Regioisomer effects of [70]fullerene mono-adduct acceptors in bulk heterojunction polymer solar cells

Regioisomer separations of [70]fullerene mono-adducts for polymer solar cell (PSC) applications were conducted for the first time.

Despite numerous organic semiconductors being developed during the past decade, C₇₀ derivatives are predominantly used as electron acceptors in efficient polymer solar cells (PSCs). However, as-prepared C₇₀ mono-adducts intrinsically comprise regioisomers that would mask individual device performances depending on the substituent position on C₇₀. Herein, we separate the regioisomers of C₇₀ mono-adducts for PSC applications for the first time. Systematic investigations of the substituent position effect using a novel symmetric C₇₀ mono-adduct ([70]NCMA) and a prevalent, high-performance one ([70]PCBM) reveals that we can control the structures of the blend films with conjugated polymers and thereby improve the PSC performances by regioisomer separation. Our approach demonstrates the significance of exploring the best-matching regioisomer of C₇₀ mono-adducts with high-performance conjugated polymers, which would achieve a remarkable progress in PSC devices.

Resonant energy transfer of triplet excitons from pentacene to PbSe nanocrystals

The efficient transfer of energy between organic and inorganic semiconductors is a widely sought after property, but has so far been limited to the transfer of spin-singlet excitons. Here we report efficient resonant-energy transfer of molecular spin-triplet excitons from organic semiconductors to inorganic semiconductors. We use ultrafast optical absorption spectroscopy to track the dynamics of triplets, generated in pentacene through singlet exciton fission, at the interface with lead selenide (PbSe) nanocrystals. We show that triplets transfer to PbSe rapidly (<1 ps) and efficiently, with 1.9 triplets transferred for every photon absorbed in pentacene, but only when the bandgap of the nanocrystals is close to resonance (±0.2 eV) with the triplet energy. Following triplet transfer, the excitation can undergo either charge separation, allowing photovoltaic operation, or radiative recombination in the nanocrystal, enabling luminescent harvesting of triplet exciton energy in light-emitting structures.

Temperature-independent singlet exciton fission in tetracene

We use transient absorption spectroscopy to demonstrate that the dynamics of singlet exciton fission in tetracene are independent of temperature (10–270 K). Low-intensity, broad-band measurements allow the identification of spectral features while minimizing bimolecular recombination. Hence, by directly observing both species, we find that the time constant for the conversion of singlets to triplet pairs is ~90 ps. However, in contrast to pentacene, where fission is effectively unidirectional, we confirm that the emissive singlet in tetracene is readily regenerated from spin-correlated "geminate" triplets following fission, leading to equilibrium dynamics. Although free triplets are efficiently generated at room temperature, the interplay of superradiance and frustrated triplet diffusion contributes to a nearly 20-fold increase in the steady-state fluorescence as the sample is cooled. Together, these results require that singlets and triplet pairs in tetracene are effectively degenerate in energy, and begin to reconcile the temperature dependence of many macroscopic observables with a fission process which does not require thermal activation.

Nanosecond intersystem crossing times in fullerene acceptors: implications for organic photovoltaic diodes

Triplet-exciton formation through intersystem crossing of photogenerated singlet excitons in fullerene acceptors can compete with charge generation in organic photovoltaic diodes. This article reports the intersystem crossing timescale (τISC ) of the most commonly used fullerene acceptors, PC60 BM and PC70 BM, in solutions and in spin-coated films. These times are on the nanosecond timescale, and are longer than the characteristic times for charge generation (τd ).

How disorder controls the kinetics of triplet charge recombination in semiconducting organic polymer photovoltaics

Theoretical models of polymer–fullerene interfaces indicate that inhomogeneous broadening of fullerene energy levels introduces strong coupling between the interfacial³CT and nearby fullerene triplet excitons that can enhance the decay of these states in systems with higher degrees of energetic disorder.

Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes

Understanding the charge-separation mechanism in organic photovoltaic cells (OPVs) could facilitate optimization of their overall efficiency. Here we report the time dependence of the separation of photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems. By tracking the modulation of the optical absorption due to the electric field generated between the charges, we measure ~200 millielectron volts of electrostatic energy arising from electron-hole separation within 40 femtoseconds of excitation, corresponding to a charge separation distance of at least 4 nanometers. At this separation, the residual Coulomb attraction between charges is at or below thermal energies, so that electron and hole separate freely. This early time behavior is consistent with charge separation through access to delocalized π-electron states in ordered regions of the fullerene acceptor material.

The role of spin in the kinetic control of recombination in organic photovoltaics

In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.

