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Mn-doped CdS quantum dots sensitized hierarchical TiO2 flower-rod for solar cell application
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Mn-doped CdS quantum dots sensitized hierarchical TiO2 flower-rod for solar cell application Libo Yu, Zhen Li, Yingbo Liu, Fa Cheng, Shuqing Sun ∗ Department of Chemistry, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 3 December 2013 Received in revised form 13 March 2014 Accepted 14 March 2014 Available online xxx Keywords: TiO2 flower-rod Hydrothermal method Mn-doped CdS quantum dots Solar cells
a b s t r a c t A double-layered TiO2 film which three dimensional (3D) flowers grown on highly ordered self-assembled one dimensional (1D) TiO2 nanorods was synthesized directly on transparent fluorine-doped tin oxide (FTO) conducting glass substrate by a facile hydrothermal method and was applied as photoanode in Mn-doped CdS quantum dots sensitized solar cells (QDSSCs). The 3D TiO2 flowers with the increased surface areas can adsorb more QDs, which increased the absorption of light; meanwhile 1D TiO2 nanorods beneath the flowers offered a direct electrical pathway for photogenerated electrons, accelerating the electron transfer rate. A typical type II band alignment which can effectively separate photogenerated excitons and reduce recombination of electrons and holes was constructed by Mn-doped CdS QDs and TiO2 flower-rod. The incident photon-to-current conversion efficiency (IPCE) of the Mn-doped CdS/TiO2 flower-rod solar cell reached to 40% with the polysulfide electrolyte filled in the solar cell. The power conversion efficiency (PCE) of 1.09% was obtained with the Mn-doped CdS/TiO2 flower-rod solar cell under one sun illumination (AM 1.5G, 100 mW/cm2 ), which is 105.7% higher than that of the CdS/TiO2 nanorod solar cell (0.53%). © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, with the ever increasing demand for energy and serious environmental pollution caused by extensive fossil fuels consumption, solar energy is becoming a promising energy resource to take the place of the conventional ones. The photovoltaic cells are the most effective approach to realize the utilization of solar energy [1,2]. Among various solar cells, quantum dot sensitized solar cells (QDSSCs) have attracted much attention because of their low manufacturing costs and ease of fabrication [3,4]. Narrowband gap semiconductor quantum dots (QDs) are usually used as sensitizers in QDSSCs on account of their extraordinary optical and electrical properties, such as the tunable band gap of QDs, higher extinction coefficients, larger intrinsic dipole moments, multiple exciton generation (MEG) with a single photon by impact ionization, and hot electron injection [5–10]. Among the semiconductor QDs, CdS is a desirable candidate in photovoltaic application due to its narrow band gap of 2.25 eV and high absorption coefficient in the visible light region. Moreover, doping Mn2+ can alter the charge separation and recombination dynamics in QDSSCs by creating electronic states in the midgap region of the ODs according
∗ Corresponding author. Tel.: +86 13920690912. E-mail address: [email protected] (S. Sun).
to previous literature [11], which is in favor of the improvement of power conversion efficiency. In this work, we have succeeded in doping CdS QDs with Mn2+ as sensitizer for designing high efficiency QDSSCs. TiO2 is one of the most commonly used wide gap semiconductor substrates in QDSSCs [12–15] because of its stabilities of chemical and physical. In view of efficient charge transport property, the one-dimensional (1D) TiO2 nanorod [16–19] has been a focus of investigation in preparing photoanode substrate for QDSSCs. This can be attributed to the electrons diffusion coefficient of 1D TiO2 nanorod is larger than randomly oriented titania nanoparticles, leading to fast transport of excited electrons [20]. In addition, 1D TiO2 nanorods have less grain boundaries that can scatter or trap the electrons, resulting in the reduction of electrons loss during diffusion process [5]. However, despite the 1D TiO2 nanorods possess such advantages that are mentioned above, the power conversion efficiency of QDSSC with vertically aligned 1D TiO2 nanorod arrays as photoanode is relatively lower. The main reason was that 1D TiO2 nanorods have low internal surface area, which result in insufficient QDs loading and light harvesting [17,21]. In order to compensate for the disadvantages of TiO2 nanorods, we designed a hierarchical double-layered TiO2 film which 3D TiO2 flowers are grown on highly ordered 1D TiO2 nanorods. The TiO2 flowers enlarged the surface areas, which can improve the QDs loading and light harvesting; meanwhile the nanorods beneath TiO2 flowers remain its
http://dx.doi.org/10.1016/j.apsusc.2014.03.090 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: L. Yu, et al., Mn-doped CdS quantum dots sensitized hierarchical TiO2 flower-rod for solar cell application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.090
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superiorities for charges separation. To our best knowledge, the application of this kind of hierarchical TiO2 flower-rod film directly grown on the FTO glass in QDSSCs has been reported rarely. In this work, a novel hierarchical TiO2 flower-rod film on FTO glass was successfully synthesized by a facile hydrothermal method and was used as photoanode in QDSSCs. Mn-doped CdS QDs were sensitized onto the TiO2 flower-rod film by in situ successive ionic layer adsorption and reaction (SILAR). Our Mn-doped CdS/TiO2 flower-rod solar cell has shown better photoelectrochemical performance than the CdS/TiO2 nanorod cell, which verified the potential value of TiO2 flower-rod film in fabricating high efficiency QDSSCs. 2. Experimental 2.1. Materials Titanium butoxide (Ti(OC4 H9 )4 ), cadmium nitrate (Cd(NO3 )2 ·4H2 O) with analytical grade were purchased from Sigma–Aldrich China. Concentrated hydrochloric acid (HCl, 36.5–38 wt%), sodium sulfide (Na2 S·9H2 O), sodium chloride (NaCl), sodium hydroxide (NaOH), sulfur powder (S), manganese acetate (Mn(CH3 COO)2 ·4H2 O), copper sulfate (CuSO4 ·5H2 O) and thiourea (H2 NCSNH2 ) with analytical grade were purchased from Tianjin Chemical Reagents Co. Ltd. All chemicals were used directly in experiments without further purification. Deionized water (DI water, resistivity of 18.2 M cm) was obtained from MilliQ ultra-pure water system (Millipore, USA). 2.2. Preparation of hierarchical TiO2 flower-rod electrodes Hierarchical TiO2 flower-rod film was prepared by a facile hydrothermal method and the details of the synthetic procedure were similar to that described by Liu and Aydil [22]. In brief, Fluorine-doped tin dioxide (FTO) (F: SnO2 , 14 /square, Nippon Sheet Glass Group, Japan) conducting glasses were thoroughly cleaned by sonication in a mixed solution of DI water, acetone, and 2-propanol (volume ratios of 1:1:1) for 30 min, and finally dried in air. Then the FTO conducting glass was transferred to the Teflonlined stainless steel autoclave at an angle against the wall of the Teflon-liner with the conductive side facing up. Subsequently, a transparent mixed solution consisted of 25 ml of DI water, 30 ml of concentrated hydrochloric acid, 5 ml of saturated NaCl aqueous solution and 1 ml of titanium butoxide was added into the Teflon-lined stainless steel autoclave, filling the 80% volume of the autoclave. Then the hydrothermal synthesis reaction was conducted at the temperature of 150 ◦ C for 12 h in an electric oven. Afterwards, the autoclave was cooled to room temperature under flowing water and the product was taken out, rinsed thoroughly with DI water and ethanol, respectively. Finally, the product was dried in ambient air. We also prepared TiO2 nanorods film on FTO glass by this hydrothermal method with the conductive surface of FTO glass facing down to the autoclave under the identical reaction condition for contrastive consideration. 2.3. Sensitization of Mn-doped CdS QDs on TiO2 flower-rod electrodes by SILAR In situ growth of Mn-doped CdS QDs on TiO2 flower-rod electrode was carried out by the successive ionic layer adsorption and reaction (SILAR) method [23]. Typically, TiO2 flower-rod electrode was dipped alternatively into each of cation and anion precursors solutions for 5 min, the cation solution consisted of 0.075 M Mn(CH3 COO)2 and 0.1 M Cd(NO3 )2 dissolved in mixed solution of ethanol and DI water with volume ratio of 1:1, and the anion
