Novel Imidazole Substituted Bodipy-Based Organic Sensitizers in Dye-Sensitized Solar Cells

A comparative study on the photophysical, electrochemical properties and photovoltaic performances of pure imidazole dyes containing varying linker groups is done. Two new organic dyes containing 4,5-bis(4-methoxyphenyl)-1H-imidazole (BPI) unit as an electron donor, boron dipyrromethene (Bodipy) chromophore as a conjugate bridge, cyanoacetic acid as an electron acceptor, and phenylene (BPI-P) or thienyl (BPI-T) as a additional linker have been synthesized for fabricating dye-sensitized solar cells (DSSCs). A reference dye (DPI-T) with 6,9-dimethoxy-1H-phenanthro[9,10-d]imidazole as the donor has also been synthesized for comparison. The overall conversion efficiencies of 0.18%, 0.32%, and 1.28% were obtained for DSSCs based on BPI-P, BPI-T, and DPI-T, respectively. DPI-T was found to be more efficient than BPI-P and BPI-T because of its enhanced light harvesting efficiency and better coplanar geometry of the electronic structure.


Introduction
Global energy consumption and environmental pollution are leading to the increasing demand for viable renewable energy sources [1][2][3][4].Dye-sensitized solar cells (DSSCs) are attractive solar energy conversion devices because of their peculiarity in terms of low-cost production, relative ease of fabrication, environmental friendliness, and stability, which paves the way to novel applications [5][6][7][8][9].Besides the electrolyte and semiconductor, the performance of DSSCs is vitally influenced by the structural engineering of the sensitizing dyes, which absorb sunlight to generate electrons and transmit generated electrons [10][11][12][13][14][15].Enormous studies have been focusing on searching novel fully organic dyes, providing advantages in terms of the molecularly tailored design flexibility and raw material abundance comparable to their inorganic counterparts.Most metal-free organic sensitizers with high efficiency contain dye molecules with linearly shaped structures comprising a strong D-π-A dipole with an electron donor (D), a π-bridge, and an electron acceptor (A), which owns photoinduced intramolecular charge transfer natures [16,17].
Imidazole, an archetypical heterocyclic molecule, is well known in the field of medicine as a drug with anticancer, antibacterial, and antifungal activities and as an antioxidant [18][19][20][21].Recently, researchers have already introduced imidazole into organic light-emitting diode [22][23][24][25][26] and DSSC application [27][28][29][30][31] because of its attractive properties.According to previous studies, introducing electron donors in the 4,5-site and an electron acceptor in the 2-site of imidazole is propitious to form conjugated dipolar sensitizers as well as strengthen their light-harvesting capability.Moreover, as a result of weakening positive charge density at the donor group by electronic delocalization of the two substituents in the 4,5-site of the imidazolyl ring, charge recombination after electron injection may be retarded [27,30].Nevertheless, light sensitivity of imidazole photosensitizers in near-infrared spectra is limited.Boron dipyrromethene (Bodipy), known as the "little sister of porphyrins," is a unique chromophore with an intensive absorption profile in the visible/near-infrared region and could be conveniently and flexibly tuned by chemical modification at five different points of the Bodipy core [32].Extensive efforts have been devoted to the development of Bodipy-modified dyes with a D-π-A system to increase the light-harvesting properties [33][34][35][36].With the aforementioned in mind, synthesis of some properties of new 4,5-di-substituted imidazole which is conjugated with Bodipy for broadening the spectral range could be worthwhile.
In the present work, two new organic sensitizers containing 4,5-bis(4-methoxyphenyl)-1H-imidazole (BPI) and Bodipy as an electron donor and a conjugated bridge, respectively, along with different additional linker groups (phenylene for BPI-P and thienyl for BPI-T) incorporated into the bridge to further expand the absorption spectra and improve the solubility were synthesized and applied in DSSCs.6,9-Dimethoxy-1H-phenanthro [9,10-d]imidazole (DPI) is structurally analogous to the BPI unit except for an additional single C-C bond that links two phenyl rings.For the purpose of comparison, dye DPI-T [37] with DPI as the electron donor, Bodipy as the conjugated bridge, and thienyl unit as the additional linker was prepared.The corresponding molecular structures are shown in Figure 1.The structural and electronic properties of these sensitizers are also investigated using density functional theory (DFT) calculations.

