Porphyrins Containing a Triphenylamine Donor and up to Eight Alkoxy Chains for Dye-Sensitized Solar Cells: A High Efficiency of 10.9%

Molecular Engineering Strategies of Spectral Matching and Structure Optimization for Efficient Metal-Free Organic Dyes in Dye-Sensitized Solar Cells: A Theoretical Study

Metal-free organic dyes are promising dyes that can be applied widely in dye-sensitized solar cells (DSSCs). The rational design and selection of dyes with complementary absorption can promote the development of methods that can enhance the utilization of incident light by DSSCs, such as cosensitization and tandem devices. Based on these opinions, the structure of the reported high-performance metal-free organic dye ZL003 is used as a template to design two new metal-free organic dyes, HX-1 and HX-2, by replacing its donor unit with a 2-phenothiazine-phenylamine unit and fusing its three independent π-bridge units into a whole with the aim of driving the red shift and the blue shift of the absorption spectra of ZL003, respectively. Through theoretical investigation, it is demonstrated that the perfect complementary optical absorption of HX-1 and HX-2 can be realized as the shift of the absorption spectra of ZL003 to different directions, which means their feasibility to the application in cosensitization or tandem dye-sensitized solar cells (T-DSSCs). Furthermore, it is hypothesized that HX-1 may be the dye with better photovoltaic performance than ZL003 by modeling their intramolecular charge-transfer (ICT) processes, TiO2 surface adsorption, and photovoltaic parameters. The short-circuit current density (Jsc) and photoelectric conversion efficiency (PCE) of HX-1 are 23.10 mA·cm-2 and 21.26% in theory, compared to those of 20.68 mA·cm-2 and 19.64% in ZL003 at the same computational level, respectively. In view of the complementary optical properties, the combination of HX-1 with HX-2 may be a reasonable option for dyes for the development of a highly efficient cosensitization system or T-DSSCs in the future. In terms of such findings, these two novel metal-free organic dyes may have bright prospects in the research of highly efficient DSSCs, and this work can provide a reference for the design of dyes with complementary absorption through simple structural adjustments of the realistic dyes with high photovoltaic performance.

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.

Redox Shuttle-Based Electrolytes for Dye-Sensitized Solar Cells: Comprehensive Guidance, Recent Progress, and Future Perspective

A redox electrolyte is a crucial part of dye-sensitized solar cells (DSSCs), which plays a significant role in the photovoltage and photocurrent of the DSSCs through efficient dye regeneration and minimization of charge recombination. An I^-/I₃ ^- redox shuttle has been mostly utilized, but it limits the open-circuit voltage (V _oc) to 0.7-0.8 V. To improve the V _oc value, an alternative redox shuttle with more positive redox potential is required. Thus, by utilizing cobalt complexes with polypyridyl ligands, a significant power conversion efficiency (PCE) of above 14% with a high V _oc of up to 1 V under 1-sun illumination was achieved. Recently, the V _oc of a DSSC has exceeded 1 V with a PCE of around 15% by using Cu-complex-based redox shuttles. The PCE of over 34% in DSSCs under ambient light by using these Cu-complex-based redox shuttles also proves the potential for the commercialization of DSSCs in indoor applications. However, most of the developed highly efficient porphyrin and organic dyes cannot be used for the Cu-complex-based redox shuttles due to their higher positive redox potentials. Therefore, the replacement of suitable ligands in Cu complexes or an alternative redox shuttle with a redox potential of 0.45-0.65 V has been required to utilize the highly efficient porphyrin and organic dyes. As a consequence, for the first time, the proposed strategy for a PCE enhancement of over 16% in DSSCs with a suitable redox shuttle is made by finding a superior counter electrode to enhance the fill factor and a suitable near-infrared (NIR)-absorbing dye for cosensitization with the existing dyes to further broaden the light absorption and enhance the short-circuit current density (J _sc) value. This review comprehensively analyzes the redox shuttles and redox-shuttle-based liquid electrolytes for DSSCs and gives recent progress and perspectives.

