Research On Post-processing Perovskite Films And Their Application In Perovskite Solar Cell

Perovskite solar cells(PSCs)have been becoming the most promising candidate for the next generation photovoltaic technology for their low cost of fabrication,ease of processing and high performance.Perovskite light absorbance layer plays a key role in a PSC.High-quality perovskite film is a prerequisite to achieve desirable photovoltaic performance for corresponding device.The methodologies for high-quality perovskite films formation could be roughly divided into two categories:(i)optimizing deposition processes of perovskite films;(ii)adopting effective post-treatment strategies.This thesis is based on post-treatment strategies to control the growth and morphology of perovskite thin films.A one-step antisolvent method was used to prepare the CH3NH3PbI3(MAPbI3)perovskite films,and a multi-step annealing strategy based on DMF/DMSO mixed solvent vapor was used for post-processing the film for the first time.Compared with pure DMF solvent or pure DMSO solvent,a certain proportion of DMF/DMSO mixed solvents exhibit synergistic effect,which show a better control of the crystal growth and film morphology.As a result,the mean efficiency of devices was increased from 15.85%to 19.51%,and the efficiency distribution of the optimized devices fabricated in different batches was narrow,proving that this post-processing process has good repeatability and reliability.Therefore,it provides a feasible approach for dramatically improving the quality of perovskite films in industrial up-scaling fabrication.The antisolvent method based on the spin-coating process is not scalable and can not be applied to industrial production.Therefore,the development of antisolvent-free preparation process towards high-quality perovskite films is of great significant.In this work,pristine MAPbI3 perovskite films with terrible morphology were deposited by a one-step direct spincoating method.Then,a joint post-treatment strategy containing MGT and MSA processes was used to reconstruct and repair the initial films.As a consequence,the discontinuous,rough and low coverage initial films were finally transformed into homogeneous,dense,smooth ones with large grains.The devices based on the pristine films showed no power conversion efficiency.By constract,when the films were reshaped,corresponding solar cells chould afford efficiencies beyond 17%with an active area of 0.16 cm2.Moreover,efficiencies as high as 15%could be still delivered when the working area of a single device is scaling up to 10cm2.In consequence,the joint post-treatment strategy is undoubtedly a facile,cost-efficient and robust approach for the production of high-quality,large-area perovskite films.It can be effectively combined with industrial-scale deposition techniques,enabling a low requirement of film quality(eg.,film morphology,grain size)in the film formation stage.Due to the conduction band minimum(CBM)of the SnO2 electron transport layer is deep,it exhibits a large energy level offset compared with the CBM of MAPbI3.Besides,the humidity and thermal stability of MAPbI3 is poor.In this context,triple-cation perovskite material(CsFAMA)was employed as light absorption layer in my next works,for its lower CBM,better stability and photoelectric property.The crystal growth and morphology of the CsFAMA perovskite thin films were regulated.The effects of annealing and annealing time on CsFAMA perovskite films and their solar cells were studied.In a certain annealing time,the device efficiency increases gradually with the annealing time prolonged,and the hysteresis is obviously suppressed.The 45 min annealed devices showed the lowest hysteresis and best working stability.Further extending the annealing time,the efficiency of devices remained stable or increased slightly,but the J-V hysteresis aggravated.The annealing time exhibited little effect on the surface morphology of the thin films,but the grain boundaries on the cross section gradually fused with the extension of annealing time.Meanwhile,the thermal induced PbI2 increased gradually,which is considered to be the main reason for the aggravation of J-V hysteresis and deterioration of operation stability.More interestingly,we found that the unannealed CsFAMA perovskite films exhibited ultra-strong PL emission intensities,which gradually decreased as a function of anneaing time.We infer that it is the quantum confinement effect of crystal nuclei and the cage effect of PbI2-DMSO based intermediate phase impeded the transport and non-radiative recombination of the photo-generated carriers,and thus induced highly efficient radiative recombination.