- Email: [email protected]
TiO2 hierarchical porous film constructed by ultrastable foams as photoanode for quantum dot-sensitized solar cells
Journal of Power Sources 332 (2016) 1e7
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
TiO2 hierarchical porous film constructed by ultrastable foams as photoanode for quantum dot-sensitized solar cells Xing Du a, Xuan He a, *, Lei Zhao a, Hui Chen a, Weixin Li a, Wei Fang a, Wanqiu Zhang a, Junjie Wang b, Huan Chen b a b
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, PR China Wuhan Iron and Steel (Group) Refractory Materials Limited Liability Company, Wuhan 430081, PR China
h i g h l i g h t s Foams are first used to introduce micron-size hierarchical pores into TiO2 film. Foam component-dodecanol is innovatively utilized to synthesize CdSe QDs. PCE values of 2.20% is achieved in QDSSC based on the TiO2 hierarchical porous film.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 August 2016 Received in revised form 13 September 2016 Accepted 19 September 2016
It reported a novel and simple method for the first time to prepare TiO2 hierarchical porous film (THPF) using ultrastable foams as a soft template to construct porous structures. Moreover, dodecanol as one foam component was creatively used as solvent during the synthesis of CdSe quantum dots (QDs) to decrease reaction temperature and simplify precipitation process. The result showed that hierarchical pores in scale of microns introduced by foams were regarded to benefit for high coverage and unimodal distribution of QDs on the surface of THPF to increase the efficiencies of light-harvesting, chargecollection and charge-transfer. The increased efficiencies caused an enhancement in quantum efficiency of the cell and thus remarkably increased the short circuit current density (Jsc). In addition, the decrease of charge recombination resulted in the increase of the open circuit voltage (Voc) as well. The QDSSC based on THPF exhibited about 2-fold higher power conversion efficiency (h ¼ 2.20%, Jsc ¼ 13.82 mA cm2, Voc ¼ 0.572 V) than that of TiO2 nanoparticles film (TNF) (h ¼ 1.06%, Jsc ¼ 6.70 mA cm2, Voc ¼ 0.505 V). It provided a basis to use foams both as soft template and carrier to realize simultaneously construction and in-situ sensitization of photoanode in further work. © 2016 Elsevier B.V. All rights reserved.
Keywords: Foams Hierarchical porous film Dodecanol CdSe quantum dots Sensitized solar cell
1. Introduction Quantum dot-sensitized solar cells (QDSSCs), as a viable alternative to dye-sensitized solar cells (DSSCs) have attracted considerable attention in recent years due to the unique properties of QD light-absorbers [1e5]. Although considerable efforts have been made to improve the performance of QDSSCs with some of tremendous achievements [6e9], the reported power conversion efficiency (PCE) of QDSSCs were far less than the theoretical value (66%) and lagged behind that of dye-sensitized solar cells (DSSCs)
* Corresponding author. Present address: No. 947, Heping Avenue, Qingshan District, Wuhan, Hubei Province, PR China. E-mail address: [email protected] (X. He). http://dx.doi.org/10.1016/j.jpowsour.2016.09.103 0378-7753/© 2016 Elsevier B.V. All rights reserved.
[7,10]. Clark has pointed out that low performance of QDSSCs was a design issue and not a fundamental one [11]. In fact, the configuration and preparation processing of photoanodes in QDSSCs was initially drawing on the experience of DSSCs. However, the conventional TiO2 nanoparticles film widely used as photoanode in DSSCs was not suitable for highly efficient QDSSCs because of the significant differences between QDs and dyes [12,13]. The sizes of QDs tethered onto photoanode were much larger than that of dyes [14,15], making a hard deposition of QDs into inner pores of nanostructured TiO2 film on account of blocked outer pores at first by QDs. It normally impeded further deposition of QDs and penetration of the electrolyte [16,17] to result in a low PCE. Therefore, the monotonous distribution of pore size of TiO2 nanostructured photoanode became the key limitation to get high efficiency.
2
X. Du et al. / Journal of Power Sources 332 (2016) 1e7
Furthermore, studies have pointed out that the direct growth of QDs on the TiO2 films via chemical bath deposition (CBD) or successive ion layer absorption and reaction technique (SILAR) resulted in a low PCE because of high density of surface defects and poor size distribution of QDs in recent years [18]. Recently, the development of QDSSCs indicates that the mainstream route to tether QDs onto TiO2 films transferred from direct growth approach to post synthesis assembly method due to the accessible high quality and high loading amount of QDs via ex-situ ligand exchange route [6,9,19e22]. Design and construct a suitable photoanode to match the post-synthesis assembly route was thus urgently needed for further improvement of PCE. Now, the most popular photoanode matched post synthesis assembly approach was TiO2 mesoporous films consisting of a transparent layer and a light scattering layer [6e9,20e23]. The main contribution of the scattering layer was to adjust the pore size distribution of TiO2 photoanode for QDs deposition and polysulfide electrolyte penetration. And on this basis M.Q. Wang et al. developed a simple mechanical mixing method which not only simplified the fabrication process but also maintained similar light scattering when compared with doublelayer TiO2 film [13]. Actually, foam ceramics possessed unique pore structures due to the presence of foams with hierarchical pore distribution [24]. Therefore, the hierarchical macropores can be easily introduced into TiO2 film and well maintained through adding foams into P25 slurry without destroying nanostructure pores formed by the stacking of P25. Furthermore, in fact, the solvent trioctylphosphine (TOP), traditionally used as the reaction medium to synthesize of CdSe QDs, is expensive, hazardous, unstable, and not an environmentally friendly [25]. The cheaper solvent paraffin liquid as reaction medium can significantly reduce the temperature and simplify the process of synthesis of the CdSe QDs [25,26]. However, the precipitation process for the CdSe QDs proved not to be an effective because paraffin liquid was insoluble in methanol [26], and the obtained QDs have been barely used in QDSSCs. Herein, this work presented a novel and simple method to fabricate TiO2 hierarchical porous film by introducing ultrastable foams [27] (Scheme 1) in P25 slurry. Besides, foam componentdodecanol, once utilized to synthesize Mn3O4 nanocrystals dots [28], was selected as solvent to prepare CdSe QDs. Finally, the post
synthesis assembly approach was adopted to deposit the QDs onto the TiO2 hierarchical porous film electrode. As shown in Scheme 1, the micron-size hierarchical pores introduced by foams could avoid the unfavorable clogging of pores by CdSe QDs during its deposition, resulting in uniform distribution of QDs and rapid penetration of the electrolyte throughout the TiO2 photoanode. The result showed the ultrastable foams could be successfully used as soft template to construct TiO2 hierarchical porous film. And the high-quality CdSe QDs were successfully synthesized by utilizing foam component-dodecanol which provided a possibility to use foam liquid film as carrier to introduce QDs. On this basis, the ultrastable foams is expected to be used not only as a soft template to construct hierarchical pore structures but also as carrier to introduce QDs for simultaneous in-situ sensitization of TiO2 film in the further work. 2. Experimental section 2.1. Materials and reagents Selenium powder (Se, 200 mesh, 99.99%) and cadmium oxide (CdO, 99.99%) were purchased from Sigma Aldrich. Dodecanol (DDA, 99.0%), oleic acid (OA, 90%) 3-Mercaptopropionic acid (MPA, 99%), sodium dodecyl sulfonate (SDS, 99%) and polyving akohol (PVA, alcoholysis degree: 99.8e100%) were purchased from Aladdin reagents (Shanghai) co., ltd.. P25 (a mixed phase of 80% anatase and 20% rutile; average size 25 nm) was purchased from Degussa (China) co., ltd.. All reagents were used without any further purification. 2.2. Preparation of the TiO2 hierarchical porous film constructed by ultrastable foams The foams was prepared according to the previous work [27] with minor modification. Typically, SDS (0.1 g), PVA (0.03 g) and DDA (0.1 g) were added into deionized water (2 mL) and then violently stirring to get ultrastable foams. For preparing the porous TiO2 films, P25 (1.2 g) was evenly dispersed in deionized (4 mL) water by magnetic stirrers for 20 min to get TiO2 slurry. The slurry was then equally divided into two portions. The prepared foams were added into to one portion to form TiO2 foam slurry. Then, the as-prepared TiO2 slurry and TiO2 foam slurry were coated on clean F:SnO2-coated (FTO, 14 U square1) glass substrates by doctor blade technique. Finally, all samples were sintered at 450 C for 30 min in a muffle furnace to construct TiO2 nanoparticle film (TNF) and TiO2 hierarchical porous film (THPF), respectively. 2.3. Synthesis of CdSe QDs using dodecanol as reaction solvent
Scheme 1. Schematic design of the TiO2 hierarchical porous film photoanode.
