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Voltage-assisted SILAR deposition of CdSe quantum dots into mesoporous TiO2 film for quantum dot-sensitized solar cells
Chemical Physics Letters 735 (2019) 136764
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Voltage-assisted SILAR deposition of CdSe quantum dots into mesoporous TiO2 film for quantum dot-sensitized solar cells
T
⁎
Bin Bin Jina, , Dan Jun Wangb, Shu Ying Konga, Guo Qing Zhanga, Hui Sheng Huanga, Yan Liua, Hai Quan Liua, Jing Wua, Liang Hong Zhaoa, Deng Hea a
Chongqing Key Laboratory of Inorganic Special Functional Materials, College of Chemistry and Chemical Engineering, Yangtze Normal University, Chongqing 408100, China b Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan’an University, Yan’an 716000, China
H I GH L IG H T S
voltage drives CdSe QDs growth quickly in the mesoporous TiO film at room temperature. • External optimal efficiency of 3.27% for the CdSe QDSSCs is received. • An • This work provides a fast and efficient assembly strategy to enhance the performance of QDSSCs. 2
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanoparticles Semiconductors Electrodeposition Solar energy materials Energy storage and conversion
CdSe quantum dots (QDs) grows slowly in mesoporous TiO2 film deriving from a large lattice mismatch (11%) and poor interaction between the two, which resulting the low power conversion efficiency (PCE) of quantum dot-sensitized solar cells (QDSSCs). An external voltage is applied in the successive ionic layer adsorption reaction (SILAR) process, which drives the Cd2+ and SeSO32− ions quickly diffused into the mesoporous TiO2 film and reacted to form CdSe QDs at room temperature. The CdSe QDSSCs yields a PCE of 3.27%. This work provides an efficient strategy to assemble QDs for enhancing the performance of QDSSCs.
1. Introduction In quantum dot-sensitized solar cells (QDSSCs), assembling quantum dots (QDs) into mesoporous TiO2 film is a key to determine the charge separation and transport, which significantly affect the performance of the cells [1–3]. Linker-assisted assembly and in situ deposition as two common strategies are widely used to assemble QDs into the mesoporous films. The former involves assembling pre-synthesized colloidal QDs using bifunctional molecule as linkers [4,5]. This approach requires precise control of QDs synthesis and ligand exchange process under harsh condition. The latter is directly growing QDs into TiO2 film by chemical bath deposition (CBD) or successive ionic layer adsorption reaction (SILAR) [6,7]. This strategy bases on rapid cationanion reaction in solution, which provides a simple and economic route to assemble growth of QDs into the mesoporous film. The in situ deposition is mainly applied for deposition of metal chalcogenides QDs to enhance the performance of QDSSCs. Typically, in situ depositions of CdS and CdSe into the mesoporous films as
⁎
photoanodes are widely used to enhance light harvesting and control charge recombination [8–13]. In addition, other metal chalcogenides QDs, such as PbS, CuS, CoS, NiS, are deposited on various substrates by in situ deposition to construct counter electrode (CE) for improving electrocatalytic activity and cell stability [14–17]. Among the metal chalcogenides QDs, CdSe has been paid attention in QDSSCs because of it can capture the whole visible light region of solar spectrum by tuning particle size [18]. But because of the large lattice mismatch (11%) between CdSe and TiO2, the CdSe QDs grows very slowly in the mesoporous TiO2 film [19]. It usually has to spend several hours to increase amount of QDs loading under high temperature condition [8–12]. The low growth rate of CdSe is associated with the lower efficiency of QDSSCs because a low deposition rate implies a poor interaction of the deposited layer (CdSe) to the substrate (TiO2) [20]. Currently, using in situ deposition assemble single species sensitizer of CdSe into the mesoporous TiO2 film as electrode, the obtained efficiency of QDSSCs is around 2% [11–13,20]. Very limited studies reported beyond 3%, the performance of CdSe QDSSCs is still in a poor
Corresponding author. E-mail address: [email protected] (B.B. Jin).
https://doi.org/10.1016/j.cplett.2019.136764 Received 15 August 2019; Received in revised form 9 September 2019; Accepted 10 September 2019 Available online 11 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 735 (2019) 136764
B.B. Jin, et al.
