Theoretical Study Of Typical Organic Charge Carrier Transporting Materials

Organic Charge Carrier Transporting materials (OCCTM) are organic semi- conductor materials in which charge carrier (electron or hole) can be controlled to directionally move at the electric field when one type of charge carrier is injected. Charge carrier transport in organic materials is one of the most important properties such as in the organic light-emitting diodes (OLEDs), organic field effect transistor (OFET) and organic solar cell (OSC). So the performance of all such devices depends critically on the rate of charge carrier transporting in organic materials. It is of great significance to study theoretically charge carrier transporting efficiency of organic materials.Bathocuproine (BCP) is currently widely used as an electron transporting and hole/exciton-blocking material and has played an important role in the highly efficient OLEDs based on phosphorescent emitters. 5,12-Dihydro-quino[2,3-b]acridone- 7,14-dione, generally has been named quinacridone (or abbreviated as QA), is the most important organic pigment. In recent years, quinacridone and its derivatives have been widely used to act as doping materials in high performance OLEDs. The majority of small molecule hole transporting materials are triarylamine derivatives, they also act as electroluminescent materials and have good prospects in the developing of OLEDs. Tris(8-hydorxyquinoline)aluminum(III) (Alq3), acts as either a electron transporting material or electroluminescent material for two decades of intensive research and development of OLEDs, still continues to be the workhorse in low-molecular weight materials for optoelectronic devices.In order to understand in-depth the influence of molecular structure and molecular stacking (material structure) on the charge carrier transporting properties, the density functional methods and Marcus charge transport theory combined classical particle hopping mechanisms are used to study charge transporting efficiencies for four types of typical charge carrier transporting materials, especially the relationship between the polymorphism and charge carrier transporting properties.Our studies mainly focus on the four aspects:1. The photophysical properties, charge carrier mobility and mechanism of blocking have been investigated. And Five BCP analogues, o-phenanthroline (1), 2,9-dimethyl-1,10-phenanthroline (2), 2,9-diphenyl-1,10-phenanthroline (3), 4,7- diphenyl-1,10-phenanthroline (4), and 2,9-bis(trifluoromethyl)-1,10-phenanthroline (5) have also been studied in order to select more suitable candidates of efficient hole/blocking materials.2. The photophysical properties and charge carrier transporting ability in the polymorphism of quinacridone have been studied. The relationships between the molecular packing and charge carrier transport were quantitatively characterized3. The charge carrier mobilties of six typical triarylamine derivatives, N,N,N',N'-Tetraphenyl-1,1'-biphenyl-4,4'-diamine (TPBA), N,N'-bis(3-methylphenyl) -N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (TPD-β), N,N'-Diphenyl-N,N'-bis(4- methy-lphenyl)-1,1'-biphenyl-4,4'-diamine (TPD-γ), N,N'-bis(1-naphthyl)-N,N'- diphenyl-1,1'-biphenyl-4,4'-diamine(NPB), 2,7-bis(N,N-Diphenylamino)-9,9-diethyl- fluorene (TBAF), 4,4'-Dicarbazole-1,1'-biphenyl (CBP), have been calculated in order to explore the reason why they transport hole efficiently.4. The charge carrier transporting ability in the polymorphism of Alq3 have been studied for an in-depth understanding the reason of magnitude for charge carrier mobilty.In conclusions:1. The calculated energies of BCP are good agreement with the experimental values, suggesting that the optimized geometries are reliable. The orbital patterns revealed that the absorption and emission transitions could be attributed to the charge transfer between phenanthroline and phenyl groups or the intraphenanthroline moiety charge transfer. The S0(the ground state)→T1(excited-triplet state) excitation energies (energy gap, ET1) could be controlled by manipulating the energy level of the highest occupied orbitals (HOMO) and the lowest unoccupied orbitals (LUMO). To introduce phenyl groups on the 2,9 positions of phenanthroline could lead to the decrease of theλ(the reorganization energy) value, while neither the addition of electron pushing groups nor electron withdrawing groups on the 2,9 or 4,7 positions of phenanthroline would result in the increase of theλvalue. The dimer center mass (CM) distance and the intermolecular contact model in the hopping partners have a dramatic effect on the charge transfer integral values. The calculated drift mobility demonstrated that all compounds studied in this paper are efficient electron-transfer materials. Furthermore, compounds 1, 4, and 5 may be efficient hole/exciton blocking materials compared with. BCP.2. A suitable functionals and basis sets for description of photophysical properties of quinacridone was discovered. At the absorption and emission process, the electron transfer mainly occurs between N and O transitions. Both single-molecule charge transfer and dimer charge transfer formed through hydrogen bonding orπ-πinteraction are involved in. The effect of solvent on the photophysical properties of quinacridone is achieved by changing its energy level of HOMO. Quinacridone has good electron transport ability and electron mobilities of all the polymorphism are at 10-2 magnitude. But the hole mobilities of quinacridone which varied with the different molecular packing are at range of 10-1 to 10-3 magnitude. The difference of charge carrier mobilities among the polymorphism is originated in the packing mode.3. The charge carrier mobilities of six triarylamine are at 10-2 - 10-3 magnitude and agreement with the experimental reports. Orbital analysis shows that LUMO orbital contributions come mainly from the biphenyl part of all the compounds. Increasing the planarity of biphenyl part can reduce its electron reorganization energy and enhance the electron mobilities; the HOMO orbital contributions come from the biphenyl part and the surrounding benzene rings connected with N atoms, the hole reorganization energy can be effectively reduced and hole mobilities be improved with the increasing planarity of whole molecule. The introduction of N atoms into the compounds has a major impact on increasing hole mobilities, because it is the key nodes connected to the periphery of benzene rings and N atoms contribution to all the HOMOs in each compound is very large (more than 20%). NPB is not only a hole transporting material but also an electron transporting materials. Weak interactions between molecules such as C-H…π, hydrogen bonding andπ-πinteraction can greatly enhance the charge carrier mobilities.4. Charge carrier mobilities in polymorphisms of Alq3 are different: the electron mobility ofβphase is three times of the hole mobility; The hole and electron mobilities inαandεare in the same order of magnitude. The hole mobilities ofγandδare greater than the electron mobilities. The reason that the mobilities in polymorphisms of Alq3 are different is: First, the reorganization energy of charge carrier inα,βandεare larger than that inγandδ. This difference comes from the isomers of Alq3. Second, the charge transfer integrals are different in polymorphisms. The main reason for this difference lies in the different orbit interaction which depends on the orbit sign and contacting mode which due to the different weak interaction between the molecules in the charge carrier transporting way. The difference of charge carrier mobilities in polymorphisms can be qualitatively explained from the frontier orbital diagram of a single molecule. Auxiliary information derived from calculation shows that the dipole moment of fac-Alq3 is greater than that in mer-Alq3 and the LUMO and HOMO orbital energy gap in fac-Alq3 is larger by 0.2 eV than in mer-Alq3.