Abstract
We demonstrate nearly 30% power conversion efficiency in ultra-thin (~200 nm) gallium arsenide photonic crystal solar cells by numerical solution of the coupled electromagnetic Maxwell and semiconductor drift-diffusion equations. Our architecture enables wave-interference-induced solar light trapping in the wavelength range from 300–865 nm, leading to absorption of almost 90% of incoming sunlight. Our optimized design for 200 nm equivalent bulk thickness of GaAs, is a square-lattice, slanted conical-pore photonic crystal (lattice constant 550 nm, pore diameter 600 nm, and pore depth 290 nm), passivated with AlGaAs, deposited on a silver back-reflector, with ITO upper contact and encapsulated with SiO2. Our model includes both radiative and non-radiative recombination of photo-generated charge carriers. When all light from radiative recombination is assumed to escape the structure, a maximum achievable photocurrent density (MAPD) of 27.6 mA/cm2 is obtained from normally incident AM 1.5 sunlight. For a surface non-radiative recombination velocity of 103 cm/s, this corresponds to a solar power conversion efficiency of 28.3%. When all light from radiative recombination is trapped and reabsorbed (complete photon recycling) the power conversion efficiency increases to 29%. If the surface recombination velocity is reduced to 10 cm/sec, photon recycling is much more effective and the power conversion efficiency reaches 30.6%.
Highlights
Wave interference effects that can provide stronger light-trapping and solar absorption using considerably thinner photonic crystal solar cells[7,12]
As we show below, this provides up to 2% additive increase in overall power conversion efficiency using only 200 nanometers equivalent bulk thickness of Gallium Arsenide (GaAs)
We present full 3D numerical simulations of photo-current, voltage and power conversion efficiencies in thin-film photonic crystal solar cells based on slanted conical nano-pores in GaAs8
Summary
We calculate solar absorption inside our solar cell using the standard FDTD algorithm[32,33], in which a plane wave impulse having Berenger’s form[32] with a broad spectrum impinges onto the structure. Assuming that each absorbed photon leads to the generation of a single e-h pair, we calculate an initial charge carrier generation rate (in units of number per unit time, per unit volume) produced by the incident sunlight. This generation rate is obtained by the integration of the calculated absorption α (ω, r ) with the incident Air Mass Global Spectrum intensity I(ω) over the wavelength range 300–865 nm:. We assume that all light from radiative recombination is trapped by the photonic crystal architecture and re-absorbed very close to the emitter The latter is implemented by setting Grecycle =Rrad throughout the sample. Our light-trapping photonic crystal architecture enables re-absorption of nearly 70% of the re-emitted light
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