Philippe Blanchard

Molecular Materials for Organic Photovoltaics: Small is Beautiful

An overview of some recent developments of the chemistry of molecular donor materials for organic photovoltaics (OPV) is presented. Although molecular materials have been used for the fabrication of OPV cells from the very beginning of the field, the design of molecular donors specifically designed for OPV is a relatively recent research area. In the past few years, molecular donors have been used in both vacuum-deposited and solution-processed OPV cells and both fields have witnessed impressive progress with power conversion efficiencies crossing the symbolic limit of 10 %. However, this progress has been achieved at the price of an increasing complexity of the chemistry of active materials and of the technology of device fabrication. This evolution probably inherent to the progress of research is difficult to reconcile with the necessity for OPV to demonstrate a decisive economic advantage over existing silicon technology. In this short review various classes of molecular donors are discussed with the aim of defining possible basic molecular structures that can combine structural simplicity, low molecular weight, synthetic accessibility, scalability and that can represent possible starting points for the development of simple and cost-effective OPV materials.

3,4-Ethylenedioxythiophene (EDOT) as a versatile building block for advanced functional π-conjugated systems

The potentialities offered by EDOT as a building block for the synthesis of functional π-conjugated systems are reviewed. The first part underlines the specific advantages of the EDOT unit for the design of precursors of electrogenerated functional conducting polymers combining high reactivity and low polymerization potential. This topic is illustrated by some recent examples of polymers with specific electrochemical properties and/or a reduced band gap. The second part is focused on the interest of EDOT as a building block for the synthesis of various classes of molecular functional π-conjugated systems. Examples of fluorophores, push–pull chromophores for nonlinear optics and extended π-donors are presented. Emphasis is placed on the optical and crystallographic structure of these compounds, which shows that a major advantage of the EDOT building block lies in an unique combination of strong electron donor properties and self-structuring effects related to intramolecular non-covalent interactions between oxygen and sulfur. Such intramolecular interactions also exert a determining influence on the structure and electronic properties of π-conjugated oligomers incorporating EDOT units which represent some of the more recent uses of the EDOT building block. The structure–property relationships of various classes of EDOT-containing conjugated oligomers are discussed in relation to their potential use as organic semi-conductors. In a brief last section, various synthetic approaches devoted to the chemistry of EDOT itself, or to the modification of its chemical structure, are discussed in relation with possible future research directions.

Push–pull chromophores based on 2,2′-bi(3,4-ethylenedioxythiophene) (BEDOT) π-conjugating spacer

Abstract Replacement of 2,2′-bithiophene by BEDOT in push–pull NLO-phores produces a red shift of the absorption maximum accompanied with a large increase of the quadratic nonlinear optical susceptibility.

EXTENDED THIENYLENEVINYLENE OLIGOMERS AS HIGHLY EFFICIENT MOLECULAR WIRES

A much smaller band gap than in the parent polymer is observed for the π-conjugated hexadecamer 1, which can be reversibly charged up to the hexacation in a small potential range.

Synthesis and Electronic Properties of Adducts of Oligothienylenevinylenes and Fullerene C60

) together with preliminary investigations oftheir electrochemical and optical properties. Owing to thesmall highest occupied molecular orbital–lowest unoccupiedmolecular orbital (HOMO–LUMO) gap of nTV, these com-pounds are potentially interesting for light harvesting in or-ganic photovoltaic devices.The target compounds have been synthesized according tothe method of Prato et al.

S‐Position Isomers of BEDT‐TTF and EDT‐TTF: Synthesis and Influence of Outer Sulfur Atoms on the Electrochemical Properties and Crystallographic Network of Related Organic Metals

The synthesis and characterization of new modified tetrathiafulvalenes (TTF), the S-position isomers of BEDT-TTF and EDT-TTF, are described. The synthetic strategy presented in this work is based on an efficient and unprecedented two-step sequence for the conversion of a vicinal bis(hydroxymethyl) functionality into a disulfide ring. Different routes are discussed in terms of efficiency for the synthesis of the symmetric S-position isomer of BEDT-TTF and that of EDT-TTF. Their electrochemical properties are combined with data obtained from UV/Vis spectroscopy and orbital calculations, and the electronic influence of peripheral sulfur atoms on the neutral and oxidized species is discussed. The introduction of these outer sulfur atoms at the periphery of the TTF core gives rise to specific intermolecular S...S interactions in the corresponding organic materials. Crystallographic studies of radical cation salts synthesized upon electrocrystallization clearly showed that the network obtained is dictated by the outer sulfur atoms, which are responsible for a characteristic and unprecedented windmill array.

