Porphyrins occur in a number of important biomolecules and are also synthetically made for use as probe component of chemical and biological sensors. The performance of dye sensitized solar cells with two different porphyrin dyes was investigated in this work. The two porphyrin complexes comprised of a metal-free 5, 10, 15, 20-meso-tetrakis-(9H-2-fluorene-yl) porphyrin (H 2TFP) and its Zinc complex (ZnTFP). UV-Vis, Fluorescence, and Fourier transformed infrared measurements of the two dyes were carried out to evaluate their absorption, emission and binding characteristics. Both dyes absorbed light in the UV-visible region all the way to the near-infrared. The surface morphology and elemental analysis of the porphyrin dye sensitized photoanodes were determined using Field Emission Scanning Electron Microscopy Imaging and Transmission Electron Microscopy Imaging. Cyclic voltammetry studies, current-voltage characteristics and the electrochemical impedance spectroscopic studies were also carried out. Solar-to-electric energy efficiency of H 2TFP dye sensitized solar cell was higher (0.11%) than that of the zinc complex (0.08%). Thus the metal free porphyrin generated more power than the zinc complex under similar conditions. The impedance measurement also displayed less overall resistance for the free porphyrin (50 Ω) compared with the zinc complex (130 Ω). The LUMO levels of H 2TFP and ZnTFP sensitizers were -0.87 eV and -0.77 eV respectively. Both of these LUMO values are higher than the lower bound level of the conduction band of TiO 2 (-4.0 eV), ensuring the efficient injection of an electron from the excited porphyrin dye to the conduction band of the titanium dioxide.
Dye sensitized solar cells (DSSCs) use dyes to absorb energy from the sunlight to produce excited electrons for the generation of electricity [
In this work, a free base 5, 10, 15, 20-meso-tetrakis-(9H-2-fluorene-yl) porphyrin (H2TFP) and its Zinc complex (Zn-TFP) have been synthesized and characterized to study the photo-physical and current-voltage properties of the DSSC.
Reagents were purchased commercially and used as received, except for pyrrole and DMF that were redistilled by literature procedures [
Steady-state fluorescence spectra were recorded on the fluorescence Nanolog Spectrofluorometer System from Horiba Scientific (FL3-22 iHR, Nanolog). The morphology of each film was analyzed using field emission scanning electron microscopy (FESEM; JSM-7100FA JEOL USA, Inc.). Transmission Electron Microscopy (TEM) images were acquired on JEM-1400 Plus (JEOL USA, Peabody, Massachusetts). The images were viewed using Digital Micrograph software from Gatan (Gatan. Inc, Pleasanton, CA). HOMO and LUMO calculations were carried out using Spartan’14 software from Wave function, Inc. Irvine, CA, and USA. TiO2 paste was printed on FTO glass using WS-650 Series Spin Processor from Laurell Technologies Corporation.
The synthesis of 5, 10, 15, 20-meso-tetrakis-(9H-2-fluorene-yl) porphyrin, denoted as H2TFP, was carried out by the synthetic route shown in Scheme 1. In a 400 mL three-neck round-bottom flask equipped with a magnetic stirring bar and surmounted by a reflux condenser, freshly distilled pyrrole (1.45 g, 21.6 mmol), was added to a solution of fluorene-2-carboxaldehyde (4.2 g, 21.6 mmol), and propionic acid (180 mL) previously degassed through N2. After reagents were mixed, the system was immersed in an oil bath and temperature was raised to 130˚C. The reaction mixture was refluxed for 6 h to complete the reaction. The reaction progress was monitored by taking aliquots of the solution and recording its UV-vis spectra. After a constant pattern was observed for the Soret (=424 nm) and Q bands, the reaction was stopped, allowed to reach room
Scheme 1. Synthesis of H2TFP freebase.