spectroscopy (EIS) analyses of the cells were recorded by a CHI 660E electrochemical workstation (Shanghai, China) under the dark condition.
level [21]. It needs to develop a fast and efficient assembly strategy to enhance the performance of the CdSe QDSSCs. In this paper, we assembled CdSe QDs into the mesoporous TiO2 film using a simple and rapid voltage-assisted SILAR method at room temperature condition. In the conventional SILAR method, the Cd cations are first adsorbed on the TiO2 film at room temperature for 10 min and then react with the SeSO32− to grow CdSe QDs at 50 °C for 20 min [8–10]. To accelerate the grow rate of CdSe, we applied a 2 V voltage in the Cd2+ and SeSO32− precursor solution at room temperature for 30 s, respectively. Benefiting from the assistance of electric field, a great number of Cd2+ and SeSO32− ions quickly diffused into the mesoporous TiO2 film and reacted to form CdSe QDs. By depositing three layers of CdSe QDs, an optimal efficiency of 3.27% for the QDSSCs is received.
3. Results and discussion CdSe QDs deposited into the mesoporous FTO/TiO2 photoanodes at different number of cycles are denoted as nCdSe, respectively, where n represent the number of cycles. The crystal structures of the prepared nCdSe electrodes were characterized by XRD (Fig. 2a). All the electrodes consist of anatase (JCPDS card no. 21-1272)-rutile (JCPDS card no. 89-0553) TiO2 and SnO2 (JCPDS card no. 46-1088) structure. The diffraction peaks of anatase-rutile are from the mesoporous TiO2 film, and the diffraction peaks of SnO2 are from the FTO glass substrate. The characteristic peaks of CdSe do not appear in the XRD pattern. This is due to the diffraction peak of (1 1 1) of zinc-blende CdSe (JCPDS card no. 19-0191) overlaps with the diffraction peak of (1 0 1) of anatase TiO2, the strong diffraction peak of TiO2 covers the characteristic peak of CdSe [23–25]. Further composition analysis of the electrode was carried out using EDS and XPS. Fig. 2b shows the EDS and the cross-sectional SEM images of the 3CdSe. The corresponding Cd and Se elemental EDS mappings are shown in inset of Fig. 2b. The EDS image shows that only Ti, O, Cd, and Se present in the electrode without other element, and the atomic ratio of Cd:Se is close to 1:1 (Cd: 1.99 at.%, Se: 1.89 at.%). Cd and Se elemental EDS mappings further show that Cd and Se distributed across entire mesoporous TiO2 film. The XPS characterization shows that the binding energies of the Cd 3d5/2 and Cd 3d3/2 are at 404.7 eV and 411.4 eV, respectively (Fig. 2c), while the binding energy of the Se 3d is around 54.0 eV (Fig. 2d). This result is consistent with the previously reported for CdSe QDs [23–25]. XRD, EDS and XPS results confirmed that the CdSe QDs were successfully deposited into the mesoporous TiO2 film by the voltage-assisted SILAR method. The light absorption properties of the electrodes were evaluated using UV–vis-NIR absorption spectra (Fig. 3). There are significant red shift and absorption enhancement in the wavelength from 380 to 800 nm with the increase of the number of CdSe deposition cycles from 0 to 4. Increased number of cycles directly resulted in increased QDs loading on the TiO2 film, which implies that the red shift and absorption enhancement are closely related to the increased amount of QDs [1,26]. The red shift and absorption enhancement are also confirmed by the color change of the nCdSe electrodes from bright red to dark red (inset Fig. 3). The amount of CdSe QDs loading increased with the number of
2. Experimental section 2.1. Voltage-assisted SILAR depositon and assembly of QDSSCs Voltage-assisted SILAR deposition of CdSe QDs was carried out by applying 2 V voltage in a two-electrode system on a Maynuo M8811 DC source meter. The mesoporous TiO2 film and a graphite rod were used as the working electrode and the counter electrode, respectively. Under 2 V voltage, the mesoporous TiO2 film were alternately dipped into 0.1 M Cd(NO3)2·4H2O methanol solution and 0.3 M Na2SeSO3 aqueous solution for 30 s. The film was thoroughly rinsed with pure solvent after each immersion. The process is considered to form one layer CdSe. The deposition amount of CdSe QDs can be controlled by adjusting the number of cycles. The deposition device and process are shown in Fig. 1. Subsequent electrodes passivation, the PbSe counter electrode (CE) preparation and the QDSSCs assembly were according to our previous work [8–10,22]. 2.2. Characterization The structure and composition of the as-prepared FTO/TiO2/CdSe electrodes were characterized by X-ray diffraction (XRD, Shimadzu XRD-7000), X-ray photoelectron spectroscopy (XPS, Thermo Fisher KAlpha), scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS) (JEOL-6701). The ultraviolet–visible-near infrared (UV–vis-NIR) absorption spectra of the electrodes were measured on a UV-3600 Plus spectrometer (Shimadzu). The current–voltage (J-V) characteristics were measured under an irradiation of simulated solar light (AM 1.5G, 100 mW/cm2). The electrochemical impedance
Fig. 1. Experimental device and voltage-assisted SILAR process of depositing CdSe QDs into the mesoporous TiO2 film. 2
Chemical Physics Letters 735 (2019) 136764
B.B. Jin, et al.
