External Electric Field Effects On The Charge Transfer Properties Of Organic Semiconductors: A Theoretical Study

In recent years,organic semiconductors have attracted enormous attention due to their applications in organic field-effect transistors,organic light-emitting diodes,etc.Although device performances are constantly improving,the charge carrier mobilities and air stabilities of organic semiconductors still far lag behind their inorganic counterparts.For real applications meanwhile taking full advantages?e.g.flexible and light in weight?from the organic nature,it is therefore urgent to improve the material electrical property and relevant device fabrication.In this dissertation,by utilizing the density functional theory?DFT?at the M06-2X/6-31+g*level,we performed an in-depth study on two prototypesof organic semiconductors,quaterthiophene?p-type?and perylene diimide?n-type?.Distinct from the reported,this work systematically investigated the impacts of the external electric field on the charge transfer preference and air stability of targeted materials.It is found external electric field can alter the material charge transport efficiency by effectively tuning the hole/electron reorganization energy.F_x lowers the??552 meV?of quaterthiophene at the intensity of 1 V/?down to a value?125 meV?.In contrast,F_z applied to quaterthiophene elevates?by 48%when the field intensity increases from 0 to 0.5 V/?.Furthermore,aiming to solve the problem that perylene diimide easily degrade when exposed to air,we investigated the relevant degradation mechanisms at the molecular level.We discovered external electric field can either accelerate or hamper the degradation process depending on the field direction.For O₃?O₂?,B2 reaction site requires 10.3?26.5?kcal/mol to initiate the electrophilic addition when F=0 V/?.When F_-z=0.75 V/?,the reaction barrier is 17.7?37.7?kcal/mol.However,When F_+z=0.75 V/?,the reaction barrier is only 4.4?16.9?kcal/mol,indicating at strong F_+zz perylene diimide easily reacts with ambient oxygens hence degrades device performance.The external electric field is a double-edged sword.To fully enjoy the property promotion brought by the proper external field,it is necessary to ensure long-range order and properly placed electrodes.We hope our results and discoveries can provide design principles to experimentalists,who intend to design or improve organic-based devices with superior semiconducting properties.