In Silico Optimization of Charge Separating Dyes for Solar Energy Conversion

Dye-sensitized photoelectrochemical cells are promising devices in solar energy conversion. However, several limitations still have to be addressed, such as the major loss pathway through charge recombination at the dye-semiconductor interface. Charge separating dyes constructed as push-pull systems can increase the spatial separation of electron and hole, decreasing the recombination rate. Here, a family of dyes, consisting of polyphenylamine donors, fluorene bridges, and perylene monoimide acceptors, was investigated in silico using a combination of semi-empirical nuclear dynamics and a quantum propagation of photoexcited electron and hole. To optimize the charge separation, several molecular design strategies were investigated, including modifying the donor molecule, increasing the π-bridge length, and decoupling the molecular components through steric effects. The combination of a triphenylamine donor, using an extended 2-fluorene π-bridge, and decoupling the different components by steric hindrance from side groups resulted in a dye with significantly improved charge separation properties in comparison to the original supramolecular complex.

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.

Fluorenyl Indoline as an Efficient Electron Donor for Concerted Companion Dyes: Enhanced Light-Harvesting and Photocurrent

Concerted companion dyes (CC dyes) like XW61 have been demonstrated to be an effective platform for developing efficient DSSCs. However, the moderated phenothiazine-based electron donor in XW61 results in unsatisfactory J_sc. To address this problem, a stronger fluorenyl indoline-based electron donor has been used to construct porphyrin dye XW68 and organic dyes Y1-Y2. The stronger electron-donating character of the fluorenyl indoline unit leads to an enhanced J_sc value (20.48 mA·cm^-2) for the individual dye XW68. On this basis, CC dyes XW69-XW70-C8 have been designed and synthesized by combining the frameworks of Y1 and Y2 with XW68. The complementary absorption characters of the porphyrin and the organic dye moieties lead to panchromatic absorption with a strong light-harvesting capability from 350 to 700 nm and the onset wavelength extended to ca. 840 nm in the IPCE curves. As a result, excellent J_sc values have been achieved (>22 mA·cm^-2). In addition to the advantages of high J_sc, bulky octyl groups have been introduced into the donor of XW70-C8 to reduce dye aggregation and suppress charge recombination. Finally, a highest PCE of 11.1% with a satisfactory J_sc (22.25 mA·cm^-2) and an enhanced V_oc (750 mV) has been achieved upon coadsorption of XW70-C8 with CDCA. In addition, the CC dye XW70-C8-based solar cells exhibit excellent long-term photostability. These results provide an effective method for rationally improving the photovoltaic behavior, especially the J_sc of CC dyes, by introducing strong electron donor moieties with suitable substituents.

Synthesis, Optical Properties, and Electrochemical Behavior of 5,10,15,20-Tetraaryl-5,15-diazaporphyrin-Amine Hybrids

This work reports a series of covalently linked hybrids comprising 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (M-TADAP; M = Ni, Zn, Cu) and amines. M-TADAP-amine hybrids were prepared via the metal-templated cyclization of the corresponding metal(II)-dipyrrin complexes and redox reactions on the DAP unit. In the UV/vis/near-IR absorption spectra of the hybrids containing an 18π-electron DAP ring, broad charge-transfer bands were observed, reflecting the electron-donating property of the para-aminophenyl groups and the electron-accepting property of the 18π TADAP dication. The electrochemical behavior of the M-TADAP-amine hybrids was strongly dependent on the structure of the peripheral amine units. Further electrochemical oxidation of the hybrids bearing N-Ph groups conceivably generated amine-centered radicals, which sequentially underwent irreversible coupling to form benzidine-linked M-TADAP polymer films. The Ni-TADAP-benzidine polymer exhibited the electric conductivity of 1×10^-3  S m^-1 .

Enhancing the Photoelectric Properties of Zinc Porphyrin Dyes by Introducing Five-Membered Heterocyclic Rings into the Electron Donor: A Density Functional Theory and Time-Dependent Density Functional Theory Study