Fig. 2. (a) XRD patterns of the nCdSe electrodes; (b) EDS images, cross-sectional SEM and corresponding Cd and Se elemental EDS mappings of the 3CdSe electrode. XPS spectra: (c) Cd 3d and (d) Se 3d for 3CdSe electrode.
requires spending several hours at higher temperatures to realize the deposition of CdSe QDs. Fig. 2S is digital photo of CdSe electrodes with different number of cycles which obtained by the SILAR method. During the first five cycles, there were no obvious color change on the front and back side of the electrodes. The brown red CdSe QDs begin appearing on the back side of the electrodes until after seven cycles. However, there is still no obvious color change in the front side of the electrodes, which confirms that the CdSe QDs grow very slowly in the TiO2 film without voltage assistance. In voltage-assisted SILAR method, an electric field created by applied voltage drives the Cd2+ to diffuse/ adsorb into the TiO2 film. More importantly, in anion precursors, SeSO32− rapidly releases Se2− into the TiO2 film through a cathode reduction reaction as follows [27–29]: SeSO32− + 2e− → Se2− + SO32−
(1)
Then, the adsorbed Cd2+ reacted with Se2− to form CdSe in the TiO2 film: Fig. 3. The UV–vis-NIR absorption spectra of the nCdSe electrodes.
Cd2+ + Se2− → CdSe(s)
(2)
The performance of QDSSCs is closely related to the amount of QDs loading. J-V curves of cells with the nCdSe electrodes are recorded in Fig. 5a. The short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and the power conversion efficiency (ƞ) are listed in Table 1. The best performance of the cell (ƞ = 3.27%) is achieved when the number of cycles increased to 3. The enhanced performance of the cell is mainly due to the increase of Jsc (12.35 mA/cm2) which is direct relation to amount of QDs loading. The high amount of loading can capture more photons to generate more electrons [26]. However,
cycles was further confirmed by SEM and elemental EDS mappings. Fig. 4 shows the surface SEM images and corresponding Cd and Se elemental EDS mappings of the TiO2 film with different number of CdSe deposition cycles. Compared with the bare surface of the TiO2 film (Fig. S1), the surface of the electrodes became more compact and the contents of Cd and Se became higher with increasing number of cycles. The above results show that high CdSe QDs loading amount can be obtained in a few minutes by voltage-assisted SILAR method at room temperature. Removing the voltage, the conventional SILAR method 3
Chemical Physics Letters 735 (2019) 136764
B.B. Jin, et al.
