Patricia Chávez

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

Solution-processed bulk heterojunction solar cells have experienced a remarkable acceleration in performances in the last two decades, reaching power conversion efficiencies above 10%. This impressive progress is the outcome of a simultaneous development of more advanced device architectures and of optimized semiconducting polymers. Several chemical approaches have been developed to fine-tune the optoelectronics and structural polymer parameters required to reach high efficiencies. Fluorination of the conjugated polymer backbone has appeared recently to be an especially promising approach for the development of efficient semiconducting polymers. As a matter of fact, most currently best-performing semiconducting polymers are using fluorine atoms in their conjugated backbone. In this review, we attempt to give an up-to-date overview of the latest results achieved on fluorinated polymers for solar cells and to highlight general polymer properties’ evolution trends related to the fluorination of their conjugated backbone.

Synthesis by Direct Arylation of Thiazole–Derivatives: Regioisomer Configurations–Optical Properties Relationship Investigation

The synthesis of thiazole(Tz)-based regioisomer materials using selective direct arylation to avoid any protection steps has been developed. A series of trimers in which the Tz groups sandwich either an electron-rich or an electron-deficient unit, with a regioselective orientation of the respective Tz unit, has therefore been synthesized. This chemical strategy has also been followed to synthesize a second series of pentamers in which the Tz group is used as a π-conjugated bridge between an electron-rich central unit and electron-deficient end-capping groups and vice versa. On both trimers and pentamers, the effect of Tz orientation on the conjugation properties of the synthesized materials was investigated by a combination of experimental measurements and density functional theory calculations. This study highlights that control of the orientation of the Tz unit leads to the synthesis of the most conjugated regioisomer derivative. The present work gives chemical synthesis tools for the synthesis of selectively oriented Tz-based materials as well as a general guideline for the design of Tz-based materials with the highest conjugation length, including the Tz-orientation effect.

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 %.

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.

Tuning crystallochromism in diketopyrrolopyrrole-co-thieno[3,2-b]thiophene derivatives by the architecture of their alkyl side chains

Two diketopyrrolopyrrole-co-thieno[3,2-b]thiophene derivatives substituted with either branched ethylhexyl (TTDPP-EH) or linear hexyl side chains (TTDPP-C6) have been synthesized. The impact of the side chain architecture on the structure and optical properties has been evaluated. TTDPP molecules crystallize in triclinic unit cells observed in both single crystals and in thin films. The most striking difference between the two compounds is the packing of the molecules. For TTDPP-EH, pairs of molecules overlap only at their thienothiophene (TT) ring tips leading to a weak excitonic coupling of the J-type character. In contrast, TTDPP-C6 molecules stack in a 1D columnar structure with extended molecular overlapping. A transverse displacement of the molecules along their molecular axis allows a partial overlap of electron-rich TT and electron-poor DPP units. This leads to a stronger excitonic coupling with apparent coexistence of H- and J-like absorption features. Interestingly, both single crystals and oriented thin films change color with light polarization. This sensitivity to light polarization is related to the presence of two different excitonic couplings within TTDPP-C6.

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.