To fabricate highly efficient dye sensitizers for dye-sensitized solar cells, new zinc porphyrin dye sensitizers were designed based on one of the most efficient dyes, YD2-o-C8, by introducing electron-rich heterocyclic rings into the electron donor. Five potentially efficient dyes, Dye1-5, were obtained by replacing the phenyl group of the donor in YD2-o-C8 with pyrrolyl, furyl, and thienyl groups. The electronic structures, absorption spectra, intramolecular charge-transfer characteristics, and excited-state lifetimes of the designed dyes were investigated using the density functional theory and time-dependent density functional theory methods. All the designed dyes exhibit better photoelectric properties than those of YD2-o-C8. Compared with YD2-o-C8, the designed new dyes have smaller frontier molecular orbital energy gaps and obvious red-shifting absorption spectra in the Q band. The analyses of charge density difference plots and intramolecular charge-transfer characteristics indicated that the designed dyes can better promote intramolecular charge transfer and electron-hole separation. Among the five designed dyes, Dye1 with a pyrrolyl group exhibits the best performance. Dye3 and Dye5 with methyl-furyl and methyl-thienyl groups, respectively, exhibit the next best performance. Dye2 and Dye4 with furyl and thienyl groups, respectively, are the worst performers. The introduced methyl group can further improve the electron-donating ability of heterocyclic rings and promote the red shift of the Q bands and intramolecular charge transfer of dyes. The excited-state lifetimes of the new dyes were in the following order: YD2-o-C8 < Dye4 < Dye2 < Dye5 < Dye3 < Dye1, which shows their stronger abilities to inject electrons into semiconductor films.

Accurate Prediction of the Excited States in the Fully Conjugated Porphyrin Tapes across the Full Spectral Range: A Story of the Interplay between π-π* and Intramolecular Charge-Transfer Transitions in Soft Chromophores

The ability of density functional theory (DFT) and time-dependent DFT (TDDFT) methods for the accurate prediction of the energies and oscillator strengths of the excited states in a series of fully conjugated meso-meso β-β β-β triple-linked porphyrin oligomers (porphyrin tapes 2-12) was probed in the gas phase and solution using several exchange-correlation functionals. It was demonstrated that the use of the hybrid B3LYP functional provides a good compromise for the accurate prediction of the localized π-π* and intramolecular charge-transfer transitions, thus allowing confident interpretation of the UV-vis-NIR spectra of porphyrin oligomers. The TDDFT-based sum-over-state (SOS) calculations for the porphyrin tape dimer 2 and trimer 3 as well as parent monomer 1 correctly predicted the signs and shapes of the magnetic circular dichroism (MCD) signals in the low-energy region of the spectra.

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.

Light-Absorbing Pyridine Derivative as a New Electrolyte Additive for Developing Efficient Porphyrin Dye-Sensitized Solar Cells

To fabricate efficient dye-sensitized solar cells (DSSCs), 4-tert-butylpyridine (TBP) is commonly used as an additive in the electrolytes for improving the photovoltages (V_OC). However, TBP cannot play a positive role in improving the photocurrent (J_SC) because of the lack of absorption in the visible-wavelength range. We herein report a light-absorbing pyridine derivative N1 as an additive for the axial coordination with porphyrin dyes. N1 was synthesized by introducing a (bis(4-methoxyphenyl)amino)anthryl moiety into the para-position of pyridine via an acetylene bridge, and porphyrin dye XW64 containing meso-3,5-disubstituted phenyl groups was synthesized considering that the meta-substituted phenyl groups may induce weaker steric hindrance with the axial pyridyl ligand, as compared with wrapped and strapped porphyrin dyes. Thus, N1 was used as an electrolyte additive together with TBP. When optimized concentrations of 6 mM N1 and 0.5 M TBP were used for fabricating DSSCs based on XW64, enhanced photovoltaic performance was achieved, with J_SC, V_OC, and efficiency of 15.65 mA·cm^-2, 0.701 V, and 7.35%, respectively, superior to those of the corresponding DSSCs without using the additives (J_SC = 14.86 mA·cm^-2, V_OC = 0.599 V, and efficiency = 5.94%). The enhancement of J_SC can be ascribed to the improved light-harvesting ability induced by the axially coordinated N1. Furthermore, the two additives also can be used to fabricate efficient solar cells based on the wrapped porphyrin dye XW42, achieving high efficiency of 10.3%, indicative of their general applicability in fabricating high-performance DSSCs. These results indicate that the simultaneous employment of the traditional TBP additive and a pyridyl ligand with light-harvesting ability in the electrolyte for the axial coordination to a porphyrin dye is a promising approach for developing efficient DSSCs.

