- Email: [email protected]
CdSe co-sensitized SnO2 photoelectrodes for quantum dots sensitized solar cells
Optics Communications 346 (2015) 64–68
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
CdS/CdSe co-sensitized SnO2 photoelectrodes for quantum dots sensitized solar cells Yibing Lin, Yu Lin n, Yongming Meng, Yongguang Tu, Xiaolong Zhang College of Material Science and Engineering, Engineering Research Center of Environment-Friendly Functional Materials for Ministry of Education, Huaqiao University, Xiamen, Fujian 361021, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 December 2014 Received in revised form 2 February 2015 Accepted 17 February 2015 Available online 18 February 2015
SnO2 nanoparticles were synthesized by hydrothermal method and applied to photo-electrodes of quantum dots-sensitized solar cells (QDSSCs). After sensitizing SnO2 films via CdS quantum dots, CdSe quantum dots was decorated on the surface of CdS/SnO2 photo-electrodes to further improve the power conversion efficiency. CdS and CdSe quantum dots were deposited by successive ionic layer absorption and reaction method (SILAR) and chemical bath deposition method (CBD) respectively. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to identify the surface profile and crystal structure of SnO2 photo-electrodes before and after deposited quantum dots. After CdSe co-sensitized process, an overall power conversion efficiency of 1.78% was obtained in CdSe/CdS/SnO2 QDSSC, which showed 66.4% improvement than that of CdS/SnO2 QDSSC. & 2015 Elsevier B.V. All rights reserved.
Keywords: SnO2 CdS CdSe Quantum dots-sensitized solar cells
1. Introduction Energy crisis has been an eager issue to be solved in the world. Solar energy, a type of clean and abundant natural power source, is regarded as the most promising energetic resource for the solution of energy problems currently. Dye-sensitized solar cell (DSSC), a neoteric power conversion device, has attracted substantial attention since its first appearance in 1991 due to its low cost, simple fabrication and enormous application prospect [1]. But during the past decades, the improvement of the power convention efficiency (PCE) of DSSCs is not obvious [2–4]. So the research on more functional metal oxides and sensitizers to further improve the efficiency has been studied extensively. With the research going further, many researchers begin to concern about the influence of quantum dot-sensitized solar cells (QDSSCs) [5–7]. Due to the intrinsic attractive properties of quantum dots (QDs) with tunable band gap energy, high extinction coefficients and large dipole moment, a wide variety of QDs have been investigated as sensitizers for QDSSCs, such as CdSe [8], PbS [9], CdTe [10], SnS [11] and Sb2S3 [12]. Among the semiconductor QDs, CdS and CdSe with high potential in light harvesting have been paid much attention. Recently, Pralay et al. use the coupled QDs (CdS and CdSe) to sensitize TiO2 nanoparticles solar cells and provide a noteworthy value of PCE as high as 5.4% [13]. More research have suggested that coupled QDs-sensitized n
Corresponding author. Fax: þ86 592 6162225. E-mail address: [email protected] (Yu Lin).
http://dx.doi.org/10.1016/j.optcom.2015.02.031 0030-4018/& 2015 Elsevier B.V. All rights reserved.
solar cells systems are advantageous over single QDs-sensitized cells [14,15]. Simultaneously as the potential substitute for TiO2, other appropriate semiconductor oxides are being searched. SnO2 has a faster electron diffusion rate compared to TiO2 because of its higher electron-mobility [16,17]. And the larger band gap of SnO2 (3.6 eV) can decrease oxidative holes in the valence band to facilitate the stability of QDSSCs than TiO2 (3.2 eV). Herein, SnO2 nanoparticles were prepared through the hydrothermal method and used as the photo-electrodes materials in QDSSCs. The CdS and CdSe QDs were deposited on the surface of SnO2 photo-electrodes as the sensitizers by SILAR and CBD method respectively. The surface morphology, crystal structure, UV–vis absorption properties and the photovoltaic performance of the CdS sensitized and CdS/CdSe co-sensitized SnO2 photo-electrodes were investigated and compared.
2. Experimental section 2.1. Materials All reagents including sodium stannate terahydrate (Na2SnO3 4H2O), cadmium nitrate terahydrate [Cd(NO3)2 4H2O], sodium sulfide nonohydrate (Na2S 9H2O), triethanolamine (TEA), zinc nitrate hexahydrate (Zn(NO3)2 6H2O), potassium chloride (KCl) and sulfur sublimed (S) were AR. Grade, purchased from Aladdin Chemical Reagent Co., Ltd., and used without further treatment.
Y. Lin et al. / Optics Communications 346 (2015) 64–68
65
Fig. 1. (A and B) FE-SEM images of SnO2 nanoparticles, (C and D) images of the CdS(16)/SnO2 photo-electrodes, (E and F) images of CdSe QDs-sensitized CdS(16)/SnO2 photoelectrodes, and (G and H) the energy-dispersive X-ray spectroscopy (EDS) spectrum of CdS(16)/SnO2 photo-electrodes before and after deposit CdSe QDs.
