Coplanar Donor-π-Acceptor Dyes Featuring a Furylethynyl Spacer for Dye-Sensitized Solar Cells

Coplanar metal-free organic dyes featuring a furylethynyl spacer with different donor residues (MeO-, MeS-, and Me2N-) have been synthesized. Density functional theory (DFT) calculations predicted that the Me2N- residue would facilitate more effective charge transfer from donor to acceptor than the MeO- and MeS- residues. In agreement with DFT calculations, the dye-sensitized solar cells (DSSCs) fabricated with the Me2N- functionalized dye exhibited the best power conversion efficiency (η), 2.88%. Furthermore, the effect of the furan spacer on the photophysical properties and DSSC parameters are discussed in comparison to a previously reported thiophene counterpart.


Introduction
Incoming solar irradiation possesses a huge amount of energy and as a consequence, photovoltaic (PV) devices have been extensively developed to meet our ongoing demand for clean energy.Among next-generation PV devices, dye-sensitized solar cells (DSSCs) have been extensively explored because of their tunable color/transparency and good performance under low light and variable solar incident angles [1,2].Photosensitizers play a prominent role in harvesting sunlight and mediating the interaction between a wide-bandgap semiconductor and a redox shuttle.Recently, metal-free organic photosensitizers have embraced a donor-π bridge-acceptor (D-π-A) structure, which promotes intramolecular charge separation and effective injection of excited electrons to the conduction band of the semiconductor.They also have notable advantages, such as high molar extinction coefficients, rich synthetic protocols, and cost-effective mass production.In these systems, dialkylamine or diphenylamine moieties are commonly used as the electron donor and a cyanoacrylic acid acts as the electron acceptor and anchoring unit [3][4][5].However, various π-conjugation segments have been tested to modulate the photophysical properties of the metal-free D-π-A.
Chalcogenophene units, such as furan, thiophene, and selenophene, have been employed as stable and efficient heterocycle units because of effective π-conjugation, low resonance stabilization energy, and a bathochromic shift in light absorption [6][7][8].For example, Li et al. have indicated that furan as the π-bridge provides a negligible dihedral angle with adjacent molecular units and thus results in a higher open circuit voltage (V oc ) in the DSSCs than thiophene and selenophene [7].Qu et al. reported that the diketopyrrolopyrrole (DPP) dye containing a furan bridge exhibits better stability and photovoltaic performance than that with benzene or thiophene [8].Recently, furan has been reported to give rise to greater DSSC efficiency in porphyrin dyes when compared to thiophene [9].Despite these studies, the furan moiety has not been widely employed, and knowledge of its effect on photophysical and photovoltaic properties still is lacking for coplanar dyes.We propose that furan would play a vital role as a π-spacer in metal-free D-π-A coplanar photosensitizers since it exhibits superior coplanarity compared to other heterocycle units.Accordingly, the electron donor group will play a more significant role than that of nonplanar molecules.We report the synthesis of three metal-free organic compounds featuring a furylethynyl spacer and their optical and electrochemical properties, which have been compared to their DFT-predicted values.We also investigate how different donor groups (MeO-, MeS-, and Me 2 N-) affect the photophysical properties of the synthesized dyes and the photovoltaic performance of the resultant DSSCs.Furthermore, we compare the influence of furan and thiophene on Me 2 N-dye properties and DSSC performance.

Synthesis
The synthesis of the dyes used in this study is outlined in Scheme 1. Building blocks 3, 5, and 7 were synthesized using Sonogashira reactions, which upon subsequent Knoevenagel condensation with cyanoacrylic acid provided dyes LS-361, LS-362, and LS-365 in good overall yields.stability and photovoltaic performance than that with benzene or thiophene [8].Recently, furan has been reported to give rise to greater DSSC efficiency in porphyrin dyes when compared to thiophene [9].Despite these studies, the furan moiety has not been widely employed, and knowledge of its effect on photophysical and photovoltaic properties still is lacking for coplanar dyes.We propose that furan would play a vital role as a π-spacer in metal-free D-π-A coplanar photosensitizers since it exhibits superior coplanarity compared to other heterocycle units.Accordingly, the electron donor group will play a more significant role than that of nonplanar molecules.We report the synthesis of three metal-free organic compounds featuring a furylethynyl spacer and their optical and electrochemical properties, which have been compared to their DFT-predicted values.We also investigate how different donor groups (MeO-, MeS-, and Me2N-) affect the photophysical properties of the synthesized dyes and the photovoltaic performance of the resultant DSSCs.Furthermore, we compare the influence of furan and thiophene on Me2N-dye properties and DSSC performance.