Cause, Regulation and Utilization of Dye Aggregation in Dye-Sensitized Solar Cells

As an important member of third generation solar cell, dye-sensitized solar cells (DSSCs) have the advantages of being low cost, having an easy fabrication process, utilizing rich raw materials and a high-power conversion efficiency (PCE), prompting nearly three decades as a research hotspot. Recently, increasing the photoelectric conversion efficiency of DSSCs has proven troublesome. Sensitizers, as the most important part, are no longer limited to molecular engineering, and the regulation of dye aggregation has become a widely held concern, especially in liquid DSSCs. This review first presents the operational mechanism of liquid and solid-state dye-sensitized solar cells, including the influencing factors of various parameters on device efficiency. Secondly, the mechanism of dye aggregation was explained by molecular exciton theory, and the influence of various factors on dye aggregation was summarized. We focused on a review of several methods for regulating dye aggregation in liquid and solid-state dye-sensitized solar cells, and the advantages and disadvantages of these methods were analyzed. In addition, the important application of quantum computational chemistry in the study of dye aggregation was introduced. Finally, an outlook was proposed that utilizing the advantages of dye aggregation by combining molecular engineering with dye aggregation regulation is a research direction to improve the performance of liquid DSSCs in the future. For solid-state dye-sensitized solar cells (ssDSSCs), the effects of solid electrolytes also need to be taken into account.

Efficient Solar Cells Based on Concerted Companion Dyes Containing Two Complementary Components: An Alternative Approach for Cosensitization

With the purpose to achieve panchromatic absorption for constructing efficient dye-sensitized solar cells (DSSCs), the cosensitization approach of using two dyes with complementary absorption has been developed with great success. However, this approach usually requires time-consuming optimization of a number of parameters for controlling the ratio and distribution of the two coadsorbed dyes on TiO₂ film, which limits the potentials of this strategy. We herein report an alternative approach for developing efficient DSSCs by designing a class of "concerted companion dyes" with two complementary dye components linked covalently. Thus, a newly synthesized organic dye Z2 was linked to a recently reported doubly strapped porphyrin dye XW51 through flexible chains with various lengths to afford XW60-XW63. These dyes exhibit excellent absorption and efficiencies in the range of 8.8%-11.7%. Notably, upon coadsorption with chenodeoxycholic acid, XW61 affords an impressive efficiency of 12.4%, a record for iodine electrolyte-based DSSCs, to the best of our knowledge. In addition, these dyes also exhibit the advantages of easy cell fabrication, simple optimization, as well as excellent photostability.

A new type of multibenzyloxy-wrapped porphyrin sensitizers for developing efficient dye-sensitized solar cells

Porphyrin dyes have been widely used for the fabrication of efficient dye-sensitized solar cells (DSSCs). However, dye aggregation and charge recombination still exert negative effects on photovoltaic performance, resulting in unsatisfactory power conversion efficiencies (PCEs). Herein, we report a new class of porphyrin sensitizers, XW52 and XW53 employing four benzyloxy groups to wrap the porphyrin cores. As a result, an efficiency of 7.6% was obtained for XW52, with [Formula: see text] and [Formula: see text] of 668 mV and 16.63 mA cm[Formula: see text], respectively. Compared with XW52, an additional 2,6-dialkoxyphenyl group has been introduced to the N-atom of the phenothiazine donor to furnish XW53 with the aim to further improve the anti-aggregation character and the solubility, and thus the [Formula: see text] was improved to 674 mV, and a higher efficiency of 7.9% was achieved for XW53. Upon cosensitization with PT-C6, the[Formula: see text] and [Formula: see text] were synergistically enhanced to 727 mV and 18.67 mA cm[Formula: see text], respectively. As a result, a high efficiency of 9.6% was successfully achieved for the cosensitization system of XW53 + PT-C6. These results provide an effective novel strategy for designing efficient porphyrin dyes by introducing multiple benzyloxy groups to the meso-phenyl groups.

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 (V_oc) and short-circuit current (J_sc), 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 J_sc and V_oc. Hence, tailoring the factors that govern the improvement of J_sc and V_oc 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 V_oc from 672 (SQS1) to 718 mV (SQS6) and J_sc 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% (V_oc = 715 mV, J_sc = 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.

