Dye Molecule Sandwich

Jul 4, 2012 - Key Laboratory of Ministry of Education for Application Technology of Chemical Materials in Hainan Superior Resources, College of Materi...
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Study of Quantum Dot/Inorganic Layer/Dye Molecule Sandwich Structure for Electrochemical Solar Cells Heping Shen,† Hong Lin,*,† Yizhu Liu,† Jianbao Li,†,‡ and Dan Oron*,§ †

State Key Laboratory of New Ceramics & Fine Processing, Department of Material Science and Engineering, Tsinghua University, Beijing 100084, China ‡ Key Laboratory of Ministry of Education for Application Technology of Chemical Materials in Hainan Superior Resources, College of Materials Science and Chemical Engineering, Hainan University, Hainan 570228, China § Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel S Supporting Information *

ABSTRACT: A highly efficient quantum dot (QD)/inorganic layer/dye molecule sandwich structure was designed and applied in electrochemical QD-sensitized solar cells. The key component TiO2/CdS/ZnS/ N719 hybrid photoanode with ZnS insertion between the two types of sensitizers was demonstrated not only to efficiently extend the light absorption but also to suppress the charge recombination from either TiO2 or CdS QDs to electrolyte redox species, yielding a photocurrent density of 11.04 mA cm−2, an opencircuit voltage of 713 mV, a fill factor of 0.559, and an impressive overall energy conversion efficiency of 4.4%. More importantly, the cell exhibited enhanced photostability with the help of the synergistic stabilizing effect of both the organic and the inorganic passivation layers in the presence of a corrosive electrolyte.

1. INTRODUCTION Semiconductor quantum dots (QDs) such as CdS,1−3 CdSe,4,5 PbS,6 and InP7 have been widely employed as sensitizers of photochemical solar cells due to their specific advantages as compared with dye molecules, such as easily tunable energy gap, broadband optical absorption, high extinction coefficients,8,9 larger intrinsic dipole moment facilitating rapid charge separation,1,10 and potentially higher stability and resistivity against oxygen and water. Among the semiconductor candidates, CdS is a promising material whose conduction band edge lies above that of TiO2, which is propitious to the injection of excited electrons from CdS into TiO2. However, the bulk band gap of CdS is 2.25 eV, which limits its absorption threshold below ca. 550 nm, leaving much of the visible and near-IR spectrum unutilized. Cosensitization with molecular dyes is a viable strategy to extend the spectral absorption to the near-infrared. This has been recently employed using CdS, and the common dye sensitizer N719 {(Bu 4 N) 2 [Ru(dcbpyH)2(NCS)2, where dcbpy = 4,4′-dicarboxy-2,2′-bipyridine}, which serves as an ideal partner due to its absorption range, extended to ca. 750 nm11 and also in a FRET-based system where CdSe QDs served as donors to SQ02 dye acceptors rather as sensitizers themselves.12 However, when cosensitized photoelectrodes were applied in solar cells, the drastic chemical divergence of QDs and dyes made it difficult to select an appropriate electrolyte redox couple that is efficient and noncorrosive for both sensitizers.13 Recently, Zaban and co-workers11 reported the insertion of a thin layer of © 2012 American Chemical Society

amorphous TiO2 serving as a QDs stabilizer between CdS and N719 cosensitizers, enabling the iodine-based electrolyte to be a feasible one and improving the full cell conversion efficiency from 0.57% for CdS sensitization alone to 1.51% for cosensitized solar cell. Nevertheless, such a configuration left charge recombination from CdS QDs to electrolyte species as a potentially significant loss mechanism14 due to the lower lying conduction band position of the TiO2 coating. In contrast, ZnS, resembling some other wide-bandgap semiconductor coating layers in DSCs,15−17 has been reported to successfully block such a recombination pathway in QD-sensitized solar cells.18 To further promote the photovoltaic performances of QD− dye-cosensitized solar cells, we propose here a TiO2/CdS/ZnS/ N719 structure (Figure 1a), in which ZnS is expected to effectively protect excited-state electrons in CdS QDs from undesired recombination without impairing electron injection from N719 into TiO2. In this system, upon light irradiation, electrons are injected directly from the CdS QDs or N719 into the TiO2 films. If the TiO2 is partially covered by ZnS, then electrons from dye molecules adsorbed on the ZnS must tunnel through the ZnS to be injected to the TiO2.11 It should be noted that the energetic diagram of the bisensitizer nanoporous electrode shown in Figure 1b exhibits a type II band alignment between the two sensitizers.19,20 This stepwise energy relationReceived: December 15, 2011 Revised: July 4, 2012 Published: July 4, 2012 15185