solution consisted of 0.1 M Na2 S with the same mixed solvent. Following each immersion, the electrode was rinsed with ethanol for 2–3 min to remove excess precursors and dried at 150 ◦ C for 10 min before the next dipping. The entire procedure was termed as one SILAR cycle; several times of the SILAR cycle were repeated to investigate the optimal cycles of Mn-doped CdS QDs for the performance of QDSSCs. As a control experiment, we also prepared CdS/TiO2 flower-rod and CdS/TiO2 nanorod photoanodes without doping of Mn2+ . 2.4. Solar cell fabrication Quantum dot-sensitized solar cells were assembled into a sandwich-type fashion. The Surlyn film with 60 m thickness was used as a spacer between photoanodes and counter electrodes in order to avoid evaporation of electrolyte. The Cu1.8 S/CuS-coated FTO (the synthesis details can be found in the literature [24]) counter electrode was predrilled with two holes for the injection of electrolyte. 0.1 M S, 1 M Na2 S, and 0.1 M NaOH in the mixed solution of water and methanol with volume ratio of 3:7 was used as polysulfide electrolyte [25], which was injected into the solar cell through one hole on the counter electrode. The illuminated active surface area of the cell was 0.16 cm2 . 2.5. Characterization The crystal structure of the samples was identified by X-ray diffraction (XRD) analysis on a Bruker D8 Advance X-ray diffrac˚ from 10◦ to 90◦ at tometer using Cu K␣ radiation ( = 1.5416 A) a scan rate of 2.4◦ min−1 . The surface morphology of the samples was characterized by field emission scanning electron microscopy (FE-SEM) operating at a voltage of 5.0 kV. Transmission electron microscopy (TEM) was carried out by a JEOL JEM-2100F microscope. The Mn-doped CdS/TiO2 flower-rod sample was detached from the FTO substrate, then dispersed in ethanol, and dropped onto a carbon film supported on a copper grid. Energy-dispersive spectrometer (EDS) equipped on TEM was used to analyze the elemental composition of the samples. Diffuse reflectance absorption spectra of bare TiO2 flower-rod, CdS/TiO2 nanorod, CdS/TiO2 flower-rod and Mndoped CdS/TiO2 flower-rod electrodes were recorded in the range from 250 to 800 nm by a Hitachi U-3010 spectroscope. 2.6. Photoelectrochemical measurements The photocurrent density–voltage (J–V) curves were measured by Oriel I–V test station under simulated AM 1.5G illumination with a solar simulator calibrated by standard silicon solar cell. The active illuminated area of the QDSSC was fixed to 0.16 cm2 . The incident photo to current conversion efficiency (IPCE) measurements were performed with a monochromator to select the illumination wavelength, a 500 W xenon arc lamp (Oriel) served as a light source. 3. Results and discussion 3.1. Morphology and structure characterization of the as-prepared TiO2 flower-rod and TiO2 nanorod structures A double-layered TiO2 flower-rod and TiO2 nanorod arrays on FTO substrates were synthesized using a simple hydrothermal method with conducting surface facing up and down separately. Morphologies of TiO2 flower-rod and TiO2 nanorod electrodes were recorded by FE-SEM. Fig. 1a and b are top view of the TiO2 flowerrod electrode. Two parts, 3D TiO2 flowers on the top and 1D TiO2 nanorod arrays at the bottom constitute the hierarchical TiO2 nanostructure. As shown in Fig. 1a, TiO2 flowers are homogeneously
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Fig. 1. FE-SEM images of TiO2 flower-rod and TiO2 nanorod electrodes: typical top view of TiO2 flower-rod at (a) low and (b) high magnifications; (c) cross-sectional view of TiO2 flower-rod and (d) magnified view of red outline in c; (e) top view of TiO2 nanorod arrays, the inset represents a higher magnification of such arrays; (f) cross-sectional view of the well-aligned TiO2 nanorod arrays. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
distributed on the nanorod arrays and nearly cover the entire surface of TiO2 film. In the magnified image of the TiO2 flower in Fig. 1b, it is observed that the flower is composed of nanorods. The flowers exhibit open structure with numerous nanorods extended outside, and become gradually compact inside. Obviously, this flower-rod structure enlarges the surface area of the TiO2 film, implying its potential application value in QDSSCs. The cross-sectional image of the TiO2 flower-rod is shown in Fig. 1c, it can be clearly seen that the TiO2 flowers were grown on top of TiO2 nanorods, which provide the evidence for the formation of the double-layered TiO2 nanostructure. Fig. 1d is the magnified view of red outline in (c), which shows that the well-aligned nanorods are nearly vertically grown on the FTO substrate. The vertically grown TiO2 nanorods
beneath the flowers provide another evidence for the formation of hierarchical structure of TiO2 flower-rods film on FTO glass. Fig. 1e reveals that only TiO2 nanorods were synthesized on FTO glass during the hydrothermal reaction with the conductive side facing down. The inset of Fig. 1e represents a higher magnification of such arrays, which shows that the nanorods are highly ordered. Fig. 1f is a cross-sectional image of the TiO2 nanorod arrays, showing that the nanorods are nearly perpendicular to the FTO substrate, and the length of nanorod is about 3 m, which is the same as the length of 1D nanorod in TiO2 flower-rod structure. Fig. 2 displays the XRD patterns of the FTO glass, TiO2 nanorod, TiO2 flower-rod and CdS/TiO2 flower-rod photoelectrodes. It is worth noting that both the XRD patterns of TiO2 flower-rod and
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diffraction peaks indexed to cubic CdS (JCPDF card no. 80-0019) can be discerned apart from the diffraction peaks of rutile TiO2 . The reduced intensity of diffraction peaks of TiO2 flower-rod which sensitized by CdS QDs through SILAR also indicates that the successfully deposition of CdS QDs on TiO2 flower-rods. Furthermore, there is no other peaks can be observed in XRD pattern of CdS/TiO2 flower-rod, indicating the high purity of the sample. 3.2. Morphology and elemental composition characterization of Mn-doped CdS/TiO2 flower-rod structure
Fig. 2. XRD patterns of FTO, TiO2 nanorod, TiO2 flower-rod and CdS/TiO2 flower-rod electrodes.
TiO2 nanorod show the same diffraction peaks which can be ascribed to the TiO2 tetragonal rutile phase (JCPDF card no. 211276). The sharp and intense diffraction peaks suggest that the TiO2 flower-rod and TiO2 nanorod are well crystallized. In contrast with XRD patterns of FTO glass, the appearance of the diffraction peaks belonging to tetragonal rutile phase confirms that rutile TiO2 flower-rod can be successfully synthesized by hydrothermal method. In the XRD patterns of CdS/TiO2 flower-rod, additional
The variation of the Mn-doped CdS/TiO2 flower-rod structure was identified by FE-SEM. As shown in Fig. 3a, the surface of the Mn-doped CdS/TiO2 flower-rod becomes rougher than that of bare TiO2 flower-rod which is shown in Fig. 1b. It is obvious that the Mn-doped CdS QDs can be uniformly adsorbed on TiO2 flowerrod through SILAR. The inset is the higher magnification image of the flower; it clearly displays that Mn-doped CdS QDs were not only homogeneously deposited on the top of TiO2 flower but also deposited on the surface of each nanorod that constituted the TiO2 flower. Fig. 3b is the typical TEM image of the sensitized flowerrod unit which constituted the Mn-doped CdS/TiO2 flower-rod, showing that there are small nanoparticles distributed on its surface. Chemical elemental compositions of the Mn-doped CdS/TiO2 flower-rod unit were analyzed by energy-dispersive spectroscopy (EDS) equipped on TEM, which is shown in Fig. 3c. The C, O, Cu, S, Cd, Ti and Mn peaks can be observed in the spectrum, of which, C and Cu came from carbon film supported on the copper grid, Ti and O peaks result from the TiO2 , and the atomic ratio of Mn plus Cd versus S is close to 1:1, which confirms the stoichiometric
Fig. 3. (a) Morphology of Mn-doped CdS/TiO2 flower-rod, the inset represents a higher magnification of such structure; (b) TEM image of Mn-doped CdS sensitized TiO2 flower-rod unit; and (c) EDS spectra of Mn-doped CdS/TiO2 flower-rod.
