Solvent Engineering For Improving Crystallization Of Perovskite Films And Performance Of Perovskite-based Optoelectronic Devices

Perovskite material materials have a varity of advantages,such as excellent photoelectric properties,low cost and simple preparation process and etc.In recent years,perovskite materials have already become one of the most attractive materials which are studied by researchers for fabricating photoelectonic devices.The efficiency of perovskite solar cells?PSCs?is increased from 3.8%in 2009 to more than 25%recently.High-performance perovskite photodetectors with wide spectrum and fast response speed have also been extensively studied.However,in the research process of perovskite materials,the crystallization of perovskite films is always one of the most important problems for the preparation of high-performance optoelectronic devices.The solution procedure is the most widely used because of its simple process and low cost.The improved crystallinity of films here should be assigned to larger grain sizes,fewer grain boundaries based on the morphology of films and better crystallization quality of crystallites.The non-radiative recombination of carriers in the perovskite can be efficiently suppressed when the improved crystallinity of films is obtained.At the same time,the photoelectric performances of the devices are also enhanced.The residual lead iodide precursor is also one of the key problems for improving the crystallinity of perovskite films.Too much residual Pb I2 seriously influences the performances of perovskite optical electronic devices.Meanwhile,stable crystalline phase of perovskite is also required.This dissertation mainly focuses on the crystallinity control of perovskite films via solvent engineering.We mainly improve the crystallinity and quality of perovskite films and systemically study the performance of perovskite optoelectronic devices including solar cells or photodetectors based on solvent engineering.The specific content is mainly summarized into the following four aspects:1.The solvent annealing process is studied in sequential deposition method to reduce the residual Pb I2 and improve the crystallization of perovskite films.It is mainly divided into two aspects:First,solvent annealing process of dimethyl sulfoxide?DMSO?is employed for the preparation of lead iodide precursor films.DMSO solvent annealing shows unique phenomenon.The crystallization of Pb I2 films are suppressed due to the strong coordination ability between DMSO molecules and Pb²⁺ions.After annealing,mesoporous morphology of Pb I2 films is obtained because of the volatilization of DMSO molecules.This specific mesoporous morphology increases the specific surface area of Pb I2 films which leads to a better reaction between Pb I2 and methyl ammonium iodide?MAI?.The perovskite films with better crystallinity and less residual Pb I2 are successfully made.PSCs with the power conversion efficiency?PCE?of 18.5%have been obtained.Second,the sequential weak coordination solvent?isopropanol,IPA?annealing for both Pb I2 and perovskite films is studied.It is found that the IPA solvent mainly influences the atmosphere of the crystallization of Pb I2 and perovskite films.The IPA atmosphere is also beneficial for the formation of perovskite nuclei because of the solubility of IPA for organic precursor in perovskites.Perovskite films with better crystallization and denser coverage are obtained after IPA solvent annealing process.Perfermances of both CH3NH3Pb I3 and Cs Pb I3 solar cells are also improved.In summary,our results show that the solvent annealing process can effectively control the crystallization of the perovskite precursor films and perovskite films in sequential deposition process.Our research is very meaningful for investigating the formation dynamics of perovskites films via the solution process.2.The mixed anti-solvent process is studied to improve the crystallization of the perovskite films and the performance of devices.The following two aspects are mainly studied:First,IPA is added into the anti-solvent chlorobenzene to form mixed solvent.This mixed anti-solvent is employed into the anti-solvent process to control the morphology and crystallization of triple cation perovskite films.It is found that the isopropanol solvent with a weakly solubility of perovskite,low-boiling point is the ideal additive solvent for effectively improve the crystallization and surface morphology of perovskite films.Based on this method,the crystallization and surface morphology of perovskite films are improved.PSCs with the best PCE of 19.4%are obtained.Second,[6,6]-phenyl-C61-butyric acid methyl ester?PCBM?is added into chlorobenzene to prepare PCBM-perovskite heterojunction films.It is found that PCBM is distributed at grain boundaries and the surface of perovskite films.The defects at grain boundaries are passivated.The introduction of PCBM effectively improves the crystallization and stability of perovskite films.Based on PCBM-perovskite heterojunction,photodetectors with the responsibility of 0.18?AW^-1 and the response time of 123?ms are fabricated.Our research results show that mixed solvent process is an efficient way to improve the crystallization of perovskite films and promote the performance of optoelectronic devices.3.In this part we use the solution method to make two-dimensional and three-dimensional perovskites.Based on the previous work of DMSO solvent annealing,butylamine iodide?BAI?is studied in sequential deposition process to squeeze and slice the lattice of Pb I2 precursor.The crystallization of lead iodide is inhibited by BAI molecules.In the second step,BAI re-dissolves and diffuses into the IPA solvent.After annealing,the BA2MAPb2I7?n=2?two-dimensional?2D?perovskite is formed and doped into three-dimensional?3D?MAPb I3 perovskites.In this process,the reaction between MAI and lead iodide is also promoted.PSCs with the best PCE of 18%are fabricated.The introduction of BAI and 2D perovskites enhances the stability of PSCs.This research is very meaningful for the modification of Pb I2 and the fabrication of2D/3D perovskites.4.A solvent selective approach is used here to make 2D/3D perovskite heterojunctions.This method is used for selecting appropriate solvents that maintains a balance between the formation of the 2D perovskite film and the dissolution of the 3D perovskite layer.Acetonitrile as the solvent in this method shows the best result because of the weak solubility and the low boiling point.Finally,a clear interface between the2D and the 3D perovskite layer is observed by using Time of Flight Secondary Ion Mass Spectrometry and Grazing Incidence Wide Angle X-ray Scattering measurement,indicating a formation of the 2D/3D perovskite heterojunction.The 2D perovskite improves the crystallization of 3D perovskites.PSCs with the best PCE of 17.2%are achieved.Our research supplies a new pathway of fabricating 2D/3D perovskites through solution methods.In summary,the research of improving the crystallization and quality of perovskite films by using solvent engineering is deeply studied in this thesis.Stable,high-performance PSCs and photodetectors are also successfully fabricated in our research.The efforts in solvent engineering provide a unique and interesting way to improve solution procedures?sequential deposition,anti-solvent process et.al?of fabricating perovskites and devices.Our research is also meaningful for investigating the growth mechanism of high-quality perovskite filmsand the commercial application of perovskite solar cells.