Application research of CdS: Eu3+ quantum dots-sensitized TiO2 nanotube solar cells

Materials Science in Semiconductor Processing 46 (2016) 53–58

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Application research of CdS: Eu3 þ quantum dots-sensitized TiO2 nanotube solar cells Qian Chen, Jiahui Song, Chunyan Zhou, Qi Pang, Liya Zhou n Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, 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 10 October 2015 Received in revised form 18 January 2016 Accepted 11 February 2016 Available online 17 February 2016

Trivalent Eu3 þ -doped CdS quantum dot (CdS: Eu3 þ QD)-sensitized TiO2 nanotube arrays (TNTAs) solar cells are prepared by using the direct adsorption method. The influences of sensitization time, sensitization temperature, and Eu3 þ ion concentrations are investigated systematically. The photo-current of the CdS: Eu3 þ QDs/TiO2 nanotubes appear at the main absorption region of 320–480 nm, and the maximum incident photon to the current conversion efficiency (IPCE) value is 21% at 430 nm when the sensitization condition is 4% doping Eu3 þ concentration, 60 °C sensitization temperature, 8 h sensitization time. Compared with the un-doped CdS QD-sensitized TNTAs, the conversion efficiency and IPCE of CdS: Eu3 þ QDs/TNTAs are two times and three times than that of un-doped CdS QDs sensitized TNTAs. This scenario exhibits the potential applications of rare earth elements in QD-sensitized solar cells. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Quantum dots Nanotubes Solar cells

1. Introduction Exploring renewable energy sources has become an important issue because of increasing energy demand. Quantum dot (QD)sensitized solar cells (QDSSCs) have piqued public interests because such materials provide a power production method for sunlight conversion into electrical energy. TiO2 nanotubes (TNTs) are attractive photo-anode materials for QDSSCs because of their inertness, non-toxicity, and inexpensive cost [1,2]. However, TNTs only take advantage of ultraviolet (UV) light (λ o 388 nm), which accounts for 4–5% of sunlight, and cannot absorb visible light, which accounts for 45% of the solar spectrum, because TNTs are high-bandgap semiconductors [3,4]. Different modification materials have been used to decrease the bandgap of TNTs and widen the absorption spectra to the visible region. Among the many materials that sensitize TNTs, doped semiconductor QDs have obtained the attention of scientists because of their broad absorption spectra [5]. CdS QDs doped with different types of impurities have recently been reported [6–8]. These QDs are doped with different ions that can cause spectral changes. CdS: Cu2 þ QDs show a redshift in the UV absorption and spectra, whereas both the UV absorption and spectra of CdS: Zn2 þ QDs show a blueshift [9]. The UV absorption edge of CdS: Eu3 þ QDs shift to the visible light range with the introduction of Eu3 þ ions [5]. CdS QDs works as a sensitizer in n

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

http://dx.doi.org/10.1016/j.mssp.2016.02.005 1369-8001/& 2016 Elsevier Ltd. All rights reserved.

enhancing the visible light absorption and photoelectric conversion efficiency with the lower bandgap (2.4 eV) than TiO2 (3.2 eV) [10,11]. For instance, the deposition of CdS QDs extends the absorption of TNTs into the visible light region, with the three times increases of the photoelectric conversion efficiency [12]. Furthermore, dopants increase the photoelectrochemical performance of CdS QDs/TNTs [13]. In the said reference, Eu3 þ -doped CdS QDs (CdS: Eu3 þ ) were synthesized by using a one-pot aqueous approach. CdS: Eu3 þ QDs were then deposited on TNTs by direct adsorption. The schematic of CdS: Eu3 þ QDs deposited on TNTs is shown in Fig. 1. The influences of sensitization time, sensitization temperature, and Eu3 þ ion concentrations were investigated to improve the photoelectric conversion performance of TNT solar cells sensitized by CdS: Eu3 þ QDs.

2. Experiments 2.1. Fabrication of the TNT arrays TNTs were grown on Ti foil surfaces by the anodic oxidation of Ti foils in glycerol electrolyte containing 0.5 wt% NH4F and 13.73 wt% deionized (DI) water at a constant voltage of 45 V for 10 h [14–16]. Prior to anodization, the Ti foils (1.0 cm  1.0 cm) were polished in an ultrasonic bath for 15 min and cleaned with acetone, ethanol, and DI water, followed by nitrogen drying. The highly ordered TNTs were performed in a two-electrode configuration with plumbago as the cathode and Ti foils as the anode.

