Study On The Preparation Of Large-area Methylammonium Lead Iodide Thin Films And Photovoltaic Devices Via Chemical Vapor Deposition

Organic-Inorganic hybrid perovskite solar cells(PSCs)are one of the biggest advances in photovoltaics in recent years.The reported record efficiencies of PSCs rapidly increased from 3.8% in 2009 to over 20% in 2015.In the fabrication of high-efficiency PSCs,it is critical to prepare highly crystalized,pure,dense and homogeneous MAPbI3 films.MAPb I3 is usually synthesized in two types of methods: solution methods and vapor-solid reaction(VSR)methods.VSR methods can be used to fabricate uniform MAPb I3 thin films with controllable grain sizes at a low cost.Among them,chemical vapor deposition(CVD)has become the major VSR method of preparing MAPbI3 films for its low cost,less pollution and controllable process.However,MAPb I3 films prepared by traditional tubular CVD have many disadvantages,including inhomogeneous reaction processes,small grain sizes,and poor reproducibility.To overcome these issues,a parallel-hot-plate(PHP)apparatus was built in this thesis,using which uniform MAPb I3 films with area of 8 × 8 cm2 were successfully prepared.The corresponding photovoltaic devices showed negligible J-V hysteresis with respect to the forward and reverse scans.The contents of the thesis are as below.Firstly,the processes of preparing functional layers in inverted-structure planar heterojunction PSCs were optimized,including the PbI2,PEDOT:PSS and PCBM layers.It was found that characteristics of PbI2 layers are almost the same either on ITO or on PEDOT:PSS.Substrate temperature has no effect on the film thickness of spin-coated PEDOT:PSS.The optimized layer thickness of PCBM is 43 nm.After that,a series of semi-closed tubular CVD apparatus were built for producing perovskite films.We found the film surface is very easy to become hazy.Pb I2 close to MAI source reacts first to form perovskite.When PbI2 far from the MAI source is transformed to perovskite,Pb I2 that is closer to the MAI source has already overreacted.Accordingly,we conclude that conventional tubular CVD methods are difficult to prepare large-area,uniform perovskite films.A parallel-hot-plate(PHP)apparatus was then built by our own,using which uniform MAPb I3 films with area of 8 × 8 cm2 were successfully prepared.The method overcomes disadvantages of traditional tubular CVD.The main difference between PHP and tubular CVD is that PHP can generate uniform MAI flow and its substrate holder can stay at a constant temperature.We fabricated PSCs with a champion efficiency of 12.2% under simulated solar light(AM1.5,100 mW cm-2)with an effective area of 0.11 cm2.Fabricated devices show negligible hysteresis with respect to the forward and reverse scans.Finally,chemical reaction mechanisms between MAI vapor and PbI2 films were investigated in our study.Characterizations with XRD,SEM,EDX and UV-visible absorption confirmed that MAPbI3 might form low-dimensional perovskite(LDP)materials with excess MAI.Such material exists in the form of large white spots under SEM observations.Because of its scattering of visible light,the film surface looks hazy.Insertation of a PEDOT:PSS layer below PbI2 can effectively suppress the formation of LDP.