- Email: [email protected]
Growth of ZnO nanowires and their applications for CdS quantum dots sensitized solar cells
Optik 149 (2017) 63–68
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Growth of ZnO nanowires and their applications for CdS quantum dots sensitized solar cells Zukang Mo, Ying Huang, Shanshan Lu, Yuechun Fu ∗ , Xiaoming Shen, Huan He Center of Ecological Collaborative Innovation for Aluminum Industry in Guangxi, Guangxi Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, College of Materials Science and Engineering, Guangxi University, Nanning 530004, China
a r t i c l e
i n f o
Article history: Received 22 May 2017 Accepted 8 September 2017 Keywords: ZnO nanowires CdS quantum dots Solar cells Energy conversion efficiency
a b s t r a c t ZnO nanowires grown on ITO conductive glass substrates by chemical bath deposition (CBD) method were used as photoanodes to assemble the CdS quantum dots sensitized solar cells (QDSSCs). The growth mechanism of ZnO nanowires and the photovoltaic performance of CdS QDSSCs were investigated. The results show that the c-axis oriented seed layer and the growth process both contribute to the preferential alignment of ZnO nanowires along the [0001] direction. The average length and diameter of nanowires increase with increasing growth time, and the maximum aspect ratio is 20.56 at 9 h. CdS quantum dots deposited on ZnO nanowires enhance the absorbance and extend the absorption range to the visible region. ZnO nanowires with higher aspect ratio effectively increase the energy conversion efficiency () of CdS QDSSCs, and the best is 0.401% with an aspect ratio of 20.56. In such a solar cell, the short-circuit current density is significantly improved due to more electron-hole pairs and strong light trapping effect of ZnO nanowires. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction CdS quantum dots sensitized ZnO nanowires solar cells have attracted intense attention recently for the potential applications as photovoltaic devices. One-dimensional (1D) ZnO nanowires possess high electron mobility and surface area, which provide the direct paths for charge transport and efficient light harvesting [1]. CdS quantum dots own a reasonable band gap and high conduction band edge in contrast to the ZnO photoanodes, which are favorable for absorbing visible light and inducing a high open-circuit voltage [2,3]. Thus, high photovoltaic performance of this solar cell can be expected. Though much effort has been made to the development of ZnO nanowires or nanorods based CdS quantum dots sensitized solar cells (QDSSCs), their energy conversion efficiencies are still relatively low [4–6]. One major challenge is how to prepare well crystallized and aligned 1D ZnO nanostructures. Among various synthesis methods for 1D ZnO nanostructures, chemical bath deposition (CBD) technique has drawn much attention because of its simple coating process, low temperature, reproducibility, and low cost of equipment [7]. In this process, growth parameters such as seed layer, reactant concentration, growth temperature, etc., have been reported to affect the quality of 1D ZnO nanostructures [8–11]. Nevertheless, their growth mechanism should be further understood by exploring the forming process. In this paper, ZnO nanowires were grown on ITO conductive glass substrates by CBD method for different time. The crystallinity and morphology evolutions of ZnO nanowires and the photovoltaic performance of CdS QDSSCs were investigated.
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Fu). http://dx.doi.org/10.1016/j.ijleo.2017.09.035 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
64
Z. Mo et al. / Optik 149 (2017) 63–68
Fig. 1. XRD patterns of ZnO seed layer and ZnO nanowires as a function of growth time. The inset is the enlarged XRD pattern of ZnO seed layer.