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Fig. 1. The schematic diagram of CdS: Eu3 þ QDs deposited on TiO2 nanotubes.

The 0.02 M CdCl2  2H2O and 0.02 M Eu(NO3)3 were mixed in 100 mL DI water in certain proportions, stirred uniformly, and then poured into a 250 mL three-necked flask. Thioglycolic acid was used as a stabilizer. Approximately 1.0 M NaOH solution was used to adjust the pH value of the mixed solution. Approximately 0.1203 g Na2S  9H2O was added into the mixture solution when the pH value of the mixed solution was 11. The mixture solution was refluxed for 5 h at 100 °C and stirred constantly [18]. CdS: Eu3 þ QDs were then obtained, and the Eu3 þ contents were 0%, 1%, 2%, 3%, 4%, and 5%. 2.3. CdS: Eu3 þ QD-sensitized TNT arrays TNTs were placed in Petri dishes that contained 20 mL CdS: Eu3 þ QDs. The Petri dishes were then placed into an oven. The entire procedure was called Cycle 1. The TNTs films on the Ti foils with sensitization of CdS: Eu3 þ QDs were prepared and characterized. The properties of the CdS: Eu3 þ QDs/TNTs were studied by changing the Eu3 þ content, sensitization temperature, and sensitization time. 2.4. Characterization

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The crystal phases of the samples were studied by using a Rigaku Uitima IV X-ray diffractometer (XRD). The surface morphologies of the products were conducted with an FEI Quanta 200 scanning electron microscope (SEM). The diffuse reflectance UV– vis absorption spectra of the samples were recorded with a TU1901PC spectrometer with an integrating sphere attachment. The X-ray photoelectron spectroscopy (XPS) data were determined by an Al Ka micro-focused monochromator. The absorption spectra of the samples were recorded with a TU-1901PC spectrometer. The incident photon to electron carrier conversion efficiency (IPCE) measurement was characterized by using a Zolix IPCE test system. The photovoltaic characteristics were recorded on an electrochemical station (model LK98BII). The simulation sunlight was provided by a 500 W Xe lamp with AM 1.5 irradiance conditions. The photoelectric conversion efficiency of the prepared samples was performed by using a PEC test system with a saturated Ag/ AgCl reference electrode and a Pt counter electrode. The polysulfide electrolyte was composed of 0.125 M S, 0.5 M Na2S, and 0.2 M KCl.

3. Results and discussions 3.1. XRD and SEM characterization Fig. 2A shows the XRD patterns of the CdS: Eu3 þ QDs with

Fig. 2. (A) The XRD patterns of CdS: Eu3 þ QDs with various Eu3 þ molar concentrations: 0% (a), 2% (b), 4% (c). (B) The XRD patterns of bare TiO2 (a) as well as CdS: Eu3 þ QDs/TiO2 nanotubes with different sensitization conditions (b–e).

different Eu3 þ molar concentrations: (a) 0%, (b) 2%, and (c) 4%. The analysis of Fig. 2A suggests that the un-doped CdS QDs (a) and CdS: Eu3 þ QDs (b and c) had the same crystal structures. Furthermore, the peak located at 26.5°, 43.9°, and 52.1° corresponded to the (111), (220), and (311) crystal planes of CdS (JCPDS: No.100454). The results showed that the Eu3 þ was successfully incorporated into the CdS QDs lattice. Fig. 2B shows the XRD patterns of (a) bare TiO2 and CdS: Eu3 þ QDs/TNTs with different sensitization conditions: (b) 1% doping Eu3 þ concentration at 60 °C for 10 h, (c) 4% doping Eu3 þ concentration at 60 °C for 10 h, (d) 4% doping Eu3 þ concentration at 60 °C for 8 h, and (e) 4% doping Eu3 þ concentration at 100 °C for 8 h. Fig. 2B(a) shows that the locations of the XRD peaks were consistent with those of the TiO2 anatase phase (JCPDS: No. 21-1272). The XRD spectra of the CdS: Eu3 þ QDs/TNTs maintained the characteristic peaks of TNTs and did not exhibit a new diffraction peak compared with pure TNTs. This observation demonstrated that the changes in sensitization time, sensitization temperature, and Eu3 þ ion concentrations had no effect on the TNT crystal structure. However, the XRD spectra of CdS: Eu3 þ QDs/TNTs had a broader spectrum at 25.31°. The broadened spectra indicated the existence of CdS: Eu3 þ QDs [19].

