Boosting the cell efficiency of CdSe quantum dot sensitized solar cell via a modified ZnS post-treatment

Electrochimica Acta 111 (2013) 179–184

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Boosting the cell efficiency of CdSe quantum dot sensitized solar cell via a modified ZnS post-treatment Chunmei Liu a , Lili Mu a , Jianguang Jia a,∗ , Xiaowen Zhou b,∗ , Yuan Lin b a b

Department of Physical Chemistry, School of Science, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e

i n f o

Article history: Received 22 May 2013 Received in revised form 24 July 2013 Accepted 26 July 2013 Available online 11 August 2013 Keywords: Quantum dots sensitized solar cell Lattice matching Post-treatment ZnSe ZnS

a b s t r a c t We report here a large improvement of cell performance of CdSe quantum dot sensitized solar cell (QDSSC) by a modified ZnS post-treatment, being carried out by introducing a ZnSe thin layer before ZnS deposition through a successive ion layer adsorption reaction (SILAR) method. CdSe quantum dots were deposited onto TiO2 surface using a chemical bath deposition method. Photovoltaic measurements showed that the introduction of ZnSe layer can significantly increase the photocurrent of CdSe QDSSC, resulting in a large enhancement of the solar energy conversion efficiency of the cell. On variation of the numbers of ZnSe deposition cycle, the effect of the thickness of ZnSe was investigated. The maximum energy conversion efficiency of 3.46% was achieved for CdSe QDSSC with ZnSe/ZnS treatment, showing a 22% increment compared to that of with ZnS treatment. Moreover, it was found that the introduction of ZnSe improved the stability of CdSe QDSSC. The benefit role of ZnSe was ascribed to its intermediate lattice parameter to CdSe and ZnS, which leads to the suppression of defects at CdSe/ZnS interfaces and facilitating the growth of ZnS with higher quality. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Quantum dot sensitized solar cells (QDSSCs), in which a semiconductor quantum dots (QDs) of narrow bandgap served as the photosensitizer, have recently attracted growing interests as an alternative to dye-sensitized solar cell (DSSC), owing to their high flexibility of bandgap tuning, large light absorption coefficient, high stability as well as potential multiple exciton generation of the QDs [1–7]. Similar to DSSC, the overall principal of QDSSC is based on the charge separation at the interface between QDs and the wide bandgap semiconductors such as TiO2 . Under illumination, QDs absorb light and inject electrons into the TiO2 conduction band, while holes are removed by the reductant in electrolyte and further regenerated at the counter electrode. Although large progress have been achieved, which leads to a significant increase of the energy conversion efficiency from less than 1% to over 5% [8,9], the cell efficiency of QDSSC is still far below either the efficiency of DSSC (∼12%) [10] or the theoretical thermodynamic efficiency of QDSSC (∼66%) [11]. One of the important reasons is ascribed to the recombination path induced by the surface states in the cell. The modification of QDSSC photoanode by surface coating is thought to be one of the most efficient routes to suppress the recombination. Several approaches were adopted using either

∗ Corresponding author. E-mail addresses: [email protected] (J. Jia), [email protected] (X. Zhou). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.07.220

inorganic or organic materials [12–18]. By modifying the TiO2 /CdS/CdSe with diisooctyl phosphonic acid and methoxythiophenol, Chi et al. found the cell efficiency of QDSSC was improved [14]. Yu et al. reported that the deposition of a very thin layer of TiO2 can promote the efficiency of CdS sensitized QDSSC [15]. Nevertheless, ZnS is the most studied material for surface coating, which was firstly reported by Yang et al. in 2002 [16]. ZnS thin layer is usually coated by a successive ion layer adsorption reaction (SILAR) method. It was proved that ZnS treatment can efficiently cover the bare TiO2 as well as QDs, resulting in strong inhibition of the electron leakage from either TiO2 or QDs to electrolyte, thereby significantly enhance the cell performance of QDSSCs [17]. For instance, Toyoda et al. found the cell efficiency of CdSe sensitized TiO2 QDSSC was increased from 1.12% to 2.02% upon ZnS coating [18]. However, it needs to be noted that the lattice parameters of ZnS and QD sensitizers are in most cases quite different. For example, lattice mismatch for CdSe and ZnS is reached to ∼12%. Such a large lattice mismatch can induce the formation of interface defects at CdSe/ZnS interface while ZnS is deposited, and thus decrease the cell efficiency of QDSSC. In this manuscript, we reported a modified ZnS treatment by introducing an intermediate layer of ZnSe before ZnS coating. CdSe quantum dots were used as the photosensitizer, being deposited onto the surface of TiO2 crystals through a chemical bath deposition route. Photovoltaic investigations showed that the cell performance of CdSe sensitized QDSSC was obviously improved by the introduction of ZnSe, which leads to a solar energy conversion

