Ultrafast Dynamics Study Of Polymer Solar Cell Materials

Charge transfer(CT)in conjugated polymer solar cell materials is a very important photophysical process.It is valuable to improve the photoelectric conversion efficiency by specifically analyzing the charge transfer process.At present,although researchers have been studying the ultrafast kinetics processes of polymer solar cells,there is still no consistent understanding of the ultrafast kinetics of the excited state of conjugated polymers.In this paper,the ultrafast excited state kinetics of donor and acceptor D-?-A conjugated polymers were studied by femtosecond transient absorption spectroscopy.In the course of experimental research,we mainly involved instruments such as femtosecond laser system,TOPAS system and Helios Fire pump detection system.The femtosecond laser system provides the light source,which is the basis of the optical experiment.The TOPAS system can tune the wavelength of the pump light.To achieve the wavelength we need,the Helios Fire Pump Detection System acquires transient absorption spectral data by adjusting the delay between the pump light and the probe light.The pump light is tunable in the range of 240-2600 nm,which is continuously adjustable from the ultraviolet to the infrared.The wavelength of the probe light can be divided into four wavelengths: ultraviolet(320-650 nm),visible(400-780 nm),800(760-840 nm)and infrared(820-1600 nm).These pumping and detection bands are designed to meet the ultra-fast dynamics of various material systems.The conjugated polymer solar cell material samples we tested were carried out under the condition of a pump light wavelength of 500 nm.The detected light was in the visible and infrared bands,respectively,and finally we obtained the femtosecond transient absorption spectrum data.By making use of Surface Xplorer software to deal with and analyze the collected transient absorption data,the ultrafast dynamic process of conjugated polymer solar cell materials is obtained,and the exciton transition paths are determined and their ultrafast relaxation processes are fitted.In addition,we also determined the generation and complex kinetics of free carriers in conjugated polymers,which are produced by Coulomb-bound charge transfer(CT)states.The CT state has a lower energy local state,which is based on disorder.Fluorescence is emitted during the CT state recombination,which is called geminate recombination.Free carrier recombination is a non-geminate recombination.We analyze ultrafast kinetic processes based on geminate recombination of CT states and non-geminate recombination of free carriers.From the perspective of recombination kinetics,we finally analyze the time constant of the CT state transition to the free carrier state and the time constant of the non-geminate recombination lifetime of the free carrier.These parameters are important for improving the energy conversion efficiency of solar cells.In addition,by analyzing the ultrafast relaxation path and the recombination kinetics,the energy level structure of the polymer solar cell material is finally determined,there is very important guiding significance for solar cell to improve energy conversion efficiency.Therefore,by making use of femtosecond transient absorption spectroscopy technology,we have a deeper understanding of the microscopic mechanism of conjugated polymer solar cells,which is conducive to the further development of organic solar cells.