2.2. Synthesis of SnO2 nanoparticles The SnO2 nanoparticles were prepared by the hydrothermal method. The typical procedure is as follows: 0.075 g of Na2SnO3 4H2O was dissolved in 35 ml deionized water containing 3–5 ml TEA. The solution was stirred violently for 30 min until a white suspension was obtained. The suitable solvent system was transferred into an autoclave and kept 180 °C for 24 h. After the reaction, the autoclave was cooled naturally and the product was collected by centrifugation after washing with distilled water several times. After being dried over night at 80 °C, the whole products were annealed at 450 °C for 30 min. 2.3. Fabrication of CdS and CdSe/CdS coupled QDs-sensitized SnO2 photoelectrodes SnO2 paste was prepared by mixing with 2 ml terpineol, 0.2 g ethyl cellulose, 5 ml ethanol and 1.0 g SnO2 nanoparticles. The
solution was thoroughly mixed by stirring and keeping at 100 °C for 3 h to yield a slurry. SnO2 photo-electrodes for QDSSCs were prepared by coating the SnO2 paste on the FTO with doctor-blade technique [18] and annealing at 450 °C for 30 min to remove the organic matters. The SnO2 photo-electrodes were sensitized with CdS QDs by SILAR method. SnO2 films on FTO glasses were dipped in a solution containing 0.1 M Cd(NO3)2 4H2O in ethanol for 2 min and rinsed with ethanol. Subsequently, the photo-electrodes were dipped in a methyl alcohol containing 0.1 M Na2S 9H2O for another 2 min and wished with ethanol. All these steps were one SILAR cycle. After 16 cycles, the production denoted by CdS(16)/SnO2 was ready and the 16 was defined as the SILAR cycles in number. For the deposition of CdSe, the CdS(16)/SnO2 photo-electrodes were immersed in a Se2 source solution and keeping at 80 °C for 4 h. The solution contained 0.03 g of Se and 0.1 g of NaBH4 in 60 ml of dissolved in water. In order to acquire a ZnS passivation layer, the former prepared QDs sensitized SnO2 photo-electrodes were immersed
66
Y. Lin et al. / Optics Communications 346 (2015) 64–68
Fig. 2. TEM images of (A) bare SnO2 nanoparticles, (B) CdS(16)/SnO2 photo-electrodes and (C) CdSe(4 h)/CdS(16)/SnO2 photo-electrodes.
Fig. 3. XRD patterns of (a) bare SnO2 nanoparticles, (b) CdS(16)/SnO2 photo-electrodes and (c) CdSe(4 h)/CdS(16)/SnO2 photo-electrodes.
Fig. 4. UV–vis absorption spectra of bare SnO2 nanoparticles, CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes.
2.4. Assembly of QDSSCs into 0.1 M Zn(NO3)2 alcohol solution and 0.1 M Na2S methanol solution for 1 min alternately and rinsed with alcohol. After each immersion in the precursor solution process, the ZnS deposition cycles were done for 2 times and dried in the air.
After being sensitized, the composite electrodes were used as the photo-electrodes. The photo-electrodes were sandwiched with platinum electrodes and then the electrolyte was dropped into the aperture between the two electrodes. The liquid electrolyte containing 0.5 M Na2S, 2 M S and 0.2 M KCl in a methanol/water (7:3 by volume) solution.
Y. Lin et al. / Optics Communications 346 (2015) 64–68
67
UV–vis spectrophotometer (UV-2550, Shimadzu, Japan). Photoelectrochemical tests were carried out by measuring the current density–voltage (J–V) characteristic curves under simulated AM 1.5 solar illumination at 100 mW cm 2 from a xenon arc lamp (CHFXM500, Trusttech Co. Ltd., China) in an ambient atmosphere. Electrochemical impedance spectroscopy (EIS) measurements were analyzed in the dark. All measurements were performed at a CHI660E electrochemical workstation (CH Instrument Inc., China).
3. Results and discussion 3.1. Microstructure and optical performance of SnO2 photoelectrodes
Fig. 5. J–V curves of the QDSSCs CdSe(4 h)/CdS(16)/SnO2 photo-electrodes.
