Ultrafast Kinetics Of Cuprous Thiocyanate (CuSCN) Semiconductor Thin Films

As a P-type semiconductor material,copper thiocyanate(CuSCN)has become one of the potential and replacable materials for organic hole transport materials due to its excellent optical transparency and hole mobility,which has been successfully used as hole transport layer of the photovoltaic device.Most of researches were focused on improving the conversion efficiency of CuSCN-based solar cells or other photovoltaic devices.Few researches has been done at the molecular level basis to understand the working principle of CuSCN as a photovoltaic device.However,the improvement of its conversion efficiency is inseparable from the rapid recombination and migration of carriers in CuSCN semiconductors.In addition,the physical process of this kind of carrier usually takes place on a very short time scale(fs-ns:10'12-10-9 s),the commonly used characterization methods,such as NMR or XRD,cannot monitor this ultrafast change.The ultrafast multi-dimensional vibrational spectroscopy technology has been proved to be a suitable method for studying the dynamic processes of solid-state thin film materials at the molecular level.Based on this,the home-made multi-dimensional ultrafast spectrum platform was used to study the ultrafast dynamics process in CuSCN thin films at the molecular level in this thesis.In addition,the carrier dynamics in CuSCN thin films were investigated in combination with femtosecond transient absorption spectroscopy technology.The effects of the addition of carbon-based substrate materials on the carrier dynamics of CuSCN films were compared,and corresponding energy level models were established to help understand the photophysical processes of the carriers.The main research content can be divided into the following two parts:1.Vibrational relaxation dynamics of semiconductor CuSCN thin filmsThe vibrational relaxation dynamics of CN groups in CuSCN films with different thicknesses were studied using pump-probe technology.The FTIR spectra of the film showed a sharp CN vibrational stretching mode,and the crystal form of the film with different thickness were unchanged.The results of IR pump-IR probe showed that the vibrational life of CN vibrational stretching is significantly reduced compared with that in KSCN film,which was due to the rapid energy transfer in CuSCN film;at the same time,anisotropy decay had been observed abnormal behavior,which was due to the thermal effects caused by CN vibrational stretching.More specific analysis showed that the anisotropy curve can be fitted well with a tri-exponential decay function,which represented three processes,namely the resonance energy transfer of the thin film,the heating process of the CuSCN lattice and the rotation of the SCN molecules.In addition,the experiment of Vis pump-IR probe discussed the relationship between carrier and CN vibrational relaxation.The results showed that the carrier scattering and relaxation processes in CuSCN films are closely related to the vibrational excitation and relaxation dynamics of the CN stretching vibrational mode.2.The effect of different substrates on the carrier dynamics of semiconductor CuSCN thin filmsThe carrier dynamics of CuSCN films and different carbon-based material substrates on the carrier dynamics in CuSCN films were studied by femtosecond transient absorption technology.The CuSCN film was excited at 300 nm.The results showed that the CuSCN film undergone two ultrafast processes,namely exciton stimulated emission and exciton recombination.The exciton recombination was mainly achieved through nonradiative electron-hole recombination.At this time,there was heat release in the film.By fitting the dynamics curve of CuSCN thin film,dynamics parameter information of each process were obtained,and a simplified energy level model was proposed.In addition,the effects of the addition of different carbon-based materials under CuSCN films on the carrier dynamics in CuSCN films were discussed.The simplified energy level models and the fit of dynamics curves indicated that the addition of different carbon-based materials could affect the heat transfer process of CuSCN films and cause different carrier dynamics in CuSCN films.