Ureasil organic–inorganic hybrids as photoactive waveguides for conjugated polyelectrolyte luminescent solar concentrators

Abstract

The development of an efficient luminescent solar concentrator (LSC), with minimised optical losses, requires careful consideration of its principal constituting materials, a waveguide and a luminophore, in tandem. Here, a series of LSCs are fabricated utilising a poly(fluorene-alt-phenylene) copolymer containing on-chain perylenediimide (PDI) chromophore units as the luminophore (PBS-PFP-PDI) immobilised within a poly(oxyalkylene)/siloxane organic–inorganic hybrid, known as a ureasil, as the waveguide. PBS-PFP-PDI and the ureasil both function as photoactive components, offering the possibility of energy transfer between the ureasil host and/or the PBS-PFP donor chains to the PDI acceptor, leading to reduced re-absorption losses and harvesting a broader wavelength range of the solar spectrum. A combination of studies using UV/vis absorption, Fourier transform infrared, steady-state and time-resolved photoluminescence spectroscopies revealed that the branching of the ureasil framework influences the packing of the polymer chains, with the tri-podal ureasil structure facilitating improved dispersion of the PBS-PFP-PDI chains, while the linear di-ureasil structure promotes more intimate mixing of the PBS-PFP-PDI and the ureasil. Picosecond time-correlated single photon counting measurements showed that strong spectral overlap, combined with efficient electronic coupling results in efficient excitation energy transfer from the ureasil to emissive trap sites localised on the PBS-PFP unit. This process inhibits subsequent energy transfer to the PDI chromophore, but leads to high solid-state photoluminescence quantum yields of >50%. The optical efficiency of the PBS-PFP-PDI–ureasil composites as LSCs was evaluated under AM1.5G solar simulated light delivering values of up to 5.6% using a scattering background, which could be boosted to 13.1% by increasing the percentage of PDI units per PBS-PFP chains using a model system. The results demonstrate that consideration of the combined photophysical properties of the luminophore and the waveguide are crucial to the design of next generation LSCs.