Current Applied Physics 18 (2018) 546–550
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ZnO/TiO2 core–shell heterojunction for CdS and PbS quantum dot-cosensitized solar cells
Fangfang Gao, Qian Chen, Xiaoshan Zhang, Huan Wang, Tianjiao Huang, Liya Zhou∗ School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO nanorods Heterojunction Quantum dots sensitized Photovoltaic performance
ZnO nanorods (NRs) with regular morphology were prepared through hydrothermal method, and the TiO2 shell was assembled onto the surface of ZnO NRs by spin coating to the ZnO/TiO2 core–shell heterojunction. CdS and PbS quantum dots (QDs) were used to cosensitize the ZnO/TiO2 nanostructure by direct adsorption (DA) and successive ionic layer adsorption and reaction, respectively. SEM, TEM, and HRTEM images show that the samples possessed a rough surface and four lattice fringes indicating the successful synthesis of the ZnO/TiO2/ CdS/PbS composite structure. The ZnO/TiO2(10T)/CdS/PbS sample showed a high absorption intensity at a broad range of wavelength to visible light region. The ZnO/TiO2(10T)/CdS/PbS photoelectrode with QDSSCs showed the highest IPCE of 36.04% and photoelectric eﬃciency (η) of 1.59%; these values increased by approximately 550% and 150% compared with those of unsensitized ZnO (0.29%) and ZnO/TiO2(10T) (1.04%) and about 146% and 120% compared with those of ZnO/TiO2(10T)/CdS and ZnO/TiO2(10T)/PbS, respectively. The ﬁll factor was 0.36, and the photocurrent density (Jsc) and open circuit voltage (Voc) reached the maximum values of 9.73 mA cm−2 and 0.46 V, respectively.
1. Introduction Quantum dot-sensitized solar cells (QDSSCs) are similar to dyesensitized solar cells (DSSCs) and are composed of working electrode, electrolyte, and counter electrode; QDSSCs are a promising material for transformation of solar energy to electric energy at a low cost [1,2]. In general, semiconductor metal oxide materials with wide band gaps (Eg), such as ZnO (Eg of approximately 3.3 eV)  and TiO2 (Eg of approximately 3.2 eV) [4,5], exhibit superior properties and electronic structures for transferring charge carrier but cannot absorb visible light and infrared solar photon. Various promising semiconductor QDs with narrow Eg exhibit distinct optical and electrical properties; these QDs include CdS [6,7], CdSe [8,9], PbS [10,11], cadmium telluride , and InAs . QDs are mainly characterized by (1) quantum size eﬀect, (2) surface eﬀect, and (3) multiple exaction excitation eﬀect. Semiconductor QDs sensitizing TiO2 or ZnO can broaden their absorption of the solar spectrum. Therefore, multiple semiconductor QDs with narrow Eg have been used for sensitizing TiO2 or ZnO to enhance electrical conduction in the composite structure. Scholars have extensively studied the fabrication and application of ZnO and TiO2 nanostructures [14–18]. TiO2 is the earliest and most widely studied photocatalytic material. TiO2 nanoparticles have attracted considerable research interest because of their high
photocatalytic activity, acid–base resistance property, resistance to photochemical corrosion, low cost, non-toxicity, and other advantages [19,20]. ZnO semiconductor materials possess higher electron transport capacity than traditional TiO2 semiconductor materials and thus can better inhibit the electronic composite and improve the photoelectric properties of the equipment . However, single ZnO nanomaterials with wide band Eg are prone to chemical etching , and their light absorption is limited to wavelength ultraviolet light (λ < 387 nm), thereby limiting the availability of sunlight. To overcome these limitations, researchers have focused on synthesis of ZnO/TiO2 core–shell heterostructure composites. The ZnO/TiO2 core–shell heterojunction not only can broaden the range of light absorption wavelength of ZnO nanomaterials but can also prevent or slow down material corrosion caused by the external environment (such as acid, alkali solutions, or organic dyes) to a certain extent. Coating a layer of ZnO led to superior chemical stability and extended service life of the material. Among developed QDs, CdS is the semiconductor material of direct band-gap. Compared with that of TiO2 (approximately 3.2 eV), the Eg of CdS (approximately 2.4 eV) is more negative. Therefore, electrons produced by the excitation are transmitted rapidly to the conduction band of TiO2, thereby preventing the recombination of photogenerated electrons and hole pairs. Several studies reported that the photoelectric conversion eﬃciency of QDSSCs can be further improved if they are
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.cap.2018.02.020 Received 2 October 2017; Received in revised form 25 January 2018; Accepted 28 February 2018 1567-1739/ © 2018 Published by Elsevier B.V. on behalf of Korean Physical Society.
