N3 Dye Molecules: Dual

Apr 9, 2009 - We developed dual photosensitization, in which CdS quantum dots serve as a sandwich layer between the ZnO nanoparticles (NPs) and N3 ...
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J. Phys. Chem. C 2009, 113, 7666–7669

ZnO Nanoparticles-CdS Quantum Dots/N3 Dye Molecules: Dual Photosensitization T. Ganesh, Rajaram S. Mane,† Gangeri Cai, Jin-Ho Chang, and Sung-Hwan Han* Inorganic Nano-Materials Laboratory, Department of Chemistry, Hanyang UniVersity, Seongdong-Gu, Haengdang 17, Seoul 133-791, Republic of Korea ReceiVed: February 10, 2009; ReVised Manuscript ReceiVed: March 12, 2009

We developed dual photosensitization, in which CdS quantum dots serve as a sandwich layer between the ZnO nanoparticles (NPs) and N3 dye molecules, alleviating the serious issues of CdS photocorrosion and the Zn2+/N3 dye complex formation leading to insulating surface aggregation. The improvement in the solarto-electrical conversion efficiency from 0.11 and 0.85 to 3.04% when compared to ZnO/N3 and ZnO-CdS, respectively, is attributed to the effective coupling between the ZnO NPs/N3 dye molecules in the presence of CdS QDs, which in turn enhances the rate of electron injection due to the faster electron injection and transfer rate. Strong ZnO NPs-CdS QDs linking is explored systematically using UV-vis spectroscopy and is interpreted with a charge transfer resistance obtained from impedance spectroscopy studies. Introduction Dye-sensitized solar cell (DSSCs) is a conspicuous member of the larger group of thin film photovoltaic systems, which operate differently from other modes, that is, artificial photosynthesis. The sensitizer, dye molecule functions like chlorophyll in plants, absorbs the incident light and generates charge carriers in conjunction with wide band gap semiconductors having mesoporous or nanocrystalline structure. Most of the dyesensitized solar cells to date have focused on TiO2 with a solarto-electrical energy conversion efficiency (η) of 11% and have intensely scrutinized other wide band gap semiconductors.1 The ZnO-based photoanodes resemble the TiO2 photoanodes with their similar band gaps and conduction band positions, but with different electronic properties they have been considered for their good electron mobility (ZnO, 115-155 cm2 V-1 s-1; TiO2, ∼10-5 cm2 V-1 s-1).2 However, its poor stability in acidic ruthenium(II) cis-di(thiocyanato)bis(2,2′-bypyridyl-4,4′-dicarboxylic acid) (N3) dye leads to the formation of Zn2+/dye complex, a relatively nonconductive/insulating layer that generally blocks the overall electron injection efficiency of the dye molecules, produces inferior device conversion efficiency.3 To utilize ZnO as an electron-accepting layer in DSSCs, the chemical protection of ZnO surface from acidic environment is crucial for the formation of ideal interfacial contact between the ZnO and the dye molecules. Incorporation of semiconductor quantum dots (QDs) with a conduction band level comparatively higher than the electron transporting semiconducting layer, photoanode, facilitates the passage of photogenerated electrons into the photoanode and impedes back transfer of electrons to a great extent, whereas the holes, back transferred from the n-type QDs to the redox electrolyte (I-/I3- redox couple in organic solvent), are regenerated at the counter electrode (CE) interface. If the semiconductor material is chemically unstable in the acidic media, it is essential to use QDs on the surface of ZnO to protect the ZnO surface, which ultimately increases the performance of the DSSCs.4 Herein, in this article, a simple ZnO surface passivation approach using CdS QDs is explored. * Corresponding author. Phone: + 822 2292 5212. Fax: +822 2299 0762. E-mail: [email protected]. † Present address: Clarendon Laboratory, Department of Physics, Oxford University, Oxford OX1 3PU, UK.

