Solar energy trapping with modulated silicon nanowire photonic crystals

Abstract

We demonstrate the efficacy of nanostructured thin film silicon solar cells to trap and absorb approximately 75% of all sunlight incident (400 nm–1200 nm) with an equivalent bulk thickness of only 1 micron of silicon. This is achieved by sculpting the collection zone into a three-dimensional, simple-cubic-symmetry, photonic crystal consisting of modulated silicon nanowires embedded in SiO2 and sitting on a quartz substrate with no metallic mirrors. A specific modulation of the radius of nanowires provides antireflection, strong light trapping, and back-reflection mechanisms in targeted spectral regions. This modulation is linear at the top of the nano-rods leading to nanocones at the solar cell to air boundary. These silicon nanocones are very good absorbers at short wavelengths and act as broadband coupler to a light-trapping region below at longer wavelengths. In the light trapping region the modulation is periodic to form a simple cubic photonic crystal exhibiting a broad spectrum of strong parallel interface refraction resonances. Here, light incident from most angles is deflected into slow group velocity modes with energy flow nearly parallel to the interface, long dwell times, and strong light intensity enhancement (up to 150 times the incident intensity) in specific regions. Finally, a stronger and chirped modulation of the nanowire underneath provides back-reflection by means of a one-dimensional depth-dependent photonic stop-gap. The possibility of absorbing light at energies below the electronic band gap of silicon is illustrated using a graded index SixGe1−x alloy in the bottom section of each nanowire. Each nanowire is amenable to a radial P-N junction for proximal charge carrier separation and efficient collection of photo-generated current.