Theoretical Study On Charge-transport Properties Of Organic Molecular Semiconductors

Organic molecular semiconductors have great application potentialin (opto)electronics, which attract wide attentions from both academiaand industry. The charge-transfer properties directly dictate theperformance of organic (opto)electronics devices. And the theoreticalstudy on charge-transfer properties of organic semiconductors has beencarried out since the1960’s. Although the full understanding of chargetransport mechanisms in organic semiconductors has yet been reached,we have been able to qualitatively predict or give reasonable theoreticalexplanation to the charge-transfer properties，and put forward somesuggestions towards the design and synthesis of new materials. Comparewith hole transport materials, electron or ambipolar charge transportmaterials are relatively few, thus they become the research hotspots inorganic electronics. In addition, nonlocal electron-phonon couplings,which have been less investigated before, are found to be an importantfactor in determining the charge-transport properties. In this thesis, wewill introduce the following two achievements:1. Good ambipolar charge-transport characteristics have beenunraveled for the halogenated anthanthrone crystals by the combinationof density functional theory and molecular dynamics calculations. In particular, for the iodinated derivative, the electronic couplings for holesand electrons are very similar and as large as about110meV along the π-stacking direction and the resulting effective masses are as small as about1.0m0. In case of chlorinated and brominated derivatives, although theelectronic coupling for electrons is larger than that for holes along the π-stacking direction, to some extent, the strong nonlocal electron-phononcouplings will lead to more balanced hole and electron transport.2. Phonon dispersions have important impact on nonlocal electron-phonon couplings. In naphthalene crystal, mainly because of the absenceof counterparts of acoustic phonons, the overall strength of nonlocalcouplings is underestimated by taking sole consideration of opticalphonons at the point. In particular, for the parallel-type molecularpairs, acoustic phonons contribute more than40per cent to the overallstrengths of nonlocal couplings (L parameters) for both holes andelectrons.