This approach not only provides an effective linkage between dye molecules and ZnO NPs surface but also prevents the ZnO surface from the surface aggregation effect, generally limiting the electron transport from dye molecule to semiconductor nanostructures.5 It is also inferred that the surface passivation is found to have an important significant impact on charge transfer resistance (Rct). Apart from organic dye molecules, semiconductor QDs that absorb light in the visible region as sensitizers to improve the conversion efficiencies of DSSCs have also been reported.6 By embedding CdS QDs between the ZnO NPs and N3 dye molecules, surface aggregation can be prevented, and also the additive (dual photosensitization) hybrid organic/inorganic photosensitization effect is expected. Second, CdS is a well-known photocorrosive material on account of CdS + hν f CdS (h+ + e-) and 2h+ + CdS f S + Cd2+, where h+, e-, and S denote a hole in the valence band, electron in the conduction band, and a sulfur atom deposited on the electrode surface, respectively.7 When the CdS QDs-coated ZnO comes in contact with an electrolyte, photocorrosion can be prevented to a great extent when covered by dye molecules. Herein, using this novel approach, we report a few orders enhancement in energy conversion efficiency of ZnO. The rationale for this change is explored thoroughly by considering surface-related characteristics such as change in morphology, optical density, impedance, and, finally, current density versus applied voltage (J-V) measurements. Experimental Details To prepare ZnO electrode of nanoparticles (purchased from Alfa Aesar), ZnO, NanoShield ZN-5060, 50% in H2O, colloidal dispersion was preferred as received. Via a drop-cast method, the ZnO NPs electrodes were prepared on to a cleaned indium-tin oxide (ITO) glass substrates by fixing the rotary pump at 10-3 Torr and adjusting the speed of the motor to 2000 rpm. Electrodes were then annealed at 300 °C for 1 h in air to remove the residual solvents and organic chemicals on ZnO NPs. CdS QDs were synthesized through a wet chemical process using 0.01 M CdCl2 and 0.01 M thiourea for 60 min and for successive steps at room temperature. The ZnO NPs-CdS QDs (ZnO-CdS) electrodes were annealed in the presence of air at 200 °C for 30 min. The ZnO NPs and ZnO-CdS electrodes

10.1021/jp901224n CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009

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Figure 1. (a) The SEM morphologies of ZnO NPs of irregular dimensions; left and right insets indicate the cross-section and ZnO NPs at high resolution, respectively. (b) ZnO NPs-CdS QDs; the right inset indicates the ZnO NPs-CdS QDs at high resolution. (c) ZnO-CdS film after 20 h dipping in N3 dye. (d) ZnO NPs after 20 h dipping in N3 dye.

were intentionally sensitized for 20 h into 0.3 mM N3 dye solution (1:1 mixture of acetonitrile and tert-butyl alcohol) for understanding the surface aggregation effect. The ZnO NPs and CdS QDs were examined with a Philips Japan MPD 1880 X-ray powder diffractometer (XRD) using Cu KR radiation (V ) 40 kV and I ) 100 mA), along with scanning electron microscopy (SEM) JEOL. To gain information about CdS QDs, energydispersive X-ray analysis (EDX) measurement, linked to SEM unit, was elucidated. To confirm the change in band-bending positions, the UV-vis spectra of ZnO NPs, ZnO + N3, and ZnO-CdS + N3 systems were recorded using a Cary Japan model 100 CONC spectrophotometer. The influence of charge transport and recombination on the performance of DSSCs using electrochemical impedance spectroscopy (EIS) and transient voltage decay measurement, which is based on the measurement of the current response to a harmonically modulated voltage superimposed on a constant applied bias voltage, and the resulting frequency analysis typically reveals three wellseparated semicircles in the Nyquist diagram, was recently studied by Gratzel et al.,8 as EIS is a useful technique that has been widely employed to investigate the charge transfer kinetic process occurring in the DSSCs in the dark. Therefore, the EIS measurement was performed using a BAS-Zahner IM6 Impedance analyzer. The potential amplitude of ac was applied at 10 mV; meanwhile, its frequency region was maintained from 0.01 Hz to 100 kHz. Electrolyte of 0.1 M tetrabutylammonium tetrafluoroborate (TBA/TFB) in acetonitrile solution purged in argon gas for 15 min was used along with platinum spiral wire and Ag/AgCl as the counter and the reference electrodes, respectively. The applied potential for EIS measurement was taken from the open circuit voltage versus time from the cyclic voltammetry (Bioanalytical systems Inc.), using the same solvent in the presence and in the absence of light. The ZnO NPs and ZnO-CdS electrodes sensitized in dye were sandwiched with 100 nm thick Pt-sputtered ITO electrode, separated by Surlynbased polymer sheet (thickness: 80 µm), and sealed, into which

