Hierarchical TiO2 Submicrorods Improve the Photovoltaic

Dec 29, 2016 - Composite photoanodes in dye-sensitized solar cells (DSSCs) were prepared by ... Solar Energy Materials and Solar Cells 2018 183, 193-1...
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Research Article pubs.acs.org/journal/ascecg

Hierarchical TiO2 Submicrorods Improve the Photovoltaic Performance of Dye-Sensitized Solar Cells Daipeng Guo, Shengqiang Xiao, Ke Fan,* and Jiaguo Yu*

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State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, P. R. China ABSTRACT: Hierarchical TiO2 submicrorods (HTRs) assembled from tiny nanoparticles and nanorods were synthesized through a facile hydrothermal method using titanate glycolate rods as a self-template. The as-prepared hierarchical TiO2 submicrorods possessed a higher surface area (103 m2 g−1) than P25 nanoparticles (NPs) (55 m2 g−1). Composite photoanodes in dye-sensitized solar cells (DSSCs) were prepared by integrating the prepared HTRs and P25 NPs, and a photovoltaic conversion efficiency of 8.09% was obtained, which was obviously higher than that of the pristine P25 NPs-based photoanode (5.37%). The incorporated hierarchical TiO2 submicrorods in the hybrid photoanode film showed three functions in the enhancing photovoltaic performance of DSSCs: (i) increasing the specific surface area for effective adsorption of dye molecules, (ii) enhancing the light harvesting efficiency, and (iii) accelerating the electron transport by the films. This work highlights the importance of tuning the structure of the photoanode and exhibits an efficient strategy for enhanced energy conversion of DSSCs. KEYWORDS: TiO2, Hierarchical submicrorods, Composite photoanode, Dye-sensitized solar cells, Photovoltaic performance

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INTRODUCTION Dye-sensitized solar cells (DSSCs), as one of the candidates to substitute silicon-based solar cells, are very promising in thirdgeneration photovoltaic devices because of their easy fabrication, low cost, stability, and relatively high energy conversion efficiency.1 Normally, a DSSC contains three main components: a dye-adsorbed nanocrystalline titanium oxide photoanode, an I−/I3− electrolyte solution, and a platinized (Pt) counter electrode.2,3 The principle of DSSCs can be expressed as follows: after light absorption, the dye molecules generate photoinduced electrons, and then these electrons are injected into the conduction band (CB) of TiO2 and are collected by fluorine-doped tin oxide (FTO) glass, after which the electrons move through the external circuit to the Pt counter electrode. The I− in the electrolyte solution is transformed to I3− to regenerate the excited dye sensitizers, while I3− ions are reduced to I− by the electrons at the Pt counter electrode.4,5 Typically, TiO2 nanoparticles (NPs), which possess a high specific surface area for adsorbing dye molecules, are widely applied in the photoanodes of DSSCs. However, the scattering of visible light by such photoanodes based on TiO2 NPs is usually weak because of the small particle size (10−50 nm), and a significant part of the input light is directly transmitted through the TiO2 NP film without being harvested and utilized.6 Additionally, the electron traps existing in the grain boundaries between nanosized particles lead to a low electron transfer rate and serious charge recombination losses in the film of TiO2 NPs, which limit the efficiency of DSSCs.7,8 To © 2016 American Chemical Society

