Highly efficient GaAs solar cells by limiting light emission angle

In a conventional flat plate solar cell under direct sunlight,light is received from the solar disk,but is re-emitted isotropically.This isotropic emission corresponds to a significant entropy increase in the solar cell,with a corresponding drop in efficiency.Here,using a detailed balance model,we show that limiting the emission angle of a high-quality GaAs solar cell is a feasible route to achieving power conversion efficiencies above 38%with a single junction.The highest efficiencies are predicted for a thin,light trapping cell with an ideal back reflector,though the scheme is robust to a non-ideal back reflector.Comparison with a conventional planar cell geometry illustrates that limiting emission angle in a light trapping geometry not only allows for much thinner cells,but also for significantly higher overall efficiencies with an excellent rear reflector.Finally,we present ray-tracing and detailed balance analysis of two angular coupler designs,show that significant efficiency improvements are possible with these couplers,and demonstrate initial fabrication of one coupler design.

Spontaneous and stimulated electron-photon interactions in nanoscale plasmonic near fields

The interplay between free electrons,light,and matter offers unique prospects for space,time,and energy resolved optical material characterization,structured light generation,and quantum information processing.Here,we study the nanoscale features of spontaneous and stimulated electron–photon interactions mediated by localized surface plasmon resonances at the tips of a gold nanostar using electron energy-loss spectroscopy(EELS),cathodoluminescence spectroscopy(CL),and photon-induced near-field electron microscopy(PINEM).Supported by numerical electromagnetic boundary-element method(BEM)calculations,we show that the different coupling mechanisms probed by EELS,CL,and PINEM feature the same spatial dependence on the electric field distribution of the tip modes.However,the electron–photon interaction strength is found to vary with the incident electron velocity,as determined by the spatial Fourier transform of the electric near-field component parallel to the electron trajectory.For the tightly confined plasmonic tip resonances,our calculations suggest an optimum coupling velocity at electron energies as low as a few keV.Our results are discussed in the context of more complex geometries supporting multiple modes with spatial and spectral overlap.We provide fundamental insights into spontaneous and stimulated electron-light-matter interactions with key implications for research on(quantum)coherent optical phenomena at the nanoscale.