Synthesis
The synthesis of the dyes used in this study is outlined in Scheme 1. Building blocks 3, 5, and 7 were synthesized using Sonogashira reactions, which upon subsequent Knoevenagel condensation with cyanoacrylic acid provided dyes LS-361, LS-362, and LS-365 in good overall yields.Scheme 1. Synthetic route toward dyes LS-361, LS-362, and LS-365.

Characterization
The UV-vis spectra of the three compounds recorded in dimethylformamide (DMF) are provided in Figure 1a.Two absorption peaks were seen in LS-361, whereas one absorption peak was observed in LS-362 and LS-365.The absorption band of LS-361 around 320 nm is likely due to the π-π* transition of the conjugated aromatic moieties, while the absorption bands around 360 nm (for LS-362 and LS-365) and around 392 nm (for LS-361) correspond to an intramolecular charge transfer (ICT) between the donor and acceptor groups.Recently, we reported an organic dye featuring a thienylethynyl spacer (denoted as Control) analogous to LS-361 [10,11].The wavelength of absorption maxima decreases in the order of Control > LS-361 > LS-365 > LS-362 in agreement with the electronic absorption spectra from TDDFT calculations shown in Figure 1b.This trend is in line with the better electron donating ability of the dimethylamino moiety, which increases ICT behaviors in D-π-A molecules.The different electronegativity of the heteroatoms between Control Scheme 1. Synthetic route toward dyes LS-361, LS-362, and LS-365.

Characterization
The UV-vis spectra of the three compounds recorded in dimethylformamide (DMF) are provided in Figure 1a.Two absorption peaks were seen in LS-361, whereas one absorption peak was observed in LS-362 and LS-365.The absorption band of LS-361 around 320 nm is likely due to the π-π* transition of the conjugated aromatic moieties, while the absorption bands around 360 nm (for LS-362 and LS-365) and around 392 nm (for LS-361) correspond to an intramolecular charge transfer (ICT) between the donor and acceptor groups.Recently, we reported an organic dye featuring a thienylethynyl spacer (denoted as Control) analogous to LS-361 [10,11].The wavelength of absorption maxima decreases in the order of Control > LS-361 > LS-365 > LS-362 in agreement with the electronic absorption spectra from TDDFT calculations shown in Figure 1b.This trend is in line with the better electron donating ability of the dimethylamino moiety, which increases ICT behaviors in D-π-A molecules.The different electronegativity of the heteroatoms between Control and LS-361 influences the absorption peak position and molar extinction coefficient [7].This change in electronegativity results in the red-shift of the absorbance maximum.However, the extinction coefficient of LS-361 is higher than that of Control.It is likely due to the decrease in the torsion angle between the ethynyl unit and the heterocycle (i.e., 0.66 • for furan and 0.78 • for thiophene).and LS-361 influences the absorption peak position and molar extinction coefficient [7].This change in electronegativity results in the red-shift of the absorbance maximum.However, the extinction coefficient of LS-361 is higher than that of Control.It is likely due to the decrease in the torsion angle between the ethynyl unit and the heterocycle (i.e., 0.66° for furan and 0.78° for thiophene).The electrochemical properties of the dyes were explored using cyclic voltammetry (CV) and square wave voltammetry (SWV) (Figure 2), and their estimated ionization potentials (IP) and electron affinities (EA) obtained from the SWV measurements are provided in Table 1 [12].The stronger donating ability of the amino moiety allowed for more up-shift of the IP energy of LS-361, which is consistent with the oxidation potential (Eox) obtained from CV and SWV, and, thus, the smaller energy gap (Egap).Egap increases in the order of Control > LS-361 > LS-365 > LS-362, and this is consistent with the trend seen in the optical bandgap (Eopt) measurements.The electrochemical properties of the dyes were explored using cyclic voltammetry (CV) and square wave voltammetry (SWV) (Figure 2), and their estimated ionization potentials (IP) and electron affinities (EA) obtained from the SWV measurements are provided in Table 1 [12].The stronger donating ability of the amino moiety allowed for more up-shift of the IP energy of LS-361, which is consistent with the oxidation potential (E ox ) obtained from CV and SWV, and, thus, the smaller energy gap (E gap ).E gap increases in the order of Control > LS-361 > LS-365 > LS-362, and this is consistent with the trend seen in the optical bandgap (E opt ) measurements.
and LS-361 influences the absorption peak position and molar extinction coefficient [7].This change in electronegativity results in the red-shift of the absorbance maximum.However, the extinction coefficient of LS-361 is higher than that of Control.It is likely due to the decrease in the torsion angle between the ethynyl unit and the heterocycle (i.e., 0.66° for furan and 0.78° for thiophene).The electrochemical properties of the dyes were explored using cyclic voltammetry (CV) and square wave voltammetry (SWV) (Figure 2), and their estimated ionization potentials (IP) and electron affinities (EA) obtained from the SWV measurements are provided in Table 1 [12].The stronger donating ability of the amino moiety allowed for more up-shift of the IP energy of LS-361, which is consistent with the oxidation potential (Eox) obtained from CV and SWV, and, thus, the smaller energy gap (Egap).Egap increases in the order of Control > LS-361 > LS-365 > LS-362, and this is consistent with the trend seen in the optical bandgap (Eopt) measurements.