Firstly, CdO (2 mmol), OA (2 mL), DDA (8 mL) were added into a three-neck flask and heated to 150 C with constantly stirring. The color of CdO powder contained solution transformed to pale yellow from crimson when the uniform Cd precursor solution formed. Secondly, Se powder (0.8 mmol) and DDA (30 mL) were loaded into another three-neck flask and heated to 220 C with vigorous stirring to generate orange Se precursor solution. Thirdly, 8 mL of solution contained about 1.6 mmol of Cd precursor was quickly injected into Se precursor solution with rapidly stirring. The reaction system stayed at 220 C for 5 min before removing the heater and cooling to room temperature. Then, 38 mL chloroform used as extraction solvent were added to the reaction products to separate the nanocrystals from unreacted precursors. The obtained CdSe QDs were precipitated by adding methanol into the chloroform solution and further purified by repeated centrifugation and
X. Du et al. / Journal of Power Sources 332 (2016) 1e7
3
Fig. 1. Top-view SEM images of (a) TNF and (b) THPF at low magnification. Inset: Top-view SEM images of TNF and THPF at high magnification respectively.
decantation with addition of methanol and chloroform [19]. Finally, the purified CdSe QDs were dispersed into 60 mL chloroform to form CdSe QDs chloroform solution (containing 0.8 mmol of QDs). The MPA-capped CdSe QDs were prepared by a ligand exchange route according to the method reported by pervious literature [29,30].
DK240 monochrometer. The fluorescence lifetime study was carried out by Horiba Jobin Yvon Fluoromax-4. Electrochemical impedance spectroscopy (EIS) measurements of cells were measured using the electrochemical workstation with 0.5 V bias potential and 10 mV of amplitude under dark condition, and the frequency range was set from 0.1 Hz to 100 kHz.
2.4. Fabrication of CdSe QDs-sensitized TiO2 photoanodes 3. Results and discussion After immersed in the as-prepared MPA-capped CdSe QDs aqueous dispersion for 2 h and sequentially rinsed with water and ethanol and then dried with nitrogen, TNF and THPF were coated with CdSe QDs, H2PtCl4 ethanol solution (8 mM) was deposited on FTO glass by spin-coating and then heat treated at 400 C for15 min [23] to prepare the Pt counter electrode,. The polysulfide electrolyte solution was composed of Na2S (2.0 M), S (2.0 M), and KCl (0.2 M) in methanol/deionized water (3:7, v/v) solution. The cells were constructed by sandwiching the QDs-sensitized TiO2 photoanodes and Pt electrode using a 50 mm-thickness 3M adhesive tape as a spacer to separate the two electrodes and permeating with a 10 mL of polysulfide electrolyte. 2.5. Characterization Transition electron microscopy (TEM) image was taken using a JEOL JEM-2100UHR high-resolution transmission electron microscope (HRTEM). UV-Vis absorption and steady state photoluminescence (PL) emission spectra were obtained on a Shimadzu UV-2550 UV-Vis spectrophotometer and a Shimadzu RF-6000 fluorescence spectrophotometer, respectively. An X-ray diffraction spectrum was recorded with a Philips X'Pert Pro Xray powder diffractometer with Cu-Ka radiation (1.5406 Å). Scanning electron microscopy (SEM) was performed using a FEI Nova 400 Nano SEM system equipped with an energy dispersive spectroscopy (EDS). Brunauer - Emmett - Teller (BET) surface area was evaluated by using a Quantachrome Autosorb-1MP system. The specific surface area was calculated by the singlepoint BET method. The BET measurement was detected for times for each sample to evaluate the specific surface area by averaged value. Current-voltage characteristics (J-V curves) of cells were recorded on a Zahner Elektrik IM6 electrochemical workstation under illumination of AM 1.5 G solar simulator (Newport 91160). The power of the simulated light was calibrated to 100 mW cm2 by a NREL standard Si solar cell. The photoactive area was 0.25 cm2. Incident photon-to-current conversion efficiency (IPCE) spectra were obtained in the range of 300e750 nm by a Keithley 2000 multimeter under the illumination of a 300 W tungsten lamp with a Spectral Product
The microstructure morphology of the TNF and THPF are shown in Fig. 1. It observed that the addition of foams gave rise to the presence of micron-size hierarchical pores in TiO2 films. The average specific surface areas of TNF and THPF were 50.407 and 51.678 m2 g1, respectively, which indicates that the added foams did not destroy the nanostructure pores formed by P25. It illustrated that the hierarchical pore structure (macro-micro-nano) was successfully constructed by the ultrastable foams and P25. Fig. 2(a) presented PL emission and UV-Vis absorption spectra of CdSe QDs colloidal solution. The UV-Vis absorption band and PL emission peak respectively located at 565 nm and 580 nm indicated that the obtained CdSe QDs can appropriately be used as sensitizer for QDSSCs. And the TEM and HRTEM images of the CdSe QDs revealed that the size of QDs was almost monotonous distributed and all particles was uniformly shaped in spherical as shown in Fig. 2(b). The well-distinguished lattice fringes of CdSe QDs (the inset in Fig. 2(b)) suggested their good crystallinity. The XRD pattern of CdSe QDs was presented in Fig. 2(c) and the three diffraction features appeared at about 25.5 , 42.4 , and 50.0 , corresponding to (111), (220), and (311) planes of the cubic zinc-blende phase of CdSe, respectively [26]. Thus it can be concluded that the dodecanol could be successfully used as reaction solvent to synthesize CdSe QDs which could effectively reduce the reaction temperature and simplify the synthesis and precipitation process. In addition, as the foam formation, dodecanol is conduce to foam stability, which can provided a possibility to use foam liquid film as carrier to introduce QDs to realize in-situ sensitization of TiO2 film in the further work. According to the growth mechanism of forming CdSe nanocrystals in the paraffin liquid reported by Tang et al. [25], the possible chemical reactions involved in the formation of CdSe QDs using DDA and OA as the reaction medium are shown in Equations (1)e(3).
CdO þ Oleic acid / Cd oleate D
(1)
4
X. Du et al. / Journal of Power Sources 332 (2016) 1e7
Fig. 2. (a) UV-Vis absorption (magenta line) and PL emission (green line, lex ¼ 400 nm) spectra (b) TEM and HRTEM (inset in panel b) and (c) X-ray powder diffraction pattern of the CdSe QDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. UV-Vis absorption spectra of CdSe QDs sensitized TNF and THPF. Inset: photographs of TNF and THPF after being sensitized.
5Se þ 2CH3 ðCH2 Þ10 CH2 OH
dehydrogenation 2CH3 ðCH2 CH ¼ CHCH2 Þ2:5 OH / D
þ 5H2 Se (2) Cd oleate þ H2 Se / CdSe nuclei þ byproducts D
(3)
Fig. 5. The excited state electron radiative decay of the TNF and THPF films loaded with CdSe QDs.
The UV-Vis absorption spectra of TNF and THPF after sensitized by the MPA-capped CdSe QDs were presented in Fig. 3. In case of THPF, change tendency of profile and sharp excitonic feature of absorption curve was reserved comparing to that in Fig. 2(a). It reflected that the particle size and size distribution of the deposited QDs on THPF remained their original state. However, the sharp excitonic feature of TNF disappeared because of the aggregation of QDs in some extent when deposited on TNF that pores of nanostructured films was easily clogged by QDs to impede the further deposition. In this regards, it pointed out that porous structure of
Fig. 4. SEM images of a typical cross-section of QD sensitized (a) TNF and (b) THPF. (c) Cd/Ti ratio along the cross section of TNF and THPF.
X. Du et al. / Journal of Power Sources 332 (2016) 1e7
5
Fig. 6. (a) Nyquist plot and (b) Bode plot curves of the cells based on TNF and THPF photoanodes at 0.5 V forward bias. Inset: equivalent circuit adopted for fitting the EIS data obtained.