Figure 2. The XRD pattern of ZnO-CdS film on indium-tin oxide substrate. The inset indicates the EDX analysis confirming the presence of CdS on ZnO NPs.

an electrolyte solution was infiltrated using a fine 10 mL nontoxic Kovax syringe. The liquid electrolyte contained 0.6 M 1-hexyl-2,3-dimethyl-imidazolium iodide (C6DMI), 0.1 M lithium iodide (LiI), 0.05 M iodine (I2), and 0.5 M 4-tertbutylpyridine (t-BPy) in 15 mL of methoxyacetonitrile (98%) solvent. DSSCs measurements were performed with a photointensity of 80 mW/cm2 with an effective electrode area of 0.28 cm2. Results and Discussion The electrode thickness of ∼2.5 µm of ZnO was found to be similar in the absence and in the presence of CdS QDs with a noticeable change in surface texture under the close resolution (inset of Figure 1a, left). The coarse surface of ZnO NPs is

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Figure 3. (a) UV-vis optical density spectra of ZnO NPs (black), CdS QDs deposition on ZnO NPs through wet chemical process for every subsequent 15 min time interval. (b) Optical density spectra of ZnO NPs, ZnO/N3, ZnO-CdS, and ZnO-CdS/N3 dye systems, respectively.

Figure 4. (a) C-V curves of indium-tin oxide, CdS QDs, ZnO NPs, and ZnO-CdS electrodes, obtained in the electrolyte containing 0.1 M TBA/TFB in acetonitrile, at a scan rate of 50 mV s-1. (b) Nyquist plots of the ZnO NPs, CdS QDs, and ZnO-CdS photoanodes with Pt as the counter electrode and Ag/AgCl as the reference electrode. The plots were recorded over a frequency range of 0.01 Hz to 100 kHz, with ac amplitude of 10 mV and applied voltage taken from the cyclic voltammogram of Voc versus time, in the presence and in the absence of light.

changed to smooth due to the incorporation of CdS QDs as seen in Figure 1a and b, inset, right. No appreciable surface change in ZnO-CdS electrode surface after N3 loading is noticed, which qualitatively supports the fact that an incorporation of CdS QDs prevents the ZnO surface from surface damage when dipped in N3 dye even for 20 h. This could be observed in relevance to the clear aggregation effect of N3 dye on ZnO NPs by the formation of Zn2+/N3 complex, as seen in Figure 1d, whereas Figure 1c verifies the aggregation-free effect of CdS QDs between ZnO NPs and N3 dye molecules. The coarse surfaces observed on the ZnO NPs were due to the pores; these pores are generally filled with Zn2+/N3 precipitates (inactive dye molecules).9 The CdS QDs fill these pores by acting as an effective coupler between ZnO NPs and N3 dye molecules. Furthermore, when compared to our previous results for ZnO rods, the (002) peak intensity of ZnO NPs observed in the XRD analysis shown in Figure 2 is less dominant than (110) and (101) peak intensities, indicating that the present ZnO electrode could be more conducting on account of their oxygen vacancies.10 The XRD analysis of the ZnO NPs protrudes the amorphous nature of CdS QDs on ZnO NPs. The EDX spectrum for Cd and S shows a 48:52 ratio, supporting the formation of CdS QDs on ZnO NPs (Figure 2, inset). This clearly indicates that on ZnO NPs, CdS QDs were located. The change in optical density in the parent ZnO electrode UV-vis spectrum with CdS QDs immersion time is shown in Figure 3a. The optical density change was recorded at every 15 min interval of the CdS QDs growth. An increase in optical density was observed that conveys the growth of CdS QDs, with every subsequent time interval, which was proportional to the CdS QDs incorporation time. The coarse ZnO NPs generally offer a high surface area and pores equivalent to the size of the CdS QDs. An impulsive increase in absorbance observed between 45 and 60 min replenishes the uncovered surface area of ZnO NPs with CdS QDs and further

