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.

Control of Geminate Recombination by the Material Composition and Processing Conditions in Novel Polymer: Nonfullerene Acceptor Photovoltaic Devices.

Herein, we report on the charge dynamics of photovoltaic devices based on two novel small-molecule nonfullerene acceptors featuring a central ketone unit. Using ultrafast near-infrared spectroscopy with optical and photocurrent detection methods, we identify one of the key loss channels in the devices as geminate recombination (GR) of interfacial charge transfer states (CTSs). We find that the magnitude of GR is highly sensitive to the choice of solvent and annealing conditions. Interestingly, regardless of these processing conditions, the same lifetime for GR (∼130 ps) is obtained by both detection methods upon decomposing the complex broadband transient optical spectra, suggesting this time scale is inherent and independent of morphology. These observations suggest that the CTSs in the studied material blends are mostly strongly bound, and that charge generation from these states is highly inefficient. We further rationalize our results by considering the impact of the processing on the morphology of the mixed donor and acceptor domains and discuss the potential consequences of the early charge dynamics on the performance of emerging nonfullerene photovoltaic devices. Our results demonstrate that careful choice of processing conditions enables enhanced exciton harvesting and suppression of GR by more than 3 orders of magnitude.

Incorporation of 2,6-Connected Azulene Units into the Backbone of Conjugated Polymers: Towards High-Performance Organic Optoelectronic Materials.

Azulene is a promising candidate for constructing optoelectronic materials. An effective strategy is presented to obtain high-performance conjugated polymers by incorporating 2,6-connected azulene units into the polymeric backbone, and two conjugated copolymers P(TBAzDI-TPD) and P(TBAzDI-TFB) were designed and synthesized based on this strategy. They are the first two examples for 2,6-connected azulene-based conjugated polymers and exhibit unipolar n-type transistor performance with an electron mobility of up to 0.42 cm2 V-1 s-1 , which is among the highest values for n-type polymeric semiconductors in bottom-gate top-contact organic field-effect transistors. Preliminary all-polymer solar cell devices with P(TBAzDI-TPD) as the electron acceptor and PTB7-Th as the electron donor display a power conversion efficiency of 1.82 %.