Rachel C. Huber

Long-lived photoinduced polaron formation in conjugated polyelectrolyte-fullerene assemblies

Photoinduction of long-lived polarons Photosynthetic complexes and organic photovoltaics can rapidly create separated charges upon photoexcitation. However, unproductive charge recombination often occurs in the human-made system. This is in part because the charge acceptor and donor structures are much larger. Huber et al. created aqueous micelles that pair conjugated polyelectrolyte charge donors with fullerene acceptors at a much smaller interface. They observed the photoinduced formation of polarons—stable pairs of separated charges—with lifetimes of several days. Science, this issue p. 1340 An aqueous micelle can stabilize separated charges created by photoinduced electron transfer for several days. The efficiency of biological photosynthesis results from the exquisite organization of photoactive elements that promote rapid movement of charge carriers out of a critical recombination range. If synthetic organic photovoltaic materials could mimic this assembly, charge separation and collection could be markedly enhanced. We show that micelle-forming cationic semiconducting polymers can coassemble in water with cationic fullerene derivatives to create photoinduced electron-transfer cascades that lead to exceptionally long-lived polarons. The stability of the polarons depends on the organization of the polymer-fullerene assembly. Properly designed assemblies can produce separated polaronic charges that are stable for days or weeks in aqueous solution.

Beyond PCBM: methoxylated 1,4-bisbenzyl[60]fullerene adducts for efficient organic solar cells

Organic solar cells have been based mostly on conjugated polymers and the classic fullerene derivative PCBM and are characterized by modest open circuit voltages (Voc). Increasing Voc requires fullerene acceptors with higher LUMOs than PCBM. To date, most fullerene derivatives synthesized for this purpose either do not achieve the high photocurrent afforded by PCBM or show relatively poor compatibility with the next-generation low bandgap conjugated polymers used in high-efficiency organic solar cells. Here, we report the facile synthesis of methoxylated 1,4-bisbenzyl fullerene adducts and their application as efficient electron acceptors in conjugated polymer-based solar cells. The methoxy groups are found to be essential to increasing the LUMO levels, and accordingly the Voc, of the devices compared to the parent 1,4-bisbenzyl fullerene, and more importantly, to PCBM. The best fullerene 1,4-bisadduct provides a ∼20% enhancement in power conversion efficiency over PCBM when used with the classic crystalline polymer P3HT. When used in combination with a higher-performance low bandgap polymer, PTB7, the bisadduct both increases the device open-circuit voltage and maintains the high photocurrent provided by the more traditional PCBM. We also examine 10 different 1,4-fullerene bisadducts and show that the photovoltaic device performance is strongly influenced by the number and relative position of the methoxy substituents on the benzyl addends: moving a single methoxy substituent by one position on the benzyl rings can change the device efficiency by over a factor of 2.

Time resolved small angle X-ray scattering experiments performed on detonating explosives at the advanced photon source: Calculation of the time and distance between the detonation front and the x-ray beam

Time resolved Small Angle X-ray Scattering (SAXS) experiments on detonating explosives have been conducted at Argonne National Laboratorys Advanced Photon Source Dynamic Compression Sector. The purpose of the experiments is to measure the SAXS patterns at tens of ns to a few μs behind the detonation front. Corresponding positions behind the detonation front are of order 0.1–10 mm. From the scattering patterns, properties of the explosive products relative to the time behind the detonation front can be inferred. This report describes how the time and distance from the x-ray probe location to the detonation front is calculated, as well as the uncertainties and sources of uncertainty associated with the calculated times and distances.

Structural evolution of detonation carbon in composition B by X-ray scattering

Products evolved during the detonation of high explosives are primarily a collection of molecular gases and solid carbon condensates. Electron microscopy studies have revealed that detonation carbon (soot) can contain a variety of unique carbon particles possessing novel morphologies, such as carbon onions and ribbons. Despite these observations very little is known about the conditions that leads to the production of these novel carbon nanoparticles. A fuller understanding on conditions that generate such nanoparticles would greatly benefit from time-resolved studies that probe particle formation and evolution through and beyond the chemical reaction zone. Herein, we report initial results employing time-resolved X-ray scattering (TRSAXS) measurements to monitor nanosecond time-scale carbon products formed from detonating Composition B (60% TNT, 40% RDX). These studies were performed at the Dynamic Compression Sector (DCS, Sector 35) at the Advanced Photon Source (Argonne National Laboratory). Analysis o...