2. Experimental ZnO nanowires were grown on ITO conductive glass substrates by CBD method. Prior to the growth of nanowires, ZnO seed layers (∼130 nm) were deposited on the substrates in a pulsed laser deposition (PLD) system which can be evacuated to a base pressure of 10−5 Pa. A pulsed KrF excimer laser beam ( = 248 nm, = 10 ns) with the repetition rate of 5 Hz was focused on the ZnO (99.9% purity) target which was mounted 5 cm apart from the substrate. Depositions were carried out at the substrate temperature of 200 ◦ C, laser energy of 180 mJ/p, and working pressure of 0.8 Pa using O2 (99.999% purity) as the background gas. Subsequently, the seeded substrates with an area of about 0.7 cm2 were dipped face-down in an aqueous solution containing 0.05 M zinc nitrate hexahydrate, 0.05 M methenamine and 4.5 mM polyethyleneimine to grow ZnO nanowires. After growth at 95 ◦ C in a water bath for 3–12 h, the samples were thoroughly rinsed with deionized water and dried in air. CdS quantum dots were deposited on ZnO nanowires by successive ion layer adsorption and reaction (SILAR) method. ZnO nanowires were successively dipped in 0.05 M CdCl2 ethanol solution, ethanol, 0.05 M Na2 S methanol solution and methanol for 30 s, and the dipping procedure was repeated for 12 cycles. For the assembly of QDSSCs, the Pt counter electrodes were prepared by drop-casting an ethanolic H2 PtCl6 ·6H2 O solution onto the ITO substrates and annealing at 500 ◦ C for 30 min. A seal film under heating (100 ◦ C) was sandwiched between the CdS quantum dots sensitized ZnO nanowires photoanode and the Pt-coated ITO counter electrode. An electrolyte (0.3 M tetra-butyl-ammonium iodide, 0.06 M LiI, 0.03 M I2 , 0.5 M 4-tert-butylpyridine in acetonitrile solvent) was then injected into the space between the two electrodes. The crystallinity and morphology of ZnO nanowires were characterized by X-ray diffraction (XRD, Rigaku D/MAX-RB) and scanning electron microscopy (SEM, FEI Quanta-400). The optical absorption spectra of CdS quantum dots sensitized ZnO nanowires were measured by UV–vis spectrophotometer (PerkinElmer Lambda 950). Current-voltage (I–V) characteristics of QDSSCs were recorded by semiconductor device analyzer (Agilent B1500A), and the light source was a 500 W xenon lamp with the illumination intensity of 100 mW cm−2 . The incident photon-to-current conversion efficiency (IPCE) was measured by solar cell quantum efficiency measurement system (Zolix, Solar Cell Scan 100). 3. Results and discussion 3.1. Growth of ZnO nanowires Fig. 1 shows the XRD patterns of ZnO seed layer and ZnO nanowires as a function of growth time. All diffraction peaks are assigned to ITO conductive glass substrate and the randomly oriented hexagonal wurtzite phase of ZnO. But the ZnO (0002) peak is relatively more intense, indicating that the seed layer and nanowires both grow preferentially along the [0001] direction. The result confirms that the vertical growth of ZnO nanowires is strongly related to the (0002) orientation of the seed layer [12,13]. It should be noted that the intensity of ZnO (0002) peak becomes stronger with increasing growth time. In general, the diffraction intensity of (0002) plane depends linearly on the amount of ZnO crystals with c-axis aligned paralleling to the substrate normal. As the growth time prolongs, longer nanowires would be obtained to enhance the (0002) peak intensity. ¯ peak height ratio increases from 1.27 (seed layer) to 24.2 (12 h), indicating that nanowires Meanwhile, the (0002)/(1011) becomes more vertical to the substrate. This increase of verticality also contributes to the (0002) peak intensity. Therefore, it can be inferred that the oriented seed layer helps to a certain preferential alignment of nanowires at the initial growth stage, and non-vertical nanowires in the following process would stop growing when encounter the others. This observation differs from the non-preferential orientation of ZnO nanowires without refreshing the growth solution [9]. Cross-sectional and top view images of ZnO nanowires grown for different time are shown in Fig. 2. For each growth period, most of the nanowires are vertically aligned on the substrate, and all nanowires exhibit a regular hexagonal shape,
Z. Mo et al. / Optik 149 (2017) 63–68
65
Fig. 2. Cross-sectional and top view images of ZnO nanowires grown for different time: (a and b) 3 h, (c and d) 6 h, (e and f) 9 h, (g and f) 12 h. A higher magnification is adopted in (a) to clearly show the morphology of nanowires.
66
Z. Mo et al. / Optik 149 (2017) 63–68
Fig. 3. The average length and diameter of ZnO nanowires grown for different time.
Fig. 4. (a) Absorption spectra and (b) (˛h)2 versus h plots of pure ZnO nanowires grown for 9 h and CdS quantum dots sensitized ZnO nanowires as a function of growth time.