THPF was more suitable for QDs deposition than that of TNF due to the introduced micron-size hierarchical pores, which was demonstrated by the absorption intensity and the colors of sensitized films in the inset of Fig. 3 as well. In order to investigate if porous structure of THPF was more suitable for the deposition of QDs, the coverage of QDs along the film thickness was verified by elemental analysis. The elemental distribution depending on depth in cross-section of the sensitized TNF and THPF were measured by EDS equipped on SEM. As presented in Fig. 4(c), the Cd/Ti ratio (20%) throughout the total THPF cross-section maintained nearly constant indicating homogeneous distribution of QDs on the sensitized THPF. The Cd/Ti ratio of sensitized TNF was dropped dramatically on depth while the value of Cd/Ti ratio was much higher adjacent the surface than that in deep side. Additionally, the Cd/Ti ratio on THPF was apparently higher than that on TNF because of higher content of QDs on THPF and more TiO2 nanoparticles on TNF. It suggested that the pores close to the surface of TNF were probably blocked by QDs to make them difficult to diffuse into the inner pores while the porous structure of THPF was favorable for the deposition of QDs at the same condition. The excited state electron radiative decay of the TNF and THPF films loaded with CdSe QDs were shown in Fig. 5. Shorter emission decay of the CdSe QDs on THPF photoanode than that on TNF photoanode indicated that the PL lifetime of QDs was shortened when deposited on the THPF photoanode. It supposed that the porous structure of THPF may play an important role on it. Acturally, the structural feature of THPF was benefit for the deposition of QDs without aggregation while QDs were aggregated in some extent when deposited on TNF. Since the (conduction band energy) Ecb of smaller QDs is higher than that of bigger ones [15], the driving force of the electrons from QDs to TiO2 in the case of THPF was enhanced which also resulted in the increase of the injection kinetics and displayed short-lived excited state of QDs on THPF as in Fig. 5 [4,31]. EIS was applied to elucidate charge recombination processes in the cells. Fig. 6(a) displayed Nyquist curves of cells based on TNF and THPF photoanodes in the dark under a forward bias of 0.5 V. The semicircles in the high frequency region reflected the electron injection at the counter electrode/electrolyte interface and transport in the electrolyte (R1) while the Table 1 Fitted impedance values at 0.5 V forward bias for TNF and THPF photoanodes. Electrode
R 1 ( U)
R2 (U)
tn (ms)
TNF THPF
58.4 80.2
107.6 174.8
28.3 41.6
semicircles in the low frequency region reflected electron transfer at the TiO2/QDs/electrolyte interface and transport in the TiO2 film (R2) [32]. The equivalent circuit was shown in the inset in Fig. 6(a) and the fitting results were listed in Table 1. In general, R2 was mainly determined by the recombination resistance of electrons. A higher value indicated less back electron transfer at the TiO2/QDs/electrolyte interface, that meant the suppressed interfacial charge recombination [33,34]. The results showed that the value of Rct of the cell based on THPF (174.8 U) was higher than that of the cell based on TNF (107.6 U), which resulted in the decrease of charge recombination. The increase of R2 can be attributed to the decreased contact area of TiO2 with electrolyte [32]. The poor QDs loading on TNF caused the direct exposure of TiO2 to the electrolyte. On the contrary, hierarchical porous structure resulted in high content of QDs deposited on the surface of TiO2 and was favored for the permeation of the electrolyte. The charge recombination between the injected electrons in the conduction band of TiO2 and the oxidized ions (S2 n ) in the electrolyte was consequently suppressed. Fig. 6(b) showed the Bode plots of the cells, and the curve peak of the spectrum can be used to determine the electron lifetime according to Equation (4) [35].
tn ¼ 1
. 2pfpeak
(4)
The corresponding results were listed in Table 1. It observed that the electron lifetime of the cell based on THPF photoanode is 41.6 ms, which was much higher than that of the cell based on TNF photoanode (28.3 ms). Consequently, the THPF photoanode can enhance the charge recombination resistance and thus prolong the electron lifetime. The IPCE of cells were measured and results were shown in Fig. 7. The maximum value of IPCE of CdSe QDSSC based on THPF photoanode reached 74% at 595 nm, which was enhanced by ~64% compared to the maximum IPCE of 45% at 580 nm for CdSe QDSSC based on TNF photoanode. The IPCE is calculated by equation (5), where LHE is the light-harvesting efficiency, hct is charge-transfer efficiency, and hcc is charge-collection efficiency [4].
IPCE ¼ LHEhct hcc
(5)
The LHE of the solar cell is closely related to the absorbance of the photoelectrode [4]. As shown in Fig. 3, the absorbance of CdSe QDs sensitized THPF film was higher than that of CdSe QDs sensitized TNF film, which indicated that the LHE of cell based on THPF was higher than that of cell based on TNF accordingly. The hct can be deduced by the excited state electron radiative decay of QDs, and
6
X. Du et al. / Journal of Power Sources 332 (2016) 1e7 Table 2 Average photovoltaic parameters obtained from the J-V curves of the QDSSCs based on TNF and THPF photoanodes.
Fig. 7. IPCE curves of CdSe QDSSCs constructed using TNF and THPF photoanodes.
the increase of electron injection kinetics was helpful for improving hct [4]. Thus, the hct of cell based on THPF was higher than that of TNF due to the increase of electron injection kinetics (Fig. 5). The hct was mainly contributed by charge recombination [4]. EIS results (Fig. 6 and Table 1) revealed that the THPF photoanode can enhance the charge recombination resistance and prolong the electron lifetimeto cause an improvement in charge-collection efficiency (hcc). As a result, it can be concluded that the increased lightharvesting, charge-transfer and charge-collection efficiencies can efficiently improve the IPCE of cells. To compare the photovoltaic performance of as-prepared TNF and THPF photoanodes, sandwich-type thin layer cells were fabricated with Pt-coated FTO as the counter electrode with polysulfide electrolyte as hole transporter. Fig. 8 showed the J-V curves corresponding to cell devices based on different photoanodes. The open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and PCE (h) of those QDSSCs were summarized in Table 2. As expected, the QDSSC based on TNF photoanode exhibited a Jsc of 6.70 mA cm2, Voc of 0.505 V and h of 1.06%, whereas the QDSSC based on THPF photoanode achieved a Jsc of 13.82 mA cm2, Voc of 0.572 V and h of 2.20%, manifesting 107% improvement of PCE compared to that of the cells based on TNF photoanode. It was mainly attributed to the significant increase of short-circuit current
Electrode
Jsc (mA cm2)
Voc(V)
FF
h (%)
TNF THPF
6.70 13.82
0.505 0.572
0.313 0.278
1.06 2.20
(Jsc) of the cell based on THPF photoanode. The remarkable increase of Jsc was attributed to the excited charge characteristics, such as generation, injection or collection [4]. As mentioned above, the hierarchical pores in THPF facilitating the penetration of QDs improved the light-harvesting capability of photoanode and charge-transfer efficiency. The high content of QDs deposited on the surface of TiO2 and the rapid permeation of the electrolyte suppressed the charge recombination at TiO2/QDs/electrolyte interface and prolonged the electron lifetime, led to an enhancement in charge-collection efficiency. As a result, the increase of light-harvesting, charge-transfer and charge collection gave rise to the improvement of the IPCE of the solar cell, resulting in enhanced Jsc [4]. In addition, the suppressed charge recombination could result in the increase of Voc as well. 4. Conclusions In summary, TiO2 hierarchical porous film was successfully fabricated using the ultrastable foams in this work. The highquality zinc-blende (cubic) CdSe QDs were successfully synthesized utilizing foam component-dodecanol. The hierarchical pores in micron-size introduced by foams can benefit for high coverage of QDs on the surface of TiO2, maintaining unimodal distribution of QDs size and accelerating the penetration of the electrolyte throughout the TiO2 photoanode. High coverage of QDs on the surface of TiO2 not only improved the light-harvesting capability of photoanode but also decreased the direct exposure of TiO2 to electrolyte to enhance charge-collection efficiency. The maintained unimodal distribution of QDs size can increase the injection kinetics to improve the charge-transfer efficiency. The increase of the efficiencies of light-harvesting, charge-collection and chargetransfer were contributed to enhance the external quantum efficiency (IPCE) of the cell, resulted in remarkable increase of Jsc. In addition, the decrease of charge recombination resulted in the increase of Voc as well. As a result, the cell based on TiO2 hierarchical porous film photoanode achieved a PCE of 2.20%, which was obviously higher than that of the cell based on TiO2 nanoparticle film photoanode (1.06%). The results offered an evidence that ultrastable foams can be used as soft templates to construct hierarchical pore structures of TiO2 photoanode, and dodecanol as reaction solvent to synthesize CdSe QDs. This paves the way for the further work to use foams both as soft template and carrier to simultaneously realize the construction and in-situ sensitization of photoanode. Acknowledgements The work was financially supported by the China Postdoctoral Science Foundation (2015M572210), the National Natural Science Foundation of China (61604110), the Hubei Provincial Natural Science Foundation of China (ZRMS2016000349) and the Foundation of Wuhan University of Science and Technology (2016XZ002). References
Fig. 8. J-V curves of CdSe QDSSCs constructed using TNF and THPF photoanodes.