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Fig. 1. Schematic of experimental process for ZnO/TiO2/CdS/PbS composite structure.
2.2. Preparation of ZnO/TiO2 core-shell heterojunction Fluorine-doped tin oxide (FTO, 10 Ω/cm2) sheet glass was used as substrate, cleaned ultrasonically by deionized water and alcohol for 30 min, and dried at room temperature. ZnO seed layer was prepared on the FTO glass substrate by dipping it into the solution with 0.05 M zinc acetate and 0.05 M diethanol amine ethyl alcohol for 10 s. ZnO nanoparticles were obtained by annealing at 400 °C for 2 min and used as seed layer. ZnO NRs were prepared on the FTO substrates via onestep hydrothermal method. The two substrates were immersed into a Teﬂon bottle with autoclavable screw caps and ﬁlled with 40 mL of aqueous solution, with the conductive side facing down. The aqueous solution contained 12 mL of ammonia (25%) and 240 mL of 0.1 M zinc acetate. The bottle was heated at 90 °C for 180 min. The samples were removed and dried in air at room temperature. TiO2 layer was prepared as a shell of the ZnO NRs via a sol–gel process after dissolving titanium isopropoxide [Ti(OC3H7)4] in 2methoxy ethanol. The acid was used to catalyze the hydrolysis of Ti (OC3H7)4. The TiO2 layer was spin-coated on the surface of ZnO NRs at 2000 rpm. The TiO2 layer was spin coated for 8, 10, and 12 times to obtain diﬀerent numbers of surface coating layers, and the samples were labeled ZnO/TiO2(8T), ZnO/TiO2(10T), and ZnO/TiO2(12T), respectively. Finally, the substrate was placed in a furnace and calcined at 500 °C for 2 h to form ZnO/TiO2. 2.3. Preparation of QD-sensitized ZnO/TiO2 core–shell heterojunction CdS QD-sensitized ZnO/TiO2 nanostructure by DA: the above samples were immersed in CdS QDs solution for 6 h at 60 °C and dried in air at room temperature. PdS QDs were grown on the samples by SILAR. First, CdS QD-sensitized ZnO/TiO2 core–shell samples were placed on a watch glass with 0.1 M Pb(NO3)2 aqueous solutions for 3 min. The samples were removed, washed with deionized water, and dried in air. Secondly, the samples were dropped on a watch glass with 0.1 M Na2S·9H2O aqueous solution for 3 min and washed by deionized water. These steps are circulates of SILAR. In this work, PdS QDs were treated for three repeated SILAR cycles. Fig. 1 shows the ﬂow chart of experimental reaction.
Fig. 2. XRD patterns of ZnO/TiO2 core–shell nanocables(a) and ZnO/TiO2/CdS/PbS composite structure coatings diﬀerent times(b).
sensitized by multiple QDs [23,24]. Moreover, the light absorption range of PbS can be extended to the near infrared region of the solar spectrum to produce numerous photogenerated carriers . Researchers prefer PbS because of its multiple exciton generation eﬀects [26,27]. In this work, CdS QDs were used to sensitize ZnO/TiO2 nanostructure by direct adsorption (DA). PbS QDs were assembled onto the surface of the samples via successive ionic layer adsorption and reaction (SILAR).
