Study Of The Charge Transport Properties Of Amorphous Small-molecule Organic Semiconductors

Organic semiconductors are widely used in electronic devices, and they are often in amorphous forms in applications. Current understanding of the charge transport process in amorphous organic semiconductors is far from complete. This makes molecular engineering and device optimizations largely blind. In this dissertation, the influences of energetic disorder on charge transport and current characteristics were studied systematically, and methods to understand the charge transport properties of small-molecule organic semiconductors were explored. The main contents of this dissertation are as follows:1. The influences of energetic disorder on the charge transport and current characteristics were studied in detail, and the mobilities from the time-of-flight(TOF) method, the dark-injection space-charge-limited current(DI-SCLC) method and the space-charge-limited current(SCLC) method were compared. Space charge perturbation leads to the formation of a cusp in transient currents, and results in lower mobilities for TOF measurements. At low temperatures or when the energetic disorder is high, mobilities from the DI-SCLC method will be higher than those from the TOF method. Due to the effects of the injection barrier and traps, charge mobilities from the SCLC method may be lower than those from the TOF method. Transition metal oxide(TMO) doping leads to wider distribution of density of states(DOS), which lowers mobility at low doping ratios. The SCLC method overestimates charge mobilities for TMO-doped systems.2. Charge transport properties of N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine(TPD), N,N’-diphenyl-N,N’-bis(1-naphthyl)-1,1’-biphenyl-4,4’-diamine(NPB), 4,4’-bis(N-carbazolyl)-1,1’-biphenyl(CBP) and 4,7-diphenyl-1,10-phenanthroline(Bphen) were studied based on their crystal structures. Results are 6%-31% lower if molecular packing is taken into account when calculating the intra-molecular reorganization energies. This is related to the conformational change during the charge transfer process. The calculated hole versus electron mobility is in agreement with the reported values of amorphous films from literature.3. Methods to directly calculate the charge mobilities and current characteristics of amorphous small-molecule organic semiconductors were explored. The multi-scale approach combines Born-Oppenheimer molecular dynamics(BOMD), non-adiabatic molecular dynamics(NAMD), master equation method and Monte Carlo simulations. The calculated hole and electron mobilities of 9,10-di-(2-naphthyl)anthracene(ADN) are in good agreement with those from TOF measurements. For NPB, two amorphous structures are investigated. The differences of the calculated hole mobilities of 1 structure with those from TOF measurements are within 1 order of magnitude. The calculated current density were compared with that of a hole-only device from literature. It is found that when an appropriate injection barrier is assumed, the results agrees well with experiment.