Alternating Copolymers Incorporating Dithienogemolodithiophene for Field-Effect Transistor Applications

We report the synthesis of an electron-rich fused dithienogemolodithiophene monomer containing straight chain tetradecyl solubilizing groups. Copolymers were prepared with four different electron accepting monomers of varying reduction potential. We report how the choice of acceptor influences the optical properties and molecular energy levels as well as the solid state packing. Field effect transistor devices were fabricated using silver source-drain electrodes, with a promising charge carrier mobility up to 0.26 cm² V⁻¹ s⁻¹ for films deposited from non-chlorinated solvents. These results suggest dithienogemolodithiophene is a useful building block for the development of high performance semiconducting polymers

The influence of microstructure on charge separation dynamics in organic bulk heterojunction materials for solar cell applications

Light-induced charge formation is essential for the generation of photocurrent in organic solar cells. In order to gain a better understanding of this complex process, we have investigated the femtosecond dynamics of charge separation upon selective excitation of either the fullerene or the polymer in different bulk heterojunction blends with well-characterized microstructure. Blends of the pBTTT and PBDTTPD polymers with PCBM gave us access to three different scenarios: either a single intermixed phase, an intermixed phase with additional pure PCBM clusters, or a three-phase microstructure of pure polymer aggregates, pure fullerene clusters and intermixed regions. We found that ultrafast charge separation (by electron or hole transfer) occurs predominantly in intermixed regions, while charges are generated more slowly from excitons in pure domains that require diffusion to a charge generation site. The pure domains are helpful to prevent geminate charge recombination, but they must be sufficiently small not to become exciton traps. By varying the polymer packing, backbone planarity and chain length, we have shown that exciton diffusion out of small polymer aggregates in the highly efficient PBDTTPD:PCBM blend occurs within the same chain and is helped by delocalization

The fate of electron–hole pairs in polymer:fullerene blends for organic photovoltaics

There has been long-standing debate on how free charges are generated in donor:acceptor blends that are used in organic solar cells, and which are generally comprised of a complex phase morphology, where intermixed and neat phases of the donor and acceptor material co-exist. Here we resolve this question, basing our conclusions on Stark effect spectroscopy data obtained in the absence and presence of externally applied electric fields. Reconciling opposing views found in literature, we unambiguously demonstrate that the fate of photogenerated electron–hole pairs—whether they will dissociate to free charges or geminately recombine—is determined at ultrafast times, despite the fact that their actual spatial separation can be much slower. Our insights are important to further develop rational approaches towards material design and processing of organic solar cells, assisting to realize their purported promise as lead-free, third-generation energy technology that can reach efficiencies over 10%

Collision entre amateurs de cyclotourisme : les vertus de l'article 1384, alinéa 1 !

Note sous CA Metz, 1ère ch., 4 février 2010, RG n°06/01490. Sport ; cyclotourisme ; responsabilité civile ; accident ; article 1384 du code civil ; acceptation des risques ; garde de la chose

Effects of a Heavy Atom on Molecular Order and Morphology in Conjugated Polymer:Fullerene Photovoltaic Blend Thin Films and Devices

We study the molecular order and morphology in poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS) thin films and their blends with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). We find that substitution of the sulfur atoms in the thiophene rings of P3HT by heavy selenium atoms increases the tendency of the molecules to form better ordered phase; interestingly, their overall fraction of ordered phase is much lower than that of P3HT-based films. The higher tendency of P3HS molecules to order (aggregate) is consistent with more planar chain conformation simulated. The lower fraction of ordered phase (or the higher fraction of disordered phase) in P3HS-based films is clearly identified by in-plane skeleton Raman modes under resonant excitation conditions, such as a smaller ratio of the CC modes associated with the ordered (∼1422 cm^–1) and disordered (∼1446 cm^–1) phases (<i>I</i>_1422cm^–1/<i>I</i>_1446cm^–1 = 1.4 for P3HS and 0.6 for P3HS:PCBM), compared with P3HT-based films (<i>I</i>_1449cm^–1/<i>I</i>_1470cm^–1 = 2.5 for P3HT and 1.0 for P3HT:PCBM) and a larger Raman dispersion of the CC mode: P3HS (17 cm^–1) <i>versus</i> P3HT (6 cm^–1) and P3HS:PCBM (36 cm^–1) <i>versus</i> P3HT:PCBM films (23 cm^–1). The higher fraction of disordered phase in P3HS prevents the formation of micrometer-sized PCBM aggregates in blend films during thermal annealing. Importantly, this lower fraction but better quality of ordered phase in P3HS molecules strongly influences P3HS:PCBM photovoltaic performance, producing smaller short-circuit current (<i>J</i>_sc) in pristine devices, but significantly larger increase in <i>J</i>_sc after annealing compared to P3HT:PCBM devices. Our results clarify the effects of heavy atom substitution in low band gap polymers and their impact on blend morphology and device performance. Furthermore, our study clearly demonstrates resonant Raman spectroscopy as a simple, but powerful, structural probe which provides important information about “fraction/quantity of ordered phase” of molecules, not easily accessible using traditional X-ray-based techniques

Additive-assisted supramolecular manipulation of polymer : fullerene blend phase morphologies and its influence on photophysical processes

It is well known that even small variations in the solid-state microstructure of polymer:fullerene bulk heterojunctions can drastically change their organic solar cell device performance. We employ pBTTT:PC₆₁BM as a model system and manipulate co-crystal formation of 1 : 1 (by weight) blends with the assistance of fatty acid methyl esters as additives. This allows us to evaluate the role of the intermixed phase in such binary blends through manipulation of their phase morphology—from fully intercalated to partially and predominantly non-intercalated systems—and its effect on the exciton- and carrier- dynamics and the efficiency of charge collection, with relevance for future device design and manufacturing.10 page(s

Direct Correlation of Charge Transfer Absorption with Molecular Donor:Acceptor Interfacial Area via Photothermal Deflection Spectroscopy

Here we show that the charge transfer (CT) absorption signal in bulk-heterojunction solar cell blends, measured by photothermal deflection spectroscopy, is directly proportional to the density of molecular donor:acceptor interfaces. Since the optical transitions from the ground state to the interfacial CT state are weakly allowed at photon energies below the optical gap of both the donor and acceptor, we can exploit the use of this sensitive linear absorption spectroscopy for such quantification. Moreover, we determine the absolute molar extinction coefficient of the CT transition for an archetypical polymer:fullerene interface. The latter is ∼100 times lower than the extinction coefficient of the donor chromophore involved, allowing us to experimentally estimate the transition dipole moment as 0.3 D and the electronic coupling between the ground and CT states to be on the order of 30 meV