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Transparent conducting oxides and multiferroic perovskites for solar energy conversion applications.


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Huang, Wei (2018). Transparent conducting oxides and multiferroic perovskites for solar energy conversion applications. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 176 p.

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Clean and sustainable solar energy is regarded as one of the most reliable and abundant energy sources to replace fossil fuels. To harvest this energy, in the past several decades, the research trend has been towards exploring novel materials and photon-to-electron mechanisms to achieve higher efficiency in solar energy conversion applications, such as photovoltaic (PV) cells. The PV effect is used to directly harvest solar energy by converting the incident photons into following free charge carriers and thus produce electricity. The emerging PV cells mainly differ in how they operate in practice, that is, in the mechanisms that make the sequence of the generation, separation and transport of electronic charge carriers possible. Elucidating those mechanisms is of fundamental importance for understanding the working principle of each solar cell type, and for its further performance optimization. In conventional p-n junction based PV cells, the charge separation is determined by the existence of a gradient in the electrochemical potential, also known as intrinsic built-in electric fields. Moreover, solar-driven water splitting and hydrogen generation technologies that produce hydrogen (H₂ gas) and oxygen (O₂ gas) by directly decomposing water using an artificial photocatalytic electrode have been extensively studied as a fundamental technology for the future, due to its simplicity, low-cost operation and the use of nearly neutral pH water, such as seawater for large scale solar-fuel production. In the semiconductor photocatalysts, the carrier extraction is no longer limited by thermodynamic diffusion, but the transport of spatially separated electron-hole pairs to the photocatalyst surface is determined by the surface band bending. Ferroelectric (FE) oxides have recently emerged as a new alternative pathway to achieve the separation of photo-generated charge carriers, and their application in photon-to-current (e.g., PV cells, photodetectors) and photoelectrochemical (PEC) devices was recently started to be explored. Since the discovery of PV effect in these materials over 50 years ago, ferroelectric devices including solar cells (FESC) and PEC cells have attracted significant attention owing to many unique advantages. This technology involves a simplified structure and fabrication methods as well as stable and abundant materials. Different from the p-n diode or Schottky diode solar cells, in the FESC, FE polarization plays a dominant role in charge separation and transport. FESC provides reversible directions of photocurrent and photovoltage by polarization direction switching, and also generates above bandgap open-circuit voltages (Voc up to 40 V), which potentially permit to surpass the Shockley-Queisser limit observed in traditional semiconductors cells. While for a long time such devices were characterized by weak performance values, recent advances led to significant breakthroughs by using multiferroic materials. Multiferroics combine two ferroic functionalities (specifically FE, ferromagnetic or ferroelastic), and typically possess a magnetic order parameter besides the FE one and the electron-electron interaction regulating the magnetic ordering induced a smaller gap (e.g., 2.6–2.7 eV for BiFeO₃ and 1.4–2.4 eV for Bi₂FeCrO₆) than other FE materials (e.g., 3.2–3.6 eV for La–doped Pb(Zr,Ti)O₃ and 3.2 eV for BaTiO₃). Among these multiferroics, double-perovskite Bi₂FeCrO₆ (BFCO) is highly promising in PV and photocatalytic applications due to its nontoxicity, chemical stability, narrow bandgap, huge light absorption coefficient (2.5 × 10⁵ cm⁻¹), large remnant polarization (55 μC cm⁻² at room temperature (RT)) and magnetization (160 emu cm⁻³). A breakthrough in the field of PV and PEC devices occurred through our recent study on multiferroic BFCO epitaxial thin films, which demonstrated a high efficiency solar energy conversion (solar to electrical or chemical energy). In such material, lower band gap and large FE polarization promote an effective charge carrier generation and separation. Single crystalline BFCO with higher ratio of Fe/Cr cation ordering (R≈0.5−5.1%) showed a lower bandgap (Eg≈1.4−1.8 eV) accompanied with weak FE polarization (P≈5−20 μC cm⁻²), which exhibit thus a semiconductor behavior. In despite of a larger absorption of sun light by semiconducting BFCO films, the weak FE polarization induced internal electric field decreases the separation power of photogenerated carriers and increases the recombination loss. This dramatically affects the performance of the devices. These results clearly suggest a