based
on
CdS(16)/SnO2
and
Table 1 Parameters of QDSSCs based on CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photoelectrodes. Samples
Jsc (mA cm 2)
Voc (V)
FF
PCE (%)
CdS(16)/SnO2 CdSe(4 h)/CdS(16)/SnO2
4.98 5.76
0.611 0.612
0.351 0.505
1.07 1.78
Fig. 6. EIS spectra of the QDSSCs based on CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes under dark condition. Table 2 Performances determined by EIS measurements. QDSSCs
Rs (Ω cm2)
R1 (Ω cm2)
R2 (Ω cm2)
CdS(16)/SnO2 CdSe(4 h)/CdS(16)/SnO2
4.46 4.95
16.5 25.4
26.3 27.8
The FE-SEM images of the SnO2 nanoparticles samples (Fig. 1A and B) illustrate that a typical morphology of SnO2 nanostructure compose of nanoparticles with high porosity which is much suitable for deposition of CdS or CdSe QDs to boost the performance of the SnO2 photo-electrodes. After CdS and CdSe QDs sensitize SnO2 photo-electrodes as seen in Fig. 1C–F, the appearances of SnO2 films are covered by some nanoparticles compared with the bare SnO2 films (Fig. 1A and B). Fig. 1E and F shows the surface of CdSe(4)/CdS(16)/SnO2 photo-electrodes via using CBD method to sedimentate CdSe QDs on CdS(16)/SnO2 samples. From the energydispersive X-ray spectroscopy (EDS) images in Fig. 1G and H, it proves that the Cd2 þ , S2 and Se2 ions have been deposited on the SnO2 photo-electrodes successfully. Fig. 2A shows the TEM images of SnO2 nanoparticles without QDs deposition. After the CdS and CdSe QDs deposition as seen in Fig. 2B and C, the surface of the SnO2 nanaoparticles adhere to a thin film, which indicates that QDs have been effectively deposited on the SnO2 photoelectrodes. To clarify the QDs deposited successfully further, we offer a very typical testing method XRD in the next step. Fig. 3 displays the XRD patterns of the bare SnO2 nanoparticles, CdS(16)/SnO2 photo-electrodes and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes. It can be seen that the diffraction peaks are indexed to the tetragonal rutile structure of SnO2 as (110), (101), (200) and (211) etc. in contrast with JCPDS card no. 41-1445 in Fig. 3A. The sharp diffraction peaks point the high crystallinity of the prepared SnO2 nanoparticles. In Fig. 3B, the (101) and (110) diffraction peaks situated at 28.2° and 43.7° are weak, which facilitate the identification of successful precipitation for CdS QDs and the layer is very thin. After 4 h of CBD deposition, the CdSe QDs overlayer is also thin, so the (100) and (102) diffraction peaks at 23.9° and 35.1° are particularly weak as shown in Fig. 3C. Obviously, it presents that the CdS and CdSe QDs successfully are deposited as a thin layer combined with the EDS and TEM. The UV–vis absorption spectra of bare SnO2 nanoparticles, CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/ SnO2 photo-electrodes are revealed in Fig. 4. The absorption edge has been greatly enhanced in the wavelength from 300 nm to 550 nm with CdS and CdSe deposition on the SnO2 films. It shows that after covering CdSe QDs on the CdS(16)/SnO2 photo-electrodes, a further increase in absorbance is obtained. Especially, the optical absorption is heightened and desirable for solar light harvesting, so the cosensitization effect of CdS and CdSe can enhance QDSSCs performance.
2.5. Characterization 3.2. Photoelectrochemical performance of SnO2 photo-electrodes The surface morphology of SnO2 photo-electrodes were characterized by field-emission scanning electron microscope (FE-SEM, Hitachi SU8010) and a transmission electron microscope (TEM, H-7650, Hitachi, Japan). The crystal structures were analyzed by X-ray diffraction (XRD, Cu Kα radiation, SmartLab 3 kW, Rigaku, Japan). The optical performances of the samples were studied by a
The J–V curves of the QDSSCs based on CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes are shown in Fig. 5. The total cycles of CdS deposition are acquired to 16 SILAR cycles (the optimal PCE of the CdS deposition are controlled through increasing the cycles of SILAR method). The open circuit potential
68
Y. Lin et al. / Optics Communications 346 (2015) 64–68
(Voc), fill factor (FF), short circuit current (Jsc) and the PCE of QDSSCs assembled with the different QDs and cell structure are listed in Table 1. The efficiencies measured for the CdS and CdS/ CdSe co-sensitized cells are 1.07% and 1.78%, respectively. The higher efficiency of CdS/CdSe co-sensitized cells is attributed to the FF, probably due to the higher driving force for the electron injection. On the other hand, the broader light-absorption range leads to a higher Jsc (5.76 mA/cm2) compared with that of CdS sensitized cells (4.98 mA/cm2). Fig. 6 shows the Nyquist plot of cells based on CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes at an applied bias of Voc and a frequency range from 10 1 Hz to 106 Hz under dark condition. Two well-defined arcs extending from total series resistance (Rs) are observed in the measured frequency range. The first arc (R1) in the high frequency region stands for the electrons transfer at the counter electrode/electrolyte interface, while the second semicircle (R2) in the intermediate frequency stands for the electrons transfer at the photo-electrode/electrolyte interface [19]. The fitted data are shown in Table 2. It can be observed from Table 2 that the R2 value (27.8 Ω cm2) of CdSe(4 h)/CdS(16)/SnO2 photo-electrodes is relatively larger than that of CdS(16)/SnO2 photo-electrodes (26.3 Ω cm2). Therefore, it reflects the increase of the charge transfer resistance of CdSe(4 h)/CdS(16)/SnO2 photoelectrodes on the electrode/electrolyte interface under dark condition. The higher electron transfer resistance in the CdSe(4 h)/CdS(16)/SnO2 photo-electrodes can decrease the probability of electron recombination at the SnO2/QDs/electrolyte interface, which is contributed to its better photoelectric properties.