Characterization and Measurement.
The NMR spectra were obtained using a Bruker AV spectrometer operating at 300 MHz for 1 H NMR and 75 MHz for 13 C NMR. MALDI-TOF MS spectra were recorded on the Thermo LTQ Orbitrap mass spectrometer.UV-Vis absorption spectrum was performed on a Shimadzu UV/Vis-2450 spectrometer.

Synthesis and Characterization of New Compounds.
The synthetic routes of BPI-P and BPI-T are depicted in Figure 2. We note that the synthesis routes of BPI-P and BPI-T are similar with the only difference in the linker unit.The synthetic routes start from a Suzuki coupling reaction of 3a and 3b with 4 to afford the important precursor aldehyde intermediates 5a and 5b in favorable yields, respectively.Afterwards, the target product dyes BPI-P and BPI-T were obtained through a Knoevenagel condensation reaction between 5 and cyanoacetic acid in the presence of piperidine.The structures of the two compounds were characterized using spectroscopy.
2.3.1.Synthesis and Characterization of 5a and 5b.Under N 2 , compound 3 (0.6 mmol) was reacted with 4 (0.5 mmol) by Suzuki coupling reaction using Pd(PPh 3 ) 4 (50 mg) and 2 M K 2 CO 3 (3 mL) aqueous solution as catalysis in the mixture solution of THF (15 mL) and toluene (15 mL) at 90 °C for 12 h.After cooling, water (50 mL) was added and the reaction mixture was extracted with CH 2 Cl 2 (100 mL).The combined organic layer was washed with brine, dried over anhydrous MgSO 4 , and evaporated under reduced pressure.The crude product was purified by column chromatography on silica gel using CH 2 Cl 2 and EtOAc as eluent.

Synthesis and Characterization of BPI-P and BPI-T.
Compound 5 (0.20 mmol) and 2-cyanoacetic acid (0.60 mmol) were mixed with 20 mL MeCN and 20 mL CHCl 3 .The mixture was heated to reflux for 24 h in the presence of a few drops of piperidine.After cooling to room temperature, solvents were removed by rotary evaporation.The residue was extracted with CH 2 Cl 2 , washed with brine, and dried over anhydrous Na 2 SO 4 .The solvent was evaporated under reduced pressure, and the crude product was purified by column chromatography on silica gel with CH 2 Cl 2 and MeOH as eluent to afford target dyes.

Fabrication and Characterization of the DSSC Devices.
The nanocrystalline titanium dioxide (TiO 2 ) electrode and the platinum (Pt) cathode were prepared according to our previous report [37].Nanocrystalline electrodes are about 14.5 μm thick [determined by a profilometer (XP-2, AMBIOS Technology Inc.)].The TiO 2 electrodes were immersed in a dry CHCl 3 solution containing 0.3 mM dye sensitizer for 12 h at the room temperature to ensure complete dye uptake and were then rinsed with anhydrous CHCl 3 and EtOH to remove the unbound dye.The dye-coated TiO 2 films were placed under vacuum for further drying and used as the photo-anode in the DSSCs.A thermally Pt counter  3 International Journal of Photoenergy electrode was then combined with the dye-adsorbed TiO 2 electrode by use of a hot-melt Surlyn film as a spacer to produce a sandwich-type electrochemical cell, and then an electrolyte was injected into the seam between two electrodes.An acetonitrile (CH 3 CN) solution containing 1,2-dimethyl-3-n-propylimidazolium iodide (0.6 M), LiI (0.1 M), I 2 (0.05 M), and tert-butylpyridine (TBP, 0.5 M) was used as an electrolyte.The opening was sealed with Araldite glue after filling the electrolyte.To prevent inflated photocurrents arising from stray light, a black metal mask surrounding the active area was a testing cell during all measurements.The active geometrical area of the DSSCs was 0.25 cm 2 (0.5 cm × 0.5 cm).The cell parameters were obtained under incident light with an intensity of 100 mW•cm −2 using a Keithley 2420 3A source meter controlled by Test-point software under a solar simulator (solar AAA simulator, oriel USA, calibrated with a standard crystalline silicon solar).