enhance the efficiency of DSSCs, most of the state-of-the-art photoanodes are based on the development of various advanced electrode materials with multiple advantages such as an outstanding light-scattering effect, larger amounts of adsorbed dye, porous structures, and/or fast electron transfer capability.9−19 Among these materials for photoanodes, onedimensional (1D) nanomaterials, e.g., nanowires,20,21 nanorods,22,23 nanofibers,24,25 and nanotubes,26,27 become more and more popular in DSSCs because of their enhanced light scattering and fast electron transfer capability. For instance, Wang et al.23 prepared a DSSC using TiO2 nanorods as an overlayer of the photoanode, which showed strong light scattering ability and supplied a fast and lengthened charge carrier transport channel, resulting in dramatically enhanced photovoltaic conversion efficiency. However, the 1D nanostructured materials (mainly nanowires and nanorods) usually have small specific surface areas, leading to insufficient dye loading compared with nanoparticles.28−30 For example, Bai et al.28 introduced ZnO nanowires into TiO2 nanoparticles to improve the photovoltaic conversion efficiency, but this resulted in decreased specific surface area and lower dye loading because the size of the ZnO nanowires was much larger. At present, the preservation of 1D structure without sacrificing the surface area of the photoanode of DSSCs is still challenging. Fortunately, materials with hierarchical structure, Received: July 18, 2016 Revised: December 23, 2016 Published: December 29, 2016 1315

DOI: 10.1021/acssuschemeng.6b01671 ACS Sustainable Chem. Eng. 2017, 5, 1315−1321

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°C for 12 h. The excess dye molecules of the dye-sensitized composite photoanode were then removed by anhydrous ethanol. Finally, the open solar cell was prepared by placing a Pt-coated FTO glass on the sensitized photoanode separated by a ca. 50 μm thick polymer spacer, and the active area of the cell was 0.14 cm2. The electrolyte was then injected into the open cell, and the photovoltaic performance was immediately measured. Characterization. The morphologies of the as-prepared HTRs and NP/HTR composite film were observed by field-emission scanning electron microscopy (FESEM) on a JEOL JSM-7500 microscope and SEM on a JEOL JSM-5610LV microscope. Transmission electron microscopy (TEM) images were observed through a JEM-2100F microscope. X-ray diffraction (XRD) results were obtained using a Rigaku D/Max-RB X-ray diffractometer with Cu Kα irradiation. Nitrogen isotherms were obtained by Brunauer− Emmett−Teller (BET) measurements on a Micromeritics ASAP 2020 nitrogen adsorption apparatus. UV−vis diffuse reflectance experiments were performed using a Shimadzu UV-2600 spectrophotometer. To determine the adsorbed dye loading on photoanodes, the dye-sensitized photoanodes were immersed in a 0.1 M NaOH water/ ethanol solution (1:1 v/v) at room temperature for 1 h, and then the absorbance A was obtained on a Shimadzu UVmini-1240 spectrophotometer. The obtained solution was analyzed by UV−vis spectroscopy to determine the dye concentration c using Beer’s law (A = εcl, where A is the absorbance at the wavelength of 515 nm, l is the length of the cell, and ε is the N719 absorptivity (1.41 × 104 dm3 mol−1 cm−1)).35 The current density−voltage (J−V) curves and electrochemical impedance spectra with a 10 mV AC amplitude were all measured on a CHI660C electrochemical workstation. All of the cells were characterized under 100 mW cm−2 irradiation with a Newport 91160 solar simulator. The incident photoelectric conversion efficiency (IPCE) data were obtained using a 300 W Xe lamp (model 6258) fitted with a monochromator (Newport, Irvine, CA, USA).

in which building blocks, e.g., nanoparticles (0D), nanofibers, nanorods, nanowires, and nanotubes (1D), and nanosheets (2D), self-assemble into highly ordered structures with various morphologies, have attracted much attention in DSSCs because of their porosity and high specific surface area.31−33 The high surface area is beneficial for adsorbing dye molecules, and the porous structure provides an accessible diffusion pathway for the electrolyte. Li et al.31 prepared a three-dimensional hierarchical TiO2 microsphere structure assembled from numerous nanorods and found that DSSCs based on this hierarchical microstructure showed greatly enhanced photovoltaic conversion efficiency due to a higher specific surface area for effective adsorption of dyes and enhanced light scattering. On the basis of the above considerations, we successfully prepared a novel 1D anatase hierarchical TiO2 submicrorod (HTR) with a high specific surface area (103 m2 g−1) through a facile hydrothermal method. The HTRs assembled from numerous tiny nanorods and nanoparticles were mixed with P25 NPs to fabricate composite photoanodes to enhance the photovoltaic conversion efficiency of DSSCs. Because of the increased amount of adsorbed dye, enhanced light harvesting, and facilitated charge transfer derived from the incorporated HTRs, the composite photoanode containing 20 wt % HTRs showed the highest photovoltaic conversion efficiency of 8.09%, which is far superior to that of the pristine P25 NP-based photoanode. Our work was proven to be an efficient and promising approach for improving the photovoltaic efficiency of DSSCs.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Phase Structure and Morphology. The crystalline features of the obtained samples were determined by XRD (as shown in Figure 1). As noted, all of the peaks in the XRD