Theoretical Calculations
Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were conducted to gain insight into the electronic structure and optical properties of the organic dyes [13].Figure S1 illustrates that all of the optimized molecular geometries are entirely coplanar.It originates from the rigid nature of a carbon-carbon triple bond (C≡C) and conjugated (hetero)aromatic moieties (i.e., phenyl and furan groups).Interestingly, all of the dihedral angles between the carbon chains are less than 1 • .This coplanar structure is conducive to the stability of the molecule and the intramolecular charge transfer (ICT) between donor and acceptor groups.Second order perturbation theory (SOPT) analysis of the Fock matrix on the natural bond orbital (NBO) basis can determine the charge transfer characteristics between different parts of the molecule [14].Specific carbon atoms were selected to investigate the electronic delocalization process, and they are numbered (C 1 ~C8 ).Table S1 summarizes the NBO parameters, conjugative interaction energies (∆E) between the π and π* orbitals, the energy difference between the interacting NBO and matrix element (E acc -E don ), and the off-diagonal element associated with the NBO Fock matrix (F(acc,don)).High conjugative interaction energies imply more charge transfer from donor to acceptor parts.Table S2 summarizes the NBO population charge for the electron donor, π-bridge, and electron acceptor, which are denoted as q donor , q π-bridge , and q acceptor , respectively.The most significant charge variance between the natural charges on the donor and acceptor groups is represented as ∆q D-A .In this study, both ∆E and ∆q D-A values can be used to determine the electron donating ability of the moiety.The values of LS-361 are higher than those of LS-362 and LS-365, implying that the Me 2 N-moiety facilitates more effective charge transfer from the donor to the acceptor than the MeO-and MeS-moieties.
The highest occupied molecular orbital (HOMO) is associated with the electron donating ability of a molecule, and the lowest unoccupied molecular orbital (LUMO) represents its electron accepting ability.Figure 3 displays the frontier molecular orbitals from HOMO to LUMO energy state of the three dyes (LS-361, LS-362, and LS-365) and the Control.At the HOMO level, electrons are extended from the donor to the acceptor.On the contrary, at the LUMO level, the excited electron is localized in the π-conjugation and acceptor groups.This spatial molecular orbital distribution is beneficial to the photo-driven ICT process.For DSSC operation, the HOMO of the molecule should be located below the iodide redox potential, and its LUMO should be above the conduction band (CB) of the TiO 2 semiconductor [1].Interestingly, the calculated energy levels reveal that LS-361 is entirely different from LS-362 and LS-365.Table 2 summarizes the absorption wavelengths and oscillator strengths calculated at the CAM-B3LYP level.The excitation at λ max is mainly due to a HOMO→LUMO transition and, thus, it can be interpreted as the ICT peak.The red-shift transitions can be seen as the donor strength increases.Although the oscillator strength (f ) is sensitive to the basis set choice, a large f indicates large light harvesting efficiency (LHE) that is directly related to short-circuit current density (J sc ) [14].

