Photonic band engineering in absorbing media for spectrally selective optoelectronic films.

Abstract

Spectrally selective materials are of great interest for optoelectronic devices in which wavelength-selectivity of the photoactive material is necessary for applications such as multi-junction solar cells, narrow-band photodetectors, transparent photovoltaics, and tailored emission sources. Achieving controlled transparency or opacity within multiple wavelength bands in the absorption, reflection, and transmission spectra are difficult to achieve in traditional semiconductors that typically absorb at all energies above their electronic band gap and is generally realized by the use of external bandpass filters. Here, we propose an alternate method for achieving spectral selectivity in optoelectronic thin films: the use of photonic band engineering within the absorbing region of a semiconductor in which resonant photonic bands are strongly coupled to the external reflectivity and transmission spectra. As a first step, we use optical simulations to systematically study the effect of material absorption on the properties of the photonic bands in a photonic crystal slab structure. We find that adding a weak loss to the materials model does not appreciably change the frequencies of the photonic bands but does reduce the quality factor of the associated photonic modes. Critically, the radiating photonic bands induce strong Fano resonance features in the transmission and reflection spectra, even in the presence of material absorption, due to coupling between the bands and external electromagnetic plane waves. These resonances can be tuned by adjusting the photonic crystal structural properties to induce spectral selectivity in the absorbing region of semiconductors. Lastly, we demonstrate this tuning method experimentally by fabricating a proof-of-principle photonic structure consisting of a self-assembled polystyrene bead monolayer infiltrated with PbS CQDs that displays both near-infrared absorption enhancement and visible transparency enhancement over a homogeneous control film, qualitatively matching predictions and showing promise for optoelectronic applications.