Effect of Position of Donor Units and Alkyl Groups on Dye-Sensitized Solar Cell Device Performance: Indoline-Aniline Donor-Based Visible Light Active Unsymmetrical Squaraine Dyes.

Indoline (In) and aniline (An) donor-based visible light active unsymmetrical squaraine (SQ) dyes were synthesized for dye-sensitized solar cells (DSSCs), where the position of An and In units was changed with respect to the anchoring group (carboxylic acid) to have In-SQ-An-CO2H and An-SQ-In-CO2H sensitizers, AS1-AS5. Linear or branched alkyl groups were functionalized with the N atom of either In or An units to control the aggregation of the dyes on TiO2. AS1-AS5 exhibit an isomeric π-framework where the squaric acid unit is placed in the middle, where AS2 and AS5 dyes possess the anchoring group connected with the An donor, and AS1, AS3, and AS4 dyes having the anchoring group connected with the In donor. Hence, the conjugation between the middle squaric acid acceptor unit and the anchoring -CO2H group is short for AS2, AS5, and AK2 and longer for AS1, AS3, and AS4 dyes. AS dyes showed absorption between 501 and 535 nm with extinction coefficients of 1.46-1.61 × 105 M-1 cm-1. Further, the isomeric π-framework of An-SQ-In-CO2H and In-SQ-An-CO2H exhibited by means of changing the position of In and An units a bathochromic shift in the absorption properties of AS2 and AS5 compared to the AS1, AS3, and AS4 dyes. The DSSC device fabricated with the dyes contains short acceptor-anchoring group distance (AS2 and AS5) showed high photovoltaic performances compared to the dyes having longer distance (AS1, AS3, and AS4) with the iodolyte (I-/I3-) electrolyte. DSSC device efficiencies of 5.49, 6.34, 6.16, and 5.57% have been achieved for AS1, AS2, AS3, and AS4 dyes, respectively; without chenodeoxycholic acid (CDCA), small changes have been observed in the device performance of the AS dyes with CDCA. Significant changes have been noted in the DSSC parameters (open-circuit voltage VOC, short-circuit current JSC, fill factor ff, and efficiency η) for the AS5 dye while sensitized with CDCA and showed highest DSSC efficiency of 8.01% in the AS dye series. This study revealed the potential of shorter SQ acceptor-anchoring group distance over the longer one and the importance of alkyl groups on the overall DSSC device performance for the unsymmetrical squaraine dyes.

Far-red active unsymmetrical squaraine dyes containing N-arylated indoline donors for dye sensitized solar cells.

Squaraine dyes possess sharp far-red active transition with high extinction coefficient and form aggregates on TiO2 surface. Aggregation of dyes on TiO2 has been considered as a detrimental factor for DSSC device performance, which can be controlled by appending alkyl groups to the dye structures. Hence by integrating alkylated (alkyl groups with both in-plane and out-of-plane) aryl group with indoline moiety to make it compatible with other electrolytes and for controlling the dye-aggregation, a series of squaraine acceptor-based dyes SQA4-6 have been designed and synthesized. SQA4-6 dyes showed absorption between 642 and 653 nm (λmax), photophysical and electrochemical studies indicated that the HOMO energy levels of this sets of dyes are well aligned with the potentials of I-/ I 3 - and [Co(bpy)3]2+/3+ redox shuttles for better dye regeneration process. DSSC device efficiency of 3% has been achieved for SQA5 dye with iodolyte (I-/ I 3 - ) electrolyte in the presence of 0.3 mM of chenodeoxycholic acid (CDCA). The IPCE profile of DSSC device fabricated with SQA4-6 dyes indicated the contribution of aggregated structures for the photocurrent generation.

Photoactivatable Fluorophores for Bioimaging Applications.

