Optical Properties and Light-Induced Charge Transfer in Selected Aromatic C60 Fullerene Derivatives and in Their Bulk Heterojunctions with Poly(3-Hexylthiophene).

Fullerene derivatives offer great scope for modification of the basic molecule, often called a buckyball. In recent years, they have been the subject of numerous studies, in particular in terms of their applications, including in solar cells. Here, the properties of four recently synthesized fullerene C60 derivatives were examined regarding their optical properties and the efficiency of the charge transfer process, both in fullerene derivatives themselves and in their heterojunctions with poly (3-hexylthiophene). Optical absorption, electron spin resonance (ESR), and time-resolved photoluminescence (TRPL) techniques were applied to study the synthesized molecules. It was shown that the absorption processes in fullerene derivatives are dominated by absorption of the fullerene cage and do not significantly depend on the type of the derivative. It was also found by ESR and TRPL studies that asymmetrical, dipole-like derivatives exhibit stronger light-induced charge transfer properties than their symmetrical counterparts. The observed inhomogeneous broadening of the ESR lines indicated a large disorder of all polymer-fullerene derivative blends. The density functional theory was applied to explain the results of the optical absorption experiments.

Towards designing polymers for photovoltaic applications: A DFT and experimental study of polyazomethines with various chemical structures.

Theoretical studies of polyazomethines (PAZs) with various chemical structures designated for photovoltaic applications are presented. PAZ energy levels and optical properties were calculated within density-functional theory (DFT and TDDFT) framework for 28 oligomers (monomer, dimer and trimer) of PAZs. The correlations between chemical structure of PAZ and location of its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were examined. It turned out that the presence of triaminophenylene, dimethoxydiphenylene and fluorine group raises the orbital energies. As a consequence, it is a factor which improves the photovoltaic efficiency of solar cell built on the base of the corresponding PAZ and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). On the contrary, quinone, 1,3,5-triazine and perfluorophenylene groups lower orbital energies and have negative influence on the photovoltaic efficiency. Moreover, calculations for methyl, ethyl and butyl analogs of P3HT as well as polythiophenes were performed and compared with the results obtained for PAZs. In addition experimental data are presented, which cover optical, electrochemical and electrical transport properties of the studied PAZs, allowing to determine HOMO and LUMO energies of the polymers and their conductivity. Finally, comparison between calculated and experimental results were made and discussed.