Study On N-type Electrochromism And Electrochemical Doping Behavior Of Organic Semiconductors

Compared with inorganic semiconductors,organic semiconductors are widely concerned by researchers with advantages of easy modification,easy processing,high flexibility and low cost.In addition to representative applications such as organic field effect transistors(OFET),organic photovoltaic(OPV)and organic light-emitting diodes(OLED),electrochromism which produces reversible changes in optical properties under an applied electric field is also one of the important applications of organic semiconductors.The working process of electrochromism is accompanied by the process of doping and de-doping of organic semiconductors.Through the interaction between electrode injected charge and electrolyte ions,stable ionic species could form,which are easy to be detected by electrochemical,spectral and spectroelectrochemistry measurements.Thus,quantitatively studies of the doping behavior of organic semiconductors can be performed.Among organic electrochromic materials,high-performance p-doped materials are widely studied,while stable n-doped materials are extremely scarce.Therefore,aiming at the design of high-performance n-type organic electrochromic materials and the qualitative and quantitative research on the doping behavior of organic semiconductors,the following research contents have been carried out:(1)Considering that the instability of n-type doping is caused by energy level,utilizing chlorine substituted perylene bisimide with low lowest unoccupied orbital energy level(-4.1e V)as chromophore and thiophene with high polymerization efficiency as crosslinking group,n-type organic electrochromic electrode films with high optical contrast and long cycle life were prepared on ITO by electropolymerization.The optical contrast of poly(Th-Cl-PBI)with a thickness of 280 nm at 680 nm wavelength is 95.7%,and the electrochemical cycle life and spectral cycle life are more than 2000 cycles and 500 cycles,respectively.By using tetraoctyl ammonium tetrafluoroborate with larger cation size,the damage of over-doping and overpotential can be effectively limited,resulting an increase to more than 2000 cycles spectral cycle life at the cost of reducing a certain optical contrast.(2)Based on the characteristics of absorption spectrum and electrochromic process,four theoretical calculation models of the relationship between optical contrast and thickness are established.Through the derivation of graphic and mathematical relations,it has been pointed out that the maximum optical contrast is positively correlated with the ratio of linear absorption coefficient of bleached state to colored state.The larger the linear absorption coefficient of colored state,the smaller the optimal thickness required to reach the maximum.Based on poly(Th-Cl-PBI),poly(3,4-ethylene dioxythiophene)and polypyrrole,the mathematical relationship between relative doping degree and thickness is explored,and the calculation model of optical contrast in incomplete doping state is further improved.(3)A new concept of "non-equilibrium energy level" is proposed.For the different situations of electrically injected carriers and photogenerated carriers,as well as p-doped materials and n-doped materials,the influence of charge accumulation caused by current density on the energy level displacement of organic semiconductors due to the large difference of electron and hole carrier mobility in the same material has been analyzed,respectively.Moreover,the generation process of high-energy exciton state in OLED and electromotive force under small energy level difference in OPV are explained by non-equilibrium energy level model.