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in bulk (2.25 eV), indicating that the size of CdS particle assembled on the TiO2 nanorod electrode was within the scale of QD. According to the empirical equations proposed by Yu et al. [27], the average diameter of CdS particle was calculated to be about 8.796 nm. Compared with the absorption spectrum of CdS/TiO2 nanorod electrode, the CdS/TiO2 flower-rod electrode (spectrum c, Fig. 4) shows a red shift in the absorption with onset around 530 nm and a significant increased absorbance. This increased absorbance can be ascribed to that the TiO2 flower-rod provided the enlarged surface areas and could adsorb more QDs than TiO2 nanorod, which resulted in increasing light absorption. Further red shift of absorption edge and increased absorbance are observed with Mn-doped CdS/TiO2 flower-rod electrode (spectrum d, Fig. 4), and the onset of absorption occurred at around 550 nm indicates that doping of Mn in CdS QDs can further extend the absorption range and absorbance. 3.4. Photoelectrochemical characterization Fig. 4. Diffuse reflectance absorption spectra of bare TiO2 flower-rod, CdS/TiO2 nanorod, CdS/TiO2 flower-rod and Mn-doped CdS/TiO2 flower-rod electrodes.
formation of Mn-doped CdS. There was no other element can be distinguished in EDS, suggesting that the obtained product was in high purity. 3.3. Optical property of Mn-doped CdS/TiO2 flower-rod photoanode The UV–vis diffuse reflectance absorption spectra were used to record the difference light absorption properties of the bare TiO2 flower-rod, CdS/TiO2 nanorod, CdS/TiO2 flower-rod and Mn-doped CdS/TiO2 flower-rod electrodes. Each of the sensitized electrodes was obtained with 10 SILAR cycles. As shown in Fig. 4, the onset optical absorption of the bare TiO2 flower-rod electrode (spectrum a, Fig. 4) occurs at around 410 nm and the main light absorption centered on ultraviolet light region. This result was consistent with the band gap of 3.0 eV for rutile TiO2 [26]. The absorption spectrum of CdS deposited TiO2 nanorod electrode (spectrum b, Fig. 4) shows absorption onset around 520 nm, and the corresponding band gap was calculated to be 2.387 eV, which is higher than the value of CdS
The J–V characteristics of the assembled QDSSCs were measured by Oriel I–V test station under 1 sun (=100 mW/cm2 AM 1.5G solar illumination) with the active area of 0.16 cm2 . Fig. 5a displays the performance variation of TiO2 flower-rod solar cells sensitized by different SILAR cycles of Mn-doped CdS QDs. It is found that the short circuit current density (Jsc ) increased gradually with extending deposition cycles at initial stage (from 5 cycles to 10 cycles) and the open circuit voltage (Voc ) reduced first (from 5 cycles to 7 cycles) and then increased (from 7 cycles to 10 cycles). However, both the Jsc and Voc declined as the further increase of the SILAR cycles from 10 cycles to 15 cycles. Because the value of Jsc changed significantly and the value of Voc varied little, the variation trend of power conversion efficiency () is consistent with Jsc , as is shown in Fig. 5b and c. This variation trend can be interpreted as follows: first, with extending SILAR cycles of Mn-doped CdS QDs from five to ten, more QDs were deposited on TiO2 flower-rod, which can enhance the light harvesting, leading to the increase of Jsc . In addition, according to Ref. [28], the Voc can be determined by the following equation: Voc =
EFn − Eredox kB T = ln(n/n0 ) e e
Fig. 5. (a) J–V characteristics of different SILAR cycles of Mn-doped CdS QDs sensitized TiO2 flower-rod solar cells. The variation of photocurrent density; (b) and power conversion efficiency; and (c) with different SILAR cycles of Mn-doped CdS QDs sensitized TiO2 flower-rod solar cells.
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Table 1 Parameters obtained from the J–V curves of the QDSSCs using different photoanodes. Samples
Jsc (mA/cm2 )
Voc (V)
FF
(%)
CdS/TiO2 nanorod CdS/TiO2 flower-rod Mn-doped CdS/TiO2 flower-rod
4.18 6.04 8.02
0.37 0.40 0.42
0.34 0.35 0.32
0.53 0.84 1.09
where EFn is the quasi-Fermi level of the electrons in semiconductor photoanode under illumination; Eredox is the potential of redox electrolyte; e is the positive elementary charge; kB T is the thermal energy; n is the electron concentration in conduction band of the semiconductor photoanode under illumination; n0 is the electron concentration in the dark condition. With increasing amount of Mn-doped CdS QDs, more electrons were injected into the TiO2 conduction band under illumination and the value of n can be enhanced, which lead to more negative shift of the EFn , while the Eredox remains unchangeable, causing the improvement of Voc (from 7 cycles to 10 cycles). However, the excessive SILAR cycles can lead to the aggregation of Mn-doped CdS QDs, which increase the possibilities of the recombination between the electrons and holes when the photogenerated electrons diffuse across the QD layers to the TiO2 film. Furthermore, the excessive SILAR cycles can also limit the efficiency of charge separation and charge extraction as a result of hindering the diffusion of the polysulfide electrolyte [29], which will deteriorate the entire performance of the solar devices. According to the variation of the J–V curves to SILAR cycles in Fig. 5a, the best solar cell performance with the Jsc of 8.02 mA/cm2 and Voc of 0.42 V was obtained by 10 SILAR cycles of Mn-doped CdS QDs. It has already been obtained that the Mn-doped CdS/TiO2 flower-rod solar cell with 10 SILAR cycles has the optimal Jsc and Voc . In order to assess whether Mn-doped CdS QDs sensitizer and TiO2 flower-rod substrate play a significant role in improving performance of QDSSCs, we prepared the other two types of semiconductor photoanodes as a control experiment: 10 SILAR cycles of CdS/TiO2 flower-rod and 10 SILAR cycles of CdS/TiO2 nanorod solar cells without doping of Mn2+ . Fig. 6 depicts the J–V curves of CdS/TiO2 nanorod, CdS/TiO2 flower-rod and Mn-doped CdS/TiO2 flower-rod solar cells, respectively. The photovoltaic parameters of these solar cells are summarized in Table 1. In contrast with CdS/TiO2 nanorod cell, the value of Voc and FF of CdS/TiO2 flower-rod cell increased modestly, but the value of Jsc increased remarkably (from 4.18 mA/cm2 to 6.04 mA/cm2 ), resulting in the enhancement of power conversion efficiency (from 0.53% to 0.84%). The augment of all parameters in CdS/TiO2 flower-rod solar cell shows that TiO2 flower-rod structure is beneficial to the improvement of the solar cell performance compared with TiO2 nanorod structure. The hierarchical TiO2 flower-rod has influence on solar cell in two aspects, for one thing, 3D TiO2 flowers provide an increased surface area to adsorb more ODs, which is
Fig. 6. J–V curves of the QDSSCs using different photoanodes measured under AM 1.5G condition.
in favor of enhancing the light-harvesting efficiency; for another, 1D TiO2 nanorods beneath the TiO2 flowers offer the direct transport pathways for photogenerated electrons to enhance the charges separation efficiency. After Mn-doping, the Mn-doped CdS/TiO2 flower-rod cell exhibited the best performance and the power conversion efficiency can reach to 1.09%, which is 29.8% higher than CdS/TiO2 flower-rod solar cell without doping of Mn2+ . This can be attributed to the midgap states created by Mn2+ , causing the electrons to get trapped and screen them from charge recombination with holes [11]. This result suggested that doping Mn2+ in CdS QDs is helpful for a further enhancement of the solar cell performance. Fig. 7 shows the photoelectrical conversion structure of our Mn-doped CdS/TiO2 flower-rod solar cell, which consists of Mndoped CdS QDs sensitized TiO2 flower-rod photoanode, polysulfide electrolyte, and Cu1.8 S/CuS/FTO counter electrode. Under illumination, photons are captured by the Mn-doped CdS QDs, yielding electron–hole pairs (excitons). The electrons are injected into the oxide film and transported to the transparent conductive FTO, while holes are transferred via the electrolyte to the counter electrode, where the oxidized counterpart of the redox couple is reduced [30]. From the working mechanism of Mn-doped CdS QDs sensitized TiO2 flower-rod solar cell, we can see that the amount of adsorbed QDs and electron transport are the key factors affecting the performance of QDSSC. The hierarchical TiO2 flower-rod structure is an ideal material for QDSSCs, on account of the 3D TiO2 flowers can enlarge the surface area for QDs loading, and 1D TiO2 nanorods can accelerate the movement of electrons and reduce the recombination of electrons and holes.
