| dc.description.abstracteng | The overarching goal of this dissertation is to investigate new principals
to increase photovoltaic efficiency. In this context, three topics are presented, one
concerning the application of carrier selective contacts to silicon solar cells and the
other two dealing with phase transitions and charge carrier separation in manganite-titanate
based hot carrier solar cells. In the first part, we investigate electron selective contacts based on SiO$_\text{x}$/TiO$_\text{y}$/Al layers deposited on n-type crystalline Si in terms of photovoltaic properties and
their relation to composition and electronic structure of the involved layers, using
the analytical capabilities of transmission electron microscopy. In particular, we investigate how annealing of
the samples improves the carrier selectivity and hence the efficiency of the TiO$_\text{y}$/Al
contact. As the reasons for the improvement we identify, for one thing, an improved
interface passivation of the annealed contact and, even more important, a decreased
contact resistance due to a thinner SiO$_\text{x}$ layer between Si and TiO$_\text{y}$. Regarding the
manganite-titanate solar cells, we investigate two types calcium-doped praseodymium manganite (PCMO) thin films deposited on niobium-doped strontium titanate (STNO) substrates. One is based on
PCMO with perovskite structure (3D-PCMO), the other is based on PCMO with the
Ruddlesden-Popper structure (RP-PCMO). For RP-PCMO, we present temperature dependent electron diffraction measurements to follow structural and electronic phase transitions. These measurements show several contributions to a sophisticated set of phase transitions for temperatures between 100-300 K. In particular, we identify a change in the tilt
system of the MnO$_6$ octahedra and a connection between nanostructural defects and
orbital ordered phases. To improve the understanding of charge carrier separation at
STNO/3D-PCMO and STNO/RP-PCMO interfaces, we apply a variety of advanced
scanning transmission electron microscopy techniques to the samples. The
aim of this investigation is to determine the most important electrical properties,
such as the local electron affinity, the extent of the space charge region and the
potential distribution across the junction. The results show that the space charge
region on the STNO side has an extent of a few tens of nanometres, while the space
charge region on the PCMO side is only a few Ångström wide. Furthermore, we
identified a similar offset of the conduction band edges at the heterointerface for
both samples, which might play a crucial role for charge carrier separation at these
interfaces. | de |