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glass substrate serving as a photoanode with an active area of 0.16 cm2. CdS QD-sensitized TiO2 film was prepared by alternately dipping TiO2-loaded glass substrates into an ethanol solution of Cd(NO3)2 (0.5 M) or a methanol solution of Na2S (0.5 M) for 5 min each and rinsing with methanol.2 The above cycle, named the successive ionic layer adsorption and reaction (SILAR) cycle, was repeated four times. For ZnS deposition, the CdS-sensitized TiO2 thin film was dipped into an aqueous solution of Zn (NO3)2 (0.5M) for 1 min, washed with deionized water thoroughly, dipped again into a methanol solution of Na2S (0.5 M) for 1 min, and at last washed with methanol.18 In this paper, hybrid film after only one cycle of ZnS deposition was denoted as CdS−ZnS(1), while that after two cycles was denoted as CdS−ZnS(2). For N719 cosensitization, the as-prepared film was immersed into an ethanol solution of N719 (5 × 10−4 M) for about 20 h. 2.2. Cell Fabrication. The counter electrode was prepared by sputtering platinum onto the FTO glass for 20 s and sealed into a cell together with respective hybrid photoanode using a 20 μm thick Surlyn hot-melt ring (DuPont, United States). The electrolyte consisting of 0.025 M I2, 0.6 M 1-propyl-3methylimidazolium iodide (PMII), 0.1 M guanidine thiocyanate (GNCS), and 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile (AN) was injected. 2.3. Photovoltaic Performance Characterization. The photocurrent density−voltage (J−V) characteristics of the cells were measured with a solar simulator (91192, Oriel, United States), under illumination of AM 1.5 (100 mW cm−2). The start voltage is −0.1 V, and the stop voltage is 1 V, during which 200 points were obtained. The sweep delay is 30 ms. The incident monochromatic photon-to-current conversion efficiency (IPCE) spectra were measured by IPCE measurement system (PEC-S20, Peccell Technologies, Inc., Japan). A standard silicon solar cell (Si PDS1337-1010BQ, Bunkoukeiki Co., Ltd.) was used as reference, and the IPCE values were obtained by comparing the current ratio and the IPCE value of the reference cell at each wavelength. 2.4. Other Characterization Methods. High-resolution transmission electron microscopy (HRTEM) spectra were obtained from JEOL 2011 (JEOL, Japan). TiO2 photoelectrodes' absorbance spectra were obtained by an UV/vis/ NIR spectrophotometer (Lambda 950, Perkin-Elmer, United States). N719 dye loading amounts were determined by desorbing the adsorbed dyes from the stained TiO2 films

Figure 1. (a) Schematic drawing of TiO2/CdS/N719 cosensitized system (the solid circles represent CdS QDs, and the five-pointed stars represent N719 molecules) and (b) the energetic diagram of the bisensitizer nanoporous electrode (TiO2/CdS/ZnS/N719). Upon light irradiation, electrons are injected directly from the CdS QDs or N719 into the TiO2 films. After the charge separation, the QDs are regenerated with the hole transfer from the QD to the dye. Besides, undesired direct recombination channels from the TiO2 and the CdS QDs to the electrolyte, which are strongly suppressed by the ZnS, are indicated by dashed lines.

ship featured the occupied and unoccupied electronic states descending from the molecular dye via the QD to the TiO2 and was deemed to be favorable for charge separation,19 which enables the QDs to be regenerated by the hole transfer from the QD to the dye. Besides, undesired direct recombination channels from the TiO2 and the CdS QDs to the electrolyte, which are strongly suppressed by the ZnS, are indicated by dashed lines.