Fig. 7. Schematic diagram of the photoelectrical conversion structure of our Mn-doped CdS/TiO2 flower-rod solar cell.
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and collection efficiency (cc ). Therefore, the IPCE of our Mn-doped CdS/TiO2 flower-rod solar cell can reach to such a high level. 4. Conclusion In summary, the hierarchical TiO2 flower-rod film was successfully prepared on FTO glass by a facile hydrothermal method. The TiO2 flower-rod film sensitized with Mn-doped CdS QDs by SILAR was used as photoanode in solar cell and the efficiency has reached to 1.09% under one sun illumination (AM 1.5G, 100 mW/cm2 ), which was 29.8% higher than CdS/TiO2 flower-rod solar cell, and was 105.7% higher than CdS/TiO2 nanorod solar cell. The enhanced power conversion efficiency can mainly be ascribed to the hierarchical TiO2 flower-rod substrate and Mn-doped CdS QDs sensitizer. Our Mn-doped CdS QDs sensitized TiO2 flower-rod solar cell with the acceptable efficiency of 1.09% reported here has shown a great potential value to design high efficiency QDSSCs. Fig. 8. IPCE spectrum of QDSSC fabricated from Mn-doped CdS/TiO2 flower-rod.
Acknowledgment This work was supported by Key Project of Tianjin Sci-Tech Support Program (No. 08ZCKFSH01400). References
Fig. 9. Diagrammatic drawing of the electron transfers from Mn-doped CdS QD into TiO2 .
The incident photon to current conversion efficiency (IPCE) of Mn-doped CdS/TiO2 flower-rod solar cell is shown in Fig. 8. It can be seen that the profile of IPCE plot corresponds well with the UV–vis absorption spectrum of the Mn-doped CdS/TiO2 flower-rod in Fig. 4. As we can see, the maximum IPCE of our device can reach to 40% using a polysulfide electrolyte. Three factors determine the IPCE: the light-harvesting efficiency (LHE), the charge injection efficiency (inj ), and the charge collection efficiency (cc ) [31]. In our solar cell, the hierarchical TiO2 flower-rod structure provides a large surface area for adsorption of sensitizers, leading to the enhanced light harvesting efficiency (LHE). A typical type II heterojunction was constructed by Mn-doped CdS QDs and TiO2 flower-rod due to the matching of the conduction band energies. A shown in Fig. 9, following absorption of photons by Mn-doped CdS QDs, electron–hole pairs (excitons) are generated, and then the electrons are emitted from valance band (VB) to conduction band (CB) of CdS QDs. Because of the heterojunction at the Mn-doped CdS QD/TiO2 interface, electrons in the CB of CdS QDs were injected into the CB of wide band gap semiconductor TiO2 , this is a extremely fast process that typically occurs on the picosecond time scale [32], and 1D nanorods beneath TiO2 flowers can improve electrons transfer rate as a result of offering a direct pathway for excited electrons, leading to the improvement of the charge injection efficiency (inj )
[1] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385–2393. [2] A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 130 (2008) 4007–4015. [3] G.S. Paul, J.H. Kim, M.-S. Kim, K. Do, J. Ko, J.-S. Yu, ACS Appl. Mater. Interfaces 4 (2012) 375–381. [4] Y.-L. Lee, Y.-S. Lo, Adv. Funct. Mater. 19 (2009) 604–609. [5] M. Li, Y. Liu, H. Wang, H. Shen, W. Zhao, H. Huang, C. Liang, J. Appl. Phys. 108 (2010) 094304. [6] K. Prabakar, H. Seo, M. Son, H. Kim, Mater. Chem. Phys. 117 (2009) 26–28. [7] H. Hu, H. Shen, C. Cui, D. Liang, P. Li, S. Xu, W. Tang, J. Alloys Compd. 560 (2013) 1–5. [8] J. Tian, R. Gao, Q. Zhang, S. Zhang, Y. Li, J. Lan, X. Qu, G. Cao, J. Phys. Chem. C 116 (2012) 18655–18662. [9] M.C. Beard, J. Phys. Chem. Lett. 2 (2011) 1282–1288. [10] W.A. Tisdale, K.J. Williams, B.A. Timp, D.J. Norris, E.S. Aydil, X.Y. Zhu, Science 328 (2010) 1543–1547. [11] P.K. Santra, P.V. Kamat, J. Am. Chem. Soc. 134 (2012) 2508–2511. [12] H. Wang, Y. Bai, H. Zhang, Z. Zhang, J. Li, L. Guo, J. Phys. Chem. C 114 (2010) 16451–16455. [13] F. Shao, J. Sun, L. Gao, S. Yang, J. Luo, ACS Appl. Mater. Interfaces 3 (2011) 2148–2153. [14] A. Ramadoss, S.J. Kim, J. Alloys Compd. 561 (2013) 262–267. [15] S. Wang, G. Xia, H. He, K. Yi, J. Shao, Z. Fan, J. Alloys Compd. 431 (2007) 287–291. [16] F. Xu, Y. Wu, X. Zhang, Z. Gao, K. Jiang, Micro Nano Lett. 7 (2012) 826–830. [17] Z.-J. Zhou, J.-Q. Fan, X. Wang, W.-H. Zhou, Z.-L. Du, S.-X. Wu, ACS Appl. Mater. Interfaces 3 (2011) 4349–4353. [18] A. Kumar, A.R. Madaria, C. Zhou, J. Phys. Chem. C 114 (2010) 7787–7792. [19] A.M. More, T.P. Gujar, J.L. Gunjakar, C.D. Lokhande, O.-S. Joo, Appl. Surf. Sci. 255 (2008) 2682–2687. [20] D.R. Baker, P.V. Kamat, Adv. Funct. Mater. 19 (2009) 805–811. [21] H. Yu, J. Pan, Y. Bai, X. Zong, X. Li, L. Wang, Chemistry 19 (2013) 13569–13574. [22] B. Liu, E.S. Aydil, J. Am. Chem. Soc. 131 (2009) 3985–3990. [23] H. Lee, M. Wang, P. Chen, D.R. Gamelin, S.M. Zakeeruddin, M. Grätzel, M.K. Nazeeruddin, Nano Lett. 9 (2009) 4221–4227. [24] S.S. Kalanur, S.Y. Chae, O.S. Joo, Electrochim. Acta 103 (2013) 91–95. [25] M. Shalom, Z. Tachan, Y. Bouhadana, H.-N. Barad, A. Zaban, J. Phys. Chem. Lett. 2 (2011) 1998–2003. [26] M. Xu, P. Da, H. Wu, D. Zhao, G. Zheng, Nano Lett. 12 (2012) 1503–1508. [27] W.W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 15 (2003) 2854–2860. [28] T. Zeng, H. Tao, X. Sui, X. Zhou, X. Zhao, Chem. Phys. Lett. 508 (2011) 130–133. [29] P. Sudhagar, J.H. Jung, S. Park, Y.-G. Lee, R. Sathyamoorthy, Y.S. Kang, H. Ahn, Electrochem. Commun. 11 (2009) 2220–2224. [30] T. Toyoda, Q. Shen, J. Phys. Chem. Lett. 3 (2012) 1885–1893. [31] S.W. Jung, J.-H. Kim, H. Kim, C.-J. Choi, K.-S. Ahn, Curr. Appl. Phys. 12 (2012) 1459–1464. [32] A. Kumar, K.-T. Li, A.R. Madaria, C. Zhou, Nano Res. 4 (2011) 1181–1190.
