TiO2 composite counter electrode for CdS quantum dot-sensitized ZnO nanorods solar cells

TiO2 composite counter electrode for CdS quantum dot-sensitized ZnO nanorods solar cells

Materials Letters 172 (2016) 171–174 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet A...

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Materials Letters 172 (2016) 171–174

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

A novel PbS/TiO2 composite counter electrode for CdS quantum dot-sensitized ZnO nanorods solar cells Chunyan Zhou, Yue Geng, Qian Chen, Jiehua Xu, Ni Huang, Yufei Gan, Liya Zhou n School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China

art ic l e i nf o

a b s t r a c t

Article history: Received 16 October 2015 Received in revised form 23 January 2016 Accepted 21 February 2016 Available online 3 March 2016

A series of novel PbS/TiO2 nanotubes was fabricated as counter electrodes (CEs) through a combination of anodic oxidation and successive ionic-layer adsorption and reaction methods. The photovoltaic-conversion efficiency of ZnO nanorods sensitized with CdS quantum dots using the optimized PbS(6)/TiO2 CE increased by nearly 440.8% and 227.5% compared with the efficiency of nanorods using planar Pt and bare TiO2 CE, respectively. Thus, the novel PbS/TiO2 composite CE was a promising catalytic electrode with excellent electrocatalytic activity for the reduction of charge carriers in polysulfide electrolyte. & 2016 Elsevier B.V. All rights reserved.

Keywords: Counter electrode Quantum dot-sensitized solar cell PbS/TiO2 nanotubes

1. Introduction Quantum-dot-sensitized solar cells (QDSSCs) are attracting significant attention because of the advantages of semiconductor quantum dots (QDs) [1–3]. CdS QDs are reported to be relatively more stable than other materials and have been studied for their potential application in photovoltaic devices [4]. ZnO is widely used to fabricate solar cell devices because of their band-gap energy (3.43 eV), stability against photocorrosion, electron affinity, and environment-friendly properties [5,6]. Ray et al. [7] used CdSdecorated ZnO nanorod (NR) heterostructures in improved hybrid photovoltaic devices. The photovoltaic-conversion efficiency of QDSSCs is still much lower than that of dye-sensitized solar cells (DSSCs), the poor charge transfer at the counter electrode (CE)/ electrolyte interface is considered to be a major hurdle in obtaining a high efficiency of QDSSCs [8,9]. An aqueous polysulfide (S2 /Sn2 ) redox electrolyte is usually used for CdS QD-sensitized ZnO NR solar cells. However, Pt CE is inappropriate in QDSSC systems because of the chemisorption of sulfur compounds on the Pt surface [9]. Therefore, the development of a novel CE for polysulfide solution is necessary to further enhance the efficiency of QDSSCs. Various electrocatalytic materials such as carbon [10] and Cu2S [11] materials have been extensively studied. The basic requirements are low cost, chemical durability, high electrical conductivity, and catalytic activity [12]. In the present study, PbS/TiO2 nanotubes (NAs) were fabricated n

Corresponding author. E-mail address: [email protected] (L. Zhou).

http://dx.doi.org/10.1016/j.matlet.2016.02.092 0167-577X/& 2016 Elsevier B.V. All rights reserved.

using successive ionic-layer adsorption and reaction (SILAR) method and applied as CEs for QDSSCs. To the best of our knowledge, promising PbS/TiO2 CEs with low cost have not yet been used in polysulfide electrolyte. Effects of the PbS/TiO2 CEs on the photovoltaics performance of QDSSCs were investigated.

2. Experimental ZnO NRs were prepared using the hydrothermal method, and CdS QD-sensitized ZnO NRs were prepared by direct adsorption (DA) for use as photoanode. TiO2 NAs were prepared by anodic oxidation. Then, PbS/TiO2 composite CEs were prepared by the typical SILAR method. TiO2 NAs were first immersed into 0.1 M Pb(NO3)2 and then into 0.1 M Na2S aqueous solution for 3 min. This procedure was deemed as 1 cycle, which was repeated 2, 4, 6, 8 and 10 times and denoted as PbS(2)/TiO2, PbS(4)/TiO2, PbS(6)/ TiO2, PbS(8)/TiO2, and PbS(10)/TiO2, respectively. The crystalline structure of samples was evaluated by X-ray diffraction (XRD; Rigaku/Dmax-2500). High-resolution transmission electron microscopy (HRTEM) images, morphologies, and elemental compositions of the samples were obtained using a scanning electron microscopy (SEM) system (JEOL-2100F) with an energy-dispersive spectrometer (Hitachi S-3400N). The current density–voltage (J–V) curves of the cell devices were recorded on a Keithley 2400 source meter under illumination by an AM 1.5 g solar simulator. The power of the simulated light was calibrated to 100 mW/cm2 by an NREL standard Si solar cell.

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3. Results and discussion

+

&TiO2 +PbS

&

& (220)

& +

# #Ti (200)

Relative intensity (a.u.)

(a)

(111)

Fig. 1a shows the XRD patterns of bare TiO2 NAs and PbS/TiO2 CEs prepared with different SILAR cycles. For the bare TiO2 NAs, all diffraction peaks can be attributed to the anatase TiO2 phase (JCPDS No. 21-1272) and metal Ti phase resulting from the Ti

substrate. After PbS deposition, three new peaks identified as cubic PbS phase (JPCDS No. 05-0592) were observed. In addition, the intensity of the main PbS (200) peak became sharper and narrower with increased number of SILAR cycles, indicating that more PbS nanoparticles were deposited onto the TiO2 NAs. Fig. 1b shows the energy-dispersive spectra of the PbS/TiO2 NAs. The obvious

# &

#

#&

&

&

+

P PbS(6)/TiO 2 PbS(4)/TiO P 2

TiO2

20

30

40

50

60

70

80 0

2θ(degre ee) Fig. 1. (a) XRD patterns and (b) energy-dispersive spectra of PbS/TiO2 NAs.

