Photonic crystal-driven spectral concentration for upconversion photovoltaics

The main challenge for applying upconversion (UC) to silicon photovoltaics is the limited amount of solar energy harvested directly via erbium-based upconverter materials (24.5 W m–2). This could be increased up to 87.7 W m–2 via spectral concentration. Due to the nonlinear behavior of UC, this could increase the best UC emission by a factor 13. In this paper, the combined use of quantum dots (QDs)—for luminescent down-shifting—and photonic crystals (PCs)—for reshaping the emission—to achieve spectral concentration is shown. This implies dealing with the coupling of colloidal QDs and PC at the high-density regime, where the modes are shifted and broadened. In the first fabricated all-optical devices, the spectral concentration rises by 67%, the QD emission that matches the absorption of erbium-based upconverters increases by 158%, and the vertical emission experiences a 680% enhancement. Remarkably, the PC redshifts the overall emission of the QDs, which could be used to develop systems with low reabsorption losses. In light of this, spectral concentration should be regarded as one of the main strategies for UC photovoltaics.