Concepts to enhance the efficiency of upconversion for solar applications

Upconversion of otherwise lost sub-band-gap photons is a promising approach for more efficient solar cells. We investigate upconverter materials based on lanthanides, especially trivalent erbium. They are known for high upconversion efficiency of infrared photons under laser excitation at a wavelength around 1520 nm. However, the achieved upconversion efficiency is still not large enough and the absorption range of these materials is too narrow for an application in photovoltaics. Herein, we present an overview of different possibilities to enhance the efficiency of upconversion for silicon solar cells. The concepts discussed can be divided into two groups. The first group comprises internal concepts, e.g., the host material itself, size effects and dopant concentration. The second group consists of external methods, which change the physical environment around the upconverter to improve the absorption properties and enhance the upconversion quantum yield. By considerin g the different effects in a sophisticated rate equation model of the upconverting material -NaY 0.8Er 0.2F 4, and comparing the results with experimental data, we show that there is a big potential to improve the upconversion properties for solar applications. Furthermore we show variety opportunities to increase the upconversion quantum yield are.