Crystallization And Properties Of Perovskite CH₃NH₃PBI₃

The efficiency of organic-inorganic hybrid perovskite solar cells has been rapidly improved over the past few years and its efficiency has been close to silicon solar cells.Organic-inorganic hybrid perovskites were usd to replace dyes in dye-sensitized solar cells when firstly used in solar cells.The device used liquid electrolyte as the hole transport layer,and little attention were recieved due to the low efficiency（3.8%.in 2009）and poor stability.In 2012,a relatively high efficiency（～9%）solid-state device was obtained by replacing the liquid electrolyte with solid-state HTL（Spiro-OMeTAD）.This breakthrough led to the so-called"perovskite fever" in the next few years and attracted a lot of research,finally,the efficiency increased to 22.1%in 2016.However,many problems also exists for the perovskite material such as easy to deliquescence,hard to package,lead halogen toxic and other shortcomings,so perovskite solar cells still need improvement.In the past,most studies have focused on optimizing the structure and crystallization of the perovskite film by adjusting the deposition method,selecting the appropriate barrier and support layer materials,optimizing preparation processes,modifying the hole transport layer,and engineering the interface/bandgap behavior.However,due to the relatively short development time of perovskite solar cells,its research is not sufficient,especially the crystallization kinetics,the existence of ferroelectric,thermal conversion and other aspects;On the other hand,TiO₂ is commonly used as the electron transport layer in perovskite solar cells,the structure of TiO₂ will have a great impact on the efficiency and stability of the battery.In this paper,the crystallization process,the effect of annealing temperature to ferroelectric,the role of clorine to thermal conversion of perovskite materials and the influence of Sbulk/Ssurface on the photoelectric process of TiO₂ were studied.The detailed work of this paper as follows:（1）Crystallization is one of the most important properties for materials.The efficiency of perovskite solar cells depends on the crystallinity and crystallinity morphology of organo-inorganic hybrid perovskites,and its crystallization behavior partly depends on the processing conditions.We used a one-step spin-coating process to prepare CH₃NH₃PbI₃ samples.The isothermal and non-isothermal crystallization kinetics of CH₃NH₃PbI₃ was studied by differential scanning calorimetry（DSC）.The isothermal crystallization process is described by the JMA model.The results show that the isothermal crystallization behavior has a strong dependence on temperature.The value of Avrami index n indicates that the growth of CH₃NH₃PbI₃ is two-dimensional lamellar mode,and its rate is determined by nucleation and growth.The non-isothermal crystallization process is described by the Jeziorny and Mo models.With the increase of cooling rate,the crystallization kinetics constant increases and the crystallization rate become fast.The value of n decreased with the increasing of the cooling rate,which indicates a low degree of full crystallization.（2）The research of ferroelectricity is relatively rare for CH₃NH₃PbI₃ and the existence of ferroelectricity in CH₃NH₃PbI₃ is still controversial.We investigated the effect of preparation conditions on the ferroelectricity of CH₃NH₃PbI₃ thin fi]ms by piezoforce microscopy（PFM）.In the PFM measurement,the hysteresis loop,the butterfly curve and the ferroelectric domain structure were all obtained,which indicated the existence of ferroelectricity of CH₃NH₃PbI₃.In addition,with the increase of annealing temperature,the crystallinity of the film and the uniformity of the crystal gradually improved,and the phase difference between the positive and negative regions was gradually strengthened.Therefore,the flip-flop of the CH₃NH₃PbI₃ ferroelectric domain is also different with the change of annealing temperatures.（3）Chloride is usually used as dopants to improve the transport performance of CH₃NH₃PbI₃.However,the role of chloride in improving efficiency and stability is not clear.We used the DSC technique to study the stability and energy loss of CH₃NH₃PbI₃ and CH₃NH₃PbI₃-xClx.We determined that CH3NH3Pbl3-xClx was stable than CH₃NH₃PbI₃ in the working optical wavelength range of the cell.In addition,photothermal studies have shown that 20 percent more light energy was transformed to heat in CH₃NH₃PbI₃ than in CH₃NH₃PbI₃-xClx.Moreover,the more heat released in CH₃NH₃PbI₃ may cause the occurrence of phase transition,and the decrease of the efficiency further.These results provide a new sight to explain why the addition of chloride can improve the battery efficiency.（4）The special structure of TiO₂ nanosheets array with {116} exposed facets may improve the performance of perovskite solar cell.The TiO₂ nanosheets were prepared on the FTO substrate by hydrothermal method,and the relationship between the defects and the annealing temperature was discussed preliminarily.Combined with positron annihilation spectroscopy.EPR and XPS,we conclude that bulk defects are Ti3+ related vacancy defectss,and the presence of adsorbed oxygen on the surface can inhibit the separation and migration of electrons-holes and thus reduce the photocatalytic efficiency.By changing the concentration ratio of Sbulk/Ssurface defects in TiO₂ nanosheets,the electron-hole separation efficiency can be improved,thereby significantly improving the photocurrent and photocatalytic efficiency.The above results show that the Sbulk/Ssurtace has a great influence on the photoelectric process in the TiO₂ nanosheets and therefore will affects the performance of the solar cells used the TiO₂ nanosheets.However,due to time and conditions restriction,we have not study the application of TiO₂ nanosheets in perovskite solar cells.