Photoactivatable fluorophores provide the opportunity to switch fluorescence on exclusively in a selected area within a sample of interest at a precise interval of time. Such a level of spatiotemporal fluorescence control enables the implementation of imaging schemes to monitor dynamic events in real time and visualize structural features with nanometer resolution. These transformative imaging methods are contributing fundamental insights on diverse cellular processes with profound implications in biology and medicine. Current photoactivatable fluorophores, however, become emissive only after the activation event, preventing the acquisition of fluorescence images and, hence, the visualization of the sample prior to activation. We developed a family of photoactivatable fluorophores capable of interconverting between emissive states with spectrally resolved fluorescence, instead of switching from a nonemissive state to an emissive one. We demonstrated that our compounds allow the real-time monitoring of molecules diffusing across the cellular blastoderm of developing embryos as well as of polymer beads translocating along the intestinal tract of live nematodes. Additionally, they also permit the tracking of single molecules in the lysosomal compartments of live cells and the visualization of these organelles with nanometer resolution. Indeed, our photoactivatable fluorophores may evolve into invaluable analytical tools for the investigation of the fundamental factors regulating the functions and structures of cells at the molecular level.

Photoactivatable BODIPYs for Live-Cell PALM.

Photoactivated localization microscopy (PALM) relies on fluorescence photoactivation and single-molecule localization to overcome optical diffraction and reconstruct images of biological samples with spatial resolution at the nanoscale. The implementation of this subdiffraction imaging method, however, requires fluorescent probes with photochemical and photophysical properties specifically engineered to enable the localization of single photoactivated molecules with nanometer precision. The synthetic versatility and outstanding photophysical properties of the borondipyrromethene (BODIPY) chromophore are ideally suited to satisfy these stringent requirements. Specifically, synthetic manipulations of the BODIPY scaffold can be invoked to install photolabile functional groups and photoactivate fluorescence under photochemical control. Additionally, targeting ligands can be incorporated in the resulting photoactivatable fluorophores (PAFs) to label selected subcellular components in live cells. Indeed, photoactivatable BODIPYs have already allowed the sub-diffraction imaging of diverse cellular substructures in live cells using PALM and can evolve into invaluable analytical probes for bioimaging applications.

D-A-D-based Unsymmetrical Thiosquaraine Dye for the Dye-Sensitized Solar Cells†.

In dye-sensitized solar cell, modulating the electronic properties of the sensitizer by varying the donor, π-spacer, acceptor and anchoring groups help optimizing the structure of the dye for better device performance. Here, a donor-acceptor-donor-based unsymmetrical thiosquaraine sensitizer (SQ5S) has been designed and synthesized. Photophysical, electrochemical, theoretical and photovoltaic characterizations of SQ5S dye have been compared with its oxygen analog, SQ5. The incorporation of the sulfur atom in the acceptor unit of SQ5S dye showed an intense peak at 688 nm, which was 38 nm of red-shifted and showed the panchromatic light harvesting response with the onset of 850 nm compared with SQ5 dye. The LUMO and HOMO energy levels are well aligned with the conduction band of TiO2 and the redox potential of electrolyte for the charge injection and the dye-regeneration processes, respectively. Photovoltaic efficiency of 1.51% (VOC 610 mV, JSC 3.07 mA cm-2 , ff 81%) has been achieved for SQ5S dye, whereas SQ5 showed the device performance of 5.43% (VOC 723 mV, JSC 9.3 mA cm-2 , ff 80%). The decreased device performance for the dye SQ5S has been attributed to the favorable intersystem crossing process associated with the photoexcited SQ5S that reduces the driving force for the charge injection process.

Solvent-Dependent Functional Aggregates of Unsymmetrical Squaraine Dyes on TiO2 Surface for Dye-Sensitized Solar Cells.