Fig. 4. The surface SEM images of the nCdSe electrodes and the corresponding Cd and Se elemental EDS mappings: (a) 1CdSe electrodes; (b) 2CdSe electrodes; (c) 3CdSe electrodes and (d) 4CdSe electrodes.
when the number of cycles increased to 4, Jsc and ƞ of the cell were reduced to 11.93 mA/cm2 and 2.81%, respectively. This indicates the overload of QDs results in the poor performance of the cell. The overloaded QDs increase recombination sites to consume photogenerated electrons [30]. The other reason is the mesopores of TiO2 film can be blocked by the overloaded QDs, which obstruct the penetration of the electrolyte into the mesopores of TiO2 film and delay the CdSe QDs from regeneration [1,23,25,26]. The above reasons result in increasing the charge recombination at TiO2/QDs/electrolyte interface, which reduce the numbers of photogenerated electrons transport to external circuits and reduce Jsc. FF from 52.44% reduces to 46.18% with the increase number of cycles also shows that the overloaded QDs increase charge recombination in the cell. The charge recombination was further confirmed by dark current curves and EIS characterizations. Fig. 5b shows that the increasing trend of dark current is proportional to the number of cycles, which suggests that increase recombination with increase the amount of QDs loading. EIS analyses were performed in the dark and the Nyquist plots are shown in Fig. 5c. The inset of Fig. 5c is the corresponding equivalent circuit. The recombination resistance at TiO2/QDs/electrolyte interface (Rct) derives from the intermediate frequency of Nyquist plots. The values of the Rct of the nCdSe electrodes are listed in Table 1. The order of the Rct of the electrodes is: 1CdSe > 2 CdSe > 3CdSe > 4CdSe. The high value of the Rct shows that the electrons in the photoanode are difficult to recombine with the electrolyte and TiO2/QDs interface, the
low value of the Rct indicates an increased charge recombination at the interface [31,32]. Furthermore, the electron lifetime (τe) can be calculated by τe = 1/2πfmid [32–34]. The fmid is the frequency of midfrequency peak in the Bode phase plot (Fig. 5d). With increase the number of cycles, τe of the nCdSe electrodes are gradually shortened (Table 1). The shorter electron lifetime in QDSSCs means that more electron recombination occurs at the interface [34]. The values of the Rct and τe of the nCdSe electrodes both suggest that charge recombination of the interface increase with the number of cycles. The results of EIS are agreement with dark current, but, are contrary to the J-V tests. This is due to the competition between photon capture and charge recombination in the cell caused by the amount of QDs loading. In the first three cycles, photon capture and electron transport are dominant. So, more electrons are generated and transferred to external circuits, which obtained large Jsc. But in the fourth cycle, recombination are dominates, a large number of electrons are recombined at the interface, leading to deterioration of the cell performance. 4. Conclusions In summary, the electric field created by applied voltage in the voltage-assisted SILAR method accelerates the growth of CdSe QDs and enhances the amount of QDs loading in the mesoporous TiO2 film. The amount of CdSe QDs loading increases with the number of cycles. The light absorption and photovoltaic properties of these films as electrodes 4
Chemical Physics Letters 735 (2019) 136764
B.B. Jin, et al.
Fig. 5. (a) J-V curves; (b) dark current curves; (c) Nyquist plots and (d) Bode plots of cells with the nCdSe electrodes. The nCdSe electrodes with PbSe CE were filled with a polysulfide electrolyte (0.1 M NaOH, 1.0 M S and 1.0 M Na2S aqueous solution).
Cooperative Research Projects of Ministry of Education of China, the Scientific Research Funding Project of Yangtze Normal University (Grant No. 2017KYQD88) and the Growth Support Plan for Young Scientific Talents from Yangtze Normal University.
Table 1 The photovoltaic and EIS parameters of the prepared electrodes. Electrodes 1 2 3 4
CdSe CdSe CdSe CdSe
Jsc (mA/cm2)
Voc (V)
FF (%)
η (%)
Rct (Ω)
τe (ms)
7.03 9.46 12.35 11.93
0.51 0.52 0.57 0.51
52.44 46.34 46.45 46.18
1.88 2.28 3.27 2.81
43.53 30.28 24.55 18.85
192.90 108.47 73.92 41.56
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.136764.
