Optimization and characterization of n- and p-type DSSCs: A road toward feasible tandem DSSCs

Together, coal, gas, and oil possess the largest footprint of greenhouse gases on this planet. These fossil resources are mainly used to generate heat and electricity. Subsequently, the energy sector has proven hazardous to our planet's sensitive ecosystem and susceptible to shortages due to political, geographical and social conflicts. To secure a stable and reliant supply of heat and energy mankind must leave fossil fuels in the past and embrace renewable energy. Especially the sun represents a practically undepletable reservoir of energy that nearly every country in the world has access to. The photovoltaic technology, however, still leaves much to be desired. In order to guarantee a stable energy supply in the future, efficiencies must be greatly improved and novel working principles must evolve. Dye-sensitized solar cells are one promising technology, which researchers have been focusing on since 1991. This thesis covers the investigation and optimization of different components of the dye-sensitized solar cell technology. In order to fully study these compounds an arsenal of characterization methods was undertaken. First, the photophysical properties were determined utilizing steady-state absorption as well as emission spectroscopy, transient absorption spectroscopy on the femto- and nano-second timescale, and diffuse reflectance. Second, the photoelectric properties were characterized with current-voltage measurements, incident-photon-to-current efficiency, and electrochemical impedance spectroscopy. Additional topographic information was gained by profilometry and atomic force microscopy. This thesis comprises two parts covering a total of four projects. Each part focuses on one component of a dye-sensitized solar cell. In the first part, the investigation and optimization of both n- and p-type semiconductors generated a good foundation for further research. The first project compared the performance of two p-type semiconductors, NiO and CuO, with each other. Single-junction and double-junction architectures were investigated. The goal was to successfully control the fill factor and interfacial charge transfer in order to optimize the performance of these devices. The second project shifted the attention to n-type semiconductors and focused on the combination of intrinsic and extrinsic incorporation into ZnO-based devices. Here, the main idea was fine-tuning energy levels in order to control the charge injection and recombination processes as well as to enhance the conductivity of the semiconductor layer. For the second part of this thesis, relying on the foundation established in the first part, the focus shifted towards the photosensitizers. In the third project, a perylene-monoimide dye was investigated in a p-type single-junction device. Focusing on multi-exciton generation the main goal of this project was the elucidation and subsequently the control of the deactivation pathway. The final project changed directions and focused on broadening the absorption wavelengths making the nIR region accessible for solar energy conversion schemes. The fusion of polycyclic aromatic hydrocarbons to porphyrins via 5-membered rings leads to an increase in biradicaloid character, which greatly changes the deactivation pathway creating fast deactivating non-radiative chromophores.