Self-powered UV Photoelectric Response Of NiO/ZnO Nanorod Arrays Heterojunctions Based Devices

Self-powered UV photodetectors?UV PDs?without power supply can realized miniaturization and intelligence,which will play an important role in self-powered wireless remote nano-devices in the future.The self-powered heterojunction UV PDs based on the photovoltaic effect can promote the separation of photogenerated carriers under the effect of built-in electric field,realizing a UV light detection without any external power supply.One-dimensional ZnO nanorod array?NRs?and NiO nanoflakes can be fabricated II-type heterojunction UV PDs with photovoltaic effect.The nanostructure heterojunctions can provide more separation interfaces for the photogenerated carriers.The ordered array structure is favorable for the directional transport of the carriers.However,as a result of the large specific surface area and the relatively high density of surface states and defects states of nanostructure semiconductor,the carrier will recombine easily at the interface of NiO/ZnO heterojunction,resulting in the large dark current and the low photoresponsivity of the heterojunction devices without external power supply.In this thesis,NiO nanoflakes were prepared by simple chemical bath deposition?CBD?methods on ZnO NRs surface and Au/NiO/ZnO NRs/FTO heterojunction were fabricated.The self-powered UV photoresponse properties of the heterojunction devices were investigated without external electric field.The strategies such as the improved crystal quality of ZnO NRs and the dielectric insertion layer at the heterojunction interface are used to reduce the carriers recombination center,decrease the carrier recombination at the heterojunction interface,and then enhance the photocurrent and reduce the dark current of the heterojunction devices,which will improve the performance of self-powered heterojunction devices.The main research contents and results are as follows:?1?The surface states and defect states of ZnO NRs are important factors to affect the performance of NiO/ZnO NRs heterojunctions devices.The surface states and defect states of ZnO NRs are reduced recombination of carrier at the surface states and defect states decreased after the vacuum annealing.The NiO/ZnO NRs heterojunction devices showed the good rectifying characteristics and the photovoltaic effect under the UV light illumination,which can realized the self-powered UV light detection.The heterojunctions devices exhibited good spectral selectivity without an external electric field under the UV light radiation.The dark current at zero bias generated by thermal excitation of ZnO NRs heterojunction devices was reduced after 300 ^oC vacuum annealing,resulting in the improved sensitivity of the devices.The results can be attributed to the reduced surface states and defect states of ZnO NRs after vacuum annealing treatment,the decreased carriers recombination centers,and the reduced carriers recombination in the heterojunction interfaces.?2?MgO nanoparticles were deposited on ZnO NRs surface with different time by the magnetron sputtering method and the NiO/MgO/ZnO NRs heterojunction based PDs were fabricated.The effect of MgO dielectric layer on the self-powered UV photoresponse properties of the PDs was investigated.The MgO insertion layer plays two roles in the heterojunction devices:one is modify the surface of ZnO NRs that effectively suppress the carriers recombination of the heterojunction interface caused by the ZnO NRs surface defects states.The reduced dark current caused by the thermal effect at zero bias improves the signal-to-noise ratio of devices.The other is as a barrier layer of carriers transport between NiO nanoflakes and ZnO NRs.The photogenerated holes of ZnO NRs reach NiO layer by tunneling through MgO.The NiO/MgO/ZnO NRs heterojunction devices with 30 min MgO deposition exhibited the high photoresponsivity R?378 nm,73.9 mA/W?and high detectivity D*(5.84?10¹¹ cm?Hz^1/2/W)under the weak UV light radiation.?3?High quality ZnO NRs were obtained by adding Al³⁺ions into the precursor of ZnO NRs growth to regulate the nucleation and growth of nanorods.The self-powered UV photoresponse properties of NiO/ZnO NRs heterojunction devices were studied.ZnO NRs with better crystal quality grown in the precursor with 0.5%Al³⁺ions addition show less the surface defect states,higher carrier concentration,and the Fermi level is closer to the bottom of the conduction band.The NiO/ZnO NRs heterojunctions devices show a narrow spectral selectivity?FWHM of 9 nm?,high photoresponsivity R?85.1 mA/W?and detectivity D*(1.74?10¹² cm?Hz^1/2/W)and a fast response speed?² ms?without external bias under the weak light radiation?0.2 mW/cm² of 378 nm?.The strong built-in electric field of the heterojunction can promote the separation of the photogenerated carriers and the directed transport,inhibit the recombination of the photogenerated carriers in the interface.