Nanosecond-microsecond Time Scale Research Articles

Influence of Charge Transport Layers on Open-Circuit Voltage and Hysteresis in Perovskite Solar Cells

Summary Perovskite materials have experienced an impressive improvement in photovoltaic performance due to their unique combination of optoelectronic properties. Their remarkable progression, facilitated by the use of different device architectures, compositional engineering, and processing methodologies, contrasts with the lack of understanding of the materials properties and interface phenomena. Here we directly target the interplay between the charge-transporting layers (CTLs) and open-circuit potential ( V OC ) in the operation mechanism of the state-of-the-art CH 3 NH 3 PbI 3 solar cells. Our results suggest that the V OC is controlled by the splitting of quasi-Fermi levels and recombination inside the perovskite, rather than being governed by any internal electric field established by the difference in the CTL work functions. In addition, we provide novel insights into the hysteretic origin in perovskite solar cells, identifying the nature of the contacts as a critical factor in defining the charge accumulation at its interface, leading to either ionic, electronic, or mixed ionic-electronic accumulation.

An Acrobatic Substrate Metamorphosis Reveals a Requirement for Substrate Conformational Dynamics in Trypsin Proteolysis

The molecular basis of enzyme catalytic power and specificity derives from dynamic interactions between enzyme and substrate during catalysis. Although considerable effort has been devoted to understanding how conformational dynamics within enzymes affect catalysis, the role of conformational dynamics within protein substrates has not been addressed. Here, we examine the importance of substrate dynamics in the cleavage of Kunitz-bovine pancreatic trypsin inhibitor protease inhibitors by mesotrypsin, finding that the varied conformational dynamics of structurally similar substrates can profoundly impact the rate of catalysis. A 1.4-Å crystal structure of a mesotrypsin-product complex formed with a rapidly cleaved substrate reveals a dramatic conformational change in the substrate upon proteolysis. By using long all-atom molecular dynamics simulations of acyl-enzyme intermediates with proteolysis rates spanning 3 orders of magnitude, we identify global and local dynamic features of substrates on the nanosecond-microsecond time scale that correlate with enzymatic rates and explain differential susceptibility to proteolysis. By integrating multiple enhanced sampling methods for molecular dynamics, we model a viable conformational pathway between substrate-like and product-like states, linking substrate dynamics on the nanosecond-microsecond time scale with large collective substrate motions on the much slower time scale of catalysis. Our findings implicate substrate flexibility as a critical determinant of catalysis.

Effectiveness of External Electric Field Treatment of Conjugated Polymers in Bulk-Heterojunction Solar Cells.

External electric field treatment (EFT) on P3HT:PCBM bulk heterojunction (BHJ) devices was recently found to be a viable approach for improving the power conversion efficiencies (PCEs) through modulating the blend nanomorphology. However, its effectiveness over the broad family of polymer-fullerene blends remains unclear. Herein, we investigate the effects of external EFT on various polymer-fullerene blends with distinct morphologies stemming from the difference in molecular structure of the polymers (i.e., semicrystalline vs amorphous) in a bid to establish a clear morphology-function-charge dynamics relationship to the photovoltaic performance. Our findings reveal that EFT promotes self-organization of the semicrystalline thiophene-based conjugated polymers (i.e., P3HT and P3BT) while it was ineffective for the amorphous polymers (i.e., PTB7 and PCPDTBT) even at the maximum applied E-field of 8 kV cm-1. Transient absorption spectroscopy shows an improvement in the initial charge-carrier and polaron formation from delocalized excitons in the E-field treated semicrystalline blends compared to their untreated reference samples. Interfacial trap-assisted monomolecular and trap-free bimolecular recombination at nanosecond-microsecond time scale in the E-field treated P3BT:PC60BM devices are significantly suppressed. Importantly, our findings shed new light and provide guidelines on the effectiveness of utilizing external EFT to enhance the PCEs of a larger family of conjugated polymer-based BHJ OSCs.

