- Nano Express
- Open Access
Efficient Dye-Sensitized Solar Cells Made from High Catalytic Ability of Polypyrrole@Platinum Counter Electrode
© Ma et al. 2015
- Received: 12 May 2015
- Accepted: 18 July 2015
- Published: 14 August 2015
Polypyrrole@platinum (PPy@Pt) composite film was successfully synthesized by using a one-step electrochemical method and served as counter electrode (CE) for efficient dye-sensitized solar cells (DSSCs). The PPy@Pt CE with one-dimensional structure exhibited excellent electrocatalytic activity and superior charge transfer resistance for I−/I3 − electrolyte after being the cyclic voltammetry and electrochemical impedance spectroscopy tested. The photocurrent-photovoltage curves were further used to calculate the theoretical photoelectric performance parameters of the DSSCs. The DSSC based on the PPy@Pt CE achieved a remarkable power conversion efficiency of 7.35 %, higher about 19.9 % than that of conventional Pt CE (6.13 %). This strategy provides a new opportunity for fabricating low-cost and highly efficient DSSCs.
- Dye-sensitized solar cell
- Photo-electric conversion efficiency
Since the report of dye-sensitized solar cells (DSSCs) in 1991 have attracted considerable attention due to their simple fabrication process, low production costs, relatively high conversion efficiency, and being environmental friendly [1–4]. So far, the highest photo-electric conversion efficiency of DSSCs has achieved of over 13 %  by depositing platinum (Pt) on a transparent conductive substrate. However, since Pt is a kind of limited resource to induce increase in cost and hider commercialize application, replacement or reduction of Pt has emerged as an important issue for further development of DSSCs.
As alternative cost-efficient materials, various counter electrode (CE) materials including carbon-based materials, conducting polymers, sulfides, nitrides, and carbides have been integrated into DSSCs [6–15]. Polypyrrole (PPy) has attracted much research attention due to its high conductivity, low cost, large electrochemical surface area, and good electrocatalytic activity for I3 − reduction enabling application in electronics, catalysis, energy storage, and sensing [6–8]. Wu et al.,  have prepared PPy nanoparticles and applied as CE catalyst in DSSCs and got remarkable power conversion efficiency. Yue et al.  reported a composite CE composed of poly (3, 4-ethylenedioxythiophene):polystyrenesulfonate and PPy by using electrochemical polymerization route, which showed a good catalytic ability in I−/I3 − electrolyte and an improved photovoltaic performance for DSSC.
At present, although several groups have developed some alternative efficient Pt-free materials for DSSCs, Pt CE still is the most excellent and stable catalytic material. Also, a one-step route for synthesizing high-quality Pt-based hybrids as enhanced platform for electroanalytical applications is rarely a concern. Thus, in this report, we have developed a low temperature method and efficiently synthesized PPy@Pt hybrid counter electrode using one-step electrochemical deposition route, by which the obtained PPy@Pt hybrid counter electrode would exhibit excellent electrocatalytic activity and high conductivity served for DSSCs. The electrochemical performance of the PPy@Pt hybrid counter electrode were investigated by cyclic voltammetry (CV), electrochemical impendence spectroscopy (EIS), and Tafel polarization and showed excellent electrocatalytic activity and lower charge transfer resistance of 2.47 Ω·cm2. The DSSC with PPy@Pt CE exhibited an enhanced photovoltaic conversion efficiency of 7.35 % under irradiation of 100 mW·cm−2 (AM 1.5 G).
Preparation of PPy@Pt Hybrid Electrode
Briefly, the PPy@Pt hybrid electrode was prepared by using the electrodeposition method which outlined below. All experiments were implemented in a three-electrode cell, including one Pt foil as CE, one Ag/AgCl electrode as reference electrode, and fluorine-doped tin oxide (FTO) glass with an exposed area of 1 cm2 as working electrode. The base electrodeposition solution consisted of 0.1 M of pyrrole, 0.1 M of lithium perchlorate, and 0.1 M of oxalic acid in 50 ml deionized water and treated by ultrasonication for 30 min. Then, the prepared of 0.01 M chloroplatinic acid isopropanol solution was mixed into the above PPy base polymerization solution by ultrasonication for 1 h. A constant current density of 10 mA·cm−2 was served for electrodeposition. The obtained PPy@Pt hybrid electrode was put into anhydrous ethanol for 2 h and vacuum oven at 100 °C for 12 h, respectively. For comparison, the PPy and Pt CEs were prepared using similar method as well as the PPy@Pt hybrid electrode.