temperature and crunched ice added to precipitate a deep green-violet solid. The cold solution was filtered and the residue washed several times with methanol until no more colored solution was observed in the filtrate. The resulting tarry residue was triturated with toluene (50 ml) and allowed to stand in the refrigerator overnight. A fine purple powder was precipitated, filtered and washed thoroughly with cold toluene. The dried powder was dissolved in dichloromethane (100 ml), treated with trifluoroacetic acid (0.15 ml), and allowed to stir at room temperature for 4 h. The reaction mixture was refluxed for an additional 2.5 h and allowed to cool down to room temperature after which, a saturated solution of sodium carbonate, NaHCO3 (100 mL) was added and the mixture was washed with water (2 × 100 mL) to attain an organic face after separation. Methanol (100 mL) was added over the organic layer to precipitate the free base H2TFP that was collected by filtration, washed several times with cold methanol and dried in desiccator under CaCl2. The solid was dissolved in a little amount of dichloromethane (~7 mL) followed by purification on an alumina column. Eluting with pure dichloromethane then ensued affording a pure compound that was also characterized by UV-Vis giving a sharp Soret band at 424 nm. The solvent was evaporated on rotavapor system and the solid was re-dissolved once more in dichloromethane and filtered over cotton to remove any impurities. The filtrate was recovered in a vial and kept in a dark place at room temperature for several days under gentle evaporation, yielding a deep green-foiled material. Yield, 1.06 g (25.2%). Anal. Calcd for C72H46N4∙CH2Cl2, H2TFP (MW = 967.22 g/mol): C, 83.32; H, 4.60; N, 5.32. Found: C, 83.31; H, 4.30; N, 5.61.
Zn-5, 10, 15, 20-meso-tetrakis-(9H-2-fluorene-yl) porphyrinato (II), denoted as ZnTFP, was prepared according to the synthetic route shown in Scheme 2. In a 50 mL two-neck round-bottom flask, 50 mg (0.22 mmol) of Zn(Oac)2∙2H2O was dissolved in 25 mL dichloromethane previously degassed under anitrogen atmosphere for 10 minutes. To this solution, 21 mg (0.02 mmol) of 5, 10, 15, 20-meso-tetrakis-(9H-2-fluorene-yl) porphyrin was added and degassed with nitrogen for about 10 min. Under these conditions, the reaction flask was capped with rubber septum and kept stirring for 12 h protected from light. After that period the flask was connected to a reflux condenser surmounted by a CaCl2 trap
Scheme 2. Synthesis of Zinc-porphyrin complex.
and refluxed for more two additional hours. The solvent was evaporated and the resulting solid dried overnight on desiccator and dissolved in dichloro-methane. This solution was washed with a concentrated sodium hydrogen carbonate (10% solution, 100 mL) and after separation the resulting organic phase was dried on sodium sulfate anhydrous for 4 h protected from light. The solvent was evaporated and the solid dissolved in a small amount of dichloromethane. Silica gel was used for flash filtration employing dichloromethane: methanol (9:1 v/v) as eluent solution. After solvent evaporation, the solid was treated with dichloromethane (~10 mL), and filtered on cotton to remove any remaining impurities. The filtrate was recovered in a vial, placed inside of a flask capped with rubber septum under an atmosphere of methanol and left to stand for several days at room temperature protected from light. The obtained blue wish-violet solid was washed twice with cold methanol and dried in vacuum over fused CaCl2. The yield was 19.3 mg (88%). Anal. Calcd for C72H44N4Zn (1030.61g/mol): C, 83.90; H, 4.31; N, 5.44; Zn, 6.34; Found: C, 82.62; H, 6.18; N, 3.49; Zn, 2.57. The 1H-NMR spectrum of this compound is displayed in
The dye samples were dissolved in 3 mL of ethanol for fluorescence lifetime solution. To prevent inner filter effect, absorption measurements were carried out to ensure that the absorbances of the dyes were less or equal to 0.15 absorbance unit. Fluorescence decay was measured using Horiba Deltaflex fluorescence lifetime system using the time-correlated single-photon counting (TCSPC) technique with the PPD-850 picosecond photon detection module. The excitation source was 340 nm light-emitting diodes (Delta LED).