relationship between the optical property, ferroelectricity and crystal structure of BFCO thin films. To advantageously exploit the semiconducting properties of BFCO for PV application, the p-i-n device architectures have to be designed and developed. Here, an intrinsic semiconductor is sandwiched between a heavily doped p- and n-layers. Semiconductor material absorbs a certain portion of the solar spectrum and adjacent layers (electrodes) are required to extract the photo-generated carriers for conveyance to an external electrical load. The p-type layer in p-i-n devices is possible incident photons enter the intrinsic absorber layer (i-) of the device for conversion into charge carriers. This requires that all other “photo-passive” but functional layers in the PV cell that lie in the path of photons traversing to the absorber should not absorb or back reflect any of the light before it reaches the intrinsic absorber. That is, the window layers must be highly transparent, i.e., having a wider bandgap than the absorber and thus a much lower absorption coefficient in the spectral range of light used for photo-conversion. Electrically, the p-layer should have hole conduction in order to transport the photogenerated holes and block the electrons. Based on these requirements, the p-type layer in p-i-n devices should be the p-type transparent conducting oxides (p-TCOs). The work performed in this thesis was therefore driven by two main objectives: (1) synthesis and understanding the fundamental physical properties (i.e., structural, electrical, and optical) of p-TCOs thin films; (2) design and investigating the multiferroic BFCO thin-film absorber based devices for efficient solar energy conversion applications including photodetectors, PV and PEC devices. The results obtained in this work are resumed in two sections as follows: In the first section, we synthesized the p-type transparent conducting thin films and studied their structural, electrical and optical properties. (1) Perovskite p-type In-doped SrTiO₃ (SrInxTi1-xO₃ with In concentration in the range 0≤x≤0.15) thin films were grown on LaAlO₃ substrates using a pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) combination technique. (2) The In-STO (ISTO, 0.1≤x≤0.15) films grown under oxygen pressure of 10⁻⁷ − 10⁻³ Torr show an optimal range of high transmittance (~70%), low resistivity (~10⁻² Ω cm), high carrier concentration of ~3×10¹⁹ cm⁻³ with mobility of ~5 cm²/Vs, and large Eg≥3.2 eV. The second section is focused on solar energy conversion applications of BFCO thin films based devices: (1) the photodiodes based on the epitaxial BFCO/SrRuO₃ (SRO) thin films were fabricated by using PLD techniques. The photodiodes with a large ideality factor (n≈5.0) exhibits a substantial photocurrent at forward biased voltages (photoconductive effect) and a fast transient response (in the order of 10⁻² s). The tailoring of photoelectric performance was achieved by switching FE polarization state of BFCO (Eg≈2.5 eV; P≈40 μC cm⁻²). The ultrafast charge transfer at BFCO/SRO heterojunction was investigated by time-resolved photoluminescence. A peak sensitivity was measured as 0.38 mA/W at 500 nm by photoresponsivity spectroscopy. (2) We reported the fabrication of multiferroic BFCO thin films (Eg≈1.7−2.4 eV; P≈14−44 μC cm⁻²) based PV devices with p-i-n heterojunction by PLD. A p-type NiO thin film acting as hole-transporting/electron-blocking layer and a typical n-type Nb-STO acting as electron-transporting layer were used to form the double-interfaced heterojunctions. Under 1 sun illumination, the optimized p-i-n devices yielded an open-circuit voltage of ~0.53 V and a short-circuit current density of ~8.0 mA cm⁻², leading to a PCE of ~2.0%, a four-fold enhancement compared to that of the i-n device architecture. (3) The n-type BFCO thin films coated with a p-type transparent conducting NiO layer were implemented as a heterojunction photoelectrode by PLD for solar-driven water splitting. The tailoring of PEC performance of the bare BFCO (Eg≈1.8 eV; P≈20 μC cm⁻²) based photoanodes was achieved by effectively tuning the FE polarization state, and thus resulted in a 1.5-fold increase in photocurrent density. A 4-fold enhancement of photocurrent density, up to 0.4 mA cm⁻² (at +1.23 V vs. RHE) in 1 M Na₂SO₄ (pH 6.8) electrolyte under 1 sun illumination was carried out by coating the bare BFCO photoanodes with a p-type transparent conducting NiO layer acting as an electron-blocking and protection layer. The stable operation of p-NiO/n-BFCO heterojunction photoanodes was confirmed by observing a constant current density over 4 hours.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Rosei, Federico
Co-directeurs de mémoire/thèse: Chaker, Mohamed; Nechache, Riad
Mots-clés libres: solar energy; photovoltaic cells;
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 29 janv. 2019 16:16
Dernière modification: 29 sept. 2021 19:44
URI: https://espace.inrs.ca/id/eprint/7642

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