Fast and Stable Photochromic Oxazines for Fluorescence Switching

The stringent limitations imposed by diffraction on the spatial resolution of fluorescence microscopes demand the identification of viable strategies to switch fluorescence under optical control. In this context, the photoinduced and reversible transformations of photochromic compounds are particularly valuable. In fact, these molecules can be engineered to regulate the emission intensities of complementary fluorophores in response to optical stimulations. On the basis of this general design logic, we assembled a functional molecular construct consisting of a borondipyrromethene fluorophore and a nitrospiropyran photochrome and demonstrated that the emission of the former can be modulated with the interconversion of the latter. This fluorophore-photochrome dyad, however, has a slow switching speed and poor fatigue resistance. To improve both parameters, we developed a new family of photochromic switches based on the photoinduced opening and thermal closing of an oxazine ring. These compounds switch back and forth between ring-closed and -open isomers on nanosecond-microsecond timescales and tolerate thousands of switching cycles with no sign of degradation. In addition, the attachment of appropriate chromophoric fragments to their switchable oxazine ring can be exploited to either deactivate or activate fluorescence reversibly in response to illumination with a pair of exciting beams. Specifically, we assembled three dyads, each based on either a borondipyrromethene or a coumarin fluorophore and an oxazine photochrome, and modulated their fluorescence in a few microseconds with outstanding fatigue resistance. The unique photochemical and photophysical properties of our fluorophore-photochrome dyads can facilitate the development of switchable fluorophores for superresolution imaging and, ultimately, provide valuable molecular probes for the visualization of biological samples on the nanometer level.

Quantum Chemical Characterization of Low-Lying Excited States of an Aryl Peroxycarbonate: Mechanistic Implications for Photodissociation

Recent experiments have revealed the existence of an excited state dissociative mechanism for certain peroxycarbonates, with the demonstration that the lifetime of the excited state matches the picosecond time scale for appearance of nascent carbon dioxide product. The data infer that the photoreaction proceeds via an effectively concerted three-body dissociation within the lifetime of the singlet excited state. Many other arylperoxides decay sequentially via [(aryloxy)carbonyl]oxy radical intermediates on nanosecond-microsecond time scales. Uncertainty as to the lifetime of the excited state relates to the character and the relative energetic ordering of states of the parent molecule, since the spectra and photochemistry imply that low-lying states may exist on each of the aryl, carbonate, and peroxide chemical functionalities. We employ many-body electronic structure calculations to determine the energies and characters of the low-lying valence states of a minimal aryl peroxycarbonate model germane to the above-mentioned experiments, methyl phenyl peroxycarbonate (MPC). Our results indicate that the lowest-lying state is an intrinsically nondissociative aryl pipi* excited state. We identify additional low-lying states that are expected to be dissociative in nature and propose that the time scales observed for the dissociation reaction may correspond to the time scale for transfer of excited state population to these states.

Intermediate rate atomic trajectories of RNA by solid-state NMR spectroscopy.

Many RNAs undergo large conformational changes in response to the binding of proteins and small molecules. However, when RNA functional dynamics occur in the nanosecond-microsecond time scale, they become invisible to traditional solution NMR relaxation methods. Residual dipolar coupling methods have revealed the presence of extensive nanosecond-microsecond domain motions in HIV-1 TAR RNA, but this technique lacks information on the rates of motions. We have used solid-state deuterium NMR to quantitatively describe trajectories of key residues in TAR by exploiting the sensitivity of this technique to motions that occur in the nanosecond-microsecond regime. Deuterium line shape and relaxation data were used to model motions of residues within the TAR binding interface. The resulting motional models indicate two functionally essential bases within the single-stranded bulge sample both the free and Tat-bound conformations on the microsecond time scale in the complete absence of the protein. Thus, our results strongly support a conformational capture mechanism for recognition: the protein does not induce a new RNA structure, but instead captures an already-populated conformation.

Additive Effect of Mutations Affecting the Rate of Phylloquinone Reoxidation and Directionality of Electron Transfer within Photosystem I†

