AbstractsPhysics

It is well known that the efficiency of photovoltaic cells can benefit from upconversion of sub-bandgap light. Upconversion by triplet-triplet-annihilation is a very promising mechanism for this application. It employs two molecular species, one of which emits higher energy photons via fluorescence upon triplet-triplet-annihilation, whereas the second species sensitizes this process by supplying excitations for the annihilation process. These sensitizers absorb low energy photons with an efficiency that enables successful upconversion already with incoherent sunlight. In the first part of this thesis, we aim to improve the efficiency of the upconversion process by developing a novel structure that better supports upconversion than the state-of-the-art random placement of emitter and sensitizer molecules. To this end, we first establish theoretical approaches for the description of the dynamics within an upconverter material. The triplet-triplet interaction, which requires two triplets and, therefore, scales non-linearly with the number of triplets, prevents an exact solution of the theoretical equations. We show that the state-of-the-art approximate solutions based on effective rate equations are valid only in certain parameter regimes. This motivates the exact simulation of dynamics with numerical methods. By performing kinetic Monte Carlo simulations, we find that the upconversion efficiency can be optimized by tuning the ratio of emitter and sensitizer molecules. Furthermore, we present a structure with new topology designed to minimize losses in the material. This advanced structure prevents the loss mechanisms that are typically introduced in random structures when placing more sensitizer molecules in the material, thus enabling a better absorption of incoming light and ultimately a higher efficiency. We identify the most efficient ratio of emitters-to-sensitizers for our advanced structure and find power law relations for it as a function of the material parameters and light conditions. Moreover, we study the transient behaviour that upconverters exhibit upon turning off the light source. The transient signals can be used to characterize the dynamics. Within our model, we reproduce recent experimental findings on the transient behaviour of systems where excitation trapping occurs. In the second part of this thesis, we study the photosynthetic light harvesting mechanism of the purple bacterium Rhodospirillum photometricum. The light energy is harvested and transformed into chemical energy by photosynthetic membranes. It is known that this bacterium adapts to the environmental light conditions by expressing two different membrane structures. Here, we perform model simulations in order to resolve contradicting results of previous studies regarding the purpose of this adaptation. We find that the two membrane types exhibit a crossing in their efficiency, as a function of the illumination intensity, and therefore perform more efficiently in the respective light regime that they are adapted to. Es ist bekannt,…