Effect of a long alkyl group on cyclopentadithiophene as a conjugated bridge for D-A-π-A organic sensitizers: IPCE, electron diffusion length, and charge recombination

The role of the donor group and electron-accepting substitutions inserted in π-linkers in tuning the optoelectronic properties of D-π-A dye-sensitized solar cells: a DFT/TDDFT study

The design of low-cost and high-efficiency sensitizers is one of the most important factors in the expansion of dye-sensitized solar cells (DSSCs). To obtain effective sensitizer dyes for applications in dye-sensitized solar cells, a series of metal-free organic dyes with the D–π–A–A arrangement and with different donor and acceptor groups have been designed by using computational methodologies based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT). We have designed JK-POZ(1–3) and JK-PTZ(1–3) D–π–A–A organic dyes by modifying the donor and π-linker units of the JK-201 reference dye. Computational calculations of the structural, photochemical properties and electrochemical properties, as well as the key parameters related to the short-circuit current density and open-circuit voltage, including light-harvesting efficiency (LHE), singlet excited state lifetime (τ), reorganization energies (λ_total), electronic injection-free energy (ΔG^inject) and regeneration driving forces (ΔG^reg) of dyes were calculated and analyzed. Moreover, charge transfer parameters, such as the amount of charge transfer (q^CT), the charge transfer distance (D^CT), and dipole moment changes (μ^CT), were investigated. The results show that ΔG^reg, λ_max, λ_total and τ of JK-POZ-3 and JK-PTZ-3 dyes are superior to those of JK-201, indicating that novel JK-POZ-3 and JK-PTZ-3 dyes could be promising candidates for improving the efficiency of the DSSCs devices.

A series of metal-free organic dyes with the D–π–A–A arrangement and with different donor and acceptor groups have been designed theoretically.

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

Water oxidation is still one of the most important challenges to develop efficient artificial photosynthetic devices. In recent decades, the development and study of molecular complexes for water oxidation have allowed insight into the principles governing catalytic activity and the mechanism as well as establish ligand design guidelines to improve performance. However, their durability and long-term stability compromise the performance of molecular-based artificial photosynthetic devices. In this context, heterogenization of molecular water oxidation catalysts on electrode surfaces has emerged as a promising approach for efficient long-lasting water oxidation for artificial photosynthetic devices. This review covers the state of the art of strategies for the heterogenization of molecular water oxidation catalysts onto electrodes for (photo)electrochemical water oxidation. An overview and description of the main binding strategies are provided explaining the advantages of each strategy and their scope. Moreover, selected examples are discussed together with the the differences in activity and stability between the homogeneous and the heterogenized system when reported. Finally, the common design principles for efficient (photo)electrocatalytic performance summarized.

Novel anthracene-based organic dyes as co-sensitizers of porphyrins for developing efficient dye-sensitized solar cells

A high efficiency of 10.22% has been achieved through the cosensitization of anthracene-based organic dyes with a porphyrin dye.

The influence of π-linkers configuration on properties of 10-hexylphenoxazine donor-based sensitizer for dye-sensitized solar cell application - Theoretical approach

10-Hexylphenoxazine based dyes with A-(π)_n-D-(π)_n-A architecture is designed and investigated systematically for dye-sensitized solar cell (DSSC) application by Density Functional Theory (DFT) and Time-dependent Density Functional Theory (TD-DFT). The designed sensitizers consist of 10-Hexylphenoxazine as electron donor and cyanoacrylic acid as an acceptor, connected by the Thiophene and Cyanovinyl π-spacers configurations with symmetrical and asymmetrical form. The effect of π-spacers configurations on the electronic and optical properties of the dyes is also investigated. The optimized structure, electronic properties and absorption characteristics of A-(π)_n-D-(π)_n-A dyes were investigated. The charge separation and polarization properties are analyzed by co-planarity, natural bond orbital (NBO), dipole moment and linear polarizability studies. The free energy change of electron injection and dye regeneration of the sensitizers are also calculated. The addition of a greater number of π-spacers improves the electronic, spectroscopic, optical, and free energy properties of the designed sensitizer. The DFT studies also reveal that the position of the π-spacers plays an important role in the electronic and spectroscopic properties.

