Polydiacetylene-nested Porphyrin Conjugate for Dye-sensitized Solar Cells

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Introduction
Porphyrins are attractive materials due to their adjustable photophysical and electrochemical properties via modification of peripheral substituents and central metals, 1 as well as thermal and photo-stability.Up to date, several porphyrinic derivatives have been proposed for being used as key constituents in (opto)electronic devices, such as photovoltaic cells, 2 organic light-emitting diodes (OLEDs), 3 and organic fieldeffect transistors (OFETs). 4Recently, they have proven to be promising candidates for dye-sensitized and bulkheterojunction solar cells (DSSCs 5 and BHJ-SCs 6 , respectively).
Our recent work reported the synthesis and use of polydiacetylene (PDA)-nested zinc-porphyrin for the BHJ-SCs, showing that the presence of a PDA network significantly enhanced absorption of the porphyrin at the region of 530-600 nm and suggesting the potential use as an electron donating material in the BHJ-SCs. 7The objective of this work is to explore the possibility of using a novel PDA-nested zincporphyrin derivative as a dye in the DSSCs.The porphyrin monomer contains three meso-C 25 alkyl chains having diacetylene (DA) units, which can be photopolymerized to give a porphyrin embedded PDA web, and one carboxyl anchoring group at the remaining meso position of the porphyrin.
According to previous studies, 7,8 this system is designed with three main advantages: (i) π-stacking of the porphyrin macrocycle can facilitate the local orientation of the DA units for the desirable photopolymerization, (ii) the long chain of the DA-containing C 25 alkyl chains can serve as solubilizing groups in organic solvents, and (iii) the formation of the PDA network can enhance the absorption at 500-650 nm, where the porphyrin has low absorptivity.To the best of our knowledge, PDA was previously studied as a hole transport material in a solid state DSSCs by Wang et al, 9 but a porphyrin-PDA conjugate has not been investigated for using in the DSSCs.In this work, we report the novel PDA-nested zinc-porphyrin conjugate exhibiting suitable photophysical and electrochemical properties for DSSC application, and giving favorable device performance.The results from these studies will become a useful guideline for the development of other porphyrin-based and PDA-based materials for optoelectronic applications.

Experimental Materials and Methods.
All chemicals were analytical grade, purchased from commercial suppliers and used as received without further purification. 1H-NMR and 13 C-NMR spectra were obtained in deuterated chloroform (CDCl 3 ) using a NMR spectrometer operated at 400 megahertz (MHz) for 1 H and 100 MHz for 13 C nuclei.Chemical shifts (δ) are reported in parts per million (ppm) relative to the residual CHCl 3 peak (7.26 ppm for 1 H-NMR and 77.0 ppm for 13 C-NMR).Coupling constants (J) are reported in Hz.Mass spectra were obtained by matrix-assisted This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins laser desorption ionization mass spectrometry (MALDI-MS) using dithranol as a matrix.Absorption and emission spectra of dye solutions were measured in toluene at room temperature.Absorption data of a film of porphyrin-PDA conjugate was collected from the film prepared by submerging a TiO 2 film into a 0.3 mM monomer solution in a mixed solvent of tetrahydrofuran (THF) and ethanol (v/v, 1/4) at room temperature for 16 h, followed by exposing the resulting dry film under ambient light for at least 1 h or under a 254 nm ultraviolet radiation for 15 min.Non-commercial Compounds.5,10,15,20-tetra (4-carboxyphenyl) porphyrin (1) 10 and 1-amino-10,12-pentacosadiyne (3) 11 were prepared following literature procedures.C, and then submerged into the dye solution (0.3 mM) using a mixed solvent of THF and ethanol (v/v, 1/4) for 16 h.The resulting dyed TiO 2 film was exposed to the ambient light for at least 1 h to proceed the photopolymerization.To fabricate the devices, the stained photoanode was assembled with the platinized counter electrode using a 25 μm thick Surlyn hot melt ring (Dupont, USA) and sealed by heating press.The internal gap between two glasses was filled with a volatile liquid electrolyte (1.0 M 1,3-dimethylimidazolium iodide (DMII), 0.5 M tertbutylpyridine (tBP), 0.1 M guanidiniumthiocyanate (GNCS), 0.1 M LiI and 0.03 M I 2 in the mixed solvent of acetonitrile and valeronitrile (v/v, 85/15)).To characterize the DSSCs, a 450 W xenon light source (Oriel, USA) was used.

