Sadiara Fall

High-Performance Solution-Processed Solar Cells and Ambipolar Behavior in Organic Field-Effect Transistors with Thienyl-BODIPY Scaffoldings

Green-absorbing dipyrromethene dyes engineered from bis-vinyl-thienyl modules are planar molecules, exhibiting strong absorption in the 713-724 nm range and displaying comparable electron and hole mobilities in thin films (maximum value 1 × 10(-3) cm(2)/(V·s)). Bulk heterojunction solar cells assembled with these dyes and a fullerene derivative (PC(61)BM) at a low ratio give a power conversion efficiency as high as 4.7%, with short-circuit current values of 14.2 mA/cm(2), open-circuit voltage of 0.7 V, and a broad external quantum efficiency ranging from 350 to 920 nm with a maximum value of 60%.

Absorption tuning of monosubstituted triazatruxenes for bulk heterojunction solar cells.

A series of triazatruxene (TAT)-functionalized Bodipy dyes were prepared by a sequence of reactions involving either cross-coupling reactions promoted by Pd complexes or a Knoevenagel reaction leading to a vinyl linker. The new dyes show large absorption coefficients and fluorescence quantum yields as well as interesting electrochemical properties. The blue dyes of this series exhibit interesting photovoltaic effects (V(OC) = 0.83 V, J(SC) = 3.6 mA/cm(2), efficiency 0.9%) in bulk heterojunction solar cells, due to the good hole mobility imported by the TAT entity.

Thiazole-based scaffolding for high performance solar cells

An interesting way of decreasing both HOMO and LUMO energy levels simultaneously while keeping the band-gap constant in soluble electron-donor small molecules for photovoltaic applications is presented. This consists in the replacement of thiophene rings by thiazole units in small molecules based on the alternation of electron-rich and electron-deficient moieties. A new diketopyrrolopyrrole-based dumbbell-shaped electron-donor soluble molecule for organic photovoltaic applications has been synthesized and characterized. It includes thiazole units as linkers between the bis-lactam core and the triazatruxene moieties used as π-stacking platforms. A power conversion efficiency of 6.3% has been attained with this thiazole derivative and in particular with an increase of the open-circuit voltage of 0.15 V with respect to the thiophene-based organic semiconducting counterpart. This open-circuit voltage increase is due to the lowering of the HOMO level of the thiazole derivative while its LUMO level has also been stabilized as highlighted by the similar band-gap measured for the thiazole and thiophene derivatives.

Tailoring the microstructure and charge transport in conjugated polymers by alkyl side-chain engineering

Charge transport in conjugated polymers is critical to most optoelectronic devices and depends strongly on the polymer structure and conformation in the solid state. Understanding the correlations between charge carrier mobility, energy disorder and molecular assembly is therefore essential to improve device performances. Alkyl side-chains contribute to intermolecular interactions and are key to controlling the polymer microstructure and electronic properties. Investigating a set of polymers with common conjugated units but different side-chain functionalization provides new insights into the complex structure–transport relationship. Here, field-effect transistors and space-charge-limited current devices are used together with in situ grazing-incidence wide-angle X-ray scattering to study charge transport and morphology in a series of donor–acceptor copolymers. Probing hole mobility as a function of carrier density and orientation permits us to assess energy disorder and hopping rate anisotropy, while X-ray diffraction allows us to link transport properties to the polymer microstructure. We show that branched side-chains enhance structural and energy disorder and lead to isotropic transport, whereas linear chains induce either a common lamellar structure or a more exceptional pseudo-hexagonal columnar phase with a helicoidal polymer conformation. The latter enhances out-of-plane mobility but increases energy disorder possibly due to larger interring torsion angles.

Ambipolar charge transport in polymer:fullerene bulk heterojunctions for different polymer side-chains

We use field-effect transistors to investigate electron and hole mobilities in polymer:fullerene blends. Low-band-gap polymers with a common conjugated backbone and differing side-chains are utilized in order to clarify the link between the side-chain molecular structure and grafting position, and the power-conversion efficiency of related bulk heterojunction solar cells. The results show that, at a fixed polymer:fullerene weight ratio, the electron mobility increases by more than four orders of magnitude when changing from linear to branched side-chains. As a consequence, the photovoltaic performances are highest at low fullerene contents for branched chains while the opposite is true for linear chains.