Fig. 2. (a) Top-view, (b) cross-sectional SEM images, (c) TEM and (d) HRTEM images of PbS (6)/TiO2 NAs.

C. Zhou et al. / Materials Letters 172 (2016) 171–174

173

2

Photocurrent density(mA/cm ) Ph t td it ( A/

(b) (

TiO O2

4

Pb bS(2)/TiO2 Pb bS(4)/TiO2

3

Pb bS(6)/TiO2 Pb bS(8)/TiO2

2

Pb bS(10)/TiO2 Pt

1

0 0.0

0.1

0.2 2 0.3 0.4 Ph hotovoltage e(V)

0.5 5

0.6

Fig. 3. (a) Schematic diagram of QDSSC assembled with PbS/TiO2 CE and (b) J–V curves of QDSSC with different CEs.

Table 1 Photovoltaic parameters of QDSSC with different CEs. Counter electrode

Jsc(mA/cm2)

Voc(V)

FF

η(%)

Pt TiO2 PbS(2)/TiO2 PbS(4)/TiO2 PbS(6)/TiO2 PbS(8)/TiO2 PbS(10)/TiO2

1.8174 1.7428 2.0357 3.2004 3.8134 3.5706 2.3343

0.250 0.458 0.475 0.492 0.505 0.480 0.430

0.3251 0.3586 0.3319 0.3169 0.3381 0.3328 0.3251

0.1477 0.2862 0.3209 0.4990 0.6511 0.5703 0.3263

signals showed that the nanotube arrays were composed of the elements Ti, O, Pb, and S. Thus, PbS QDs were successfully deposited onto TiO2 NAs. Fig. 2a shows the SEM image of vertically grown PbS/TiO2 NAs fabricated with six SILAR cycles from the surface of Ti foils; the average tube diameter was about 170 nm, and the top surface of TiO2 NAs was covered with PbS QDs. The tube uniform length was about 3 mm, as shown in the corss-sectional SEM image in Figs. 2b. c shows TEM image of PbS/TiO2 NAs. Clearly, a large number of PbS QDs was deposited on the tube wall. Fig. 2d reveals that the clearly marked interplanar d spacing of 0.35 and 0.30 nm corresponded to the (101) lattice plane of tetragonal TiO2 and the PbS (220) crystal plane. Fig. 3a illustrates the structure of CdS/ZnO photoanode based on PbS/TiO2 CE. Upon photoexcitation of the QDs, electrons from an excited state of CdS QDs entered ZnO NRs, the FTO, and finally the cathode through the external circuit. The electrons then participated in the reaction at the electrolyte/CE interface. Moreover, the holes produced Sx2 by oxidizing the polysulfide electrolyte. To evaluate the photovoltaics performance of PbS/TiO2 CEs with a saturated Ag/AgCl reference electrode, and polysulfide electrolyte composed of 0.5 m Na2S, and 0.7 m Na2SO3, the current density–voltage (J–V) curves and photovoltaic parameters of QDSSC with PbS/TiO2 and Pt CEs were obtained, as shown in Fig. 3b and Table 1. Compared with the devices having PbS/TiO2 CEs, the cell with Pt CE exhibited much lower photoelectrochemical performance with a photovoltaic-conversion efficiency (η) of about 0.1477%. This finding was due to the strong absorption of S2- on the Pt surface in polysulfide electrolyte, which reduced the surface activity of Pt [10]. Then, using bare TiO2 as CE,

the open-circuit voltage (VOC) and η increased to about 0.458 V and 0.2862%, respectively. With increased SILAR cycle of PbS, the short-circuit current density (JSC) and η of the cell increased; the cell with PbS(6)/TiO2 CE showed the highest η of 0.6511%, VOC of 0.505 V and JSC of 3.8134 mA/cm2 among all cells. However, further increased number of PbS cycles reduced cell performance because JSC and VOC deteriorated, which due to the blockage of the interstices between TiO2 NAs. Results confirmed that the catalytic activity toward the polysulfide redox shuttle of PbS/TiO2 CEs was superior to that of Pt CE, and that charge transfer at the PbS/TiO2 CEs was greatly improved. In addition, it can be concluded that under illumination, at open circuit potential, the cell aged has almost stable parameters, which can be observed on photovoltaic parameters (Fig. 3b). However, the dissolution of PbS from CE, desorption of ions of polysulfide electrolyte from surface of PbS/TiO2 add to the change in performance of the cell with aging over time. The chemical change of the CdS QDs and ZnO NRs are also responsible for decrease in performance of the cells aged under illumination [13].

4. Conclusion A series of PbS/TiO2 NAs was designed and fabricated by a SILAR method and used for the first time as efficient CEs for QDSSCs. As a result, the η of CdS QD-sensitized ZnO NRs using the optimized PbS(6)/TiO2 CE increased by 440.8% and 227.5% compared with the η of NRs using planar Pt and TiO2 CE, respectively. The novel PbS/TiO2 composite CE exhibited high conductivity and good electrocatalytic activity for the reduction of charge carriers in polysulfide electrolyte, indicating highly promising applications.

Acknowledgment This work was financially supported by Grants from the National Natural Science Foundation of China (No. 61264003); the Students Innovation and Entrepreneurship Training Program of Guangxi University (201510593082 and 201410593101). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.02.092.

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