4. Conclusions Above all, SnO2 nanoparticles had been successfully synthesized by a hydrothermal method. Using the SILAR and CBD method, CdS and CdSe QDs were prepared and deposited on the SnO2 films. CdS and CdSe co-sensitized cells could reach 1.78% of PEC, which showed 66.4% improvement than that of single CdS sensitized solar cells. The analysis from EIS showed that CdSe covering on the CdS sensitized SnO2 films weakened the electron recombination in the interface of photo-electrode and electrolyte, leading to increase the photocurrent of the cells.
Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (Grant no. JB-ZR1137).
References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] A. Yella, H.W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E. W. Diau, C. Yeh, S.M. Zakeeruddin, M. Grätzel, Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency, Science 334 (2011) 629–634. [3] X. Tian, Y. Hao, Y. Zhang, Y. Cui, T. Ji, H. Wang, B. Wei, W. Huang, Omnidirectional and broadband optical absorption enhancement in small molecule organic solar cells by a patterned MoO3/Ag/MoO3 transparent anode, Opt. Commun. 338 (2015) 226–232. [4] Z. Yang, C. Chen, P. Roy, H. Chang, Quantum dot-sensitized solar cells incorporating nanomaterials, Chem. Commun. 47 (2011) 9561–9571. [5] J. Tian, G. Cao, Semiconductor quantum dot-sensitized solar cells, Nano Rev. 4 (2013) 22578. [6] H.K. Jun, M.A. Careem, A.K. Arof, Quantum dot-sensitized solar cells-perspective and recent developments: a review of Cd chalcogenide quantum dots as sensitizers, Renew. Sustain. Energy Rev. 22 (2013) 148–167. [7] J. Zhou, B. Song, G. Zhao, W. Dong, G. Han, TiO2 nanorod arrays sensitized with CdS quantum dots for solar cell applications: effects of rod geometry on photoelectrochemical performance, Appl. Phys. A 107 (2012) 321–331. [8] A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, P.V. Kamat, Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe–TiO2 architecture, J. Am. Chem. Soc. 130 (2008) 4007–4015. [9] 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. [10] L. Sai, X.Y. Kong, Electrostatic assembly of CdTe quantum dots with different charged ligands into TiO2 porous film for solar cells, Appl. Phys. A 114 (2014) 1153–1160. [11] Y. Xu, N. Al-Salim, C.W. Bumby, R.D. Tilley, Synthesis of SnS quantum dots, J. Am. Chem. Soc. 131 (2009) 15990–15991. [12] Y. Itahaik, O. Niitsoo, M. Page, G. Hodes, Sb2S3-sensitized nanoporous TiO2 solar cells, J. Phys. Chem. C 113 (2009) 4254–4256. [13] K.S. Pralay, V.K. Prashant, Mn-doped quantum dot sensitized solar cells: a strategy to boost efficiency over 5%, J. Am. Chem. Soc. 134 (2012) 2508–2511. [14] Q. Zhang, G. Chen, Y. Yang, X. Shen, Y. Zhang, C. Li, R. Yu, Y. Luo, D. Li, Q. Meng, Toward highly efficient CdS/CdSe quantum dots-sensitized solar cells incorporating ordered photoanodes on transparent conductive substrates, Phys. Chem. Chem. Phys. 14 (2012) 6479–6486. [15] R.E. Chandler, A.J. Houtepen, J. Nelson, D. Vanmaekelbergh, Electron transport in quantum dot solids: Monte Carlo simulations of the effects of shell filling, Coulomb repulsions, and site disorder, Phys. Rev. B 75 (2007) 085325. [16] S.S. Bhande, E.K. Kim, D.V. Shinde, S. Patil, R.S. Mane, S.H. Han, Uprightstanding SnO2 nanosheets glazed CdSe nanocrystallites: high photocurrent generating photoelectrochemical cells, Int. J. Electrochem. Sci. 8 (2013) 11596–11605. [17] S. Hao, J. Wu, L. Fan, Y. Huang, J. Lin, Y. Wei, The influence of acid treatment of TiO2 porous film electrode on photoelectric performance of dye-sensitized solar cell, Sol. Energy 76 (2004) 745–750. [18] Q. Wang, S. Ito, M. Gratzel, F. Fabregat-Santiago, I. Mora-Sero, J. Bisquert, T. Bessho, H. Imai, Characteristics of high efficiency dye-sensitized solar cells, J. Phys. Chem. B 110 (2006) 25210–25221. [19] N. Huang, Y. Liu, T. Peng, X. Sun, B. Sebo, Q. Tai, H. Hu, B. Chen, S. Guo, X. Zhao, Synergistic effects of ZnO compact layer and TiCl4 post-treatment for dyesensitized solar cells, J. Power Sources 204 (2012) 257–264.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
CdS/CdSe co-sensitized SnO2 photoelectrodes for quantum dots sensitized solar cells Yibing Lin, Yu Lin n, Yongming Meng, Yongguang Tu, Xiaolong Zhang College of Material Science and Engineering, Engineering Research Center of Environment-Friendly Functional Materials for Ministry of Education, Huaqiao University, Xiamen, Fujian 361021, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 December 2014 Received in revised form 2 February 2015 Accepted 17 February 2015 Available online 18 February 2015
SnO2 nanoparticles were synthesized by hydrothermal method and applied to photo-electrodes of quantum dots-sensitized solar cells (QDSSCs). After sensitizing SnO2 films via CdS quantum dots, CdSe quantum dots was decorated on the surface of CdS/SnO2 photo-electrodes to further improve the power conversion efficiency. CdS and CdSe quantum dots were deposited by successive ionic layer absorption and reaction method (SILAR) and chemical bath deposition method (CBD) respectively. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to identify the surface profile and crystal structure of SnO2 photo-electrodes before and after deposited quantum dots. After CdSe co-sensitized process, an overall power conversion efficiency of 1.78% was obtained in CdSe/CdS/SnO2 QDSSC, which showed 66.4% improvement than that of CdS/SnO2 QDSSC. & 2015 Elsevier B.V. All rights reserved.