which are consistent with the XRD results. But a different tapered shape along the axis at 3 h is also noticed, which can be clearly seen at a higher magnification in Fig. 2(a). The corresponding average length and diameter of ZnO nanowires are plotted with growth time in Fig. 3. In the first 3 h, the length and diameter both increase rapidly. As the growth time prolongs to 9 h, the length maintains the high growth rate, but the diameter keeps nearly constant. With the further extension of time to 12 h, the increase of length slows down, but the diameter shows a rapid growth. The resulting aspect ratios of ZnO nanowires are 7.37, 14.88, 20.56 and 16.02 respectively, showing a maximum value at 9 h. At the beginning of growth, the (0002) preferred orientation of ZnO seed layer is conducive to the vertical growth of ZnO nanowires. Within a short period of time, high growth rate in [0001] direction and the relatively slow lateral growth result in the tapered shape of nanowires. As the increase of growth time, nanowires have uniform diameter for the sufficient mobility and diffusion of ions. Meanwhile, the preferential adsorption of polyethyleneimine on the lateral planes limits the radial growth of nanowires [11]. However, decreased reactant concentrations after a relatively long time retard the growth rate, while the diameter increases relatively obvious for the absence of polyethyleneimine adsorption. 3.2. Optical properties of CdS quantum dots sensitized ZnO nanowires Fig. 4(a) presents the absorption spectra of pure ZnO nanowires grown for 9 h and CdS quantum dots sensitized ZnO nanowires as a function of growth time. Pure ZnO nanowires grown for different time exhibit the similar UV absorption region with a threshold wavelength of ∼ 380 nm. After the deposition of CdS quantum dots on ZnO nanowires, the absorption range extends to the visible region, indicating a red-shift. With the increasing of growth time, the absorbance increases and the absorption edge shifts from 390 nm to 540 nm. According to Tauc equation [14]:
˛ (hv) = A hv − Eg
n
(1)
where ␣ is the absorption coefficient, h is the energy of the incident photon, A is a constant, and n is 0.5 for the direct band gap semiconductor. By plotting the curves of (˛h)2 versus h as shown in Fig. 4(b), the band gaps (Eg ) of pure ZnO nanowires and CdS quantum dots sensitized ZnO nanowires grown for 3 h to 12 h are extracted to be 3.22 eV, 3.12 eV, 2.48 eV, 2.22 eV and 2.23 eV respectively. Obviously, narrower band gap can be obtained after the deposition of CdS quantum dots, which effectively widen the absorption range of ZnO nanowires. It can also be observed that though the nanowires grown for 12 h have the maximum diameter and length, the highest absorbance and the widest absorption range belong to the nanowires
Z. Mo et al. / Optik 149 (2017) 63–68
67
Fig. 5. I–V characteristics of CdS QDSSCs with different aspect ratio of ZnO nanowires. 7.37, 14.88, 16.02 and 20.56 in parentheses are the aspect ratios of ZnO nanowires. Table 1 Photovoltaic parameters of CdS QDSSCs with different aspect ratio of ZnO nanowires. Photoanodes
Isc (mA/cm2 )
Voc (V)
Fill factor
(%)
ZnO(7.37)/CdS ZnO(14.88)/CdS ZnO(16.02)/CdS ZnO(20.56)/CdS
0.341 1.766 2.031 3.551
0.186 0.368 0.481 0.454
0.264 0.295 0.302 0.248
0.017 0.192 0.295 0.401
Fig. 6. IPCE spectra of CdS QDSSCs with different aspect ratio of ZnO nanowires.
having the maximum aspect ratio of 20.56 grown for 9 h. This implies that the aspect ratio of nanowires in this condition plays an important effect on their absorption properties. It is well known that the absorption spectra of CdS quantum dots sensitized photoanodes strongly depend on the size and loading amount of CdS [3,15,16]. Therefore, nanowires with high aspect ratio have high specific surface area, which can greatly improve the CdS loading amount to widen the absorption range and enhance the absorbance. 3.3. Performance of QDSSCs Fig. 5 displays the I–V characteristics of CdS QDSSCs with different aspect ratio of ZnO nanowires, and the calculated photovoltaic parameters are listed in Table 1. The energy conversion efficiency () increases with increasing aspect ratio of ZnO nanowires, and the best is 0.401% with an aspect ratio of 20.56. This energy conversion efficiency is higher than that of dye sensitized or CdSe quantum dots sensitized solar cells with a higher aspect ratio of ZnO nanowires [9,17], which is mainly attributed to the significant enhancement of the short-circuit current density (Isc ). As shown in Table 1, the Isc increases about 10.4 times when the aspect ratio increases from 7.37 to 20.56, while the open-circuit voltage (Voc ) increases only about 2.4 times. Moreover, the IPCE spectra of CdS QDSSCs show the similar increasing trend as shown in Fig. 6. The QDSSCs with higher aspect ratio of ZnO nanowires exhibit a relatively high IPCE value and a broad photo-electricity response range. A maximum IPCE of 39% is observed in a range of 380–420 nm with an aspect ratio of 20.56.