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Mn-doped CdS quantum dots sensitized hierarchical TiO2 flower-rod for solar cell application Libo Yu, Zhen Li, Yingbo Liu, Fa Cheng, Shuqing Sun ∗ Department of Chemistry, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 3 December 2013 Received in revised form 13 March 2014 Accepted 14 March 2014 Available online xxx Keywords: TiO2 flower-rod Hydrothermal method Mn-doped CdS quantum dots Solar cells
a b s t r a c t A double-layered TiO2 film which three dimensional (3D) flowers grown on highly ordered self-assembled one dimensional (1D) TiO2 nanorods was synthesized directly on transparent fluorine-doped tin oxide (FTO) conducting glass substrate by a facile hydrothermal method and was applied as photoanode in Mn-doped CdS quantum dots sensitized solar cells (QDSSCs). The 3D TiO2 flowers with the increased surface areas can adsorb more QDs, which increased the absorption of light; meanwhile 1D TiO2 nanorods beneath the flowers offered a direct electrical pathway for photogenerated electrons, accelerating the electron transfer rate. A typical type II band alignment which can effectively separate photogenerated excitons and reduce recombination of electrons and holes was constructed by Mn-doped CdS QDs and TiO2 flower-rod. The incident photon-to-current conversion efficiency (IPCE) of the Mn-doped CdS/TiO2 flower-rod solar cell reached to 40% with the polysulfide electrolyte filled in the solar cell. The power conversion efficiency (PCE) of 1.09% was obtained with the Mn-doped CdS/TiO2 flower-rod solar cell under one sun illumination (AM 1.5G, 100 mW/cm2 ), which is 105.7% higher than that of the CdS/TiO2 nanorod solar cell (0.53%). © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, with the ever increasing demand for energy and serious environmental pollution caused by extensive fossil fuels consumption, solar energy is becoming a promising energy resource to take the place of the conventional ones. The photovoltaic cells are the most effective approach to realize the utilization of solar energy [1,2]. Among various solar cells, quantum dot sensitized solar cells (QDSSCs) have attracted much attention because of their low manufacturing costs and ease of fabrication [3,4]. Narrowband gap semiconductor quantum dots (QDs) are usually used as sensitizers in QDSSCs on account of their extraordinary optical and electrical properties, such as the tunable band gap of QDs, higher extinction coefficients, larger intrinsic dipole moments, multiple exciton generation (MEG) with a single photon by impact ionization, and hot electron injection [5–10]. Among the semiconductor QDs, CdS is a desirable candidate in photovoltaic application due to its narrow band gap of 2.25 eV and high absorption coefficient in the visible light region. Moreover, doping Mn2+ can alter the charge separation and recombination dynamics in QDSSCs by creating electronic states in the midgap region of the ODs according
∗ Corresponding author. Tel.: +86 13920690912. E-mail address: [email protected] (S. Sun).
to previous literature [11], which is in favor of the improvement of power conversion efficiency. In this work, we have succeeded in doping CdS QDs with Mn2+ as sensitizer for designing high efficiency QDSSCs. TiO2 is one of the most commonly used wide gap semiconductor substrates in QDSSCs [12–15] because of its stabilities of chemical and physical. In view of efficient charge transport property, the one-dimensional (1D) TiO2 nanorod [16–19] has been a focus of investigation in preparing photoanode substrate for QDSSCs. This can be attributed to the electrons diffusion coefficient of 1D TiO2 nanorod is larger than randomly oriented titania nanoparticles, leading to fast transport of excited electrons [20]. In addition, 1D TiO2 nanorods have less grain boundaries that can scatter or trap the electrons, resulting in the reduction of electrons loss during diffusion process [5]. However, despite the 1D TiO2 nanorods possess such advantages that are mentioned above, the power conversion efficiency of QDSSC with vertically aligned 1D TiO2 nanorod arrays as photoanode is relatively lower. The main reason was that 1D TiO2 nanorods have low internal surface area, which result in insufficient QDs loading and light harvesting [17,21]. In order to compensate for the disadvantages of TiO2 nanorods, we designed a hierarchical double-layered TiO2 film which 3D TiO2 flowers are grown on highly ordered 1D TiO2 nanorods. The TiO2 flowers enlarged the surface areas, which can improve the QDs loading and light harvesting; meanwhile the nanorods beneath TiO2 flowers remain its
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superiorities for charges separation. To our best knowledge, the application of this kind of hierarchical TiO2 flower-rod film directly grown on the FTO glass in QDSSCs has been reported rarely. In this work, a novel hierarchical TiO2 flower-rod film on FTO glass was successfully synthesized by a facile hydrothermal method and was used as photoanode in QDSSCs. Mn-doped CdS QDs were sensitized onto the TiO2 flower-rod film by in situ successive ionic layer adsorption and reaction (SILAR). Our Mn-doped CdS/TiO2 flower-rod solar cell has shown better photoelectrochemical performance than the CdS/TiO2 nanorod cell, which verified the potential value of TiO2 flower-rod film in fabricating high efficiency QDSSCs. 2. Experimental 2.1. Materials Titanium butoxide (Ti(OC4 H9 )4 ), cadmium nitrate (Cd(NO3 )2 ·4H2 O) with analytical grade were purchased from Sigma–Aldrich China. Concentrated hydrochloric acid (HCl, 36.5–38 wt%), sodium sulfide (Na2 S·9H2 O), sodium chloride (NaCl), sodium hydroxide (NaOH), sulfur powder (S), manganese acetate (Mn(CH3 COO)2 ·4H2 O), copper sulfate (CuSO4 ·5H2 O) and thiourea (H2 NCSNH2 ) with analytical grade were purchased from Tianjin Chemical Reagents Co. Ltd. All chemicals were used directly in experiments without further purification. Deionized water (DI water, resistivity of 18.2 M cm) was obtained from MilliQ ultra-pure water system (Millipore, USA). 2.2. Preparation of hierarchical TiO2 flower-rod electrodes Hierarchical TiO2 flower-rod film was prepared by a facile hydrothermal method and the details of the synthetic procedure were similar to that described by Liu and Aydil [22]. In brief, Fluorine-doped tin dioxide (FTO) (F: SnO2 , 14 /square, Nippon Sheet Glass Group, Japan) conducting glasses were thoroughly cleaned by sonication in a mixed solution of DI water, acetone, and 2-propanol (volume ratios of 1:1:1) for 30 min, and finally dried in air. Then the FTO conducting glass was transferred to the Teflonlined stainless steel autoclave at an angle against the wall of the Teflon-liner with the conductive side facing up. Subsequently, a transparent mixed solution consisted of 25 ml of DI water, 30 ml of concentrated hydrochloric acid, 5 ml of saturated NaCl aqueous solution and 1 ml of titanium butoxide was added into the Teflon-lined stainless steel autoclave, filling the 80% volume of the autoclave. Then the hydrothermal synthesis reaction was conducted at the temperature of 150 ◦ C for 12 h in an electric oven. Afterwards, the autoclave was cooled to room temperature under flowing water and the product was taken out, rinsed thoroughly with DI water and ethanol, respectively. Finally, the product was dried in ambient air. We also prepared TiO2 nanorods film on FTO glass by this hydrothermal method with the conductive surface of FTO glass facing down to the autoclave under the identical reaction condition for contrastive consideration. 2.3. Sensitization of Mn-doped CdS QDs on TiO2 flower-rod electrodes by SILAR In situ growth of Mn-doped CdS QDs on TiO2 flower-rod electrode was carried out by the successive ionic layer adsorption and reaction (SILAR) method [23]. Typically, TiO2 flower-rod electrode was dipped alternatively into each of cation and anion precursors solutions for 5 min, the cation solution consisted of 0.075 M Mn(CH3 COO)2 and 0.1 M Cd(NO3 )2 dissolved in mixed solution of ethanol and DI water with volume ratio of 1:1, and the anion
