Geneviève Sauvé

Standardization protocol of process efficiencies and activation parameters in heterogeneous photocatalysis: relative photonic efficiencies ζr

The quantum yield Φ is crucial in homogeneous photochemistry; in heterogeneous photocatalysis this term remains elusive since the number of absorbed photons remains experimentally difficult to asses. A comprehensive method to standardize and compare process efficiencies in heterogeneous photocatalysis was proposed earlier by describing the relative photonic efficiency ζr (J. Photochem. Photobiol., A: Chem., 73 (1993) 11). The method of determining ζr is herein tested for the photocatalyzed degradation of phenol as the standard process and Degussa P25 TiO2 as the standard photocatalyst. The effects of light intensity, reactor geometry, pH, temperature, concentration of reactant and concentration of TiO2 on ζr were examined for the photodegradation of three substituted phenols: 2-methylphenol, 2,4-dimethylphenol and 4-chlorophenol. Relative photonic efficiencies of other phenolic substrates are reported for a given set of conditions. Efficiencies on varying the nature and the source of TiO2 for the photodegradation of phenol were also explored. These ζr are useful to assess process quantum yields once the actual quantum yield for a standard process (Φstand, for a given photocatalyst and a standard organic substrate) has been rigorously determined; thus Φ = ζrΦstand.

Beyond Fullerenes: Designing Alternative Molecular Electron Acceptors for Solution-Processable Bulk Heterojunction Organic Photovoltaics.

Organic photovoltaics (OPVs) are promising candidates for providing a low cost, widespread energy source by converting sunlight into electricity. Solution-processable active layers have predominantly consisted of a conjugated polymer donor blended with a fullerene derivative as the acceptor. Although fullerene derivatives have been the acceptor of choice, they have drawbacks such as weak visible light absorption and poor energy tuning that limit overall efficiencies. This has recently fueled new research to explore alternative acceptors that would overcome those limitations. During this exploration, one question arises: what are the important design principles for developing nonfullerene acceptors? It is generally accepted that acceptors should have high electron affinity, electron mobility, and absorption coefficient in the visible and near-IR region of the spectra. In this Perspective, we argue that alternative molecular acceptors, when blended with a conjugated polymer donor, should also have large nonplanar structures to promote nanoscale phase separation, charge separation and charge transport in blend films. Additionally, new material design should address the low dielectric constant of organic semiconductors that have so far limited their widespread application.

Azadipyrromethene-based Zn(II) complexes as nonplanar conjugated electron acceptors for organic photovoltaics.

The effectiveness of new a electron acceptor for organic solar cells is demonstrated. The acceptor is a homoleptic zinc(II) complex of 2,6-diphenylethynyl-1,3,7,9-tetraphenylazadipyrromethene. The high power-conversion efficiency obtained is attributed to the acceptors 3D structure, which prevents crystallization and promotes a favourable nanoscale morphology, its high Voc , and its ability to contribute to light harvesting at 600-800 nm.

On the usage of turnover numbers and quantum yields in heterogeneous photocatalysis

Abstract This paper discusses two issues which have led to, and continue to cause, confusion in the heterogeneous photochemical literature: (i) turnover numbers (turnover rates, turnover frequencies) and (ii) quantum yields. Attention is called to the use of these two parameters when describing the catalytic activity of a photocatalyst on the one hand and the efficiency of a photon on the other. A parameter is defined (relative photonic efficiency) and a simple method is proposed which provides a means of validating data between experiments and between laboratories. In addition, the process efficiency can be assessed and the photocatalytic activity of different materials and a given photocatalyst material from various sources can be validated.

Azadipyrromethene-based conjugated oligomers with near-IR absorption and high electron affinity.

Solution-processable conjugated oligomers incorporating red-light absorbing azadipyrromethenes (aza-DIPY) within the main chain were synthesized via palladium-catalyzed Sonogashira coupling reactions. Thin films of these compounds absorbed light up to ∼1000 nm and displayed reversible reductions as ascertained by cyclic voltammetry experiments. Reactions with trifluoroboron etherate yielded materials displaying a unique combination of good solubility in organic solvents, low optical band gaps (∼1.3 eV), and high electron affinity (∼4.5 eV).

Well-defined, high molecular weight poly(3-alkylthiophene)s in thin-film transistors: side chain invariance in field-effect mobility

In this paper, the effect of side chain length on transistor performance was analyzed by using a series of well-defined, regioregular poly(3-alkylthiophene) (P3AT) samples (with PDI values as low as 1.1). Both untreated and octyltrichlorosilane-treated (OTS) SiO2 transistor dielectric layers were compared for all samples. On untreated SiO2, P3AT with hexyl side chains showed the best mobility, with mobilities as high as 0.1 cm2 V−1 s−1, and mobility values then decreased slightly with longer side-chain length. When using OTS-8 treated SiO2, on the other hand, mobility values remained high even with polymers of longer side-chain length, obtaining mobilities as high as 0.2 cm2 V−1 s−1 for hexyl, octyl and dodecyl side chains, which were much higher than previously reported values. Carrier mobilities of P3ATs with longer side chains were seen to be more sensitive to surface chemistry than for P3ATs with shorter side chains. Using morphological and GIXS studies, we found that for longer side chains, self-assembly is governed by two competing processes—backbone packing (π-stacking) and side-chain packing—that resulted in polymorphic behavior and disorder. On an OTS-treated surface, interactions between dielectric surface and side chains were reduced, resulting in better order at the interface and higher mobility.

