Photovoltaic Effect Of Bi-based Superconductor Bi-2212

The phenomenon that the superconducting material can generate photovoltaic voltage under light irradiation has been observed in many kinds of superconductor.But there are different conclusions in different experimental reports,and a common theory cannot be derived to explain all experimental phenomena.In the previous study,the photovoltaic effect of YBa₂Cu₃O_?superconductors have been experimentally verified in ceramic samples and thin-film samples.It was proposed that the interfacial potential between the metal and the superconductor is the cause of photovoltaic effect.This theory is verified in iron-based superconducting materials which carriers are electrons.This theory is also applicable to Bi-2223 material photovoltaic effect.However,whether this theory is equally applicable to explaining the photovoltaic effect of other types of superconducting materials has not been studied.In this thesis,Bismuth-based superconductors Bi-2212 was chosen as the research object.First,we prepared ceramic samples of Bi-2212 having a different doping concentration at different atmospheres of air,oxygen,argon,by a solid phase reaction method.After characterizing the structure and morphology of the ceramic samples respectively,the resistance of the samples at various temperatures and the IV characteristic curve were tested by a comprehensive physical property measurement system?PPMS?using a standard four-lead method.The light at the junction of the sample and the metal wire in the experiment was provided by a violet laser??=405 nm?through a quartz clear window.Experiment observed photovoltage significant effect,up to 0.1mV,much larger than the thermoelectric power generated by the laser.In the meantime,when the laser irradiates different electrodes?I+,V+or I-,V-?,an open circuit voltage with the opposite direction is generated,and in the superconducting state,there is still a photovoltaic voltage.By comparing the photo-induced voltage effects of Bismuth-based superconductors Bi-2212 samples with different oxygen content,it is found that the photovoltaic effects produced by the samples with different doping levels also differ greatly.In the over-doped samples,the open circuit voltage generated by the laser irradiation of the positive electrode of the sample is always a positive value,and the value of the open circuit voltage gradually becomes smaller as the temperature decreases.In the under-doped sample test,the photovoltaic effect is significantly different from that of the over-doped sample.In the temperature range of 20 K to 300 K,the polarity of the open circuit voltage is reversed twice,and there exist a peak at 45K.The open-circuit voltage observed in the sample which close to the ideal doping is consistent with the change trend of the open-circuit voltage in the over-doped sample.But also exists a peak at 45 K.It is considered that under different doping concentration conditions,the carrier type of the material is due to the competition between the internal charge reservoir layer and the conductive layer,and the dominant carrier types of the samples with different doping concentrations are inconsistent:the carrier of the over-doped sample.With electronic features,under-doped samples have more hole-like features.The change of the carrier type has a great influence on the built-in electric field,which is the reason for the change of the direction of the photovoltaic voltage.The peak value of the open circuit voltage appearing at 45 K may be related to the inclusion of Bi-2223.In order to further study the photo-induced voltage effect of Bi-2212 material,we explored the technology of pulsed laser deposition superconducting thin film on the SrTiO₃?100?.The morphology and structure of the films were characterized.The results show that high-quality Bi-2212 thin films can be grown by depositing at 820?and in suit annealed at 830?for 40 min.To produce higher quality films requires further exploration.However,there was no photovoltaic effect been observed in the film sample,and it needs further exploration.