increase in optical density increases the size of the CdS QDs.11 Figure 3b investigates the optical density changes in the presence of N3, CdS QDs, and CdS + N3 on ZnO NPs film. The optical density of ZnO + N3 is less intense than with ZnO-CdS and ZnO-CdS + N3. The CdS QDs incorporated on ZnO NPs present more optical density than that of ZnO NPs film, due to the sensitizing property of CdS QDs. Further increased optical density was observed with N3 dye on ZnO NPs-CdS QDs (ZnO-CdS) substantiates the linkage of N3 dye molecules with CdS QDs.12 The cyclic voltammetry (C-V) measurement performed in acetonitrile solvent with 0.1 M TBA/TFB in the potential region from 0 to -1.8 V, at a scanning rate of 50 mV/s, confirms the the presence of CdS involvement on ZnO NPs, the reduction peaks of ZnO and CdS, are clearly visible in ZnO-CdS, shown in Figure 4a. The extremely large current density in ZnO-CdS is supposed to be from the synergistic effect, which was further confirmed by the EIS measurement. The Rct of ZnO-CdS electrode is considerably lesser than only the CdS QDs or the ZnO NPs (Figure 4b). Here, the solvent conditions were the same as those used in the C-V measurement. The extremely large current density observed in C-V concords with the least Rct offered by the synergistic effect in ZnO-CdS electrode. The photovoltaic behavior observed in these electrodes through current density versus applied voltage (J-V) characteristics provided substantial evidence and involvement of CdS QDs and N3 dye on ZnO NPs for increased photocurrent and therein efficiency (shown in Figure 5a). The curve corresponding to ZnO/N3 dye indicates a short circuit current (Jsc) ) 1.01 mA/ cm2, an open circuit voltage (Voc) ) 0.44 V, fill factor (ff) ) 0.20, and η ) 0.11%. The low fill factor is due to high series resistance of the solar cell, attributed to the high sheet resistance of the ZnO,13 in this case, insulating ZnO/N3 dye complex or aggregates, which is also supported by Rct (discussed later). The

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Figure 5. (a) J-V characteristics of ZnO/N3, ZnO-CdS, and ZnO-CdS/N3 photoanodes in the presence of redox electrolyte with a photointensity of 80 mW/cm2. (b) Nyquist plots of the DSSCs with ZnO/N3, ZnO-CdS, and ZnO-CdS/N3 photoanodes fabricated with the Pt counter electrodes and I-/I3- redox electrolyte. The right inset magnifies the plot of ZnO-CdS- and ZnO-CdS/N3-based DSSCs.

trend changed completely by the incorporation of CdS QDs on ZnO NPs, as an increased photocurrent density of 3.65 mA/ cm2, Voc of 0.60 mV, and ff of 0.30 were obtained to reach an efficiency of 0.85%. This change in solar to electrical energy conversion efficiency is attributed to the presence of the CdS far UV light absorbing layer.6 Here, the Jsc value is higher than that of previously reported ZnO nanowires-CdSe-based quantum dot photoelectrochemical cells.14 Henceforth, the surface recombination has a significant effect on the Jsc and Voc values of the DSSCs. The CdS QDs act as a surface passivation layer on the ZnO film to reduce the impact of acidic N3 dye in the formation of Zn2+/N3 complex, and also to reduce the surface recombination and to increase the forward bias current. The electrons injected from the N3 dye molecules into the CdS QDs conduction band transfer the electrons to ZnO NPs rather than direct transfer from N3 dye molecules to the ZnO NPs. The holes are readily transferred from the dye to the imidazolium electrolyte (I-/I3-). Combined with its light absorption characteristics in the visible region of N3 molecules, enhancement in performance with Jsc ) 12.32 mA/cm2, ff ) 0.34, Voc ) 0.54 mV, and η ) 3.04%, less than and comparable to others,15 is observed. The ff value was increased from 0.20 for ZnO/N3 to 0.34 for ZnO-CdS/N3, indicating a reduction in the effective series resistance of ZnO/N3 electrode in the presence of CdS QDs. The increase in Voc value is attributed to favorable intermediate conduction band levels of CdS QDs and provides an effective coupling and therefore facilitates fast charge injection kinetics. The impedance measured in dye-sensitized ZnO NPs-based solar cells is interpreted in terms of Rct (Figure 5b), which is found to be a crucial feature for describing the dynamic operation of DSSCs based on ZnO NPs. The initial lag in high frequency region was considered to be the sheet resistance offered by the transparent conducting oxide, that is, ITO in the present case. The smaller semicircle observed at the high frequency region is supposed to be from the Rct of the counter electrode-electrolyte interface, that is, reduction of I3to 3I- at the CE interface. The semicircle 1 is smaller when the conversion of I- to I3- is faster and impedes the Rct. Comparably, the intermediate semicircle 2 is larger, implying the Rct of the working electrode-electrolyte interface is much slower than at the CE interface; that is, the reduced dye gets electrons or regenerated by the conversion of 3I- to I3- ions. The semicircle 3 at low frequency region due to the I-/I3- diffusion decreases in size and merges with the growing charge transfer circle in the intermediate region.16 From the above perceptions, the ZnOCdS + N3 system offers the least Rct and is considered to be the pre-eminent system to offer high efficiency in correlation