D-π-A-Structured Porphyrins with Extended Auxiliary π-Spacers for Highly Efficient Dye-Sensitized Solar Cells

Zn(II)-porphyrin dyes (SGT-030 and SGT-031) with extended auxiliary π-spacers in the donor (D) part have been prepared and applied to dye-sensitized solar cells (DSSCs). The porphyrin dyes contained the same D-ethynyl-zinc porphyrinyl (ZnP)-ethynyl-benzothiadiazole-acceptor platform, but their donor groups varied from phenylene (Ph) in SGT-053 as a reference dye to the thieno[3,2-b]benzothiophene (TBT) and 4-hexyl-4H-thieno[3,2-b]indole (TI) moieties in SGT-030 and SGT-031, respectively. The effects of the extended auxiliary π-spacer in the D-π-A-structured porphyrin sensitizers on the molecular and photovoltaic properties were investigated via photophysical and electrochemical experiments as well as theoretical calculations. With the trend in conjugation length (Ph < TBT ≈ TI) and the donating ability of the π-spacer (Ph < TBT < TI), the absorption maxima and molecular absorptivity increased in the order SGT-053 (Ph) < SGT-030 (TBT) < SGT-031 (TI). The incorporation of TBT and TI promoted significant enhancements in the light-harvesting properties by reducing the energy gap and efficiently improving electronic communication. The DSSCs based on SGT-030 (10.80%) and SGT-031 (10.89%) with coadsorption of 4-(3,6-bis(4-((2-ethylhexyl)oxy)phenyl)-9H-carbazol-9-yl)benzoic acid in conjunction with the [Co(bpy)₃]^2+/3+-based electrolyte showed better power conversion efficiency than that of SGT-053 (9.10%). Electrochemical impedance spectroscopy analysis unveiled that the difference in J_sc and V_oc originates mainly from the twisted orientation between D and ZnP by the introduction of TBT and TI. This result indicated that the introduction of an extended auxiliary π-spacer in the donor part is a rational molecular design approach to improve photovoltaic performance by enhancing the light-harvesting ability and hindering charge recombination on the TiO₂ photoanode.

Solar Cells Sensitized with Porphyrin Dyes Containing Oligo(Ethylene Glycol) Units: A High Efficiency Beyond 12

A series of new porphyrin dyes (XW42-XW44) containing oligo(ethylene glycol) units have been designed and synthesized. Two triethylene glycol units were introduced into the phenothiazine moiety of XW42, whereas diethylene glycol (DEG) and ethylene glycol chains were introduced to afford XW43 and XW44, respectively. Interestingly, the efficiencies of the DSSCs were clearly dependent on the chain lengths. Among the three dyes, XW42 and XW43 exhibited relatively high open-circuit voltages of 751 and 750 mV, respectively, and XW43 exhibited the highest efficiency of 10.32 % owing to the presence of the DEG chains with suitable lengths and excellent ability to trap Li⁺ . Furthermore, through a combined coadsorption and cosensitization approach, the efficiencies were dramatically enhanced. As a result, a highest efficiency of 12.10 % was obtained for the XW43+chenodeoxycholic acid+PT-C6 (a metal-free organic dye) system, which ranks among the highest efficiencies of cells based on the traditional iodine electrolyte.

Self-Assembly by Coordination with Organic Antenna Chromophores for Dye-Sensitized Solar Cells

The development of new sensitizers and new sensitization methods is one of the important means to enhance the conversion efficiency of dye-sensitized solar cells (DSSCs); the ultimate goal is to broaden the spectral response of dyes, reduce electron recombination, and suppress dye aggregation. In this study, we have developed a series of new self-assembled dyes and applied them in DSSCs. We prepared two organic antenna chromophores S1 and S2 and coordinated them with two acceptors A1 and A2 via zinc to construct A-Zn-S series self-assembled dyes. This method is very simple and feasible and can avoid the complex synthesis steps of traditional dyes; the results show that the light-harvesting ability of devices can be improved and charge recombination can be reduced by adjusting the structures of the antenna chromophores and acceptors. The device with A2-Zn-S1 gave a power conversion efficiency of 4.25%, which was higher than those with A1-Zn-S1 (3.88%), A1-Zn-S2 (3.21%), and A2-Zn-S2 (3.52%); the main reason for this is that the different coordination combinations between the antenna chromophore and the acceptor show great differences in V_oc and J_sc. The device based on A2-Zn-S1 showed a high V_oc of 632 mV and a high J_sc of 9.54 mA cm^-2; one reason for this is that S1 has better spectral responsiveness and another reason is that A2 has better steric resistance that effectively reduces charge recombination. Besides, IR spectra indicate that these self-assembled dyes anchored on a TiO₂ surface by bicarboxyl anchoring groups are also very beneficial for improving the performance of dyes.