Photovoltaic Performance
Figure 4a shows J-V characteristics of DSSCs fabricated from the three dyes.The photovoltaic parameters are summarized in Table 3. LS-361 exhibited the best photovoltaic performance, although its open-circuit voltage (Voc) was lower than that of LS-362 and LS-365.The considerable increase in short-circuit current density (Jsc) coincides with the computational prediction described above.Dark current in the DSSC mainly results from the loss of the injected electron from TiO2 to I3 - and the dark current is related to the Voc under illumination.Contrary to our initial hypothesis, the photovoltaic performance of the Control is better than that of LS-361, even if the former has worse light harvesting ability than the latter.Figure 4b shows incident photon-to-current efficiency (IPCE) spectra for the DSSCs.The IPCE of LS-361 displays a broader band between 300 nm and 700 nm than that of LS-362 and LS-365.It is in accordance with the UV-vis spectra in solution mentioned above.It should be noted that there is a significant drop in overall efficiency between LS-361 and Control, likely due to self-quenching of excitons and charge recombination induced by close π-π aggregation between coplanar dye molecules [15].
Figure 4c shows the Nyquist plots of the electrochemical impedance spectroscopy (EIS) analysis in the dark.The EIS parameters estimated by a pertinent equivalent circuit are summarized in Table S3.The first semicircle in the high-frequency region corresponds to the reaction of the

