Five types of conjugated phenylene polymer-modified photoanodes for quantum dot-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. The open-circuit voltage and fill factor in QDSSCs with the conjugated phenylene polymer with -COOH photoanodes were 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.
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 [
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 [
In this work, we synthesized conjugated phenylene polymer-modified photoanodes by immobilization of L-cysteine-stabilized QDs after electrochemical polymerization of functionalized phenyldiazonium salts onto ITO glass electrodes. The prepared conjugated phenylene polymer-modified 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.
The 4-aminobenzoic acid, 4-aminothiophenol, sodium nitrite (NaNO2), 3-thiopheneethanol, and L-cysteine (L-cys) were purchased from Sigma-Aldrich (USA), while 1,4-phenylenediamine and 4-aminophenol were purchased from Tokyo Chemical Industry Co., Ltd. (Japan). Cadmium chloride was obtained from Junsei Chemical Co., Ltd. (Japan). Selenium was supplied from Acros Organics (Belgium). Sodium borohydride was provided from Samchun Pure Chemical Co., Ltd. (Korea). All other chemicals were of analytical grade. Water was purified using a Millipore purification system (Millipore Co., Ltd., MA, USA).
Synthesis of L-cys-capped CdSe QDs was carried out in aqueous solution by a chemical reduction method [
The precursors for preparation of the conjugated phenylene polymer-modified photoanodes are shown in Figure
Precursors for preparation of the conjugated phenylene polymer-modified photoanodes.
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 SnO2, 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 K3Fe(CN)6/K4Fe(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.
The morphology of the composites was observed using a field emission transmission electron microscope (FE-TEM, Tecnai G2 F30 S-Twin, FEI, USA) at 200 kV. The absorption spectrum of L-cys-capped CdSe QDs was obtained using a UV-VIS spectrometer (UV-3101PC, Shimadzu Corp., Japan). The photoluminescence (PL) measurement was carried out by a fluorescence spectrometer (FluoroMate FS-2, Scinco Co., Ltd., Korea). DLS analysis was performed using a DynaPro NanoStar (Wyatt Technology Corp., USA). FT-IR spectrum of the prepared photoanodes was obtained using a Nicolet iS10 (Thermo Fisher Scientific Inc., USA). Surface properties were characterized by a scanning electron microscopy (SEM, S-4800, Hitachi Co., Ltd., Japan), contact angle (Phoenix 300, Surface Electro Optics Co., Ltd., Korea), and X-ray photoelectron spectroscopy (MultiLab. ESCA 2000, Thermo Fisher Scientific Inc., USA).
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).
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.
HR-TEM images and DLS data of the synthesized L-cys-stabilized CdSe QDs.
Photoluminescence and UV-vis spectrum of the synthesized L-cys-stabilized CdSe QDs.
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 [
Cyclic voltammograms during electrochemical polymerization of precursors in PBS electrolyte with scan rate 100 mV/s.
Electrochemical polymerization mechanism onto the surface of ITO electrode.
FT-IR spectra of the conjugated phenylene polymer-modified ITO glass electrode prepared by electrochemical polymerization were obtained (Figure
FT-IR spectra of the conjugated phenylene polymer-modified ITO electrodes.
The C 1s high-resolution XPS spectra of the conjugated phenylene polymer-modified ITO glass electrode prepared by electrochemical polymerization are shown in Figure
XPS spectra of the C 1s on the conjugated phenylene polymer-modified ITO electrodes.
The cross-section SEM images of the conjugated phenylene polymer-modified ITO electrodes prepared by electrochemical polymerization are displayed in Figure
Cross-section SEM images of the conjugated phenylene polymer-modified ITO electrodes.
The electrochemical impedance spectra of the conjugated phenylene polymer-modified ITO electrodes on a frequency range of 10−2~104 Hz with an amplitude of 10 mV in 0.1 M PBS buffer (pH = 7.0) containing 1.0 mM K3Fe(CN)6/K4Fe(CN)6 are shown in Figure
Electrochemical impedance spectra of the conjugated phenylene polymer-modified ITO electrodes on the frequency range of 10−2~104 Hz with the amplitude of 10 mV in 0.1 M PBS buffer (pH = 7.0) containing 1.0 mM K3Fe(CN)6/K4Fe(CN)6.
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 (
Band gap of the conjugated phenylene polymer-modified ITO electrodes determined by UV spectroscopy1.
Electron carriers | Band gap (eV) |
---|---|
| 3.91 |
| 3.73 |
| 2.90 |
| 3.90 |
| 3.95 |
Changes in the contact angles (Figure
Contact angels of the bare ITO, the conjugated phenylene polymer-modified ITO electrodes, and photoanodes.
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
In order to confirm the successful introduction of CdSe QDs on the photoanode, we carried out an analysis by high-resolution XPS spectroscopy and obtained high-resolution XPS spectra of the Se 3d and Cd 3d of the conjugated phenylene polymer-modified photoanodes (Figures
(a) XPS spectra of the Se 3d on the conjugated phenylene polymer-modified ITO electrodes. (b) XPS spectra of the Se 3d and the Cd 3d on the conjugated phenylene polymer-modified ITO electrodes.
For confirmation of CdSe QDs on the surface of the conjugated phenylene polymer-modified photoanodes, we observed the morphology of the prepared photoanodes (Figure
SEM images of the conjugated phenylene polymer-modified ITO electrodes. Elemental mapping of Cd and Se to
Evaluation of QDSSCs prepared by the conjugated phenylene polymer-modified photoanodes
Power conversion efficiency (
QDSSCs | | | FF (%) | |
---|---|---|---|---|
| 0.52 | 6.83 | 76.8 | 2.73 |
| 0.49 | 6.81 | 77.5 | 2.60 |
| 0.49 | 6.79 | 75.8 | 2.54 |
| 0.52 | 5.16 | 41.7 | 1.12 |
| 0.49 | 6.81 | 74.7 | 2.50 |
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 (
The authors declare that they have no competing interests.
This work was supported by the National Research Foundation of Korea grant, funded by the Korean Government (NRF-2015N043) and Hannam University Research Fund (2016).