Optical pump-probe spectroscopy in the nanosecond-microsecond timescale has been used to study the electron transfer reactions taking place within the Photosystem I reaction center of intact Chlamydomonas reinhardtii cells. The biphasic kinetics of phylloquinone (PhQ) reoxidation were investigated in double mutants that combine a mutation (PsaA-Y696F) near the primary acceptor chlorophyll, ec3A, with those near PhQA (PsaA-S692A, PsaA-W697F). The PsaA-S692A and PsaA-W697F mutations selectively lengthened the 200 ns lifetime component observed in the wild-type (WT). The reverse similar 20 ns component was unaltered in the single mutant, both in terms of lifetime and relative amplitude. However, both double mutants possessed a reverse similar 20 ns component (PhQB(-) reoxidation) with increased amplitude compared with the WT and the individual PhQA mutants. The component assigned to PhQA(-) reoxidation was slowed, like the individual PhQA mutants, and of lower amplitude, as observed in the single ec3A mutant. Hence, the effects of these mutations are almost entirely additive, providing strong support for the previously proposed bidirectional electron transfer model, which attributes the reverse similar 20 and reverse similar 200 ns phases to reoxidation of PhQB or PhQA, respectively. Moreover, in all the mutants investigated, it was also possible to observe an intermediate (approximately 180 ns) component, as previously reported for mutants of the PhQ(A) binding pocket (Biochim. Biophys. Acta [2006] 1757, 1529-1538), which we have tentatively attributed to forward electron transfer between the iron-sulfur clusters FX and FA/B.

Probing the Role of Dynamics in Hydride Transfer Catalyzed by Lactate Dehydrogenase

The dynamic nature of the interconversion of pyruvate to lactate as catalyzed by lactate dehydrogenase (LDH) is characterized by laser-induced temperature jump relaxation spectroscopy with a resolution of 20 ns. An equilibrium system of LDH·NADH plus pyruvate and LDH·NAD + plus lactate is perturbed by a sudden T-jump, and the relaxation of the system is monitored by NADH emission and absorption changes. The substrate binding pathway is observed to be similar, although not identical, to previous work on substrate mimics: an encounter complex is formed between LDH·NADH and pyruvate, which collapses to the active Michaelis complex. The previously unresolved hydride transfer event is characterized and separated from other unimolecular isomerizations of the protein important for the catalytic mechanism, such as loop closure, a slower step, and faster events on the nanosecond-microsecond timescales whose structural basis is not understood. The results of this study show that this approach can be applied quite generally to enzyme systems and report on the dynamic nature of proteins over a very wide time range.

Differential Line Broadening in MAS Solid-State NMR due to Dynamic Interference

Many MAS (magic angle spinning) solid-state NMR investigations of biologically relevant protein samples are hampered by poor resolution, particularly in the 15N chemical shift dimension. We show that dynamics in the nanosecond-microsecond time scale in solid-state samples can induce significant line broadening of 15N resonances in solid-state NMR experiments. Averaging of 15NH(alpha/beta) multiplet components due to 1H decoupling induces effective relaxation of the 15N coherence in case the N-H spin pair undergoes significant motion. High resolution solid-state NMR spectra can then only be recorded by application of TROSY (Transverse Relaxation Optimized Spectroscopy) type techniques which select the narrow component of the multiplet pattern. We speculate that this effect has been the major obstacle to the NMR spectroscopic characterization of many membrane proteins and fibrillar aggregates so far. Only in very favorable cases, where dynamics are either absent or very fast (picosecond), high-resolution spectra were obtained. We expect that this approach which requires intense deuteration will have a significant impact on the quality and the rate at which solid-state NMR spectroscopic investigations will emerge in the future.

Submillisecond kinetics of protein folding

New experimental methods permit observation of protein folding and unfolding on the previously inaccessible nanosecond—microsecond timescale. These studies are beginning to establish times for the elementary motions in protein folding — secondary structure and loop formation, local hydrophobic collapse, and global collapse to the compact denatured state. They permit an estimate of about one microsecond for the shortest time in which a protein can possibly fold.

Picosecond–microsecond structural relaxation dynamics in polypropylene glycol: Impulsive stimulated light-scattering experiments

Picosecond time-resolved impulsive stimulated light-scattering experiments are conducted on polypropylene glycol at temperatures above the glass transition temperature Tg. Through the use of a wide range of scattering angles, longitudinal acoustic waves are characterized in the 20 MHz–5 GHz frequency range. In addition, time-dependent thermal expansion is observed on nanosecond–microsecond time scales. The results are consistent with an empirical description of structural relaxation dynamics in terms of a stretched exponential relaxation function with exponent β=0.4 and with the average relaxation time given by the Vogel–Tamman–Fulcher form. Comparisons to Brillouin scattering and photon correlation spectroscopy results indicate that this description holds for a dynamic range of over 10 orders of magnitude. Comparison to dielectric relaxation measurements which probe mainly orientational motions of polymer segments indicates that density fluctuations involve different segmental motions with significantly faster dynamics for T>Tg.