Photovoltaic Performance
Figure 4a shows J-V characteristics of DSSCs fabricated from the three dyes.The photovoltaic parameters are summarized in Table 3. LS-361 exhibited the best photovoltaic performance, although its open-circuit voltage (V oc ) was lower than that of LS-362 and LS-365.The considerable increase in short-circuit current density (J sc ) coincides with the computational prediction described above.Dark current in the DSSC mainly results from the loss of the injected electron from TiO 2 to I 3 -and the dark current is related to the V oc under illumination.Contrary to our initial hypothesis, the photovoltaic performance of the Control is better than that of LS-361, even if the former has worse light harvesting ability than the latter.Figure 4b shows incident photon-to-current efficiency (IPCE) spectra for the DSSCs.The IPCE of LS-361 displays a broader band between 300 nm and 700 nm than that of LS-362 and LS-365.It is in accordance with the UV-vis spectra in solution mentioned above.It should be noted that there is a significant drop in overall efficiency between LS-361 and Control, likely due to self-quenching of excitons and charge recombination induced by close π-π aggregation between coplanar dye molecules [15].Figure 4c shows the Nyquist plots of the electrochemical impedance spectroscopy (EIS) analysis in the dark.The EIS parameters estimated by a pertinent equivalent circuit are summarized in Table S3.The first semicircle in the high-frequency region corresponds to the reaction of the iodine/iodide redox couple at the interface between the counter electrode and the electrolyte.The second semicircle in the middle frequency region reflects the charge recombination between ions in the electrolyte and electrons in the TiO 2 semiconductor [16,17].The radius of the second semicircle decreases in the order of LS-362 > Control > LS-365 > LS-361, yielding recombination resistances (R rec ) of 126.00 Ω, 92.51 Ω, 85.08 Ω, and 75.91 Ω, respectively.The back reaction between photoinjected electrons was more suppressed in the electrolyte/dye/TiO 2 interface of LS-362.It is well known that the dye itself can suppress the dark current at the electrolyte/TiO 2 interface by forming a blocking layer [5,11].Accordingly, it is likely that MeO-, which is comprised of highly electronegative lone pairs, may inhibit recombination toward redox electrolytes on the TiO 2 surface.This not only corresponds to the trend in the dark current in Figure 4a, but also explains the V oc of LS-362 and LS-365, 0.544 V and 0.535 V, respectively.Interestingly, LS-362 exhibited better charge-collection efficiency (η cc ) than LS-361 and LS-365.We think that the MeO-substituent with two lone pair electrons is more useful in preventing the charge recombination mentioned above.Nevertheless, the Me 2 N-substituent leads to the best power conversion efficiency due to considerable light harvesting ability and strong electron donating ability despite fast charge recombination [18].
Materials 2019, 12, x FOR PEER REVIEW 6 of 11 iodine/iodide redox couple at the interface between the counter electrode and the electrolyte.The second semicircle in the middle frequency region reflects the charge recombination between ions in the electrolyte and electrons in the TiO2 semiconductor [16,17].The radius of the second semicircle decreases in the order of LS-362 > Control > LS-365 > LS-361, yielding recombination resistances (Rrec) of 126.00 Ω, 92.51 Ω, 85.08 Ω, and 75.91 Ω, respectively.The back reaction between photoinjected electrons was more suppressed in the electrolyte/dye/TiO2 interface of LS-362.It is well known that the dye itself can suppress the dark current at the electrolyte/TiO2 interface by forming a blocking layer [5,11].Accordingly, it is likely that MeO-, which is comprised of highly electronegative lone pairs, may inhibit recombination toward redox electrolytes on the TiO2 surface.This not only corresponds to the trend in the dark current in Figure 4a, but also explains the Voc of LS-362 and LS-365, 0.544 V and 0.535 V, respectively.Interestingly, LS-362 exhibited better charge-collection efficiency (ηcc) than LS-361 and LS-365.We think that the MeO-substituent with two lone pair electrons is more useful in preventing the charge recombination mentioned above.Nevertheless, the Me2N-substituent leads to the best power conversion efficiency due to considerable light harvesting ability and strong electron donating ability despite fast charge recombination [18].

General
Nuclear magnetic resonance spectra were recorded on a Bruker AVIII (400 MHz) spectrometer (Billerica, MA, USA), and the chemical shifts are relative to tetramethylsilane and are reported in ppm.Mass spectrometry was either recorded on a Thermo Scientific LTQ Orbitrap XL (Waltham, MA, USA) using electrospray (negative mode, ESI, or a JEOL-700 MStation (EI, Akishima, Tokyo, Japan).Infrared spectra were recorded on a Perkin-Elmer FTIR (Waltham, MA, USA) and transmittance maxima are reported in wavenumbers (cm −1 ).UV-vis absorbance spectra were recorded on a JASCO UV/Vis/NIR spectrophotometer (Tokyo, Japan) and the estimated optical bandgaps (Eg) were determined using the absorption edge of the longest wavelength absorption (λ) using equation Eg = 1240/λ.The solution electrochemistry measurements were undertaken using a CH Instruments 440 A electrochemical analyzer (Austin, TX, USA).A platinum working electrode,