Investigations of New Phenothiazine-Based Compounds for Dye-Sensitized Solar Cells with Theoretical Insight

New D-π-D-π-A low-molecular-weight compounds, based on a phenothiazine scaffold linked via an acetylene unit with various donor moiety and cyanoacrylic acid anchoring groups, respectively, were successfully synthesized. The prepared phenothiazine dyes were entirely characterized using nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The compounds were designed to study the relationship between end-capping donor groups’ structure on their optoelectronic and thermal properties as well as the dye-sensitized solar cells’ performance. The effect of π-conjugation enlargement by incorporation of different heterocyclic substituents possessing various electron–donor affinities was systematically experimentally and theoretically examined. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were implemented to determine the electronic properties of the novel molecules.

Modulation of Acceptor Position in Organic Sensitizers: The Optimization of Intramolecular and Interfacial Charge Transfer Processes

With respect to cyanoacryclic acid as the traditional acceptor and anchoring group in dye-sensitized solar cells, the introduction of stronger electron-deficient groups has greatly expanded the scope of molecular design and increased their efficiencies for organic dyes. In this contribution, benzothiadiazole (BTD) was selected as a representative acceptor to illustrate the influence of its position on the photophysical properties and corresponding photovoltaic performances. Through the insertion of BTD in different positions of the framework with the same composition units and orders, four sensitizers were designed and synthesized. The structure-property relationship demonstrated the preference of the suitable position of an electron acceptor for optimization of the spectra response and interfacial charge transfer. In our system, dye LI-96 with BTD in the middle of the conjugated bridge showed the broadest spectrum and achieved the best photovoltaic performance (8.25%), which may pave a new way to design or optimize the efficient sensitizers by rational design of the acceptor position.

Efficient solar cells sensitized by a promising new type of porphyrin: dye-aggregation suppressed by double strapping

Doubly strapped porphyrin improved efficiency from 8.6% to 9.3% and finally to 10.6% through a combined approach of coadsorption and cosensitization.

Porphyrin sensitizers play essential roles in the development of efficient dye-sensitized solar cells (DSSCs). To further improve power conversion efficiency (PCE), it is vital to reduce undesirable dye aggregation that causes serious charge recombination and lowered open-circuit voltages (V_oc). To this end, we herein report a new class of porphyrin-based dyes XW40 and XW41, with the porphyrin cores strapped with two circle chains. Compared with the reference sensitizer XW10 which contains a porphyrin core wrapped in four dodecoxyl chains, the double strapping in XW40 not only effectively suppresses the dye aggregation but also improves the dye loading amount. As a result, the V_oc and photocurrent (J_sc) were improved by 19 mV and 0.8 mA cm^–2, respectively, compared with the corresponding values of XW10, and the efficiency was improved from 8.6% obtained for XW10 to 9.3% for XW40. To further extend the spectral response, an electron-withdrawing benzothiadiazole (BTD) unit was introduced as an auxiliary acceptor in XW41. Impressively, the onset wavelength of its IPCE spectrum was dramatically red-shifted to 830 nm. However, the extended π-conjugation framework results in aggravated dye aggregation, and thus a lowered efficiency of 8.2% was obtained for XW41. Through a combined approach of coadsorption and cosensitization, the efficiencies were dramatically enhanced to 10.6% and 10.2% for XW40 and XW41, respectively, as a result of simultaneously enhanced V_oc and J_sc. The results of this work provide a novel strategy for developing efficient DSSCs by employing strapped porphyrin dyes.

Organic Sensitizers with Extended Conjugation Frameworks as Cosensitizers of Porphyrins for Developing Efficient Dye-Sensitized Solar Cells