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efficiency of 3.46%, showing a 22% increment as compared to that of ZnS treatment. Meanwhile, we found that the introduction of ZnSe also improved the stability of CdSe QDSSC. 2. Experimental 2.1. Preparation of CdSe sensitized TiO2 electrodes Nanoporous TiO2 film was prepared as reported previously [19]. Cleaned FTO glass substrate (F-doped SnO2 , resistance 7  sq−1 , Hake New Energy Co. Ltd., Harbin) was firstly coated with a dense TiO2 underlayer using spray-pyrolysis method. TiO2 porous film was deposited by doctor-blading a TiO2 paste, being prepared by mixing TiO2 power (Degussa P25), n-butyl alcohol and Triton. Subsequently, the TiO2 film was annealed at 450 ◦ C for 30 min. The thickness of the TiO2 film was ca. 8 ␮m determined by SEM measurements. CdSe QDs were grown on porous TiO2 film using a chemical bath deposition (CBD) method as reported before [20]. Briefly, TiO2 film electrode was immersed into an aqueous solution containing sodium selenosulphate (Na2 SeSO3 ) (generated by the reaction of Se and Na2 SO3 at 80 ◦ C), trisodium salt of nitilotriacetic acid [N(CH2 COONa)3 ] and CdSO4 at ∼10 ◦ C for 5 h. After thoroughly rinsed with distilled water, the CdSe sensitized film electrode was vacuum dried at 60 ◦ C for 2 h.

Fig. 1. XRD patterns of TiO2 and CdSe/TiO2 films on FTO. The line spectra give the cubic CdSe bulk phase (JCPDS No. 88-2346).

off with an automatic shutter. Intensity-modulated photocurrent spectroscopy (IMPS) were performed using a green light emitting diode (max = 520 nm) driven by a solartron 1255B frequencyresponse analyzer. The LED provided both DC and AC components of the illumination.

2.2. Post-treatments for CdSe sensitized TiO2 electrodes

3. Results and discussion

Two kinds of post-treatments were performed using a successive ionic layer adsorption and reaction (SILAR) method. For ZnSe treatment, CdSe/TiO2 electrodes were alternately dipped into 0.1 M zinc acetate (Zn(CH3 COO)2 , Aladdin) for 1 min at room temperature and 0.2 M Na2 SeSO3 for 5 min at 50 ◦ C, while the samples were thoroughly rinsed with distilled water between dips. ZnS treatments were carried out by dipping CdSe/TiO2 or ZnSe treated electrodes into 0.1 M zinc acetate and 0.1 M sodium sulfide (Na2 S) aqueous solution for 1 min/dip at room temperature similar to that of ZnSe treatmen. The thicknesses of ZnS and ZnSe coatings were adjusted by controlling the repeating cycles of the SILAR procedure.

3.1. Structure and optical property of CdSe sensitized TiO2 photoanode

2.3. Fabrication of QDSSCs QDSSCs were assembled into a sandwich structure by using a CdSe sensitized TiO2 (with or without post-treatment) as the working electrode and a Pt plate as the counter electrode. Polysulfide electrolyte composed of 0.5 M S, 2.0 M Na2 S and 0.2 M KCl in water/ethanol (7:3 in volume) mixed solution was used as the redox electrolyte. 2.4. Characterizations The morphologies of TiO2 and CdSe/TiO2 films were characterized by scanning electron microscopy (SEM, S-4800F, Hitachi). High-resolution transmission electron microscopy (HRTEM) was performed using Tecnai G2 F20 (FEI Co.). The crystalline structures were characterized by an X-ray powder diffractometer (XRD, Riguku D/max 2500) with a CuKa irradiation. Photocurrent–voltage measurements were performed with a computer-programmed Keithley 2611 Source Meter at room temperature under illumination of simulated sunlight (Oriel, 91160-1000, AM1.5, 100 mW cm−2 ). The incident light was calibrated with a power meter (model 350) and a detector (model 262). The active area of the cell is 0.2 cm2 . Neutral filters were used to adjust the light intensity. To measure the time dependent photocurrent, the irradiating light was periodically turned on and