Synthesis of
The current density-voltage (J-V) characteristics were obtained by applying external potential bias to the cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter (Keithley, USA).The devices were masked to attain an illuminated active area of 0.159 cm 2 .
Loss of light reflection from the photoanode glass was reduced by applying a self-adhesive fluorinated polymer anti-reflecting film (ARKTOP, Asahi glass).Up to four devices were fabricated for each experimental variable change to give accurate statistics.The detailed methods for the TiO 2 film preparation, the device fabrication and the current-voltage measurements are described elsewhere.Normally, the decay follows closely a monoexponential form, thus, a recombination rate constant can be extracted from a slope of a semilogarithmic plot. 15

Results and discussion
Synthesis.The target compound was obtained from a series of three reactions as shown in Scheme 1.A reaction of the tetracarboxylic acid 1 10 with NHS and EDC•HCl yielded ester 2 which immediately reacted with diacetylene 3 11 , due to its hygroscopicity, to afford 4. The formation of 4 was confirmed by the presence of its molecular peak at m/z 1817.311 on a MALDI-TOF mass spectrum corresponding to its molecular mass, and characteristic patterns of free base porphyrin on its absorption and emission spectra.Photophysical and Electrochemical Properties.The photophysical properties, namely absorption as a function of a wavelength of an incident light, were determined for the monomer solution and the polymer film.The resulting curves are presented in Figure 1.The UV-Vis absorption spectra of a Zn-4 solution in toluene exhibited a characteristic pattern of Zn-porphyrin having a B-band at 425 nm, and Q-bands at 557 and 598 nm (black solid line).A poly-Zn-4 film, prepared from the 254 nm ultraviolet radiation of a Zn-4-coated TiO 2 film at room temperature for 15 min, gave the similar absorption pattern with slightly broader bands and higher intensity of the first Q-band (red dashed line).The broadening of the absorption bands can be explained by the possible macrocycle aggregation.Although the film was relatively thin on the TiO 2 surface, the enhancement of the Q-band, possibly resulted from the co-absorption of the PDA formed in the film, can be seen.These results are in a good agreement with our previous report. 7The electrochemical properties of the poly-Zn-4 film on the ITO/glass glass substrate were studied by cyclic voltammetry.Results revealed that the poly-Zn-4 film can be both electrochemically oxidized and reduced (Figure 2).In the oxidation domain (Figure 2a, black solid line), two quasireversible anodic peaks of poly-Zn-4 were observed at the peak potentials of +0.7 V and +1.3 V, related to two successive one-electron oxidation processes.Due to the dissolvation of the film into the electrolyte solution during the measurement, the positive scanning was not performed beyond the potential of +1.5 V. Compared with 5-(4carboxyphenyl)-10,15,20-(triphenyl)porphinatozinc(II) (ZnTPP-COOH; Figure 2a, red dashed line), it was observed that the first oxidation of poly-Zn-4 occurred in the similar potential (+0.7 V vs. +0.8V), while the second oxidation of poly-Zn-4 occurred at 0.2 V higher potential (+1.3 V vs. +1.1 V).In the reduction process of poly-Zn-4 (Figure 2b), the dissolvation of This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins the film was observed beyond the potential of -1.8 V, and therefore the measurement was carried out from 0.0 V to -1.8 V (Figure 2b, black solid line).In this region, poly-Zn-4 gave one irreversible peak at -1.5 V, which is corresponding to the first cathodic potential of ZnTPP-COOH (-1.5 V; Figure 2b, red dashed line).In case of ZnTPP-COOH, the second cathodic signal was observed at -1.8 V with the indefinable anodic signal from -0.4 and -0.8 V.This is likely to be resulted from the possible formation of unknown products from the reduction process(es) of ZnTPP-COOH, which is attributed to the molecular cleavage, as this feature was more conspicuous at higher cycle numbers.In comparison with the results obtained from ZnTPP-COOH, these small shifts of the redox potentials and the film instability observed from poly-Zn-4 film indicate a significant effect of the presence of the PDAcontaining alkyl chains on the electrochemical characteristics of the porphyrin ring.Following the previous studies, 16 a highest occupied molecular orbital (HOMO) energy level of a dye can be represented by its E 1/2 value of the first oxidation or E 1/2 (ox1), while a lowest unoccupied molecular orbital (LUMO) energy level can be estimated from an excited state oxidation potential (E 0-0 * ) by a following equation: When E 0-0 is an absorption onset of the dye.Since poly-Zn-4 was obtained in the form of film, the values from the abovementioned cyclic voltammetry measurement were used in this calculation.The results revealed that poly-Zn-4 were found to have the energy gap of 2.0 V with the LUMO and HOMO energy levels of -1.The detailed description is given in the device fabrication section.Figure 3 shows the photovoltaic performance of the devices under standard AM 1.5 G 1000 W•m -2 illumination.
From the current-voltage (J-V) curve in Figure 3a, the best poly-Zn-4-based solar cell exhibited respectively a short-circuit photocurrent density (J sc ), an open-circuit voltage (V oc ) and a fill factor (FF) of 4.2 mA•cm -2 , 0.7 V and 0.78, respectively, which yielded an overall power conversion efficiency (PCE) value of 2.3%.It is important to note that the PCE was increased by 35% of the original value (from 1.7% to 2.3%) when the device was illuminated under the standard AM 1.5 G 1000 W•m -2 light intensity for ~40 minutes before the J-V measurement was performed.According to the previous study, 17 the substitution of the DA-containing alkyl chains at the meso-phenyl groups of the porphyrin ring resulted in retarding the charge recombination processes and enhancing the interfacial charge separation.Moreover, compared with the previous study of ZnTPP-COOH-based DSSCs, 18 the PDAcontaining alkyl chains in poly-Zn-4 resulted in a remarkable increase in PCE value (1.8% vs. 2.3%).This phenomenon could be attributed to the solid-state photo-induced PDA formation in the poly-Zn-4 film, leading to the increase in the number of π-conjugation network and, as a result, raising the light harvesting efficiency of the dye.However, a relatively low value of PCE might be resulted from the porphyrin aggregation on the TiO 2 surface as indicated by the broadened absorption bands of the poly-Zn-4 film on TiO 2 .A similar observation has also been found in the case of ZnTPP-COOH.18   The aggregation can cause the quenching processes of the excited states to the adjacent porphyrin units, subsequently increases unfavorable charge recombination processes and reduces the electron injection efficiency.Please do not adjust margins Please do not adjust margins

