Structure Control And Optoelectronic Performance Of The Vertically Ordered M_xS_y/TiO₂ Inorganic Bulk Heterojunctions

All inorganic bulk heterojunctions,in which donor and acceptor materials are mixed on a large-scale,benefit of high extinction coefficient,wide spectral response and adjustable structure.The mixed structure can realize fast carrier separation,reduce carrier recombination,improve device response,and thus be widely used in photovoltaic and optoelectronic fields.However,conventional bulk heterojunctions are usually based on the disordered network formed by metal oxide nanoparticles,leading to serious carrier scattering and recombination.Meanwhile,the mesoporous network results in low filling rate of sulfides,heterojunction discontinuity and lower utilization rate of incident photon.In this study,the vertically ordered TiO₂ nanotubes or nanorods are prepared.TiO₂/ZnS heterojunctions with core-shell feature are formed by coating ZnS onto TiO₂ via successive ionic layer adsorption and reaction?SILAR?method.Then,ZnS is converted to p-typed narrow bandgap sulfides by cation exchange and forms the vertically ordered M_xS_y/TiO₂ bulk heterojunctions.The structure control and photoelectric performance of the bulk heterojunctions are studied.The main research results are as following:The anodization is performed to prepare TiO₂ nanotubes under alternating applied potential in the form of square wave,through which TiO₂ nanotubes synthesized are highly ordered.Meanwhile,the TiO₂ nanotube film has a good adhesion with Ti matrix.The ZnS/TiO₂ bulk heterojunctions are prepared by coatingthe TiO₂ nanotubes with ZnS via SILAR method,and the ZnS is then transformed into CuS by cation exchange to obtain the CuS/TiO₂ bulk heterojunctions.Studies have shown that conformal coating of TiO₂ nanotubes with dense ZnS films is achieved using zinc acetate as the precursor solution,while ZnS nanoparticles are formed using Zinc nitrate as the precursor solution.The presence of hydroxyl ions released through hydrolysis of acetate ions in aqueous solutions results in dense ZnS films.The exchanged CuS/TiO₂ dense heterojunction exhibits superior rectifying behavior,with a diode ideality factor of 3.4,the reverse saturation current density of 5.6×10^-6 A cm^-2,as a result of reduced hole scattering.The influence of three-dimensional interfaces on rectifying effect in bulk heterojunctions is studied.TiO₂ nanorods with different geometric properties are synthesized via hydrothermal method.And then the corresponding CuS/TiO₂heterojunctions are prepared.It reveals that the interface defects of the heterojunctions increase with the increase of three-dimensional interface,leading to the high carrier recombination rate.Therefore,the diode ideality factor and reverse saturation current density increase.Compared with the ideal two-dimensional interface,the diode ideality factor and reverse saturation current density of the three-dimensional interface are relatively larger under the dark condition.The stability of CuS/TiO₂ inorganic solar cells is studied.It shows that the photoelectric conversion efficiency of the CuS/TiO₂ based device first increases and then decreases over the time.For CuS/TiO₂ electrodes,the Cu deficient sulfide is slowly converted into CuS and the content of CuS increases in the initial stage.Thus,the conversion efficiency of the relevant CuS/TiO₂ based device increases.Afterwards,the CuS is partly transformed into CuO,leading to the decrease of CuS content.Therefore,the performance of the device is weakened.The cation exchange kinetics of different sulfides are studied systematically.As for the coversion from ZnS to Ag₂S,PbS or CdS,it takes two Ag⁺to replace one Zn²⁺due to the low valence of Ag⁺,and thus the transformation of ZnS into Ag₂S is slower compared with that from ZnS to PbS or CdS.Furthermore,the solubility product constant of Ag₂S is the smallest,that of PbS is centered,and CdS is the largest.Therefore,ZnS is converted into Ag₂S in succession with the conversion rate of 60%,when the cation exchange time is 300 min.By comparision,the conversion from ZnS to PbS has been completed,and the maximum conversion rate is of 71.4%at the exchange time of 180 min.The conversion from ZnS to CdS is finished within 30 min and the maximum conversion rate is of 6.5%.In addition,the Ag₂S/ZnS/TiO₂,PbS/ZnS/TiO₂and CdS/ZnS/TiO₂ heterojunctions are fabricated at the cation exchange time of 1 h,and their electric properties are studied.This paper lays a certain experimental and theoretical foundation for the structural regulation and application of inorganic bulk heterojunctions.