Relatively high efficiencies have been achieved for porphyrin-based dye-sensitized solar cells. For the purpose of designing efficient cosensitizers, we herein report systematically optimized dyes XC1-XC5 employing a triphenylamine donor, a benzothiadiazole moiety as the auxiliary acceptor, and a benzoic acid acceptor. One hexyl and four hexyloxy groups were introduced, and an ethynylene moiety was introduced between the donor and the auxiliary acceptor to afford XC1. To further extend the absorption wavelength, a second ethynylene moiety was introduced between the acceptor and the auxiliary acceptor to afford XC2. XC3 and XC4 were designed by introducing one and two methyl substituents, respectively, into the meta-positions of the anchoring carboxyl group. XC5 was further synthesized by inserting a cyano substituent into one of the ortho-positions of the carboxyl group with the purpose to strengthen the intramolecular charge-transfer effect and thus broaden the absorption wavelength. As expected, compared with the J_sc (14.29 mA·cm^-2) of XC1, the corresponding J_sc values for XC2-XC5 were enhanced to 16.50, 16.95, 16.73, and 17.74 mA·cm^-2, respectively. Moreover, XC4 exhibits the highest V_oc of 770 mV owing to the presence of a maximum of seven chains, which can effectively suppress dye aggregation. As a result, compared with XC1, XC2-XC5 exhibit improved efficiencies of 8.67, 9.05, 8.78, and 9.30%, respectively. In addition, the efficiencies of XC3 and XC5 were further enhanced by cosensitizing them with our previously reported porphyrin dye XW28 under various conditions. Finally, a remarkable efficiency of 10.50% was achieved for the cells cosensitized with XC5 and XW28. These results indicate that the combination of good planarity of the extended D-π-A framework with multiple alkoxy/alkyl chains may compose an effective optimizing strategy for designing and synthesizing excellent cosensitizers for porphyrins.

Novel D-A-π-A-Type Organic Dyes Containing a Ladderlike Dithienocyclopentacarbazole Donor for Effective Dye-Sensitized Solar Cells

A novel ladderlike fused-ring donor, dithienocyclopentacarbazole (DTCC) derivative, is used to design and synthesize three novel donor-acceptor-π-acceptor-type organic dyes (C1-C3) via facile direct arylation reactions, in which the DTCC derivative substituted by four p-octyloxyphenyl groups is served as the electron donor and the carboxylic acid group is used as the electron acceptor or anchoring group. To fine-tune the optical, electrochemical, and photovoltaic properties of the three dyes, various auxiliary acceptors, including benzo[2,1,3]thiadiazole (BT), 5,6-difluorobenzo[2,1,3]thiadiazole (DFBT), and pyridal[2,1,3]thiadiazole (PT), are incorporated into the dye backbones. The results indicate that all of the three dyes exhibit strong light-capturing ability in the visible region and obtain relatively high molar extinction coefficients (>31 000 M^-1 cm^-1) due to their strong charge transfer (CT) from donor to acceptor. Moreover, theoretical model calculations demonstrate fully separated highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels for the three dyes, which is helpful for efficient charge separation and electron injection. Using the three dyes as sensitizers, conventional dye-sensitized solar cells (DSSCs) based on liquid iodide/triiodide redox electrolytes are fabricated. Our results indicate that the BT-containing dye C1 affords the highest power conversion efficiency of up to 6.75%, much higher than that of the DFBT-containing dye C2 (5.40%) and the PT-containing dye C3 (1.85%). To our knowledge, this is the first example reported in the literature where the DTCC unit has been used to develop novel organic dyes for DSSC applications. Our work unambiguously demonstrates that the ladderlike DTCC derivatives are the superb electron-donating blocks for the development of high-performance organic dyes.

Orthogonally Functionalized Donor/Acceptor Homo- and Heterodimeric Dyes for Dye-Sensitized Solar Cells: An Approach to Introduce Panchromaticity and Control the Charge Recombination

