The Conjugated Phenylene Polymer-Modified Photoanodes for Quantum Dot-Sensitized Solar Cells

Five types of conjugated phenylene polymer-modified photoanodes for quantumdot-sensitized solar cells (QDSSCs) were prepared by immobilization of CdSe QDs after electrochemical polymerization of functionalized phenyldiazonium salts onto ITO glass electrodes. The successful preparation of the conjugated phenylene polymer-modified photoanodes for QDSSCs was confirmed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), FT-IR spectroscopy, UV-visible spectroscopy, contact angles, and electrochemical impedance spectroscopy.Theopen-circuit voltage and fill factor inQDSSCswith the conjugated phenylene polymerwith -COOHphotoanodeswere achieved at 0.52 V and 76.8%, respectively, and the energy conversion efficiency was improved to 2.73% using the conjugated phenylene polymer with -COOH photoanodes.


Introduction
Quantum dot-sensitized solar cells (QDSSCs) have attracted considerable attention over the past few years as a promising candidate for the development of next generation solar cells because of the superior intrinsic properties of quantum dots (QDs), which include high molar extinction coefficients, easily tunable band gaps, large intrinsic dipole moments, and possible multiple carrier generation [1][2][3].However, the power conversion efficiency of QDSSCs is not yet competitive with that of dye-sensitized solar cells (DSSCs).The poor performance of QDSSCs is mainly due to weak electron collection by the TiO 2 from the QDs and charge recombination at the QD-electrolyte interface.Therefore, an effective way to improve the efficiency of QDSSCs would be to prevent back electron transfer or recombination processes and accelerate the transportation of separated charges to the contact electrode.
In order to increase the efficiency of QDSSCs, there is a need to search for novel electron transfer materials for their working electrodes.To improve the power conversion efficiency of QDSSCs, various structures have been employed to fabricate the photoanodes, such as metal complexes [4], conjugated polymer [5], mesoporous films, hierarchical spheres, nanoflowers [6], nanowires (NWs) [7][8][9], nanorods (NRs) [10][11][12][13], and nanotube (NT) arrays [14][15][16].Recently, many new strategies, especially surface and interface treatments, have been also employed to modify the conjugated polymer of the photoanodes with the aim of reducing recombination and improving the transport of photoexcited carriers at the interface.The introduction of a phenylene polymer on the electrode surface by radical polymerization [17][18][19][20] could provide an electron transfer mediator because the phenylene site has a conjugated bond, which means there are alternating double and single bonds, and it has a low band gap of about 2.9-3.9 eV.Among various conductive polymers, polythiophene is of great interest because of a simple preparatory process, ease of mass production, and low cost.Nevertheless, its relatively high charge transfer resistance is the main barrier toward its practical application.
In this work, we synthesized conjugated phenylene polymer-modified photoanodes by immobilization of Lcysteine-stabilized QDs after electrochemical polymerization of functionalized phenyldiazonium salts onto ITO glass electrodes.The prepared conjugated phenylene polymermodified photoanodes were characterized by FT-IR spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), contact angle, and electrochemical impedance in order to confirm whether or not the preparation was successful.Furthermore, the performance of solar cells assembled with the conjugated phenylene polymer-modified photoanodes was determined with a solar simulator.

Synthesis of L-Cysteine-Capped CdSe
QDs. Synthesis of L-cys-capped CdSe QDs was carried out in aqueous solution by a chemical reduction method [21].In detail, in a round flask CdCl 2 (0.24 mmol, 0.048 g) and L-cysteine (0.24 mmol, 0.029 g) were dissolved in distilled water (80 mL), and 0.1 M NaOH solution was added until the L-cysteine was completely dissolved.This solution was stirred and kept at 100 ∘ C. In another round flask, Se powder (0.02 g) and NaBH 4 (0.10 g) were dissolved in anhydrous ethanol (15 mL) under stirring and N 2 gas.After this solution had been completely dissolved, H 2 SO 4 (0.1 M, 20 mL) was dropped into the flask.H 2 Se gas was generated by the reaction of H 2 SO 4 with Se and NaBH 4 and carried to another flask via a glass tube, which was connected between the round reaction flasks.Finally, CdSe QDs could be synthesized by this reaction.