Structure and Conductivity of Semiconducting Polymer Hydrogels

Poly(fluorene-alt-thiophene) (PFT) is a conjugated polyelectrolyte that self-assembles into rod-like micelles in water, with the conjugated polymer backbone running along the length of the micelle. At modest concentrations (∼10 mg/mL in aqueous solutions), PFT forms hydrogels, and this work focuses on understanding the structure and intermolecular interactions in those gel networks. The network structure can be directly visualized using cryo electron microscopy. Oscillatory rheology studies further tell us about connectivity within the gel network, and the data are consistent with a picture where polymer chains bridge between micelles to hold the network together. Addition of tetrahydrofuran (THF) to the gels breaks those connections, but once the THF is removed, the gel becomes stronger than it was before, presumably due to the creation of a more interconnected nanoscale architecture. Small polymer oligomers can also passivate the bridging polymer chains, breaking connections between micelles and dramatically weakening the hydrogel network. Fits to solution-phase small-angle X-ray scattering data using a Dammin bead model support the hypothesis of a bridging connection between PFT micelles, even in dilute aqueous solutions. Finally, time-resolved microwave conductivity measurements on dried samples show an increase in carrier mobility after THF annealing of the PFT gel, likely due to increased connectivity within the polymer network.

Characterization of detonation soot produced during steady and overdriven conditions for three high explosive formulations

The detonation of high explosives (HE) produces a dense fluid of molecular gases and solid carbon. The solid detonation carbon contains various carbon allotropes such as detonation nanodiamonds, onion-like carbon, graphite and amorphous carbon, with the formation of the different forms dependent upon pressure, temperature and the environmental conditions of the detonation. We have collected solid carbon residues from controlled detonations of three HE formulations (Composition B-3, PBX 9501, and PBX 9502). Soot was collected from experiments designed to produce both steady and overdriven conditions, and from detonations in both an ambient (air) atmosphere and in an inert Ar atmosphere. Differences in solid carbon residues were quantified using X-ray photoelectron spectroscopy and carbon isotope measurements. Environmental conditions, HE formulation, and peak pressures influenced the amount of and isotopic composition of the carbon in the soot. Detonations in an Ar atmosphere produced greater amounts of ca...

Single-bunch imaging of detonation fronts using scattered synchrotron radiation

A centimeter-scale field of view for transmission X-ray radiography from a sub-millimeter-focused synchrotron X-ray beam is achieved by placing a strongly scattering material upstream of the sample. Combining the scattered beam with a detector system synchronized and gated to acquire images from single X-ray pulses provides the capability for time-resolved observations of transient phenomena in samples larger than the native X-ray beam. Furthermore, switching between this scatter-beam imaging (SBI) and scattering modes is trivial compared to switching between unfocused white beam imaging and scattering using a focused pink beam. As a result, SBI additionally provides a straightforward method to precisely align samples relative to the focused X-ray beam for subsequent small-angle X-ray scattering measurements. This paper describes the use of glassy carbon for SBI to observe phenomena during detonation of small-scale high explosive charges and compares the technique to conventional white beam imaging. SBI i...

Carbon Bonding Determination with XES Using a TES Microcalorimeter Detector

TES microcalorimeter detectors are capable of high-resolution X-ray emission spectroscopy (XES) which rivals XANES spectroscopic probes found only within the confines of synchrotrons. Commercial microcalorimeters offer spectral resolution around 5-7 eV which rivals that of wavelength dispersive XRF instruments yet provide full spectra of the material of interest not merely a single element, thereby surpassing WDXRF systems capabilities.