Abstract
Intermediate band solar cells hold the promise of ultrahigh power conversion efficiencies using a single semiconductor junction. Many current implementations use materials with bandgaps too small to achieve maximum efficiency or use cost-prohibitive substrates. Here we demonstrate a material system for intermediate band solar cells using InGaN/GaN quantum-dot-in-nanowire heterostructures grown directly on silicon to provide a lower cost, large-bandgap intermediate band solar cell platform. We demonstrate sequential two-photon current generation with sub-bandgap photons, the hallmark of intermediate band solar cell operation, through vertically stacked quantum dots in the nanowires. Near-infrared light biasing with an 850 nm laser intensity up to 200 W/cm2 increases the photocurrent above and below the bandgap by up to 19% at 78 K, and 44% at room temperature. The nanostructured III-nitride strategy provides a route towards realistic room temperature intermediate band solar cells while leveraging the cost benefits of silicon substrates.
Highlights
Intermediate band solar cells hold the promise of ultrahigh power conversion efficiencies using a single semiconductor junction
To realize efficient intermediate band solar cell (IBSC), the absorption in the quantum dots must be increased through light trapping and/or adding more quantum dots
While perfectly Lambertian scattering is known to provide a maximum path length enhancement of 4n2, where n is the index of refraction, absorption increase via light trapping has been shown to surpass the 4n2 limit in certain nanostructures[28]
Summary
Intermediate band solar cells hold the promise of ultrahigh power conversion efficiencies using a single semiconductor junction. While traditional solar cells are subject to the Shockley–Queisser limit[10], intermediate band solar cell (IBSC) concepts increase both current and voltage while still using a single junction[11]. Such designs enable the harvesting of energy from sub-bandgap photons through intermediate states deep inside the semiconductor bandgap that act as steppingstones for photogenerated carriers to reach the conduction band while operating at the higher voltage associated with the large bandgap. With a 6000 K black body spectrum, the optimal bandgaps for an IBSC should be 1.95 eV and 0.7 eV under full concentration, and 2.4 eV and 0.9 eV under 1-sun illumination[12]; these high bandgaps are unavailable with most III–V semiconductors
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