with the J-V characteristics. The optimization of CdS QDs concentration for still better performance is underway. Conclusion In the presence of CdS quantum dots, the formation of insulating Zn2+/N3 dye complexes can be avoided to a great extent, and dual photosensitization on ZnO nanoparticles can be obtained. CdS quantum dots not only link N3 molecules over ZnO nanoparticles but also contribute their optical absorption wavelength. The formation of CdS is confirmed from EDX analysis. The improvement in the solar-to-electrical conversion efficiency from 0.11 and 0.85 to 3.04% when compared to ZnO/ N3 and ZnO-CdS, respectively, is attributed to the dual photosensitization caused by an effective coupling between ZnO NPs/N3 dye molecules in the presence of CdS QDs. Acknowledgment. We greatly acknowledge financial support from the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R11-2008088-03001-0). References and Notes (1) Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M. Nat. Mater. 2008, 7, 626. (2) Chou, T. P.; Zhang, Q.; Fryxell, G. E.; Cao, G. AdV. Mater. 2007, 19, 2588. (3) Quintana, M.; Edvinsson, T.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2007, 111, 1035. (4) Zhu, K.; Kopidakis, N.; Neale, N. R.; Van De Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2006, 110, 25174. (5) Chou, T. P.; Zhang, Q.; Russo, B.; Fryxell, G. E.; Cao, G. J. Phys. Chem. C 2007, 111, 6296. (6) Chang, H.; Lee, Y. L. Appl. Phys. Lett. 2007, 91, 053503. (7) Lee, Y. L.; Chang, C. H. J. Photochem. Photobiol., A: Chem. 2008, 185, 584. (8) Wang, M.; Chen, P.; Humphry-Baker, R.; Zakeeruddin, S. M.; Gratzel, M. ChemPhysChem 2009, 10, 290. (9) Lee, W.; Min, S. K.; Shin, S.; Han, S. H.; Lee, S. H. Appl. Phys. Lett. 2008, 92, 023507. (10) Asadov, A.; Gao, W.; Li, Z.; Lee, J.; Hodgson, M. Thin Solid Films 2005, 476, 201. (11) Chang, C. H.; Lee, Y. L. Appl. Phys. Lett. 2007, 91, 053503. (12) Lin, S. C.; Lee, Y. L.; Chang, C. H.; Shen, Y. J.; Yang, Y. M. Appl. Phys. Lett. 2007, 90, 143517. (13) Wu, J. J.; Chen, G. R.; Yang, H. H.; Ku, C. H.; Lai, Y. J. Appl. Phys. Lett. 2007, 90, 213109. (14) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Barry Carter, C.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 7, 1793. (15) Zhang, Q.; Chou, T. P.; Russo, B.; Jenekhe, S. A.; Cao, G. Z. Angew. Chem. 2008, 120, 2402. (16) Fabregat-Santiago, F.; Bisquert, J.; Garcia-Balmonte, G.; Boschloo, G.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2005, 87, 117.

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