The study of photovoltaic effect and electronic structure of Neodymium doped Bismuth ferrite

• Main Authors: Peng, Yi Ting, 彭憶婷
• Other Authors: Ouyang, Hao
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/78718378629475265465

碩士 === 國立清華大學 === 材料科學工程學系 === 104 === The BiFeO3 (BFO) materials have received intensive interests in the photovoltaic effect due to the high Curie temperature and the smaller direct band gap than other ferroelectric oxide ceramics. The group of Prof. Tu Chi-Shun has shown the enhanced photovoltaic effect 56 times higher in neodymium doped BFO. Using first-principles calculations, we offered physical insights into the origins of the enhanced photovoltaic effect. The doping effect of Nd in reducing the movable charge density due to oxygen vacancies in BiFeO3 was confirmed by shifting down of Fermi level of electronic structure, resulting in more electrons or holes accumulated on the two sides of p-n junction to enhance Voc and Jsc for increasing photovoltaic effect. The Rietveld analysis showed the concentration Nd around 1.24%. Besides,the scanning transmission electron microscopy image simulation analysis of BFONd and first-principles calculations of total free energy suggested that the Nd atom replaced the Bi atom on the A site of perovskite ABO3.The electronic structure showed that the calculated direct band gaps are respectively about 2.257 and 2.227 eV for BFO and BFONd. We considered that the oxygen vacancy due to the fabrication at high temperature around 870℃, forming the sub-bandgap defect states which lowered the photovoltaic effect. The doped Nd caused the sub-bandgap defect states shifted closer to conduction band easier to overcome the trap states by thermal perturbation, and reduced the movable charge density by shifting down of Fermi level to enhance photovoltaic power conversions. In addition, the optical calculations showed that the absorption coefficient of BFONd was higher than BFO to enhance the photocurrent density, which was proportional to absorption coefficient based on Glass Law. In the quantitative analysis,we can estimated the ratio of Jsc of BFONd and BFO at the laser intensity above 800W/m2 with wavelength 405nm, which was around 30 times higher from the Voc and band gap values for same thickness of BFO and BFONd samples.