68
Z. Mo et al. / Optik 149 (2017) 63–68
As the aspect ratio of ZnO nanowires increase, more electron-hole pairs are generated due to the increasing CdS loading amount on nanowires. Meanwhile, strong light trapping effect of ZnO nanowires enhances the photon absorption, generating more photogenerated charge carriers. As a result, the overall conversion efficiency of QDSSCs is largely limited to the aspect ratio of ZnO nanowires. It is predicted that higher energy conversion efficiency will be obtained for further enhancing the aspect ratio of ZnO nanowires by refreshing the growth solution. 4. Conclusions Vertically aligned ZnO nanowires were grown on ITO conductive glass substrates by CBD method. With the increasing of growth time, the verticality of ZnO nanowires along the [0001] direction increases due to the c-axis oriented seed layer and the growth process, and the average length and diameter also increase, showing a maximum aspect ratio of 20.56 at 9 h. CdS quantum dots deposited on ZnO nanowires enhance the absorbance and extend the absorption range to the visible region. ZnO nanowires based CdS QDSSCs show the best energy conversion efficiency of 0.401% with an aspect ratio of 20.56, which is mainly attributed to the significant enhancement of the short-circuit current density. Reasons for that are more electron-hole pairs and strong light trapping effect of ZnO nanowires with high aspect ratio. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant number 61474030) and Guangxi Natural Science Foundation (Grant number 2015GXNSFAA139265). References [1] L. Li, T.Y. Zhai, Y. Bando, D. Golberg, Recent progress of one-dimensional ZnO nanostructured solar cells, Nano Energy 1 (2012) 91–106. [2] H.N. Chen, L.Q. Zhu, H.C. Liu, W.P. Li, Growth of ZnO nanowires on fibers for one-dimensional flexible quantum dot-sensitized solar cells, Nanotechnology 23 (2012) 075402. [3] L.L. Yang, Z.Q. Zhang, J.H. Yang, Y.S. Yan, Y.F. Sun, J. Cao, M. Gao, M.B. Wei, J.H. Lang, F.Z. Liu, Z. Wang, Effect of tube depth on the photovoltaic performance of CdS quantum dots sensitized ZnO nanotubes solar cells, J. Alloys Compd. 543 (2012) 58–64. [4] W. Lee, S.K. Min, V. Dhas, S.B. Ogale, S.H. Han, Chemical bath deposition of CdS quantum dots on vertically aligned ZnO nanorods for quantum dots-sensitized solar cells, Electrochem. Commun. 11 (2009) 103–106. [5] J.J. Qi, W. Liu, C.D. Biswas, G.J. Zhang, L.F. Sun, Z.Z. Wang, X.F. Hu, Y. Zhang, Enhanced power conversion efficiency of CdS quantum dot sensitized solar cells with ZnO nanowire arrays as the photoanodes, Opt. Commun. 349 (2015) 198–202. [6] C.Y. Chou, C.T. Li, C.P. Lee, L.Y. Lin, M.H. Yeh, R. Vittal, H.C. Ho, ZnO nanowire/nanoparticles composite films for the photoanodes of quantum dot-sensitized solar cells, Electrochim. Acta 88 (2013) 35–43. [7] F.D. Nayeri, E.A. Soleimani, F. Salehi, Synthesis and characterization of ZnO nanowires grown on different seed layers: the application for dye-sensitized solar cells, Renew. Energy 60 (2013) 246–255. [8] G. Kenanakis, M. Pervolaraki, J. Giapintzakis, N. Katsarakis, The use of pulsed laser deposited seed layers for the aqueous solution growth of highly oriented ZnO nanowires on sapphire substrates at 95 ◦ C: Study of their photocatalytic activity in terms of octadecanoic (stearic) acid degradation, Appl. Catal. A: Gen. 467 (2013) 559–567. [9] J.B. Baxter, A.M. Walker, K.V. Ommering, E.S. Aydil, Synthesis and characterization of ZnO nanowires and their integration into dye-sensitized solar cells, Nanotechnology 17 (2006) S304–S312. [10] Z.T. Han, S.S. Li, J.J. Li, J.K. Chu, Y. Chen, Facile synthesis of ZnO nanowires on FTO glass for dye-sensitized solar cells, J. Semicond. 34 (2013) 074002. [11] J.P. Deng, M.Q. Wang, X.H. Song, J. Liu, Controlled synthesis of aligned ZnO nanowires and the application in CdSe-sensitized solar cells, J. Alloys Compd. 588 (2014) 399–405. [12] J.J. Song, S.W. Lim, Effect of seed layer on the growth of ZnO nanorods, J. Phys. Chem. C 111 (2007) 596–600. [13] Y.H. Kang, C.G. Choi, Y.S. Kim, J.K. Kim, Influence of seed layers on the vertical growth of ZnO nanowires, Mater. Lett. 63 (2009) 679–682. [14] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous germanium, Phys. Status Solidi B 15 (1966) 627–637. [15] L.M. Sai, X.Y. Kong, Type II hybrid structures of TiO2 nanorods conjugated with CdS quantum dots: assembly and optical properties, Appl. Phys. A: Mater. Sci. Process. 