for QDSSCs are comprehensively researched by UV–vis-NIR absorption spectra, J-V curves and EIS. The results show that photon capture and charge recombination are closely related to the amount of QDs loading. An optimized PCE of 3.27% is received, when the photon capture and charge recombination are balanced in the third cycle. The present work provides a simple and rapid method to improve the performance of QDSSCs, which has potential for further research.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the Natural Science Foundation Project of Chongqing Science and Technology Commission (Grant No. cstc2017jcyjAX0100), the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJQN201801405), Fuling District Science and Technology Research Projects (Grant No. FLKJ, 2018BBA3044), the “Chunhui Plan” 5
Chemical Physics Letters 735 (2019) 136764
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6
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Voltage-assisted SILAR deposition of CdSe quantum dots into mesoporous TiO2 film for quantum dot-sensitized solar cells
T
⁎
Bin Bin Jina, , Dan Jun Wangb, Shu Ying Konga, Guo Qing Zhanga, Hui Sheng Huanga, Yan Liua, Hai Quan Liua, Jing Wua, Liang Hong Zhaoa, Deng Hea a
Chongqing Key Laboratory of Inorganic Special Functional Materials, College of Chemistry and Chemical Engineering, Yangtze Normal University, Chongqing 408100, China b Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan’an University, Yan’an 716000, China
H I GH L IG H T S
voltage drives CdSe QDs growth quickly in the mesoporous TiO film at room temperature. • External optimal efficiency of 3.27% for the CdSe QDSSCs is received. • An • This work provides a fast and efficient assembly strategy to enhance the performance of QDSSCs. 2
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanoparticles Semiconductors Electrodeposition Solar energy materials Energy storage and conversion
CdSe quantum dots (QDs) grows slowly in mesoporous TiO2 film deriving from a large lattice mismatch (11%) and poor interaction between the two, which resulting the low power conversion efficiency (PCE) of quantum dot-sensitized solar cells (QDSSCs). An external voltage is applied in the successive ionic layer adsorption reaction (SILAR) process, which drives the Cd2+ and SeSO32− ions quickly diffused into the mesoporous TiO2 film and reacted to form CdSe QDs at room temperature. The CdSe QDSSCs yields a PCE of 3.27%. This work provides an efficient strategy to assemble QDs for enhancing the performance of QDSSCs.
1. Introduction In quantum dot-sensitized solar cells (QDSSCs), assembling quantum dots (QDs) into mesoporous TiO2 film is a key to determine the charge separation and transport, which significantly affect the performance of the cells [1–3]. Linker-assisted assembly and in situ deposition as two common strategies are widely used to assemble QDs into the mesoporous films. The former involves assembling pre-synthesized colloidal QDs using bifunctional molecule as linkers [4,5]. This approach requires precise control of QDs synthesis and ligand exchange process under harsh condition. The latter is directly growing QDs into TiO2 film by chemical bath deposition (CBD) or successive ionic layer adsorption reaction (SILAR) [6,7]. This strategy bases on rapid cationanion reaction in solution, which provides a simple and economic route to assemble growth of QDs into the mesoporous film. The in situ deposition is mainly applied for deposition of metal chalcogenides QDs to enhance the performance of QDSSCs. Typically, in situ depositions of CdS and CdSe into the mesoporous films as
⁎
photoanodes are widely used to enhance light harvesting and control charge recombination [8–13]. In addition, other metal chalcogenides QDs, such as PbS, CuS, CoS, NiS, are deposited on various substrates by in situ deposition to construct counter electrode (CE) for improving electrocatalytic activity and cell stability [14–17]. Among the metal chalcogenides QDs, CdSe has been paid attention in QDSSCs because of it can capture the whole visible light region of solar spectrum by tuning particle size [18]. But because of the large lattice mismatch (11%) between CdSe and TiO2, the CdSe QDs grows very slowly in the mesoporous TiO2 film [19]. It usually has to spend several hours to increase amount of QDs loading under high temperature condition [8–12]. The low growth rate of CdSe is associated with the lower efficiency of QDSSCs because a low deposition rate implies a poor interaction of the deposited layer (CdSe) to the substrate (TiO2) [20]. Currently, using in situ deposition assemble single species sensitizer of CdSe into the mesoporous TiO2 film as electrode, the obtained efficiency of QDSSCs is around 2% [11–13,20]. Very limited studies reported beyond 3%, the performance of CdSe QDSSCs is still in a poor
Corresponding author. E-mail address: [email protected] (B.B. Jin).
https://doi.org/10.1016/j.cplett.2019.136764 Received 15 August 2019; Received in revised form 9 September 2019; Accepted 10 September 2019 Available online 11 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 735 (2019) 136764
B.B. Jin, et al.
