2, Trinity College Dublin, Dublin, , Ireland
Single junction photovoltaic devices exhibit a bottleneck in their efficiency due to incomplete or inefficient harvesting of photons in the low- or high-energy regions of the solar spectrum. This can be overcome through the retro-fitting of a spectral converter to the device, which is used to convert solar photons into energies that are more effectively captured by the solar cell through a photoluminescence process. However, while a lumophore may show seemingly ideal optical characteristics for spectral conversion in an ideal solution (high emission quantum yield, strong absorption), disappointment frequently awaits on its translation to the solid-state, where aggregation and quenching effects lead to significantly reduced photoluminescence yields.
In an effort to overcome this limitation, our research focusses on the bottom-up design of integrated lumophore-host materials for solar spectral converters, in which materials chemistry design strategies are used to control the packing, orientation and placement of p-conjugated lumophores in solid-state host materials. Since the electronic properties depend explicitly on the arrangement and packing of the p-conjugated species, this approach provides a means of modulating the optical properties. In this talk, we will report our recent results on the design of Π-conjugated composite materials that utilise a family of organic-inorganic hybrid polymers known as the ureasilsas the host. Ureasils are comprised of a siliceous skeleton that is chemically-grafted to poly(ethylene oxide) (PEO)/poly(propylene oxide) (PPO) chains through urea cross-linkages, the number of which depends on the degree of branching in the organic polymer precursor. Ureasils are intrinsically photoluminescent, exhibit high refractive indices and function as optical waveguides. Through judicious selection of the degree of branching and length of the organic backbone and the incorporation method (grafting vsimmobilization vs permeation), we can control the packing, orientation and placement of the Π-conjugated species in the ureasil host. This in turn provides a means of modulating the optical properties. For example, a dramatic enhancement in the emission quantum yield to >60% is observed due to exciton localization at isolated nanodomains of a conjugated polyelectrolyte entrapped within the ureasil host. Similarly, Förster resonance energy transfer from the ureasil to embedded or grafted conjugated lumophores can be exploited to tune the emission color and even obtain white-light emission. These characteristics can be exploited to improve light-harvesting and trapping within the integrated material, which can be used to develop highly efficient LSCs to enhance the performance of silicon solar cells.