Keywords: SnO2 CdS CdSe Quantum dots-sensitized solar cells
1. Introduction Energy crisis has been an eager issue to be solved in the world. Solar energy, a type of clean and abundant natural power source, is regarded as the most promising energetic resource for the solution of energy problems currently. Dye-sensitized solar cell (DSSC), a neoteric power conversion device, has attracted substantial attention since its first appearance in 1991 due to its low cost, simple fabrication and enormous application prospect [1]. But during the past decades, the improvement of the power convention efficiency (PCE) of DSSCs is not obvious [2–4]. So the research on more functional metal oxides and sensitizers to further improve the efficiency has been studied extensively. With the research going further, many researchers begin to concern about the influence of quantum dot-sensitized solar cells (QDSSCs) [5–7]. Due to the intrinsic attractive properties of quantum dots (QDs) with tunable band gap energy, high extinction coefficients and large dipole moment, a wide variety of QDs have been investigated as sensitizers for QDSSCs, such as CdSe [8], PbS [9], CdTe [10], SnS [11] and Sb2S3 [12]. Among the semiconductor QDs, CdS and CdSe with high potential in light harvesting have been paid much attention. Recently, Pralay et al. use the coupled QDs (CdS and CdSe) to sensitize TiO2 nanoparticles solar cells and provide a noteworthy value of PCE as high as 5.4% [13]. More research have suggested that coupled QDs-sensitized n
Corresponding author. Fax: þ86 592 6162225. E-mail address: [email protected] (Yu Lin).
http://dx.doi.org/10.1016/j.optcom.2015.02.031 0030-4018/& 2015 Elsevier B.V. All rights reserved.
solar cells systems are advantageous over single QDs-sensitized cells [14,15]. Simultaneously as the potential substitute for TiO2, other appropriate semiconductor oxides are being searched. SnO2 has a faster electron diffusion rate compared to TiO2 because of its higher electron-mobility [16,17]. And the larger band gap of SnO2 (3.6 eV) can decrease oxidative holes in the valence band to facilitate the stability of QDSSCs than TiO2 (3.2 eV). Herein, SnO2 nanoparticles were prepared through the hydrothermal method and used as the photo-electrodes materials in QDSSCs. The CdS and CdSe QDs were deposited on the surface of SnO2 photo-electrodes as the sensitizers by SILAR and CBD method respectively. The surface morphology, crystal structure, UV–vis absorption properties and the photovoltaic performance of the CdS sensitized and CdS/CdSe co-sensitized SnO2 photo-electrodes were investigated and compared.
2. Experimental section 2.1. Materials All reagents including sodium stannate terahydrate (Na2SnO3 4H2O), cadmium nitrate terahydrate [Cd(NO3)2 4H2O], sodium sulfide nonohydrate (Na2S 9H2O), triethanolamine (TEA), zinc nitrate hexahydrate (Zn(NO3)2 6H2O), potassium chloride (KCl) and sulfur sublimed (S) were AR. Grade, purchased from Aladdin Chemical Reagent Co., Ltd., and used without further treatment.
Y. Lin et al. / Optics Communications 346 (2015) 64–68
65
Fig. 1. (A and B) FE-SEM images of SnO2 nanoparticles, (C and D) images of the CdS(16)/SnO2 photo-electrodes, (E and F) images of CdSe QDs-sensitized CdS(16)/SnO2 photoelectrodes, and (G and H) the energy-dispersive X-ray spectroscopy (EDS) spectrum of CdS(16)/SnO2 photo-electrodes before and after deposit CdSe QDs.
