Facile solution growth of vertically aligned ZnO nanorods sensitized with aqueous CdS and CdSe quantum dots for photovoltaic applications

  • Chunyan Luan1, 2,

    Affiliated with

    • Aleksandar Vaneski1, 3,

      Affiliated with

      • Andrei S Susha1, 3,

        Affiliated with

        • Xueqing Xu1, 3,

          Affiliated with

          • Hong-En Wang1, 2,

            Affiliated with

            • Xue Chen1, 2,

              Affiliated with

              • Jun Xu1, 2,

                Affiliated with

                • Wenjun Zhang1, 2,

                  Affiliated with

                  • Chun-Sing Lee1, 2,

                    Affiliated with

                    • Andrey L Rogach1, 3Email author and

                      Affiliated with

                      • Juan Antonio Zapien1, 2Email author

                        Affiliated with

                        Nanoscale Research Letters20116:340

                        DOI: 10.1186/1556-276X-6-340

                        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 [1] and sensors [2] to field-emission devices [3]. 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 [6]. 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 [4]. Low cost and large-scale chemical solution-based techniques have been developed to synthesize anisotropic single crystalline ZnO nanostructures on a variety of substrates [5]. 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 [4]. On the other hand, semiconductor nanocrystal quantum dots (QDs) [7] have been considered as promising photosensitizers for TiO2 and ZnO-based quantum dot sensitized solar cells (QDSCs) [8] 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 [812]. 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 [15] 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% [16]. Multilayers of TGA-capped CdTe QDs have been deposited on ZnO nanorods in combination with positively charge polyelectrolyte to improve the light-harvesting ability [17].

                        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 [18]. In the alkaline solution, carboxylic groups of TGA are deprotonated, serving as anchor points and facilitating the binding of QDs to the oxide surface [15]. 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 [19].

                        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 [20]. 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 [5]. 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 [4]. 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 [23]. 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.

                        We have modified a previously reported method of growing large-scale, vertically aligned ZnO NRAs on Zn substrates by a one-step solution-based approach [25]. In the simplified method, this process is improved by employing pure water without the use of additives like the previously used ammonia or hydrogen peroxide. Figure 1a shows the XRD pattern of ZnO NRAs grown on Zn foil. All diffraction peaks can be indexed to hexagonal wurtzite ZnO phase (JCPDS card No. 36-1451) except those marked with * which originate from the zinc substrate (JCPDS card No. 04-0831). Morphological characterization by SEM, Figure 1b, indicates the formation of arrays of ZnO NRs with a preferential growth direction nearly perpendicular to the Zn substrate. The nanorods are uniform in length (~3 μm) and possess a characteristic hexagonal cross-section with diameter in the range of approximately 50 to 450 nm. Higher magnification of a single ZnO nanorod by TEM is shown in Figure 1c; the corresponding FFT pattern indicates that the hexagonal ZnO nanorod grows along the [001] direction. The growth direction is further confirmed by high-resolution TEM image, shown in Figure 1d, which exhibits well-resolved fringes in directions parallel and perpendicular to the nanorod axis, further confirming that the ZnO nanorod is single crystalline. The lattice fringe spacing are 0.524 and 0.287 nm, which agree well with the interplanar spacing of the (001) and (100) planes of hexagonal (wurtzite) ZnO crystals. Selected ZnO NRAs were further treated with an Al2O3 coating prepared by 15 cycles of an ALD process. Figure 1e shows conformal Al2O3 deposition with average film thickness approximately 2 nm.
                        Figure 1

                        Structural characterisation of ZnO NRAs. (a) XRD spectra of ZnO nanorods on Zn substrate and of the bare Zn substrate; (b) top-view SEM image of vertically aligned ZnO nanorods; (c) TEM image of a ZnO nanorod; the inset shows the corresponding FFT pattern that identifies the growth direction as the (001) axis; (d) HRTEM image of a ZnO nanorod showing the lattice spacing and confirming the growth direction; and (e) HRTEM image of a ZnO nanorod coated with a approximately 2 nm Al2O3 film prepared by ALD.