CdSe were grown on porous TiO2 film using a chemical bath deposition (CBD) method. Fig. 1 showed the XRD pattern CdSe/TiO2 film. The appearance of diffraction peaks at 25.36◦ , 42.02◦ and 49.72◦ are well fitted to (1 1 1), (2 2 0) and (3 1 1) faces of cubic CdSe according to the standard CdSe (JCPDS 88-2346), indicating the formation of cubic CdSe on TiO2 . The crystallite size was estimated to be ca. 9 nm according to Scherrer equation. Fig. 2 presented the SEM imagines of TiO2 and CdSe/TiO2 films. TiO2 film is composed of TiO2 nanoparticles with average diameters of ∼30 nm, possessing a porous structure (Fig. 2a). After the deposition of CdSe, the sizes of the constituent particles were found to be increased, having an average diameter of ∼50 nm (Fig. 2b). An ∼20 nm increment in particle size is attributed to the deposition of CdSe QDs onto TiO2 surface, in accordance with the sizes of CdSe QDs calculated from XRD. Note that, the CdSe/TiO2 film possesses porous structure although slightly decreases in pore sizes could be observed, which can facilitate the penetration of electrolyte. Fig. 3 displayed the typical HRTEM images of CdSe/TiO2 samples post-treated by ZnS (Fig. 3a) and ZnSe/ZnS (Fig. 3b). Clear lattice fringes with interplanar spacings of 0.351 nm can be distinguished in both cases. The measured d-spacing is consistent with the (1 1 1) lattice plane of cubic CdSe phase (JCPDS 88-2346), further confirmed the deposition of cubic CdSe on TiO2 . For the sample with ZnS treatment, we found that CdSe QD was covered by a thin layer, which displayed clear lattice fringe with d-spacing 0.310 nm (Fig. 3a). Same lattice fringes were also observed for ZnSe/ZnS treatment sample as shown in Fig. 3b, which are consistent with the (1 1 1) lattice plane of cubic ZnS (JCPDS 79-0043), indicating the formation of cubic ZnS nanocrystals during ZnS post-treatments. Compared to the ZnS treated sample shown in Fig. 3a, the sample treated by ZnSe/ZnS showed an additional thin layer in between CdSe and ZnS, exhibiting clear lattice fringes (Fig. 3b). The measured interplanar spacings of 0.330 nm is in consistent with the (1 1 1) plane of cubic ZnSe (JCPDS 37-1463), suggesting the formation of cubic ZnSe by ZnSe treatment.

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Fig. 2. SEM images of TiO2 (a) and CdSe/TiO2 (b). CdSe was grown on TiO2 through a chemical bath deposition.

Fig. 3. HRTEM images of CdSe/TiO2 films post-treated with ZnS (a) and ZnSe/ZnS (b).

The optical properties of the CdSe/TiO2 film before and after post-treatments were investigated by measuring the diffused reflection spectra. As shown in Fig. 4, all the samples showed obvious absorption in visible range with the absorption edge at about 600 nm, attributed to the absorption of CdSe. The blue-shifts of the absorption spectra related to CdSe bulk (1.7 eV), is attributed to the quantum size effect. Compared to CdSe/TiO2 without posttreatment, however, we found that post-treatments by ZnS and ZnSe/ZnS resulted in slight red-shifts of the absorption spectra. This is ascribed to a lower degree of quantum confinement due to the deposition of ZnSe or ZnS [21,22]. 3.2. Photovoltaic property of CdSe-sensitized QDSSC

were characterized by measuring the photocurrent–voltage (J–V) behavior. Fig. 5 shows the typical J–V curves of QDSSCs of different configurations, where solid lines displayed QDDSCs with ZnS treatment of various SILAR cycles and dashed lines illustrated the cell treated by ZnSe or ZnSe/ZnS. The corresponding open-circuit voltages (Voc ), short-circuit current densities (JSC ), fill factors (FF), and overall energy conversion efficiencies () were gathered in Table 1. As the numbers of ZnS deposition cycle increased up to 4 cycles, obvious increases in Voc , JSC and FF were observed, which lead to a large increase of the overall energy conversion efficiency from  = 1.09% to  = 2.83%. Further increase of ZnS deposition cycle decreased Voc , JSC , FF and thus the cell efficiency. This is in accordance with that of reported by Toyoda et al., which showed an similar tendency of first increase followed by decrease in cell