solution consisted of 0.1 M Na2 S with the same mixed solvent. Following each immersion, the electrode was rinsed with ethanol for 2–3 min to remove excess precursors and dried at 150 ◦ C for 10 min before the next dipping. The entire procedure was termed as one SILAR cycle; several times of the SILAR cycle were repeated to investigate the optimal cycles of Mn-doped CdS QDs for the performance of QDSSCs. As a control experiment, we also prepared CdS/TiO2 flower-rod and CdS/TiO2 nanorod photoanodes without doping of Mn2+ . 2.4. Solar cell fabrication Quantum dot-sensitized solar cells were assembled into a sandwich-type fashion. The Surlyn film with 60 m thickness was used as a spacer between photoanodes and counter electrodes in order to avoid evaporation of electrolyte. The Cu1.8 S/CuS-coated FTO (the synthesis details can be found in the literature [24]) counter electrode was predrilled with two holes for the injection of electrolyte. 0.1 M S, 1 M Na2 S, and 0.1 M NaOH in the mixed solution of water and methanol with volume ratio of 3:7 was used as polysulfide electrolyte [25], which was injected into the solar cell through one hole on the counter electrode. The illuminated active surface area of the cell was 0.16 cm2 . 2.5. Characterization The crystal structure of the samples was identified by X-ray diffraction (XRD) analysis on a Bruker D8 Advance X-ray diffrac˚ from 10◦ to 90◦ at tometer using Cu K␣ radiation ( = 1.5416 A) a scan rate of 2.4◦ min−1 . The surface morphology of the samples was characterized by field emission scanning electron microscopy (FE-SEM) operating at a voltage of 5.0 kV. Transmission electron microscopy (TEM) was carried out by a JEOL JEM-2100F microscope. The Mn-doped CdS/TiO2 flower-rod sample was detached from the FTO substrate, then dispersed in ethanol, and dropped onto a carbon film supported on a copper grid. Energy-dispersive spectrometer (EDS) equipped on TEM was used to analyze the elemental composition of the samples. Diffuse reflectance absorption spectra of bare TiO2 flower-rod, CdS/TiO2 nanorod, CdS/TiO2 flower-rod and Mndoped CdS/TiO2 flower-rod electrodes were recorded in the range from 250 to 800 nm by a Hitachi U-3010 spectroscope. 2.6. Photoelectrochemical measurements The photocurrent density–voltage (J–V) curves were measured by Oriel I–V test station under simulated AM 1.5G illumination with a solar simulator calibrated by standard silicon solar cell. The active illuminated area of the QDSSC was fixed to 0.16 cm2 . The incident photo to current conversion efficiency (IPCE) measurements were performed with a monochromator to select the illumination wavelength, a 500 W xenon arc lamp (Oriel) served as a light source. 3. Results and discussion 3.1. Morphology and structure characterization of the as-prepared TiO2 flower-rod and TiO2 nanorod structures A double-layered TiO2 flower-rod and TiO2 nanorod arrays on FTO substrates were synthesized using a simple hydrothermal method with conducting surface facing up and down separately. Morphologies of TiO2 flower-rod and TiO2 nanorod electrodes were recorded by FE-SEM. Fig. 1a and b are top view of the TiO2 flowerrod electrode. Two parts, 3D TiO2 flowers on the top and 1D TiO2 nanorod arrays at the bottom constitute the hierarchical TiO2 nanostructure. As shown in Fig. 1a, TiO2 flowers are homogeneously
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Fig. 1. FE-SEM images of TiO2 flower-rod and TiO2 nanorod electrodes: typical top view of TiO2 flower-rod at (a) low and (b) high magnifications; (c) cross-sectional view of TiO2 flower-rod and (d) magnified view of red outline in c; (e) top view of TiO2 nanorod arrays, the inset represents a higher magnification of such arrays; (f) cross-sectional view of the well-aligned TiO2 nanorod arrays. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
distributed on the nanorod arrays and nearly cover the entire surface of TiO2 film. In the magnified image of the TiO2 flower in Fig. 1b, it is observed that the flower is composed of nanorods. The flowers exhibit open structure with numerous nanorods extended outside, and become gradually compact inside. Obviously, this flower-rod structure enlarges the surface area of the TiO2 film, implying its potential application value in QDSSCs. The cross-sectional image of the TiO2 flower-rod is shown in Fig. 1c, it can be clearly seen that the TiO2 flowers were grown on top of TiO2 nanorods, which provide the evidence for the formation of the double-layered TiO2 nanostructure. Fig. 1d is the magnified view of red outline in (c), which shows that the well-aligned nanorods are nearly vertically grown on the FTO substrate. The vertically grown TiO2 nanorods
beneath the flowers provide another evidence for the formation of hierarchical structure of TiO2 flower-rods film on FTO glass. Fig. 1e reveals that only TiO2 nanorods were synthesized on FTO glass during the hydrothermal reaction with the conductive side facing down. The inset of Fig. 1e represents a higher magnification of such arrays, which shows that the nanorods are highly ordered. Fig. 1f is a cross-sectional image of the TiO2 nanorod arrays, showing that the nanorods are nearly perpendicular to the FTO substrate, and the length of nanorod is about 3 m, which is the same as the length of 1D nanorod in TiO2 flower-rod structure. Fig. 2 displays the XRD patterns of the FTO glass, TiO2 nanorod, TiO2 flower-rod and CdS/TiO2 flower-rod photoelectrodes. It is worth noting that both the XRD patterns of TiO2 flower-rod and
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diffraction peaks indexed to cubic CdS (JCPDF card no. 80-0019) can be discerned apart from the diffraction peaks of rutile TiO2 . The reduced intensity of diffraction peaks of TiO2 flower-rod which sensitized by CdS QDs through SILAR also indicates that the successfully deposition of CdS QDs on TiO2 flower-rods. Furthermore, there is no other peaks can be observed in XRD pattern of CdS/TiO2 flower-rod, indicating the high purity of the sample. 3.2. Morphology and elemental composition characterization of Mn-doped CdS/TiO2 flower-rod structure
Fig. 2. XRD patterns of FTO, TiO2 nanorod, TiO2 flower-rod and CdS/TiO2 flower-rod electrodes.
TiO2 nanorod show the same diffraction peaks which can be ascribed to the TiO2 tetragonal rutile phase (JCPDF card no. 211276). The sharp and intense diffraction peaks suggest that the TiO2 flower-rod and TiO2 nanorod are well crystallized. In contrast with XRD patterns of FTO glass, the appearance of the diffraction peaks belonging to tetragonal rutile phase confirms that rutile TiO2 flower-rod can be successfully synthesized by hydrothermal method. In the XRD patterns of CdS/TiO2 flower-rod, additional
The variation of the Mn-doped CdS/TiO2 flower-rod structure was identified by FE-SEM. As shown in Fig. 3a, the surface of the Mn-doped CdS/TiO2 flower-rod becomes rougher than that of bare TiO2 flower-rod which is shown in Fig. 1b. It is obvious that the Mn-doped CdS QDs can be uniformly adsorbed on TiO2 flowerrod through SILAR. The inset is the higher magnification image of the flower; it clearly displays that Mn-doped CdS QDs were not only homogeneously deposited on the top of TiO2 flower but also deposited on the surface of each nanorod that constituted the TiO2 flower. Fig. 3b is the typical TEM image of the sensitized flowerrod unit which constituted the Mn-doped CdS/TiO2 flower-rod, showing that there are small nanoparticles distributed on its surface. Chemical elemental compositions of the Mn-doped CdS/TiO2 flower-rod unit were analyzed by energy-dispersive spectroscopy (EDS) equipped on TEM, which is shown in Fig. 3c. The C, O, Cu, S, Cd, Ti and Mn peaks can be observed in the spectrum, of which, C and Cu came from carbon film supported on the copper grid, Ti and O peaks result from the TiO2 , and the atomic ratio of Mn plus Cd versus S is close to 1:1, which confirms the stoichiometric
Fig. 3. (a) Morphology of Mn-doped CdS/TiO2 flower-rod, the inset represents a higher magnification of such structure; (b) TEM image of Mn-doped CdS sensitized TiO2 flower-rod unit; and (c) EDS spectra of Mn-doped CdS/TiO2 flower-rod.