                        Decoration of the ZnO NRAs with QDs results in significant increase in their surface roughness as presented for the case of CdSe QDs in the SEM image of Figure 2a; furthermore, it is obvious that coverage of the ZnO NRAs has been achieved over large areas. Closer inspection, by TEM of a single ZnO NR in Figure 2b, demonstrates almost complete surface coverage by the CdSe QDs. Figure 2c shows a representative HRTEM image taken at the nanorod's edge and provides further evidence that single crystalline QDs with 2-2.5 nm diameter are directly and tightly attached to the ZnO surface.
                        Figure 2

                        Structural characterization of QD-decorated ZnO NRAs. (a) Top-view SEM image of ZnO nanorods decorated with CdSe QDs; (b) TEM of a ZnO nanorod coated with CdSe QDs; (c) HRTEM image of CdSe QDs on a ZnO nanorod, indicated by white circles.

                        We have also grown ZnO NRAs in situ in a water bath containing CdSe QDs. The SEM image in Figure 3a shows that uniform NRAs with orientation which is even closer to the substrate's normal as compared to the pristine ZnO NRAs in Figure 1b. A higher magnification SEM of several NRs detached from the substrate, inset in Figure 3a, shows that the CdSe QDs are incorporated into the ZnO surface resulting in a larger surface roughness. The surface coverage of ZnO with QDs is also confirmed by TEM as shown in Figure 3b. As expected, the incorporation of QDs during ZnO NR growth results in more intimate contact between the ZnO NRs' surface and the QDs as observed in the HRTEM images of Figure 3c,d.
                        Figure 3

                        Structural characterization of ZnO NRAs grown in-situ in CdSe QD solution. (a) SEM image (inset: higher magnification SEM of several nanorods detached from the Zn substrate); (b) TEM image of a ZnO nanorod from sample in (a); (c, d) HRTEM images of a ZnO nanorod surface with incorporated CdSe QDs.

                        Figure 4 shows diffuse reflectance absorption spectra of ZnO NRAs prior and after decoration with CdS and CdSe QDs. The absorption in Kubelka-Munk units of the different ZnO electrodes sensitized with QDs has been extracted from their diffuse reflectance using the relation: F(R) = (1-R)2/2R, where R is the measured diffuse reflectance. This presentation allows a direct comparison of the amount of QDs adsorbed on each sample. The intrinsic absorption of ZnO nanorods can be seen as a steep increase below 400 nm. The QD decorated ZnO NRAs show increased absorption from 400 to 450 nm for CdS and from 400 to 520 nm for CdSe, respectively, with additional features around 425 and 480 nm due to the size-dependent electronic transitions of QDs. These spectral features closely match the absorption maxima of aqueous colloidal solutions of CdS and CdSe QDs, inset in Figure 4, with a slight red-shift which is likely caused by the close packing of QDs deposited on ZnO [26]. From the data of Figure 4, we estimate approximately the same amount of CdS and CdSe QDs adsorbed on ZnO NRAs, which is also expected owing to their same surface ligands and a similar concentration of nanoparticles in solution. The absorption features of the both QDs materials are present for samples sequentially immersed in CdS and in CdSe QD solutions. Remarkably, the in situ ZnO NRA growth in a bath solution containing CdSe QDs results in a strong absorption enhancement with respect to the NRAs decorated with QDs.
                        Figure 4

                        Kubelka-Munk diffuse reflectance absorption spectra of different samples. ZnO nanorods (open circle), ZnO nanorods decorated with CdS QDs (filled circle), ZnO nanorods decorated with CdSe QDs (open triangle), ZnO nanorods decorated with both CdS and CdSe QDs (filled triangle), and ZnO nanorods grown in an aqueous solution containing CdSe QDs (square). The absorbance spectra of the pristine CdS and CdSe QDs in aqueous solutions used for the NRAs treatment are shown in inset.