[1] P.V. Kamat, J.A. Christians, J.G. Radich, Quantum dot solar cells: hole transfer as a limiting factor in boosting the photoconversion efficiency, Langmuir 30
X. Du et al. / Journal of Power Sources 332 (2016) 1e7 (20) (2014) 5716e5725. [2] B. Liu, Y. Sun, X. Wang, L. Zhang, D. Wang, Z. Fu, Y. Lin, T. Xie, Branched hierarchical photoanode of anatase TiO2 nanotubes on rutile TiO2 nanorod arrays for efficient quantum dot-sensitized solar cells, J. Mater. Chem. A 3 (8) (2015) 4445e4452. [3] K. Meng, P.K. Surolia, O. Byrne, K.R. Thampi, Efficient CdS quantum dot sensitized solar cells made using novel Cu2S counter electrode, J. Power Sources 248 (2014) 218e223. [4] J. Tian, L. Lv, C. Fei, Y. Wang, X. Liu, G. Cao, A highly efficient (>6%) Cd1xMnxSe quantum dot sensitized solar cell, J. Mater. Chem. A 2 (46) (2014) 19653e19659. [5] A. Sahasrabudhe, S. Bhattacharyya, Dual sensitization strategy for highperformance core/shell/quasi-shell quantum dot solar cells, Chem. Mater. 27 (13) (2015) 4848e4859. [6] G. Wang, H. Wei, Y. Luo, H. Wu, D. Li, X. Zhong, Q. Meng, A strategy to boost the cell performance of CdSexTe1x quantum dot sensitized solar cells over 8% by introducing Mn modified CdSe coating layer, J. Power Sources 302 (2016) 266e273. [7] J. Du, Z. Du, J.-S. Hu, Z. Pan, Q. Shen, J. Sun, D. Long, H. Dong, L. Sun, X. Zhong, ZneCueIneSe quantum dot solar cells with a certified power conversion efficiency of 11.6%, J. Am. Chem. Soc. 138 (12) (2016) 4201e4209. [8] K. Zhao, Z. Pan, I. Mora-Sero, E. Canovas, H. Wang, Y. Song, X. Gong, J. Wang, M. Bonn, J. Bisquert, X. Zhong, Boosting power conversion efficiencies of quantum-dot-sensitized solar cells beyond 8% by recombination control, J. Am. Chem. Soc. 137 (16) (2015) 5602e5609. [9] Z. Ren, J. Wang, Z. Pan, K. Zhao, H. Zhang, Y. Li, Y. Zhao, I. Mora-Sero, J. Bisquert, X. Zhong, Amorphous TiO2 buffer layer boosts efficiency of quantum dot sensitized solar cells to over 9%, Chem. Mater. 27 (24) (2015) 8398e8405. [10] S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B.F. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M.K. Nazeeruddin, M. Gratzel, Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers, Nat. Chem. 6 (3) (2014) 242e247. [11] J.T. Margraf, A. Ruland, V. Sgobba, D.M. Guldi, T. Clark, Quantum-dot-sensitized solar cells: understanding linker molecules through theory and experiment, Langmuir 29 (7) (2013) 2434e2438. [12] Z. Du, H. Zhang, H. Bao, X. Zhong, Optimization of TiO2 photoanode films for highly efficient quantum dot-sensitized solar cells, J. Mater. Chem. A 2 (32) (2014) 13033. [13] X. Song, M. Wang, T. Xing, J. Deng, J. Ding, Z. Yang, X. Zhang, Fabrication of micro/nano-composite porous TiO2 electrodes for quantum dot-sensitized solar cells, J. Power Sources 253 (2014) 17e26. [14] Q. Zhang, X. Guo, X. Huang, S. Huang, D. Li, Y. Luo, Q. Shen, T. Toyoda, Q. Meng, Highly efficient CdS/CdSe-sensitized solar cells controlled by the structural properties of compact porous TiO2 photoelectrodes, Phys. Chem. Chem. Phys. 13 (10) (2011) 4659e4667. [15] P.V. Kamat, Quantum dot solar cells. Semiconductor nanocrystals as light harvesters, J. Phys. Chem. C 112 (48) (2008) 18737e18753. , J. Bisquert, R. Go mez, CdSe [16] N. Guijarro, T. Lana-Villarreal, I. Mora-Sero quantum dot-sensitized TiO2 electrodes: effect of quantum dot coverage and mode of attachment, J. Phys. Chem. C 113 (10) (2009) 4208e4214. nez, P.P. Boix, Q. Shen, M.E. Calvo, N. Taghavinia, [17] M. Samadpour, S. Gime , Effect of nanostructured electrode A.I. Zad, T. Toyoda, H. Míguez, I. Mora-Sero architecture and semiconductor deposition strategy on the photovoltaic performance of quantum dot sensitized solar cells, Electrochim. Acta 75
7
(2012) 139e147. [18] B. Liu, D. Wang, L. Wang, Y. Sun, Y. Lin, X. Zhang, T. Xie, Glutathione-assisted hydrothermal synthesis of CdS-decorated TiO2 nanorod arrays for quantum dot-sensitized solar cells, Electrochim. Acta 113 (2013) 661e667. [19] H. Zhang, K. Cheng, Y.M. Hou, Z. Fang, Z.X. Pan, W.J. Wu, J.L. Hua, X.H. Zhong, Efficient CdSe quantum dot-sensitized solar cells prepared by a postsynthesis assembly approach, Chem. Commun. 48 (91) (2012) 11235e11237. [20] H. Wei, G. Wang, Y. Luo, D. Li, Q. Meng, Investigation on interfacial charge transfer process in CdSexTe1-x alloyed quantum dot sensitized solar cells, Electrochim. Acta 173 (2015) 156e163. [21] S. Jiao, Q. Shen, I. Morasero, J. Wang, Z. Pan, K. Zhao, Y. Kuga, X. Zhong, J. Bisquert, Band engineering in core/shell ZnTe/CdSe for photovoltage and efficiency enhancement in Exciplex quantum dot sensitized solar cells, Acs Nano 9 (1) (2015) 908e915. [22] Z. Pan, I. Mora-Sero, Q. Shen, H. Zhang, Y. Li, K. Zhao, J. Wang, X. Zhong, J. Bisquert, High-efficiency “green” quantum dot solar cells, J. Am. Chem. Soc. 136 (25) (2014) 9203e9210. [23] K. Zhao, H. Yu, H. Zhang, X. Zhong, Electroplating cuprous sulfide counter electrode for high-efficiency long-term stability quantum dot sensitized solar cells, J. Phys. Chem. C 118 (11) (2014) 5683e5690. [24] F. Krauss Juillerat, U.T. Gonzenbach, A.R. Studart, L.J. Gauckler, Self-setting particle-stabilized foams with hierarchical pore structures, Mater. Lett. 64 (13) (2010) 1468e1470. [25] Z. Deng, L. Cao, F. Tang, B. Zou, A new route to zinc-blende CdSe nanocrystals: mechanism and synthesis, J. Phys. Chem. B 109 (35) (2005) 16671e16675. [26] B. Xing, W. Li, H. Dou, P. Zhang, K. Sun, Systematic study of the properties of CdSe quantum dots synthesized in paraffin liquid with potential application in multiplexed bioassays, J. Phys. Chem. C 112 (37) (2008) 14318e14323. [27] X. Du, L. Zhao, H. Xiao, F. Liang, H. Chen, X. Wang, J. Wang, W. Qu, Z. Lei, Stability and shear thixotropy of multilayered film foam, Colloid. Polym. Sci. 292 (10) (2014) 2745e2751. [28] P. Li, C. Nan, Z. Wei, J. Lu, Q. Peng, Y. Li, Mn3O4 nanocrystals: facile synthesis, controlled assembly, and application, Chem. Mater. 22 (14) (2010) 4232e4236. [29] L. Liu, X. Guo, Y. Li, X. Zhong, Bifunctional multidentate ligand modified highly stable water-soluble quantum dots, Inorg. Chem. 49 (8) (2010) 3768e3775. [30] Z. Pan, K. Zhao, J. Wang, H. Zhang, Y. Feng, X. Zhong, Near infrared absorption of CdSexTe1ex alloyed quantum dot sensitized solar cells with more than 6% efficiency and high stability, ACS Nano 7 (6) (2013) 5215e5222. [31] P.R.F. Barnes, A.Y. Anderson, S.E. Koops, J.R. Durrant, B.C. O'Regan, Electron injection efficiency and diffusion length in dye-sensitised solar cells derived from incident photon conversion efficiency measurements, J. Phys. Chem. C 113 (113) (2009) 1126e1136. [32] R. Zhou, Q. Zhang, E. Uchaker, J. Lan, M. Yin, G. Cao, Mesoporous TiO2 beads for high efficiency CdS/CdSe quantum dot co-sensitized solar cells, J. Mater. Chem. A 2 (8) (2014) 2517. [33] T. Shu, P. Xiang, Z.-M. Zhou, H. Wang, G.-H. Liu, H.-W. Han, Y.-D. Zhao, Mesoscopic nitrogen-doped TiO2 spheres for quantum dot-sensitized solar cells, Electrochim. Acta 68 (2012) 166e171. [34] J. Tian, Q. Zhang, L. Zhang, R. Gao, L. Shen, S. Zhang, X. Qu, G. Cao, ZnO/TiO2 nanocable structured photoelectrodes for CdS/CdSe quantum dot cosensitized solar cells, Nanoscale 5 (3) (2013) 936e943. [35] R. Kern, R. Sastrawan, J. Ferber, R. Stangl, J. Luther, Modeling and interpretation of electrical impedance spectra of dye solar cells operated under opencircuit conditions, Electrochim. Acta 47 (26) (2002) 4213e4225.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
TiO2 hierarchical porous film constructed by ultrastable foams as photoanode for quantum dot-sensitized solar cells Xing Du a, Xuan He a, *, Lei Zhao a, Hui Chen a, Weixin Li a, Wei Fang a, Wanqiu Zhang a, Junjie Wang b, Huan Chen b a b
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, PR China Wuhan Iron and Steel (Group) Refractory Materials Limited Liability Company, Wuhan 430081, PR China
h i g h l i g h t s Foams are first used to introduce micron-size hierarchical pores into TiO2 film. Foam component-dodecanol is innovatively utilized to synthesize CdSe QDs. PCE values of 2.20% is achieved in QDSSC based on the TiO2 hierarchical porous film.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 August 2016 Received in revised form 13 September 2016 Accepted 19 September 2016
It reported a novel and simple method for the first time to prepare TiO2 hierarchical porous film (THPF) using ultrastable foams as a soft template to construct porous structures. Moreover, dodecanol as one foam component was creatively used as solvent during the synthesis of CdSe quantum dots (QDs) to decrease reaction temperature and simplify precipitation process. The result showed that hierarchical pores in scale of microns introduced by foams were regarded to benefit for high coverage and unimodal distribution of QDs on the surface of THPF to increase the efficiencies of light-harvesting, chargecollection and charge-transfer. The increased efficiencies caused an enhancement in quantum efficiency of the cell and thus remarkably increased the short circuit current density (Jsc). In addition, the decrease of charge recombination resulted in the increase of the open circuit voltage (Voc) as well. The QDSSC based on THPF exhibited about 2-fold higher power conversion efficiency (h ¼ 2.20%, Jsc ¼ 13.82 mA cm2, Voc ¼ 0.572 V) than that of TiO2 nanoparticles film (TNF) (h ¼ 1.06%, Jsc ¼ 6.70 mA cm2, Voc ¼ 0.505 V). It provided a basis to use foams both as soft template and carrier to realize simultaneously construction and in-situ sensitization of photoanode in further work. © 2016 Elsevier B.V. All rights reserved.
