Oxide perovskites for solar energy conversion.

Inorganic ABO3 perovskite materials exhibit exotic physical and chemical properties due to their exceptional crystal structures and thus offer exciting opportunities for spintronics, computer memories, sensors, microwave and photovoltaic (PV) device applications. Since the discovery of bulk photovoltaic (BPV) effect in non-centrosymmetric crystals, the perovskite oxides including BaTiO₃ (BTO), LiNbO₃ (LNO), BiFeO₃ (BFO), BiMnO₃ (BMO), and Pb(Zr,Ti)O₃ (PZT) drew enormous interest in fundamental science research, as they exhibit either a suitable band gap or larger than band gap photovoltages (up to 1000 V) under homogeneous light illumination. However, owing to their insulating characteristics retain even at the dimensions down to nanometer scale, the charge carrier conduction in polar perovskites is extremely poor resulting in the generation of low photocurrent density, typically in the μA/cm² range and below. Driven by the challenges of improving photocurrent density, in this project, we have investigated BMO and BFO compounds in view of their integration as active materials in PV devices in order to improve the photocurrent density and thus solar power conversion efficiency (PCE). BMO in thin film form possesses ferroelectric behavior i.e. a remnant polarization of ~6 μC/cm² at room temperature and its low band gap value (~1.2 eV) efficiently absorbs light wavelengths exceeding the visible wavelengths of the solar spectrum. In contrast, BFO thin films exhibit strong ferroelectricity with a remnant polarization of ~ 55 μC/cm² and a band gap of ~2.7 eV that corresponds to visible light of solar spectrum. Although the physical and chemical properties of various forms of BFO, from bulk to nanoparticles are widely investigated in solar cells, the potential of BMO alone or in combination with other perovskites is rarely investigated and reported so far in the field of solar energy conversion. In the first part of this thesis, we demonstrate a bilayer-stacking scheme, more specifically a heterostructure based on BFO/BMO epitaxial bilayer thin films grown by pulsed laser deposition (PLD) onto Niobium doped (111)-oriented SrTiO₃ (STO) substrates. The BFO/BMO bilayers as photoactive materials in solid-state PV devices show remarkable PCEs up to 1.43% under 1 sun solar radiation (AM 1.5G), which is higher than those reported values for individual BFO or BMO thin films. The fill factor (FF) is determined to be 0.72 which is a remarkable value for ferroelectric perovskite-based devices. The bilayers exhibit prominent ferroelectric behavior (~100 μC/cm²) compared to individual BFO or BMO films. To describe the PV responses, we use a traditional interfacial model where an interface generated electric field is significantly modulated by spontaneous polarization of the materials. In the second part, we demonstrate a PV device based on Bi-Mn-O composite thin films which exhibits a PCE of 4.20%. The composite materials are made of two different types of crystal phases: BMO and BiMn₂O₅. The former is ferroelectric with a band gap of ~1.20 eV as specified previously, while BiMn₂O₅ is semiconducting with a band gap of ~1.23 eV. The composite films are grown by PLD on (100) oriented Niobium doped STO single crystal substrates. The crystal structure of both phases in the composite films is characterized by transmission electron microscopy (TEM) and further confirmed by Raman spectroscopy (RS) analysis. Conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM) under illumination show the increased photocurrent and photovoltage generations across BMO /BiMn₂O₅ grain boundaries (GB) compared to the interior of the grains. The ferroelectricity of BMO does not play a significant role in the PV effect, as confirmed by combined Piezoresponse force microscopy (PFM) and KPFM measurements. The results are described in the framework of GB barrier potentials. In the last part, we describe the photocatalytic properties of BMO thin films and nanostructures, both grown epitaxially on (111) oriented Niobium doped STO substrates by PLD. The nanostructures were achieved using nanostencils i.e. shadow masks with a periodic array of nanometer-scale circular features. Photoelectrochemical properties of films and nanostructures as working electrodes are investigated by linear sweep voltammetry (LSV) measurements under chopped illumination. The nanostructures exhibit photocurrent density of ~0.9 mA/cm² at 0.8 V vs Ag/AgCl (1.38 V vs reverse hydrogen electrode (RHE)) which is substantially higher than the values recorded in thin films, ~10 μA/cm² at 0.4 V vs Ag/AgCl (0.98 V vs RHE) under 1 Sun radiation. Band alignments with respect to the redox potential of water and gas chromatograph measurements demonstrate that the BMO photoelectrodes (both films and nanostructures) are suitable for oxygen evolution reactions.