Facile solution growth of vertically aligned ZnO nanorods sensitized with aqueous CdS and CdSe quantum dots for photovoltaic applications
© Luan et al; licensee Springer. 2011
Received: 9 February 2011
Accepted: 14 April 2011
Published: 14 April 2011
Vertically aligned single crystalline ZnO nanorod arrays, approximately 3 μm in length and 50-450 nm in diameter are grown by a simple solution approach on a Zn foil substrate. CdS and CdSe colloidal quantum dots are assembled onto ZnO nanorods array using water-soluble nanocrystals capped as-synthesized with a short-chain bifuncional linker thioglycolic acid. The solar cells co-sensitized with both CdS and CdSe quantum dots demonstrate superior efficiency compared with the cells using only one type of quantum dots. A thin Al2O3 layer deposited prior to quantum dot anchoring successfully acts as a barrier inhibiting electron recombination at the Zn/ZnO/electrolyte interface, resulting in power conversion efficiency of approximately 1% with an improved fill factor of 0.55. The in situ growth of ZnO nanorod arrays in a solution containing CdSe quantum dots provides better contact between two materials resulting in enhanced open circuit voltage.
As an n-type semiconductor with a direct and wide bandgap of 3.3 eV, ZnO is an attractive material for a variety of applications ranging from ultraviolet lasers  and sensors  to field-emission devices . In recent years, vertically aligned one-dimensional ZnO nanostructures have gained great interest for dye-synthesized solar cells [4, 5], as a promising alternative to mesoporous TiO2 films . Both ZnO and TiO2 have similar bandgaps, while the higher electron mobility and direct electrical pathways provided by vertically aligned ZnO nanorods/nanowires are favorable for electronic transport . Low cost and large-scale chemical solution-based techniques have been developed to synthesize anisotropic single crystalline ZnO nanostructures on a variety of substrates . Despite the expected advantages, the use of ZnO nanostructures in combination with dyes has been hampered due to their instability in acidic dyes leading to the formation of Zn2+/dye agglomerates, an insulating layer blocking the electron injection efficiency from the dye molecules to ZnO . On the other hand, semiconductor nanocrystal quantum dots (QDs)  have been considered as promising photosensitizers for TiO2 and ZnO-based quantum dot sensitized solar cells (QDSCs)  due to their intrinsic attractive properties: bandgap tunable both by the choice of material and by the size offering the possibility to match the solar spectrum, and to align energy levels both in respect to the conduction level of the electron-conducting nanostructure and to redox potential of the electrolyte, and high extinction coefficients [8–12]. Photosensitization of ZnO nanowires/nanorods with CdSe QDs has been reported, with relatively low photocurrents for a photoelectrochemical cell with a liquid triiodide/iodide (I3 -/I-) electrolyte due to the low QD coverage resulting in power conversion efficiencies in the range of 0.4-0.6% [13, 14]. Those works, however, relied on the use of QDs originally synthesized in organic solvents and thus capped with long-chain organic molecules which had to be post-preparatively exchanged for bifunctional short-chain ligands or thioglycolic acid (TGA) serving as molecular linkers  to the oxide surface. Recently, Chen et al. reported an improved QDSC by direct loading of mercaptopropionic acid-capped CdSe QDs on TiO2 substrates from aqueous solution with a power conversion efficiency of 1.19% . Multilayers of TGA-capped CdTe QDs have been deposited on ZnO nanorods in combination with positively charge polyelectrolyte to improve the light-harvesting ability .
In this paper, we demonstrate an efficient coverage of ZnO nanorod arrays (NRAs) grown on a Zn foil substrate by a simple solution approach with CdS or CdSe QDs using water-soluble nanocrystals capped as-synthesized with a short-chain bifuncional TGA linker. We show that the simultaneous use of CdS and CdSe QDs has an advantage of synergetic effect in the light harvest resulting in higher performance for co-sensitized structure compared with the solar cells using only one type of QDs. Furthermore, we demonstrate the modification of the QDSCs by depositing a thin (2 nm) Al2O3 layer before QDs anchoring to avoid spurious charge transfer at the interface between the electrolyte and Zn metal. Power conversion efficiencies of approximately 1% were obtained using ZnO/Al2O3/CdSe electrode with an improved fill factor (FF) of 0.55. Besides, in situ fabrication of the ZnO NRAs in a solution containing the CdSe QDs results in the enhanced open circuit voltage (V OC) of approximately 0.72 V.
Preparation of CdS and CdSe QDs
The CdS and CdSe QDs capped by a short-chain ligand TGA have been synthesized in water as previously reported . In the alkaline solution, carboxylic groups of TGA are deprotonated, serving as anchor points and facilitating the binding of QDs to the oxide surface . Size of the QDs varied between 2 and 2.5 nm.
Preparation of ZnO NRAs
Zinc foil (99.9%) was ultrasonically washed three times in absolute ethanol, placed in a sealed glass bottle containing 20 ml of de-ionized water, kept at 50°C for 24 h, washed several times with distilled water and ethanol, and finally dried in air. The procedure is a simplified version of a previously reported technique based on the fact that water has the ability to oxidize Zn, in the presence of oxygen, to form ZnO nanorods .
For comparison, we have also grown ZnO NRAs using the same procedure as above but adding CdSe QDs to the fabrication solution (15 μl CdSe QDs solution with particle concentration of approximately 10-4 M). The preparation time in this case was 48 h to compensate for a slower growth rate.