Materials. Tetrabutyl titanate (TBT, 98%, analytical reagent), ethylene glycol, sodium hydroxide (NaOH, analytical reagent), ethanol, and poly(ethylene glycol) (PEG 2000) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ruthenium dye (N719) was bought from Solaronix. Lithium iodide (LiI, 99.99%), iodine (I2, 99.99%), and tert-butylpyridine were obtained from Sigma-Aldrich. 1Propyl-3-methylimidazolium iodide was obtained from Suzhou Zhongsheng Chemical Co., Ltd., and anhydrous acetonitrile was purchased from Shanghai Chemical Reagent Factory of China. P25 is commercial-grade TiO2 powder (Degussa AG, 80% anatase and 20% rutile). Synthesis of HTRs. The HTRs were prepared as in our previous work.34 Typically, TBT (2 mL) and ethylene glycol (200 mL) were heated at 150 °C in an oil bath for 2 h. After being washed with distilled water several times, the precipitates were dried at 80 °C for 6 h. Subsequently, 0.2 g of the dried titanium glycolate precursor was put into 40 mL of 0.1 M NaOH aqueous solution, and then the stirred solution was moved to a 50 mL Teflon-lined autoclave. The autoclave was sealed and kept at 180 °C for 24 h. The resulting white precipitates were centrifuged and washed with 0.1 M HCl aqueous solution and distilled water until the pH reached 7. Finally, the precipitates were heated at 80 °C for 6 h and calcined at 450 °C for 1 h for further characterization. Assembly of NP/HTR Composite Film DSSCs. The Nippon FTO glass (14−22 Ω/square) was washed ultrasonically with surfactant, ethanol, and acetone to obtain a clean surface. Five pastes containing different HTR contents were obtained by grinding mixtures of ethanol, PEG, and mixed commercial P25 NPs and HTRs for ca. 40 min. Through the doctor blade method, the pastes were then deposited on FTO glass to form the NP/HTR composite films. After that, the films were calcined at 450 °C for 30 min. The as-obtained NP/HTR composite films are denoted as P100R0, P90R10, P80R20, P50R50, and P0R100 on the basis of the fraction of HTRs in the mixture (0, 10, 20, 50, and 100 wt %). Dye sensitization was done by dipping the composite films in 0.3 mM N719 ethanol solution at 50

Figure 1. XRD patterns of (a) pure HTR powders, (b) the P80R20 film, (c) the P100R0 film, and (d) the FTO glass substrate.

pattern of the pure HTR powders (Figure 1a) can be assigned to the anatase TiO2 phase (JCPDS no. 21-1272), and no impurities can be detected. Figure 1b,c shows the XRD patterns of P80R20 and P100R0 films on FTO glass, respectively. There is no evident difference between the XRD patterns of these two films except that the intensity of the (110) peak at 2θ = 27.4° 1316

DOI: 10.1021/acssuschemeng.6b01671 ACS Sustainable Chem. Eng. 2017, 5, 1315−1321

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ACS Sustainable Chemistry & Engineering (which belongs to the rutile TiO2 phase in P25, JCPDS no. 211276) for P80R20 is slightly weaker than that for P100R0. This can be ascribed to the relatively lower content of P25 in P80R20 than in P100R0, leading to a lower content of the rutile TiO2 phase in the P80R20 film. The SEM image of the as-prepared HTRs (Figure 2a) demonstrates that the surface of the HTRs is very rough, the

Figure 3. (a, b) Cross-sectional SEM images of the NP/HTR composite film (P50R50) and (c) a schematic illustration of the NP/ HTR composite film structure.

