PL enhanced ferroelectric photovoltaic effect.
Creators: name: Mai, Haoxin
Publisher: name: The Australian National University
Year: 2020

description: Photovoltaic (PV) is an effect converting light energy to electrical energy. Nowadays, the traditional solar cells based on narrow bandgap semiconductors show strong photocurrent but weak photovoltage due to their narrow bandgap, and alternatively, the ferroelectric (FE) photovoltaics have high photovoltage because of the bulk photovoltaic effects but low photocurrent. Such a dilemma inspires my research. Before fabricating new FEPV devices, the dynamics of FE domains under illumination must be elucidated. BiFeO3 (BFO) thin films are used as a model to illustrate the effect of above-bandgap photons on the domains. By virtue of the piezoresponse force microscopy and Kelvin probe force microscopy, it is found that the relaxation time of the polarization state will be extended while the effective polarizing voltage for the pristine domains is reduced under the above bandgap illumination, and we confirm that this photo-induced stabilization is potentially universal rather than specific to BiFeO3. Thus, this study will enrich the knowledge of photovoltaic phenomena and meanwhile provide a new route to promote the stability of photovoltaic and ferroelectric materials. One strategy to increase the solar absorption without varying the structure of the FE materials is to introduce a light harvesting layer in the PV devices. In this regard, a ferroelectric-luminescent heterostructure is designed to convert infrared light into electric power. In this heterostructure, BFO is integrated with the upconversion layer, and switchable and stable PV effects under 980 nm illumination is detected. The construction of ferroelectric-luminescent heterostructure is consequently proposed as a promising route to enhance the photovoltaic effects of ferroelectric materials by extending the absorption of the solar spectrum. Another strategy is to build the internal fields in narrow bandgap semiconductors. A new strategy therefore is established to create the bulk PV effects in the narrow bandgap semiconductors. By taking advantage of the local-chemistry-induced polar nano-regions, this strategy enables the narrow bandgap semiconductors to have a large output voltage and thus the efficiency will be improved a lot. Moreover, to further improve the power efficiency, the homogeneous heterostructure is promoted, by which the light absorbers and internal fields can work collaboratively and thus power efficiency will be even higher. This new strategy will enlarge the candidate pool of FEPV materials and provide a new path to build the high efficiency PV devices. Moreover, ferroelectricity is also discovered in more centrosymmetric oxides thin films when their thickness is below a critical value. It is suggested that this thickness-dependent ferroelectrcity may relate to the emerging interfacial effects when the thickness is reduced. This will be a very important study in that it will not only extend the candidate materials for the non-volatile memories and complementary metal-oxide-semiconductor but also provide an insight in the origin of ferroelectricity. In terms of the aforementioned investigations in the FEPV materials, a primary difficulty of the FEPV devices, that how to improve the photocurrent meanwhile maintain the high voltage output, is partially solved. On the one hand, an upconversion layer is used to enlarge the solar absorption of the FE thin films without changing the structure of the FE material; on the other hand, the inversion symmetry of the narrow bandgap centrosymmetric materials can be broken by reducing thickness or introducing oxygen vacancies, by which strong PV effects are detected in these materials. Such investigations will not only contribute to the fabrication of low dimension materials, the development of defect engineering, and the knowledge of PV phenomena, but also hew out some new approaches to construct highly efficient PV devices.