114 (2014) 605–609. [16] J. Jiao, Z.J. Zhou, W.H. Zhou, S.X. Wu, CdS and PbS quantum dots co-sensitized TiO2 nanorod arrays with improved performance for solar cells application, Mater. Sci. Semicond. Process. 16 (2013) 435–440. [17] K.S. Leschkies, R. Divakar, J. Basu, E. Enache-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) 1793–1798.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Growth of ZnO nanowires and their applications for CdS quantum dots sensitized solar cells Zukang Mo, Ying Huang, Shanshan Lu, Yuechun Fu ∗ , Xiaoming Shen, Huan He Center of Ecological Collaborative Innovation for Aluminum Industry in Guangxi, Guangxi Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, College of Materials Science and Engineering, Guangxi University, Nanning 530004, China
a r t i c l e
i n f o
Article history: Received 22 May 2017 Accepted 8 September 2017 Keywords: ZnO nanowires CdS quantum dots Solar cells Energy conversion efficiency
a b s t r a c t ZnO nanowires grown on ITO conductive glass substrates by chemical bath deposition (CBD) method were used as photoanodes to assemble the CdS quantum dots sensitized solar cells (QDSSCs). The growth mechanism of ZnO nanowires and the photovoltaic performance of CdS QDSSCs were investigated. The results show that the c-axis oriented seed layer and the growth process both contribute to the preferential alignment of ZnO nanowires along the [0001] direction. The average length and diameter of nanowires increase with increasing growth time, and the maximum aspect ratio is 20.56 at 9 h. CdS quantum dots deposited on ZnO nanowires enhance the absorbance and extend the absorption range to the visible region. ZnO nanowires with higher aspect ratio effectively increase the energy conversion efficiency () of CdS QDSSCs, and the best is 0.401% with an aspect ratio of 20.56. In such a solar cell, the short-circuit current density is significantly improved due to more electron-hole pairs and strong light trapping effect of ZnO nanowires. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction CdS quantum dots sensitized ZnO nanowires solar cells have attracted intense attention recently for the potential applications as photovoltaic devices. One-dimensional (1D) ZnO nanowires possess high electron mobility and surface area, which provide the direct paths for charge transport and efficient light harvesting [1]. CdS quantum dots own a reasonable band gap and high conduction band edge in contrast to the ZnO photoanodes, which are favorable for absorbing visible light and inducing a high open-circuit voltage [2,3]. Thus, high photovoltaic performance of this solar cell can be expected. Though much effort has been made to the development of ZnO nanowires or nanorods based CdS quantum dots sensitized solar cells (QDSSCs), their energy conversion efficiencies are still relatively low [4–6]. One major challenge is how to prepare well crystallized and aligned 1D ZnO nanostructures. Among various synthesis methods for 1D ZnO nanostructures, chemical bath deposition (CBD) technique has drawn much attention because of its simple coating process, low temperature, reproducibility, and low cost of equipment [7]. In this process, growth parameters such as seed layer, reactant concentration, growth temperature, etc., have been reported to affect the quality of 1D ZnO nanostructures [8–11]. Nevertheless, their growth mechanism should be further understood by exploring the forming process. In this paper, ZnO nanowires were grown on ITO conductive glass substrates by CBD method for different time. The crystallinity and morphology evolutions of ZnO nanowires and the photovoltaic performance of CdS QDSSCs were investigated.
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Fu). http://dx.doi.org/10.1016/j.ijleo.2017.09.035 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
64
Z. Mo et al. / Optik 149 (2017) 63–68
Fig. 1. XRD patterns of ZnO seed layer and ZnO nanowires as a function of growth time. The inset is the enlarged XRD pattern of ZnO seed layer.
