Schiff Base Metal Complexes Used As Hole Transport Materials For Perovskite Solar Cells

As one of new photovoltaic technologies,perovskite solar cells(PSCs)have attracted extensive research interests and efforts due to the characteristics of high power conversion efficiency(PCE),easy preparation,low cost and environmentally friendly property.Hole transport layers(HTLs)play a key role in enhancing the cell efficiencies and device stability for PSCs.As one class of metal complexes,Schiff base metal complexes possess the advantages of easy synthesis,excellent semiconducting properties as well as the appropriate energy levels,which make them ideal candidates as hole transport materials(HTMs)for use in PSCs.In this work,a series of new Schiff base metal complexes that are used for the fabrication of HTLs in PSCs are presented.It has been found that the physical,chemical and electronic properties of the materials can be systematically tuned via the change of the central metals as well as the introduction of alkyl chains.The n-i-p PSCs based on these materials have been fabricated and characterized,and the correlation between the molecular structures of the Schiff base metal complexes and device performances was systematically studied.In this work,a series of Schiff base metal complexes containing different central metals,i.e.PtMePy,PdMePy,NiMePy,CuMePy,PtPy,PdPy and CuPy,were designed and successfully synthesized.The molecular structures were determined by nuclear magnetic resonance(NMR),mass spectrometry(MS),fourier-transform infrared spectroscopy(FTIR)technologies etc.,and the purities of the materials were studied by elemental analysis technology.The influences of the substitution of peripheral methyl and the change of central metals on the electronic energy levels,thermal/chemical stabilities,optical properties and solubilities in organic solvents of the materials were systematically studied.The thin films of these materials fabricated by thermal evaporation method were studied by grazing incidence X-ray diffraction(GIXRD).The results showed that the intermolecular arrangements of the molecules in the thin films are dominated by ?-? stacking interactions,leading to the formation of highly oriented polycrystalline films,which would be favorable for charge transport.The surface morphology study together with the energy level test of the thin films were preformed to probe the feasibility of the use of theses materials as HTMs for PSCs.The devices based on PtMePy,PdMePy,CuMePy,NiMePy,PtPy,PdPy and CuPy were fabricated and tested.It has been found that except CuPy-based device,all the devices exhibited obvious photovoltaic properties,and the PCEs of 7.35%?6.88%?8.00%?8.15%?4.73% and 3.55% were found for PtMePy-,PdMePy-,CuMePy-,NiMePy-,PtPy-and PdPy-based devices,respectively.The device based on NiMePy exhibited the higest PCE,and the result was well consistent with the result of the single crystal analysis.Also,it has been found that the HTLs based on the Schiff base metal complexes can give an effective protection to perovskite film due to the excellent thermal/chemical stabilities of the materials as well as the hydrophobic nature of methyl group,which can effectively impede the invasion of water and oxygen into the perovskite thin film,and thus improved the device stability.The protection effects of various encapsulating materials such as UV film,glass cover,Teflon solution and copper phthalocyanine on the perovskite thin film were also studied in this thesis.Among these materials,the UV film exhibited the best performance,and the relatively high stability was observed for the sample encapsulated with UV film after exposure in ambient condition for about 300 days.