Photoresponsive polar LuMnO3 and nonpolar LaFeO3 oxides in non-centrosymmetric structures

Abstract

The low power conversion efficiency is an important limiting factor for extensive photovoltaic applications. Within this context, ferroelectric photovoltaics using ferroelectric materials, are receiving a new interest. Indeed, some unique properties that they display may contribute solving some current bottlenecks in photoconversion-related applications. For instance, the possible presence of an internal electric field (depoling) extending all over the material and the presence of bulk photovoltaic effect (BPE) associated to their inherent non-centrosymmetric character. While the first may lead to more efficient charge extraction from a photoresponsive material, the second can lead to new opportunities such as the achievement of open circuit voltage (Voc) larger than the bandgap of the involved semiconductors. In recent years, the BPE research in ferroelectrics has been mainly focused on BiFeO3 with a bandgap of 2.7 eV, which implies limited responsivity at the visible range. Obviously, ferroelectrics with narrower gap could lead to improved photoconversion and BPE can introduce additional benefit. Among those materials, a remarkably simple family are the hexagonal manganites h-ReMnO3 (Re = Lu, Y, etc.), that having a narrower bandgap (≈ 1.5 eV), could be promising candidates. In h-ReMnO3, BPE had not been yet documented, and this is the first objective of this thesis. Non-centrosymmetry can be intrinsic to the photo absorber, as in photoferroelectrics, but it can also be engineered by the design of the photovoltaic cell. Still a narrow bandgap material is required. The nonpolar LaFeO3 (bandgap ≈ 2.4 eV) is a good candidate to explore its photoresponsivity, to carefully analyze the role of electronic band alignment with electrodes on the short circuit current density (Jsc) and open circuit voltage and ultimately compare its photoresponsivity to that of polar materials. _x000D_ With these goals in mind, we have systematically studied the photovoltaic response of hexagonal LuMnO3 single crystals and thin films aiming at identifying fingerprints of BPE and evaluate their contribution to the overall photoresponse. Next, the transport properties in dark and under illumination of heterostructures containing LaFeO3 thin films sandwiched between asymmetric electrodes, have provided the opportunity to identify the connection between the built-in potential at interfaces LaFeO3 and their photovoltaic properties.

Date of Award	27 Oct 2022
Original language	English
Supervisor	Fontcuberta Griñó, Josep (Director) & Fina Martinez, Ignasi (Director)