2. Experimental ZnO nanowires were grown on ITO conductive glass substrates by CBD method. Prior to the growth of nanowires, ZnO seed layers (∼130 nm) were deposited on the substrates in a pulsed laser deposition (PLD) system which can be evacuated to a base pressure of 10−5 Pa. A pulsed KrF excimer laser beam ( = 248 nm, = 10 ns) with the repetition rate of 5 Hz was focused on the ZnO (99.9% purity) target which was mounted 5 cm apart from the substrate. Depositions were carried out at the substrate temperature of 200 ◦ C, laser energy of 180 mJ/p, and working pressure of 0.8 Pa using O2 (99.999% purity) as the background gas. Subsequently, the seeded substrates with an area of about 0.7 cm2 were dipped face-down in an aqueous solution containing 0.05 M zinc nitrate hexahydrate, 0.05 M methenamine and 4.5 mM polyethyleneimine to grow ZnO nanowires. After growth at 95 ◦ C in a water bath for 3–12 h, the samples were thoroughly rinsed with deionized water and dried in air. CdS quantum dots were deposited on ZnO nanowires by successive ion layer adsorption and reaction (SILAR) method. ZnO nanowires were successively dipped in 0.05 M CdCl2 ethanol solution, ethanol, 0.05 M Na2 S methanol solution and methanol for 30 s, and the dipping procedure was repeated for 12 cycles. For the assembly of QDSSCs, the Pt counter electrodes were prepared by drop-casting an ethanolic H2 PtCl6 ·6H2 O solution onto the ITO substrates and annealing at 500 ◦ C for 30 min. A seal film under heating (100 ◦ C) was sandwiched between the CdS quantum dots sensitized ZnO nanowires photoanode and the Pt-coated ITO counter electrode. An electrolyte (0.3 M tetra-butyl-ammonium iodide, 0.06 M LiI, 0.03 M I2 , 0.5 M 4-tert-butylpyridine in acetonitrile solvent) was then injected into the space between the two electrodes. The crystallinity and morphology of ZnO nanowires were characterized by X-ray diffraction (XRD, Rigaku D/MAX-RB) and scanning electron microscopy (SEM, FEI Quanta-400). The optical absorption spectra of CdS quantum dots sensitized ZnO nanowires were measured by UV–vis spectrophotometer (PerkinElmer Lambda 950). Current-voltage (I–V) characteristics of QDSSCs were recorded by semiconductor device analyzer (Agilent B1500A), and the light source was a 500 W xenon lamp with the illumination intensity of 100 mW cm−2 . The incident photon-to-current conversion efficiency (IPCE) was measured by solar cell quantum efficiency measurement system (Zolix, Solar Cell Scan 100). 3. Results and discussion 3.1. Growth of ZnO nanowires Fig. 1 shows the XRD patterns of ZnO seed layer and ZnO nanowires as a function of growth time. All diffraction peaks are assigned to ITO conductive glass substrate and the randomly oriented hexagonal wurtzite phase of ZnO. But the ZnO (0002) peak is relatively more intense, indicating that the seed layer and nanowires both grow preferentially along the [0001] direction. The result confirms that the vertical growth of ZnO nanowires is strongly related to the (0002) orientation of the seed layer [12,13]. It should be noted that the intensity of ZnO (0002) peak becomes stronger with increasing growth time. In general, the diffraction intensity of (0002) plane depends linearly on the amount of ZnO crystals with c-axis aligned paralleling to the substrate normal. As the growth time prolongs, longer nanowires would be obtained to enhance the (0002) peak intensity. ¯ peak height ratio increases from 1.27 (seed layer) to 24.2 (12 h), indicating that nanowires Meanwhile, the (0002)/(1011) becomes more vertical to the substrate. This increase of verticality also contributes to the (0002) peak intensity. Therefore, it can be inferred that the oriented seed layer helps to a certain preferential alignment of nanowires at the initial growth stage, and non-vertical nanowires in the following process would stop growing when encounter the others. This observation differs from the non-preferential orientation of ZnO nanowires without refreshing the growth solution [9]. Cross-sectional and top view images of ZnO nanowires grown for different time are shown in Fig. 2. For each growth period, most of the nanowires are vertically aligned on the substrate, and all nanowires exhibit a regular hexagonal shape,
Z. Mo et al. / Optik 149 (2017) 63–68
65
Fig. 2. Cross-sectional and top view images of ZnO nanowires grown for different time: (a and b) 3 h, (c and d) 6 h, (e and f) 9 h, (g and f) 12 h. A higher magnification is adopted in (a) to clearly show the morphology of nanowires.
66
Z. Mo et al. / Optik 149 (2017) 63–68
Fig. 3. The average length and diameter of ZnO nanowires grown for different time.
Fig. 4. (a) Absorption spectra and (b) (˛h)2 versus h plots of pure ZnO nanowires grown for 9 h and CdS quantum dots sensitized ZnO nanowires as a function of growth time.