                        The photovoltaic performance of QDSCs based on the vertically aligned ZnO NRAs fabricated on Zn substrates and decorated with CdS and CdSe QDs has been evaluated in a photoelectrochemical cell configuration with a liquid electrolyte containing the I3 -/I- redox couple with illumination through the Pt-coated FTO glass. The current density-voltage (J-V) characteristics for solar cells assembled from ZnO NRAs with and without QDs, which represent the performance averaged over 3-5 sample preparations are shown in Figure 5. For the non-sensitized ZnO NRAs based solar cells (curve 1 in Figure 5), the open-circuit voltage (V OC) and short circuit current (J SC) are low. The solar cells decorated with CdSe QDs (curve 3) perform better than those with CdS QDs (curve 2) in terms of the open-circuit voltage and the short-circuit current density, resulting in higher power conversion efficiency (0.46% for CdSe vs. 0.29% for CdS), which is attributed to the broader light absorption of CdSe compared with that of CdS (Figure 4). On the other hand, the FF of CdSe QDs based solar cells is smaller than that of CdS based solar cells, which can be related to the different electron recombination mechanism for the ZnO/CdS and ZnO/CdSe electrodes [27]. At the same time, as CdS solar cells present lower photocurrents, the voltage drop in the series resistance is lower for this type of solar cells, which also enhance the FF. The best performance has been achieved for ZnO nanorods simultaneously decorated with both CdS and CdSe QDs (curve 4), with V OC of 0.68 V and J SC of 4.36 mA/cm2. This shows an advantage of synergetic employment of different types of QDs as it has been discussed recently [12, 28]. The performance of the CdSe QD decorated solar cell could be further improved by introducing an ultrathin (~2 nm) layer of Al2O3 grown by ALD on the surface of ZnO nanorods prior to their decoration with QDs. The deposition of Al2O3 shell resulted in a slight increase in V OC and decrease in J SC (curve 5 in Figure 5). However, the FF significantly improved from 0.24 to 0.55, leading to about 50% increase in power conversion efficiency from 0.46 to 0.99%. The increase in V OC has been attributed to the reduction of the electron recombination at the semiconductor/electrolyte interface with the passivation of the recombination sites at the ZnO surface by the Al2O3 coating, while the decrease in J SC resulted from partial inhibition of the electron injection at the semiconductor/QD interfaces [4, 29]. Besides, the charge transfer at the Zn/electrolyte interface has been retarded resulting in a platform of the J-V curve for the lower potentials. Accordingly, an intimate contact between the ZnO NRAs and the CdSe QDs (as for the ZnO NRAs in situ grown in the presence of CdSe QDs which are presented in Figure 3c,d) could lead to an increase in V OC and FF due to the eliminated recombination sites at the ZnO/CdSe interface. Indeed this is observed in Figure 5 (curve 6), where V OC has further increased to 0.72 V without significant change in J SC (~2.81 mA/cm2). The performance characteristics of the solar cells illustrated in Figure 5 are summarized in Table 1.
                        Figure 5

                        J-V characteristics of different QDSC. ZnO nanorods (curve 1: filled circle), ZnO nanorods decorated with CdS QDs (curve 2: open circle), ZnO nanorods decorated with CdSe QDs (curve 3: filled triangle), ZnO nanorods decorated with both CdS and CdSe QDs (curve 4: open triangle), ZnO nanorods coated with Al2O3 and then decorated with CdSe QDs (curve 5: filled square), and ZnO nanorods in situ grown in a solution containing CdSe QDs (curve 6: open square).

                        Table 1

                        Photovoltaic performance of QDSC made of the vertically aligned ZnO nanorods fabricated on Zn substrates and decorated with CdS and CdSe QDsM


                        V OC(V)

                        J SC(mA/cm2)


















                        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 [30], 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 [32].



                        atomic layer deposition


                        fast Fourier transform


                        fill factor


                        high-resolution transmission electron microscope


                        nanorod arrays


                        quantum dots


                        quantum dot sensitized solar cells


                        scanning electron microscopy


                        transmission electron microscope


                        thioglycolic acid


                        X-ray diffraction.



                        This work was supported by GRF projects 102810 and 103208 from the Research Grants Council of Hong Kong.