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Fig. 3. SEM micrographs of bare TiO2 nanotubes (a), SEM (b), EDS (c) of the 4% Eu3 þ doped CdS QDs sensitized TiO2 nanotubes, and XPS of the CdS: Eu3 þ QDs (d).

The SEM micrographs of bare TNTs (a), SEM (b), and EDS (c) of the 4% Eu3 þ -doped CdS QD-sensitized TNTs are shown in Fig. 3. Fig. 3a shows that the top and side views (inset) present a highly ordered TNT array, the average nanotube diameter and wall thickness was estimated to be approximately 230 nm and 40 nm, respectively. The 1D feature of the TNTs not only promoted the QD deposition but also benefited unidirectional electron transport [20]. Fig. 3b shows that the TNT mouths were covered with 4% Eu3 þ -doped CdS QDs and that the wall thickness of the TNTs increased. The EDS results (Fig. 3c) showed that the Cd, S, Ti, O, and Eu were contained in the materials. These phenomena showed that the Eu3 þ -doped CdS QDs were successfully embedded into the TNT structure [21,22]. The XPS of CdS: Eu3 þ is also showed in Fig. 3d, indicating the presence of Eu3 þ in the CdS QDs, and the Na, S, C, and O come from the NaOH, Na2S  9H2O, and thioglycolic acid.

peak of UV–vis at 410 nm corresponding to the absorption of CdS: 3% Eu3 þ QDs, which caused a slight redshift with increasing sensitization time compared with bare TiO2. The increase in sensitization time evidently led to the increase in the UV–vis absorption intensity of CdS: Eu3 þ QDs/TNTs when the sensitization time was less than 8 h. Fig. 4a shows that the absorption intensity decreased as the sensitization time exceeded 8 h. A variational trend was also observed in testing the IPCE of the CdS: Eu3 þ QDs/TNTs. The CdS: Eu3 þ QDs/TNTs had a photo-current at wavelengths in the main absorption region of 320–480 nm, and the maximum IPCE value was 16% at 410 nm when the sensitization time was 8 h in Fig. 4 (B)–(e). Many QDs gathered on the TNT surface when the sensitization time was excessive. Therefore, the absorption intensity and IPCE of the CdS: Eu3 þ QDs/TiO2 decreased [23]. The results demonstrated that the proper sensitization time can increase the usage efficiency of visible light.

3.2. Influence of experimental variables 3.2.1. Influence of sensitization time Fig. 4 shows the UV–visible light (UV–vis) diffuse reflectance UV–vis absorption spectra (A) and IPCE spectra (B) of bare TNTs (a), UV–vis absorption spectra of CdS: 3% Eu3 þ QDs (g), as well as the diffuse reflectance UV–vis absorption spectra of CdS: 3% Eu3 þ QDs/TNTs with different sensitization times at 80 °C: 2 (b), 4 (c), 6 (d), 8 (e), and 10 h (f). The sensitized TiO2 exhibited an absorption

3.2.2. Influence of sensitization temperature Fig. 5 shows the diffuse reflectance UV–vis absorption spectra (A) and IPCE spectra (B) of bare TNTs (a), as well as the diffuse reflectance UV–vis absorption spectra of CdS: 3% Eu3 þ QDs/TNTs with different sensitization temperatures for 8 h: 40 (b), 60 (c), 80 (d), and 100 °C (e). Fig. 5(A) shows that CdS: Eu3 þ QDs/TNTs had the best UV–vis absorption at the wavelength of 420 nm at 60 °C. The absorption intensity decreased when the temperature was

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lower or higher than 60 °C. This scenario was probably due to many QDs that were easily agglomerated when the sensitization temperature was higher than 60 °C, which can lower the UV–vis absorption. Fig. 5(B) shows that the changing laws of the IPCE curves coincide with the UV–vis absorption curves. The highest IPCE value was 17% at 420 nm when the sensitization temperature was 60 °C. The results illustrated that proper sensitization temperature can increase the usage efficiency of visible light.