which are consistent with the XRD results. But a different tapered shape along the axis at 3 h is also noticed, which can be clearly seen at a higher magnification in Fig. 2(a). The corresponding average length and diameter of ZnO nanowires are plotted with growth time in Fig. 3. In the first 3 h, the length and diameter both increase rapidly. As the growth time prolongs to 9 h, the length maintains the high growth rate, but the diameter keeps nearly constant. With the further extension of time to 12 h, the increase of length slows down, but the diameter shows a rapid growth. The resulting aspect ratios of ZnO nanowires are 7.37, 14.88, 20.56 and 16.02 respectively, showing a maximum value at 9 h. At the beginning of growth, the (0002) preferred orientation of ZnO seed layer is conducive to the vertical growth of ZnO nanowires. Within a short period of time, high growth rate in [0001] direction and the relatively slow lateral growth result in the tapered shape of nanowires. As the increase of growth time, nanowires have uniform diameter for the sufficient mobility and diffusion of ions. Meanwhile, the preferential adsorption of polyethyleneimine on the lateral planes limits the radial growth of nanowires [11]. However, decreased reactant concentrations after a relatively long time retard the growth rate, while the diameter increases relatively obvious for the absence of polyethyleneimine adsorption. 3.2. Optical properties of CdS quantum dots sensitized ZnO nanowires Fig. 4(a) presents the absorption spectra of pure ZnO nanowires grown for 9 h and CdS quantum dots sensitized ZnO nanowires as a function of growth time. Pure ZnO nanowires grown for different time exhibit the similar UV absorption region with a threshold wavelength of ∼ 380 nm. After the deposition of CdS quantum dots on ZnO nanowires, the absorption range extends to the visible region, indicating a red-shift. With the increasing of growth time, the absorbance increases and the absorption edge shifts from 390 nm to 540 nm. According to Tauc equation [14]:
˛ (hv) = A hv − Eg
n
(1)
where ␣ is the absorption coefficient, h is the energy of the incident photon, A is a constant, and n is 0.5 for the direct band gap semiconductor. By plotting the curves of (˛h)2 versus h as shown in Fig. 4(b), the band gaps (Eg ) of pure ZnO nanowires and CdS quantum dots sensitized ZnO nanowires grown for 3 h to 12 h are extracted to be 3.22 eV, 3.12 eV, 2.48 eV, 2.22 eV and 2.23 eV respectively. Obviously, narrower band gap can be obtained after the deposition of CdS quantum dots, which effectively widen the absorption range of ZnO nanowires. It can also be observed that though the nanowires grown for 12 h have the maximum diameter and length, the highest absorbance and the widest absorption range belong to the nanowires
Z. Mo et al. / Optik 149 (2017) 63–68
67
Fig. 5. I–V characteristics of CdS QDSSCs with different aspect ratio of ZnO nanowires. 7.37, 14.88, 16.02 and 20.56 in parentheses are the aspect ratios of ZnO nanowires. Table 1 Photovoltaic parameters of CdS QDSSCs with different aspect ratio of ZnO nanowires. Photoanodes
Isc (mA/cm2 )
Voc (V)
Fill factor
(%)
ZnO(7.37)/CdS ZnO(14.88)/CdS ZnO(16.02)/CdS ZnO(20.56)/CdS
0.341 1.766 2.031 3.551
0.186 0.368 0.481 0.454
0.264 0.295 0.302 0.248
0.017 0.192 0.295 0.401
Fig. 6. IPCE spectra of CdS QDSSCs with different aspect ratio of ZnO nanowires.
having the maximum aspect ratio of 20.56 grown for 9 h. This implies that the aspect ratio of nanowires in this condition plays an important effect on their absorption properties. It is well known that the absorption spectra of CdS quantum dots sensitized photoanodes strongly depend on the size and loading amount of CdS [3,15,16]. Therefore, nanowires with high aspect ratio have high specific surface area, which can greatly improve the CdS loading amount to widen the absorption range and enhance the absorbance. 3.3. Performance of QDSSCs Fig. 5 displays the I–V characteristics of CdS QDSSCs with different aspect ratio of ZnO nanowires, and the calculated photovoltaic parameters are listed in Table 1. The energy conversion efficiency () increases with increasing aspect ratio of ZnO nanowires, and the best is 0.401% with an aspect ratio of 20.56. This energy conversion efficiency is higher than that of dye sensitized or CdSe quantum dots sensitized solar cells with a higher aspect ratio of ZnO nanowires [9,17], which is mainly attributed to the significant enhancement of the short-circuit current density (Isc ). As shown in Table 1, the Isc increases about 10.4 times when the aspect ratio increases from 7.37 to 20.56, while the open-circuit voltage (Voc ) increases only about 2.4 times. Moreover, the IPCE spectra of CdS QDSSCs show the similar increasing trend as shown in Fig. 6. The QDSSCs with higher aspect ratio of ZnO nanowires exhibit a relatively high IPCE value and a broad photo-electricity response range. A maximum IPCE of 39% is observed in a range of 380–420 nm with an aspect ratio of 20.56.
