Research On Density Of States In Organic Semiconductors And Related Properties Of Material And Device

Organic electronics is emerging as a high potential technology for low-cost and large-area electronics.Disordered organic semiconductors are widely used in organic devices such as organic light-emitting diodes,field-effect transistor,and photovoltaic devices,and sensors.Charge-carrier transport is a fundamental issue of disordered organic semiconductors.The density of states(DOS)is the key factor to determine the charge transport and photoelectric properties of these materials.Although the Gaussian DOS is the most popular,some works support the exponential DOS or combination of both forms.The results show that the experimental mobility-density data cannot be fitted by using single Gaussian and exponential-type DOS.Based on this question,in this dissertation we proposed four exponential-type DOSs,and applied those DOSs to investigate the mobily of carriers,the general Einstein relation and Seebeck coefficient in organic semiconductor.The main contents of this dissertation are as follows:1.Four exponential-type DOSs are proposed.One has complete exponential tail,and other three clearly cut tails at some places.The variations of mobility with carrier density are obtained through numerically solving variable range hopping equations.2.It is shown that the relationships of mobility with density and Fermi level are very different among results from Gaussian,un-cutting and cutting exponential-type DOSs.Comparing with experimental data,it is shown that the mobility-density data can be well fitted by using single clearly cutting exponential-type DOS in the wide ranges of density,but cannot be fitted by using single Gaussian and un-cutting exponential-type DOS.Instead of the Gaussian and pure exponential DOS,the DOSs with exponential core and clearly cutting tail are recommended.3.The general Einstein relation(GER)is calculated based on the Gaussian and cutting exponential-type DOS.The variations of the GRE coefficient ? with temperature and density are analyzed.It is shown that the ? are a gradually decreasing function with temperature and similar for both DOSs.But variations of ? with density are very different for both DOSs.The ? is a gradually increasing function of density for the Gaussian DOS,but a non-monotonously increasing function of density for the CCME DOS with a platform located in the typical range of density.The ? is assumed as a constant to analyze the data of ideality factor for two organic diodes,the theoretical results are in agreement with experimental data.4.Seebeck coefficient is analyzed based on the cutting exponential-type DOS,general Einstein relation and genuine transport energy.The analytic expressions of carrier concentration,conductivity and Seebeck coefficient are obtained.The dependence of Seebeck coefficient on carrier concentration and temperature can be evaluated.The numerical results with optimized parameters describe the experimental data well.