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in bulk (2.25 eV), indicating that the size of CdS particle assembled on the TiO2 nanorod electrode was within the scale of QD. According to the empirical equations proposed by Yu et al. [27], the average diameter of CdS particle was calculated to be about 8.796 nm. Compared with the absorption spectrum of CdS/TiO2 nanorod electrode, the CdS/TiO2 flower-rod electrode (spectrum c, Fig. 4) shows a red shift in the absorption with onset around 530 nm and a significant increased absorbance. This increased absorbance can be ascribed to that the TiO2 flower-rod provided the enlarged surface areas and could adsorb more QDs than TiO2 nanorod, which resulted in increasing light absorption. Further red shift of absorption edge and increased absorbance are observed with Mn-doped CdS/TiO2 flower-rod electrode (spectrum d, Fig. 4), and the onset of absorption occurred at around 550 nm indicates that doping of Mn in CdS QDs can further extend the absorption range and absorbance. 3.4. Photoelectrochemical characterization Fig. 4. Diffuse reflectance absorption spectra of bare TiO2 flower-rod, CdS/TiO2 nanorod, CdS/TiO2 flower-rod and Mn-doped CdS/TiO2 flower-rod electrodes.
formation of Mn-doped CdS. There was no other element can be distinguished in EDS, suggesting that the obtained product was in high purity. 3.3. Optical property of Mn-doped CdS/TiO2 flower-rod photoanode The UV–vis diffuse reflectance absorption spectra were used to record the difference light absorption properties of the bare TiO2 flower-rod, CdS/TiO2 nanorod, CdS/TiO2 flower-rod and Mn-doped CdS/TiO2 flower-rod electrodes. Each of the sensitized electrodes was obtained with 10 SILAR cycles. As shown in Fig. 4, the onset optical absorption of the bare TiO2 flower-rod electrode (spectrum a, Fig. 4) occurs at around 410 nm and the main light absorption centered on ultraviolet light region. This result was consistent with the band gap of 3.0 eV for rutile TiO2 [26]. The absorption spectrum of CdS deposited TiO2 nanorod electrode (spectrum b, Fig. 4) shows absorption onset around 520 nm, and the corresponding band gap was calculated to be 2.387 eV, which is higher than the value of CdS
The J–V characteristics of the assembled QDSSCs were measured by Oriel I–V test station under 1 sun (=100 mW/cm2 AM 1.5G solar illumination) with the active area of 0.16 cm2 . Fig. 5a displays the performance variation of TiO2 flower-rod solar cells sensitized by different SILAR cycles of Mn-doped CdS QDs. It is found that the short circuit current density (Jsc ) increased gradually with extending deposition cycles at initial stage (from 5 cycles to 10 cycles) and the open circuit voltage (Voc ) reduced first (from 5 cycles to 7 cycles) and then increased (from 7 cycles to 10 cycles). However, both the Jsc and Voc declined as the further increase of the SILAR cycles from 10 cycles to 15 cycles. Because the value of Jsc changed significantly and the value of Voc varied little, the variation trend of power conversion efficiency () is consistent with Jsc , as is shown in Fig. 5b and c. This variation trend can be interpreted as follows: first, with extending SILAR cycles of Mn-doped CdS QDs from five to ten, more QDs were deposited on TiO2 flower-rod, which can enhance the light harvesting, leading to the increase of Jsc . In addition, according to Ref. [28], the Voc can be determined by the following equation: Voc =
EFn − Eredox kB T = ln(n/n0 ) e e
Fig. 5. (a) J–V characteristics of different SILAR cycles of Mn-doped CdS QDs sensitized TiO2 flower-rod solar cells. The variation of photocurrent density; (b) and power conversion efficiency; and (c) with different SILAR cycles of Mn-doped CdS QDs sensitized TiO2 flower-rod solar cells.
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Table 1 Parameters obtained from the J–V curves of the QDSSCs using different photoanodes. Samples
Jsc (mA/cm2 )
Voc (V)
FF
(%)
CdS/TiO2 nanorod CdS/TiO2 flower-rod Mn-doped CdS/TiO2 flower-rod
4.18 6.04 8.02
0.37 0.40 0.42
0.34 0.35 0.32
0.53 0.84 1.09
where EFn is the quasi-Fermi level of the electrons in semiconductor photoanode under illumination; Eredox is the potential of redox electrolyte; e is the positive elementary charge; kB T is the thermal energy; n is the electron concentration in conduction band of the semiconductor photoanode under illumination; n0 is the electron concentration in the dark condition. With increasing amount of Mn-doped CdS QDs, more electrons were injected into the TiO2 conduction band under illumination and the value of n can be enhanced, which lead to more negative shift of the EFn , while the Eredox remains unchangeable, causing the improvement of Voc (from 7 cycles to 10 cycles). However, the excessive SILAR cycles can lead to the aggregation of Mn-doped CdS QDs, which increase the possibilities of the recombination between the electrons and holes when the photogenerated electrons diffuse across the QD layers to the TiO2 film. Furthermore, the excessive SILAR cycles can also limit the efficiency of charge separation and charge extraction as a result of hindering the diffusion of the polysulfide electrolyte [29], which will deteriorate the entire performance of the solar devices. According to the variation of the J–V curves to SILAR cycles in Fig. 5a, the best solar cell performance with the Jsc of 8.02 mA/cm2 and Voc of 0.42 V was obtained by 10 SILAR cycles of Mn-doped CdS QDs. It has already been obtained that the Mn-doped CdS/TiO2 flower-rod solar cell with 10 SILAR cycles has the optimal Jsc and Voc . In order to assess whether Mn-doped CdS QDs sensitizer and TiO2 flower-rod substrate play a significant role in improving performance of QDSSCs, we prepared the other two types of semiconductor photoanodes as a control experiment: 10 SILAR cycles of CdS/TiO2 flower-rod and 10 SILAR cycles of CdS/TiO2 nanorod solar cells without doping of Mn2+ . Fig. 6 depicts the J–V curves of CdS/TiO2 nanorod, CdS/TiO2 flower-rod and Mn-doped CdS/TiO2 flower-rod solar cells, respectively. The photovoltaic parameters of these solar cells are summarized in Table 1. In contrast with CdS/TiO2 nanorod cell, the value of Voc and FF of CdS/TiO2 flower-rod cell increased modestly, but the value of Jsc increased remarkably (from 4.18 mA/cm2 to 6.04 mA/cm2 ), resulting in the enhancement of power conversion efficiency (from 0.53% to 0.84%). The augment of all parameters in CdS/TiO2 flower-rod solar cell shows that TiO2 flower-rod structure is beneficial to the improvement of the solar cell performance compared with TiO2 nanorod structure. The hierarchical TiO2 flower-rod has influence on solar cell in two aspects, for one thing, 3D TiO2 flowers provide an increased surface area to adsorb more ODs, which is
Fig. 6. J–V curves of the QDSSCs using different photoanodes measured under AM 1.5G condition.
in favor of enhancing the light-harvesting efficiency; for another, 1D TiO2 nanorods beneath the TiO2 flowers offer the direct transport pathways for photogenerated electrons to enhance the charges separation efficiency. After Mn-doping, the Mn-doped CdS/TiO2 flower-rod cell exhibited the best performance and the power conversion efficiency can reach to 1.09%, which is 29.8% higher than CdS/TiO2 flower-rod solar cell without doping of Mn2+ . This can be attributed to the midgap states created by Mn2+ , causing the electrons to get trapped and screen them from charge recombination with holes [11]. This result suggested that doping Mn2+ in CdS QDs is helpful for a further enhancement of the solar cell performance. Fig. 7 shows the photoelectrical conversion structure of our Mn-doped CdS/TiO2 flower-rod solar cell, which consists of Mndoped CdS QDs sensitized TiO2 flower-rod photoanode, polysulfide electrolyte, and Cu1.8 S/CuS/FTO counter electrode. Under illumination, photons are captured by the Mn-doped CdS QDs, yielding electron–hole pairs (excitons). The electrons are injected into the oxide film and transported to the transparent conductive FTO, while holes are transferred via the electrolyte to the counter electrode, where the oxidized counterpart of the redox couple is reduced [30]. From the working mechanism of Mn-doped CdS QDs sensitized TiO2 flower-rod solar cell, we can see that the amount of adsorbed QDs and electron transport are the key factors affecting the performance of QDSSC. The hierarchical TiO2 flower-rod structure is an ideal material for QDSSCs, on account of the 3D TiO2 flowers can enlarge the surface area for QDs loading, and 1D TiO2 nanorods can accelerate the movement of electrons and reduce the recombination of electrons and holes.