2.2. Synthesis of SnO2 nanoparticles The SnO2 nanoparticles were prepared by the hydrothermal method. The typical procedure is as follows: 0.075 g of Na2SnO3 4H2O was dissolved in 35 ml deionized water containing 3–5 ml TEA. The solution was stirred violently for 30 min until a white suspension was obtained. The suitable solvent system was transferred into an autoclave and kept 180 °C for 24 h. After the reaction, the autoclave was cooled naturally and the product was collected by centrifugation after washing with distilled water several times. After being dried over night at 80 °C, the whole products were annealed at 450 °C for 30 min. 2.3. Fabrication of CdS and CdSe/CdS coupled QDs-sensitized SnO2 photoelectrodes SnO2 paste was prepared by mixing with 2 ml terpineol, 0.2 g ethyl cellulose, 5 ml ethanol and 1.0 g SnO2 nanoparticles. The
solution was thoroughly mixed by stirring and keeping at 100 °C for 3 h to yield a slurry. SnO2 photo-electrodes for QDSSCs were prepared by coating the SnO2 paste on the FTO with doctor-blade technique [18] and annealing at 450 °C for 30 min to remove the organic matters. The SnO2 photo-electrodes were sensitized with CdS QDs by SILAR method. SnO2 films on FTO glasses were dipped in a solution containing 0.1 M Cd(NO3)2 4H2O in ethanol for 2 min and rinsed with ethanol. Subsequently, the photo-electrodes were dipped in a methyl alcohol containing 0.1 M Na2S 9H2O for another 2 min and wished with ethanol. All these steps were one SILAR cycle. After 16 cycles, the production denoted by CdS(16)/SnO2 was ready and the 16 was defined as the SILAR cycles in number. For the deposition of CdSe, the CdS(16)/SnO2 photo-electrodes were immersed in a Se2 source solution and keeping at 80 °C for 4 h. The solution contained 0.03 g of Se and 0.1 g of NaBH4 in 60 ml of dissolved in water. In order to acquire a ZnS passivation layer, the former prepared QDs sensitized SnO2 photo-electrodes were immersed
66
Y. Lin et al. / Optics Communications 346 (2015) 64–68
Fig. 2. TEM images of (A) bare SnO2 nanoparticles, (B) CdS(16)/SnO2 photo-electrodes and (C) CdSe(4 h)/CdS(16)/SnO2 photo-electrodes.
Fig. 3. XRD patterns of (a) bare SnO2 nanoparticles, (b) CdS(16)/SnO2 photo-electrodes and (c) CdSe(4 h)/CdS(16)/SnO2 photo-electrodes.
Fig. 4. UV–vis absorption spectra of bare SnO2 nanoparticles, CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes.
2.4. Assembly of QDSSCs into 0.1 M Zn(NO3)2 alcohol solution and 0.1 M Na2S methanol solution for 1 min alternately and rinsed with alcohol. After each immersion in the precursor solution process, the ZnS deposition cycles were done for 2 times and dried in the air.
After being sensitized, the composite electrodes were used as the photo-electrodes. The photo-electrodes were sandwiched with platinum electrodes and then the electrolyte was dropped into the aperture between the two electrodes. The liquid electrolyte containing 0.5 M Na2S, 2 M S and 0.2 M KCl in a methanol/water (7:3 by volume) solution.
Y. Lin et al. / Optics Communications 346 (2015) 64–68
67
UV–vis spectrophotometer (UV-2550, Shimadzu, Japan). Photoelectrochemical tests were carried out by measuring the current density–voltage (J–V) characteristic curves under simulated AM 1.5 solar illumination at 100 mW cm 2 from a xenon arc lamp (CHFXM500, Trusttech Co. Ltd., China) in an ambient atmosphere. Electrochemical impedance spectroscopy (EIS) measurements were analyzed in the dark. All measurements were performed at a CHI660E electrochemical workstation (CH Instrument Inc., China).
3. Results and discussion 3.1. Microstructure and optical performance of SnO2 photoelectrodes
Fig. 5. J–V curves of the QDSSCs CdSe(4 h)/CdS(16)/SnO2 photo-electrodes.