spectroscopy (EIS) analyses of the cells were recorded by a CHI 660E electrochemical workstation (Shanghai, China) under the dark condition.
level [21]. It needs to develop a fast and efficient assembly strategy to enhance the performance of the CdSe QDSSCs. In this paper, we assembled CdSe QDs into the mesoporous TiO2 film using a simple and rapid voltage-assisted SILAR method at room temperature condition. In the conventional SILAR method, the Cd cations are first adsorbed on the TiO2 film at room temperature for 10 min and then react with the SeSO32− to grow CdSe QDs at 50 °C for 20 min [8–10]. To accelerate the grow rate of CdSe, we applied a 2 V voltage in the Cd2+ and SeSO32− precursor solution at room temperature for 30 s, respectively. Benefiting from the assistance of electric field, a great number of Cd2+ and SeSO32− ions quickly diffused into the mesoporous TiO2 film and reacted to form CdSe QDs. By depositing three layers of CdSe QDs, an optimal efficiency of 3.27% for the QDSSCs is received.
3. Results and discussion CdSe QDs deposited into the mesoporous FTO/TiO2 photoanodes at different number of cycles are denoted as nCdSe, respectively, where n represent the number of cycles. The crystal structures of the prepared nCdSe electrodes were characterized by XRD (Fig. 2a). All the electrodes consist of anatase (JCPDS card no. 21-1272)-rutile (JCPDS card no. 89-0553) TiO2 and SnO2 (JCPDS card no. 46-1088) structure. The diffraction peaks of anatase-rutile are from the mesoporous TiO2 film, and the diffraction peaks of SnO2 are from the FTO glass substrate. The characteristic peaks of CdSe do not appear in the XRD pattern. This is due to the diffraction peak of (1 1 1) of zinc-blende CdSe (JCPDS card no. 19-0191) overlaps with the diffraction peak of (1 0 1) of anatase TiO2, the strong diffraction peak of TiO2 covers the characteristic peak of CdSe [23–25]. Further composition analysis of the electrode was carried out using EDS and XPS. Fig. 2b shows the EDS and the cross-sectional SEM images of the 3CdSe. The corresponding Cd and Se elemental EDS mappings are shown in inset of Fig. 2b. The EDS image shows that only Ti, O, Cd, and Se present in the electrode without other element, and the atomic ratio of Cd:Se is close to 1:1 (Cd: 1.99 at.%, Se: 1.89 at.%). Cd and Se elemental EDS mappings further show that Cd and Se distributed across entire mesoporous TiO2 film. The XPS characterization shows that the binding energies of the Cd 3d5/2 and Cd 3d3/2 are at 404.7 eV and 411.4 eV, respectively (Fig. 2c), while the binding energy of the Se 3d is around 54.0 eV (Fig. 2d). This result is consistent with the previously reported for CdSe QDs [23–25]. XRD, EDS and XPS results confirmed that the CdSe QDs were successfully deposited into the mesoporous TiO2 film by the voltage-assisted SILAR method. The light absorption properties of the electrodes were evaluated using UV–vis-NIR absorption spectra (Fig. 3). There are significant red shift and absorption enhancement in the wavelength from 380 to 800 nm with the increase of the number of CdSe deposition cycles from 0 to 4. Increased number of cycles directly resulted in increased QDs loading on the TiO2 film, which implies that the red shift and absorption enhancement are closely related to the increased amount of QDs [1,26]. The red shift and absorption enhancement are also confirmed by the color change of the nCdSe electrodes from bright red to dark red (inset Fig. 3). The amount of CdSe QDs loading increased with the number of
