Etude DFT des proprietes structurales et electroniques d'un cristal de poly(3-hexylthiophene) regioregulier.

摘要

The search for new clean and renewable energy source targets several technologies, one of which is photovoltaic cells. However, a widespread use of photovoltaic cells is greatly hindered by their manufacturing costs. The use of polymeric semiconductor in photovoltaic cells could take advantage of the large-scale and low-cost production techniques associated with polymers to overcome this limitation. Current organic photovoltaic cells, composed of a bulk heterojunction combining an elecron donor polymer and an electron acceptor molecule or polymer, are not efficient enough to be viable. One way for increasing the efficiency of the organic photovoltaic cells is to improve charge carrier transport in the hetero junction. However, since the organic semiconductors structural and electronic properties are not yet well established in the literature, it is necessary to address this issue before.;The theoretical study is performed with density functional theory (DFT) calculations within the local density approximation (LDA). The softwares used, SIESTA and ABINIT, employ pseudopotentials built according to the Troullier-Martins method by using the Ceperley-Alder exchange-correlation functional. The different parameters defining the planewave basis set (ABINIT) or the atomic orbital basis set (SIESTA) are optimized until the error on the total energy is less than 0.02%. Finally, the geometry is optimized until the maximum force in the system is lower than 2, 5 x 10-3 eV/A (4 pN).;The geometry as well as the electronic properties of the wavefunctions obtained for polythiophene are very close to those previously described in the literature. The differences in geometry between the DFT calculations from the literature and the present research project are lower than 1,7%. Moreover, the band gap of 1,05 eV (1,09 eV) calculated with SIESTA (ABINIT), is consistent with the expected underestimation with respect to the experimental value within DFT-LDA limits. Thereafter, the 21 screw axis symmetry present in the polythiophene unit cell is used to explain in detail the band foldings and the avoided band crossings observed in the band structure.;The configuration and geometry study of the rrP3HT crystal have helped to corroborate a model proposed in the literature which presumes that the alkyl side chains are tilted in order to adapt to the unit cell dimension. The calculated equilibrium spacing between two consecutive polymer chains in the pi stacking direction of 3,42 A leads to relatively strong intrachain wavefunction overlap and band dispersion in this direction. This overlap causes the band gap and the HOMO and LUMO bands to be greatly influenced by the unit cell dimension in the pi stacking direction. Near the equilibrium geometry, a 1 A reduction of the dimension in the pi stacking direction closes the band gap by 0,49 eV due to the increasing dispersion of the HOMO and LUMO bands. The equilibrium band gap is 0,145 eV. The HOMO band dispersion has variation rates of 0,485 eV/A and 0,362 eV/A whereas the LUMO band dispersion has variation rates of 0.671 eV/A and 0,420 eV/A respectively in the Gamma-Y and the Gamma-Z directions. It is interesting to note that the dimension variation in the pi stacking direction does not only influences the band dispersion in this direction, but it also influences the band dispersion in the intrachain direction. The change in the pi stacking direction dimension that requires a pressure of 210 MPa causes a variation of the band gap up to 53% of its equilibrium value. Under the same unit cell deformation, the HOMO and LUMO band dispersion varies from 8 to 13% in the Gamma-Y diection and from 2 to 3% in the Gamma-Z direction. Such variations of the electronic properties associated with relatively weak geometrical deformations of the system are of great importance because they prove that the design of the electronic properties of typical materials used in organic photovoltaic cells is technically feasible. The present results significantly contributes to the advance in the engineering of high performance organic photovoltaic cells that requires an optimization of the electronic properties of the materials.;The main goal of this research project is to theoretically study the structure and electronic properties of regioregular poly(3-hexylthiophene) (rrP3HT), a typical organic semiconductor used in organic photovoltaic cells. This system is studied under various constraints in order to obtain its electronic properties for a broad range of external perturbations. The accuracy of the results are ensured by calculation on polythiophene, a well described simple system similar in nature to rrP3HT. The long term goal of this study is to understand and engineer the electronic and structural properties of organic semiconductor blends in order to achieve higher efficiency in organic photovoltaic cells.