based
on
CdS(16)/SnO2
and
Table 1 Parameters of QDSSCs based on CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photoelectrodes. Samples
Jsc (mA cm 2)
Voc (V)
FF
PCE (%)
CdS(16)/SnO2 CdSe(4 h)/CdS(16)/SnO2
4.98 5.76
0.611 0.612
0.351 0.505
1.07 1.78
Fig. 6. EIS spectra of the QDSSCs based on CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes under dark condition. Table 2 Performances determined by EIS measurements. QDSSCs
Rs (Ω cm2)
R1 (Ω cm2)
R2 (Ω cm2)
CdS(16)/SnO2 CdSe(4 h)/CdS(16)/SnO2
4.46 4.95
16.5 25.4
26.3 27.8
The FE-SEM images of the SnO2 nanoparticles samples (Fig. 1A and B) illustrate that a typical morphology of SnO2 nanostructure compose of nanoparticles with high porosity which is much suitable for deposition of CdS or CdSe QDs to boost the performance of the SnO2 photo-electrodes. After CdS and CdSe QDs sensitize SnO2 photo-electrodes as seen in Fig. 1C–F, the appearances of SnO2 films are covered by some nanoparticles compared with the bare SnO2 films (Fig. 1A and B). Fig. 1E and F shows the surface of CdSe(4)/CdS(16)/SnO2 photo-electrodes via using CBD method to sedimentate CdSe QDs on CdS(16)/SnO2 samples. From the energydispersive X-ray spectroscopy (EDS) images in Fig. 1G and H, it proves that the Cd2 þ , S2 and Se2 ions have been deposited on the SnO2 photo-electrodes successfully. Fig. 2A shows the TEM images of SnO2 nanoparticles without QDs deposition. After the CdS and CdSe QDs deposition as seen in Fig. 2B and C, the surface of the SnO2 nanaoparticles adhere to a thin film, which indicates that QDs have been effectively deposited on the SnO2 photoelectrodes. To clarify the QDs deposited successfully further, we offer a very typical testing method XRD in the next step. Fig. 3 displays the XRD patterns of the bare SnO2 nanoparticles, CdS(16)/SnO2 photo-electrodes and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes. It can be seen that the diffraction peaks are indexed to the tetragonal rutile structure of SnO2 as (110), (101), (200) and (211) etc. in contrast with JCPDS card no. 41-1445 in Fig. 3A. The sharp diffraction peaks point the high crystallinity of the prepared SnO2 nanoparticles. In Fig. 3B, the (101) and (110) diffraction peaks situated at 28.2° and 43.7° are weak, which facilitate the identification of successful precipitation for CdS QDs and the layer is very thin. After 4 h of CBD deposition, the CdSe QDs overlayer is also thin, so the (100) and (102) diffraction peaks at 23.9° and 35.1° are particularly weak as shown in Fig. 3C. Obviously, it presents that the CdS and CdSe QDs successfully are deposited as a thin layer combined with the EDS and TEM. The UV–vis absorption spectra of bare SnO2 nanoparticles, CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/ SnO2 photo-electrodes are revealed in Fig. 4. The absorption edge has been greatly enhanced in the wavelength from 300 nm to 550 nm with CdS and CdSe deposition on the SnO2 films. It shows that after covering CdSe QDs on the CdS(16)/SnO2 photo-electrodes, a further increase in absorbance is obtained. Especially, the optical absorption is heightened and desirable for solar light harvesting, so the cosensitization effect of CdS and CdSe can enhance QDSSCs performance.
2.5. Characterization 3.2. Photoelectrochemical performance of SnO2 photo-electrodes The surface morphology of SnO2 photo-electrodes were characterized by field-emission scanning electron microscope (FE-SEM, Hitachi SU8010) and a transmission electron microscope (TEM, H-7650, Hitachi, Japan). The crystal structures were analyzed by X-ray diffraction (XRD, Cu Kα radiation, SmartLab 3 kW, Rigaku, Japan). The optical performances of the samples were studied by a
The J–V curves of the QDSSCs based on CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes are shown in Fig. 5. The total cycles of CdS deposition are acquired to 16 SILAR cycles (the optimal PCE of the CdS deposition are controlled through increasing the cycles of SILAR method). The open circuit potential
68
Y. Lin et al. / Optics Communications 346 (2015) 64–68
(Voc), fill factor (FF), short circuit current (Jsc) and the PCE of QDSSCs assembled with the different QDs and cell structure are listed in Table 1. The efficiencies measured for the CdS and CdS/ CdSe co-sensitized cells are 1.07% and 1.78%, respectively. The higher efficiency of CdS/CdSe co-sensitized cells is attributed to the FF, probably due to the higher driving force for the electron injection. On the other hand, the broader light-absorption range leads to a higher Jsc (5.76 mA/cm2) compared with that of CdS sensitized cells (4.98 mA/cm2). Fig. 6 shows the Nyquist plot of cells based on CdS(16)/SnO2 and CdSe(4 h)/CdS(16)/SnO2 photo-electrodes at an applied bias of Voc and a frequency range from 10 1 Hz to 106 Hz under dark condition. Two well-defined arcs extending from total series resistance (Rs) are observed in the measured frequency range. The first arc (R1) in the high frequency region stands for the electrons transfer at the counter electrode/electrolyte interface, while the second semicircle (R2) in the intermediate frequency stands for the electrons transfer at the photo-electrode/electrolyte interface [19]. The fitted data are shown in Table 2. It can be observed from Table 2 that the R2 value (27.8 Ω cm2) of CdSe(4 h)/CdS(16)/SnO2 photo-electrodes is relatively larger than that of CdS(16)/SnO2 photo-electrodes (26.3 Ω cm2). Therefore, it reflects the increase of the charge transfer resistance of CdSe(4 h)/CdS(16)/SnO2 photoelectrodes on the electrode/electrolyte interface under dark condition. The higher electron transfer resistance in the CdSe(4 h)/CdS(16)/SnO2 photo-electrodes can decrease the probability of electron recombination at the SnO2/QDs/electrolyte interface, which is contributed to its better photoelectric properties.