Orientational effect on the photophysical properties of quaterthiophene - C60 dyads

Two quaterthiophene-[60]fullerene dyads in which C60 is singly (4TsC) or doubly (4TdC) connected to the inner beta-position of the terminal thiophene rings have been synthesized. The electronic properties of these donor-acceptor compounds were analyzed by UV/Vis spectroscopy and cyclic voltammetry, and their photophysical properties in solution and in the solid state by (time-resolved) photoluminescence (PL) and photoinduced absorption (PIA) spectroscopy. Both the flexible and geometrically constrained 4TsC and 4TdC dyads exhibit photoinduced charge transfer from the quaterthiophene to the fullerene in toluene and o-dichlorobenzene (ODCB). In toluene, charge transfer occurs in both dyads by an indirect mechanism, the first step of which is a singlet-energy transfer from the 4T(S1) state to the C60(S1) state. In the more polar ODCB, direct electron transfer from 4T(S1) competes with energy transfer, and both direct and indirect charge transfers are observed. The geometrical fixation of the donor and acceptor chromophores in 4TdC results in rate constants for energy and electron transfer that are more than an order of magnitude larger than those of the flexible 4TsC system. For both dyads, charge recombination is extremely fast, as inferred from picosecond-resolved temporal evolution of the excited state absorption of the 4T.+ radical cation both in toluene and ODCB.

Phthalimide end-capped thienoisoindigo and diketopyrrolopyrrole as non-fullerene molecular acceptors for organic solar cells

Two acetylene-bridged molecules, built by grafting phthalimides on thienoisoindigo (TII) and diketopyrrolopyrrole (DPP) blocks, have been synthesized, characterized and evaluated as electron acceptor materials in air-processed inverted organic solar cells. Once blended with poly(3-hexylthiophene), power conversion efficiencies (PCEs) of ca. 0.4% and 3.3% were achieved for TII and DPP based devices, respectively. To the best of our knowledge these PCEs (i) rank amongst the highest reported so far for diketopyrrolopyrrole based acceptors and (ii) make this contribution the very first example of thienoisoindigo based materials used as non-fullerene electron acceptors.

Small D-π-A systems with o-phenylene-bridged accepting units as active materials for organic photovoltaics.

Donor-acceptor (D-π-A) systems that combine triarylamine donor blocks and dicyanovinyl (DCV) acceptor groups have been synthesized. Starting from the triphenylamine (TPA)-thiophene-DCV compound (1) as a reference system, various synthetic approaches have been developed for controlling the light-harvesting properties and energy levels of the frontier orbitals in this molecule. Thus, the introduction of methoxy groups onto TPA, the replacement of one phenyl ring of TPA by a thiophene ring, or the extension of the π-conjugating spacer group lead to the modulation of the HOMO level. On the other hand, the fusion of the DCV group onto the vicinal thiophene ring by an ortho-phenylene bridge allows for a specific fine-tuning of the LUMO level. The electronic properties of the molecules were analyzed by using UV/Vis spectroscopy and cyclic voltammetry and the compounds were evaluated as donor materials in basic bilayer planar heterojunction solar cells by using C60 as acceptor material. The relationships between the electronic properties of the donors and the performance of the corresponding photovoltaic devices are discussed. Bilayer planar heterojunction solar cells that used reference compound 1 and C70 afforded power-conversion efficiencies of up to 3.7u2009%.

Proquinoid acceptors as building blocks for the design of efficient π-conjugated fluorophores with high electron affinity

The association of aromatic electron donor groups with nproquinoid acceptors leads to π-conjugated fluorophores combining ntunable emission wavelength at constant geometry, high fluorescence nefficiency and high electron affinity.