General
Nuclear magnetic resonance spectra were recorded on a Bruker AVIII (400 MHz) spectrometer (Billerica, MA, USA), and the chemical shifts are relative to tetramethylsilane and are reported in ppm.Mass spectrometry was either recorded on a Thermo Scientific LTQ Orbitrap XL (Waltham, MA, USA) using electrospray (negative mode, ESI, or a JEOL-700 MStation (EI, Akishima, Tokyo, Japan).Infrared spectra were recorded on a Perkin-Elmer FTIR (Waltham, MA, USA) and transmittance maxima are reported in wavenumbers (cm −1 ).UV-vis absorbance spectra were recorded on a JASCO UV/Vis/NIR spectrophotometer (Tokyo, Japan) and the estimated optical bandgaps (E g ) were determined using the absorption edge of the longest wavelength absorption (λ) using equation E g = 1240/λ.The solution electrochemistry measurements were undertaken using a CH Instruments 440 A electrochemical analyzer (Austin, TX, USA).A platinum working electrode, a platinum wire counter electrode, and a silver wire pseudo-reference electrode were used in all electrochemical measurements.Ferrocene was used as an internal standard.The redox couples are reported versus the ferrocene/ferrocenium (Fc/Fc + = −4.8eV) redox couple (adjusted to 0.0 V).The supporting electrolyte used was electrochemical grade tetrabutylammonium hexafluorophosphate (0.1 M) and was dissolved in dry dimethylformamide.Electrochemical data were recorded following purging the solution with nitrogen gas for 3 min.All chemicals and solvents were obtained from suppliers and used without prior purification, Sigma-Aldrich (UK), TCI (Europe) or Alfa-Easar (UK).

Computational Details
All of DFT and TDDFT calculations were carried out with the Gaussian '09 package (Wallingford, Connecticut, USA) [19].The ground-state geometries were fully optimized at the DFT level of B3LYP functional using 6-311G ++(d, p) basis set on all atoms without additional diffused function.Frequency calculations were performed at the same level of theory as the geometry optimization to prove that the optimized geometry corresponds to the lowest point of the potential energy surface.NBO characteristics were obtained by the SOPT analysis.The excitation energies and oscillator strengths for the lowest 20 singlet-singlet transitions on the basis of the ground-state geometry were calculated with the TDDFT method under CAM-B3LYP functional and 6-311G++(d, p) basis set.Solvent atmosphere parameters were considered only for TDDFT calculations using the PCM model (Acetonitrile: E= 35.688).

DSSC Fabrication and Photovoltaic Measurements
All acetylene-black TiO 2 paste was prepared using a paste blending method [20].The paste was screen-printed onto transparent fluorine-doped SnO 2 (FTO)-coated conducting glass (TEC 8, Pilkington, 2.2-mm-thick, sheet resistance = 8 Ω/sq).The resulting layer was placed in a muffle furnace (Thermo Scientific, Waltham, MA, USA) and gradually heated to 300 • C over a 30 min period, heated at 300 • C for 1 h, heated to 575 • C for 1 h, and then cooled to room temperature after 3 h.After that, another paste composed of 400 nm TiO 2 nanoparticles was screen-printed as a scattering layer.Then, an approximately 15-µm-thick nanocrystalline TiO 2 photoanode could be achieved.The active areas of the electrodes were 0.2025 cm 2 .The prepared TiO 2 electrodes were immersed in a 0.04 M TiCl 4 aqueous solution at 75 • C for 30 min.After that, they were rinsed several times with deionized water and ethanol followed by heat treatment at 500 • C for 30 min on a hot plate.O 2 plasma treatment was conducted to the electrodes for 10 min, and then they were immersed in 0.1 M HNO 3

Figure 4 .
Figure 4. (a) J-V characteristics, (b) incident photon-to-current efficiency (IPCE) spectra, and (c) Nyquist plots in the dark of the dye-sensitized solar cells (DSSCs) sensitized with LS-361, LS-362, and LS-365 dyes.Those results of the Control (grey dotted line) are provided for comparison.The equivalent circuit model for EIS data fitting is inserted.

Figure 4 .
Figure 4. (a) J-V characteristics, (b) incident photon-to-current efficiency (IPCE) spectra, and (c) Nyquist plots in the dark of the dye-sensitized solar cells (DSSCs) sensitized with LS-361, LS-362, and LS-365 dyes.Those results of the Control (grey dotted line) are provided for comparison.The equivalent circuit model for EIS data fitting is inserted.

Table 1 .
Summary of optical and electrochemical properties of the three dyes and Control for comparison.

Table 2 .
Selected excitation energies, electronic transition configurations, and oscillator strengths for LS

Table 3 .
Dye-sensitized solar cell (DSSC) performance parameters of LS