Connecting Direct C-H Arylation Reactions with Dye-Sensitized Solar Cells: A Shortcut to D-A-π-A Organic Dyes

A Four-Component Reaction for the Synthesis of Thienopyrrolediones under Transition Metal Free Conditions

A three-starting-material four-component reaction strategy is described to construct thienopyrrolediones (TPDs) from the simplest raw materials, elemental sulfur, aldehydes, and β-ketoamides, under transition metal free conditions. Compared with traditional multistep reaction sequences, this process is simple, efficient, environmentally friendly, and atom-economic and has laid the foundation for further development of an easily synthesized TPD unit.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Development of Step-Saving Alternative Synthetic Pathways for Functional π-Conjugated Materials

Synthetic organic chemists endeavor to develop new reaction conditions, improve product yields, and enhance atom economy (synthetic methodologies), whereas the material scientists strive to create novel functional molecules/structures, increase device stabilities, and promote power conversion efficiencies via device engineering (organic optoelectronics). However, these two prominent research fields seem to have no intersections. Since joining national central university in 2012, our research philosophy aims to narrow, or rather to bridge the gap between synthetic methodologies and π-functional organic materials. In contrast to using multistep synthetic approaches based on Suzuki- or Stille coupling reactions, this personal account describes various step-saving and viable synthesis-shortcuts developed by our group, to access thiophene-based small molecules for optoelectronic applications. We expect these succinct and user-friendly alternative pathways designed by synthetic chemists would help material scientists to reach their target molecules in a more step-economical manner.

Synthesis of Novel 3,6-Dithienyl Diketopyrrolopyrrole Dyes by Direct C‒H Arylation

Direct C-H arylation coupling is potentially a more economical and sustainable process than conventional cross-coupling. However, this method has found limited application in the synthesis of organic dyes for dye-sensitized solar cells. Although direct C-H arylation is not an universal solution to any cross-coupling reactions, it efficiently complements conventional sp²−sp² bond formation and can provide shorter and more efficient routes to diketopyrrolopyrrole dyes. Here, we have applied palladium catalyzed direct C-H arylation in the synthesis of five new 3,6-dithienyl diketopyrrolopyrrole dyes. All prepared sensitizers display broad absorption from 350 nm up to 800 nm with high molar extinction coefficients. The dye-sensitized solar cells based on these dyes exhibit a power conversion efficiency in the range of 2.9 to 3.4%.

Combined LCA and Green Metrics Approach for the Sustainability Assessment of an Organic Dye Synthesis on Lab Scale

New generation photovoltaic devices have attracted much attention in the last decades since they can be efficiently manufactured employing abundant raw materials and with less-energy intensive processes. In this context, the use of powerful environmental assessment is pivotal to support the fine-tuning of solar cells fabrication and hit the target of manufacturing effective sustainable technological devices. In this work, a mass-based green metrics and life cycle assessment combined approach is applied to analyze the environmental performances of an innovative synthetic protocol for the preparation of organic dye TTZ5, which has been successfully proposed as sensitizer for manufacturing dye sensitized solar cells. The new synthetic strategy, which is based on the C-H activation process, has been compared with the previously reported synthesis employing classic Suzuki-Miyaura cross-coupling chemistry. Results highlight the contribution of direct energy consumption and purification operations in organic syntheses at lab scale. Furthermore, they demonstrate the usefulness of the environmental multifaceted analytic tool and the power of life cycle assessment to overcome the intrinsic less comprehensive nature of green metrics for the evaluation of organic synthetic protocols.

A new class of triphenylamine-based novel sensitizers for DSSCs: a comparative study of three different anchoring groups

Triphenylamine-based dyes with chloro groups suitably placed on their π-linkers affect DSSC efficiency.

Sn- and Pd-Free Synthesis of D-π-A Organic Sensitizers for Dye-Sensitized Solar Cells by Cu-Catalyzed Direct Arylation

<A variety of D-π-A-type functional organic dyes are facilely synthesized by direct C-H arylation catalyzed by inexpensive copper salts. Under optimized reaction conditions, a broad substrate scope with good functional group compatibility was demonstrated. Based on this synthetic strategy, three new dye sensitizers (CYL-5-7) were designed and fabricated for use in dye-sensitized solar cells (DSSCs). Photovoltaic characterization showed that these devices gave open-circuit voltages of 0.65-0.75 V, short-circuit currents of 5.90-12.60 mA cm^-2 , and fill factors of 65.6-76.9 %, corresponding to power conversion efficiencies (PCEs) of 2.95-6.20 %. This work represents the first use of Cu-catalyzed C-H arylation for a step-saving, Sn-free synthesis of precursor dyes for DSSC applications.

Molecular engineering and sequential cosensitization for preventing the "trade-off" effect with photovoltaic enhancement

In dye-sensitized solar cells (DSSCs), it is essential to use rational molecular design to obtain promising photosensitizers with well-matched energy levels and narrow optical band gaps. However, the "trade-off" effect between the photocurrent and photovoltage is still a challenge. Here we report four benzoxidazole based D-A-π-A metal-free organic dyes (WS-66, WS-67, WS-68 and WS-69) with different combinations of π-spacer units and anchoring-acceptor groups. Either extending the π-spacer or enhancing the electron acceptor can efficiently modulate the molecular energy levels, leading to a red-shift in the absorption spectra. The optimal dye, WS-69, containing a cyclopentadithiophene (CPDT) spacer and cyanoacetic acid acceptor, shows the narrowest energy band gap, which displays a very high photocurrent density of 19.39 mA cm-2, but suffers from a relatively low photovoltage of 696 mV, along with the so-called deleterious "trade-off" effect. A cosensitization strategy is further adopted for enhancing the device performance. Optimization of the dye loading sequence is found to be capable of simultaneously improving the photocurrent and photovoltage, and distinctly preventing the "trade-off" effect. The superior cosensitized cell exhibits an excellent power-conversion efficiency (PCE) of 10.09% under one-sun irradiation, and 11.12% under 0.3 sun irradiation, which constitutes a great achievement in that the efficiency of a pure metal-free organic dye with iodine electrolyte can exceed 11% even under relatively weak light irradiation. In contrast with the previous cosensitization strategy which mostly focused on compensation of light-harvesting, we propose a novel cosensitization architecture, in which the large molecular-sized, high photocurrent dye WS-69 takes charge of broadening the light-harvesting region to generate a high short-circuit current (JSC) while the small molecular-sized, high photovoltage dye WS-5 is responsible for retarding charge recombination to generate a high open-circuit voltage (VOC). In addition, adsorption amount and photo-stability studies suggest that the cyano group in the anchoring acceptor is important for the stability since it is beneficial towards decreasing the LUMO levels and enhancing the binding of dyes onto TiO2 nanocrystals.

Direct Arylation Strategies in the Synthesis of π-Extended Monomers for Organic Polymeric Solar Cells

π-conjugated macromolecules for organic polymeric solar cells can be rationally engineered at the molecular level in order to tune the optical, electrochemical and solid-state morphology characteristics, and thus to address requirements for the efficient solid state device implementation. The synthetic accessibility of monomers and polymers required for the device is getting increasing attention. Direct arylation reactions for the production of the π-extended scaffolds are gaining importance, bearing clear advantages over traditional carbon-carbon forming methodologies. Although their use in the final polymerization step is already established, there is a need for improving synthetic accessibility to implement them also in the monomer synthesis. In this review, we discuss recent examples highlighting this useful strategy.