Density Functional Theory Study Predicts Low Reorganization Energies for Azadipyrromethene-Based Metal Complexes.

Small internal reorganization energy is desirable for high-performance optoelectronic materials, as it facilitates both charge separation and charge transport. However, only a handful of n-type electron accepting materials are known to have small reorganization energies. Here, DFT calculations were performed to predict the reorganization energy of azadipyrromethene-based dyes and their complexes. All compounds studied were most stable in their anionic state and had high electron affinity, indicating their potential as n-type material. The homoleptic zinc(II) complexes had significantly lower reorganization energies than either the free ligands or the BF2(+) chelates. The low reorganization energies of the zinc(II) complexes are explained by the large and rigid π conjugated system that extends across the two azadipyrromethene ligands via interligand π-π interactions. This work suggests that Zn(II) complexation is a novel strategy for obtaining materials that combine low internal reorganization energy with high electron affinity for the development of novel n-type optoelectronic materials.

Introducing 3D conjugated acceptors with intense red absorption: homoleptic metal(II) complexes of di(phenylacetylene) azadipyrromethene

A novel strategy for the design and synthesis of functional materials with excellent acceptor properties is presented. The materials are based on homoleptic metal(II) complexes of azadipyrromethene (aza-DIPY) derivatives, and exhibit intense red absorption and high electron affinity. Their strong accepting properties were demonstrated by fluorescence quenching experiments using poly(3-hexylthiophene) as the donor. DFT calculations showed that the homoleptic metal(II) complexes of 2,8-di(4-tert-butylphenylacetylene)-1,3,5,7-tetraphenylazadipyrromethene had a similar distorted tetrahedral geometry to the complexes of 1,3,5,7-tetraphenylazadipyrromethene, but with additional conjugated ‘arms’ extending in 3 dimensions (3D). A unique feature of these complexes is that the two aza-DIPY ligands are π-stacked with each other, with frontier molecular orbitals delocalized over the two ligands. These complexes can therefore easily accept electrons, delocalize the negative charge over a large conjugated structure and have the potential of transporting charges in 3D. These properties make them attractive alternatives to fullerene derivatives for use as acceptors in organic solar cells, photo-detectors and other optoelectronic applications.

Synthesis, characterization and photovoltaic properties of azadipyrromethene-based acceptors: effect of pyrrolic substituents

Azadipyrromethene derivatives are conjugated molecules with high absorptivity in the visible to near-IR region and high electron affinity that have great potential as electron acceptors for solar harvesting applications. To fully take advantage of these molecules, it is necessary to understand their structure–property relationships. We recently showed that phenylethynyl pyrrolic substituents red-shifted the visible absorption band, increased the electron affinity, and significantly improved the device performance in organic photovoltaic devices. Here, we synthesized and characterized a series of azadipyrromethene derivatives and their chelates, where the pyrrolic substituents were chosen to examine the effect of ethynyl group, aryl group and alkyl solubilizing group. Using thienylethynyl or alkoxyphenylethynyl substituents caused a small red-shift of the absorption compared to phenylethynyl substituents. The zinc(II) complexes were blended with regioregular poly(3-hexylthiophene) as the electron donor and tested in organic solar cells. All fullerene-free solar cells studied showed good photovoltaic properties, with power conversion efficiencies ranging from 2% to 4%. The blend films all had similar AFM images with no evidence of large-scale segregation. When comparing solubilizing groups on the pyrrolic substituents, we find that the performance varied as follows: H > t-butyl > 2-ethylhexyl. These results pave the way for developing higher performance non-fullerene acceptors for organic photovoltaic devices.

Dependence of field-effect mobility and contact resistance on nanostructure in regioregular poly(3-hexylthiophene) thin film transistors

The mobility and contact resistance of transistors based on regioregular poly(3-hexylthiophene) (P3HT) with Ti∕Pt electrodes were investigated as a function of the molecular weight (MW) of P3HT. For an increase in MW from 5.5to11kDa, the mobility increased from 0.04to0.16cm2V−1s−1, whereas the contact resistance decreased from 1.7to0.6MΩ. Further increases in MW yielded an apparent saturation in both the mobility and the contact resistance. A nanofibrilar morphology was observed where the width of the nanofibrils increases with MW. A qualitative model based on polymer chain folding is proposed to explain the electrical results.