Au Nanoparticles as Interfacial Layer for CdS Quantum Dot-sensitized Solar Cells
© The Author(s) 2010
Received: 26 May 2010
Accepted: 15 July 2010
Published: 28 July 2010
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© The Author(s) 2010
Received: 26 May 2010
Accepted: 15 July 2010
Published: 28 July 2010
Quantum dot-sensitized solar cells based on fluorine-doped tin oxide (FTO)/Au/TiO2/CdS photoanode and polysulfide electrolyte are fabricated. Au nanoparticles (NPs) as interfacial layer between FTO and TiO2 layer are dip-coated on FTO surface. The structure, morphology and impedance of the photoanodes and the photovoltaic performance of the cells are investigated. A power conversion efficiency of 1.62% has been obtained for FTO/Au/TiO2/CdS cell, which is about 88% higher than that for FTO/TiO2/CdS cell (0.86%). The easier transport of excited electron and the suppression of charge recombination in the photoanode due to the introduction of Au NP layer should be responsible for the performance enhancement of the cell.
Quantum dot-sensitized solar cells (QDSSCs) are considered as a promising candidate for the development of next generation solar cells because they can be fabricated by simple and low-cost techniques [1, 2]. The development of nanotechnogy, especially the synthesis and application of nanomaterials [3–8], facilitates the progress of QDSSCs and enables them to receive more and more interests. Currently, the efforts on improving the performance of QDSSCs have mainly been focused on fundamental issues, such as improved understanding of device physics , optimization of device structure by advanced processing methods [10, 11] and development of high-performance materials [12–16]. These combined efforts have led to very encouraging power conversion efficiency of 4.22% . However, such efficiency is far away for the practical applications. As a result, further exploration on the optimization of QDSSC performance is necessary.
For QDSSCs, electrons generated by the quantum dots have to pass through numerous grain boundaries and the interfaces between conductive substrate and semiconductor oxide layer to reach the conductive substrate via conduction band of semiconductor oxide. Therefore, the control of the charge carrier transportation at interfaces is one of the most challenging issues in the improvement of QDSSCs. Lee et al. [18, 19] reported the modification of QDSSCs by using single-walled carbon nanotubes (SWCNTs) on indium-doped tin oxide electrodes. The power conversion efficiency of the cell was increased by 50.0% for CdS QDSSCs and 35.6% for PbS QDSSCs due to the improved charge-collecting efficiency and reduced recombination in the presence of SWCNTs. Kim et al.  used graphene-TiO2 composite as an interfacial layer between fluorine-doped tin oxide (FTO) layer and nanocrystalline TiO2 for dye-sensitized solar cells. The introduction of graphene-TiO2 increased the efficiency from 4.8 to 5.26% due to the retardation of the back-transport reaction resulting from the direct contact of the electrolyte with the FTO substrate.
As a noble metal, nanosized Au exhibits unusual electric and optical properties as well as high chemical stability [21–23]. Therefore, Au can be considered as an interfacial layer between active layer and conductive substrate to improve the performance of cells. Kouskoussa et al. [24, 25] employed a Au ultrathin layer between FTO or aluminum-doped zinc oxide anode and organic electron donor layer to improve the interface resistance of organic solar cells. A higher conversion efficiency of cells had been achieved due to better hole collection efficiency due to the introduction of Au ultrathin layer. However, using nanosized Au as interfacial layer in the photoanode for improving the QDSSC performance has seldom been reported despite their expected potential to enhance the solar energy conversion efficiency due to favorable charge collection.
In this work, we reported CdS QDSSCs using Au nanoparticles (NPs) as interfacial layer between FTO and TiO2 layer. A large improvement in the efficiency up to 1.62% is achieved when compared with 0.86% for the QDSSC without Au NP interfacial layer. The easier transport of excited electron and the suppression of charge recombination in the photoanode due to the introduction of Au NP layer should be responsible for the performance enhancement of the cell.
FTO glass (resistivity: 14 Ω/□, Nippon Sheet Glass, Japan) was used as the substrate for nanocrystalline TiO2 (P25, Degussa) electrodes. Cadmium nitrate [Cd(NO3)2], sodium sulfide [Na2S], methanol [CH3OH] and ethanol [CH3CH2OH] (analytical grade purity) were purchased from Shanghai Chemical Reagents Co. Ltd. and were used without further purification.