2.4. Characterization X-ray powder diﬀraction (XRD) patterns were recorded using a Rigaku/Dmax-2500 system. The morphology, microstructure, and highresolution transmission electron microscopy (HRTEM) images of the samples were characterized by a JEOL-2100F scanning electron microscope. UV–visible (UV–vis) absorption was obtained by UV-2501PC (Lambda 950). A Keithley 2400 source meter was used to measure the photovoltaic performance (J–V curves) of the cell devices under an AM 1.5 G solar simulator illumination. The system was equipped with saturated Ag/AgCl as reference electrode, Pt as counter electrode (CE), and 0.5 M Na2S and 0.7 M Na2SO3 as polysulﬁde electrolyte. The power of the simulated light was regulated to 100 mW/cm2 by using an NREL standard Si solar cell. Incident photon-to-current conversion eﬃciency (IPCE) was recorded on a Zolix electrochemical station using a Zolix LSH-X150 150-W Xe lamp decorated with a monochromator.
2. Experimental 2.1. Preparation of CdS QDs and PdS QDs CdS QDs were synthesized by one-step aqueous phase process. A solution of 0.02 M CdCl2·2.5H2O (analytical reagent (AR) grade) and thioglycolic acid (AR) were added into a 250 mL three-necked ﬂask and stirred constantly. The pH of the solution was adjusted by adding an alkaline solution. The aqueous solution was added with 0.02 M Na2S·9H2O (AR) and reﬂuxed at 100 °C for 5 h. Aqueous solutions of 0.1 M Pb(NO3)2 (AR) and 0.1 M Na2S·9H2O (AR) were prepared for growing PdS QDs onto the samples. 547
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Fig. 3. SEM images of ZnO nanorods (a), ZnO/TiO2 core–shell heterojunction (b) ZnO/TiO2/CdS/PbS composite structure(c), TEM images(d) and HRTEM(e) images of ZnO/TiO2/CdS/ PbS composite structure.
3. Results and discussion
1272). The presence of both ZnO and TiO2 phases indicates the formation of the composite structure. Fig. 2(b) shows the XRD patterns of the ZnO/TiO2 core–shell heterojunction structure sensitized by CdS and PbS QDs. The peaks of ZnO NRs could be indexed to the (002), (103) planes, and the peaks of the TiO2 shell could be indexed to (101), (004) planes; moreover, the diffraction peaks of CdS and PdS QDs were detected. As indicated by the red line, the peaks around 27.23° and 52.42° correspond to the (222), and (600) planes of cubic CdS (JCPDS Card No. 65-2887), respectively.
Fig. 2(a) shows the XRD patterns of the ZnO/TiO2 core–shell heterojunction. The ﬁve main diﬀraction peaks of the pure ZnO NRs could be indexed to the (100), (002), (101), (110), and (103) planes of the hexagonal-phase (wurtzite structure) ZnO (JCPDS Card No. 36-1451). Furthermore, four peaks marked by red circles indicates the diﬀraction peaks for anatase TiO2 that correspond to the (101), (004), (200), and (204) planes of the tetragonal crystal structure (JCPDS Card No. 21548
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onto the ZnO/TiO2 core–shell heterojunction steadily. Fig. 3(a) shows the ZnO nanorods growing on the FTO vertically and arranged in a close and orderly manner. The surface of ZnO nanorods was smooth, and the length and the diameter of ZnO nanorods were approximately 6 μm and 200 nm, respectively. Fig. 3(b) shows that the surface of ZnO nanorods became rough after spin coating of the shell layer of TiO2, indicating that the core–shell nanocomposite is formed. Fig. 3(c) shows the SEM images of the ZnO/TiO2 core–shell heterojunction sensitized by CdS and PbS QDs. The surface of the heterojunction became rough, indicating that the QDs grow onto the surface. The TEM and HRTEM images of the ZnO/TiO2/CdS/PbS composite structure are shown in Fig. 3(d) and (e). Fig. 3(d) shows the QD particles on the NRs. Fig. 3(e) shows the lattice fringes corresponding to the (002) plane of the