Keywords: Foams Hierarchical porous film Dodecanol CdSe quantum dots Sensitized solar cell
1. Introduction Quantum dot-sensitized solar cells (QDSSCs), as a viable alternative to dye-sensitized solar cells (DSSCs) have attracted considerable attention in recent years due to the unique properties of QD light-absorbers [1e5]. Although considerable efforts have been made to improve the performance of QDSSCs with some of tremendous achievements [6e9], the reported power conversion efficiency (PCE) of QDSSCs were far less than the theoretical value (66%) and lagged behind that of dye-sensitized solar cells (DSSCs)
* Corresponding author. Present address: No. 947, Heping Avenue, Qingshan District, Wuhan, Hubei Province, PR China. E-mail address: [email protected] (X. He). http://dx.doi.org/10.1016/j.jpowsour.2016.09.103 0378-7753/© 2016 Elsevier B.V. All rights reserved.
[7,10]. Clark has pointed out that low performance of QDSSCs was a design issue and not a fundamental one [11]. In fact, the configuration and preparation processing of photoanodes in QDSSCs was initially drawing on the experience of DSSCs. However, the conventional TiO2 nanoparticles film widely used as photoanode in DSSCs was not suitable for highly efficient QDSSCs because of the significant differences between QDs and dyes [12,13]. The sizes of QDs tethered onto photoanode were much larger than that of dyes [14,15], making a hard deposition of QDs into inner pores of nanostructured TiO2 film on account of blocked outer pores at first by QDs. It normally impeded further deposition of QDs and penetration of the electrolyte [16,17] to result in a low PCE. Therefore, the monotonous distribution of pore size of TiO2 nanostructured photoanode became the key limitation to get high efficiency.
2
X. Du et al. / Journal of Power Sources 332 (2016) 1e7
Furthermore, studies have pointed out that the direct growth of QDs on the TiO2 films via chemical bath deposition (CBD) or successive ion layer absorption and reaction technique (SILAR) resulted in a low PCE because of high density of surface defects and poor size distribution of QDs in recent years [18]. Recently, the development of QDSSCs indicates that the mainstream route to tether QDs onto TiO2 films transferred from direct growth approach to post synthesis assembly method due to the accessible high quality and high loading amount of QDs via ex-situ ligand exchange route [6,9,19e22]. Design and construct a suitable photoanode to match the post-synthesis assembly route was thus urgently needed for further improvement of PCE. Now, the most popular photoanode matched post synthesis assembly approach was TiO2 mesoporous films consisting of a transparent layer and a light scattering layer [6e9,20e23]. The main contribution of the scattering layer was to adjust the pore size distribution of TiO2 photoanode for QDs deposition and polysulfide electrolyte penetration. And on this basis M.Q. Wang et al. developed a simple mechanical mixing method which not only simplified the fabrication process but also maintained similar light scattering when compared with doublelayer TiO2 film [13]. Actually, foam ceramics possessed unique pore structures due to the presence of foams with hierarchical pore distribution [24]. Therefore, the hierarchical macropores can be easily introduced into TiO2 film and well maintained through adding foams into P25 slurry without destroying nanostructure pores formed by the stacking of P25. Furthermore, in fact, the solvent trioctylphosphine (TOP), traditionally used as the reaction medium to synthesize of CdSe QDs, is expensive, hazardous, unstable, and not an environmentally friendly [25]. The cheaper solvent paraffin liquid as reaction medium can significantly reduce the temperature and simplify the process of synthesis of the CdSe QDs [25,26]. However, the precipitation process for the CdSe QDs proved not to be an effective because paraffin liquid was insoluble in methanol [26], and the obtained QDs have been barely used in QDSSCs. Herein, this work presented a novel and simple method to fabricate TiO2 hierarchical porous film by introducing ultrastable foams [27] (Scheme 1) in P25 slurry. Besides, foam componentdodecanol, once utilized to synthesize Mn3O4 nanocrystals dots [28], was selected as solvent to prepare CdSe QDs. Finally, the post
synthesis assembly approach was adopted to deposit the QDs onto the TiO2 hierarchical porous film electrode. As shown in Scheme 1, the micron-size hierarchical pores introduced by foams could avoid the unfavorable clogging of pores by CdSe QDs during its deposition, resulting in uniform distribution of QDs and rapid penetration of the electrolyte throughout the TiO2 photoanode. The result showed the ultrastable foams could be successfully used as soft template to construct TiO2 hierarchical porous film. And the high-quality CdSe QDs were successfully synthesized by utilizing foam component-dodecanol which provided a possibility to use foam liquid film as carrier to introduce QDs. On this basis, the ultrastable foams is expected to be used not only as a soft template to construct hierarchical pore structures but also as carrier to introduce QDs for simultaneous in-situ sensitization of TiO2 film in the further work. 2. Experimental section 2.1. Materials and reagents Selenium powder (Se, 200 mesh, 99.99%) and cadmium oxide (CdO, 99.99%) were purchased from Sigma Aldrich. Dodecanol (DDA, 99.0%), oleic acid (OA, 90%) 3-Mercaptopropionic acid (MPA, 99%), sodium dodecyl sulfonate (SDS, 99%) and polyving akohol (PVA, alcoholysis degree: 99.8e100%) were purchased from Aladdin reagents (Shanghai) co., ltd.. P25 (a mixed phase of 80% anatase and 20% rutile; average size 25 nm) was purchased from Degussa (China) co., ltd.. All reagents were used without any further purification. 2.2. Preparation of the TiO2 hierarchical porous film constructed by ultrastable foams The foams was prepared according to the previous work [27] with minor modification. Typically, SDS (0.1 g), PVA (0.03 g) and DDA (0.1 g) were added into deionized water (2 mL) and then violently stirring to get ultrastable foams. For preparing the porous TiO2 films, P25 (1.2 g) was evenly dispersed in deionized (4 mL) water by magnetic stirrers for 20 min to get TiO2 slurry. The slurry was then equally divided into two portions. The prepared foams were added into to one portion to form TiO2 foam slurry. Then, the as-prepared TiO2 slurry and TiO2 foam slurry were coated on clean F:SnO2-coated (FTO, 14 U square1) glass substrates by doctor blade technique. Finally, all samples were sintered at 450 C for 30 min in a muffle furnace to construct TiO2 nanoparticle film (TNF) and TiO2 hierarchical porous film (THPF), respectively. 2.3. Synthesis of CdSe QDs using dodecanol as reaction solvent
Scheme 1. Schematic design of the TiO2 hierarchical porous film photoanode.
