We address the binding energy of charge-transfer excitons at organic semiconductor heterojunctions by investigating a polymer blend where the energy of the intramolecular singlet exciton is just sufficient to create separated charge pairs, placing the system at the threshold for photovoltaic operation. At 10 K, we report long-lived photoluminescence arising from charge recombination and triplet-exciton bimolecular annihilation. Both mechanisms regenerate singlet excitons in the electron acceptor, but we demonstrate that charge recombination dominates singlet regeneration dynamics on e300 ns time scales. This occurs by tunnelling of separated electron and holes across heterojunctions. The separated charge pairs are therefore degenerate with repopulated singlet states. From the difference of the charge-transfer and intrachain exciton emission energies, we determine that the binding energy of charge-transfer excitons with respect to bulk charge separation is g250 meV. Directed charge flow away from the heterojunction would avoid formation of strongly bound charge-transfer excitons, that act as traps and limit current generation in organic solar cells.

The advent of ultrafast highly brilliant coherent X-ray free-electron laser sources has driven the development of novel structure-determination approaches for proteins, and promises visualization of protein dynamics on sub-picosecond timescales with full atomic resolution. Significant efforts are being applied to the development of sample-delivery systems that allow these unique sources to be most efficiently exploited for high-throughput serial femtosecond crystallography. Here, the next iteration of a fixed-target crystallography chip designed for rapid and reliable delivery of up to 11 259 protein crystals with high spatial precision is presented. An experimental scheme for predetermining the positions of crystals in the chip by means of in situ spectroscopy using a fiducial system for rapid, precise alignment and registration of the crystal positions is presented. This delivers unprecedented performance in serial crystallography experiments at room temperature under atmospheric pressure, giving a raw hit rate approaching 100% with an effective indexing rate of approximately 50%, increasing the efficiency of beam usage and allowing the method to be applied to systems where the number of crystals is limited.

A portable sample viewing and alignment system is described which provides fast and reliable motion positioning for fixed target arrays at synchrotrons and free-electron laser sources.

How does chemistry scale in complexity to unerringly direct biological functions? Nass Kovacs et al. have shown that bacteriorhodopsin undergoes structural changes tantalizingly similar to the expected pathway even under excessive excitation. Is the protein structure so highly evolved that it directs all deposited energy into the designed function? It is difficult to overstate the importance of having atomic structures to help shape our thinking and understanding of matter. Structural information constrains the number of possible solutions in trying to piece together a puzzle in how matter undergoes transformation from one structure to another and the associated changes in material properties 1,2. In terms of understanding biological processes, this question always reduces to how the protein structure surrounding an active site has evolved to direct chemical processes into biological functions, typically with efficiencies well beyond our current capabilities to exploit chemistry. In this respect, bacteriorhodopsin (bR) serves as a model system for understanding structurefunction relationships for membrane proteins 3-5. This system functions as a light-driven, outward proton pump, which can be triggered by light to use time resolved optical methods to watch it function in real time. Its structure is composed of seven transmembrane α-helices that are covalently bound to a photoactive retinal molecule via a lysine residue through a Schiff base linkage (Fig. 1b). Upon absorbing a photon, the retinal chromophore undergoes rapid isomerization from an all-trans to 13-cis form passing through the I 460 (charge separated), J 625 (highly twisted) and K 590 (isomerized) intermediates. The retinal isomerization acts like a push in changing the electrostatic and structural environment around the active site. These changes in turn lead to a series of cascaded protein conformational changes to facilitate the transport of a proton from the retinal Schiff base to the extracellular side of the membrane via L 550 and M 410 intermediates. The retinal then undergoes reprotonation and thermal re-isomerization through the N 560 and O 630 intermediates, respectively, where it can then return to the bR 568 ground state. These processes have been well characterized spectroscopically and many of the long-lived

A fixed-target approach to high-throughput room-temperature serial synchrotron crystallography with oscillation is described. Patterned silicon chips with microwells provide high crystal-loading density with an extremely high hit rate. The microfocus, undulator-fed beamline at CHESS, which has compound refractive optics and a fast-framing detector, was built and optimized for this experiment. The high-throughput oscillation method described here collects 1–5° of data per crystal at room temperature with fast (10° s−1) oscillation rates and translation times, giving a crystal-data collection rate of 2.5 Hz. Partial datasets collected by the oscillation method at a storage-ring source provide more complete data per crystal than still images, dramatically lowering the total number of crystals needed for a complete dataset suitable for structure solution and refinement – up to two orders of magnitude fewer being required. Thus, this method is particularly well suited to instances where crystal quantities are low. It is demonstrated, through comparison of first and last oscillation images of two systems, that dose and the effects of radiation damage can be minimized through fast rotation and low angular sweeps for each crystal.

