Structure and photovoltaic properties of strongly correlated manganite/titanite heterojunctions
von Benedikt Ifland
Datum der mündl. Prüfung:2018-05-17
Erschienen:2018-08-21
Betreuer:Prof. Dr. Christian Jooß
Gutachter:Prof. Dr. Christian Jooß
Gutachter:Prof. Dr. Vasily Moshnyaga
Dateien
Name:Dissertation_Ifland_2018.pdf
Size:15.7Mb
Format:PDF
Zusammenfassung
Englisch
In the present thesis the photovoltaic (PV) effect in strongly correlated manganite/titanite heterostructures is investigated. As a model system thin films of Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ (PCMO) were deposited on single crystalline SrTi$_{0.998}$Nb$_{0.002}$O$_{3}$ (STNO) substrates. In particular, PCMO with a doping level of x=0.35 is a suitable material to investigate the PV effect in correlated solar cells, because PCMO reveals high absorbtance in nearly the entire solar spectrum. In addition the appearance of long living hot polaronic states after a primary excitation with light can be observed and different electric and magnetic ground states can be realized by varying the level of Ca substitution. A necessary prerequisite for this type of study is the preparation of well defined, high quality charge separating interfaces. Therefore, high quality heteroepitaxial manganite thin films are required. The thin manganite films were prepared by using ion beam sputter deposition. The stress/strain results from a combination of thermal and lattice mismatch between PCMO and STNO as well as the influence of preparation-induced defects. The influence of strain was investigated with respect to the phase formation of PCMO. Compared to bulk samples the Mn solubility is dramatically reduced in PCMO thin film samples with Mn excess. This is accompanied by strain induced MnO$_z$ precipitation observed for films deposited with a Mn excess. The precipitation seems to be a strain relaxation mechanism, if a critical stress threshold is overcome. Preparation of atomically flat surfaces was achieved performing an optimization of process parameters. Using well-defined interfaces it was possible to study electrical transport processes across the PCMO/STNO interfaces. Therefore, current-voltage (I-V) characteristics were measured under different illumination conditions, i.e. dark, polychromatic (Xe arc lamp) and monochromatic illumination. All I-V curves were analyzed within the Shockley theory for semiconductor-based diodes. The applicability of the model was tested, which is chalanged because of the polaronic nature of the charge carrier in the correlated heterostructure. I-V curves measured in the dark were evaluated by using an one diode equivalent circuit. Here, the polaronic nature of the charge carrier is visible in the temperature dependence of the determined diode parameters, the ideality factor n, saturation current density J$_S$ and the parasitic resistances, the series resistance R$_S$ and the parallel resistance R$_P$. For example the temperature dependence of R$_S$ indicates the typical thermal activated hopping conductivity of small polarons. A further example is the CER (Colossal Electro-Resistance) like behavior visible in the temperature dependence of R$_P$. The influence of correlation effects is also observable for illuminated heterostructures. According to the Shockley theory a linear dependence of the open-circuit voltage V$_{OC}$ is expected, if the diffusion length of the excess charge carrier is larger than the extension of the space charge region. For monochromatic illumination with photon energies smaller than the bandgap of STNO the linear behavior of V$_{OC}$(T) is observable below the charge ordering temperature, which can be interpreted, as a contribution of long living excess carrier generated in the bulk material of PCMO to the PV effect. PCMO undergoes a first-order transition from a disordered to a charge ordered phase. A similar trend is visible for heterostructures composed of PCMO with x=0.95 and STNO. Here a linear behavior of V$_{OC}$(T) is visible below the Neel temperature, accompanied with a transition to a magnetically ordered phase. Both indicate that the appearance of ordering effects in the manganites is essential for the photovoltaic effect in PCMO/STNO heterostructures.
Keywords: photovoltaic effect; manganites; perovskites; polaron; correlated interfaces; electrical transport; thin film deposition