Conclusions
The target diacetylene-porphyrin monomer was successfully prepared and showed satisfactory solubility characteristics.
The formation of the polydiacetylene-nested zinc-porphyrin was indicated by the slight enhancement of the absorption at the Q-band region relative to the monomer.The results from cyclic voltammetry showed that the polydiacetylene-nested zinc-porphyrin exhibited the suitable levels of the highest occupied molecular orbital and lowest unoccupied molecular orbital for using in dye-sensitized solar cells.Device studies suggested that the incorporation of the polydiacetylene network and the porphyrinic system was advantageous for the device performance with respect to the previously reported system based on 5-(4-carboxyphenyl)-10,15,20-(triphenyl)porphinatozinc(II).

14 A
modulated light intensity data acquisition system was used to control the incident photon-to-current conversion efficiency (IPCE) measurement.The modulation frequency was about 1 Hz.Light from a 300 W Xenon lamp (ILC Technology, USA) was focused through a computer controlled Gemini-180 double monochromator (JobinYvon Ltd., UK) onto the photovoltaic cell.White light bias was used to bring the total light intensity on the device closer to operating conditions.In the transient photoelectrical decay experiments, different steady-state light levels were provided by a homemade white light-emitting diode array tuning the driving voltage.A red light-emitting diode array controlled with a fast solid-state switch was used to generate a perturbation pulse of 50 ms duration.The pulsed red-and steady-state white-light were Please do not adjust margins Please do not adjust margins both incident on the working electrode side of the test cell.The intensity of the red light pulse was carefully controlled by the driving potential of the red diode array to keep the modulated photovoltage below 10 mV.In a transient photovoltage decay measurement, the cells were maintained at open circuit voltage under the white light and the transient photovoltage decay following the red light pulse was monitored.

Scheme 1 .
Scheme 1. Synthesis of the target compound.

Figure 1 .
Figure 1.Absorption spectra of a Zn-4 solution in toluene (black solid line) and a poly-Zn-4 film (red dashed line).The inset shows a magnification of the region between 525 and 625 nm with baseline calibration for a comparison purpose.

Figure 2 .
Figure 2. Cyclic voltamograms of a poly-Zn-4 film in the ranges between (a) 0 and +1.5 V and (b) 0 and -1.8 V in comparison with those of ZnTPP-COOH film in the ranges between (a) 0 and +1.8 V and (b) 0 and -2.2 V.
3 and +0.7 V, respectively.With the LUMO energy level being more negative than the conduction band of TiO 2 (-0.50 V vs. NHE) and the HOMO energy level more positive than the oxidation potential of an iodide ion/triiodide ion couple (+0.40 V vs. NHE), the electron injection of the excited dye to the TiO 2 surface and the dye regeneration by the I -/I 3 -electrolyte should be thermodynamically possible in the DSSCs based on poly-Zn-4.Photovoltaic Characteristics.The poly-Zn-4-based solar cells were assembled using a double layer TiO 2 film (8+5 μm) in conjugation with an acetonitrile-based electrolyte solution.

Figure 3 .
Figure 3. (a) Current density-voltage curves of the poly-Zn-4-based solar cell under simulated solar irradiation of 96.3 (green solid line), 49.4 (blue solid line) and 9.1 (red solid line) mW•cm -2 (AM 1.5), and in the dark (black dotted line); and (b) IPCE spectrum of the poly-Zn-4-based solar cell.

Figure 4 .
Figure 4. Plots of electron recombination lifetime (blue square) and chemical capacitance (red triangle) vs. open-circuit voltage of the poly-Zn-4-based solar cells.