Alkyl group wrapped donor-acceptor-donor (D-A-D) based unsymmetrical squaraine dyes SQ1, SQ5, and SQS4 were used to evaluate the effect of sensitizing solvents on dye-sensitized solar cell (DSSC) efficiency. A drastic change in DSSC efficiency was observed when the photo-anodes were sensitized in acetonitrile (bad solvent when considering dye solubility) and chloroform (good solvent) with an Iodolyte (I-/I3-) electrolyte. The DSSC device sensitized with squaraine dyes in acetonitrile showed better photovoltaic performance with enhanced photocurrent generation and photovoltage compared to the device sensitized in chloroform. In a good sensitizing solvent, dyes with long hydrophobic alkyl chains are deleterious forming aggregates on the TiO2 surface, which results in an incident photon-to-current conversion efficiency (IPCE) response mostly from monomeric and dimeric structures. Meanwhile, a bad sensitizing solvent facilitates the formation of well-packed self-assembled structures on the TiO2 surface, which are responsible for a broad IPCE response and high device efficiencies. The photoanode sensitized in the bad sensitizing solvent showed enhanced VOC values of 642, 675, and 699 mV; JSC values of 6.38, 11.1, and 11.69 mA/cm2; and DSSC device efficiencies of 3.0, 5.63, and 6.13% for the SQ1, SQ5, and SQS4 dyes in the absence of a coadsorbent (chenodeoxycholic acid (CDCA)), respectively, which were further enhanced by CDCA addition. Meanwhile, the photoanode sensitized in the good sensitizing solvent showed relatively low photovoltaic VOC values of 640, 652, and 650 mV; JSC values of 5.78, 6.79, and 6.24 mA/cm2; and device efficiencies of 2.73, 3.35, and 3.20% for SQ1, SQ5, and SQS4 in the absence of CDCA, respectively, which were further varied with equivalents of CDCA. The best DSSC device efficiencies of 6.13 and 3.20% were obtained for SQS4 without CDCA, where the dye was sensitized in acetonitrile (bad) and chloroform (good) sensitizing solvents, respectively.

Alkyl-Group-Wrapped Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells: Branched Alkyl Chains Modulate the Aggregation of Dyes and Charge Recombination Processes.

Electron transfer processes at the interfaces dictate the factors that improve the photovoltaic parameters, such as open-circuit voltage (Voc) and short-circuit current (Jsc), of a dye-sensitized solar cell device, besides selection of a set of suitable anode, dye, electrolyte, and cathode materials. An inefficient charge injection process at the dye-TiO2 interface and charge recombination at the TiO2-dye/electrolyte interface have detrimental effects on improving both Jsc and Voc. Hence, tailoring the factors that govern the improvement of Jsc and Voc will be an ideal approach to get the desired sensitizers with good device efficiencies. Squaraines are far-red-active zwitterionic dyes and have a high molar extinction coefficient along with unique aggregation properties due to the large dipole moment associated with them. Here, we report a series of unsymmetrical squaraine dyes, SQS1 to SQS6, with systematic variation of alkyl groups at the sp3-C and N-atoms of the indoline unit that is away from the anchoring group to control the dye-dye interactions on the TiO2 surface. The branched alkyl groups help in modulating the self-assembly of sensitizers on the TiO2 surface, besides passivating the surface that helps avoid the charge recombination processes. Light harvesting efficiency and cyclic voltammetry studies of dye-sensitized TiO2 electrodes indicate that the aggregation and charge hopping process between the dye molecules can be modulated, respectively, by systematically increasing the number of carbon atoms in the alkyl groups. Such a variation in the branched alkyl group helps enhance Voc from 672 (SQS1) to 718 mV (SQS6) and Jsc from 7.95 (SQS1) to 12.22 mA/cm2 (SQS6), with the device efficiency ranging from 3.82% to 6.23% without any coadsorbent. Dye SQS4 achieves the highest efficiency of 7.1% (Voc = 715 mV, Jsc = 13.05 mA/cm2) with coadsorbent chenodeoxycholic acid (CDCA) using an iodine (I-/I3-) electrolyte compared to its analogues. An analysis of the incident photon-to-current efficiency profiles indicates that the major contribution to photocurrent generation is from the aggregated squaraine dyes on TiO2.