Results and Discussion
3.1.Photophysical Properties.The UV-Vis absorption spectra of BPI-P, BPI-T, and DPI-T in CHCl 3 solution are depicted in Figure 3, and the characteristic parameters are listed in Table 1.The absorption spectra of all sensitizers show two distinct bands in the range of 250-675 nm.The shorter wavelength region (250-400 nm) is assigned to the aromatic π-π * electron transition of BPI or DPI with an additional linker, whereas the longer wavelength region with high-intensity absorption (420-650 nm) corresponds to the Bodipy unit with some intramolecular charge transfer transition.The λ max (absorption maximum wavelength) of BPI-P with molar absorption coefficient (ε) of 62,000 M −1 •cm −1 and BPI-T with ε of 64,000 M 1 •cm −1 are both at about 531 nm, due to the similar chemical structures except for the additional linker substituents.Remarkably, resulting from its increased donor property and smaller steric hindrance between the thiophene and BPI unit (vide infra), the thienyl-containing dye (BPI-T) shows an apparent broadening especially for longer wavelength when compared to the phenylene-containing dye (BPI-P).Generally, the better conjugation in the thiophenebased analogue would cause some small bathochromic shift of the band maximum, not only its broadening.The CHCl 3 solution is medium-polarity solvent, and aggregation of the studied dyes is very probable in it.To verify this, their absorption spectra have been measured in some highpolarity solvent, such as CH 3 CN solution.However, the UV/Vis absorption spectra of BPI-P and BPI-T in CH 3 CN which exhibit simultaneously blue shifts of ca.11 nm compared to those in the medium-polarity solvent.
Additionally, the reference dye DPI-T has a relatively red-shifted absorption maximum wavelength and greater molar absorption coefficient than BPI-T.This is presumably caused by two reasons: on the one hand, the torsion angle between the DPI and thiophene is smaller than BPI and thiophene (vide infra), leading to favorable charge transfer from the donor unit to the acceptor unit; on the other hand, the electron-donating ability of DPI is stronger than that of BPI due to the fact that four aromatic rings in the former are more propitious to conjugate extension.Based on the above analysis, the order of the light-harvesting ability is DPI-T > BPI-T > BPI-P.
Upon adsorption onto the nanocrystalline TiO 2 films (Figure 4), the λ max values for BPI-P, BPI-T, and DPI-T are 505 nm, 527 nm, and 528 nm, respectively.The absorption spectra of all three sensitizers on the TiO 2 films exhibited significantly hypsochromic shift and a broader full width at half maximum (fwhm) than absorption spectra measured in CHCl 3 due to strong interactions between the dyes and TiO 2 surfaces.Meanwhile, both in solutions and on TiO 2 film, the fwhm values of sensitizers with additional thiophene chromophore were broader in the region of 400-700 nm than those in the phenylene group, representing an advantageous spectral property for light harvesting and thus increasing the photocurrent response region.