Firstly, CdO (2 mmol), OA (2 mL), DDA (8 mL) were added into a three-neck flask and heated to 150 C with constantly stirring. The color of CdO powder contained solution transformed to pale yellow from crimson when the uniform Cd precursor solution formed. Secondly, Se powder (0.8 mmol) and DDA (30 mL) were loaded into another three-neck flask and heated to 220 C with vigorous stirring to generate orange Se precursor solution. Thirdly, 8 mL of solution contained about 1.6 mmol of Cd precursor was quickly injected into Se precursor solution with rapidly stirring. The reaction system stayed at 220 C for 5 min before removing the heater and cooling to room temperature. Then, 38 mL chloroform used as extraction solvent were added to the reaction products to separate the nanocrystals from unreacted precursors. The obtained CdSe QDs were precipitated by adding methanol into the chloroform solution and further purified by repeated centrifugation and
X. Du et al. / Journal of Power Sources 332 (2016) 1e7
3
Fig. 1. Top-view SEM images of (a) TNF and (b) THPF at low magnification. Inset: Top-view SEM images of TNF and THPF at high magnification respectively.
decantation with addition of methanol and chloroform [19]. Finally, the purified CdSe QDs were dispersed into 60 mL chloroform to form CdSe QDs chloroform solution (containing 0.8 mmol of QDs). The MPA-capped CdSe QDs were prepared by a ligand exchange route according to the method reported by pervious literature [29,30].
DK240 monochrometer. The fluorescence lifetime study was carried out by Horiba Jobin Yvon Fluoromax-4. Electrochemical impedance spectroscopy (EIS) measurements of cells were measured using the electrochemical workstation with 0.5 V bias potential and 10 mV of amplitude under dark condition, and the frequency range was set from 0.1 Hz to 100 kHz.
2.4. Fabrication of CdSe QDs-sensitized TiO2 photoanodes 3. Results and discussion After immersed in the as-prepared MPA-capped CdSe QDs aqueous dispersion for 2 h and sequentially rinsed with water and ethanol and then dried with nitrogen, TNF and THPF were coated with CdSe QDs, H2PtCl4 ethanol solution (8 mM) was deposited on FTO glass by spin-coating and then heat treated at 400 C for15 min [23] to prepare the Pt counter electrode,. The polysulfide electrolyte solution was composed of Na2S (2.0 M), S (2.0 M), and KCl (0.2 M) in methanol/deionized water (3:7, v/v) solution. The cells were constructed by sandwiching the QDs-sensitized TiO2 photoanodes and Pt electrode using a 50 mm-thickness 3M adhesive tape as a spacer to separate the two electrodes and permeating with a 10 mL of polysulfide electrolyte. 2.5. Characterization Transition electron microscopy (TEM) image was taken using a JEOL JEM-2100UHR high-resolution transmission electron microscope (HRTEM). UV-Vis absorption and steady state photoluminescence (PL) emission spectra were obtained on a Shimadzu UV-2550 UV-Vis spectrophotometer and a Shimadzu RF-6000 fluorescence spectrophotometer, respectively. An X-ray diffraction spectrum was recorded with a Philips X'Pert Pro Xray powder diffractometer with Cu-Ka radiation (1.5406 Å). Scanning electron microscopy (SEM) was performed using a FEI Nova 400 Nano SEM system equipped with an energy dispersive spectroscopy (EDS). Brunauer - Emmett - Teller (BET) surface area was evaluated by using a Quantachrome Autosorb-1MP system. The specific surface area was calculated by the singlepoint BET method. The BET measurement was detected for times for each sample to evaluate the specific surface area by averaged value. Current-voltage characteristics (J-V curves) of cells were recorded on a Zahner Elektrik IM6 electrochemical workstation under illumination of AM 1.5 G solar simulator (Newport 91160). The power of the simulated light was calibrated to 100 mW cm2 by a NREL standard Si solar cell. The photoactive area was 0.25 cm2. Incident photon-to-current conversion efficiency (IPCE) spectra were obtained in the range of 300e750 nm by a Keithley 2000 multimeter under the illumination of a 300 W tungsten lamp with a Spectral Product
The microstructure morphology of the TNF and THPF are shown in Fig. 1. It observed that the addition of foams gave rise to the presence of micron-size hierarchical pores in TiO2 films. The average specific surface areas of TNF and THPF were 50.407 and 51.678 m2 g1, respectively, which indicates that the added foams did not destroy the nanostructure pores formed by P25. It illustrated that the hierarchical pore structure (macro-micro-nano) was successfully constructed by the ultrastable foams and P25. Fig. 2(a) presented PL emission and UV-Vis absorption spectra of CdSe QDs colloidal solution. The UV-Vis absorption band and PL emission peak respectively located at 565 nm and 580 nm indicated that the obtained CdSe QDs can appropriately be used as sensitizer for QDSSCs. And the TEM and HRTEM images of the CdSe QDs revealed that the size of QDs was almost monotonous distributed and all particles was uniformly shaped in spherical as shown in Fig. 2(b). The well-distinguished lattice fringes of CdSe QDs (the inset in Fig. 2(b)) suggested their good crystallinity. The XRD pattern of CdSe QDs was presented in Fig. 2(c) and the three diffraction features appeared at about 25.5 , 42.4 , and 50.0 , corresponding to (111), (220), and (311) planes of the cubic zinc-blende phase of CdSe, respectively [26]. Thus it can be concluded that the dodecanol could be successfully used as reaction solvent to synthesize CdSe QDs which could effectively reduce the reaction temperature and simplify the synthesis and precipitation process. In addition, as the foam formation, dodecanol is conduce to foam stability, which can provided a possibility to use foam liquid film as carrier to introduce QDs to realize in-situ sensitization of TiO2 film in the further work. According to the growth mechanism of forming CdSe nanocrystals in the paraffin liquid reported by Tang et al. [25], the possible chemical reactions involved in the formation of CdSe QDs using DDA and OA as the reaction medium are shown in Equations (1)e(3).
CdO þ Oleic acid / Cd oleate D
(1)
4
X. Du et al. / Journal of Power Sources 332 (2016) 1e7
Fig. 2. (a) UV-Vis absorption (magenta line) and PL emission (green line, lex ¼ 400 nm) spectra (b) TEM and HRTEM (inset in panel b) and (c) X-ray powder diffraction pattern of the CdSe QDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. UV-Vis absorption spectra of CdSe QDs sensitized TNF and THPF. Inset: photographs of TNF and THPF after being sensitized.
5Se þ 2CH3 ðCH2 Þ10 CH2 OH
dehydrogenation 2CH3 ðCH2 CH ¼ CHCH2 Þ2:5 OH / D
þ 5H2 Se (2) Cd oleate þ H2 Se / CdSe nuclei þ byproducts D
(3)
Fig. 5. The excited state electron radiative decay of the TNF and THPF films loaded with CdSe QDs.
The UV-Vis absorption spectra of TNF and THPF after sensitized by the MPA-capped CdSe QDs were presented in Fig. 3. In case of THPF, change tendency of profile and sharp excitonic feature of absorption curve was reserved comparing to that in Fig. 2(a). It reflected that the particle size and size distribution of the deposited QDs on THPF remained their original state. However, the sharp excitonic feature of TNF disappeared because of the aggregation of QDs in some extent when deposited on TNF that pores of nanostructured films was easily clogged by QDs to impede the further deposition. In this regards, it pointed out that porous structure of
Fig. 4. SEM images of a typical cross-section of QD sensitized (a) TNF and (b) THPF. (c) Cd/Ti ratio along the cross section of TNF and THPF.
X. Du et al. / Journal of Power Sources 332 (2016) 1e7
5
Fig. 6. (a) Nyquist plot and (b) Bode plot curves of the cells based on TNF and THPF photoanodes at 0.5 V forward bias. Inset: equivalent circuit adopted for fitting the EIS data obtained.