The influence of antenna and anchoring moieties on the improvement of photoelectronic properties in Zn(ii)-porphyrin-TiO2 as potential dye-sensitized solar cells

A systematic study for the rational design of porphyrins (P4 spider-shaped derivatives) with potential application in dye-sensitized solar cells is presented. Using density functional theory (DFT) (B3LYP/6-31G*) and time-dependent DFT (M06/6-31G*) we show that the UV-vis absorption properties of a spider-shaped Zn(ii) porphyrin, previously synthesized by Stangel et al., may be greatly improved by applying some push-pull strategies in meso positions. We found that the selected triphenylamine push group induces a remarkable improvement in the absorption bands of P4 spider-shaped derivatives. The pull effect reached through the π-electron-rich phenyl group and the benzodithiazole (BTD) group allowed the Q bands to be red-shifted up to 689 nm, much longer than the 593 nm reported experimentally for the original spider-shaped porphyrin. The adsorption results of the P4 spider-shaped derivatives onto a TiO2-anatase surface model [Ti16O34H4] through the carboxylic acid group showed that the adsorptions energies were favourable and very similar in all cases. Natural bond orbitals (NBO) indicated a two-center bond (BD) O(carboxyl)-Ti(TiO2) for the porphyrin with the highest adsorption energy (8.27 kcal mol-1), and donor acceptor interactions from LP O(carboxyl) to Ti(TiO2) for the other porphyrins. The natural transition orbitals (NTO) for P4-derivatives-TiO2 confirm the nature of the excited states associated with Q and Soret bands. Finally, the frontier molecular orbitals revealed charge-separated states between those occupied and unoccupied, indicating a favourable charge-transfer process between the dyes and the surface conduction bands. In conclusion, this work showed a systematic study based on the push-pull strategy that improves the performance of porphyrins with the purpose to be used in dye-sensitized solar cells.

Multiply Wrapped Porphyrin Dyes with a Phenothiazine Donor: A High Efficiency of 11.7% Achieved through a Synergetic Coadsorption and Cosensitization Approach

Photocurrent ( J_sc) and photovoltage ( V_oc) are two important parameters for dye-sensitized solar cells (DSSCs) to achieve high power conversion efficiencies (PCEs). Herein, we synthesize four novel porphyrin dyes, XW36-XW39, using an N-phenyl-substituted phenothiazine donor to pursue higher PCE. For XW36 and XW37, the N-phenyl group is wrapped with two ortho-alkoxy chains. In contrast, it is substituted with a para-alkoxy group in XW38 and XW39. The phenothiazine wrapping in XW36 and XW37 induces more serious distortion, which is beneficial for anti-aggregation but unfavorable for the electron transfer from donor to a porphyrin framework. Thus, individual porphyrin dyes XW36 and XW37 exhibit efficiencies of 9.05 and 9.58%, respectively, lower than those of 9.51 and 10.0% achieved for XW38 and XW39, respectively. Besides, the introduction of a methyl group into a benzoic acid acceptor unit is conducive to anti-aggregation and thus improves the V_oc and efficiencies. Therefore, higher efficiencies were achieved for XW37 and XW39, compared with XW36 and XW38, respectively. Interestingly, although the individual XW36 dye shows a lowest efficiency among the four dyes, a highest efficiency of 11.7% was obtained for XW36 on the basis of synergetic adsorption with chenodeoxycholic acid and PT-C6 because of simultaneously improved J_sc and V_oc, which may be ascribed to the lowest dye-loading amount of XW36 among all of these porphyrin dyes, with the largest vacancy area left on the TiO₂ surface available for cosensitizer PT-C6, resulting in a highest J_sc. The high efficiency of 11.7% is one of the highest efficiencies using I^-/I₃^- electrolytes in DSSCs. These results provide an effective strategy for developing efficient DSSCs by the targeted coadsorption and cosensitization of porphyrin sensitizers optimized through introducing a bis( ortho-alkoxy)-wrapped phenyl group into the phenothiazine donor and/or methyl groups into the benzoic acid acceptor unit.

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.

