Electronic and Structural Properties of Heterojunctions in Solar Cells studied by In-situ and Analytical Transmission Electron Microscopy

von Christoph Flathmann
Dissertation
Datum der mündl. Prüfung:2023-10-10
Erschienen:2023-11-03
Betreuer:Prof. Dr. Michael Seibt
Gutachter:Prof. Dr. Michael Seibt
Gutachter:Prof. Dr. Christian Jooß
Zum Verlinken/Zitieren: http://dx.doi.org/10.53846/goediss-10166

 

 

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Englisch

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.
Keywords: Photovoltaic; Transmission electron microscopy; Selective contacts; Perovskite; Correlated oxides; In-situ TEM
 
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