THPF was more suitable for QDs deposition than that of TNF due to the introduced micron-size hierarchical pores, which was demonstrated by the absorption intensity and the colors of sensitized films in the inset of Fig. 3 as well. In order to investigate if porous structure of THPF was more suitable for the deposition of QDs, the coverage of QDs along the film thickness was verified by elemental analysis. The elemental distribution depending on depth in cross-section of the sensitized TNF and THPF were measured by EDS equipped on SEM. As presented in Fig. 4(c), the Cd/Ti ratio (20%) throughout the total THPF cross-section maintained nearly constant indicating homogeneous distribution of QDs on the sensitized THPF. The Cd/Ti ratio of sensitized TNF was dropped dramatically on depth while the value of Cd/Ti ratio was much higher adjacent the surface than that in deep side. Additionally, the Cd/Ti ratio on THPF was apparently higher than that on TNF because of higher content of QDs on THPF and more TiO2 nanoparticles on TNF. It suggested that the pores close to the surface of TNF were probably blocked by QDs to make them difficult to diffuse into the inner pores while the porous structure of THPF was favorable for the deposition of QDs at the same condition. The excited state electron radiative decay of the TNF and THPF films loaded with CdSe QDs were shown in Fig. 5. Shorter emission decay of the CdSe QDs on THPF photoanode than that on TNF photoanode indicated that the PL lifetime of QDs was shortened when deposited on the THPF photoanode. It supposed that the porous structure of THPF may play an important role on it. Acturally, the structural feature of THPF was benefit for the deposition of QDs without aggregation while QDs were aggregated in some extent when deposited on TNF. Since the (conduction band energy) Ecb of smaller QDs is higher than that of bigger ones [15], the driving force of the electrons from QDs to TiO2 in the case of THPF was enhanced which also resulted in the increase of the injection kinetics and displayed short-lived excited state of QDs on THPF as in Fig. 5 [4,31]. EIS was applied to elucidate charge recombination processes in the cells. Fig. 6(a) displayed Nyquist curves of cells based on TNF and THPF photoanodes in the dark under a forward bias of 0.5 V. The semicircles in the high frequency region reflected the electron injection at the counter electrode/electrolyte interface and transport in the electrolyte (R1) while the Table 1 Fitted impedance values at 0.5 V forward bias for TNF and THPF photoanodes. Electrode
R 1 ( U)
R2 (U)
tn (ms)
TNF THPF
58.4 80.2
107.6 174.8
28.3 41.6
semicircles in the low frequency region reflected electron transfer at the TiO2/QDs/electrolyte interface and transport in the TiO2 film (R2) [32]. The equivalent circuit was shown in the inset in Fig. 6(a) and the fitting results were listed in Table 1. In general, R2 was mainly determined by the recombination resistance of electrons. A higher value indicated less back electron transfer at the TiO2/QDs/electrolyte interface, that meant the suppressed interfacial charge recombination [33,34]. The results showed that the value of Rct of the cell based on THPF (174.8 U) was higher than that of the cell based on TNF (107.6 U), which resulted in the decrease of charge recombination. The increase of R2 can be attributed to the decreased contact area of TiO2 with electrolyte [32]. The poor QDs loading on TNF caused the direct exposure of TiO2 to the electrolyte. On the contrary, hierarchical porous structure resulted in high content of QDs deposited on the surface of TiO2 and was favored for the permeation of the electrolyte. The charge recombination between the injected electrons in the conduction band of TiO2 and the oxidized ions (S2 n ) in the electrolyte was consequently suppressed. Fig. 6(b) showed the Bode plots of the cells, and the curve peak of the spectrum can be used to determine the electron lifetime according to Equation (4) [35].
tn ¼ 1
. 2pfpeak
(4)
The corresponding results were listed in Table 1. It observed that the electron lifetime of the cell based on THPF photoanode is 41.6 ms, which was much higher than that of the cell based on TNF photoanode (28.3 ms). Consequently, the THPF photoanode can enhance the charge recombination resistance and thus prolong the electron lifetime. The IPCE of cells were measured and results were shown in Fig. 7. The maximum value of IPCE of CdSe QDSSC based on THPF photoanode reached 74% at 595 nm, which was enhanced by ~64% compared to the maximum IPCE of 45% at 580 nm for CdSe QDSSC based on TNF photoanode. The IPCE is calculated by equation (5), where LHE is the light-harvesting efficiency, hct is charge-transfer efficiency, and hcc is charge-collection efficiency [4].
IPCE ¼ LHEhct hcc
(5)
The LHE of the solar cell is closely related to the absorbance of the photoelectrode [4]. As shown in Fig. 3, the absorbance of CdSe QDs sensitized THPF film was higher than that of CdSe QDs sensitized TNF film, which indicated that the LHE of cell based on THPF was higher than that of cell based on TNF accordingly. The hct can be deduced by the excited state electron radiative decay of QDs, and
6
X. Du et al. / Journal of Power Sources 332 (2016) 1e7 Table 2 Average photovoltaic parameters obtained from the J-V curves of the QDSSCs based on TNF and THPF photoanodes.
Fig. 7. IPCE curves of CdSe QDSSCs constructed using TNF and THPF photoanodes.
the increase of electron injection kinetics was helpful for improving hct [4]. Thus, the hct of cell based on THPF was higher than that of TNF due to the increase of electron injection kinetics (Fig. 5). The hct was mainly contributed by charge recombination [4]. EIS results (Fig. 6 and Table 1) revealed that the THPF photoanode can enhance the charge recombination resistance and prolong the electron lifetimeto cause an improvement in charge-collection efficiency (hcc). As a result, it can be concluded that the increased lightharvesting, charge-transfer and charge-collection efficiencies can efficiently improve the IPCE of cells. To compare the photovoltaic performance of as-prepared TNF and THPF photoanodes, sandwich-type thin layer cells were fabricated with Pt-coated FTO as the counter electrode with polysulfide electrolyte as hole transporter. Fig. 8 showed the J-V curves corresponding to cell devices based on different photoanodes. The open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and PCE (h) of those QDSSCs were summarized in Table 2. As expected, the QDSSC based on TNF photoanode exhibited a Jsc of 6.70 mA cm2, Voc of 0.505 V and h of 1.06%, whereas the QDSSC based on THPF photoanode achieved a Jsc of 13.82 mA cm2, Voc of 0.572 V and h of 2.20%, manifesting 107% improvement of PCE compared to that of the cells based on TNF photoanode. It was mainly attributed to the significant increase of short-circuit current
Electrode
Jsc (mA cm2)
Voc(V)
FF
h (%)
TNF THPF
6.70 13.82
0.505 0.572
0.313 0.278
1.06 2.20
(Jsc) of the cell based on THPF photoanode. The remarkable increase of Jsc was attributed to the excited charge characteristics, such as generation, injection or collection [4]. As mentioned above, the hierarchical pores in THPF facilitating the penetration of QDs improved the light-harvesting capability of photoanode and charge-transfer efficiency. The high content of QDs deposited on the surface of TiO2 and the rapid permeation of the electrolyte suppressed the charge recombination at TiO2/QDs/electrolyte interface and prolonged the electron lifetime, led to an enhancement in charge-collection efficiency. As a result, the increase of light-harvesting, charge-transfer and charge collection gave rise to the improvement of the IPCE of the solar cell, resulting in enhanced Jsc [4]. In addition, the suppressed charge recombination could result in the increase of Voc as well. 4. Conclusions In summary, TiO2 hierarchical porous film was successfully fabricated using the ultrastable foams in this work. The highquality zinc-blende (cubic) CdSe QDs were successfully synthesized utilizing foam component-dodecanol. The hierarchical pores in micron-size introduced by foams can benefit for high coverage of QDs on the surface of TiO2, maintaining unimodal distribution of QDs size and accelerating the penetration of the electrolyte throughout the TiO2 photoanode. High coverage of QDs on the surface of TiO2 not only improved the light-harvesting capability of photoanode but also decreased the direct exposure of TiO2 to electrolyte to enhance charge-collection efficiency. The maintained unimodal distribution of QDs size can increase the injection kinetics to improve the charge-transfer efficiency. The increase of the efficiencies of light-harvesting, charge-collection and chargetransfer were contributed to enhance the external quantum efficiency (IPCE) of the cell, resulted in remarkable increase of Jsc. In addition, the decrease of charge recombination resulted in the increase of Voc as well. As a result, the cell based on TiO2 hierarchical porous film photoanode achieved a PCE of 2.20%, which was obviously higher than that of the cell based on TiO2 nanoparticle film photoanode (1.06%). The results offered an evidence that ultrastable foams can be used as soft templates to construct hierarchical pore structures of TiO2 photoanode, and dodecanol as reaction solvent to synthesize CdSe QDs. This paves the way for the further work to use foams both as soft template and carrier to simultaneously realize the construction and in-situ sensitization of photoanode. Acknowledgements The work was financially supported by the China Postdoctoral Science Foundation (2015M572210), the National Natural Science Foundation of China (61604110), the Hubei Provincial Natural Science Foundation of China (ZRMS2016000349) and the Foundation of Wuhan University of Science and Technology (2016XZ002). References
Fig. 8. J-V curves of CdSe QDSSCs constructed using TNF and THPF photoanodes.