Growth of Al2O3 for core-shell NRAs
Trimethylaluminum and distilled water, with nitrogen as a carrier gas, were used as precursor and oxidant, respectively, to deposit Al2O3 by atomic layer deposition (ALD). The deposition temperature was 150°C and the expected growth rate was 0.91 Å/cycle; a total of 15 cycles were carried out to deposit an ultrathin Al2O3 layer on the surface of selected ZnO NRAs prior to the decoration with QDs.
Sensitization of ZnO NRAs with QDs
Substrates with vertically aligned ZnO NRAs were immersed in aqueous colloidal solutions of CdS or CdSe QDs (pH 9.5, particle concentration of approximately 10-5 M) for 4 h at room temperature, and subsequently dried at 90°C for several minutes. For co-sensitized ZnO NRAs, the substrates were firstly immersed in aqueous colloidal CdS QDs solutions for 2 h and then in aqueous colloidal CdSe QDs solutions for another 2 h, resulting in the preferential adsorption of CdSe QDs on top of CdS layer. The white color of ZnO covered substrates changes to light yellow or orange after adsorption of CdS or CdSe QDs, respectively.
Fabrication of photoelectrochemical cells
The photoelectrochemical cells were fabricated as follows. A thin island-like Pt layer has been deposited by dropping 0.8 mM H2PtCl6 solution on an FTO-covered glass and subsequent annealing at 400°C for 30 min, and used as a photocathode assembled into a cell device face-to-face with ZnO/QD photoanode. The two electrodes were separated by 60 μm spacer and bonded together using compression metal clips. The cell was infiltrated with a liquid I3 -/I- electrolyte containing 0.1 M LiI, 50 mM I2 and 0.6 M 1,2-dimethyl-3-propylimidazolium iodide dissolved in acetonitrile, sealed, and characterized immediately owing to the low stability of CdS and CdSe QDs in I3 -/I- electrolyte . The effective electrode area was between 0.2 and 0.5 cm2.
Structural, optical, and electrical characterization
ZnO nanorods were characterized by X-ray diffraction (XRD) spectra recorded with a Siemens D500 diffractometer at 40 kV/30 mA, scanning electron microscopy (SEM; Philips XL 30 FEG), and transmission electron microscope (TEM; a Philips CM20). High-resolution transmission electron microscope (HRTEM) images and fast Fourier transform (FFT) pattern were obtained with a Philips CM200 FEG TEM operated at 200 kV. UV-vis spectra were obtained from diffuse reflectance measurements using an integrating sphere on a LAMBDA 750 UV-vis spectrophotometer. The reflectance spectrum of Zn substrate was used as reference. The current density-voltage (J-V) characteristics were recorded with a Ketheley 2400 SourceMeter. The assembled cells were illuminated using a solar simulator at AM 1.5 G, where the light intensity was adjusted with a NREL-calibrated Si solar cell with a KG-5 filter to 1 sun intensity (100 mWcm-2).
Results and discussion
The most widely used fabrication method to obtain vertically aligned ZnO nanostructures is the hydrothermal method . ZnO nanowires on F-doped SnO2 (FTO) or In-doped SnO2 (ITO) substrates are typically prepared by a two-step approach involving the coating of a substrate with ZnO seed nanoparticles which serve as nucleation sites for the formation of nanowires under hydrothermal treatment [21, 22]. Thus, the fabricated ZnO-covered transparent conducting electrode serves as photoanode, through which the cell is illuminated . One of the disadvantages is that the resistance of FTO or ITO glass becomes larger after growing ZnO, which can be detrimental for electronic transport . One-step methods to grow ZnO nanostructures on metal substrates such as Zn foil have been also reported [24, 25], given advantage of an easier fabrication (no seeds employed) and lower resistance of substrates.
Photovoltaic performance of QDSC made of the vertically aligned ZnO nanorods fabricated on Zn substrates and decorated with CdS and CdSe QDsM
ZnO/(CdS and CdSe)
ZnO/CdSe (in situ grown)
In summary, ZnO nanorods have been grown on Zn substrate by a simple one-step solution-based approach allowing for large-scale, low cost fabrication of vertically aligned arrays. The decoration of ZnO nanorods with CdS and CdSe QDs has been achieved by using aqueous-based QDs capped with a short ligand thioglycolic acid serving as molecular linker to ZnO nanorod surface. The photovoltaic performance of ZnO nanorods on Zn foil decorated with CdS and CdSe QDs has been evaluated in a photoelectrochemical solar cell configuration with a liquid triiodide/iodide electrolyte. The simultaneous use of CdS and CdSe QDs results in a higher open circuit voltage and short circuit current for co-sensitized structure in spite of a low FF. Power conversion efficiencies of approximately 1% were achieved using ZnO/Al2O3/CdSe electrode with an improved FF of 0.55. We have shown that simultaneous growth of the ZnO NRAs in the presence of CdSe QDs is possible and results in the improved V OC without compromising J SC and FF. Further work is on the way to develop devices with stable performance using, for example, TiO2 amorphous coating encapsulating the QDs , or QDs layer deposited on ZnO followed by a layer of Ruthenium dye. The last approach would benefit from the already mentioned advantage of improved light harvesting and charge extraction [31, 32], and at the same time alleviate the problem of photocorrosion for QDs in contact with I3 -/I- redox couple and the problem of the instability of ZnO in contact with acidic dye .
atomic layer deposition
fast Fourier transform
high-resolution transmission electron microscope
quantum dot sensitized solar cells
scanning electron microscopy
transmission electron microscope
This work was supported by GRF projects 102810 and 103208 from the Research Grants Council of Hong Kong.
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