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Organic semiconductors exhibit excellently suited material properties for a variety of applications and offer the possibility of low-cost and large-scale production. This thesis analyses photonic properties of several organic semiconductors, which are of great interest for different applications. To explore these materials, a combination of steady-state and time-resolved optical measurement techniques is employed and energy transfer dynamics within organic semiconductors are modelled. Research was focused on the following areas: The influence of the fraction of chain segments that adapt β-phase conformation on film microstructure of thin films of the blue-emitting polymer PFO is consistently investigated for the first time. An analysis of emission and absorption properties of PFO films with systematically varying β-phase content indicates that with increasing β-phase in PFO films, the chain conformation becomes more planar and includes more repeat units. Energy transfer, which occurs upon photoexcitation from glassy-phase to β-phase chain segments in PFO thin films is evaluated in terms of Förster resonant energy transfer theory. Differences in Förster radii calculated from ultrafast emission dynamics and spectral overlap between steady-state emission and absorption indicate that the energy transfer process is influenced by exciton diffusion within the glassy phase. Parasitic absorption of light in a molecular sun-facing charge extraction layer (CTL) of a metal halide perovskite (MHP)-based solar cell structure is addressed. Experimentally observed energy transfer between layers of photoexcited conjugated polymers and a MHP layer is combined with modelling the efficiency of the energy transfer process. Efficient energy transfer is found to counteract parasitic light absorption in the CTL for thin layers (≤ 10 nm) and/or high PLQE materials. Morphological changes in films of the hole-transport material P3HT deposited on MAPbI3 are explored. Changes in aggregation are observed through careful analysis of absorption and emission spectra from P3HT films of varying thickness and spin speed during their fabrication on a MAPbI3 layer.