2. EXPERIMENTAL SECTION 2.1. Hybrid Photoelectrode Preparation. The TiO2 paste was prepared by mixing TiO2 nanoparticles (18%) with terpineol (73%) and a viscous solution of ethyl cellulose in ethanol. The TiO2 nanoparticles are P25 powders (Degussa, Germany), with an average particle size of 21 nm. They consisted of both anatase and rutile phases with a weight ratio of about 80:20. The 13 μm thick TiO2 mesoscopic film was screen-printed on the fluorine-doped SnO2 (FTO) conducting

Figure 2. UV−vis absorption spectra of (a) TiO2 photoelectrodes without and with different sensitizers including CdS, CdS−ZnS(1), CdS−ZnS(2), CdS−N719, CdS−ZnS(1)−N719, CdS−ZnS(2)−N719, CdS−N719 (45 min), and CdS−N719 (20 h). (b) N719 dye loading amounts after desorbing from TiO2 hybrid films sensitized by CdS−N719, CdS−ZnS(1)−N719, CdS−ZnS(2)−N719, and N719 alone 45 min and 20 h [CdS− ZnS(1) represented hybrid film after only one cycle of ZnS deposition, while CdS−ZnS(2) denoted that after two cycles]. 15186

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Figure 3. (a) Raman spectra (0−2200 cm−1) at 488 nm, (b) Raman spectra (1200−2200 cm−1) at 488 nm, and (c) photoluminescence emission spectra at 325 nm for the noncoated and ZnS-coated CdS-sensitized and CdS-N719-cosensitized electrodes.

which compensated the QD absorbance reduction following ZnS overcoating. However, by increasing the ZnS deposition times [TiO2−CdS−ZnS(2)−N719], the absorbance became poorer than both TiO2−CdS−N719 and TiO2−CdS−ZnS(1)− N719, which is, again, possibly due to the stronger reflectance induced by ZnS deposition. As was mentioned above, ZnS deposition increased N719 loading amount revealed by Figure 2b, ascribed to the superior dye loading on ZnS relative to the CdS-coated electrode. Meanwhile, the photoelectrode stained by N719 alone for 45 min exhibited the same dye loading as that of CdS−ZnS(1)−N719 and was used as a reference. TiO2−N719 (45 min) in Figure 2a exhibited commensurable absorbance with that of TiO2−CdS−ZnS(1)−N719 in the wavelength ranging from 550 to 800 nm, due to almost the same dye loading amount among them. While under the shorter wavelength, TiO2−CdS−ZnS(1)−N719 exhibited much larger absorbance than TiO2−N719 (45 min) thanks to the absorbance of CdS QDs. Similarly, TiO2−N719 (20 h) exhibited the largest absorbance in the wavelength ranging from 550 to 800 nm but much smaller under the shorter wavelength than that of TiO2−CdS−N719 and TiO2−CdS−ZnS(1)− N719. To investigate the charge separation between CdS QD and N719 molecule, Raman and photoluminescence characterization of the various cells was executed. Figure 3 shows Raman spectra at 488 nm and photoluminescence emission (PL) spectra at 325 nm for the noncoated and ZnS-coated CdSsensitized and CdS−N719-cosensitized electrodes. For all QDsensitized samples, we can identify from the Raman spectra the intense Eg Raman mode of anatase TiO2 at 142 cm−1 and the resonantly excited longitudinal optical (LO) phonon of CdS QDs at 305 cm−1.21 N719 cosensitization brought in more Raman peaks between 1200 and 2000 cm−1 (Figure 3b). It should be noted that a weak and broad band appears at about 1370 cm−1 on TiO2−CdS−N719, TiO2−CdS−ZnS(1)−N719, and TiO2−CdS−ZnS(2)−N719 electrodes accompanied by a weak shoulder at about 1580 cm−1 on the TiO2−CdS−N719

using 0.01 M ethanol−water (volume ratio of 1:1)−NaOH solution and measuring UV−vis spectra of the obtained dye solution, which was recorded in a 1 cm length path quartz cell with a Thermo Spectronic UV500 (Thermo, United States). To measure the optical and vibrational properties of various photoanodes, Raman and PL measurements were carried out by the equipment (HORIBA JOBIN YVON HR800, France) at room temperature. Raman was measured using the laser of 488 nm. PL was measured using a 325 nm laser with an excitation power of about 20mW.