Organic dyes possessing conjugated π-framework forms closely packed monolayers on photoanode in dye-sensitized solar cell (DSSC), because of the limitation to control the orientation and the extend of intermolecular π-π interaction, self-aggregation of dyes leads to reduced cell performance. In this report, a series of homodimeric (D₁-D₁ and D₂-D₂) and heterodimeric (D₁-D₂ and D₂-D₄) donor/acceptor (D/A) dyes containing spiroBiProDOT π-spacer were designed and synthesized by utilizing Pd-catalyzed direct arylation reaction and correlates the device performance with monomeric dyes (D₁ and D₂). Both the thiophenes (π-spacer) of spiroBiProDOT were functionalized with same or different donor groups which led to homodimeric and heterodimeric chromophores in a single sensitizer. The homodimeric spiro-dye D₁-D₁ showed higher power conversion efficiency (PCE), of 7.6% with a V_oc and J_sc of 0.672 V and 16.16 mA/cm², respectively. On the other hand, the monomeric D₁ exhibited a PCE of 3.2% (V_oc of 0.64 V and J_sc of 7.2 mA/cm²), which is lower by 2.4 fold compared to dimeric analogue. The spiro-unit provides flexibility between the incorporated chromophores to orient on TiO₂ due to four sp³-centers, which arrest the molecular motions after chemisorption. This study shows a new molecular approach to incorporate two chromophores in the dimeric dye possessing complementary absorption characteristics toward panchromatic absorption. The attenuated charge recombination at TiO₂/Dye/redox couple interface in case of D₁-D₁, owing to better passivation of TiO₂ surface, was elucidated through impedance analysis. The FT-IR spectrum of D₁-D₁ adsorbed on TiO₂ film indicated both the carboxylic units were involved in chemisorption which makes strong coupling between dye and TiO₂.

Interplay between π-Bridges and Positions of Branched Alkyl Groups of Unsymmetrical D-A-D-π-A Squaraines in Dye-Sensitized Solar Cells: Mode of Dye Anchoring and the Charge Transfer Process at the TiO2/Dye/Electrolyte Interface

Far-red-absorbing squaraines possessing high molar absorptivity (>10⁵ M^-1 cm^-1) are being attracted as high-efficiency chromophores in dye-sensitized solar cells (DSSCs). A series of donor-acceptor-donor-π spacer-acceptor (D-A-D-π-A) unsymmetrical squaraines, PSQ1-5, with indoline donor and squaric/cyanoacetic acid acceptor units, were designed for sensitized solar cells. For extending the absorption toward the near-infrared region (NIR) and controlling the orientation on the TiO₂ surface, benzene (PSQ1 and PSQ2) and thiophene (PSQ3-5) π-spacers and out-of-plane branched alkyl groups at the indoline that are away (PSQ1, PSQ3, and PSQ5) or near (PSQ2 and PSQ4) the anchoring group, respectively, were introduced. Dynamic aggregation tendency of PSQ1 and PSQ3 than that of their isomers systematically modulates the orientation on the TiO₂ surface, which in turn enhances photovoltaic performance. Absorptance on a thin transparent TiO₂ film shows a visible-to-NIR response with an onset around 800 nm for PSQ3-5. Although there is close resemblance in electrochemical redox levels, their high injection efficiency and recombination resistance differentiated their impact on the way of anchoring and the dihedral angle between D-A-D units and π-spacers. DSSCs sensitized with PSQ5 achieved a PCE of 8.15% under simulated AM 1.5G illumination (100 mW cm^-2), with the current density (J_sc) and open-circuit voltage (V_oc) of 19.73 mA cm^-2 and 630 mV, respectively. A clear comparison of the incident-photon-to-current conversion efficiency versus the light-harvesting efficiency correlates the structure-property relationship with J_sc obtained for PSQ dyes. Electrochemical impedance spectroscopy was carried out to examine the TiO₂/dye/electrolyte interface for further confirmation of the enhanced PCE of top-sp³-alkylated PSQ5 over that of other dyes.

Cyclopentadithiophene organic core in small molecule organic solar cells: morphological control of carrier recombination

Two new planar and symmetrical A–D–A (electron acceptor–electron donor–electron acceptor) small molecules based on a commercial cyclopentadithiophene derivative have been synthesized for solution processed small molecule organic solar cells.