68
Z. Mo et al. / Optik 149 (2017) 63–68
As the aspect ratio of ZnO nanowires increase, more electron-hole pairs are generated due to the increasing CdS loading amount on nanowires. Meanwhile, strong light trapping effect of ZnO nanowires enhances the photon absorption, generating more photogenerated charge carriers. As a result, the overall conversion efficiency of QDSSCs is largely limited to the aspect ratio of ZnO nanowires. It is predicted that higher energy conversion efficiency will be obtained for further enhancing the aspect ratio of ZnO nanowires by refreshing the growth solution. 4. Conclusions Vertically aligned ZnO nanowires were grown on ITO conductive glass substrates by CBD method. With the increasing of growth time, the verticality of ZnO nanowires along the [0001] direction increases due to the c-axis oriented seed layer and the growth process, and the average length and diameter also increase, showing a maximum aspect ratio of 20.56 at 9 h. CdS quantum dots deposited on ZnO nanowires enhance the absorbance and extend the absorption range to the visible region. ZnO nanowires based CdS QDSSCs show the best energy conversion efficiency of 0.401% with an aspect ratio of 20.56, which is mainly attributed to the significant enhancement of the short-circuit current density. Reasons for that are more electron-hole pairs and strong light trapping effect of ZnO nanowires with high aspect ratio. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant number 61474030) and Guangxi Natural Science Foundation (Grant number 2015GXNSFAA139265). References [1] L. Li, T.Y. Zhai, Y. Bando, D. Golberg, Recent progress of one-dimensional ZnO nanostructured solar cells, Nano Energy 1 (2012) 91–106. [2] H.N. Chen, L.Q. Zhu, H.C. Liu, W.P. Li, Growth of ZnO nanowires on fibers for one-dimensional flexible quantum dot-sensitized solar cells, Nanotechnology 23 (2012) 075402. [3] L.L. Yang, Z.Q. Zhang, J.H. Yang, Y.S. Yan, Y.F. Sun, J. Cao, M. Gao, M.B. Wei, J.H. Lang, F.Z. Liu, Z. Wang, Effect of tube depth on the photovoltaic performance of CdS quantum dots sensitized ZnO nanotubes solar cells, J. Alloys Compd. 543 (2012) 58–64. [4] W. Lee, S.K. Min, V. Dhas, S.B. Ogale, S.H. Han, Chemical bath deposition of CdS quantum dots on vertically aligned ZnO nanorods for quantum dots-sensitized solar cells, Electrochem. Commun. 11 (2009) 103–106. [5] J.J. Qi, W. Liu, C.D. Biswas, G.J. Zhang, L.F. Sun, Z.Z. Wang, X.F. Hu, Y. Zhang, Enhanced power conversion efficiency of CdS quantum dot sensitized solar cells with ZnO nanowire arrays as the photoanodes, Opt. Commun. 349 (2015) 198–202. [6] C.Y. Chou, C.T. Li, C.P. Lee, L.Y. Lin, M.H. Yeh, R. Vittal, H.C. Ho, ZnO nanowire/nanoparticles composite films for the photoanodes of quantum dot-sensitized solar cells, Electrochim. Acta 88 (2013) 35–43. [7] F.D. Nayeri, E.A. Soleimani, F. Salehi, Synthesis and characterization of ZnO nanowires grown on different seed layers: the application for dye-sensitized solar cells, Renew. Energy 60 (2013) 246–255. [8] G. Kenanakis, M. Pervolaraki, J. Giapintzakis, N. Katsarakis, The use of pulsed laser deposited seed layers for the aqueous solution growth of highly oriented ZnO nanowires on sapphire substrates at 95 ◦ C: Study of their photocatalytic activity in terms of octadecanoic (stearic) acid degradation, Appl. Catal. A: Gen. 467 (2013) 559–567. [9] J.B. Baxter, A.M. Walker, K.V. Ommering, E.S. Aydil, Synthesis and characterization of ZnO nanowires and their integration into dye-sensitized solar cells, Nanotechnology 17 (2006) S304–S312. [10] Z.T. Han, S.S. Li, J.J. Li, J.K. Chu, Y. Chen, Facile synthesis of ZnO nanowires on FTO glass for dye-sensitized solar cells, J. Semicond. 34 (2013) 074002. [11] J.P. Deng, M.Q. Wang, X.H. Song, J. Liu, Controlled synthesis of aligned ZnO nanowires and the application in CdSe-sensitized solar cells, J. Alloys Compd. 588 (2014) 399–405. [12] J.J. Song, S.W. Lim, Effect of seed layer on the growth of ZnO nanorods, J. Phys. Chem. C 111 (2007) 596–600. [13] Y.H. Kang, C.G. Choi, Y.S. Kim, J.K. Kim, Influence of seed layers on the vertical growth of ZnO nanowires, Mater. Lett. 63 (2009) 679–682. [14] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous germanium, Phys. Status Solidi B 15 (1966) 627–637. [15] L.M. Sai, X.Y. Kong, Type II hybrid structures of TiO2 nanorods conjugated with CdS quantum dots: assembly and optical properties, Appl. Phys. A: Mater. Sci. Process. 114 (2014) 605–609. [16] J. Jiao, Z.J. Zhou, W.H. Zhou, S.X. Wu, CdS and PbS quantum dots co-sensitized TiO2 nanorod arrays with improved performance for solar cells application, Mater. Sci. Semicond. Process. 16 (2013) 435–440. [17] K.S. Leschkies, R. Divakar, J. Basu, E. Enache-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) 1793–1798.