3. RESULTS AND DISCUSSION For the CdS-sensitized TiO2 photoelectrode, CdS QDs were observed on the surface of TiO2 particle with diameters of about 5 nm (Figure S1 in the Supporting Information). The surface coverage of TiO2 nanoparticle by CdS QDs was relatively low, which may provide enormous sites for direct charge recombination from the TiO2 to the electrolyte redox species, resulting in poor photovoltaic performance in integrated solar cells. This will be discussed below in more detail. Absorption spectra of the TiO2 nanoparticle films coupled with the dye loading amount before and after sensitization with various sensitizers are illustrated in Figure 2. CdS QD-sensitized TiO2 electrodes showed absorption onsets at about 550 nm, mainly due to the strong visible light absorption of CdS. Deposition of ZnS caused slight attenuation in the absorbance, and the effect was even more manifest with increased ZnS deposition times [TiO2−CdS−ZnS(2)], indicating stronger reflectance of incident light by ZnS due to the higher effective refractive index of the overcoated structure. As for CdS- and N719-cosensitized TiO2 films, N719 cosensitization extended the range of light absorption to ca. 750 nm as compared with CdS QDs. As can be seen in Figure 2a, the absorbance value of the TiO2−CdS−ZnS(1)−N719 hybrid film was higher than that of the cosensitized film without ZnS deposition across the entire spectrum (350−800 nm). We attribute this to the higher N719 dye loading (Figure 2b), 15187

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electrode. They were identified with the symmetric and asymmetric COO− stretching modes, respectively. Similar bands have been observed previously in the Raman spectra of N719/CdSe QD/TiO2, which was considered to suggest that the dye complex may be bound onto the CdSe QDs through its carboxylate groups.22 In the present structure, N719 was deemed to be adsorbed both directly on the TiO2 surface and on top of the QDs exhibited in Figure 1a. The ZnS coating is deposited between the two sensitizers. All of the CdS QD/TiO2 samples exhibited the sharp PL peaks stemming from the band-edge emission of CdS QDs. In the CdS-sensitized electrodes, PL peaks were remarkably enhanced by employing ZnS as an intermediate layer with enlarged luminescent quantum yields. The ZnS deposition acted as a passivation layer of the CdS QD surface states by reducing the recombination of photoinjected electrons in TiO2 with electrolyte redox species, which was validated by the improved photovoltaic performances of the ZnS-coated solar cells discussed above. More importantly, in QD−dye hybrid system, three different basic arrangements of the energetic levels between the two sensitizers and TiO2 are generally possible, leading to different charge separation efficiencies.14 In CdS−N719-cosensitized electrodes, because the occupied and unoccupied electronic states descend from the molecular dye via the CdS QD to the TiO2, N719 could serve as regenerating agents of the CdS QD with the fast photoinduced hole transfer from QDs to the dye molecules instead of electrolyte redox species, which is similar to those in the literature.22,23 This was confirmed by the obvious reduction of the QD emission for cosensitization of the QD/TiO2 samples with the N719 dye as compared to CdS-sensitized ones, which significantly exceeds the reduction expected due to direct absorption of the 325 nm light by N719. This result serves as evidence that employing composite heterostructures consisting of QDs and molecular dyes, with thoroughly different charge transfer mechanisms, is a promising alternative to the conventional configuration of QDsensitized solar cells and dye-sensitized solar cells. Furthermore, even in the presence of ZnS deposition between two sensitizers, strong photoinduced charge transfer was possible, revealing that ZnS deposition should be an efficient method to enhance photovoltaic performances since it dramatically improves surface passivation without suffering a penalty to the photoinduced charge transfer.22 The above two speculations are verified by the J−V curves of the various fabricated cells, which are shown in Figure 4a. Evidently, the ZnS blocking layer dramatically improved the performance of both the CdS sensitized cell and the cosensitized cells. The photovoltaic parameters including short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF), and overall energy conversion efficiency (η) are listed in Table 1. In particular, the CdS-sensitized solar cell without ZnS deposition exhibits low overall energy conversion efficiency (η) of only 0.19%. This is mainly due to the extremely poor FF of 0.13, which is induced by direct contact between QD and the corrosive I−/I3− redox species.11 Recombination from the CdS QDs to electrolyte redox species probably constituted a vital electron loss pathway, ascribed to the presence of surface states in QDs acting as recombination centers. Another reason is the direct electron recombination from TiO2 to electrolyte redox species due to the low surface coverage by CdS QDs as mentioned above. Deposition of a ZnS blocking layer gave rise to significant improvement in FF and considerable enhancement in both Jsc and Voc, demonstrat-

Figure 4. (a) Photocurrent density−voltage (J−V) characteristics under AM 1.5 illumination (100 mW cm−2) and (b) incident photonto-current conversion efficiency (IPCE) action spectra of solar cells fabricated with TiO2 photoelectrodes with different sensitizers [(G) N719 alone sensitized solar cell stained for 45 min with the same dye load amount with that of CdS−ZnS(1)−N719].