2. Experimental section 2.1. Voltage-assisted SILAR depositon and assembly of QDSSCs Voltage-assisted SILAR deposition of CdSe QDs was carried out by applying 2 V voltage in a two-electrode system on a Maynuo M8811 DC source meter. The mesoporous TiO2 film and a graphite rod were used as the working electrode and the counter electrode, respectively. Under 2 V voltage, the mesoporous TiO2 film were alternately dipped into 0.1 M Cd(NO3)2·4H2O methanol solution and 0.3 M Na2SeSO3 aqueous solution for 30 s. The film was thoroughly rinsed with pure solvent after each immersion. The process is considered to form one layer CdSe. The deposition amount of CdSe QDs can be controlled by adjusting the number of cycles. The deposition device and process are shown in Fig. 1. Subsequent electrodes passivation, the PbSe counter electrode (CE) preparation and the QDSSCs assembly were according to our previous work [8–10,22]. 2.2. Characterization The structure and composition of the as-prepared FTO/TiO2/CdSe electrodes were characterized by X-ray diffraction (XRD, Shimadzu XRD-7000), X-ray photoelectron spectroscopy (XPS, Thermo Fisher KAlpha), scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS) (JEOL-6701). The ultraviolet–visible-near infrared (UV–vis-NIR) absorption spectra of the electrodes were measured on a UV-3600 Plus spectrometer (Shimadzu). The current–voltage (J-V) characteristics were measured under an irradiation of simulated solar light (AM 1.5G, 100 mW/cm2). The electrochemical impedance
Fig. 1. Experimental device and voltage-assisted SILAR process of depositing CdSe QDs into the mesoporous TiO2 film. 2
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Fig. 2. (a) XRD patterns of the nCdSe electrodes; (b) EDS images, cross-sectional SEM and corresponding Cd and Se elemental EDS mappings of the 3CdSe electrode. XPS spectra: (c) Cd 3d and (d) Se 3d for 3CdSe electrode.
requires spending several hours at higher temperatures to realize the deposition of CdSe QDs. Fig. 2S is digital photo of CdSe electrodes with different number of cycles which obtained by the SILAR method. During the first five cycles, there were no obvious color change on the front and back side of the electrodes. The brown red CdSe QDs begin appearing on the back side of the electrodes until after seven cycles. However, there is still no obvious color change in the front side of the electrodes, which confirms that the CdSe QDs grow very slowly in the TiO2 film without voltage assistance. In voltage-assisted SILAR method, an electric field created by applied voltage drives the Cd2+ to diffuse/ adsorb into the TiO2 film. More importantly, in anion precursors, SeSO32− rapidly releases Se2− into the TiO2 film through a cathode reduction reaction as follows [27–29]: SeSO32− + 2e− → Se2− + SO32−
(1)
Then, the adsorbed Cd2+ reacted with Se2− to form CdSe in the TiO2 film: Fig. 3. The UV–vis-NIR absorption spectra of the nCdSe electrodes.
Cd2+ + Se2− → CdSe(s)
(2)
The performance of QDSSCs is closely related to the amount of QDs loading. J-V curves of cells with the nCdSe electrodes are recorded in Fig. 5a. The short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and the power conversion efficiency (ƞ) are listed in Table 1. The best performance of the cell (ƞ = 3.27%) is achieved when the number of cycles increased to 3. The enhanced performance of the cell is mainly due to the increase of Jsc (12.35 mA/cm2) which is direct relation to amount of QDs loading. The high amount of loading can capture more photons to generate more electrons [26]. However,
cycles was further confirmed by SEM and elemental EDS mappings. Fig. 4 shows the surface SEM images and corresponding Cd and Se elemental EDS mappings of the TiO2 film with different number of CdSe deposition cycles. Compared with the bare surface of the TiO2 film (Fig. S1), the surface of the electrodes became more compact and the contents of Cd and Se became higher with increasing number of cycles. The above results show that high CdSe QDs loading amount can be obtained in a few minutes by voltage-assisted SILAR method at room temperature. Removing the voltage, the conventional SILAR method 3
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Fig. 4. The surface SEM images of the nCdSe electrodes and the corresponding Cd and Se elemental EDS mappings: (a) 1CdSe electrodes; (b) 2CdSe electrodes; (c) 3CdSe electrodes and (d) 4CdSe electrodes.