Push-Pull Zinc Porphyrins as Light-Harvesters for Efficient Dye-Sensitized Solar Cells

Dye-sensitized solar cell (DSSC) has been attractive to scientific community due to its eco-friendliness, ease of fabrication, and vivid colorful property etc. Among various kinds of sensitizers, such as metal-free organic molecules, metal-complex, natural dyes etc., porphyrin is one of the most promising sensitizers for DSSC. The first application of porphyrin for sensitization of nanocrystaline TiO2 can be traced back to 1993 by using [tetrakis(4-carboxyphenyl) porphyrinato] zinc(II) with an overall conversion efficiency of 2.6%. After 10 years efforts, Officer and Grätzel improved this value to 7.1%. Later in 2009, by constructing porphyrin sensitizer with an arylamine as donor and a benzoic acid as acceptor, Diau and Yeh demonstrated that this donor-acceptor framwork porphyrins could attain remarkable photovoltaic performance. Now the highest efficiencies of DSSC are dominated by donor-acceptor porphyrins, reaching remarkable values around 13.0% with cobalt-based electrolytes. This achievement is largely contributed by the structural development of donor and acceptor groups within push-pull framwork. In this review, we summarized and discussed the developement of donor-acceptor porphyrin sensitizers and their applications in DSSC. A dicussion of the correlation between molecular structure and the spectral and photovoltaic properties is the major target of this review. Deeply dicussion of the substitution group, especially on porphyrin's meso-position were presented. Furthermore, the limitations of DSSC for commercialization, such as the long-term stability, sophisticated synthesis procedures for high efficiency dye etc., have also been discussed.

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.

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 -D_sq , D_sq -D_sq were systematically designed and synthesized. The photophysical and electrochemical properties were evaluated and the E_HOMO and E_LUMO levels were determined; these energy levels determines the electron injection from E_LUMO of the dye to E_CB of TiO₂ and regeneration of oxidized dye by the electrolyte, respectively. The absorption spectra of D_sq -D_sq , D₁ -D_sq were broadened in solution compared to model dye D_sq ; this indicates that the dye-dye interaction is prominent in solution. In D₁ -D_sq excitation energy transfer between photoexcited D₁ and D_sq was explained by using Förster resonance energy transfer (FRET). The homodimeric dye showed a device performace of 2.8 % (V_oc 0.607, J_sc 6.62 mA/cm² , ff 69.3 %),whereas the heterodimeric dye D₁ -D_sq showed a device performance of 3.9 % (V_oc 0.652 V, J_sc 8.89 mA/cm² , ff 68.8 %). The increased photocurrent for D₁ -D_sq is due to the panchromatic IPCE response compared to D_sq -D_sq . The increased V_oc 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, V_oc , for the devices prepared from D_sq , D₁ -D_sq and D_sq -D_sq were systematically modulated by controlling the intermolecular π-π and intramolecular dipole-dipole interactions of the dimeric dyes.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Fused Fluorenylindolenine-Donor-Based Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells

A series of four unsymmetrical squaraine dyes, XSQ1-4, were synthesized using a fused fluorenylindolenine-based donor unit for dye-sensitized solar cells (DSSCs). The fused structure of fluorenylindolenine helped in moving the absorption toward the near-infrared (NIR) region, and the two sp³-C centers available on this donor were utilized to incorporate out-of-plane alkyl chains in opposite directions to control the dye-dye interactions on the TiO₂ surface. High extinction coefficient (ε ≥ 10⁵ M^-1 cm^-1) for absorbing NIR photons and suitable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels with respect to the conduction band of TiO₂ and electrolyte for charge injection and dye regeneration processes, respectively, make these dyes potential sensitizers for DSSCs. Introduction of branched alkyl groups in the π-framework helped in controlling dye aggregation to reduce exciton quenching and assisted in TiO₂ surface passivation to avoid the charge recombination process. Furthermore, having a naphthyl group on the indole part of the anchoring group containing segment helped to red-shift the absorption spectrum of dyes 15 nm toward the NIR region (XSQ3-4). Among all of the dyes under investigation, XSQ2 gave the best photovoltaic performance, having a short-circuit current density ( J_SC) of 13.99 mA cm^-2, open-circuit voltage ( V_OC) of 0.66 V, and a fill factor (ff) of 0.71, with a device performance (η) of 6.57%. Electrochemical impedance spectroscopy revealed higher electron lifetime on TiO₂ for XSQ2, which helps to avoid the charge recombination process.