ZnO, the (004) plane of TiO2, the (111) plane of CdS, and the (200) plane of PbS. This result conﬁrms the successful synthesis of the ZnO/TiO2/CdS/PbS composite structure. The UV–vis absorbance spectra of ZnO nanorods and ZnO/TiO2 core–shell heterojunction are shown in Fig. 4. The absorbance of ZnO NRs at UV wavelengths less than 400 nm could be mainly due to their wide Eg (3.43 eV). The absorbance spectrum of ZnO/TiO2 core–shell heterojunction became broader in the visible light region (approximately 525 nm), and the absorption intensity was markedly enhanced. These ﬁndings indicate that the TiO2 shell was attached to the surface ZnO nanorods. Moreover, the absorbance spectrum of the ZnO/ TiO2(10T)/CdS structure becomes broader to 600 nm, and the intensity of the visible absorption of ZnO/TiO2(10T)/PbS structures was improved to a certain extent. Compared with those of the four samples above, the intensity and spectra of absorption CdS and PbS QD-cosensitized ZnO/TiO2 core–shell heterojunction evidently improved. This improvement is due to the absorbance spectra of CdS in the visible region up to 600 nm and the absorbance spectra of PbS containing the total visible region, even extending to the near-IR region. The illustration in Fig. 4 shows the UV–vis absorbance spectra (from 200 nm to 2500 nm) of the ZnO/TiO2/CdS/PbS composite structures. The monochromatic IPCE spectra of the QDSSCs with ZnO, ZnO/ TiO2(10T), ZnO/TiO2(10T)/CdS, ZnO/TiO2(10T)/PbS, and ZnO/ TiO2(10T)/CdS/PbS photoelectrodes are shown in Fig. 5. Compared with that of ZnO nanorods, the IPCE of QDSSCs with the ZnO/ TiO2(10T) heterojunction photoelectrode increased from 19.25% to 28.63%, with a wide wavelength range of 300–525 nm. The IPCE of the QDSSCs with ZnO/TiO2(10T)/CdS and ZnO/TiO2(10T)/PbS photoelectrode improved to 35.77% and 29.57%, respectively. The ZnO/ TiO2(10T)/CdS photoelectrode exhibit wavelength range from 300 nm to 600 nm. The ZnO/TiO2(10T)/PbS photoelectrode shows an evident response range in a wide wavelength of visible region. The IPCE of the QDSSCs with CdS and PbS QD-cosensitized ZnO/TiO2(10T) reached 36.04% with wavelength range in the visible region. This ﬁnding is consistent with the UV–vis absorbance spectra in Fig. 4. Fig. 6 and Table 1 show the current density–voltage (J–V) characteristics and photovoltaic parameters of the QDSSCs with ZnO, ZnO/ TiO2, ZnO/TiO2(10T)/CdS, ZnO/TiO2(10T)/PbS, and ZnO/TiO2/CdS/ PbS photoelectrodes. Through calculation and analysis, the photovoltaic parameters, such as short-circuit current density (Jsc), are exhibited in Table 1. Compared with unsensitized ZnO and ZnO/TiO2, the photovoltaic parameters of CdS or PbS QD-sensitized simplex samples were enhanced. The η values of ZnO/TiO2(10T)/CdS (1.09%) and ZnO/ TiO2(10T)/PbS (1.34%) are approximately 3.8 and 4.6 times that of unsensitized ZnO (0.29%) and 1.05 and 1.3 times that of ZnO/ TiO2(10T) (1.04%), respectively. Moreover, the photovoltaic parameters of CdS and PbS QDs co-sensitized samples improved substantially owing to the improved absorption of samples in the visible light region after the attachment of QDs, as indicated by the UV–vis absorbance shown in Fig. 4. This is due to that, two kinds of QDs were attached to the surface of sample. The increase of the speciﬁc surface area of the sample and QDs' characteristics enhance the absorption intensity and absorption range of the photoanode in the visible region,
Fig. 4. UV–vis absorbance spectra of ZnO nanorods, ZnO/TiO2 core–shell heterojunction, ZnO/TiO2(10T)/CdS, ZnO/TiO2(10T)/PbS and ZnO/TiO2/CdS/PbS composite structures coating diﬀerent times.
Fig. 5. IPCE spectra of the QDSSCs with ZnO, ZnO/TiO2(10T), ZnO/TiO2(10T)/CdS, ZnO/TiO2(10T)/PbS and ZnO/TiO2(10T)/CdS/PbS photoelectrodes.