Preparation of CdSe
QDs-Sensitized Solar Cells.The precursors for preparation of the conjugated phenylene polymer-modified photoanodes are shown in Figure 1.For synthesis of 4-carboxybenzenediazonium salt, 0.01 mol of 4aminobenzoic acid was dissolved in 20 mL of warm aqueous hydrochloride solution (0.22 M).After cooling to 0 ∘ C, 20 mL of aqueous NaNO 2 (0.011 M) was added slowly to the reaction mixture with stirring.A solid powder of 4carboxybenzenediazonium salt was obtained by filtration, and then the synthesized 4-carboxyphenyldiazonium salt was stored at 4 ∘ C until using electrochemical polymerization onto an ITO glass electrode was performed in phosphate buffer solution (PBS, pH = 7.0) in the presence of the synthesized 4carboxyphenyldiazonium salt.Other conjugated phenylene polymer-modified ITO electrodes were also prepared by a similar method as that described above except that modification of the electrode was done by electrochemical polymerization of the 3 precursor in acetonitrile electrolyte containing tetrabutylammonium tetrafluoroborate.QDSSCs were assembled into a sandwich structure using a QD-anchored electrode as the working electrode, an ITO electrode as the counter electrode, and a 0.5 M Na 2 S, 2 M S, and 0.2 M KCl in water/methanol (3 : 7 in volume) mixed solution as the electrolyte.
2.4.Instruments.Cyclic voltammetry (CV) was performed using a VersaSTAT 3 potentiostat/galvanostat (Ametek PAR, USA) and a conventional three-electrode system comprising an ITO glass substrate (In-doped SnO 2 , resistance: 10 Ω sq −1 , and thickness: 0.7 mm) as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode.Electrochemical impedance spectroscopy (EIS) was performed using a PP240 and IM6ex (ZAHNER-elektrik GmbH & Co. KG, Germany).EIS measurements were performed in the presence of PBS (pH = 7.0) containing a 1.0 mM K 3 Fe(CN) 6 /K 4 Fe(CN) 6 (1 : 1) mixture as a redox probe in the frequency range between 100 mHz and 10 kHz at the amplitude of +10 mV.
Photovoltaic measurements of the QDSSCs were performed at an illumination of 100 mW cm −2 (AM 1.5 G) generating from a solar simulator (Polaronix K201/LAB50, McScience, Korea) equipped with a 200 W Xenon lamp.Current-voltage curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a photovoltaic power meter (Polaronix K101/LAB20, McScience, Korea).

Characterization of the L-Cysteine-Capped CdSe QDs.
Quantum dots are widely used not only in the photoelectrochemical field but also in biosensors, biological imaging, and bioconjugates because of their remarkable electronic and optic properties.In order to apply photoelectrochemical fields, we synthesized L-cysteine-capped CdSe QDs using a chemical reduction method as in the following process.