CdSe/TiO2 electrodes with different post-treatments were assembled to QDSSCs, and their photovoltaic properties

Fig. 4. Diffused reflection spectra of CdSe/TiO2 films with different post-treatments. The numbers in brackets indicate the number of SILAR deposition cycles.

Fig. 5. J–V curves of CdSe/TiO2 QDSSCs with different post-treatments. Solid lines indicate the ZnS post-treatments, while dashed lines represent the post-treatments by ZnSe or ZnSe/ZnS. The numbers in brackets indicate the number of SILAR deposition cycles.

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Table 1 Photovoltaic parameters of CdSe QDSSCs with different post-treatments. Post-treatment

Jsc (mA cm−2 )

Voc (mV)

FF

 (%)

None ZnS(2) ZnS(4) ZnS(6) ZnS(8) ZnSe(2) ZnSe(2)ZnS(2) ZnSe(2)ZnS(4)

5.00 7.08 9.37 7.96 5.61 8.62 10.39 11.08

405 455 495 475 465 435 475 515

0.54 0.60 0.61 0.57 0.53 0.60 0.63 0.61

1.09 1.94 2.83 2.15 1.38 2.24 3.10 3.46

efficiency for PbS sensitized QDSSC by increasing the numbers of ZnS deposition cycle [23]. The benefit effect of ZnS coating (less than 4 cycles) is attributed to the suppression of electron leakage to electrolyte due to the efficient passivation of TiO2 and CdSe QDs by ZnS coating. When excess ZnS was deposited, however, hole transfer from CdSe QDs to electrolyte is hindered due to its more positive valence band of ZnS than CdSe, thus decrease the cell efficiency [24]. Interestingly, we noticed that the introduction of ZnSe (2 cycles) in between ZnS and CdSe QDs leads to the further increases of Voc and JSC . Consequently, an overall energy conversion efficiency reached to 3.46% was achieved, showing a 22% increment compared to the maxium value of 2.83% treated by 4 cycles ZnS deposition. It needs to be mentioned that post-treatment by only ZnSe can also enhance Voc , JSC and the cell efficiency (Fig. 5), owing to its passivation effect of ZnSe similar to ZnS as mentioned above. In comparison with the ZnS treatment, however, we found that ZnSe treatment with the same SILAR cycles achieved higher JSC but lower Voc . This suggests that the effect of ZnS and ZnSe posttreatment on the improvement of photovoltaic properties of CdSe QDSSC is different, while ZnSe plays a critical role in improving the photocurrent. In separate experiments, the thickness of ZnSe coating was adjusted by varying the SILAR cycles of ZnSe deposition. Fig. 6 showed the dependence of Voc (JSC ) on the numbers of ZnSe deposition cycles. In all the cases, 4 cycles of ZnS were deposited after the ZnSe coating. Clearly, we found that the first two cycle’s ZnSe deposition resulted in obvious increases in JSC from 9.55 mA cm−2 to 11.22 mA cm−2 , showing a 17% increment, which accompanied by a slight increase in Voc . When the number of ZnSe deposition cycle was further increased, however, both Voc and JSC were decreased. This result indicated that the thickness of ZnSe layer is crucial, where a suitable thin layer is propitious to construct CdSe sensitized QDSSC with high photovoltaic properties.

Fig. 6. Dependence of JSC and Voc for CdSe QDSSC on the SILAR cycles of ZnSe deposition. ZnS treatments with 4 SILAR cycles were applied after ZnSe deposition. The dashed lines are guide for eye.

Fig. 7. Intensity modulated photocurrent spectra of CdSe/TiO2 electrodes with ZnS and ZnSe/ZnS treatments.