Electrochemical Properties.
In order to evaluate the feasibility of electron transfer from the excited dye molecule to the conduction band (CB) of TiO 2 and the regeneration of oxidized dyes, the cyclic voltammetry (CV) has been performed in CH 2 Cl 2 solutions with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as supporting electrolyte and a three-electrode configuration consisting of a glass carbon as working electrode, an auxiliary Pt wire electrode, and a saturated calomel electrode (SCE) reference electrode.All potentials reported are calibrated with Fc/Fc + as an external reference.The relevant data are compiled in Table 1.
The E 0-0 values were estimated from the onset point of UV-vis spectra.As compared to BPI-P, both BPI-T and DPI-T have narrow E 0-0 values, a trend consistent with the wider absorption ranges illustrated in Figures 3 and  4, indicating the additional linker with the thiophene moiety which reduces energy gap energies.The excited state potentials (E ox * ), which correspond to the lowest unoccupied molecular orbital (LUMO) levels, were estimated by 4 International Journal of Photoenergy virtue of the following equation: E ox * = E ox onset − E 0,0 , where E ox onset stands for the onset potential of oxidation of the ground state of a dye dissolved in CH 2 Cl 2 .As shown in Figure 5, all E ox * values are more negative than the CB edge of TiO 2 (−0.5 V versus NHE), ensuring an efficient electron injection process from the excited state of the dyes into the TiO 2 electrode.In contrast, the more positive oxidation potential of E ox onset corresponding to the highest occupied molecular orbital (HOMO) level of all photosensitizers compared with that of the I 3 − /I − redox couples (0.42 V versus NHE) indicates that regeneration of the dyes is thermodynamically feasible.All dyes can be used as sensitizers for feasible electron transfer in DSSCs.

Computational Study.
Density functional theory (DFT) calculations using the Gaussian 09 [38] software (B3LYP/ 6-31 G) were conducted in order to gain in-depth insight into the optimized geometrical configuration (Figure 6) and electron distribution of the frontier orbitals of the three molecules (Figure 7).Time-dependent density functional theory (TD-DFT) was then utilized at the same theoretical level to find vertical transitions (Table 2).
In the structure of BPI-P, the dihedral angle between the imidazole core and the adjacent phenyl ring is 41.9 o , while in the case of BPI-T, the dihedral angle between imidazole and the neighboring `thienyl ring is 19.0 o .Encouragingly, when the BPI was replaced with DPI to construct DPI-T, the torsion angle calculated between the imidazole core and thiophene linker gets smaller (9.7 o ).Based on the calculated molecular geometry, a more distinctly molecular planar can be observed for DPI-T, which may naturally increase the effective conjugation and strengthen the charge transfer from the donor to the acceptor causing a broadening absorption spectrum compared to BPI-P and BPI-T.
From Table 2, the relative trend of UV-Vis absorption spectra based on the TD-DFT calculations is well consistent with our experimental values.The pathways for excitation and electron injection process can be learned by investigating and analyzing the computational results.For example, the low-energy band located in the range of 420-675 nm can be assigned to HOMO/LUMO, HOMO-1/ LUMO, and HOMO-2/LUMO transitions.From Figure 7, the electron density of the HOMOs of all dyes is located primarily at the electron-rich BPI or DPI moiety and extended to the conjugated linker group, while the HOMO-1s and LUMOs are concentrated at electron withdrawing unit of cyanoacetic acid and its nearby Bodipy bridge.It is interesting to note that HOMO-2 of thienyl-containing dyes is localized on the whole molecular skeleton and phenylenecontaining dye is a donor part.The maximum oscillator strength (f) for lower-energy transitions is higher for DPI-T (0.75) compared to that of BPI-T (0.58), respectively.The  Overall, the results of DFT and TD-DFT calculations give a better understanding of electronic structures and optical properties and the nature of transition of synthesized molecules.Previous relevant studies have reported that a disturbance to charge delivery, through either a distortion of the molecular geometry or intramolecular aggregation, may reduce charge migration rate [39].Better coplanar geometry and hydrophobic long chains   3.
The overall η for phenylene-conjugated derivative (BPI-P) and thiophene-linked analogue (BPI-T) are 0.18%  6 International Journal of Photoenergy (J sc = 0.76 mA/cm 2 , V oc = 0.45 V, and ff = 0.54) and 0.32% (J sc = 1.04 mA/cm 2 , V oc = 0.52 V, and ff = 0.60), respectively.Under the same conditions, the solar cell based on reference dye DPI-T generated an efficiency of 1.28%, with a J sc of 3.18 mA/cm 2 , a V oc of 0.58 V, and an ff of 0.69.Despite their relatively low overall conversion efficiencies, the dyes show amusing structural dependence in their DSSC performance.Both V oc and J sc increase with the thiophene units.The difference in performance between the BPI-P and BPI-T probably stems from the difference in the coplanarity of the aromatic segment that links the donor and π bridge unit.According to the DFT study, the phenylene derivative possesses a more twisted nonplanar geometry, which leads to poor orbital overlap.As a result, the benzene ring cannot be of benefit to the conjugated system and unflavored the electron injection.It is confirmed that the introduction of phenylene lowered the extinction coefficient not only in the ultraviolet regions but also in the visible regions, compared with the thiophene-linked dye (Figure 3).Hence, the photocurrent dropped due to the inefficient light harvest.The thiophenelinked analogues possess a high planar configuration, which can lead to an increased intermolecular π-π * interaction.Compared to the devices using BPI-T and BPI-P with the BPI as the electron donor, the device based on DPI-T has dramatically improved η of 1.28%.Thus, the order of photovoltaic performance is DPI-T > BPI-T > BPI-P, consistent with their light-harvesting ability.
It is found that DPI-T shows a higher photocurrent than BPI-T due to the excellent conjugation system with relatively better electron-donating ability and smaller steric hindrance, which produces a broader absorption spectrum and a higher extinction coefficient.Previous reports have revealed that the long hydrophobic chain might effectively suppress the electron recombination and reduce the interaction between the dye molecules [40,41], and in this study, DPI-T with octyl chain in the 1-site of the imidazolyl ring exhibited higher V oc than the sensitizers with ethyl chain in the same substitution site.

