Electronic Fingerprint of the Protonated Imidazole Dimer Probed by X-ray Absorption Spectroscopy.

Protons in low-barrier superstrong hydrogen bonds are typically delocalized between two electronegative atoms. Conventional methods to characterize such superstrong hydrogen bonds are vibrational spectroscopy and diffraction techniques. We introduce soft X-ray spectroscopy to uncover the electronic fingerprints for proton sharing in the protonated imidazole dimer, a prototypical building block enabling effective proton transport in biology and high-temperature fuel cells. Using nitrogen core excitations as a sensitive probe for the protonation status, we identify the X-ray signature of a shared proton in the solvated imidazole dimer in a combined experimental and theoretical approach. The degree of proton sharing is examined as a function of structural variations that modify the shape of the low-barrier potential in the superstrong hydrogen bond. We conclude by showing how the sensitivity to the quantum distribution of proton motion in the double-well potential is reflected in the spectral signature of the shared proton.

Raman Spectroscopy of Formamidinium-Based Lead Mixed-Halide Perovskite Bulk Crystals.

In recent years, there has been an impressively fast technological progress in the development of highly efficient lead halide perovskite solar cells. Nonetheless, the stability of perovskite films and associated solar cells remains a source of uncertainty and necessitates sophisticated characterization techniques. Here, we report low- to mid-frequency resonant Raman spectra of formamidinium-based lead mixed-halide perovskites. The assignment of the different Raman lines in the measured spectra is assisted by DFT simulations of the Raman spectra of suitable periodic model systems. An important result of this work is that both experiment and theory point to an increase of the stability of the perovskite structure with increasing chloride doping concentration. In the Raman spectra, this is reflected by the appearance of new lines due to the formation of hydrogen bonds. Thus, higher chloride doping results in less torsional motion and lower asymmetric bending contributing to higher stability. This study yields a solid basis for the interpretation of the Raman spectra of formamidinium-based mixed-halide perovskites, furthering the understanding of the properties of these materials, which is essential for their full exploitation in solar cells.

Short-range organization and photophysical properties of CdSe quantum dots coupled with aryleneethynylenes.

Hybrid organic-inorganic nanomaterials composed of organic semiconductors and inorganic quantum dots (QDs) are promising candidates for opto-electronic devices in a sustainable internet of things. Especially their ability to combine the advantages of both compounds in one material with new functionality, the energy-efficient production possibility and the applicability in thin films with little resource consumption are key benefits of these materials. However, a major challenge one is facing for these hybrid materials is the lack of a detailed understanding of the organic-inorganic interface which hampers the widespread application in devices. We advance the understanding of this interface by studying the short-range organization and binding motif of aryleneethynylenes coupled to CdSe QDs as an example system with various experimental methods. Clear evidence for an incorporation of the organic ligands in between the inorganic QDs is found, and polarization-modulation infrared reflection-absorption spectroscopy is shown to be a powerful technique to directly detect the binding in such hybrid thin-film systems. A monodentate binding and a connection of neighboring QDs by the aryleneethynylene molecules is identified. Using steady-state and time resolved spectroscopy, we further investigated the photophysics of these hybrid systems. Different passivation capabilities resulting in different decay dynamics of the QDs turned out to be the main influence of the ligands on the photophysics.

Effect of static external electric field on bulk and interfaces in organic solar cell systems: A density-functional-theory-based study.

In this work, a detailed comparison of optical and electronic properties in bulk and interfaces of well-known organic semiconductor systems in presence of an external electric field is reported. We have used density functional theory (DFT) to model organic solar cell systems. The study promotes a deeper understanding of the connection between the chemical structures and the optical and electronic properties in the well-known organic solar cell systems based on thiophene and fullerene polymers. We have performed a vibration-mode analysis by simulating Raman spectra in presence of external electric fields. Time-dependent DFT has been used to investigate the effect of an external electric field on excited state properties. The charge-transfer rate controlled by the external electric field has been quantitatively extracted using the simulated excited state dipole moment, Gibbs free energy, and Marcus theory. Our results provide a detailed characterization of the effect of the external electric field on the neat polymers (bulk) and on the donor-acceptor heterojunctions (interfaces) in organic solar cell systems. This theoretical approach not only helps to understand the effect of an external field on bulk and interfaces in organic semiconductors, but it also supports the design of novel devices.

Insights into ultrafast charge-pair dynamics in P3HT:PCBM devices under the influence of static electric fields.