Effect of Out-of-Plane Alkyl Group's Position in Dye-Sensitized Solar Cell Efficiency: A Structure-Property Relationship Utilizing Indoline-Based Unsymmetrical Squaraine Dyes

Squaraine dyes are promising chromophores to harvest visible and near-infrared (NIR) photons. A series of indoline-based unsymmetrical squaraine (SQ) dyes that contain alkyl chains at sp³ C- and N- atoms of indoline moieties with a carboxylic acid anchoring group were synthesized. The optical and electrochemical properties of the SQ dyes in solution were nearly identical as there was no change in the D-A-D SQ framework; however, remarkable changes with respect to the power conversion efficiencies (PCE) were observed depending upon the position of alkyl groups in the dye. Introduction of alkyl groups to the indoline unit that was away from anchoring unit were helped in more dye loading with controlled organization of dyes on surface, increased charge transfer resistance, long electron lifetime, and hence higher PCE than that of the corresponding isomer in which the alkyl groups funtionalized indoline unit contains the carboxylic acid anchoring group. Careful analysis of incident photon-to-current conversion efficiency (IPCE) profiles indicated the presence of aggregated structure on the TiO₂ surface that contributes to the charge injection in the presence of a coadsorbent. A dye-sensitized solar cell (DSSC) device made out of SQ5 was achieved an efficiency of 9.0%, with an open-circuit potential (V_oc) of 660 mV and short-circuit current density (J_sc) of 19.82 mA/cm², under simulated AM 1.5G illumination (100 mW/cm²). The IPCE profile of SQ5 shows an onset near to 750 nm with a good quantum efficiency (>80%) in the range of 550-700 nm, indicating the importance of self-organization of dyes on the TiO₂ surface for an efficient charge injection. This present investigation revealed the importance of position of alkyl groups in the squaraine-based dyes for the better PCE.

New anthracene-based organic dyes: the flexible position of the anthracene moiety bearing isolation groups in the conjugated bridge and the adjustable cell performance

A series of 9,10-diarylsubstituted anthracene (An)-bridged with different sequences were developed to explore the suitable position of isolation groups.

Enediyne as π linker in D–π–A dyes for dye-sensitized solar cells

Three enediyne-bridged D–π–A dyes with different donor position and number are synthesized and their performance on DSSCs are investigated.

Insight into D-A-π-A Structured Sensitizers: A Promising Route to Highly Efficient and Stable Dye-Sensitized Solar Cells

The dye-sensitized solar cell (DSSC) is one of the most promising photovoltaic technologies with potential of low cost, light weight, and good flexibility. The practical application of DSSCs requires further improvement in power conversion efficiency and long-term stability. Recently, significant progress has been witnessed in DSSC research owing to the novel concept of the D-A-π-A motif for the molecular engineering of organic photosensitizers. New organic and porphyrin dyes based on the D-A-π-A motif can not only enhance photovoltaic performance, but also improve durability in DSSC applications. This Spotlight on Applications highlights recent advances in the D-A-π-A-based photosensitizers, specifically focusing on the mechanism of efficiency and stability enhancements. Also, we find insight into the additional acceptor as well as the trade-off of long wavelength response. The basic principles are involved in molecular engineering of efficient D-A-π-A sensitizers, providing a clear road map showing how to modulate the energy bands, rationally extending the response wavelength, and optimizing photovoltaic efficiency step by step.

Dithiafulvenyl–triphenylamine organic dyes with alkyl chains for efficient coadsorbent-free dye-sensitized solar cells

Dithiafulvenyl–triphenylamine (DTF–TPA) based D-D-η-A organic dyes with different alkyl chains attached on the DTF unit were synthesized to investigate the effect of alkyl chain lengths on the photovoltaic performances of dye-sensitized solar cells.

New thieno[3,2-b][1]benzothiophene-based organic sensitizers containing π-extended thiophene spacers for efficient dye-sensitized solar cells

Thieno[3,2-b][1]benzothiophene-based organic sensitizers with thiophene spacers are developed for dye-sensitized solar cells. By optimizing the performances, the sensitizer containing bithiophene π-spacer exhibited the best performance (9.69%).

Functional tuning of organic dyes containing 2,7-carbazole and other electron-rich segments in the conjugation pathway

Organic dyes containing a triarylamine donor, a cyanoacrylic acid acceptor and a conjugation pathway composed of 2,7-carbazole, thiophene and fluorene have been synthesized and characterized as sensitizers for TiO₂-based dye-sensitized solar cells.

Molecular engineering of quinoxaline dyes toward more efficient sensitizers for dye-sensitized solar cells

N-annulated perylene-containing quinoxaline sensitizer (NIQ4) displays remarkable performance in light harvesting, electron injection, and dye regeneration.