Voc). The expression was originally suggested for silicon solar cells that behave according to a single-diode model and, in addition to Voc, it requires an ideality factor as input. It is now commonly applied to silicon cells by assuming a unity ideality factor—even when the cells are not in low injection—as well as to non-silicon cells. Here, we evaluate the accuracy of the expression in several cases. In particular, we calculate the recombination-limited FF and Voc of hypothetical silicon solar cells from simulated lifetime curves, and compare the exact FF to that obtained with the approximate expression using assumed ideality factors. Considering cells with a variety of recombination mechanisms, wafer doping densities, and photogenerated current densities reveals the range of conditions under which the approximate expression can safely be used. We find that the expression is unable to predict FF generally: For a typical silicon solar cell under one-sun illumination, the error is approximately 6% absolute with an assumed ideality factor of 1. Use of the expression should thus be restricted to cells under very low or very high injection.
Phone: þ480 965 9959, Fax: þ480 965 3837Metal reflectors or electrodes in contact with optoelectronic devices can induce parasitic light absorption. A low-refractiveindex (low-n) layer inserted between the metal reflector and the optically active layer(s) reduces this absorption. We investigate the use of porous, nanoparticulate films as low-n layers, and fabricate silicon solar cells with nanoparticle/silver rear reflectors. We vary the porosity and thus n (between 1.1 and 1.5) of the nanoparticle films, which are deposited by a controllable aerosol spray process, and investigate their effectiveness in reducing infrared parasitic absorption in the solar cells. Optical test structures incorporating films with the highest n exhibit an internal reflectance of over 99%, matching best-in-class structures; lower-n layers should in theory perform better still but their rougher surfaces appear to induce plasmonic absorption in the overlying silver layer. No loss in open-circuit voltage or fill factor is observed when applying the best nanoparticle films in silicon heterojunction solar cells, enabling efficiencies similar to those achieved with reference cells that employ a thick indium tin oxide layer between the wafer and the rear silver electrode.
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