Study On Ultrafast Dynamics Of Sb₂Se₃ Under Photoexcitation

The development of ultrafast technology has brought conveniences to detect ultrafast dynamics in matter.Studying ultrafast processes such as atomic oscillation around the equilibrium position and carrier migration will help us better understand the properties of materials and improve the performance of materials.Selenium Telluride?Sb₂Se₃?are widely used in the photovoltaic field due to its attractive properties of suitable band gap,high absorption coefficient,low cost,and low toxicity.In this thesis,the dynamics of the photovoltaic material Sb₂Se₃ were measured employing the reflective pump-probe system and the transient absorption spectroscopy.The works carried out are as follows:?1?Construction of a reflective ultrafast pump-probe system.Compared to transient absorption spectroscopy,the pump-probe system with single-wavelength has a higher signal-to-noise ratio and time resolution,suitable for studying the coherent phonon oscillations.The reflective pump-probe system uses femtosecond laser pulse with wavelength of 800 nm and pulse width of 34 fs as the light source.The output from the laser is divided into two arms with precise time correlation.The arm with more energy is used as the pump light to stimulate the sample dynamics.The other arm is used as the probe light to capture the sample reflection signal.Subsequently,the critical parameters of the system are measured,including the pulse width of the laser pulse before the sample?about 48 fs?,the spot radius at the focus position?about 6.5?m?and the time zero of the system.Besides,the system is verified for long-term measurement work by repeated measurements.Finally,a graphite sample is used for demonstration experiments,confirming that the built system is capable for testing Sb₂Se₃.?2?Detection of Sb₂Se₃ coherent optical phonons.The amorphous Sb₂Se₃ sample is probed by the reflective pump-probe system.In this experiment,the laser pulse of 800 nm is used to excite and detect the sample.It is seen that the coherent optical phonon has an oscillation frequency of about 5.4 THz and a lifetime of less than 1 ps,which is excited by directly interacting of the phonon system with the excitation photons.At the same time,the oscillation amplitude of the optical phonon also increases with the increment of the pump fluence,which is consistent with the description of the theoretical formula.However,the coherent optical phonon in the amorphous Sb₂Se₃ specimen is not affected by the polarization of the excitation light,which is attributed to the random orientation of amorphous.?3?Study on the photogenerated carrier dynamics of Sb₂Se₃.This work included four parts.First,we studied the effect of the pump fluence on the band gap.By increasing the pump fluence,the blue shift of the absorption peak,corresponding to the bleaching peak,is observed in the absorption spectrum at the same delay time,which is attributed to the competition between the Burstein-Moss band filling effect and the band gap renormalization.Second,by analyzing the absorption curve of Sb₂Se₃ sample,the relaxation of Sb₂Se₃ is mainly divided into three processes:one is the rapid electron-electron scattering,which restores the equilibrium of the electronic system with a higher temperature;the other is the carrier trapped in the defect level,which is characterized by a slower decay process;the third is the long relaxation process of the carriers,and the absorption signal remains almost unchanged during the 8 ns range.The carrier lifetime of Sb2Se3 is determined approximately by a triple-exponential decay fit.Also,the carrier recombination with the blue shift of the bleaching peak occurs under the high fluence,and the shift degree is positively correlated with the fluence.We explained it by the Auger recombination.Third,we studied the relation between the carrier lifetime and the pump fluence.By increasing the pump fluence,the lifetime of the Sb2Se3 carrier decreases.Because the increase of the pump fluence will bring a higher temperatures of the excited electronic system and the balanced lattice system,more phonon modes will be stimulated,intensifying the electron-phonon scattering effect;at the same time,the high fluence will lead to the Auger recombination,thereby the relaxation time reduced.Last,we observed that the carrier lifetime increased with the increase of the sample thickness,which is attributed to the surface recombination and the effect of the deep-level defects.