Upconversion : making the invisible - visible for efficient silicon solar cells
MacDougall, Sean Kye Wallace
MetadataShow full item record
Upconversion (UC) for photovoltaics (PV) aims to mitigate the intrinsic losses associated with the transmission of sub-band gap photons. The potential enhancement of a PV device via a coupled UC layer has not been realised experimentally due to the weak absorption coefficient of ~6 cm-1 at 1523 nm across a narrow bandwidth (100 nm) and non-linear nature of the active UC erbium ion (Er3+). These limiting properties have resulted in the characterisation of the UC layer conducted with high power monochromatic lasers at the strongest resonant wavelength which is dissimilar to that of the solar spectrum. Herein, a novel method is developed to determine the photoluminescent quantum yield (iPLQY) of an UC layer under broadband excitation through the correct determination of the absorption spectrum unaffected by re-emission. This method is used to evaluate the iPLQY of the UC material under increasing bandwidths of excitation where saturation is seen for bandwidths greater than 60 nm, significantly shorter than the 100 nm absorption spectrum. Although an iPLQY of 16.2% was achieved, with the normalisation to the solar spectrum this equates to a solar concentration beyond what is attainable on Earth. Therefore, advanced photon management techniques are necessary to realise a working device. An in-depth investigation is conducted on the optimisation of an UC material across a wide range of solar concentrations and doping of the active UC ion which indicates β-NaYF4: 25% Er3+to be the most suitable for a coupled PV device.