3.3. IMPS and photocurrent analyses The above J–V investigations showed that ZnS post-treatment can efficiently improve the photovoltatic properties of CdSe QDSSC, while the introduction of ZnSe thin layer in between ZnS and CdSe QDs further enhanced the cell efficiency owing to the large improvement of photocurrent. To understand the role of ZnSe intermediate layer, IMPS measurements were performed. Fig. 7 illustrates the IMPS spectra of QDSSCs treated by ZnS and ZnSe/ZnS. In both cases, the IMPS spectra were nearly perfect semicircles. The transit time ( d ) of photogenerated electron was determined as before [25], using the equation: d =

1 2fmin

where fmin is the minimum frequency in IMPS plot. Note that the cell post-treated by ZnSe/ZnS had lower transit time ( d = 15.9 ms) as compared to that of by ZnS treatment ( d = 22.5 ms). The lower transit time for cell with ZnSe/ZnS treatment indicated the improved transportation rate of electron in TiO2 electrode due to the introduction of ZnSe in post-treatment. Fig. 8 showed the photocurrent as a function of the illumination intensity for cells with ZnS and ZnSe/ZnS treatments. ZnS treated sample showed a linear behavior at lower light intensity, but it deviated from linearity at higher light intensity (>80 mW cm−2 ). The nonlinearity behavior of the sample with ZnS treatment indicates the increased electron recombination at higher light intensity [26,27]. Oppositely, ZnSe/ZnS treated QDSSC showed a linear behavior over the entire light intensity range up

Fig. 8. Short circuit photocurrent density as a function of illumination intensity for QDSSCs post-treated by ZnS(4) and ZnSe(2)/ZnS(4).

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contact of CdSe QDs with electrolyte. Interestingly, we noted that the photocurrent of QDSSC post-treated by ZnSe/ZnS kept almost unchanged during the illumination period, indicating that the introduction of ZnSe in post-treatment can further improve the stability of QDSSC. The reason is not very clear, but possibly the improved quality of ZnS coating due to the introduction of ZnSe can be ascribable. Because of their less lattice mismatch between ZnS and ZnSe, ZnS coating with higher quality could be grown on ZnSe, which produce the better preventing role. 4. Conclusions

Fig. 9. Short circuit photocurrent density as a function of time using periodic illumination intervals.

to 100 mW cm−2 , indicating an efficient suppression of electron recombination resulting from the introduction of ZnSe intermediate layer. In principle, the photocurrent of a solar cell is close related to the light harvesting, electron injection and collection efficiency. Due to the very thin layer of ZnSe deposited in our case, the photocurrent caused by the absorption of ZnSe can be rule out. This is supported by the almost unchanged light absorptions of CdSe/TiO2 films before and after ZnSe treatment (Fig. 4), and further evidenced by the fact that 2 cycles of ZnSe deposited on TiO2 electrode produced neglected photocurrent of less than 0.06 mA cm−2 under the same illumination as shown in Fig. 5. Having changed only the post-treatment, the increased photocurrent due to the introduction of ZnSe can be ascribed to the enhanced electron injection and collection efficiency. As proved by IMPS and light intensity dependent photocurrent measurements, the introduction of ZnSe led to the increases of electron transportation rate in TiO2 and suppression of electron recombination (Figs. 7 and 8). As a result, the electron collection efficiency can be increased, and thus leading to the enhancement of photocurrent. In addition, the improved interface property due to the introduction of ZnSe intermediate layer is also ascribable. It is known that the lattice parameter of CdSe is much different to that of ZnS. The large lattice mismatch (∼12%) can induce defects at CdSe/ZnS interfaces when ZnS was coated on CdSe QDs. Upon illumination, the photoexcited electrons are trapped by the defects, thus decrease the electron injection efficiency from CdSe to TiO2 . When ZnSe intermediated layer was introduced, however, the interfacial defects could be largely removed due to its better lattice matching of ZnSe with CdSe (∼6%). Consequently, the decreased defects could increase the electron injection from CdSe to TiO2 , leading to the improvement of photocurrent. The improving of interfacial properties by introducing a intermediate layer is in accordance with that of reported by Weller et al., whereas they found that the photoluminescence of CdSe/ZnS core/shell QDs was largely enhanced by introducing a middle layer of ZnSe or CdS [28]. 3.4. Stability of QDSSC To test the stability of QDSSCs with post-treatments, the photocurrent as a function of illumination time was recorded. The illumination was periodically chopped using an automatic shutter. As shown in Fig. 9, the photocurrent of the cell without post-treatment showed continuous decreases from the second illumination cycle. After ten cycle’s illumination, the photocurrent decreased to ca. 75% of its initial value, showing a 25% decrement. In contrast, ZnS treatment resulted in much stable photocurrent, showing only 4% decrement after 10 cycles illumination. This is attributed to the blockage effect of ZnS, prevented the direct

