Theoretical Study On Carrier Transport Properties Of A Series Of Halogenated/Heterocyclic Acene Derivatives

OFETs have attracted increasing interest from both academic and industrial communities due to their attractive advantages of lightweight,low-cost,flexibility and easy processability,which make them promising for the next generation electronic circuits,such as radio-frequency identification tags?RFIDs?,organic field-effect transistors?OFETs?,organic photovoltaic cells?OPVs?,organic light emitting displays?OLEDs?and sensors.The macroscopic performance of these microelectronic and optoelectronic devices is greatly related to the microscopic charge transport process.In general,organic semiconductors can be divided into two categories 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 it should work and how to optimize it.Based on these understandings,we further propose promising strategies of molecular design and critical rules for molecule selection.Therefore,the unipolar p-type 6,13-bis?triisopropyl-silylethynyl?-pentacene?TIPS-PEN?,unipolar n-type 6,13-bis??triisopropylsilyl?ethynyl?quinoxalino[2,3-b]phenazine?TIPS-TAP,3a?derivatives and ambipolar aza-pentacene derivatives were selected.Marcus/FGR electron transfer theory and dynamic monte carlo simulation were used to systematically study the charge transfer properties of these two derivatives.We also investigated the effect of concentration of thiophene in DPP polymers on the ambipolarity of the materials.Our results not only reveal the relationship between the structure-charge transfer properties of a single molecule,but also deeply study the effect of intermolecular stacking mode and the dynamic disorder of crystal structure on charge transfer,providing a theoretical basis for the design and synthesis of organic semiconductor materials with high mobility in the future.The conclusions of our research can be summarized as follows:1.Theoretical Study on Charge Transport Properties of Intra-and Extra-Ring Substituted Pentacene DerivativesA series of pentacene derivatives,halogen-substituted and thiophene-and pyridine-substituted,have been studied on the electronic properties and charge transport properties using the density functional theory and classical Marcus charge transfer theory.The transport properties of holes and electrons have been focused to get insight into the effect of halogenation and heteroatom substitution on transport and injection of charge carrier.The calculation results revealed that fluorination and chlorination can effectively lower LUMO level,modulate the hole/electron reorganization energy??_h/?_e?,improve the stacking mode of crystal structure and enhance the ambipolar character.Chlorination gives a better ambipolar character.On the basis of halogen-substitution,the substitution of terminal benzene ring of TIPS-PEN by a thiophene or pyridine will largely lower the LUMO level,improve stacking mode,and become more suitable ambipolar materials.Hence,both intra-and extra-ring substitution are favorable to enhance the ambipolar transport property of TIPS-PEN.2.Theoretical Study of Synergy Effect between Halogenation and Pyrazine Substitutions on Transport Properties of Silylethynylated PentacenesCombining quantum tunneling effect enabled hopping theory with kinetic Monte Carlo simulation as well as dynamic disorder effects;the charge transport properties of a series of N-hetero TIPS-PEN derivatives with halogen-substitution were studied.Based on both single molecule and theoretically predicted crystal structure,electronic structures and nuclear dynamic disorder effect of halogen-and aza-systems and its impact on charge transport behavior were expounded.On the one hand,this study revealed the regularity of carrier transport by the substitution with one or two pyrazine rings from the external to internal for benzene ring in TIPS-PEN,suggesting the substitution of two internal-pyrazine rings,can sharply shrink the hole transport feature.Meanwhile,internal-pyrazine ring improves the electron transport performance mainly by enhancing air stability and making?-stacking compactness.On the other hand,the investigation elaborated the delicate tuning effect of chlorination on two kinds of carrier transport based on aza-TIPS-PEN derivatives,indicating chlorination can effectively equilibrate the electron and hole reorganization energy,decrease the slip distances along the molecular long and short axes in favor of the coming of hole transfer integral closer to electron one,and thus achieve more balanced ambipolar transport characteristic.The coordination of monopyrazine substitution and chlorination on acene is an effective way to acquire ambipolar transport property.In addition,nuclear dynamic disorder effect affects more in the hole transfer integral than in the electron transfer integral.In the case of comprehensive consideration of both the ease of charge injection and the intrinsic transportmobility,both8,9,10,11-tetrafluoro-6,13-bis??triisopropylsilyl?ethynyl?naphtho[2,3-b]phenazine?2b?and tetrachloro-6,13-bis??triisopropylsilyl?ethynyl?quinoxalino[2,3-b]phenazine?3c?manifested to be electron-dominant materials,while 8,9,10,11-tetrachloro-6,13-bis??triisopropylsilyl?ethynyl?naphtho[2,3-b]phenazine?2c?showed well ambipolar transport characteristic with the average hole/electron mobility of 3.64/2.21 cm² V^-1 s^-1.3.Theoretical Study on Charge Transport Properties of Copolymers of DPP and Oligo-thiopheneThe electronic structures and charge transport properties of the oligomers of diketopyrrolopyrrole?DPP?with the thiophene were investigated by using ab initio density functional theory and classical Marcus charge transport theory.The results show that with the increase of the DPP concentration?or decrease of the thiophene numbers?in the polymer unit both the HOMO and LUMO energy levels of the polymers decrease,and the band gaps narrow.As the DPP concentration increased,the intrachain overlap of the electron wave functions of the DPP acceptor units is improved,that facilitates the intrachain electron transport.Moreover,the increase of DPP concentration strengthens the rigidity of the molecular backbone and enhances the interchain overlap of LUMO orbitals,which strengthens the electron transfer integrals and the system is converted from p-type into ambipolar materials.