Rational Engineering of BODIPY-Bridged Trisindole Derivatives for Solar Cell Applications

Boron dipyrromethene (BODIPY) and its derivatives are known to be efficient photon-harvesting chromophores. However, their study as active materials in bulk heterojunction (BHJ) solar cells is still scarce. In this study, the development of new synthetic ways to design original BODIPY-based dumbbell-shape molecules, including a first 2,3,5,6-tetravinyl aromatic BODIPY molecule, is reported. High fill factors can be obtained in BHJ solar cells when blended with a fullerene derivative, leading to a new record BODIPY-based power conversion efficiency of 5.8 %.

Incorporation of spirobifluorene regioisomers in electron-donating molecular systems for organic solar cells

We report herein the synthesis and characterizations of new small molecules for organic photovoltaics including diketopyrrolopyrrole and spirobifluorene. Two grafting positions of the spirobifluorene units are used leading to two regioisomers with different optoelectronic properties. Despite, the moderate photovoltaic performances, we show that the incorporation of spirobifluorene as a three-dimension platform into conjugated material backbones allows the increase of dimensionality and brings a more isotropic character to the charge transport properties of the final materials. The solubility is also increased despite the low density of grafted alkyl side-chains, which is an important feature for the future of this technology.

An Electron-Transporting Thiazole-Based Polymer Synthesized Through Direct (Hetero)Arylation Polymerization

In this work, a new n-type polymer based on a thiazole-diketopyrrolopyrrole unit has been synthesized through direct (hetero)arylation polycondensation. The molar mass has been optimized by systematic variation of the the monomer concentration. Optical and electrochemical properties have been studied. They clearly suggested that this polymer possess a high electron affinity together with a very interesting absorption band, making it a good non-fullerene acceptor candidate. As a consequence, its charge transport and photovoltaic properties in a blend with the usual P3HT electron-donating polymer have been investigated.

Versatile synthesis of α-fused BODIPY displaying intense absorption in the NIR region and high electron affinity

We report the design and highly efficient and versatile synthesis of four isomers of α-fused boron dipyrromethene (BODIPY). Structure–property relationships have been established through photophysical and electrochemical properties analysis as well as first-principles calculations. A comparative study of their charge transport properties has been carried out in Organic Field Effect Transistor (OFET) devices, and this allowed the identification of a high mobility material (10−2 cm2 V−1 s−1) displaying balanced ambipolar behavior.

The role of chemical structure in indacenodithienothiophene-alt-benzothiadiazole copolymers for high performance organic solar cells with improved photo-stability through minimization of burn-in loss

It is of upmost importance to gain an in-depth understanding of the role of the polymer chemical structure in the performance of the corresponding organic solar cell (OSC) and its degradation behavior, which is currently insufficiently explored. Achieving these correlations will set new design rules towards further optimization of polymer chemical structures for OSCs exhibiting high performances and long stability. In this study, our efforts have been focused on identifying how the nature of aryl substituents and the number of fluorine atoms anchored in the backbone of indacenodithieno[3,2-b]thiophene (IDTT) based polymers influence their optoelectronic properties, the OSC performances and their degradation mechanisms. The most important outcome of this study is the demonstration that standard initial burn-in loss is primary attributed to microstructure instabilities. Furthermore, the initial burn-in loss could be substantially reduced through the rational design of the polymeric semiconductors chemical structure, leading to improved lifetimes and low (20%) initial power conversion efficiency loss. In particular, we identify the beneficial effect of the presence of the two fluorine atoms on the benzo[c][1,2,5]thiadiazole (BTD), as compared to the non-fluorinated and mono-fluorinated analogues, in decreasing the burn-in by reducing the microstructure instabilities regardless of the aryl substituent that is present in the polymer backbone.