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Multiferroic materials as active elements for photovoltaic devices.

Chakrabartty, Joyprokash (2013). Multiferroic materials as active elements for photovoltaic devices. Mémoire. Québec, Université du Québec, Institut national de la recherche scientifique, Maîtrise en sciences de l'énergie et des matériaux, 88 p.

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Résumé

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The discovery of bulk photovoltaic (BPV) effect in multiferroic (MF) materials has opened new perspectives in the field of photovoltaics (PV), because the photogenerated charge carriers are separated by an electric field which is not bound to a specific site, or interface. Instead, the mechanism of carrier separation in MFs relies on the electric field created by the spontaneous polarization present in the non-centrosymmetric material. Additionally, when the characteristic dimension of the material is decreased down to nana scale, for example in a multiferroic thin film, the polarization-induced electric field is even more intense. Thus, MF materials eliminate the need for n-p junctions that are usually employed in conventional semiconductor PV devices. However, the electrical power conversion efficiency (PCE) of BPV effect-based devices under sun light illumination is poor, and to date still not comparable with the conventional n-p junction based PV devices. The main reason for this is the low bulk photoconductivity of oxide based MFs, since the BPV originates in the ferroelectric properties, and ferroelectrics are insulating in nature. Therefore, the ultimate challenge in developing competitive devices using the BPV effect is to improve the conductivity by preserving the built-in spontaneous polarization of such type of material. Nonetheless, the energy band gap of MFs (~1.2-2.7 eV), an important parameter for photo carrier separation, is smaller than ferroelectric materials (~3-4 eV). Therefore, we dedicated our work to the development of novel MF materials or heterostructures that would result in PV devices with competitive PCE. In first part of our work we developed a multi-stacking scheme, more specifically a heterostructure based on several BiFe03/BiCr03 (BFO/BCO) epitaxial bilayers grown by pulsed laser deposition (PLD) onto (100)-oriented LaAI03 (LAO) substrates coated with conductive CaRu03 (CRO) buffer layers. When used as active layers in PV devices, these heterostructures showed a remarkable PCE of ~0.01 % when illuminated with 1 sun (AM 1.5G). The fill factor (FF) was determined to be 31 %, a prominent value for both ferroelectric- and MF-based devices. Our results demonstrate that photocurrent density and photovoltage can be tuned by varying the thickness and number of bilayers, opening new perspectives for the design of novel MF heterostructures with improved PV properties, tuned via engineered interfacial layers. In the second part, we focused our efforts to the study of the BPV effect in thin films of BiMn- O material system. The films were again grown by PLD on (111) oriented Niobium doped SrTi03 single crystal substrates. Detailed X-ray diffraction studies show that the films consist of two main phases, the perovskite BiMn03 (BMO) with a (111)m orientation, and Mn304 phase having the (101)t orientation. As revealed by Ф-scan measurements, both phases are epitaxially grown onto substrates. For PV tests we deposited transparent tin doped indium oxide (ITO) top electrodes, and we illuminated the devices using a sun simulator (AM 1.5G). Our findings show that the elemental Bi/Mn ratio in the films controls the magnitude of the photovoltage and photocurrent. Higher Bi/Mn ratio results in an increased PCE, and we obtained a maximum PCE of ~0.1 % for the Bi/Mn ratio of 0.82. Film-electrode interfacial effect, modulated by ferroelectric polarization of the film has employed as a model to describe the observed PV phenomena.

Type de document: Mémoire
Directeur de mémoire/thèse: Rosei, Federico
Informations complémentaires: Résumé avec symboles
Mots-clés libres: photovoltaïque; matériel multiferroïque; champs électriques
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 09 juill. 2014 20:59
Dernière modification: 11 nov. 2015 21:17
URI: http://espace.inrs.ca/id/eprint/2176

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