Porphyrin-sensitized solar cells: systematic molecular optimization, coadsorption and cosensitization

As a promising low-cost solar energy conversion technique, dye-sensitized solar cells have undergone spectacular development since 1991. For practical applications, improvement of power conversion efficiency has always been one of the major research topics. Porphyrins are outstanding sensitizers endowed with strong sunlight harvesting ability in the visible region and multiple reaction sites available for functionalization. However, judicious molecular design in consideration of light-harvest, energy levels, operational dynamics, adsorption geometry and suppression of back reactions is specifically required for achieving excellent photovoltaic performance. This feature article highlights some of the recently developed porphyrin sensitizers, especially focusing on the systematic dye structure optimization approach in combination with coadsorption and cosensitization methods in pursuing higher efficiencies. Herein, we expect to provide more insights into the structure-performance correlation and molecular engineering strategies in a stepwise manner.

Near-Infrared-Absorbing Metal-Free Organic, Porphyrin, and Phthalocyanine Sensitizers for Panchromatic Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are a promising source of renewable energy. However, the power conversion efficiency (PCE) of devices has been limited largely by the difficulty of producing electricity using photons from the near-infrared (NIR) spectral region. Metal-free organic sensitizers frequently employ strong electron-donating or -withdrawing moieties to tune the optical band gap to allow the absorption of lower energy wavelengths in charge-transfer systems, whereas porphyrins and phthalocyanines use substituents to shift the Soret and Q bands toward lower energy absorption. Very few devices employing precious metal-free dyes have achieved panchromatic and NIR photon conversion for electricity generation at wavelengths >750 nm despite a tremendous number of sensitizers published over the last 25 years. This Minireview seeks to compile a summary of these sensitizers to encourage assimilation, analysis, and development of efficient future sensitizers with absorption extending into the NIR. Herein, we discuss common synthetic strategies, optical properties, and electronic properties of the most successful panchromatic organic sensitizers.

Theoretical design of porphyrin sensitizers with different acceptors for application in dye-sensitized solar cells

Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, three porphyrin dyes with different acceptors, such as carboxylic acid, cyanoacrylic acid, and 2-cyano-N-hydroxyacrylamide, have been designed. Compared to the best sensitizer (YD2-o-C8) so far, these designed dyes have small highest occupied orbital to lowest unoccupied orbital (HOMO–LUMO) band gaps, and wide absorptions with large oscillator strength at porphyrin Q bands. And the designed Dye1 is similar to YD2-o-C8 in electronic coupling with TiO₂, while improved Dye2 and Dye3 are better than YD2-o-C8, thus, Dye2 and Dye3 will be much faster for electron injection in dye-sensitized solar cell systems based on their long-term stable and efficient anchor groups. All these features show that our designed dyes, especially Dye2 and Dye3, have better absorption performance and faster electron injection. In addition, our results point out that 2-cyano-N-hydroxyacrylamide is a new promising acceptor. This study is expected to assist the molecular design of new efficient dyes for the advancement of dye-sensitized solar cells.

Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, three porphyrin dyes with different acceptors, such as carboxylic acid, cyanoacrylic acid, and 2-cyano-N-hydroxyacrylamide, have been designed.

Multimolecular assemblies on high surface area metal oxides and their role in interfacial energy and electron transfer

High surface area metal oxides offer a unique substrate for the assembly of multiple molecular components at an interface.

New di-anchoring A–π-D–π-A configured organic chromophores for DSSC application: sensitization and co-sensitization studies

Three new metal-free carbazole based di-anchored dyes with A–π-D–π-A architecture (E1–3) were successfully designed, synthesized and characterized as potential photosensitizers. E1 displayed the highest photovoltaic performance, while E1–3 showed good V_OC values when co-sensitized.

Role of Co-Sensitizers in Dye-Sensitized Solar Cells

Co-sensitization is a popular route towards improved efficiency and stability of dye-sensitized solar cells (DSSCs). In this context, the power conversion efficiency (PCE) values of DSSCs incorporating Ru- and porphyrin-based dyes can be improved from 8-11 % to 11-14 % after the addition of additives, co-adsorbents, and co-sensitizers that reduce aggregation and charge recombination in the device. Among the three supporting material types, co-sensitizers play a major role to enhance the performance and stability of DSSCs, which is requried for commercialization. In this Minireview, we highlight the role co-sensitizers play in improving photovoltaic performance of devices containing Ru- and porphyrin-based sensitizers.