Ibrahim Bulut

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.

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 %.

LUMO's modulation by electron withdrawing unit modification in amorphous TAT dumbbell-shaped molecules

The synthesis and characterization of a series of dumbbell-shaped molecules constructed from paraffin-free central dyes linked to two triazatruxene (TAT) units are described. The photovoltaic properties of these new glassy electron-donating molecules in a bulk heterojunction with fullerene derivatives have been studied. For this series of molecules, the local-range molecular packing in the amorphous solid state is mainly influenced by the TAT units bearing bulky side-chains, while the optical properties and energy levels are tuned by the choice of the central core. Therefore, TAT acts as a structuration platform in thin films for pure or blended active layers and independently, the optoelectronic properties are driven by the choice of the central dye. Photovoltaic cells based on these materials exhibited power conversion efficiencies of up to 3.5%, among the top values reported so far for soluble amorphous small molecule based organic photovoltaic devices. These TAT-based derivatives are a promising platform to reach high solar cell efficiencies as well as to understand in detail the impact of the chemical structure on the optoelectronic properties.

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.

Improved structural order by side-chain engineering of organic small molecules for photovoltaic applications

Despite obvious progress in organic semiconducting material design and organic bulk-heterojunction solar cell power conversion efficiencies the rationalization of the molecular design to finely tune organic semiconductor properties is still challenging. Herein, thanks to a particular dumbbell-shaped molecular design allowing partial decoupling between the structural properties and the frontier energy level positioning and optical absorption properties, we demonstrate the impact of the nature of side chains along the conjugated backbone on the structural properties of conjugated molecules. Thus, linear side chains on the structurally cohesive triazatruxene building blocks of our molecules provide higher stacking abilities, resulting in higher charge transport abilities and photovoltaic performances. These dumbbell-shaped molecules are a promising molecular family for reaching high solar cell efficiencies as well as for understanding in detail the impact of chemical structure on optoelectronic properties.

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.

Experimental and theoretical investigations on the optical and electrochemical properties of π-conjugated donor-acceptor-donor (DAD) compounds toward a universal model

A series of nine (9) donor-acceptor-donor (DAD) π-conjugated small molecules were synthesized via palladium catalyzed Stille aromatic cross-coupling reactions by the combination of six (6) heterocycle building blocks (thiophene, furan, thiazole, 2,1,3-benzothiadiazole, 2,1,3-pyridinothiadiazole, thienothiadiazole) acting as electron donating (thiazole, furan, thiophene) and electron deficient (benzothiadiazole, pyridinethiadiazole, thienothiadiazole) units. These model compounds enable determining the correspondence between the theoretical and experimental optical and electrochemical properties for the first time, via Density Functional Theory (DFT), time-dependent DFT, UV-Vis spectroscopy, and cyclic voltammetry, accordingly. The obtained theoretical models can be utilized for the design and synthesis of new DAD structures with precise optical bandgaps, absorption maxima, and energy levels suitable for different optoelectronic applications.