Polymer-fullerene blends based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric-acid methyl ester (PCBM) have been extensively studied as promising bulk heterojunction materials for organic semiconductor devices with improved performance. In these donor-acceptor systems where the bulk morphology plays a crucial role, the generation and subsequent decay mechanisms of photoexcitation species are still not completely understood. In this work, we use femtosecond transient absorption spectroscopy to investigate P3HT:PCBM diodes under the influence of applied static electric fields in comparison to P3HT:PCBM thin films. At the same time, we try to present a detailed overview about work already done on these donor-acceptor systems. The excited state dynamics obtained at 638 nm from P3HT:PCBM thin films are found to be similar to those observed earlier in neat P3HT films, while those obtained in the P3HT:PCBM devices are affected by field-induced exciton dissociation, resulting not only in comparatively slower decay dynamics, but also in bimolecular deactivation processes. External electric fields are expected to enhance charge generation in the investigated P3HT:PCBM devices by dissociating excitons and loosely bound intermediate species like polaron pairs (PPs) and charge transfer (CT) excitons, which can already dissociate only due to the intrinsic fields at the donor-acceptor interfaces. Our results clearly establish the formation of PP-like transient species different from CT excitons in the P3HT:PCBM devices as a result of a field-induced diffusion-controlled exciton dissociation process. We find that the loosely bound transient species formed in this way also are reduced in part via a bimolecular annihilation process resulting in charge loss in typical donor-acceptor P3HT:PCBM bulk heterojunction semiconductor devices, which is a rather interesting finding important for a better understanding of the performance of these devices.

Ultrafast polaron-pair dynamics in a poly(3-hexylthiophene-2,5-diyl) device influenced by a static electric field: insights into electric-field-related charge loss.

The generation and decay mechanisms of polaron pairs in organic semiconductor-based optoelectronic devices under operational conditions are relevant for a better understanding of photophysical processes affecting the device performance, since the possible occurrence of a polaron pair introduces an intermediate step in exciton dissociation into fully separated charge carriers. The role played by static electric fields in polaron-pair dynamics is important but poorly understood or not investigated in detail. In this work, insights into the polaron-pair dynamics in neat poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films and P3HT films sandwiched between electrical contacts with an applied external static electric field are probed using femtosecond pump-probe transient absorption spectroscopy. Asymmetric contacts result in P3HT devices with application-related diode characteristics. Consistent with the electric field-induced dissociation of oppositely charged species, we show that polaron-pair dissociation into charge carriers occurs in the P3HT device more significantly with increasing reverse bias, and that this process follows an initial instantaneous polaron-pair photoabsorption quenching due to a pronounced immediate loss of primary photoexcitation species (hot excitons). Furthermore, we show that the net-electric field present in the P3HT diode (including built-in-potential at 0 V bias) results in a more complex dynamics with new findings as compared to the neat-P3HT thin film case. Indeed, besides polaron pairs directly originating from hot excitons, we experimentally observe polaron-pair formation during exciton dissociation via a field-mediated generation process, resulting in a slower contribution to the overall decay dynamics. Moreover, unlike in the external electric field-free P3HT film, bimolecular annihilation processes clearly appear as an additional loss channel when a field is applied and hence have an impact on carrier generation performance in a working device.

Impact of Size and Electronegativity of Halide Anions on Hydrogen Bonds and Properties of 1-Ethyl-3-methylimidazolium-Based Ionic Liquids.

The effect of the anion size and electronegativity of halide-based anions (Cl-, Br-, I-, and BF4-) on the interionic interaction in 1-ethyl-3-methylimidazolium-based ionic liquids (ILs) C2mim X (X = Cl, Br, I, and BF4) is studied by a combined approach of experiments (Raman, IR, UV-vis spectroscopy) and quantum chemical calculations. The fingerprint region of the Raman spectra of these C2mim X ion-pairs provides evidence of the presence of the conformational isomerism in the alkyl chain of the C2mim+ cation. The Raman and IR bands of the imidazolium C2-H stretch vibration for C2mim X (X = Cl, Br, I, and BF4) were noticeably blue-shifted with the systematic change in size of anions and the electronegativity. The observed blue shift in the C2-H stretch vibration follows the order C2mim BF4 > C2mim I > C2mim Br > C2mim Cl, which essentially indicates the strong hydrogen bonding in the C2mim Cl ion-pair. DFT calculations predict at least four configurations for the cation-anion interaction. On the basis of relative optimized energies and basis-set-superposition-error (BSSE) corrected binding energies for all ion-pair configurations, the most active site for the anion interaction was found at the C2H position of the cation. Besides information about the C2H position, our DFT results give insights into the anion interaction with the ethyl and methyl chain of the cation, which was also confirmed experimentally [ Chem. Commun. 2015 , 51 , 3193 ]. The anion interaction at the C2H site of the cation favors a planar geometry in C2mim X for X = Cl, Br, and I; however, for BF4, the system prefers a nonplanar geometry where the anion is located over the imidazolium ring. TD-DFT results were used to analyze the observed UV-vis absorption spectra in a more adequate way giving insights into the electronic structure of the ILs. Overall, a reasonable correlation between the observed and the DFT-predicted results is established.