Electronic Structures, Vibrational Spectra And Inter-molecular Combination Of Electron-acceptor Materials PC₆₁BM And PC₇₁BM

Syntheses of new oganic photovoltaic materials isin rapid progress.However,PC61BM and PC71BM, the most extensively applied acceptor materials, are still irreplaceable at present. In this thesis we have investigated some fundamental properties of isolated and crystalline PC61BM and PC71BM with density functional theory (DFT) calculations and experimental measurements.We acquire the knowledge about the electronic structures of PC61(71)BM in three phases. We first calculate the ground state electronic structures of isolated molecules.The effect of the side tail on the electronic states of the carbon cages is analyzed in detail.Then we calculate the energy band dispersion and density of states of PC61BM single crystal. The dispersion is very small for the bands derived from the LUMO and HOMO of C60, but the deeper band is substantially broadened due to the interaction of the side tail with the adajacent molecules. The interaction may include inter-molecular hydrogen bonds. Our photoemission measurement of a PC61BM film reveals that the ionization energy of solvent-free,uncontaminated PC61BM film is 6.10 eV.We study the vibrational spectraof PC61BM and PC71BM with DFT calculations and IR, Raman measurements.It is found that the measured IR spectra are mainly derived from the vibrations of the side chain, while the measured Raman spectra are mainly from the vibrations of the carbon cages. The Raman spectrm and the IR peak of the C=O stretching mode are very suitable for studing inter-molecular interaction.On the basis of the above results, we have demonstrated the existing of inter-molecular hydrogen bonds in PC61BM solids with the calculated crystal structure of PC61BM single crystal and the measured IR spectra of the C=O stretching mode.The hydrogen bonds give an explanation to the liquid phase of PC61BM. The hydrogen bonds can also be one of the driving forces for the phase separation of the PC61(71)BM/polymer blends.At last, we grow PC71BM single crystals with solution method. PC71BM molecules can combine to be single crystal. However, the quality of the single crystals is not high due to three isomers existing in the material.