Conclusions
In summary, two new 4,5-bis(4-methoxyphenyl)-1H-imidazole (BPI) organic dyes BPI-P and BPI-T containing the same Bodipy-conjugated bridges and different additional linkers (phenylene/thiophene) were designed and synthesized.Their optical DFT calculations and photovoltaic properties were systematically investigated because of their promising potential as efficient photosensitizers in DSSCs.The BPI-P-and BPI-T-based cell generated the overall η of 0.18% and 0.32%, respectively, whereas the reference dye DPI-T-based cell gave the efficiency of 1.28%.Dye DPI-T was found to be more efficient than both BPI-P and BPI-T and is attributed to its relatively outstanding light-harvesting efficiency as a result of smaller steric hindrance between the thiophene and DPI unit and better extension of π-conjugated system with four rings on the donor.

Figure 1 :
Figure 1: Molecular structures of the three dyes.

Figure 2 :
Figure 2: Synthetic route to the dyes BPI-P and BPI-T.
(n-C 8 H 17 ) in DPI-T might be favorable to reduce intermolecular aggregation and dark current.

3. 4 .
Photovoltaic Performance of DSSCs Based on the Bodipy Dyes.The typical photocurrent-voltage (J-V) curves of the devices fabricated with BPI-P, BPI-T, and DPI-T as sensitizers under the illumination of AM 1.5G (100 mW•cm −2 ) are shown in Figure 8.The photovoltaic conversion efficiency (η) of the DSSCs is calculated from short-circuit current density (J sc ), open-circuit voltage (V oc ), fill factor (ff), and intensity of the incident light (P) according to the following equation: η (%) = (J sc × V oc × ff/P) × 100.The detailed photovoltaic parameters are summarized in Table

Figure 8 :
Figure 8: The photocurrent density-voltage curve for DSSCs based on BPI-P, BPI-T, and DPI-T under AM 1.5 full simulated sunlight irradiation.

Table 1 :
Photophysical and electrochemical data of three dyes.Absorption maximum in CHCl 3 solution.b Absorption maximum on TiO 2 was obtained through measuring the dyes adsorbed on 6 μm TiO 2 nanoparticle films.c Values in CHCl 3 solutions (on TiO 2 film).d E ox onset was measured in CH 2 Cl 2 with 0.1 M TBAPF 6 as the electrolyte.e E 0,0 was estimated from the onset point of the absorption spectra.f E ox * was estimated by subtracting E 0,0 from the E ox a

Table 2 :
Calculated excitation energy characteristics of BPI-T and DPI-T.The composition means contribution of each transition for excitation energies.c H→HOMO; L→LUMO.International Journal of Photoenergy aThe oscillator strength of a transition.b