The photoanode was prepared by depositing a thin film of TiO2 on the conductive side of fluorine doped tin oxide (FTO) glass using a spin coater and sintered at 380˚C for 2 hours [
The energy efficiencies of the fabricated DSSCs were measured using 150 W fully reflective solar simulator with a standard illumination with air-mass 1.5 global filter (AM 1.5 G) having an irradiance corresponding to 1 sun (100 mW/cm2) purchased from Sciencetech Inc., London, Ontario, Canada and Reference 600 Potentiostat/Galvanostat/ZRA from Gamry Instruments (734 Louis Drive, Warminster, PA 18974). The tested solar cells were masked to an area of 5 cm2. Each cell performance value was taken as the average of three independent samples. The solar energy to electricity conversion efficiency (η) was calculated based on the equation:
where, FF is the fill factor, Isc is the short-circuit photocurrent density (mA cm?2), and Voc is the open-circuit voltage (V) as listed in
The proton NMR signals of free base H2TFP is depicted in
−2.60 and 8.94 ppm, respectively, while in the zinc-porphyrin complex (
The UV-visible absorption spectra in dichloromethane of the neutral free base (H2TFP) and its Zinc complex (ZnTFP) were carried out at room temperature and have been reported previously [
corresponding to the B(0, 0) band (Soret band, ε = 2.3 × 105 M−1 cm−1) is slightly red shifted compared to 417 nm for TPP. This tendency in red shifting is also observed for the four Q bands. The metallic porphyrin complexes in the other hand, exhibit characteristic changes on the electronic spectra compared to H2TFP.
Thus, the absorption spectra of Zn(II) porphyrin complex display a single Soret band which is a combination of the QX and QY electronic transitions due to the D4h symmetry of metallic porphyrins, instead of the D2h low symmetry characteristic of free base porphyrins, with two discernible sub-bands (Q(1,0) and Q(0-0)) [
Fluorescence spectra of H2TFP and ZnTFP deployed in the fabrication of the DSSC were measured to investigate their emission characteristics. The spectra measured in dichloromethane by scanning from 500 to 800 nm with fixed excitation at 420 nm are displayed in
emission pattern of the free porphyrin resembles its UV-Vis absorption spectrum but the emission observed with the Zn complex shows to be very different. H2TFP for example, displayed a red shift while ZnTFP exhibited a broadened fluorescence spectrum. The two emission peaks for the H2TFP occurred at 658 nm and 720 nm, whereas the fluorescence bands for ZnTFP occurred at 599 nm and 650 nm, respectively. It could also be observed from the absorption spectra that the fluorescence peaks are the result of the metallic substitution that occurred on the first Q-bands of the porphyrin and these transitions are not dependent on the Soret band.
As part of the photophysical studies on the porphyrin dyes, the fluoroscence lifetime measurement were carried out to determine the length of time the porphyrin molecules stay in the excited state before decay.
Sample Name | Lifetime (ps) | ||
---|---|---|---|
τ1(ps) | τ2(ns) | χ2 | |
H2TFP | 38.91 | 1.38 | 1.10 |
ZnTFP | 58.11 | 5.27 | 1.09 |
The FT-IR of the H2TFP and ZnTFP are displayed in
Studies on the morphology of the TiO2 electrode were further investigated using Field-Emission Scanning Microscopy Imaging (FESEM).
TiO2 film. Only minor changes are observed in the surface morphology of the samples before and after dye application. The dye adsorbed TiO2 was finer than the bare TiO2 nanocrystalline film. Energy dispersive X-ray spectroscopic (EDS) mapping analysis of a nanocrystalline TiO2 partially sensitized with porphyrin dye is displayed in
made up of carbon. The nanocrystalline nature of the TiO2 which increases the surface area of the titanium dioxide is clearly observed in
To confirm the adsorption of the porphyrin dyes on the TiO2 nanoparticles, the high-resolution images were taken by transmission electron microscopy as shown in
the anatase TiO2 powder. It is speculated that the adsorption between the TiO2 nanoparticles and the dyes results from the electrostatic binding, since the dyes do not have any functional groups which are able to induce any binding between them.
