The effects of calcium and other element substitution on the electric and photoelectric properties of bismuth ferrite

• Main Authors: Wang-JenLi, 李婉甄
• Other Authors: Xiaoding Qi
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/41204950239370020978

碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 101 === The aim of this study was to search for ferroelectric photovoltaic materials. In contrast to the semiconductor p-n junction, where the photovoltaic voltage is limited to around 0.6 V, the spontaneous polarization in ferroelectric materials can produce a much higher output voltage. Indeed, in a recent work, a large photovoltaic output of over 20 V has been demonstrated with the BiFeO3 single crystals. However, because ferroelectric materials usually have a very high DC resistivity over 108 ohm-cm, there is a need to increase the mobile carrier concentration in these materials. For this purpose, BiFeO3 (BFO) was doped with various ions in the present study. Polycrystalline pellets of BFO:M (M=Ca, Pb, Sr and Ni) were prepared by the sol-gel method. After poling, the photoelectric effects of the samples were studied. X-ray diffraction showed that after sintering at 500~600oC for 12 hours, undoped BFO and the samples doped with Ca, Pb, Sr and Ni had a pure phase with the well-known rhombohedral structure. If the sintering temperature was raised to 700oC, a second phase of the composition Bi25FeO40 appeared. The resistivities of the BFO samples doped with Ca, Pb, Sr, and Ni doped were measured to be 106~107 ohm-cm, which were1~2 orders lower than that of the undoped BFO. The samples were poled under a high electrical field at 600oC. The degree of the achieved poling was checked by the measured d33 values. I-V curves and photovoltaic effects of the samples with a large d33 were then investigated. The experiment results showed that under suitable preparation conditions (e.g. sintering temperature and time, degree of achieved poling, etc.), an open-circuit voltage was usually observed in the samples. After the samples were illuminated by the simulated-sunlight, large current increases were observed in the I-V curves. The undoped BFO showed a largest negative open-circuit voltage of -1.25 V, whereas the open-circuit voltages for Ca and Ni doped BFO were -0.75 and -0.2 (V), respectively. On the other hand, The Ca and Ni doped samples showed the largest photocurrents, which were increased from 10-9 A at dark to 10-8 A after the illumination. The undoped BFO showed a trivial photocurrent after illumination, indicating a lack of mobile charge carrier which could be excited by the light.