Table 1. Photovoltaic Characteristics of Cells Fabricated with TiO2 Photoelectrodes with Different Sensitizers under AM 1.5 Illumination (100 mW cm−2) devices A B C D E F G

sensitizers CdS CdS−ZnS(1) CdS−ZnS(2) CdS−N719 CdS−ZnS(1)−N719 CdS−ZnS(2)−N719 N719 (sensitized for 45 min)

Jsc (mA cm−2)

Voc (mV)

FF

η (%)

2.35 2.92 3.34 9.90 11.04 10.60 8.60

602 712 718 740 713 729 657

0.13 0.53 0.67 0.16 0.56 0.55 0.65

0.19 1.11 1.60 1.20 4.40 4.27 3.70

ing suppression of charge recombination from either TiO2 or CdS QDs to electrolyte redox species.24 This resulted in dramatic elevation of the energy conversion efficiency to 1.11%. Furthermore, when repeating the ZnS SILAR cycle twice, both Jsc and FF were further improved, yielding an efficiency of 1.60%. The IPCE is given by the equation25 IPCE = LHE × ηinj × ηc

where LHE is the light-harvesting efficiency at a given wavelength, ηinj is the electron injection efficiency, and ηc is the charge collection efficiency. As seen in Figure 4b, for the CdS-sensitized solar cells, the IPCE values were progressively enlarged with increased ZnS deposition times, which was in good agreement with the trend in Jsc values. As discussed above in relation to Figure 2, LHE values were gradually reduced with the increased ZnS deposition times. Thus, the improved IPCE values for ZnS deposition samples can be mostly accounted for by either an improved ηinj or an enlarged charge collection efficiency ηc. Considering the various charge recombination pathways discussed above, it can be concluded that deposition of the ZnS layer has successfully reduced the leakage current by blocking the drastic dark reaction from either TiO2 or CdS to electrolyte redox species, with the former leading to improved ηc and the latter leading to enlarged electron injection ηinj by passivating QD surface states. 15188

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Figure 5. Short circuit current density as a function of time using periodic illumination intervals (12 s interval length) for more than 400 s, (a) CdSsensitized solar cells, (b) CdS-N719-cosensitized solar cells, (c) CdS−ZnS(1)-sensitized solar cells, (d) CdS−ZnS(1)−N719-sensitized solar cells, (e) CdS−ZnS(2)-sensitized solar cells, and (f) CdS−ZnS(2)−N719-sensitized solar cells.

circuit photocurrent density of 11.04 mA cm−2, an open-circuit voltage of 713 mV, a fill factor of 0.56, and an overall energy conversion efficiency of 4.4%. Especially, the IPCE values were further increased across the entire spectrum, ranging from 400 to 800 nm and reaching up to 60% between 400 and 500 nm. Note that the use of ZnS dramatically improved the photovoltaic function of cosensitized solar cells, leading to performance significantly exceeding that of similar solar cell with an efficiency of 1.51% incorporating amorphous TiO2 layer as N719 supporter.11 Despite the higher conduction band edge position of ZnS relative to both that of the CdS QDs and the LUMO level of N719, it serves as an effective blocker of charge recombination from QDs to electrolytes rather than an undesired barrier of electron injection from N719 to TiO2 when it is thin enough. To explain the recombination inhibition by ZnS coating, we have tried to use electrochemical impedance spectroscopy (EIS), which was shown and explained in Figure S2 (in the Supporting Information). However, such a tunneling effect that allowed efficient electron injection from the N719 dye through the ZnS layer was less manifest as the ZnS deposition times increased (device F), leading to reduced Jsc and thus slightly decreased energy conversion efficiency (4.27%), which is different from the trend for above-mentioned

Similar results were obtained for CdS−N719 cosensitized solar cells. As also shown in Figure 4, cosensitization resulted in efficient photovoltaic response from 550 to 800 nm as compared with CdS-sensitized solar cells, ascribed to extended absorption spectra by N719. Unfortunately, such enhancement in photocurrent was counteracted by the still low FF in the absence of ZnS modification, yielding an overall energy conversion efficiency of only 1.20%. Theoretically speaking, it is expected that the uncovered TiO2 surface sensitized with CdS QDs alone could be compactly grafted with N719 molecules, which not only brought about incremental Jsc but also blocked charge recombination from TiO2 to electrolyte redox species acting as the first loss. However, the only slightly improved FF values (from 0.13 for device A to 0.16 for device D) indicated that QD corrosion by electrolyte and charge recombination from CdS QDs to electrolyte redox species acting as the second loss could not be suppressed by N719 molecules, while this recombination dominated the photovoltaic performances of cosensitized solar cells. This problem is adequately solved by taking advantage of our newly proposed TiO2/CdS/ZnS/N719 hybrid photoanode. Despite a small penalty in Voc, ZnS insertion between CdS and N719 dramatically enhanced the cell performance, with a short15189

