Dynamics Of CdSe Quantum Dots And Perovskite Films By Transient Absorption Spectroscopy

Energy shortage and environmental pollution have always been problems that people urgently need to solve.The emergence of clean energy has brought people the dawn.Perovskite and quantum dots are favored by people because of their low cost and ease of manufacturing.Although a lot of research has been done on perovskite and quantum dots to increase the photoelectric conversion efficiency in an exponential form,the internal mechanism has always puzzled people.The emergence of femtosecond transient absorption can solve this problem,which is very important for the progress of science.In this paper,we performed ultrafast kinetic studies on CdSe quantum dots and perovskite films using femtosecond transient absorption spectroscopy.We first studied the energy level transition of CdSe quantum dots.In this part,we tested the steadystate absorption spectrum and transient absorption spectrum of CdSe quantum dots,and found that CdSe quantum dots have four ground-state bleaching and steady-state absorption peaks.We use different pump light to excite CdSe quantum dots.When the pump wavelength is 330 nm,the four ground state bleaching peaks are 385 nm,412 nm,468 nm and 512 nm.After changing the pump wavelength(500 nm),four ground state bleaching peaks can still be observed.The data shows that the 412 nm ground state bleaching is related to 1S(e).Combined with the division of energy level transition by D.J.Norris group,we give the correct energy level transition diagram.We then used TA spectroscopy to investigate the cooling of the thermal excitons and the transfer of holes in CdSe quantum dots.Because the hole states 1S3/2 and 2S3/2 can be separated in quantum dots,the dynamics of hot hole relaxation can be directly observed.We found that the 2S3/2 state has a lifetime of about 6 ps,and there is also a lifetime of a few picoseconds in 1S3/2,which is consistent with the results of previous studies.Therefore,this experiment introduces CdSe quantum dots with separated hole states and derives the hole relaxation lifetime,which provides strong evidence for the study of relaxation dynamics.Secondly,we studied the effect of pre-heat treatment and delayed process on ITO/PEDOT:PSS/CH3NH3 Pb Ix Cl3-x thin film.After preheating,three samples were placed for 0 h,1 h and 2.5 h,and finally annealed together.Through experiments,it was found that the internal carrier recombination rate of Sample 2 placed for 1 h was slower than that of Sample 1 prepared by conventional annealing process.However,after further preheating for 2.5 h,the rate of carrier recombination is significantly accelerated,which is detrimental to photoelectric conversion.We can also know from the global fitting that after 1 h of pre-heat treatment,the carrier takes the fastest time from the perovskite layer to the hole transport layer.This rapid separation effectively avoids the direct recombination of carriers.However,after further preheating treatment for 2.5 h,the transport time of perovskite carriers from the perovskite layer to the hole transport layer is much slower.The reason for this phenomenon is the passivation of Pb I2 formed when the perovskite is degraded in water,which leads to the reduction of the defect state of the surface of the perovskite crystal,which helps the crystal growth.However,contact with too much water will reduce the number of perovskite crystals.Therefore,the quality of the desired perovskite thin film can be easily fine-tuned by controlling the delayed annealing time.In this way,carriers can be extracted quickly and avoid direct recombination.The final experimental results show that the preheating and delayed annealing process can better adjust the photoelectric conversion efficiency of the perovskite film.