Photodeposition of Ag2S on TiO2 nanorod arrays for quantum dot-sensitized solar cells
© Hu et al.; licensee Springer. 2013
Received: 5 November 2012
Accepted: 27 December 2012
Published: 3 January 2013
Ag2S quantum dots were deposited on the surface of TiO2 nanorod arrays by a two-step photodeposition. The prepared TiO2 nanorod arrays as well as the Ag2S deposited electrodes were characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope, suggesting a large coverage of Ag2S quantum dots on the ordered TiO2 nanorod arrays. UV–vis absorption spectra of Ag2S deposited electrodes show a broad absorption range of the visible light. The quantum dot-sensitized solar cells (QDSSCs) based on these electrodes were fabricated, and the photoelectrochemical properties were examined. A high photocurrent density of 10.25 mA/cm2 with a conversion efficiency of 0.98% at AM 1.5 solar light of 100 mW/cm2 was obtained with an optimal photodeposition time. The performance of the QDSSC at different incident light intensities was also investigated. The results display a better performance at a lower incident light level with a conversion efficiency of 1.25% at 47 mW/cm2.
KeywordsAg2S Quantum dot-sensitized solar cell Photodeposition TiO2 nanorod
Quantum dot-sensitized solar cells (QDSSCs) have attracted increasing attention due to their relatively low cost and potentials to construct high-efficiency energy conversion systems . Compared with organic dyes used in dye-sensitized solar cells (DSSCs), semiconductor sensitizers in the form of quantum dots (QDs) present higher extinction coefficients and adjustable absorption spectra by controlling their size [2, 3]. However, the best efficiency (approximately 5%) reached by QDSSCs is much lower than that of conventional DSSCs [4, 5]. The deposition of QD sensitizers on the electron acceptor (e.g., TiO2) related to the loading amount and the connection between QDs and electron acceptor plays a key role in the QDSSC performance. QDs with various sizes should be deposited on the surface of mesoporous TiO2 separately as a requirement for efficient charge separation . Typically, the coverage of mesoporous TiO2 by QDs is much less than a full monolayer [6, 7], which leads to insufficient light harvesting and back electron transfer from exposed TiO2 to electrolyte. Besides, deposition of typically 3 to 8 nm diameter QDs into mesoporous TiO2 with relative narrow pores is rather difficult, and large QDs that inserted into mesoporous TiO2 may also cause pore blocking and subsequently inhibit the penetration of electrolyte deep into the holes . The efficiency enhancement of QDSSCs could be achieved by applying an advanced deposition method as well as suitable TiO2 nanostructure. For the former, several deposition methods have been developed to anchor QDs on the surface of TiO2 including ex-situ and in-situ methods , where photodeposition is a promising candidate by taking advantage of the photocatalytic properties of TiO2 in the deposition process [9–11]. Photoreduction on the surface of TiO2 leads to a large and uniform coverage of QDs and intimate contact between the QDs and TiO2 for efficient interfacial charge transfer . For the latter, one-dimensional oriented arrays (nanotube or nanorod arrays) possess large surface area and efficient electron transfer property that can be employed to improve the performance of QDSSCs [12, 13]. Importantly, the high-oriented arrays provide uniform pore size that is favorable for QD anchoring with rare pore blocking.
Ag2S is an important photoelectric material and has a broad application in terms of photocatalysis and electronic devices [14–17]. With bulk bandgap of 1.0 eV, close to the optimal bandgap of 1.1 to 1.4 eV for photovoltaic devices , Ag2S is a potential sensitizer superior to others used in QDSSCs. Several researches that concentrated on the Ag2S-QDSSCs have been reported since the first application of Ag2S in QDSSCs [19–23]. However, the reported conversion efficiency (η) remains lower than that of QDSSCs based on other narrow bandgap semiconductor (e.g., CdS and CdSe) [24, 25], which is partly attributed to the low coverage of Ag2S on the surface of TiO2.
To improve the efficiency of Ag2S-QDSSCs, we apply a modified photodeposition as well as an oriented TiO2 nanorod array (NRA) on the cell. Typically, the oriented TiO2 NRA was prepared by a simple hydrothermal method. Photodeposition of Ag2S was conducted by two steps: photoreduction of Ag+ to Ag by TiO2 NRA followed by the sulfurization of Ag to Ag2S QDs. To our knowledge, this is the first report of Ag2S QD-sensitized TiO2 NRA solar cells. Results show that a large coverage of Ag2S QDs on the TiO2 NRs has been achieved by this modified photodeposition, and the photoelectrochemical properties of these electrodes suggest that Ag2S has a great potential for the improvement of QDSSCs.