The standard redox process of a free base porphyrin is characterized for the two-electron transfer reaction with two reversible waves named E1Ox and E2Ox, which are determinate by cyclic voltammetry. [
The cyclic voltammetry were carried out on a Princeton Applied Research Versa STAT-3 potentiostate with samples degassed under a nitrogen flow before each scan. The synthesis and characterization of meso-tetrakis-fluorenylpor- phyrin, H2TFP, in Scheme 1, has been previously reported [
Compound | E1/2 (V)a | Assignment |
---|---|---|
H2TfP | −0.07 | Por 0/+ |
0.31 | Por +/− | |
−0.56 | Fluorenyl 0/+ | |
−1.07 | Por +/2+ | |
−0.79 | Por 2+/+ | |
Zn-TfP | 0.57 | Zn II/III |
0.03 | Zn III/II | |
−0.63 | Fluorenyl 0/+ | |
−0.86 | Por 0/+ | |
−1.12 | Por +/0 | |
Zn-TfP + pyridine | 0.36 | Zn II/III |
−0.13 | Zn III/II | |
−0.77 | Fluorenyl 0/+ | |
−1.19 | Por 0/+ | |
−1.13 | Por +/0 |
aPotentials reported versus the SHE reference electrode in acetonitrile (0.1 M) with samples dissolved in DMF. Scans recorded at 100 mV∙s−1 and 25˚C.
behavior, that could be related with the high stability showed by this porphyrin when exposed to air for long times but light protected. The peak potential separations Ep between the anodic and cathodic peaks than for H2TFP and its complexes ranged from 53 mV to 67 mV with an average of 61 ± 0.002 mV. These values are close to the theoretical value of 59 mV for a one-electron transfer reaction, that for a reversible process the peak width is given by the following relationship:
where Ep/2 is the half-peak potential at the half value of the peak current, Ep, is the peak potential, F is the Faraday constant and n is the number of electrons in the reaction. The presence of metallic ions in the fluorenylporphyr in core makes the oxidation reaction of these complexes easier with the redox potentials shifted towards lower values.
The results of our experiments shows that zinc-fluorenyl porphyrin complex (ZnTFP) undergoes a one electron transfer process with a half wave potential, Eo1/2 = 0.57 V vs SHE in DMF (
DYE | Isc (mA/cm2) | Voc (V) | Imp (mA/cm2) | Vmp (V) | Fill Factor | Efficiency (%) |
---|---|---|---|---|---|---|
H2TFP | 0.92 | 0.31 | 0.64 | 0.18 | 0.40 | 0.11 |
ZnTFP | 1.38 | 0.25 | 0.72 | 0.11 | 0.23 | 0.08 |
(0.11%) compared with that fabricated with Zn complex (0.08%). The higher efficiency of the free porphyrin compared to the Zn complex could be due to an efficient interaction between the TiO2 surface and the free porphyrin. Such interaction favors the injection of electron from the dye into the conduction band of the titanium dioxide. Specifically,, the geometry orientation and flexibility changes when the metal is inserted into the porphyrin ring. Thus, the rigidity of the resulting coordinated porphyrin reduces the interaction of the dye with the TiO2 surface leading to a decrease in the electron transfer efficiency of the cell. Therefore, the efficiency values on these dyes could be improved when anchoring groups that facilitate charge transfers attached to these porphyrin molecules.
