Using Side‐chain Attachment Density and Backbone Curvature to Regulate D–A Co-oligomer Crystalline Orientation in Thin Film

Autor:	Zhong-Yi Xu, 徐忠義
Rok vydání:	2016
Druh dokumentu:	學位論文 ; thesis
Popis:	105 Conducting polymers are important materials in the fabrication of optoelectronic devices. The orientation of the polymer crystalline is critical to achieve high charge carrier mobility and performance in the devices. In general, the edge-on orientation is preferred for utilities of organic field-effect transistors, and the face-on orientation is desired for applications in organic solar cells. However, the detail mechanisms behind the cause of favored crystal orientation of conducting polymers are not clearly understood currently. The systematic studies of molecular design and synthesis for the desired crystal orientation are relatively few present in the literatures. In this research, we design and synthesize eight different co-oligomers, poly(thienoisoindigo-alt-(xoctylthiophene)n, PTInT-xC8, where thienoisoindigo unit (TI) acting as an acceptor is conjugated with thiophene (T) as a donor; n is the number of thiophene in a donor segment, and xC8 denotes the number of n-octyl substituted on thiophenes in a repeat unit. We could tailor the crystal orientation by varying two parameters: (1) side-chain attachment density by changing the number of n-octyl substituents (x) on the thiophenes, and (2) the backbone curvature by changing the number of thiophene (n). The co-oligomers were synthesized successfully via either Stille coupling reaction or direct arylation polycondensation, and were characterized by nuclear magnetic resonance spectroscopy (NMR) for chemical structures, matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF) for molecular weights, grazing-incidence wide angle X-ray scattering (GIWAXS) for crystalline structures, ultraviolet－visible－near infrared absorption spectroscopy (UV－Vis－NIR) for optical properties and cyclic voltammetry (CV) for band gap determination, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) for thermal properties. The differences among 8 co-oligomers are also studied in details by modeling the molecules by the density functional theory (DFT) calclations. The results indicate that: (1) increasing the number of substituted n-octyl chains (x) on the donor segment of thiophenes while the number of thiophenes (n) is fixed, the polymer backbone will be twisted dramatically as shown by molecular modeling. From the absorption spectra, the relative absorption peak intensity of electrons excited from donor’s HOMO to donor’s LUMO (λ1) is increased while the one symbolized intramolecular transfer (λ2) is decreased. The GIWAXS results also indicate that the π-π stacking distances (dπ-π) in the direction of (010) increase. For example, the dπ-π of PTI2T-0C8 and PTI2T-2C8 are 3.57 Å and 4.83 Å, respectively; the π-π interaction of PTI2T-4C8 is weakened enough that no diffraction peaks could be defined. (2) Increasing the number of thiophene (n) on donor segment while fixing the number of substituted n-octyl chains (x) on thiophenes will distort the backbone due to the increasing freedom of torsion, which shortens the π-conjugation length, and the band gap is increased accordingly. For example, Egsol of PTI2T-2C8, PTI3T-2C8 and PTI4T-2C8 is 1.00, 1.18 and 1.25 eV, respectively; Egfilm is 0.87, 1.14 and 1.20 eV, respectively. The GIWAXS study shows the lamellar distances (d(100)) of PTI3T series are larger than those series of PTI2T and PTI4T. The results are due to the comformation differences between the odd and even number of thiophene (n) in donor segment on the backbone. The even number n gives a centrosymmetric structure whereas the odd number exhibits an axisymmetric configuration. Meanwhile, the steric hindrance of side-chain also influences the packing of polymer backbones. Thus, the centrosymmetric polymers (n = 2 or 4) exhibit the edge-on crystal orientation of long-range order due to the strong π-π interaction. Their orientation will be gradually changed to face-on orientation when the number of side-chains (x) on thiophenes equal to 2 or larger. The change of orientation is far more dramatic for axisymmetric co-oligomerss when the number of side-chains is changed due to the the nonlinear characteristic of backbone which are not suitable for standing on substrate. The edge-on packing is observed for co-oligomers (PTI3T-0C8) without side-chain attached on thiophenes. However, the packing orientation becomes well-ordered face-on with two side-chains added onto the donor segment of thiophenes, and amorphous structure is obtained with adding two more side-chains. The CV results also reveal that most of the PTInT could be used as a p-type semi-conductor materials, and part of the co-oligomers are quiet stable at high temperature with the decomposition temperature as high as 390°C. The outcomes of this research provide pathways for designing and synthesizing conducting polymers with desired crystal orientation for specific optoelectronic device applications.
Databáze:	Networked Digital Library of Theses & Dissertations
Externí odkaz:	http://ndltd.ncl.edu.tw/handle/f7m7vk Zobrazit plný text záznamu