Hole-conductor-free Perovskite Solar Cells With Carbon Counter Electrodes Based On Nanorod Arrays

With the development of dye-sensitized solar cells(DSSCs),ZnO and TiO2 with different morphologies have been widely used as photoanode due to their superior performance.It is well known that one-dimensional(1D)nanostructures,such as nanorods,nanowires or nanotubes,have been attracted more attention,in view of their low trap density and the direct path to the current collecting electrode that can speed up charge migration without adversely affecting recombination.Therefore,the application of 1D ZnO and TiO2 NR in solar cells will be beneficial to improve their power conversion efficiency(PEC).Recently,perovskite solar cells(PSCs),evolved from dye-sensitized solar cells,as a cost-effective alternative to silicon-based photovoltaics have attracted considerable attention and shown promise toward the development of next generation solar cells.The certified PCE of PSCs has increased rapidly to 22.1%within the past five years.In order to make it more competitive for future commercialization,multiple research groups have demonstrated various device architectures for avoiding the use of high-cost hole transporting materials(HTMs)and/or noble metal Au counter electrodes in mesoscopic TiO2/CH3NH3PbI3 solar cells.Their results indicated that the devices can perform well.Most notably,to reduce the cell fabrication cost caused by HTMs and Au counter electrodes at the same time,an HTM free device based on a lead iodide perovskite have been developed.A mesoporous ZrO2 layer served to prevent recombination between the fully printable carbon electrode(CE)and the mesoporous TiO2 electron-transport layer,resulting in a PCE of 12.8%.Many reports indicated that morphology and architectures of semiconductor effect the performance of the PSCs.Several studies have already demonstrated the use of ZnO NR arrays for application in PSCs with HTMs and noble metal counter electrodes.In this thesis,1D ZnO and TiO2 arrays have been prepared on the FTO glass and applied as semiconductor of hole-conductor-free perovskite solar cells with carbon counter electrodes.The main contents include the following:(1)For the first time,ZnO nanorods array was used as semiconductor of hole-conductor-free PSCs with carbon counter electrodes.ZnO NR array film was fabricated on the FTO substrate through a hydrothermal method.The thickness of the film was controlled by the hydrothermal treating time.The ZnO NR array with a length of 5 μm was applied in PSCs and a conversion efficiency of 5.56%was obtained.The good performance could be attributed to the open structures of ZnO which servesed as the backbone for the direct electron transport by providing short transport channels as well as improved perovskite infiltration within the electron transport material.(2)The attainable performance of the ZnO NR based PSCs in part(1)was apparently lower owing to the high rate of undesirable recombination.Thus,TiO2 layer was coated on the surface of ZnO NR forming core-shell nanoarchitectures to passivate the defect states on the ZnO surface and suppress charge recombination during the measurement of its photovoltaic performance.The ZnO/TiO2 NR based PSCs achieved high PCE of 8.24%whereas the reference ZnO NR based PSCs exhibited PCE of 5.56%.The suppression of charge recombination and the back electron transfer from ZnO to the perovskite by the coated TiO2 lead to the increase of both the V and FF.EIS analysis indicated that the coating of the TiO2 layer successfully increased the Rctl at the ZnO/perovskite interface.(3)1D TiO2 NR array was prepared on FTO substrate successfully via hydrothermal method.The effects of crystalline seeds layer,titanium sources,temperature,time and pH on the morphologies were investigated.The results showed that the diameter of the NRs increased with the size of the seeds on the substrates.The gaps between the NRs had relation with the growth velocity of the NR.The thickness of the film was dependent with the solution of the Cl-.The TiO2 NR based PSCs achieved high PCE of 9.07%,which makes the TiO2 NR array novelty candidates as electron collectors for PSCs.(4)ZnO NRs array was used as photoanode in QDSSC and was sensitized by CdS and CdS/CdSe respectively.SEM,UV-vis,EIS and HRTEM were introduced to investigate the morphology and the structure of the sample.The as-prepared ZnO NRs array was coupled with CdSe by a SILAR process and the effect of cycle times was investigated.The conversion efficiency of 0.44%was achieved for the 30 times treatment of NR/CdS based QDSSCs.A CdS/CdSe co-sensitized ZnO solar cell was designed.ID NR based co-sensitized solar cell not only affords fast electron injection rate,but also increases the light harvesting,which yield power conversion effiencies of 0.3 much higher than that of ZnO/CdS based QDSSCs.