Fig. 6. Current density-voltage (J–V) characteristics of the QDSSCs with ZnO, ZnO/TiO2, ZnO/TiO2(10T)/CdS, ZnO/TiO2(10T)/PbS and ZnO/TiO2/CdS/PbS photoelectrodes.
The peaks marked by green circles around 30.12° and 43.12° respectively correspond to the (200) and (220) planes of cubic PbS (JCPDS Card No. 65-9496). These ﬁndings imply that CdS and PbS QDs adhered 549
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Table 1 Photovoltaic parameters of the QDSSCs with ZnO, ZnO/TiO2, ZnO/TiO2(10T)/CdS, ZnO/ TiO2(10T)/PbS and ZnO/TiO2/CdS/PbS photoelectrodes. Photoanode
ZnO ZnO/TiO2(10T) ZnO/TiO2(10T)/CdS ZnO/TiO2(10T)/PbS ZnO/TiO2(8T)/CdS/PbS ZnO/TiO2(10T)/CdS/PbS ZnO/TiO2(12T)/CdS/PbS
1.93 6.44 6.94 7.14 9.04 9.73 8.82
0.37 0.41 0.42 0.43 0.43 0.46 0.42
0.29 0.39 0.37 0.43 0.36 0.36 0.38
0.21 1.04 1.09 1.34 1.39 1.59 1.42
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which is beneﬁcial to produce more photogenerated. When the number of spin coating of TiO2 is 10 times, the η of ZnO/TiO2/CdS/PbS sample reached the maximum value (1.59%), which is approximately 5.5 and 1.5 times of ZnO (0.29%) and ZnO/TiO2 (1.04%), respectively. The ﬁll factor was 0.36. The photocurrent density (Jsc) and open circuit voltage (Voc) reached the maximum values of 9.73 mA cm−2 and 0.46 V, respectively. When the number of coatings increased to 12, the photovoltaic parameters decreased due to certain electrons captured by oxygen molecules adsorbed on the surface of TiO2 and therefore cannot be transferred to ZnO . In addition, it is diﬃcult for the excess TiO2 sol to enter the ZnO nanorods gap, reducing the electronic transmission capacity of the heterojunction. 4. Conclusions The ZnO/TiO2 core–shell heterojunction is compounded and cosensitized by CdS and PbS QDs successfully. The optical properties of ZnO/TiO2 core–shell heterojunction and ZnO/TiO2/CdS/PbS composite structures were evidently enhanced. The UV–vis absorbance spectra indicate that CdS and PbS QD-cosensitized ZnO/TiO2 core-shell heterojunction can improve the absorption eﬀect in ultraviolet–visible light. When the number of spin coating TiO2 as 10 times, the η of ZnO/ TiO2/CdS/PbS sample reached the maximum value (1.59%), which was about 5.5, 1.5, 1.46, and 1.2 times those of unsensitized ZnO (0.29%), unsensitized ZnO/TiO2(10T) (1.04%), CdS-sensitized ZnO/TiO2(10T) (1.09%), and PbS sensitized ZnO/TiO2(10T) (1.34%), respectively, with the maximum IPCE of 36.04%. The ﬁll factor was 0.36, and the photocurrent density (Jsc) and open circuit voltage (Voc) attained the maximum values of 9.73 mA cm−2 and 0.46 V, respectively. These results strongly proved the advantage of the CdS and PbS QD-cosensitized ZnO/TiO2 core–shell heterojunction structure. Acknowledgments This work was ﬁnancially supported by grants from the Science Foundation of Guangxi Province (No. 2016GXNSFDA380036); the National Natural Science Foundation of China (No. 61664002). References  B. O'Regan, M. Grätzel, A low-cost, high-eﬃciency solar cell based on dye-sensitized colloidal TiO2 ﬁlms, Nature 353 (1991) 737–740.  M. Grätzel, Recent advances in sensitized mesoscopic solar cells, Acc. Chem. Res. 42 (2009) 1788–1798.