Abstract
Phenyl C71 butyric acid methyl ester (PC71BM) has been adopted as electron acceptor materials in bulk heterojunction solar cells with relatively higher power conversion efficiency. The understanding of the mechanism and performance for the devices based upon PC71BM requires the information of conformations, electronic structures, optical properties, and so forth. Here, the geometries, IR and Raman, electronic structures, polarizabilities, and hyperpolarizabilities of PC71BM isomers are studied by using density functional theory (DFT); the absorption and excitation properties are investigated via time‐dependent DFT with B3LYP, PBE0, and CAM‐B3LYP functionals. The calculated results show that [6,6]PC71BM is more stable than [5,6]PC71BM due to the lower total energy. The vibrational modes of the isomers at IR and Raman peaks are quite similar. As to absorption properties, CAM‐B3LYP functional is the suitable functional for describing the excitations of PC71BM because the calculated results with CAM‐B3LYP functional agree well with that of the experiment. The analysis of transition configurations and molecular orbitals demonstrated that the transitions at the absorption maxima in UV/Vis region are localized π‐π* transitions in fullerenes cages. Furthermore, the larger isotropic polarizability of PC71BM indicates that the response of PC71BM to applied external electric field is stronger than that of PC61BM, and therefore resulting into better nonlinear optical properties.
Highlights
The electronic devices based upon organic materials, such as organic radiofrequency identification, light emitting diode, memory devices, and solar cells, have attracted considerable attention in the past decade due to their potential to be lower-cost, light-weight, flexible, and large-area equipment
When the C atom in the side chain of butyric acid methyl ester connects to C70 cage, two possible isomers can be formed since the C atom can connect to the most “polar” carbon-carbon double bonds in C70, and the carbon-carbon single bond in C70, and these two structures were denoted as [6,6]Phenyl C71 butyric acid methyl ester (PC71BM) and [5,6]PC71BM, respectively
The bond character of C5–C15 was changed from double bond (0.140 nm) in pure C70 fullerene to single bond (0.163 nm) in [6,6]PC71BM due to forming a carbon trigon (C5–C15–C71) through the changes of orbital hybridization, and the change of C5–C15 bond length is similar to the cases of C60-TPA [48] and N-methyl-3,4-fulleropyrrolidine [49], while, in [5,6]PC71BM, the atomic distance between C14 and C15 is about 0.213 nm, which far exceeds the typical C–C
Summary
The electronic devices based upon organic materials, such as organic radiofrequency identification, light emitting diode, memory devices, and solar cells, have attracted considerable attention in the past decade due to their potential to be lower-cost, light-weight, flexible, and large-area equipment These devices usually contain heterojunction formed by electronic donor and acceptor materials. The properties of materials in these devices, including chemical structures [1], electronic structures [2], excited states [3], charge transfer, and charge transport [4], are of particular importance for their overall performance. To provide a better understanding toward the higher performance of device, it is necessary to investigate the electronic structures of the materials, as well as the energy level alignment at the heterojunction interface [5]. Some fullerene hybrids show good nonlinear optical properties [12]
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