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(B)b to (B)g. The Eu3 þ ions could modify the physical and electronic properties of CdS QDs and cause the absorption spectrum redshift to the visible region. The maximum IPCE value was 21% at 430 nm when the Eu3 þ content was 4%. The UV–vis absorption and IPCE spectra investigation suggests that CdS: Eu3 þ QDs/TNTs can achieve the best effect of visible light absorption when the Eu3 þ content was 4%. 3.3. Photovoltaic performance of photoelectrodes

3.2.3. Influence of the Eu3 þ molar content Fig. 6 shows the diffuse reflectance UV–vis absorption spectra (A) and IPCE spectra (B) of bare TNTs (a) and the diffuse reflectance UV–vis absorption spectra of CdS: Eu3 þ QDs/TNTs with different Eu3 þ molar contents at 60 °C for 8 h: 0% (b), 2% (c), 3% (d), 4% (e), and 5% (f). Fig. 6A(a) shows that the diffuse reflectance UV–vis absorption spectra of pure TNTs mainly occurred in the UV region probably because of the large energy edge of TNTs at 3.2 eV [24]. With increasing Eu3 þ content, the absorption band of the CdS: Eu3 þ QDs/TNTs increased first and then decreased, as shown in Fig. 6(A)b to (A)f. The absorption peak edge of CdS: Eu3 þ QDs/TNTs reached 480 nm when the Eu3 þ molar content was 4%. Fig. 6 (B) shows that the pure TNTs had some photo-current at wavelengths in the main absorption region of 300–400 nm, and the maximum IPCE value was 14% at 350 nm (Fig. 6(A)(a)). Furthermore, the IPCE of CdS QDs/TNTs increased the spectral response in the range of 320–450 nm. The IPCE curves of CdS: Eu3 þ QDs/TNTs showed a wider and enhanced spectral response in the range of 300–500 nm than pure TNTs and CdS QDs/TNTs as shown in Fig. 6

The current density–voltage (J–V) curve characteristics were examined under the illumination of AM 1.5 G solar spectrum simulation (Fig. 7) to study the effect of the electronic transfer rate on the photovoltaic parameters of CdS: Eu3 þ QDs/TNT solar cells. The short-circuit current density (Jsc) increased from 0.64 mA/cm2 for CdS/TNT photoanodes to 0.98 mA/cm2 for 4% Eu3 þ -doped CdS QDs. The photoelectric conversion efficiency (η) also increased from 0.24% to 0.39%, which is higher than those previously reported [11]. These results demonstrate that CdS: Eu3 þ QDs/TNT photoanodes had significantly better photoelectric properties than CdS QDs/TNT photoanodes. The reason for this scenario can be that the appropriate amount of rare earth elements can promote the charge transformation and utilization [25]. Fig. 8 presents the j–t photoresponsive characteristics of the samples. The two “light on/light off” j–t curves reveal that the observed current for the cell was full due to the sensitizer photoactivity, thus offering a high electron transmission rate. The current density of the CdS QDs/TNT photoanodes was higher than the current density of the CdS QDs/

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Eu3 þ -doped CdS QDs with Eu3 þ for designing TNT solar cells were successfully synthesized using direct adsorption. Studying the influence of sensitization time, sensitization temperature, and Eu3 þ content shows that the optimal condition was 4% doping Eu3 þ concentration, 60 °C sensitization temperature, and 8 h sensitization time. The CdS: Eu3 þ QDs/TNTs can cause the absorption spectrum to Einstein shift to the visible region and improve the photoelectric conversion efficiency under the optimum condition compared with bare TNTs. These results showed that semiconductor QDs doped with rare earth ions can change their chemical and physical properties, thus further increasing the application range. However, studies on doped semiconductor QDsensitized solar cells are lacking. The current study showed that other doped semiconductor QDs could also be researched to improve the photoelectric conversion efficiency.

Acknowledgements

a TiO2 FF=0.60 ¨=0.19% b CdS-TiO2 FF=0.57 ¨=0.24% 3+ c CdS:Eu -TiO2 FF=0.57 ¨=0.39%

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This work was supported by grants from the National Natural Science Foundation of China (No. 61264003); the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2014K004).

References 0.4

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TNT photoanodes when the light was turned on. This condition further illustrates that the CdS: Eu3 þ QDs/TNT photoanodes were provided with significantly better photoelectric properties than CdS QDs/TNT photoanodes.

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