Characterization of the Conjugated Phenylene Polymer-Modified Photoanodes.
In order for electron transfer to be made easily onto the photoelectrode, the conjugated phenylene polymers were grafted onto the ITO glass surface by electrochemical polymerization in accordance with a similar method to that described previously [21].Consecutive CVs (Figure 4) were obtained with a scan rate of 100 mV/s during electrochemical polymerization of the precursors in PBS electrolyte (pH = 7.0), except for the CV of 3, which was obtained by electrochemical polymerization of the 3 precursor in acetonitrile electrolyte with 0.1 M tetrabutylammonium tetrafluoroborate because of insoluble properties in PBS buffer solution.In the 1 CV of Figure 4, the oxidation peak observed at around −0.50 V corresponds to a reduction of 4-carboxyphenyl diazonium salts to 4-carboxyphenyl radicals at the respective electrode on the 1st cycles (see, 1 in Figure 5).In the 5th cycles, the intensity of the −0.50 V oxidation peak was increased because of the formation of many 4-carboxyphenyl radicals.In the 10th cycles, the characteristic oxidation peak observed at 0.57 V is because of electrochemical polymerization (see structure 1 in Figure 5).Earlier reports suggested that electrochemical reduction of diazonium salts on ITO surfaces could result in the formation of thin film by heterogeneous one-electron reduction in aprotic solvents [12].In the 2 CV of Figure 4, the oxidation peaks exhibited at −0.50 V appear in similar patterns to those in the 1 CV because of 4-hydroxyphenyl radicals (2 CV in Figure 5).With increasing redox cycles, the oxidation peaks at −0.50 V were increased because of the formation of hydroxyphenyl radicals, namely, electrochemical polymerization.The oxidation peaks with the acetonitrile electrolyte were displayed around −0.25 V in the 3 CV of Figure 4 because of 4-thiophenyl radicals (see 3 in Figure 5).On the other hand, the reduction peaks appeared around −0.5 V on the CV of the 4-aminophenyl diazonium chloride (4 in Figure 4) as shown in 4 in Figure 5.With increasing redox cycles, current values were decreased because the electron was not transferred onto the 4-aminophenyl polymer grafted ITO glass electrode, which indicates that the electrons were not transferred onto the grafted polymer-modified ITO glass electrode.From these results, the structure of the grafted polymers onto the surface of the ITO glass electrode is considered to be as shown in Figure 5-4.With 3-thiopheneethanol (5 of Figure 4), the first reduction peak was at −0.5 V, and the reduction peaks moved to −0.25 V with increasing CV cycles while the current values were also increased.These results show that the conjugated phenylene polymers were successfully grafted onto the surface of the ITO glass electrode (Figure 5-5).FT-IR spectra of the conjugated phenylene polymermodified ITO glass electrode prepared by electrochemical polymerization were obtained (Figure 6).In the 1 electrode, the characteristic peak at 1720 cm −1 corresponding to carboxylic acid and the C 6 H 5 -C vibration peak at 1200 cm −1 indicating the aromatic ring appear.In the 2 electrode, a broad peak at 3400 cm −1 attributed to the O-H vibration was observed.The characteristic peak at 2950 cm −1 due to the-S-H bond was detected in the 3 electrode.In the 4 electrode, a sharp peak around 3300 cm −1 due to the N-H bond was observed.In the 5 electrode, a broad peak at 3400 cm −1 attributed to the O-H bond appeared.These results show that the conjugated phenylene polymer-modified ITO glass electrodes were successfully prepared by electrochemical polymerization.
The C 1s high-resolution XPS spectra of the conjugated phenylene polymer-modified ITO glass electrode prepared by electrochemical polymerization are shown in Figure 7.In the high-resolution XPS spectra of the 1 electrode, the C=O group appears at 288 eV and C-O group appears at 286.5 eV because of the carboxyl group.In the 2 electrode, a small amount of the C-O group appears because of C-O-H.In the 3 electrode, a small C-S peak was observed because of the thiol group as a functional group.In the 4 electrode, a C-N peak at 285.2 eV corresponding to the amine group was also observed.After electrochemical polymerization of the thiopheneethanol, a characteristic C-O peak at 286.5 eV due to a hydroxy group and C-S peaks were also observed.From these results, we confirmed that the conjugated phenylene polymer-modified ITO glass electrode could be successfully prepared by electrochemical polymerization onto an ITO glass electrode.
The cross-section SEM images of the conjugated phenylene polymer-modified ITO electrodes prepared by electrochemical polymerization are displayed in Figure 8.In the 1 electrode, the morphology of the conjugated phenylene polymer was a fiber structure, and the thickness of the conjugated phenylene polymer was 723 nm.It is considered that the electron could be expected to transfer to an ITO glass electrode.The morphology and the thickness of the conjugated phenylene polymer were dense and 364 nm, respectively, in the 2 electrode.In the 3 electrodes, the morphology of the conjugated phenylene polymer was a similar pattern to that of the 2 electrode and the thickness was 225 nm.Thicknesses of these magnitudes for conjugated polymers are expected to be effective for electron transfer to ITO glass electrodes.In the 4 electrode, the morphology of the conjugated polymers was a rigid fiber structure that looked like a polyaniline structure, and the thickness of the conjugated polymer was 75.5 m.It is considered that a conjugated phenylene polymer layer of such thickness would not be effective for electron transfer.In the 5 electrode, the morphology appeared dense and the thickness was 235 nm.These results strongly indicate that the conjugated phenylene polymer-modified ITO electrodes were successfully prepared by electrochemical polymerization.The electrochemical impedance spectra of the conjugated phenylene polymer-modified ITO electrodes on a frequency range of 10 −2 ∼10 4 Hz with an amplitude of 10 mV in 0.1 M PBS buffer (pH = 7.0) containing 1.0 mM K 3 Fe(CN) 6 /K 4 Fe(CN) 6 are shown in Figure 9.The Ret value of the bare ITO was observed at about 130 Ω; however, a relatively small Ret of 59 Ω was observed in the 2 electrode, which indicates that it is an excellent conductor because of acceleration of electron transfer.The Ret values of the 1, 3, and 5 electrodes were about 114 Ω, 165 Ω, and 142 Ω, respectively, and the extremely large Ret value of the 4 electrode was obtained because of the thickness of the conjugated phenylene polymer or presence of a mixture of polymers (see structure in Figure 5).From these results, it was concluded that the conjugated phenylene polymer-modified ITO glass electrode is suitable for use as a photoanode in a solar cell.
In order to determine the band gap of the conjugated phenylene polymer, we analyzed the conjugated phenylene polymers prepared by electrochemical polymerization in accordance with the above described equation ( 5), and the band gap was between 2.90 eV and 3.95 eV (Table 1).From these results, it was concluded that the prepared conjugated phenylene polymer-modified ITO glass electrode is suitable for use as a photoanode in a solar cell.
Changes in the contact angles (Figure 10) of the bare ITO electrode (top) after electrochemical polymerization (middle) and immobilization of CdSe QDs (bottom) were examined.The contact angle of the bare ITO electrode surfaces was shown as 77.7 ∘ , while the contact angle values for the conjugated phenylene polymer-modified ITO electrodes 1, 2, 3, 4, and 5 were 55.5 ∘ , 50.6 ∘ , 48.3 ∘ , 58.3 ∘ , and 85.7 ∘ , respectively.The contact angles after electrochemical polymerization show a dramatic decrease of about 20 ∘ ∼30 ∘ compared with that of the bare ITO electrode because of the hydrophilic properties of the conjugated phenylene polymers with functional group, except for the contact angle of the surface of the 5 electrode, which was increased because of the hydrophobic properties of the grafted thiophene polymers.
After immobilization of L-cys-capped CdSe QDs on the surface of the conjugated phenylene polymer-modified ITO electrodes, the contact angle values for the conjugated phenylene polymer-modified photoanodes 1, 2, 3, 4, and 5 showed significant decreases of 36.4 ∘ , 44.1 ∘ , 15.3 ∘ , 44.7 ∘ , and 22.2 ∘ , respectively.From these results, it was concluded that the conjugated phenylene polymer-modified photoanodes were successfully prepared by immobilization of L-cys-capped  CdSe QDs onto the surface of the conjugated phenylene polymer-modified ITO electrodes.
In order to confirm the successful introduction of CdSe QDs on the photoanode, we carried out an analysis by highresolution XPS spectroscopy and obtained high-resolution XPS spectra of the Se 3d and Cd 3d of the conjugated phenylene polymer-modified photoanodes (Figures 11(a) and 11(b)).The Se 3d peak at 55 eV, Cd 3d5/2 peak at 404 eV, and Cd 3d3/2 peak at 411 eV appeared for all of the prepared conjugated phenylene polymer-modified photoanodes.From these results, we confirmed that the conjugated phenylene polymer-modified photoanodes were successfully prepared by immobilization of QDs after electrochemical polymerization.
For confirmation of CdSe QDs on the surface of the conjugated phenylene polymer-modified photoanodes, we observed the morphology of the prepared photoanodes (Figure 12).The surface morphology of the conjugated phenylene polymer-modified photoanode was an amorphous-like structure.Element mapping showed that the Cd and Se elements were dramatically dispersed onto the surface of the conjugated phenylene polymer-modified photoanodes.From the SEM results, we confirmed the successful preparation of the conjugated phenylene polymer-modified photoanodes by immobilization of the CdSe QDs after electrochemical polymerization.
Evaluation of QDSSCs prepared by the conjugated phenylene polymer-modified photoanodes - curves of the QDSSCs prepared by the conjugated phenylene polymermodified photoanodes (Figure 13) clearly demonstrates that the deposition of the various electron transfer carrier modified CdSe thin layers significantly improved the cell performance.Power conversion efficiency (), fill factor (FF), open-circuit voltage ( oc ), and short circuit current ( sc ) to the QDSSCs prepared by photoanodes are shown in Table 2.Under one sun illumination (AM 1.5 G, 100 mW cm −2 ), the maximum conversion efficiency () of the cell was between 2.50 and 2.73% except for QDSSCs prepared by the 4 photoanode.Efficiency improved because of the prevention of recombination between the photoanode and the quantum dot sensitizer.As shown by these results, the conjugated phenylene-modified photoanodes were effective photoanodes in QDSSCs.