Fabrication of DSSC
The TiO2 anode was prepared as described previously [18, 19]. The dye was loaded by immersing the TiO2 anode in the 0.3 mM of Z907 ethanol solution for 12 h. Thus, the dye-sensitized TiO2 anode with thickness of 8–10 μm was obtained. The DSSC was fabricated by injecting the liquid electrolyte (0.05 M of I2, 0.1 M of LiI, 0.6 M of tetrabutylammonium iodide, and 0.5 M of 4-tertpbutylpyridine in acetonitrile) into the aperture between the dye-sensitized TiO2 electrode and the CE. The two electrodes were clipped together and wrapped with thermoplastic hot-melt Surlyn.
The surface morphology of the sample was observed by using JSM-7600F field emission scanning electron microscope (SEM). CV, EIS, and Tafel polarization curves were conducted by using a computer-controlled electrochemical analyzer (CHI 660D, CH Instrument). The electrolyte used in the DSSC test was also injected into the symmetric dummy cells for both EIS and Tafel measurements. EIS was carried out under the simulating open-circuit conditions at ambient atmosphere, sealing with thermoplastic hot-melt Surlyn and leaving an exposed area of 0.64 cm2. The frequency of applied sinusoidal AC voltage signal was varied from 0.1 to 105 Hz, and the corresponding amplitude was kept at 5 mV in all cases. The photovoltaic test of DSSC with an exposed area of 0.4 × 0.7 cm2 was carried out by measuring photocurrent-photovoltage (J-V) character curve under white light irradiation of 100 mW·cm−2 (AM 1.5 G) from the solar simulator (XQ-500W, Shanghai Photoelectricity Device Company, China) in ambient atmosphere.
Preparation of Nanocomposite Electrode and its Surface Morphology
EIS parameters of the dummy cells and the photoelectric properties of the DSSCs obtained from the Pt, PPy, and PPy@Pt CEs
R s (Ω·cm2)
R ct (Ω·cm2)
Z w (Ω·cm2)
V oc (V)
J sc (mA·cm−2)
Thus, from Table 1, the diffusion coefficient of I3 − for the PPy@Pt CE is much larger than that of the Pt and PPy CEs, presumably deriving from its improvement of the surface roughness and the synergistic catalytic effect of Pt and PPy. Figure 2d exhibits the Nyquist plots of the symmetrical Pt, PPy, and PPy@Pt CEs to further investigate the electrocatalytic ability for regeneration of the I − /I3 − redox couple on the aforementioned CEs. The equivalent circuit model illustrates as inset, in which the high-frequency intercept on the real axis represents the series resistance (R s); the semicircle at high frequency refers to the charge-transfer resistance (R ct) for the I3 − reduction at the CE|electrolyte interface, and the semicircle at low frequency represents the Nernst diffusion impedance (Z w ) corresponding to the diffusion resistance of the I−/I3 − redox species . As everyone knows, the CE with the smaller R ct means the less overpotential for an electron transferring from the CE to the electrolyte and great electrochemical ability. The R ct for the abovementioned CEs are 3.11, 4.69, and 2.47 Ω·cm2, respectively. Among them, the PPy@Pt CE exhibits the lowest R ct value of 2.47 Ω·cm2 compared to the pristine Pt electrode or PPy electrode, revealing a synergistic effect of Pt and PPy on the improvement of electrocatalytic activity and electrical conductivity for the hybrid CE. Additionally, it should be noted that the Z w for the PPy CE (7.35 Ω·cm2) is larger than that of the Pt electrode (1.65 Ω·cm2). This can be attributed to the conductive polymers relatively low electrical conductivity compared with that of the Pt catalyst.
Photovoltaic Performance of DSSCs With PPy@Pt CE
In this paper, we reported an efficient PPy@Pt composite film as the CE in DSSCs with power conversion efficiency of 7.35 %, which is superior to that of Pt electrode under the same conditions. The PPy@Pt CE demonstrated amazing electrocatalytic activity for the I−/I3 − redox reaction due to its high cathodic current density in the extensive electrochemical analyses made from CV measurement and the low R ct of 2.47 Ω·cm2 from EIS test. The CE with shuttle-like structure nanoparticles prepared by using electropolymerization technique is an effective strategy for accelerating the charge transfer and iodide redox. The research presented here is far from being optimized but these profound advantages along with low-cost synthesis and scalable materials promise the new PPy@Pt CE to be a great potential and strong candidate in robust DSSCs.
The authors are very grateful for the joint support by the Natural Science Foundation of Henan Educational Committee (No. 14A430023) and the Natural Science Foundation of Henan University (No. 2013YBZRO47).
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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