4. Conclusions Above all, SnO2 nanoparticles had been successfully synthesized by a hydrothermal method. Using the SILAR and CBD method, CdS and CdSe QDs were prepared and deposited on the SnO2 films. CdS and CdSe co-sensitized cells could reach 1.78% of PEC, which showed 66.4% improvement than that of single CdS sensitized solar cells. The analysis from EIS showed that CdSe covering on the CdS sensitized SnO2 films weakened the electron recombination in the interface of photo-electrode and electrolyte, leading to increase the photocurrent of the cells.
Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (Grant no. JB-ZR1137).
References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] A. Yella, H.W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E. W. Diau, C. Yeh, S.M. Zakeeruddin, M. Grätzel, Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency, Science 334 (2011) 629–634. [3] X. Tian, Y. Hao, Y. Zhang, Y. Cui, T. Ji, H. Wang, B. Wei, W. Huang, Omnidirectional and broadband optical absorption enhancement in small molecule organic solar cells by a patterned MoO3/Ag/MoO3 transparent anode, Opt. Commun. 338 (2015) 226–232. [4] Z. Yang, C. Chen, P. Roy, H. Chang, Quantum dot-sensitized solar cells incorporating nanomaterials, Chem. Commun. 47 (2011) 9561–9571. [5] J. Tian, G. Cao, Semiconductor quantum dot-sensitized solar cells, Nano Rev. 4 (2013) 22578. [6] H.K. Jun, M.A. Careem, A.K. Arof, Quantum dot-sensitized solar cells-perspective and recent developments: a review of Cd chalcogenide quantum dots as sensitizers, Renew. Sustain. Energy Rev. 22 (2013) 148–167. [7] J. Zhou, B. Song, G. Zhao, W. Dong, G. Han, TiO2 nanorod arrays sensitized with CdS quantum dots for solar cell applications: effects of rod geometry on photoelectrochemical performance, Appl. Phys. A 107 (2012) 321–331. [8] A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, P.V. Kamat, Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe–TiO2 architecture, J. Am. Chem. Soc. 130 (2008) 4007–4015. [9] 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. [10] L. Sai, X.Y. Kong, Electrostatic assembly of CdTe quantum dots with different charged ligands into TiO2 porous film for solar cells, Appl. Phys. A 114 (2014) 1153–1160. [11] Y. Xu, N. Al-Salim, C.W. Bumby, R.D. Tilley, Synthesis of SnS quantum dots, J. Am. Chem. Soc. 131 (2009) 15990–15991. [12] Y. Itahaik, O. Niitsoo, M. Page, G. Hodes, Sb2S3-sensitized nanoporous TiO2 solar cells, J. Phys. Chem. C 113 (2009) 4254–4256. [13] K.S. Pralay, V.K. Prashant, Mn-doped quantum dot sensitized solar cells: a strategy to boost efficiency over 5%, J. Am. Chem. Soc. 134 (2012) 2508–2511. [14] Q. Zhang, G. Chen, Y. Yang, X. Shen, Y. Zhang, C. Li, R. Yu, Y. Luo, D. Li, Q. Meng, Toward highly efficient CdS/CdSe quantum dots-sensitized solar cells incorporating ordered photoanodes on transparent conductive substrates, Phys. Chem. Chem. Phys. 14 (2012) 6479–6486. [15] R.E. Chandler, A.J. Houtepen, J. Nelson, D. Vanmaekelbergh, Electron transport in quantum dot solids: Monte Carlo simulations of the effects of shell filling, Coulomb repulsions, and site disorder, Phys. Rev. B 75 (2007) 085325. [16] S.S. Bhande, E.K. Kim, D.V. Shinde, S. Patil, R.S. Mane, S.H. Han, Uprightstanding SnO2 nanosheets glazed CdSe nanocrystallites: high photocurrent generating photoelectrochemical cells, Int. J. Electrochem. Sci. 8 (2013) 11596–11605. [17] S. Hao, J. Wu, L. Fan, Y. Huang, J. Lin, Y. Wei, The influence of acid treatment of TiO2 porous film electrode on photoelectric performance of dye-sensitized solar cell, Sol. Energy 76 (2004) 745–750. [18] Q. Wang, S. Ito, M. Gratzel, F. Fabregat-Santiago, I. Mora-Sero, J. Bisquert, T. Bessho, H. Imai, Characteristics of high efficiency dye-sensitized solar cells, J. Phys. Chem. B 110 (2006) 25210–25221. [19] N. Huang, Y. Liu, T. Peng, X. Sun, B. Sebo, Q. Tai, H. Hu, B. Chen, S. Guo, X. Zhao, Synergistic effects of ZnO compact layer and TiCl4 post-treatment for dyesensitized solar cells, J. Power Sources 204 (2012) 257–264.