Conclusions
In this study, conjugated phenylene polymer-modified photoanodes for QDSSCs were prepared by immobilization of L-cys-capped QDs after electrochemical polymerization, and the prepared photoanodes were characterized.The size of the L-cysteine-capped CdSe QDs prepared by an aqueous solution method was below 5 nm and they were well dispersed.The preparation mechanism of the conjugated phenylene polymer-modified ITO electrodes was determined via cyclic voltammetry, FT-IR, XPS, SEM, contact angle, and impedance analysis.The maximum conversion efficiency () of the cell was between 2.50 and 2.73% except for QDSSCs prepared by the 4 photoanode (see the structure in Figure 5).The conjugated phenylene-modified photoanodes were effective photoanodes in QDSSCs.

Figure 1 :
Figure 1: Precursors for preparation of the conjugated phenylene polymer-modified photoanodes.

Figure 2 :
Figure 2: HR-TEM images and DLS data of the synthesized L-cys-stabilized CdSe QDs.

Figure 5 :
Figure 5: Electrochemical polymerization mechanism onto the surface of ITO electrode.

Figure 7 :
Figure 7: XPS spectra of the C 1s on the conjugated phenylene polymer-modified ITO electrodes.

Figure 8 :
Figure 8: Cross-section SEM images of the conjugated phenylene polymer-modified ITO electrodes.

Table 1 :
Band gap of the conjugated phenylene polymer-modified ITO electrodes determined by UV spectroscopy 1 .