P100R0, P80R20, and P0R100 as examples, Figure 4a shows their nitrogen isotherms and (inset) pore size distribution curves. All of the nitrogen isotherms for the three samples belong to type IV (Brunauer−Deming−Deming−Teller (BDDT) classification),36,37 showing the existence of a mesoporous structure. The H3-type hysteresis loops imply slitlike pores derived from nanoparticle gathering.38 Furthermore, compared with P100R0 and P0R100, P80R20 shows a bimodal pore structure (∼7.6 and ∼49 nm mesopores), which can be ascribed to the small inner pores of the additive HTRs and the large pores constructed by P25 NPs, respectively. These porous structures could facilitate the permeation of liquid electrolyte into the film of the composite photoanode. Moreover, as listed in Table 1, the BET surface areas (SBET) of the composite samples gradually increase from 55 to 103 m2 g−1 as the HTR content increases from 0 to 100 wt %. The larger SBET and increased pore volume are favorable for the adsorption of dye molecules. UV−vis reflectance spectra of all of the photoanodes in the range of 400−800 nm are shown in Figure 4b. The reflectance intensity gradually increases with increasing HTR content, indicating that the as-prepared HTRs indeed can enhance the light scattering and light harvesting. Because the hierarchical structure can be conducted as a light scattering center of the TiO2 porous films, the incident light can be strongly scattered by the HTRs with submicron size, and the path length of the incident light can be greatly extended, leading to enhanced light harvesting for the DSSC.39−41 Electrochemical Impedance Spectroscopy. Nyquist plots were obtained to study the internal charge transfer and recombination in DSSCs. Figure 5 presents the electrochemical impedance spectroscopy (EIS) results for P100R0-, P80R20-, and P0R100-based DSSCs. In the equivalent circuit (inset in Figure 5a), Rs, R1/R2 and CPE represent the series resistance, the internal resistances, and the constant phase element, respectively. The semicircles in the high-frequency region, intermediate-frequency region, and low-frequency region, are

Figure 2. (a, b) FESEM images, (c) TEM image, and (d) HRTEM image of the as-prepared HTRs.

diameter of the HTRs is ca. 450 nm, and the length is several micrometers. The SEM image with high magnification in Figure 2b distinctly depicts that the HTR is hierarchically composed of numerous tiny nanoparticles and nanorods with a uniform diameter of ca. 30 nm. The hierarchical structure was further authenticated by the corresponding TEM image (Figure 2c). The high-resolution TEM (HRTEM) image of the TiO2 nanorods in the HTR with clear lattice fringes provides evidence of the high degree of crystallization, and the lattice spacing of 0.34 nm can be indexed to (101) of the anatase TiO2 phase (Figure 2d), which is consistent with the XRD results. The cross sections of the prepared samples were also investigated to obtain more information about the NP/HTR composite films. Taking P50R50 as a representative, the crosssectional SEM image in Figure 3a indicates that the NP/HTR composite film has a thickness of about 25 μm. The SEM image with higher magnification in Figure 3b clearly shows that the HTRs are embedded in the film of P25 NPs, and a further observation reveals that there are plenty of large voids in the TiO2 composite films, which are supposed to facilitate the penetrability of electrolytes. As a result of the hierarchical and 1D structure of the HTRs observed from these SEM and TEM images, as illustrated in Figure 3c, the HTRs incorporated in the P25 film can be expected to act as scattering centers to enhance the light harvesting because of the relatively larger submicron size. Meanwhile, it is predicted that the 1D geometry of the HTRs could offer a direct pathway for long charge carrier transfer to reduce the recombination as well. BET Analysis and UV−Vis Reflectance Spectra. To further get an insight into the porous structure of the NP/HTR composite film, the pore structures and specific surface areas of all the samples were obtained by BET measurements. Taking 1317

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Figure 4. (a) N2 isotherms and (inset) pore size curves of P100R0, P80R20, and P0R100 powders and (b) UV−vis reflectance spectra of the photoanodes.

