Research On Ambipolar,Defect State And Dielectric Constant In Polymer Semiconductor

The transport of carriers in polymer semiconductors has always been a hot topic.The transport of carriers in polymers mainly includes hole transport in highest occupied molecular orbital(HOMO)and electron transport in lowest unoccupied molecular orbital(LUMO).Through theoretical analysis,we believe that polymer semiconductors should be intrinsically ambipolar as the non-doped inorganic semiconductors.In order to verify this idea,we first study the ambipolar transmission characteristics of the traditional hole injection material PEDOT:PSS.After analyzing its semiconductor characteristics,the impedance spectroscopy method based on particle swarm algorithm is used to analyze the carrier transport characteristics of hole-only and electron-only devices.It is found that the hole and electron mobility of PEDOT:PSS is within an order of magnitude in the two different thicknesses of the film.Based on this,we continue to use impedance spectroscopy to study the carrier transport characteristics of traditional hole transport materials P3HT,MEH-PPV and PVK,and find that their hole and electron mobility are within an order of magnitude.Therefore,the ambipolar transport characteristic is the intrinsic characteristic of polymer semiconductors.On the basis of the experiment of ambipolar transmission,we observe an abnormal negative slope of??E^1/2.There is no morphological evidence that the mobility has an effect on the negative slope.The electric field should have an acceleration effect on the carriers,and the negative slope must be the result of energy loss.Simple theoretical calculations show that a single polymer segment vibrates 10⁷ to 10⁸ times during the process of applying an electric field,and this high-frequency vibration is bound to affect the microstructure of the molecule.Therefore,the concept of vibration relaxation is proposed to explain the negative slope phenomenon.When an electric field is applied,the molecular vibration becomes more intense,and the influence of carrier energy relaxation exceeds the influence of the electric field,So the mobility appears to decreases as the electric field increases.In order to better understand the vibration relaxation,infrared spectra are obtained from experimental and theoretical aspects,and the vibration modes of polymer molecules were analyzed.Defect states are ubiquitous in polymer semiconductors and closely affect the transport of carriers.Therefore,it is particularly important to understand the origin and characterization methods of defect states.We introduce two methods for detecting defect states:AS method and DLCP method.Then the defect states of the ITO/P3HT/Al device are detected by two methods respectively,and the spatial distribution,energy distribution and density of states of the defect states are presented,and the detailed data processing process is shown.In addition,it is analyzed that the DLCP method is seriously affected by the interface state from the perspective of theory and experiment.Therefore,it is concluded that the AS method is suitable for characterizing the modification effect of the interface in the device,and the DLCP method is suitable for characterizing the defect state of the material itself.Finally,the Gaussian distribution is used to fit the defect states measured by the DLCP method,and it is inferred that the P3HT film contains a large number of electronic defect states,and this defect state is attributed to the structural disorder of the polymer.All in all,we elaborate the principles and detection process of AS and DLCP in detail,analyze the advantages and disadvantages and applicable conditions of the two methods,and perform experimental comparisons with P3HT as the sample material.The photoelectric properties of organic semiconductors are characterized by their low dielectric constant.Although the dielectric constant is very important for research in the field of organic electronics,there is no in-depth study of the dielectric constant in the literature.When necessary,most studies simply approximate the relative dielectric constant?=3.Therefore,it is necessary to conduct a detailed study on the dielectric constant.We designed the hole-only and electron-only structure devices of P3HT to study the dielectric constant.The larger dielectric constant in the hole device is attributed to the larger capacitance contribution.In addition,the influence of the bias voltage is also studied,and it is found that the dependence of the dielectric constant on the electric field in the two devices is exactly opposite.In short,we study the carrier transport characteristics,defect states and dielectric constants of polymer semiconductors,with the hope to provide a little help for researchers in designing new materials and improving device performance.