Given their nanoscale dimensions, colloidal semiconductor nanocrystals provide unique systems for investigating the dynamics controlling surface chemistry and fundamental issues regarding lattice reorganization upon changes in electron distribution. These systems are particularly amenable to ultrafast electron probes, offering an atomic level picture of the lattice reorganization involved following photoexcitation. Here, we study lead sulfide (PbS) quantum dots with ultrafast electron diffraction to characterize the atomic motions following high-intensity photoexcitation. Short-range nonthermal lattice distortions and increased atomic disorder were observed in PbS colloidal quantum dots ranging from 2.4 to 8.7 nm in size. These effects scaled inversely with size and were less pronounced in nanocrystals with a chloride-containing surface rather than only organic ligands, which is consistent with an effect arising at the surface. The anisotropic, nonthermal lattice disordering occurs preferentially along the (100) crystallographic directions, which could indicate an anisotropic distribution of localized charge between the differing lattice terminations of the {111} and {100} crystal facets. This is consistent with the larger anharmonicity for the lattice potential at lattice sites with reduced ligand coordination relative to the bulk, which has been shown to cause accelerated relaxation into dynamic and static surface trap sites. Through an exploration of quantum dot size and variation in surface termination, this work provides the missing structural details to advance our understanding and control of charge-carrier formation, trapping, and recombination processes in nanoscale semiconductor systems.

For the two proteins myoglobin (MB) and fluoroacetate dehalogenase (FAcD), we present a systematic comparison of crystallographic diffraction data collected by serial femtosecond (SFX) and serial synchrotron crystallography (SSX). To maximize comparability, we used the same batch of crystals, the same sample delivery device, as well as the same data analysis software. Overall figures of merit indicate that the data of both radiation sources are of equivalent quality. For both proteins reasonable data statistics can be obtained with approximately 5000 room temperature diffraction images irrespective of the radiation source. The direct comparability of SSX and SFX data indicates that diffraction quality is rather linked to the properties of the crystals than to the radiation source. Time-resolved experiments can therefore be conducted at the source that best matches the desired time-resolution.

Dye-sensitized metal oxide polymer hybrid solar cells deliver a promising basis in organic solar cell development due to many conceptual advantages. Since the power conversion efficiency is still in a noncompetitive state, it has to be understood how the photocurrent contribution can be maximized (i.e., which dye-polymer properties are most beneficial for efficient charge generation in hybrid solar cells). By the comparison of three model systems for hybrid solar cells with TiO 2-dye-polymer interfaces, this paper was aimed at elucidating the role of the exact mechanism of charge generation. In the exciton dissociation (ED) case, an exciton that is generated in the polymer is split at the dye-polymer interface. Alternatively, this exciton can be transferred to the dye via an energy transfer (ET), upon which charge separation occurs between dye and TiO 2. For comparison, the third case is included in which the high lowest unoccupied molecular orbital of the dye does not allow exciton separation or ET from the dye to the polymer, so that the dye only is responsible for charge generation. To separate effects owing to differences in energy levels of the involved materials from the impact of local order and disorder in the polymer close to the interface, this paper further comprises a detailed analysis of the polymer crystallinity based on the H-aggregate model. While the massive impact of the poly(3-hexylthiophene) crystallinity on device function has been outlined for bare metal oxide-polymer interfaces, it has not been considered for hybrid solar cells with dye-sensitized TiO 2. The results presented here indicate that all dye molecules in general influence the polymer morphology, which has to be taken into account for future optimization of hybrid solar cells. Apart from that, it can be suggested that ED on the polymer needs an additional driving force to work efficiently; thus, energy transfer seems to be currently the most promising strategy to increase the polymer photocurrent contribution.