Figure 5. (a) Nyquist plots and (b) Bode phase plots for P100R0, P80R20, and P0R100 photoanodes of DSSCs under illumination.

Table 2. Parameters Derived from the EIS Measurements

Table 1. Data from BET Measurements on the As-Obtained Samples 2

−1

3

−1

sample

SBET (m g )

pore volume (cm g )

average pore size (nm)

P100R0 P90R10 P80R20 P50R50 P0R100

55 61 67 80 103

0.16 0.23 0.24 0.26 0.28

11.9 13.8 14.4 13.0 10.9

sample

Rs (Ω)

R2 (Ω)

f max (Hz)

τe (ms)

P100R0 P80R20 P0R100

13.0 13.7 14.2

30.4 25.0 90.2

8.1 5.5 254

19.7 29.0 0.63

HTRs incorporated in P25 NPs contribute to the reduction of charge transport resistance because the 1D structure of the HTR facilitates the interfacial charge transport and lowers the charge recombination. Moreover, the pore size distribution in the TiO2 film is also essential for the electron transport.46,47 P80R20 with a bimodal pore size distribution (inset in Figure 4a) shows the largest average pore size (14.4 nm; Table 1), which could facilitate the diffusion of the electrolyte and the regeneration of the dye, resulting in accelerated electron transport. In contrast, the P0R100 DSSC has the biggest interfacial resistance R2 because of the small inner mesopores in the HTR and the poor contact between the HTRs, which can suppress the diffusion of the electrolyte and the transfer of electrons, resulting in larger R2. Another interesting parameter, the electron lifetime (τe), was derived from the following eq 1:

assigned to the charge transfer resistance (R1) at the platinum counter electrode−electrolyte interface, the electron transfer resistance (R2) at the TiO2−dye−electrolyte interface, and the diffusion process of I−/I3− in the electrolyte, respectively.42−44 The parameters are summarized in Table 2. It can be found that Rs becomes larger with increasing HTR content in the photoanode, indicating that the interfacial contact between TiO2 and FTO becomes poorer as a result of the incorporation of HTRs. However, the change trend of R2 displays the advantage of HTRs in the films. Usually, a larger R2 indicates a larger interfacial electron transport resistance and higher current loss.45 The values of R2 for different DSSCs are as follows: P80R20 (25.0 Ω) < P100R0 (30.4 Ω) < P0R100 (90.2 Ω). The DSSC based on P80R20 has a smaller interfacial resistance than the DSSC based on P100R0, suggesting that

τe = 1/(2πfmax ) 1318

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ACS Sustainable Chemistry & Engineering where f max (obtained from the Bode phase plot) is the maximum frequency.48,49 As shown in Table 2, the electron lifetimes are as follows: P0R100 (0.63 ms) < P100R0 (19.7 ms) < P80R20 (29.0 ms). A longer electron lifetime means a longer electron transport distance and efficient charge transfer process. From the above investigations, one can see that the DSSC based on P80R20 has the lowest transport resistance and longest electron lifetime, suggesting that the P80R20-based DSSC indeed is an ideal cell. Photocurrent Density−Voltage Characteristics and IPCE Analysis. The J−V characteristics of the as-prepared solar cells were investigated, and the results are presented in Figure 6 and Table 3. The film thicknesses of the photoanodes

