Nanoscale Observation of Waveguide Modes Enhancing the Efficiency of Solar Cells

Abstract

Nanophotonic light management concepts are on the way to advance photovoltaic technologies and accelerate their economical breakthrough. Most of these concepts make use of the coupling of incident sunlight to waveguide modes via nanophotonic structures such as photonic crystals, nanowires, or plasmonic gratings. Experimentally, light coupling to these modes was so far exclusively investigated with indirect and macroscopic methods, and thus, the nanoscale physics of light coupling and propagation of waveguide modes remain vague. In this contribution, we present a nanoscopic observation of light coupling to waveguide modes in a nanophotonic thin-film silicon solar cell. Making use of the subwavelength resolution of the scanning near-field optical microscopy, we resolve the electric field intensities of a propagating waveguide mode at the surface of a state-of-the-art nanophotonic thin-film solar cell. We identify the resonance condition for light coupling to this individual waveguide mode and associate it to a pronounced resonance in the external quantum efficiency that is found to increase significantly the power conversion efficiency of the device. We show that a maximum of the incident light couples to the investigated waveguide mode if the period of the electric field intensity of the waveguide mode matches the periodicity of the nanophotonic two-dimensional grating. Our novel experimental approach establishes experimental access to the local analysis of light coupling to waveguide modes in a number of optoelectronic devices concerned with nanophotonic light-trapping as well as nanophotonic light emission.

Recent Advances in Resonant Waveguide Gratings

“…The implementation of a backside metal reflector has been shown to further raise the absorbance . Finally, the experimental observation of a single waveguide mode enhancing the light‐trapping effect through a 2D RWG was recently carried out with a scanning near‐field optical microscopy (SNOM) …”

“…[102,267] Finally, the experimental observation of a single waveguide mode enhancing the light-trapping effect through a 2D RWG was recently carried out with a scanning near-field optical microscopy (SNOM). [269] Another implementation is related to coherent perfect absorbers (Figure 14a-c), which are lossy structures that allow a complete absorption of coherent illumination, as the time reversal of a laser having the opposite (positive) imaginary part of the refractive index of its gain medium. [270] A coherent perfect absorber based on a thin-film a-Si RWG using two TM polarized beams has been developed.…”

Photonic Structures for Light Trapping in Thin Film Silicon Solar Cells: Design and Experiment

“…For TFSCs, one of the most effective approaches to achieve high performance is to devise a state-of-the-art light harvesting system including in-coupling sufficient light into the solar cells via the front surface (namely light coupling) [8][9][10] and efficiently trapping the incoming light in the thin absorption layers (namely light-trapping) [11][12][13]. For n-i-p TFSCs, light-trapping primarily occurs owing to the textured back reflector.…”

Plasmonic effects and the morphology changes on the active material P3HT:PCBM used in polymer solar cells using Raman spectroscopy

“…In the plasmonic PSC systems, field effects (also called plasmonic effects, including far‐field enhancement (strong resonant light scattering) and near‐field enhancement of electromagnetic radiation) can be detected by spectral measurements or simulation; the morphology and structure can be measured by X‐ray diffraction (XRD) or atomic force microscopy (AFM) . However, field effects and morphology or structure changes cannot be detected simultaneously by the above mentioned measurements.…”