when the number of cycles increased to 4, Jsc and ƞ of the cell were reduced to 11.93 mA/cm2 and 2.81%, respectively. This indicates the overload of QDs results in the poor performance of the cell. The overloaded QDs increase recombination sites to consume photogenerated electrons [30]. The other reason is the mesopores of TiO2 film can be blocked by the overloaded QDs, which obstruct the penetration of the electrolyte into the mesopores of TiO2 film and delay the CdSe QDs from regeneration [1,23,25,26]. The above reasons result in increasing the charge recombination at TiO2/QDs/electrolyte interface, which reduce the numbers of photogenerated electrons transport to external circuits and reduce Jsc. FF from 52.44% reduces to 46.18% with the increase number of cycles also shows that the overloaded QDs increase charge recombination in the cell. The charge recombination was further confirmed by dark current curves and EIS characterizations. Fig. 5b shows that the increasing trend of dark current is proportional to the number of cycles, which suggests that increase recombination with increase the amount of QDs loading. EIS analyses were performed in the dark and the Nyquist plots are shown in Fig. 5c. The inset of Fig. 5c is the corresponding equivalent circuit. The recombination resistance at TiO2/QDs/electrolyte interface (Rct) derives from the intermediate frequency of Nyquist plots. The values of the Rct of the nCdSe electrodes are listed in Table 1. The order of the Rct of the electrodes is: 1CdSe > 2 CdSe > 3CdSe > 4CdSe. The high value of the Rct shows that the electrons in the photoanode are difficult to recombine with the electrolyte and TiO2/QDs interface, the
low value of the Rct indicates an increased charge recombination at the interface [31,32]. Furthermore, the electron lifetime (τe) can be calculated by τe = 1/2πfmid [32–34]. The fmid is the frequency of midfrequency peak in the Bode phase plot (Fig. 5d). With increase the number of cycles, τe of the nCdSe electrodes are gradually shortened (Table 1). The shorter electron lifetime in QDSSCs means that more electron recombination occurs at the interface [34]. The values of the Rct and τe of the nCdSe electrodes both suggest that charge recombination of the interface increase with the number of cycles. The results of EIS are agreement with dark current, but, are contrary to the J-V tests. This is due to the competition between photon capture and charge recombination in the cell caused by the amount of QDs loading. In the first three cycles, photon capture and electron transport are dominant. So, more electrons are generated and transferred to external circuits, which obtained large Jsc. But in the fourth cycle, recombination are dominates, a large number of electrons are recombined at the interface, leading to deterioration of the cell performance. 4. Conclusions In summary, the electric field created by applied voltage in the voltage-assisted SILAR method accelerates the growth of CdSe QDs and enhances the amount of QDs loading in the mesoporous TiO2 film. The amount of CdSe QDs loading increases with the number of cycles. The light absorption and photovoltaic properties of these films as electrodes 4
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Fig. 5. (a) J-V curves; (b) dark current curves; (c) Nyquist plots and (d) Bode plots of cells with the nCdSe electrodes. The nCdSe electrodes with PbSe CE were filled with a polysulfide electrolyte (0.1 M NaOH, 1.0 M S and 1.0 M Na2S aqueous solution).
Cooperative Research Projects of Ministry of Education of China, the Scientific Research Funding Project of Yangtze Normal University (Grant No. 2017KYQD88) and the Growth Support Plan for Young Scientific Talents from Yangtze Normal University.
Table 1 The photovoltaic and EIS parameters of the prepared electrodes. Electrodes 1 2 3 4
CdSe CdSe CdSe CdSe
Jsc (mA/cm2)
Voc (V)
FF (%)
η (%)
Rct (Ω)
τe (ms)
7.03 9.46 12.35 11.93
0.51 0.52 0.57 0.51
52.44 46.34 46.45 46.18
1.88 2.28 3.27 2.81
43.53 30.28 24.55 18.85
192.90 108.47 73.92 41.56
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.136764.
for QDSSCs are comprehensively researched by UV–vis-NIR absorption spectra, J-V curves and EIS. The results show that photon capture and charge recombination are closely related to the amount of QDs loading. An optimized PCE of 3.27% is received, when the photon capture and charge recombination are balanced in the third cycle. The present work provides a simple and rapid method to improve the performance of QDSSCs, which has potential for further research.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the Natural Science Foundation Project of Chongqing Science and Technology Commission (Grant No. cstc2017jcyjAX0100), the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJQN201801405), Fuling District Science and Technology Research Projects (Grant No. FLKJ, 2018BBA3044), the “Chunhui Plan” 5
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