Modeling and simulations of interfacial interactions and materials design for polymeric light emitting diode


Student thesis: Doctoral Thesis

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  • Shiling SUN


Awarding Institution
Award date15 Jul 2005


Since the work of Burroughes et al. in 1990, polymer based light-emitting diodes (PLEDs) have aroused much interest in research and development for new-generation display. A series of recent breakthroughs in processing and applications of conjugated polymers have further stimulated new excitements in this field. Currently, lots of works are being done to improve the device performance by developing new materials and better device structure. To assist the experimental activities and to design new materials and predict their novel properties, systematic theoretical investigations about the interfacial interactions and materials properties were conducted in this thesis work. The theoretical methodologies demonstrated and developed in this work are also of significances and transferability to other related studies. Firstly, we chose the simplest conjugated polymer, trans-polyacetylene (t-PA), as a sample for study. The geometrical and electronic structures of an ideal PA and a defect PA (involving soliton) were studied using Density Functional Theory (DFT). Their absorption spectra were investigated using a time-dependent Hartree-Fock method. It is shown that the alternating feature in bond lengths of the PA structure turns to be weakened due to the presence of soliton in the defect PA structure. And a localized electronic state appears near the midgap in the structure involving soliton. Such a state not only enhances the photoconductivity of the polymeric material, but also generates additional absorption peak in low energy region. Although only weak photoluminescence (PL) could be observed in the infrared region from t-PA, recent experimental studies have shown that mono-substituted polyacetylenes and di-substitued polyaceltylenes could exhibit strong PL. It indicates that the optical properties of PA derivatives depend on the substituents. These substituents can also be expected to prevent the formation of solitons in the t-PA oligomer. In the thesis work, the effects of di-substitution in each monomer and in an alternate monomer of t-PA on the electronic structures and the absorption spectra were examined using the time-dependent Hartree-Fock method. It is shown that the electronic and optical properties are affected not only by the nature of the substituents but also by the number of substituents and the way in which they are substituted. The partial substitution scheme retains the characteristics of pristine PA. Whereas, a full substitution modifies the main chain due to the steric hindrance. The revealed substituent effects would be helpful for the PA-based molecular design. Based on the experiences gained using the PA, two more complicated polymeric systems, namely poly(9,9-dioctylfluorene) (PFO) and poly(p-phenylenevinylene) (PPV) were studied. These two systems are the most widely studied polymer-based light-emitting materials and continue to be the focus of recent researches. We compared the optical properties of PFO of different sizes with those of the well-studied PPV by performing calculations using time-dependent localized density matrix approach based on intermediate neglect of differential overlap/spectroscopy (INDO/S) Hamiltonian. The derived theoretical optical gap for PFO of infinite size is about 2.9 eV, while that of PPV is about 2.7 eV, agreeing well with the experimental data (2.95 eV and 2.2-2.5 eV, respectively). The actual physical sizes of the lowest excited state exciton (Wannier exciton) were obtained to be ~2.5 nm (about 4 repeating units) of PFO while ~2.7 nm for PPV (about 5 repeating units). The result indicates that the optical properties of PFO would saturate to its bulk behavior at a smaller size than that of PPV. Furthermore, the intensity of band-edge absorption of PFO is similar to that of PPV, suggesting that the PFO oligomer can perform as efficiently as PPV in opto-electronic application. It is well-known that the fabrication of organic light emitting devices involves deposition of metal cathode materials onto the organic layer. The formation of interface between cathode and organic layer is one of the factors to determine the efficiency of devices. So the geometric and electronic structures of PFO oligomer interacting with Ca atoms were studied using Møller-Plesset Perturbation Theory. A weak interaction with little charge transfer and with a relatively long Ca-C distance (about 4.0 Å) was found when only one Ca atom was attached to a PFO unit. However, when two Ca atoms were adsorbed at a PFO unit, a strong interaction with a shorter Ca-C distance (about 2.67 Å) took place with considerable charge transfer from the Ca atom to the PFO and with significant deformation in the backbone of the PFO oligomer. In the latter case, the frontier orbitals of the PFO were modified. However, the deformed PFO and its modified frontier orbitals could be recovered when oxygen was added, in good agreement with experimental observation.

    Research areas

  • Electric properties, Polymers, Materials, Light emitting diodes