Single-junction photovoltaic devices suffer from intrinsic obstacles limiting their efficiency to a top value dictated by the well-known Shockley-Queisser (SQ) limit . The development of photodiode devices on micro and nanophotonic structures has opened new possibilities over the standard technology. The impinging light is strongly confined inside those photonic structures, enhancing optical absorption and photocarrier generation, as it has been reported for planar optical cavities and, more recently, for nanowire resonators [2,3]. The most fundamental limitation is given by the energy bandgap of the semiconductor, which determines the minimum energy of photons that can be converted into electron-hole pairs. In the case of silicon a large percentage of infrared sunlight, with energy value below the fundamental absorption edge of silicon, is still useless.
Here we show the first example of a photodiode developed on a micrometer size silicon spherical cavity whose photocurrent shows the Mie modes of a classical spherical resonator. The long dwell time of resonating photons enhances the absorption efficiency of photons. Also the photocurrent response shows very rich spectra with plenty of high-Q resonant peaks in a similar manner as the scattering spectra of high order whispering gallery modes (WGMs) of spherical microcavities. Also, as a consequence of the enhanced resonant absorption, the photocurrent response extends far below the bandgap of crystalline silicon . It opens the door for developing a new generation of solar cells and photodetectors that may harvest infrared light more efficiently than silicon based photovoltaic devices.References:
1. Schockley, W. & Queisser, H. J. J. Appl. Phys. 32, 510-519, (1961).
2. Wallentin, J. et al. Science 339, 1057, (2013).
3. Krogstrup, P. et al. Nature Photon. 7, 306—310, (2013).
4. Garin, M., Fenollosa, Shi L., Alcubilla R., Marsal Ll., and Meseguer F. Submitted for publication.