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CdS-sensitized solar cells. This is further validated when scrutinizing Figure 4b that the main drop in IPCE values lays above 500 nm for CdS−ZnS(2)−N719 cosensitized cell, while the loss below 500 nm was only marginal, indicating more difficult utilization of photoexcited electrons originated from Ru(II) MLCT (metal-to-ligand charge transfer) transition.26 To analyze the N719 sensitizer's contribution on photovoltaic performance, a photoelectrode stained by N719 alone for 45 min, exhibiting the same dye loading as that of CdS−ZnS(1)− N719, was incorporated into a solar cell. This cell showed an efficiency of 3.7% and a Jsc of 8.6 mA cm−2. In fact, it was found that the Jsc of the CdS−N719-cosensitized solar cell (device E) was even slightly higher than the Jsc sum of single CdS QDs (device A) and N719 dye (device G), revealing the efficient inhibition of recombination channels afforded by this design. To evaluate the photostability of both CdS-sensitized and CdS−N719-cosensitized solar cells by using the I−/I3−-based electrolyte, short-circuit current density as a function of time using periodic illumination intervals was measured and shown in Figure 5. In CdS-sensitized solar cells, the photocurrent of the uncoated electrode already decreased during the first illumination cycle and decreased further during each following cycle. While the ZnS coating on top of the CdS-sensitized cell [CdS−ZnS(2)] somewhat slowed photodegradation, it was still significant from the second cycle onward. These results are in agreement with the findings of Shalom and co-workers.27 The ZnS-coated CdS, which was further cosensitized with N719, showed much higher stability. The cell employing TiO2−CdS− ZnS(1)−N719 began to degrade only after 250 s and the one incorporated with TiO2−CdS−ZnS(2)−N719 maintained the same photocurrent without any reduction during all of the cycles for more than 400 s with apparent degradation appearing only after 600 s (see the Supporting Information). This highlights the synergistic stabilizing effect of both the organic and the inorganic passivation layers and potentially points at the use of combined inorganic−organic system for achieving enhanced photostability in the presence of a corrosive electrolyte.

Article

ASSOCIATED CONTENT

S Supporting Information *

TEM images of TiO2 nanoparticles before and after CdS QD sensitization, EIS of the cosensitized solar cell, electrode absorbance before and after photovoltaic performance test, and the further investigation of the long-term stability of TiO2− CdS−ZnS(2)−N719. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-62772672. Fax: +86-10-62772672. E-mail: [email protected] (H.L.) or [email protected] (D.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express our gratitude for the support provided by the Ministry of Science & Technology, People's of Republic of China, the China-Israel Scientific and Strategic Research FundNo. 7 of the fifth round, and the International Cooperation S&T Cooperation Program of China (2010DFA64360).

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REFERENCES

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4. SUMMARY AND CONCLUSIONS In summary, efficient TiO2/CdS/N719 hybrid photoanodes with strong photoinduced charge transfer between the two sensitizers thanks to the type II energy alignment have been designed. Importantly, employment of ZnS between sequential deposition of the two types of sensitizers not only significantly increased the dye loading amount of N719 as compared to common cosensitization but also eliminated, to a great extent, the possibility of severe charge recombination thanks to the much higher conduction band edge of ZnS as compared to an analogous configuration utilizing a TiO2 layer.11 This highly efficient hybrid sensitization was demonstrated as a promising alternative to conventional QD-sensitized solar cells, with significant potential advantages such as spectrally splitting the absorption between QDs and dye species, enabling broader absorption spectra with an increased choice of the organic sensitizer, and excluding elaboration of simultaneous stability of the sensitizers. Besides, such a scheme is compatible with the use of alternative electrolytes such as cobalt electrolytes, which have recently exhibited outstanding photovoltaic performances in dye-sensitized solar cells28 and also high stability in QDsensitized solar cell and is expected to contribute to the longterm stability of such systems.13 Further research work on the elucidation of such fundamental issues is now underway. 15190

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp305050w | J. Phys. Chem. C 2012, 116, 15185−15191