Growth of TiO2 NRA
TiO2 NRA was grown on the fluorine-doped SnO2-coated conducting glass (FTO) substrate (resistance 25 Ω/square, transmittance 85%) by a hydrothermal method as described in the literature . Briefly, 30 mL deionized water was mixed with 30 mL concentrated hydrochloric acid (36.5% to 38.0% by weight). The mixture was stirred for 5 min followed by an addition of 1 mL titanium butoxide (98%, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China). After stirring for another 5 min, the mixture was transferred into a Teflon-lined stainless steel autoclave of 100-mL volume. The FTO substrate was placed at an angle against the wall of the Teflonliner with the conducting side facing down. After a hydrothermal treatment at 150°C for 20 h, the substrate was taken out and immersed in 40 mM TiCl4 aqueous solution for 30 min at 70°C. The TiCl4-treated sample was annealed at 450°C for 30 min.
Photodeposition of Ag2S on TiO2 NRA
Solar cell assembly
The counter electrode was prepared by dripping a drop of 10 mM H2PtCl6 (99.99%, Aldrich Company, Inc., Wyoming, USA) ethanol solution onto FTO substrate, followed by heating at 450°C for 15 min. Ag2S-sensitized TiO2 nanorod (NR) photoanode and Pt counter electrode were assembled into sandwichstructure using a sheet of a thermoplastic frame (25-μm thick; Surlyn, DuPont, Wilmington, USA) as spacer between the two electrodes. The polysulfide electrolyte consisted of 0.5 M Na2S, 2 M S, 0.2 M KCl, and 0.5 M NaOH in methanol/water (7:3 v/v). An opaque mask with an aperture was coated on the cell to ensure the illuminated area of 0.16 cm2.
X-ray diffraction (XRD) measurements were carried out using a RAD-3X (Rigaku Corporation, Tokyo, Japan) diffractometer with Cu-Kα radiation. The morphology of the films was observed by field emission scanning electron microscopy (FESEM, S4800, Hitachi Ltd., Tokyo, Japan) and transmission electron microscope (TEM, JEM-2100, JEOL Ltd., Beijing, China). To prepare the TEM sample, TiO2 NRs together with Ag2S QDs were scratched from the FTO substrate and dispersed in ethanol by sonication. The UV–vis absorption spectra of TiO2 NRA and Ag2S-deposited TiO2 NRA were recorded in the range from 350 to 800 nm using a Hitachi U-3010 spectroscopy. The photocurrent density-voltage (J-V) characteristics of solar cells were examined by a Keithley 2400 sourcemeter (Keithley Instruments, Inc., Cleveland, USA) under illumination by a solar simulator (AM 1.5 G). Incident light intensity was calibrated by standard silicon solar cell and light intensity meter (FZ-Aradiometer) simultaneously. The stability of the solar cell was measured by electrochemical workstation (pp211; Zahner, Elektrik GmbH & Co.KG, Kronach, Germany) with continuous illumination on the solar cell.
Results and discussion
Morphology of the TiO2 NRA
Photodeposition of Ag2S QDs
Photoreduction rate of Ag+ by TiO2 in ethanol solution is so rapid that the electrode turned to silvery-white within 3 min after immersing FTO/TiO2 in the solution. To verify the effect of photocatalytic properties of TiO2 on the reduction process, the ethanol solution containing Ag+ was irradiated in the same condition but in the absence of TiO2, and no silver was observed in 10 h. Similar results were also observed when immersing FTO/TiO2 in the Ag+ solution in the dark, consistent with the proposed photoreduction mechanism.
Optical and photoelectrochemical properties of Ag2S QDs-sensitized TiO2 NRA
Photovoltaic parameters of solar cells fabricated with different photoanodes under AM 1.5 illumination at 100 mW/cm 2
Photovoltaic parameters of Ag 2 S QD-sensitized solar cell measured at different light intensities
We have deposited Ag2S QDs on TiO2 NRA by a two-step photodeposition. The deposition process was conducted by photoreduction of Ag+ to Ag on the surface of TiO2 NRs followed by chemical reaction with sulfur. By controlling the photoreduction period, we have obtained Ag2S-sensitized TiO2 NRs with a large coverage and superior photoelectrochemical properties. QDSSCs based on the Ag2S-sensitized TiO2 NRAs were fabricated. Under optimal condition, the Ag2S-QDSSC yields a Jsc of 10.25 mA/cm2 with a conversion efficiency of 0.98% at AM 1.5 solar light of 100 mW/cm2. We also investigated the solar cell performance under varied incident light intensities. Results show that a drawback of these cells in full sun condition compared with the maximum efficiency achieved at lower light level. The key factor that limits the solar cell performance is the low Voc values we obtained. By employing suitable redox electrolyte, we believe the Ag2S-QDSSCs will have a great promotion with increased Voc values.
dye-sensitized solar cells
field emission scanning electron microscopy
fluorine-doped SnO2-coated conducting glass
high resolution transmission electron microscope
short circuit current density
quantum dot-sensitized solar cells
scanning electron microscope
transmission electron microscope
This work was supported by the National High Technology Research and Development Program 863 (2011AA050511), Jiangsu “333” Project, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Postgraduate Research Innovation Projects at Colleges and Universities in Jiangsu Province (CXLX12_0707).
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