Fig. 7. Schematic diagram of the photoelectrical conversion structure of our Mn-doped CdS/TiO2 flower-rod solar cell.
Please cite this article in press as: L. Yu, et al., Mn-doped CdS quantum dots sensitized hierarchical TiO2 flower-rod for solar cell application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.090
G Model APSUSC-27491; No. of Pages 7
ARTICLE IN PRESS L. Yu et al. / Applied Surface Science xxx (2014) xxx–xxx
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and collection efficiency (cc ). Therefore, the IPCE of our Mn-doped CdS/TiO2 flower-rod solar cell can reach to such a high level. 4. Conclusion In summary, the hierarchical TiO2 flower-rod film was successfully prepared on FTO glass by a facile hydrothermal method. The TiO2 flower-rod film sensitized with Mn-doped CdS QDs by SILAR was used as photoanode in solar cell and the efficiency has reached to 1.09% under one sun illumination (AM 1.5G, 100 mW/cm2 ), which was 29.8% higher than CdS/TiO2 flower-rod solar cell, and was 105.7% higher than CdS/TiO2 nanorod solar cell. The enhanced power conversion efficiency can mainly be ascribed to the hierarchical TiO2 flower-rod substrate and Mn-doped CdS QDs sensitizer. Our Mn-doped CdS QDs sensitized TiO2 flower-rod solar cell with the acceptable efficiency of 1.09% reported here has shown a great potential value to design high efficiency QDSSCs. Fig. 8. IPCE spectrum of QDSSC fabricated from Mn-doped CdS/TiO2 flower-rod.
Acknowledgment This work was supported by Key Project of Tianjin Sci-Tech Support Program (No. 08ZCKFSH01400). References
Fig. 9. Diagrammatic drawing of the electron transfers from Mn-doped CdS QD into TiO2 .
The incident photon to current conversion efficiency (IPCE) of Mn-doped CdS/TiO2 flower-rod solar cell is shown in Fig. 8. It can be seen that the profile of IPCE plot corresponds well with the UV–vis absorption spectrum of the Mn-doped CdS/TiO2 flower-rod in Fig. 4. As we can see, the maximum IPCE of our device can reach to 40% using a polysulfide electrolyte. Three factors determine the IPCE: the light-harvesting efficiency (LHE), the charge injection efficiency (inj ), and the charge collection efficiency (cc ) [31]. In our solar cell, the hierarchical TiO2 flower-rod structure provides a large surface area for adsorption of sensitizers, leading to the enhanced light harvesting efficiency (LHE). A typical type II heterojunction was constructed by Mn-doped CdS QDs and TiO2 flower-rod due to the matching of the conduction band energies. A shown in Fig. 9, following absorption of photons by Mn-doped CdS QDs, electron–hole pairs (excitons) are generated, and then the electrons are emitted from valance band (VB) to conduction band (CB) of CdS QDs. Because of the heterojunction at the Mn-doped CdS QD/TiO2 interface, electrons in the CB of CdS QDs were injected into the CB of wide band gap semiconductor TiO2 , this is a extremely fast process that typically occurs on the picosecond time scale [32], and 1D nanorods beneath TiO2 flowers can improve electrons transfer rate as a result of offering a direct pathway for excited electrons, leading to the improvement of the charge injection efficiency (inj )
[1] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385–2393. [2] A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 130 (2008) 4007–4015. [3] G.S. Paul, J.H. Kim, M.-S. Kim, K. Do, J. Ko, J.-S. Yu, ACS Appl. Mater. Interfaces 4 (2012) 375–381. [4] Y.-L. Lee, Y.-S. Lo, Adv. Funct. Mater. 19 (2009) 604–609. [5] M. Li, Y. Liu, H. Wang, H. Shen, W. Zhao, H. Huang, C. Liang, J. Appl. Phys. 108 (2010) 094304. [6] K. Prabakar, H. Seo, M. Son, H. Kim, Mater. Chem. Phys. 117 (2009) 26–28. [7] H. Hu, H. Shen, C. Cui, D. Liang, P. Li, S. Xu, W. Tang, J. Alloys Compd. 560 (2013) 1–5. [8] J. Tian, R. Gao, Q. Zhang, S. Zhang, Y. Li, J. Lan, X. Qu, G. Cao, J. Phys. Chem. C 116 (2012) 18655–18662. [9] M.C. Beard, J. Phys. Chem. Lett. 2 (2011) 1282–1288. [10] W.A. Tisdale, K.J. Williams, B.A. Timp, D.J. Norris, E.S. Aydil, X.Y. Zhu, Science 328 (2010) 1543–1547. [11] P.K. Santra, P.V. Kamat, J. Am. Chem. Soc. 134 (2012) 2508–2511. [12] H. Wang, Y. Bai, H. Zhang, Z. Zhang, J. Li, L. Guo, J. Phys. Chem. C 114 (2010) 16451–16455. [13] F. Shao, J. Sun, L. Gao, S. Yang, J. Luo, ACS Appl. Mater. Interfaces 3 (2011) 2148–2153. [14] A. Ramadoss, S.J. Kim, J. Alloys Compd. 561 (2013) 262–267. [15] S. Wang, G. Xia, H. He, K. Yi, J. Shao, Z. Fan, J. Alloys Compd. 431 (2007) 287–291. [16] F. Xu, Y. Wu, X. Zhang, Z. Gao, K. Jiang, Micro Nano Lett. 7 (2012) 826–830. [17] Z.-J. Zhou, J.-Q. Fan, X. Wang, W.-H. Zhou, Z.-L. Du, S.-X. Wu, ACS Appl. Mater. Interfaces 3 (2011) 4349–4353. [18] A. Kumar, A.R. Madaria, C. Zhou, J. Phys. Chem. C 114 (2010) 7787–7792. [19] A.M. More, T.P. Gujar, J.L. Gunjakar, C.D. Lokhande, O.-S. Joo, Appl. Surf. Sci. 255 (2008) 2682–2687. [20] D.R. Baker, P.V. Kamat, Adv. Funct. Mater. 19 (2009) 805–811. [21] H. Yu, J. Pan, Y. Bai, X. Zong, X. Li, L. Wang, Chemistry 19 (2013) 13569–13574. [22] B. Liu, E.S. Aydil, J. Am. Chem. Soc. 131 (2009) 3985–3990. [23] H. Lee, M. Wang, P. Chen, D.R. Gamelin, S.M. Zakeeruddin, M. Grätzel, M.K. Nazeeruddin, Nano Lett. 9 (2009) 4221–4227. [24] S.S. Kalanur, S.Y. Chae, O.S. Joo, Electrochim. Acta 103 (2013) 91–95. [25] M. Shalom, Z. Tachan, Y. Bouhadana, H.-N. Barad, A. Zaban, J. Phys. Chem. Lett. 2 (2011) 1998–2003. [26] M. Xu, P. Da, H. Wu, D. Zhao, G. Zheng, Nano Lett. 12 (2012) 1503–1508. [27] W.W. Yu, L. Qu, W. Guo, X. Peng, Chem. Mater. 15 (2003) 2854–2860. [28] T. Zeng, H. Tao, X. Sui, X. Zhou, X. Zhao, Chem. Phys. Lett. 508 (2011) 130–133. [29] P. Sudhagar, J.H. Jung, S. Park, Y.-G. Lee, R. Sathyamoorthy, Y.S. Kang, H. Ahn, Electrochem. Commun. 11 (2009) 2220–2224. [30] T. Toyoda, Q. Shen, J. Phys. Chem. Lett. 3 (2012) 1885–1893. [31] S.W. Jung, J.-H. Kim, H. Kim, C.-J. Choi, K.-S. Ahn, Curr. Appl. Phys. 12 (2012) 1459–1464. [32] A. Kumar, K.-T. Li, A.R. Madaria, C. Zhou, Nano Res. 4 (2011) 1181–1190.
Please cite this article in press as: L. Yu, et al., Mn-doped CdS quantum dots sensitized hierarchical TiO2 flower-rod for solar cell application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.03.090