[1] P.V. Kamat, J.A. Christians, J.G. Radich, Quantum dot solar cells: hole transfer as a limiting factor in boosting the photoconversion efficiency, Langmuir 30
X. Du et al. / Journal of Power Sources 332 (2016) 1e7 (20) (2014) 5716e5725. [2] B. Liu, Y. Sun, X. Wang, L. Zhang, D. Wang, Z. Fu, Y. Lin, T. Xie, Branched hierarchical photoanode of anatase TiO2 nanotubes on rutile TiO2 nanorod arrays for efficient quantum dot-sensitized solar cells, J. Mater. Chem. A 3 (8) (2015) 4445e4452. [3] K. Meng, P.K. Surolia, O. Byrne, K.R. Thampi, Efficient CdS quantum dot sensitized solar cells made using novel Cu2S counter electrode, J. Power Sources 248 (2014) 218e223. [4] J. Tian, L. Lv, C. Fei, Y. Wang, X. Liu, G. Cao, A highly efficient (>6%) Cd1xMnxSe quantum dot sensitized solar cell, J. Mater. Chem. A 2 (46) (2014) 19653e19659. [5] A. Sahasrabudhe, S. Bhattacharyya, Dual sensitization strategy for highperformance core/shell/quasi-shell quantum dot solar cells, Chem. Mater. 27 (13) (2015) 4848e4859. [6] G. Wang, H. Wei, Y. Luo, H. Wu, D. Li, X. Zhong, Q. Meng, A strategy to boost the cell performance of CdSexTe1x quantum dot sensitized solar cells over 8% by introducing Mn modified CdSe coating layer, J. Power Sources 302 (2016) 266e273. [7] J. Du, Z. Du, J.-S. Hu, Z. Pan, Q. Shen, J. Sun, D. Long, H. Dong, L. Sun, X. Zhong, ZneCueIneSe quantum dot solar cells with a certified power conversion efficiency of 11.6%, J. Am. Chem. Soc. 138 (12) (2016) 4201e4209. [8] K. Zhao, Z. Pan, I. Mora-Sero, E. Canovas, H. Wang, Y. Song, X. Gong, J. Wang, M. Bonn, J. Bisquert, X. Zhong, Boosting power conversion efficiencies of quantum-dot-sensitized solar cells beyond 8% by recombination control, J. Am. Chem. Soc. 137 (16) (2015) 5602e5609. [9] Z. Ren, J. Wang, Z. Pan, K. Zhao, H. Zhang, Y. Li, Y. Zhao, I. Mora-Sero, J. Bisquert, X. Zhong, Amorphous TiO2 buffer layer boosts efficiency of quantum dot sensitized solar cells to over 9%, Chem. Mater. 27 (24) (2015) 8398e8405. [10] S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B.F. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M.K. Nazeeruddin, M. Gratzel, Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers, Nat. Chem. 6 (3) (2014) 242e247. [11] J.T. Margraf, A. Ruland, V. Sgobba, D.M. Guldi, T. Clark, Quantum-dot-sensitized solar cells: understanding linker molecules through theory and experiment, Langmuir 29 (7) (2013) 2434e2438. [12] Z. Du, H. Zhang, H. Bao, X. Zhong, Optimization of TiO2 photoanode films for highly efficient quantum dot-sensitized solar cells, J. Mater. Chem. A 2 (32) (2014) 13033. [13] X. Song, M. Wang, T. Xing, J. Deng, J. Ding, Z. Yang, X. Zhang, Fabrication of micro/nano-composite porous TiO2 electrodes for quantum dot-sensitized solar cells, J. Power Sources 253 (2014) 17e26. [14] Q. Zhang, X. Guo, X. Huang, S. Huang, D. Li, Y. Luo, Q. Shen, T. Toyoda, Q. Meng, Highly efficient CdS/CdSe-sensitized solar cells controlled by the structural properties of compact porous TiO2 photoelectrodes, Phys. Chem. Chem. Phys. 13 (10) (2011) 4659e4667. [15] P.V. Kamat, Quantum dot solar cells. Semiconductor nanocrystals as light harvesters, J. Phys. Chem. C 112 (48) (2008) 18737e18753. , J. Bisquert, R. Go mez, CdSe [16] N. Guijarro, T. Lana-Villarreal, I. Mora-Sero quantum dot-sensitized TiO2 electrodes: effect of quantum dot coverage and mode of attachment, J. Phys. Chem. C 113 (10) (2009) 4208e4214. nez, P.P. Boix, Q. Shen, M.E. Calvo, N. Taghavinia, [17] M. Samadpour, S. Gime , Effect of nanostructured electrode A.I. Zad, T. Toyoda, H. Míguez, I. Mora-Sero architecture and semiconductor deposition strategy on the photovoltaic performance of quantum dot sensitized solar cells, Electrochim. Acta 75
7
(2012) 139e147. [18] B. Liu, D. Wang, L. Wang, Y. Sun, Y. Lin, X. Zhang, T. Xie, Glutathione-assisted hydrothermal synthesis of CdS-decorated TiO2 nanorod arrays for quantum dot-sensitized solar cells, Electrochim. Acta 113 (2013) 661e667. [19] H. Zhang, K. Cheng, Y.M. Hou, Z. Fang, Z.X. Pan, W.J. Wu, J.L. Hua, X.H. Zhong, Efficient CdSe quantum dot-sensitized solar cells prepared by a postsynthesis assembly approach, Chem. Commun. 48 (91) (2012) 11235e11237. [20] H. Wei, G. Wang, Y. Luo, D. Li, Q. Meng, Investigation on interfacial charge transfer process in CdSexTe1-x alloyed quantum dot sensitized solar cells, Electrochim. Acta 173 (2015) 156e163. [21] S. Jiao, Q. Shen, I. Morasero, J. Wang, Z. Pan, K. Zhao, Y. Kuga, X. Zhong, J. Bisquert, Band engineering in core/shell ZnTe/CdSe for photovoltage and efficiency enhancement in Exciplex quantum dot sensitized solar cells, Acs Nano 9 (1) (2015) 908e915. [22] Z. Pan, I. Mora-Sero, Q. Shen, H. Zhang, Y. Li, K. Zhao, J. Wang, X. Zhong, J. Bisquert, High-efficiency “green” quantum dot solar cells, J. Am. Chem. Soc. 136 (25) (2014) 9203e9210. [23] K. Zhao, H. Yu, H. Zhang, X. Zhong, Electroplating cuprous sulfide counter electrode for high-efficiency long-term stability quantum dot sensitized solar cells, J. Phys. Chem. C 118 (11) (2014) 5683e5690. [24] F. Krauss Juillerat, U.T. Gonzenbach, A.R. Studart, L.J. Gauckler, Self-setting particle-stabilized foams with hierarchical pore structures, Mater. Lett. 64 (13) (2010) 1468e1470. [25] Z. Deng, L. Cao, F. Tang, B. Zou, A new route to zinc-blende CdSe nanocrystals: mechanism and synthesis, J. Phys. Chem. B 109 (35) (2005) 16671e16675. [26] B. Xing, W. Li, H. Dou, P. Zhang, K. Sun, Systematic study of the properties of CdSe quantum dots synthesized in paraffin liquid with potential application in multiplexed bioassays, J. Phys. Chem. C 112 (37) (2008) 14318e14323. [27] X. Du, L. Zhao, H. Xiao, F. Liang, H. Chen, X. Wang, J. Wang, W. Qu, Z. Lei, Stability and shear thixotropy of multilayered film foam, Colloid. Polym. Sci. 292 (10) (2014) 2745e2751. [28] P. Li, C. Nan, Z. Wei, J. Lu, Q. Peng, Y. Li, Mn3O4 nanocrystals: facile synthesis, controlled assembly, and application, Chem. Mater. 22 (14) (2010) 4232e4236. [29] L. Liu, X. Guo, Y. Li, X. Zhong, Bifunctional multidentate ligand modified highly stable water-soluble quantum dots, Inorg. Chem. 49 (8) (2010) 3768e3775. [30] Z. Pan, K. Zhao, J. Wang, H. Zhang, Y. Feng, X. Zhong, Near infrared absorption of CdSexTe1ex alloyed quantum dot sensitized solar cells with more than 6% efficiency and high stability, ACS Nano 7 (6) (2013) 5215e5222. [31] P.R.F. Barnes, A.Y. Anderson, S.E. Koops, J.R. Durrant, B.C. O'Regan, Electron injection efficiency and diffusion length in dye-sensitised solar cells derived from incident photon conversion efficiency measurements, J. Phys. Chem. C 113 (113) (2009) 1126e1136. [32] R. Zhou, Q. Zhang, E. Uchaker, J. Lan, M. Yin, G. Cao, Mesoporous TiO2 beads for high efficiency CdS/CdSe quantum dot co-sensitized solar cells, J. Mater. Chem. A 2 (8) (2014) 2517. [33] T. Shu, P. Xiang, Z.-M. Zhou, H. Wang, G.-H. Liu, H.-W. Han, Y.-D. Zhao, Mesoscopic nitrogen-doped TiO2 spheres for quantum dot-sensitized solar cells, Electrochim. Acta 68 (2012) 166e171. [34] J. Tian, Q. Zhang, L. Zhang, R. Gao, L. Shen, S. Zhang, X. Qu, G. Cao, ZnO/TiO2 nanocable structured photoelectrodes for CdS/CdSe quantum dot cosensitized solar cells, Nanoscale 5 (3) (2013) 936e943. [35] R. Kern, R. Sastrawan, J. Ferber, R. Stangl, J. Luther, Modeling and interpretation of electrical impedance spectra of dye solar cells operated under opencircuit conditions, Electrochim. Acta 47 (26) (2002) 4213e4225.