In summary, we have reported an efficient way to improve the photovoltaic properties of CdSe sensitized QDSSC through a modified ZnS post-treatment, being carried out by introducing a middle layer of ZnSe in between CdSe QDs and ZnS out-coating layer. Photovoltaic investigations showed that the introduction of ZnSe efficiently enhanced the solar energy conversion efficiency of QDSSC resulting mainly from the increase of photocurrent. Photocurrent vs. time measurements showed that the stability of the cell was also improved. Mechanism investigations illustrated that the introduction of ZnSe can improve the electron transportation rate, decrease the recombination and facilitate the growth of ZnS with high quality due to its better lattice matching with CdSe compared to ZnS. On variation of the ZnSe deposition cycles, we noted that the thickness of ZnSe layer is crucial. With ZnSe/ZnS posttreatement, an overall energy conversion efficiency of 3.46% for CdSe QDSSC based on Pt counter electrode and polysulfide electrolyte was achieved, showing a 22% increment compared to that of with ZnS post-treatment (2.83%). Acknowledgement The authors appreciate the financial support of this work by National Research Fund for Fundamental Key Project (2012CB932903). References [1] R.S. Selinsky, Q. Ding, M.S. Faber, J.C. Wright, S. Jin, Quantum dot nanoscale heterostructures for solar energy conversion, Chem. Soc. Rev. 42 (2013) 2963. [2] P.V. Kamat, Boosting the efficiency of quantum dot sensitized solar cells through modulation of interfacial charge transfer, Acc. Chem. Res. 45 (2012) 1906. [3] J.H. Bang, P.V. Kamat, Quantum dot sensitized solar cells. A tale of two semiconductor nanocrystals: CdSe and CdTe, ACS Nano 3 (2009) 1467. [4] X. Yu, B. Lei, D. Kuang, C. Su, High performance and reduced charge recombination of CdSe/CdS quantum dot-sensitized solar cells, J. Mater. Chem. 22 (2012) 12058. [5] K.G.U. Wijayantha, L.M. Peter, L.C. Otley, Fabrication of CdS quantum dot sensitized solar cells via a pressing route, Sol. Energy Mater. Sol. Cells 83 (2004) 363. [6] N. Fuke, L.B. Hoch, A.Y. Koposov, V.W. Manner, D.J. Werder, A. Fukui, N. Koide, H. Katayama, M. Sykora, CdSe quantum-dot-sensitized solar cell with ∼100% internal quantum efficiency, ACS Nano 4 (2010) 6377. [7] K.S. Leschkies, R. Divakar, J. Basu, E.E. Pommer, J.E. Boercker, C.B. Carter, U.R. Kortshagen, D.J. Norris, E.S. Aydil, Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices, Nano Lett. 7 (2007) 1793. [8] Q.X. Zhang, X.Z. Guo, X.M. Huang, S.Q. Huang, D.M. Li, Y.H. Luo, Q. Shen, T. Toyoda, Q.B. Meng, Highly efficient CdS/CdSe-sensitized solar cells controlled by the structural properties of compact porous TiO2 photoelectrodes, Phys. Chem. Chem. Phys. 13 (2011) 4659. [9] P.K. Santra, P.V. Kamat, Mn-doped quantum dot sensitized solar cells: a strategy to boost efficiency over 5%, J. Am. Chem. Soc. 134 (2012) 2508. [10] A. Yella, H.W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeerudding, E.W.G. Diau, C.Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Porphyrin-sensitized solar cells with cobalt (II/II)–based redox electrolyte exceed 12 percent efficiency, Science 334 (2011) 629. [11] A.J. Nozik, Quantum dot solar cells, Physica E 14 (2002) 115. [12] H.J. Lee, J. Bang, J. Park, S. Kim, S.M. Park, Multilayered semiconductor (CdS/CdSe/ZnS)-sensitized TiO2 mesoporous solar cells: all prepared by successive ionic layer adsorption and reaction processes, Chem. Mater. 22 (2010) 5636.

