Understanding the effect of thionation on naphthalene diimide using first-principles predictions of near-edge x-ray absorption fine structure spectra.

The near edge X-ray absorption fine structure (NEXAFS) spectra of naphthalene diimide molecules with increasing degrees of thionation show distinct and systematic changes in the C 1s → π* manifold. However, interpretation of such spectra is difficult using experimental data alone, due to the limitation in experimental NEXAFS resolution. In this work, we have calculated the NEXAFS spectra of naphthalene diimide molecules with increasing degrees of thionation using the density functional theory-based eXcited electron and Core Hole approach. We find that the systematic broadening and intensity reduction in the peaks observed in the π* manifold with increasing thionation are the result of distinct changes in the chemical environment of the outer carbon atoms that are bonded directly to either oxygen or sulfur. Specifically, the C 1s → lowest unoccupied molecular orbital (LUMO) transition energy dramatically decreases with thionation, as the valence electron density of these carbon atoms is increased when highly electronegative oxygen atoms are replaced by less-oxidizing sulfur atoms. It is also shown that significant core level shifts present in naphthalene diimide-based molecule result in a mixing of the LUMO and LUMO + 1 character in the C 1s → π* manifold, meaning that experimentally observed peaks cannot be uniquely associated with the transitions of LUMO, LUMO + 1, etc.

Impact of Acceptor Fluorination on the Performance of All-Polymer Solar Cells.

Here, we systematically study the effect of fluorination on the performance of all-polymer solar cells by employing a naphthalene diimide (NDI)-based polymer acceptor with thiophene-flanked phenyl co-monomer. Fluorination of the phenyl co-monomer with either two or four fluorine units is used to create a series of acceptor polymers with either no fluorination (PNDITPhT), bifluorination (PNDITF2T), or tetrafluorination (PNDITF4T). In blends with the donor polymer PTB7-Th, fluorination results in an increase in power conversion efficiency from 3.1 to 4.6% despite a decrease in open-circuit voltage from 0.86 V (unfluorinated) to 0.78 V (tetrafluorinated). Countering this decrease in open-circuit voltage is an increase in short-circuit current from 7.7 to 11.7 mA/cm2 as well as an increase in fill factor from 0.45 to 0.53. The origin of the improvement in performance with fluorination is explored using a combination of morphological, photophysical, and charge-transport studies. Interestingly, fluorination is found not to affect the ultrafast charge-generation kinetics, but instead is found to improve charge-collection yield subsequent to charge generation, linked to improved electron mobility and improved phase separation. Fluorination also leads to improved light absorption, with the blue-shifted absorption profile of the fluorinated polymers complementing the absorption profile of the low-band gap PTB7-Th.