Theoretical Investigation Of Charge Transport Properties In A Series Of Typical Organic Semiconductor Materials

Organic semiconductors based on ?-conjugated materials have drawn much attention due to their great potential applications in low-cost,large-area,and flexible electronic devices.The performance of these optoelectronic and microelectronic devices is highly related to the microscopic charge transfer process.In general,there are two kinds of organic semiconductors in terms of charge carrier species: unipolar organic semiconductors including p-type(hole transport)and n-type(electron transport)materials and ambipolar organic semiconductors capable of both efficient electron and hole transport.The purpose of this dissertation is to theoretically provide the in-depth understanding of charge transfer behavior in both unipolar and ambipolar organic semiconductors and to reveal their structure-property relations,such as what factor works,why should it work and how to optimize it.On the basement of these understandings,we further propose promising strategies of molecular design and critical rules for molecule selection.Therefore,we chose typical unipolar organic semiconductors of p-type thionoacenes and n-type N-heteropentacenes as well as ambipolar organic semiconductors indigoids and their derivatives to study their charge transport properties by Marcus electron-transfer theory combined with kinetic Monte-Carlo simulations.Our results not only unravel the structure-property relations at single molecule level but also shed light on the crucial effects of molecular packings and dynamic disorders,which offers solid support to the further experimental design and synthesis for high-performance organic semiconductors.Our main studies are summarized as follows: 1.Charge transport properties in a series of p-type five-ring-fused thienoacenesPentacene and oligo/polythiophenes are the representative p-type organic semiconductors in the last twenty years.Pentacene has very good p-type charge transport properties but suffers from the oxidative instability.In contrast to pentacene,oligo/polythiophenes are generally very stable in the ambient environment but their charge carrier mobilities are quite low.In this work,the charge transport properties in a series of five-ring-fused thienoacenes have been detailedly investigated.Our results show that,with both high 3-D hole mobilities and good oxidative stability,the thiophene–benzene alternate structures should be better than other two series as p-type organic semiconductors.In addition,a theoretical explanation for the difference between experimental hole mobilities in isomers is provided based on crystal structure prediction.The effect of dynamic disorder has also been investigated and the results indicate that dynamic disorder has a central role in both the 1-D and 2-D charge transport process.2.Understanding the effects of the number of pyrazines and their positions on charge-transport properties in n-type silylethynylated N-heteropentacenesSince the complementary integrated circuits consisting of both p-type and n-type transistors can achieve faster speed,good reliability and stability than unipolar circuits,rational molecular design and synthesis of excellent n-type organic semiconducting materials is urgently required.Here,the charge-transport properties of a series of silylethynylated N-heteropentacenes(TIPS-PEN-xN;x = 2,4)were systematically investigated using Marcus electron-transfer theory coupled with kinetic Monte Carlo simulations.Electronic structure calculations showed that introducing more pyrazine rings decreases the energy levels of the lowest unoccupied molecular orbitals(LUMOs)and should aid electron transfer.The number and the positions of the pyrazine rings greatly influence the molecular packing in crystals and hence the intermolecular electronic coupling.Furthermore,the introduction of internal(rather than external)pyrazine rings leads to a better charge-transport network.Transport parameters evaluated from the hopping and band-like models both demonstrate that,among the TIPS-PEN-x N molecules,B-TIPS-PEN-4N—which has two internal pyrazine rings—is the most promising n-type material.3.Rational design of bio-inspired high-performance ambipolar organic semiconductor materials based on indigo and its derivativesAmbipolar organic semiconductors capable of both efficient electron and hole transport are essential for the development of next generation opto-electronic devices.Indigoids have received much attention as the candidates of sustainable ambipolar organic semiconductor.However,the low charge carrier mobilities extremely limit their practical applications.Therefore,in-depth understanding of their electronic-structure properties and rational molecular modifications are urgently required.Here,we propose a promising strategy to design ambipolar organic semiconductors based on indigo fragments.Moreover,we predicted the organic crystal structures by evolutionary algorithm combined with DFT-D method.Charge transport properties have been significantly improved for the designed molecules,such as narrower energy gaps,higher electron affinity,larger transfer integrals as well as much smaller reorganization energies for hole and electron.Theorefore,remarkable ambipolar charge transport behavior has been predicted,indicating the effective feasibility of our strategies.