184

C. Liu et al. / Electrochimica Acta 111 (2013) 179–184

[13] T. Zewdu, J.N. Clifforda, E. Palomares, Synergistic effect of ZnS outer layers and electrolyte methanol content on efficiency in TiO2/CdS/CdSe sensitized solar cells, Phys. Chem. Chem. Phys. 14 (2012) 13076. [14] C.F. Chi, P. Chen, Y.L. Lee, I.P. Liu, S.C. Chou, X.L. Zhang, U. Bach, Surface modifications of CdS/CdSe co-sensitized TiO2 photoelectrodes for solid-state quantum-dot-sensitized solar cells, J. Mater. Chem. 21 (2011) 17534. [15] K.H. Yu, X. Lin, G.H. Lu, Z.H. Wen, C. Yuan, J.H. Chen, Optimized CdS quantum dot-sensitized solar cell performance through atomic layer deposition of ultrathin TiO2 coating, RSC Adv. 2 (2012) 7843. [16] S.M. Yang, C.H. Huang, J. Zhai, Z.S. Wang, L. Jiang, High photostability and quantum yield of nanoporous TiO2 thin film electrodes co-sensitized with capped sulfides, J. Mater. Chem. 12 (2002) 1459. [17] N. Guijarro, J.M. Campina, Q. Shen, T. Toyoda, T. Lana-Villarreala, R. Gomez, Uncovering the role of the ZnS treatment in the performance of quantum dot sensitized solar cells, Phys. Chem. Chem. Phys. 13 (2011) 12024. [18] Q. Shen, J. Kobayashi, L.J. Diguna, T. Toyoda, Effect of ZnS coating on the photovoltaic properties of CdSe quantum dot-sensitized solar cells, J. Appl. Phys. 103 (2008) 084304. [19] F.Y. Zhao, G.S. Tang, J.B. Zhang, Y. Lin, Improved performance of CdSe quantum dot-sensitized TiO2 thin film by surface treatment with TiCl4 , Electrochim. Acta 62 (2012) 396. [20] S. Gorer, G. Hodes, Quantum size effects in the study of chemical solution deposition mechanisms of semiconductor films, J. Phys. Chem. 98 (1994) 5338.

[21] C.-H. Chuang, S.S. Lo, G.D. Scholes, C. Burda, Charge separation and recombination in CdTe/CdSe core/shell nanocrystals as a function of shell coverage: probing the onset of the quasi Type-II Regime, J. Phys. Chem. Lett. 1 (2010) 2530. [22] B.C. Fitzmorris, J.K. Cooper, J. Edberg, S. Gul, J. Guo, J.Z. Zhang, Synthesis and structure, optical, and dynamic properties of core/shell/shell CdSe/ZnSe/ZnS quantum dots, J. Phys. Chem. C 116 (2012) 25065. [23] S. Hachiya, Q. Shen, T. Toyoda, Effect of ZnS coatings on the enhancement of the photovoltaic properties of PbS quantum dot-sensitized solar cells, J. Appl. Phys. 111 (2012) 104315. [24] X. Wang, R. Liu, T. Wang, B. Wang, Y. Xu, H. Wang, Dual roles of ZnS thin layers in significant photocurrent enhancement of ZnO/CdTe nanocable arrays photoanode, ACS Appl. Mater. Interfaces 5 (2013) 3312. [25] X. Yan, L. Feng, J. Jia, X. Zhou, Y. Lin, Controllable synthesis of anatase TiO2 crystals for high-performance dye-sensitized solar cells, J. Mater. Chem. A 1 (2013) 5347. [26] M. Shalom, S. Dor, S. Ruehle, L. Grinis, A. Zaban, Core/CdS quantum dot/shell mesoporous solar cells with improved stability and efficiency using an amorphous TiO2 coating, J. Phys. Chem. C 113 (2009) 3895. [27] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, Quantum dot solar cells. Harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films, J. Am. Chem. Soc. 128 (2006) 2385. [28] D.V. Talapin, I. Mekis, S. Go1tzinger, A. Kornowski, O. Benson, H. Weller, CdSe/CdS/ZnS and CdSe/ZnSe/ZnS core–shell–shell nanocrystals, J. Phys. Chem. B 108 (2004) 18826.