                        Authors’ Affiliations

                        Department of Physics and Materials Science, City University of Hong Kong
                        Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong
                        Centre for Functional Photonics (CFP), City University of Hong Kong


                        1. Bagnall DM, Chen YF, Zhu Z, Yao T, Koyama S, Shen MY, Goto T: Optically pumped lasing of ZnO at room temperature. Appl Phys Lett 1997, 70: 2230–2232. 10.1063/1.118824View Article
                        2. Mitzner KD, Sternhagen J, Galipeau DW: Development of a micromachined hazardous gas sensor array. Sens Actuators B 2003, 93: 92–99. 10.1016/S0925-4005(03)00244-2View Article
                        3. Wang ZL, Song JH: Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 2006, 312: 242–246. 10.1126/science.1124005View Article
                        4. Zhang QF, Dandeneau CS, Zhou XY, Cao GZ: ZnO nanostructures for dye-sensitized solar cells. Adv Mater 2009, 21: 4087–4108. 10.1002/adma.200803827View Article
                        5. Gonzalez-Valls I, Lira-Cantu M: Vertically-aligned nanostructures of ZnO for excitonic solar cells: a review. Energy Environ Sci 2009, 2: 19–34. 10.1039/b811536bView Article
                        6. O'Regan B, Grätzel M: A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO 2 films. Nature 1991, 335: 737–739.View Article
                        7. Rogach AL, (Ed): Semiconductor Nanocrystal Quantum Dots: Synthesis, Assembly, Spectroscopy and Applications. New York: Springer Wien; 2008.
                        8. Rühle S, Shalom M, Zaban A: Quantum-dot-sensitized solar cells. Chem Phys Chem 2010, 11: 2290–2304.
                        9. Mora-Sero I, Bisquert J, Dittrich Th, Belaidi A, Susha AS, Rogach AL: Photosensitization of TiO 2 layers with CdSe quantum dots: correlation between light absorption and photoinjection. J Phys Chem C 2007, 111: 14889–14892. 10.1021/jp074907wView Article
                        10. Kamat PV: Quantum dot solar cells: semiconductor nanocrystals as light harvesters. J Phys Chem C 2008, 112: 18737–18753.View Article
                        11. Lee HJ, Leventis HC, Moon S, Chen P, Ito S, Haque SA, Torres T, Nüesch F, Geiger T, Zakeeruddin SM, Grätzel M, Nazeeruddin MK: PbS and CdS quantum dot-sensitized solid-state solar cells: "old concepts, new results". Adv Funct Mater 2009, 19: 2735–2742. 10.1002/adfm.200900081View Article
                        12. Mora-Sero I, Bisquert J: Breakthroughs in the development of semiconductor sensitized solar cells. J Phys Chem Lett 2010, 1: 3046–3052. 10.1021/jz100863bView Article
                        13. Leschkies S, Divakar R, Basu J, Enache-Pommer E, Boercker JE, Carter CB, Kortshagen UR, Norris DJ, Aydil ES: Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices. Nano Lett 2007, 7: 1793–1798. 10.1021/nl070430oView Article
                        14. Sun XW, Chen J, Song JL, Zhao DW, Deng WQ, Lei W: Ligand capping effect for dye solar cells with a CdSe quantum dot sensitized ZnO nanorod photoanode. Opt Exp 2010, 18: 1296–1301. 10.1364/OE.18.001296View Article
                        15. Robel I, Subramanian V, Kuno M, Kamat PV: Quantum dot solar cells. harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO 2 films. J Am Chem Soc 2006, 128: 2385–2393. 10.1021/ja056494nView Article
                        16. Chen J, Zhao DW, Song JL, Sun XW, Deng WQ, Liu XW, Lei W: Directly assembled CdSe quantum dots on TiO 2 in aqueous solution by adjusting pH value for quantum dot sensitized solar cells. Electrochem Commun 2009, 11: 2265–2267. 10.1016/j.elecom.2009.10.003View Article