Homo- and Heterodimeric Dyes for Dye-Sensitized Solar Cells: Panchromatic Light Absorption and Modulated Open Circuit Potential.

The design of dyes for panchromatic light absorption has attracted much attention in the field of dye-sensitized solar cells (DSSCs). An approach to enhance panchromatic light absorption utilizes mixtures of complementary light-absorbing dyes as well as dyes with specific anchoring groups that facilitate interfacial charge transfer with TiO2 . Dipole-dipole interactions between the dye molecules on the surface broaden the spectrum, which results in decreased DSSC device performance. However, controlled aggregation of dyes results in broadening the spectral profile along with enhanced photocurrent generation. To control the dye-dye interaction, dimeric dyes with different dipole lengths D1 -Dsq , Dsq -Dsq were systematically designed and synthesized. The photophysical and electrochemical properties were evaluated and the EHOMO and ELUMO levels were determined; these energy levels determines the electron injection from ELUMO of the dye to ECB of TiO2 and regeneration of oxidized dye by the electrolyte, respectively. The absorption spectra of Dsq -Dsq , D1 -Dsq were broadened in solution compared to model dye Dsq ; this indicates that the dye-dye interaction is prominent in solution. In D1 -Dsq excitation energy transfer between photoexcited D1 and Dsq was explained by using Förster resonance energy transfer (FRET). The homodimeric dye showed a device performace of 2.8 % (Voc 0.607, Jsc 6.62 mA/cm2 , ff 69.3 %),whereas the heterodimeric dye D1 -Dsq showed a device performance of 3.9 % (Voc 0.652 V, Jsc 8.89 mA/cm2 , ff 68.8 %). The increased photocurrent for D1 -Dsq is due to the panchromatic IPCE response compared to Dsq -Dsq . The increased Voc is due to the effective passivation of the TiO2 surface by the spirolinker, and the effective dipole moment that shifts the conduction band on TiO2 . Hence, the open circuit potential, Voc , for the devices prepared from Dsq , D1 -Dsq and Dsq -Dsq were systematically modulated by controlling the intermolecular π-π and intramolecular dipole-dipole interactions of the dimeric dyes.

Panchromatic Sensitizer for Dye-Sensitized Solar Cells: Unsymmetrical Squaraine Dyes Incorporating Benzodithiophene π-Spacer with Alkyl Chains to Extend Conjugation, Control the Dye Assembly on TiO2, and Retard Charge Recombination.

Metal-free near-infrared (NIR) active unsymmetrical squaraine dyes, RSQ1 and RSQ2, with benzodithiophene (BDT) π-spacer and cyanoacrylic acid acceptor were synthesized by utilizing palladium catalyzed direct (hetero)arylation reaction. Methyl and 2-ethylhexyl groups were strategically placed at the BDT unit for RSQ1 and RSQ2, respectively, to investigate the effect of alkylated π-spacer on dye aggregation on the TiO2 surface and recombination reactions at TiO2/dye/electrolyte interface. These dyes have strong absorption (ε > 105 M-1 cm-1) in near-infrared (NIR) region and exhibit similar optical and electrochemical properties as they have same conjugated framework. RSQ2 performed better than RSQ1 owing to its higher open-circuit voltage (Voc) and fill factor (ff) in spite of having comparable short-circuit current density (Jsc). The panchromatic incident photon-to-current conversion efficiency (IPCE) response was also observed for both the dyes. RSQ2 showed power conversion efficiency (PCE) of 6.72% with short-circuit current density (Jsc) of 18.53 mA/cm2, open circuit voltage (Voc) of 0.538 V, and fill factor (ff) of 67.4%, without any coadsorbent. Attenuation of the charge recombination for RSQ2 was revealed by electrochemical impedance analysis (EIS) and open-circuit potential decay transients (OCVD), which attributes to its higher Voc and ff in comparison to RSQ1.