Direct observation of photoinduced bound charge-pair states at an organic-inorganic semiconductor interface

It is generally considered that photoinduced charge transfer at the organic-inorganic interfaces in hybrid photovoltaic devices immediately results in a pair of free charge carriers. We extend a novel interface-selective ultrafast "optical pump-push photocurrent probe" technique to study hybrid photovoltaic systems and observe bound electron-hole pair states at the organic-inorganic interface formed between electron-accepting zinc oxide and electron-donating conjugated polymers. We estimate that ∼50% of photogenerated charges stay bound and later recombine, thus hindering the photovoltaic performance of polymer/ZnO cells. We further demonstrate that interface modification with a fullerene derivative decreases the fraction of bound charges to ∼25%, which substantially improves the device efficiency.

Measurement of thickness and composition of a solvent film on a bubble

Solvent-coated air bubbles in the air-assisted solvent extraction (AASX) process achieve the dual role of high solvent specific surface area and ease of phase separation. The properties and thickness of the solvent film control the process. As an approach to the study, the layer interferometry (in the UV-vis region) and FT-IR spectroscopy were used to measure the time dependent thickness and chemical composition, respectively, of a film formed by blowing an air bubble in kerosene-based solvents. The film was stabilized by the presence of 1.5 ppm silicone oil, as employed in AASX. The film appears to comprise two layers; an outer layer of almost constant thickness and an inner layer which decreased in thickness with time. The latter is considered relevant to AASX. Generally, the initial thickness was approximately 3 microm which decreased over several minutes to a final rupture thickness of 500 nm. The initial thickness is of the order determined indirectly. The chemical composition of the layer did not change with time.

Effect of oxidative stress on stability and structure of neurofilament proteins

Neurofilament proteins are highly phosphorylated molecules in the axonal compartment of the adult nervous system. We report the structural analysis of neurofilament proteins after oxidative damage. SDS-PAGE, immunoblotting, circular dichroism, and Fourier transform infrared spectroscopy were used to investigate the relative sensitivity of neurofilaments to oxidative stress and to identify changes in their molecular organization. An ascorbate-Fe+3-O2 buffer system as well as catechols were used to generate free radicals on a substrate of phosphorylated and dephosphorylated neurofilaments. By Fourier Transform Infrared spectroscopy and circular dichroism, we established that the neurofilament secondary structure is mainly composed of alpha-helices and that after free radical damage of the peptide backbone of neurofilaments, those helices are partly modified into beta-sheet and random coil structures. These characteristic reorganizations of the neurofilament structure after oxidative exposure suggest that free radical activity might play an important role in the biogenesis of the cytoplasmic inclusions found in several neurodegenerative diseases.

AMPA receptor properties in adult rat hippocampus following environmental enrichment

In adult rats, environmental enrichment has been shown to selectively increase -AMPA binding in the hippocampus but the molecular mechanisms underlying this effect remain unknown. We used in situ hybridization with antisense oligonucleotides to determine possible changes in the hippocampal expression of messenger RNAs for different subunits of AMPA receptors in adult rats following exposure to an enriched environment. Quantitative analysis revealed that mRNA levels for three subtypes of AMPA glutamate receptors (GluR1-3; Flip and Flop variants) were not modified in any hippocampal region after environmental enrichment. In addition, no differences were detected in the levels of GluR1 and GluR2/3 proteins in Western blots of hippocampal membranes from enriched rats. Nevertheless, quantitative ligand binding autoradiography indicated that environmental enrichment evoked a significant and uniform decrease in the capacity of calcium or phosphatidylserine (PS) to up-regulate -AMPA binding in various hippocampal regions but not in the cerebral cortex. These findings support previous observations suggesting that post-translational changes in AMPA receptor properties, as a result of the activation of calcium-dependent processes, may represent an important mechanism underlying long-term modifications of synaptic efficacy in the rat hippocampus.

Binding properties of glutamate receptors in streptozotocin-induced diabetes in rats

The biochemical mechanisms by which diabetes modulates cognitive function are not well established. Here, we determined the effects of streptozotocin (STZ) administration on the binding properties of alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) subtypes of glutamate receptors in rats, using quantitative autoradiographic analysis of (3)H-AMPA and [(3)H]glutamate binding on brain tissue sections. The STZ injection (70 mg/kg intraperitoneally) produced a reduction of (3)H-AMPA binding in various brain regions, an effect that is due to a decrease in receptor affinity. The STZ-induced reduction of (3)H-AMPA binding varied in different brain structures, being more pronounced in the striatum, cerebral cortex, and hippocampus and almost absent in the cerebellum. Western blots performed on hippocampal membranes revealed that the decrease in (3)H-AMPA binding is possibly associated with changes in immunologic properties for one glutamate receptor subunit (GluR1). Finally, the effect of STZ-induced diabetes appeared to be specific to the AMPA subtype of glutamate receptors, as the same treatment did not modify [(3)H]glutamate binding to NMDA receptors. These changes in AMPA receptor properties may have important implications for understanding the biochemical mechanisms underlying cognitive impairment in diabetes.