Electrochemical impedance measurements on the solar cells fabricated using free base H2TFP and the metal complex (ZnTFP) were carried out to understand charge transfer and energetics of charge transport and recombination. The impedance measurements were carried out at frequencies between 0 and 100 KHz.
respectively. The arc in the high frequency region is usually associated with the charge transfer resistance at the interface between the redox couple/electrolyte and counter electrode whereas the arc in the low frequency region is ascribed to the resistance of the transport of the injected electrons within the TiO2 film. The results seen here are consistent with the IV data. The impedance measurement also shows less overall resistance for the free porphyrin (50 Ω) compared with the zinc complex (130 Ω). The position of the low frequency peak of the Bode plot provides information on the lifetime of the photo induced electrons via the equation:
where, f is the frequency of superimposed ac voltage. Thus, H2TFP was found to have a longer lifetime than ZnTFP.
Density functional calculations were used to optimize the geometry of fluorenyl- porphyrin, H2TFP, molecule using the software Spartan 16 from Wave function. All geometries were computed using the ωB97X-D density functional theory method. Geometry optimization was performed using 6-31G* basis set. These calculations were used to calculate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of free base and metallic meso-tetrakis-2-(H9-fluorenyl) porphyrin coordinated to Zn (square planar complex).
Lewis and 3D structures of metallic meso-tetrakis-2-(H9-fluorenyl) porphyrin, respectively. The calculations gave a result for the HOMO as −6.29 eV and the result for the LUMO found to be −0.87 eV. Thus, the difference in the HOMO and LUMO which is the band gap was 5.42 eV. The calculations for the metallic compound gave a result for the HOMO as −6.36 eV and the result for the LUMO as −0.77 eV. The difference in the HOMO and LUMO which is the band gap was 5.59 eV. The HOMO and LUMO surfaces and orbital energy diagrams for the free base is displayed in
In the presence of solar energy, porphyrin dye molecules (S) adsorbed on the TiO2 film absorb photon and make a transition from the ground state or highest occupied molecular orbitals (HOMO) to the excited state or the lowest unoccupied molecular orbital (LUMO) state as displayed in
Two different porphyrin dyes were used to fabricate dye sensitized solar cells. The two porphyrin complexes comprised of a metal-free 5, 10, 15, 20-meso-te- trakis-(9H-2-fluorene-yl) porphyrin (H2TFP) and its Zinc complex (ZnTFP). UV-Vis, Fluorescence, and Fourier Transformed Infrared measurements of the two dyes were carried out to evaluate their absorption, emission and binding characteristics. The Soret band of the dyes was almost the same while changes in the Q-band were observed. This observation was also confirmed in the fluorescence data where changes in peak intensity were due to Q-band. The surface morphology and elemental analysis of the porphyrin dye sensitized photoanodes were determined using Field Emission Scanning Electron Microscopy Imaging and Transmission Electron Microscopy Imaging. The nanocrystalline nature of the TiO2 and their interaction with the dyes were clearly observed under the electron microscopes. Cyclic voltammetry studies, current-voltage characteristics and the electrochemical impedance spectroscopic studies were also carried out. Solar-to-electric energy efficiency of H2TFP dye sensitized solar cell was higher (0.11%) than that of the zinc complex (0.08%). Thus the metal free porphyrin generated more power than the zinc complex under similar conditions. These efficiencies could be higher when anchoring groups are attached to the porphyrins. The free base H2TFP chromophore was found to possess the best photosensitization effect among the two porphyrin dyes studied which has been attributed by the flexibility of the dye to interact.
The work was financially supported by the University of Maryland System (Wilson E. Elkins Professorship), Constellation, an Exelon Company (E2-Ener- gy to Educate grant program) and Dept. of Education (SAFRA Title III Grant). The authors are also grateful to the Institution of Advancement, Coppin State University, for administrative help. The content is exclusively the responsibility of the authors and does not necessarily represent the official views of the funding agencies.
Ghann, W., Chavez-Gil, T., Goede, C.I., Kang, H., Khan, S., Sobhi, H., Nesbitt, F. and Uddin, J. (2017) Photophysical, Electrochemical and Photovoltaic Properties of Porphyrin-Based Dye Sensitized Solar Cell. Advances in Ma- terials Physics and Chemistry, 7, 148-172. https://doi.org/10.4236/ampc.2017.75013