The Au NP colloid solution was prepared by the modified tannic acid/citrate method using chlorauric acid trihydrate, sodium citrate tribasic dehydrate, potassium carbonate anhydrous and tannic acid . The concentration of the Au NPs is about 0.3 mM.
Prior to the fabrication of TiO2 film, FTO glass was ultrasonically cleaned sequentially in HCl, acetone, ethanol and water each for 30 min. After drying in the air, the FTO glass was immersed in Au NP colloid solution at 70°C for 30 min. TiO2 film was prepared by screen printing of TiO2 paste on the FTO glass, followed by sintering at 500°C for 30 min. The thickness of TiO2 layer was about 5 μm.
CdS deposition on the TiO2 film was performed by successive ionic layer adsorption and reaction (SILAR) technique . The film was dipped into an ethanol solution containing 0.33 M Cd(NO3)2 for 30 s, rinsed with ethanol and then dipped for another 30 s into a 0.5 M Na2S methanol solution and rinsed again with methanol. The two-step dipping procedure was considered to be one cycle. It is known that the amount of CdS QDs assembled on the photoanode increases with the number of SILAR cycles. Too thin or too thick CdS layer is not beneficial to the performance of QDSSCs and thus appropriate SILAR cycle is very important [28, 29]. In our experiments, the best performance of QDSSCs can be achieved for the photoanode assembled with CdS in about 12 SILAR cycles. Direct deposition of CdS on screen-printed TiO2 (TiO2/CdS) film without Au NP interfacial layer by SILAR process with 12 cycles was also carried out for comparison.
The UV–vis transmittance spectra of FTO glass and FTO glass with Au NPs (FTO/Au) were detected using a UV–vis spectrophotometer (Hitachi U3900). The morphology and structure of TiO2 and TiO2/CdS films were characterized by using a Hitachi S-4800 field emission scanning electron microscopy (FESEM) and a JEOL-2010 high-resolution transmission electron microscope (HRTEM), respectively. Impedance spectroscopy (IS) measurements [30–32] were carried out in dark conditions at forward bias: 0–0.7 V, applying a 10 mV AC sinusoidal signal over the constant applied bias with the frequency ranging between 500 kHz and 0.1 Hz (Autolab, PGSTAT 302 N and FRA2 module).
The QDSSCs were fabricated in a sandwich structure with TiO2 film as photoanode and thin Au-sputtered FTO glass as counter electrode. Water/methanol (3:7 by volume) solution was used as a co-solvent of the polysulfide electrolyte . The electrolyte solution consists of 0.5 M Na2S, 2 M S and 0.2 M KCl. The active area of the cell was 0.25 cm2. Photocurrent–voltage measurement was performed with a Keithley model 2440 Source Meter and a Newport solar simulator system (equipped with a 1 kW xenon arc lamp, Oriel) at one sun (AM 1.5G, 100 mWcm−2), which was calibrated with a reference Si reference solar cell (P/N 91150 V, Oriel). Incident photon to current conversion efficiency (IPCE) was measured as a function of wavelength from 300 to 800 nm using an Oriel 300 W xenon arc lamp and a lock-in amplifier M 70104 (Oriel) under monochromator illumination, which was calibrated with a mono-crystalline silicon diode.
Photovoltaic parameters of FTO/TiO2/CdS and FTO/Au/TiO2/CdS cells
The Au NPs were dip-coated on FTO surface as interfacial layer between FTO and TiO2 film to improve the photovoltaic performance of QDSSCs. The performance of the cells was investigated. The results show that the η of FTO/Au/TiO2/CdS cell reaches up to 1.62% under one sun illumination, which is 88% higher than that of FTO/TiO2/CdS cell. Such an enhancement in photovoltaic performance should be ascribed to the easier transport of excited electron and the suppression of charge recombination in the photoanode due to the introduction of Au NP layer.
This work was supported by Shanghai Pujiang Program (No. 08PJ14043), Special Project for Nanotechnology of Shanghai (No. 0952nm02200) and Program of Shanghai Subject Chief Scientist (No. 08XD1421000).
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