Strongly enhanced upconversion in trivalent erbium ions by tailored gold nanostructures: toward high-efficient silicon-based photovoltaics

TL;DR

This study demonstrates a significant enhancement in erbium-based photon upconversion efficiency using tailored gold nanostructures, paving the way for more efficient silicon solar cells beyond current limits.

Contribution

It introduces a novel design of gold nanostructures optimized for maximal electric-field enhancement to boost erbium upconversion efficiency in solar applications.

Findings

Achieved upconversion enhancement factor of 913 at 1.7 Wcm$^{-2}$ excitation.

Developed a semi-empirical model for photonically enhanced upconversion efficiency.

Fabricated nanostructures via electron beam lithography based on topology optimization.

Abstract

Upconversion of sub-band-gap photons constitutes a promising way for improving the efficiency of silicon-based solar cells beyond the Shockley-Queisser limit. 1500 to 980 nm upconversion by trivalent erbium ions is well-suited for this purpose, but the small absorption cross section hinders real-world applications. We employ tailored gold nanostructures to vastly improve the upconversion efficiency in erbium-doped TiO$_2$ thin films. The nanostructures are found using topology optimization and parameter optimization and fabricated by electron beam lithography. In qualitative agreement with a theoretical model, the samples show substantial electric-field enhancements inside the upconverting films for excitation at 1500 nm for both s- and p-polarization under a wide range of incidence angles and excitation intensities. An unprecedented upconversion enhancement of 913(51) is observed at an excitation intensity of 1.7 Wcm$^{-2}$. We derive a semi-empirical expression for the photonically enhanced upconversion efficiency, valid for all excitation intensities. This allows us to determine the upconversion properties needed to achieve significant improvements in real-world solar-cell devices through photonic-enhanced upconversion.