                        17. Briscoe J, Gallardo DE, Hatch S, Lesnyak V, Gaponik N, Dunn S: Enhanced quantum dot deposition on ZnO nanorods for photovoltaics through layer-by-layer processing. J Mater Chem 2011, 21: 2517–2523. 10.1039/c0jm02279aView Article
                        18. Rogach AL, Kornowski A, Gao M, Eychmüller A, Weller H: Synthesis and characterization of a size series of extremely small thiol-stabilized CdSe nanocrystals. J Phys Chem B 1999, 103: 3065–3069. 10.1021/jp984833bView Article
                        19. Fang YP, Pang Q, Wen XG, Wang JN, Yang SH: Synthesis of ultrathin ZnO nanofibers aligned on a zinc substrate. Small 2006, 2: 612–615. 10.1002/smll.200500379View Article
                        20. Hodes G: Comparison of dye- and semiconductor-sensitized porous nanocrystalline liquid junction solar cells. J Phys Chem C 2008, 112: 17778–17787. 10.1021/jp803310sView Article
                        21. Greene LE, Law M, Tan DH, Montano M, Goldbergger J, Somorjai G, Yang PD: General route to vertical ZnO nanowire arrays using textured ZnO seeds. Nano Lett 2005, 5: 1231–1236. 10.1021/nl050788pView Article
                        22. Wu JJ, Wen HI, Tseng CH, Liu SC: Well-aligned ZnO nanorods via hydrogen treatment of ZnO films. Adv Funct Mater 2004, 14: 806–810. 10.1002/adfm.200305092View Article
                        23. Mukherjee K, Teng TH, Jose R, Ramakrishna S: Electron transport in electrospun TiO 2 nanofiber dye-sensitized solar cells. Appl Phys Lett 2009, 95: 012101–012103. 10.1063/1.3167298View Article
                        24. Tang Q, Zhou WJ, Shen JM, Zhang W, Kong LF, Qian YT: A template-free aqueous route to ZnO nanorod arrays with high optical property. Chem Commun 2004, 712–713. 10.1039/b313387g
                        25. Yang ZH, Luan CY, Zhang WX, Liu AP, Tang SP: Fabrication and optical properties of ZnO nanostructured thin films via mechanical oscillation and hydrothermal method. Thin Solid Films 2007, 516: 5974–5980. 10.1016/j.tsf.2007.10.085View Article
                        26. Gaponik NP, Talapin DV, Rogach AL: A light-emitting device based on a CdTe nanocrystal/polyaniline composite. Phys Chem Chem Phys 1999, 1: 1787–1789. 10.1039/a808619bView Article
                        27. González-Pedro V, Xu XQ, Mora-Seró I, Bisquert J: Modeling high-efficiency quantum dot sensitized solar cells. ACS Nano 2010, 4: 5783–5790.View Article
                        28. Lee YL, Lo YS: Highly efficient quantum-dot-sensitized solar cell based on co-sensitization of CdS/CdSe. Adv Funct Mater 2009, 19: 604–609. 10.1002/adfm.200800940View Article
                        29. Fabregat-Santiago F, Garcia-Canadas J, Palomares E, Clifford JN, Haque SA, Durrant JR, Garcia-Belmonte G, Bisquert J: The origin of slow electron recombination processes in dye-sensitized solar cells with alumina barrier coatings. J Appl Phys 2004, 96: 6903–6907. 10.1063/1.1812588View Article
                        30. Shalom M, Dor S, Rühle S, Grinis L, Zaban A: Core/CdS quantum dot/shell mesoporous solar cells with improved stability and efficiency using an amorphous TiO 2 coating. J Phys Chem C 2009, 113: 3895–3898. 10.1021/jp8108682View Article
                        31. Mora-Sero I, Gross D, Mittereder T, Lutich AA, Susha AS, Dittrich T, Belaidi A, Caballero R, Langa F, Bisquert J, Rogach AL: Nanoscale interaction between CdSe or CdTe nanocrystals and molecular dyes fostering or hindering directional charge separation. Small 2010, 6: 221–225. 10.1002/smll.200901494View Article
                        32. Ganesh T, Mane RS, Cai G, Chang JH, Han SH: ZnO nanoparticles--CdS quantum dots/N3 dye molecules: dual photosensitization. J Phys Chem C 2009, 113: 7666–7669. 10.1021/jp901224nView Article


                        © Luan et al; licensee Springer. 2011

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