The open-circuit voltages (Voc) of the DSSCs containing different HTR contents are compared in Table 3. The Voc of a DSSC is given by the difference between the Fermi level of the TiO2 photoanode and the redox potential of the I−/I3− solution.50,51 It can be seen that the Voc values for all of the DSSCs in this research are similar. Compared with the pristine one, the Voc values of the P90R10- and P80R20-based DSSCs are slightly larger, which could be explained by the negative shift of the Fermi level resulting from the decreased electron− hole recombination and the consequent increase in the electron density in the conducting band of TiO2. The conversion efficiency (η) of the DSSCs first increases and then decreases as the HTR content increases from 0 to 100 wt %. Significantly, the P80R20-based DSSC shows the maximum η of 8.09%. Moreover, the amounts of dye adsorbed on all of the photoanodes are shown in Table 3, and one can see that the amount of adsorbed dye increases with increasing HTR content, which is consistent with the SBET trend. From the above investigations, this improvement in the DSSCs can be ascribed to the advantages of the multiple functions of the incorporated HTRs, including the comprehensive effect of the good dye loading, the strong light scattering effect, the consequently lower charge transfer resistance, and the longer electron lifetime. Figure 7 displays the normalized IPCE characterization of DSSCs made with P100R0, P90R10, P80R20, P50R50, and

Figure 6. J−V curves of DSSCs made with P100R0, P90R10, P80R20, P50R50, and P0R100.

are similar and were optimized in the design of the photoanode for the DSSCs. It can be seen that the J−V characteristics of the DSSCs strongly rely on the HTR content. The short-circuit current density (Jsc) of the DSSCs first increases and then decreases with increasing HTR content, and the DSSC based on P80R20 has the highest Jsc (16.5 mA cm−2). As discussed earlier, the P90R10 and P80R20 films have higher surface areas (Table 1) and stronger light scattering (Figure 4b) than the P100R0 film, and therefore, the higher current densities of DSSCs based on P90R10 and P80R20 are due to the greater dye loading, the stronger light scattering, and faster electron transfer provided by the 1D structure of HTRs. However, when the content of HTRs in the photoanode is more than 20 wt % (e.g., P50R50 and P0R100), the DSSCs show relatively lower photocurrent densities compared with P100R0 even though they have greater dye loading and stronger light scattering ability. This is due to their poor interface contacts with FTO, leading to fewer collected photogenerated electrons.

Figure 7. Normalized IPCEs of the DSSCs based on P100R0, P90R10, P80R20, P50R50, and P0R100 film electrodes.

P0R100. The IPCE first increases and then decreases with increasing HTR content. In particular, the P80R20 composite photoanode shows the best IPCE performance, confirming that the incorporation of a moderate amount of HTRs (20 wt %) into TiO2 films can indeed improve the photovoltaic performance of DSSCs.

Table 3. Parameters of DSSCs Based on P100R0, P90R10, P80R20, P50R50, and P0R100 sample

film thickness (μm)

Jsc (mA cm−2)

Voc (V)

FF

η (%)

dye loading (10−7 mol cm−2)

P100R0 P90R10 P80R20 P50R50 P0R100

23 24 25 24 25

11.6 14.1 16.6 10.4 7.1

0.67 0.68 0.69 0.67 0.68

0.69 0.71 0.70 0.71 0.67

5.37 6.87 8.09 4.96 3.26

0.25 0.48 0.62 0.90 1.11

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CONCLUSION One-dimensional anatase hierarchical TiO2 submicrorods were successfully prepared and used in photoanode films of DSSCs. To optimize the performance of the DSSCs, the effect of the relative NP/HTR ratio on the performance of the solar cells was investigated. The optimized DSSC exhibited a photoelectric conversion efficiency of 8.09% at 20 wt % HTR in the composite photoanode film, which was much higher than that of 5.37% for the solar cell based on pristine P25 NPs. The improvement can be assigned to the multiple functions of the incorporated HTRs, including enhanced light harvesting, increased dye loading, and accelerated electron transport of the photoanode films. This work will offer a new perspective for performance enhancement of DSSCs.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Fax: 0086-27-87879468. Tel: 0086-27-87871029. E-mail: [email protected]. ORCID

Ke Fan: 0000-0003-2269-4042 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (51272199, 21433007, and 51320105001), the 973 Program (2013CB632402), the Natural Science Foundation of Hubei Province (2015CFA001), the Fundamental Research Funds for the Central Universities (WUT: 2015-III-034), and the Innovative Research Funds of SKLWUT (2015-ZD-1).

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