From sun to screen: time-resolved spectroscopy of organic solar cells and hyperfluorescent OLEDs

In this thesis the photophysical properties of organic semiconductors for use in organic light-emitting diodes (OLEDs) and organic solar cells are investigated. Interest in these technologies has been motivated by their low-cost material and fabrication costs. OLEDs have already become a leading display technology whilst performance of organic photovoltaics have seen significant increases over the past decade. This thesis employs a variety of experimental methods in order to characterize device performance and measure rates of photophysical processes in thin films. First the impact of terminal emitter addition on hyperfluorescent OLED performance is investigated. It is shown that the terminal emitter can cause higher rates of singlet triplet annihilation and worsen efficiency roll-off. The impact of the terminal emitter on the PL decay of the sensitizer in thin hyperfluorescent films is also studied. A novel model for the sensitizer PL decay with a time dependent rate of Förster resonance energy transfer (FRET) is derived and effects of terminal emitter aggregation are explored. FRET and aggregation are additionally shown to have a significant effect on the performance of D18:Y6 organic solar cells. A study of thin films of D18:Y6 shows that efficient charge generation in D18:Y6 solar cells is due to energy transfer from D18 to Y6 followed by exciton diffusion to low energy crystalline sites of Y6. These low energy sites provide a stronger driving force for hole transfer and improve charge generation. Finally, the performance of D18:Y6 and two novel blends of D18:SF-PDI and D18:FBR are evaluated in conditions simulating underwater spectra to assess their potential for use as power sources for underwater sensors. Spectra from a variety of geographical locations and depths are simulated and it is shown that reasonable efficiencies and power outputs can be achieved by underwater organic photovoltaics.