- Email: [email protected]
Controlled synthesis of symbiotic structured TiO2 microspheres to improve the performance of dye-sensitized solar cells
Solar Energy 183 (2019) 587–593
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Controlled synthesis of symbiotic structured TiO2 microspheres to improve the performance of dye-sensitized solar cells Yong Dinga,b, Jianxi Yaoa, Linhua Hua, Songyuan Daia,b, a b
T
⁎
Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing 102206, PR China Key Laboratory of Novel Thin Film Solar Cells, Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, PR China
ARTICLE INFO
ABSTRACT
Keywords: TiO2 microsphere Dye-sensitized solar cell Electron transfer dynamic Electron lifetime
Due to the excellent light scattering and dye loading ability, mesoporous TiO2 microspheres are vastly utilized in dye-sensitized solar cells (DSSCs) to serve as photoanode. Despite of the good electron transport properties due to the unique geometry of mesoporous TiO2 microspheres, the existence of large holes between adjacent microspheres limits the formation of necking and deteriorates the device performance. In this work, we report a simple 2-step process to synthesize symbiotic structured TiO2 microspheres treated with different ammonia concentrations. Meanwhile, the nitrogen doping of TiO2 microspheres is also introduced while ammonia treatment is performed and as a result, the charge transport property is greatly improved. By using the symbiotic structured TiO2 to substitute pure TiO2 microspheres photo anode, the photovoltaic performance of DSSCs was greatly improved from 8.77% to 9.58%. It is demonstrated that the improvement is mainly due to the improved charge transportation property of symbiotic structured TiO2 photoanodes and the effective suppression of carrier recombination. The results give us a comprehensive understanding of electron transport and recombination mechanism in mesoporous TiO2 microspheres, which will provide significant information for the optimization of TiO2 structures for DSSC and other optoelectrical applications.
1. Introduction The superior properties of the dye-sensitized solar cell, such as lightweight, cost effective, light transmittance, flexibility and so on, make it attractive for applications in portable power banks, wearable devices, as well as in the building integrated photovoltaic windows. (Heiniger et al., 2013; LeeYoon, 2018; Scalia et al., 2017) At present, the main bottle neck of their practical application is the device performance. Since DSSCs were reported in 1991, earnest efforts have been made to achieve higher light harvesting and electron collection efficiency to pursue better performance. (Ito et al., 2008; 2006; Oregan and Gratzel, 1991; Wang et al., 2009; Zhang, et al., 2009) A generous portion of them was concentrated on novel photoanode materials and architectures. Generally, to achieve highly efficient DSSC, several essential properties of photoanode materials should be taken into consideration, such as high specific surface area, efficient light scattering and rapid electron transport capabilities, and so on. Therefore, for a TiO2 hierarchical architecture, it should possess good electron transport capability within the spheres as well as high light harvesting properties, which means a strong scattering ability. Whereas, most of the reported architectures composed of
⁎
nanoparticles have numerous grain boundaries, which will act as recombination paths for charge carriers. (Adachi et al., 2004; Bisquert and Vikhrenko, 2004) Further, the point-like connection between adjacent architectures would reduce the connectivity of photoanode based on TiO2 and therefore retard the electron transport across the photoanodes. One strategy for this problem is to mix the nanoparticles with the microspheres. In this case, fast electron transport and high dye loading of composite films were witnessed. (Xi et al., 2011) Another is to synthesize branched TiO2 architectures consisting of various shapes (e.g. nanorods, nanotubes, nanosheets and nanowires) to facilitate the electrons transportation. (Han et al., 2014; Marandi and Bayat, 2018; Marandi et al., 2017; Mohammadpour et al., 2015; Poudel and Qiao, 2012; Roy et al., 2010; Sheng et al., 2014; So et al., 2015; Sun et al., 2013; Tao et al., 2015) In particular, rod-shaped TiO2 nanocrystals based DSSC have considerable advantages over spherical nanoparticles, which is embodied in the features of highly decreased inter-crystalline contacts between grain boundaries and stretched grown structure with the specified directionality. This structure could accelerate the transport of electrons in one direction. (Ding et al., 2017) At the same time, it is difficult to provide sufficient active sites to adsorb dye molecules in the limited surface-to-volume-ratio.
Corresponding author at: Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing 102206, PR China. E-mail address: [email protected] (S. Dai).
https://doi.org/10.1016/j.solener.2019.02.063 Received 27 December 2018; Received in revised form 22 February 2019; Accepted 25 February 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Here, we demonstrated a 2-step process to synthesize mesoporous TiO2 microspheres with tunable surface area, good crystallinity and large pore size by combining sol-gel with a solvothermal treatment. Novel symbiotic structure of TiO2 microspheres and single crystalline nanoparticles were obtained in the process of solvothermal treatment by adjusting the concentration of ammonia. The crystal structure was optimized with the increase of ammonia concentration. And as a result, necessary necking between microspheres was formed and the charge recombination was significantly suppressed due to the improved electron transport capability of TiO2. In addition, ammonia could also serve as nitrogen dopant source in the synthesizing process of TiO2 microspheres. And it has been clarified that the N doping would result in the conduction band shift and therefore be beneficial for the electron transport. (Guo et al., 2011; Lindgren et al., 2003; Simyaet al., 2014; Wang et al., 2009; Zhang et al., 2011) This work accurately characterized the electron transport and recombination dynamics in the symbiotic structure of TiO2 microspheres and nanoparticles. The results give us a comprehensive understanding of electron transport and recombination mechanism in mesoporous TiO2 microspheres, which will provide significant information for the optimization of TiO2 nanostructures for DSSCs. Besides, based on the photocatalytic features of TiO2, (Li et al., 2015; Moniz et al., 2015; Ranjith and Uyar, 2017; Song and Paik, 2016; Tang et al., 2008; Zhang et al., 2016) the results of our present work could also offer important implications for other optoelectrical applications.
dimethylimidazolium iodide (DMII), 0.5 M tert-butylpyridine, and 0.1 M guanidinium thiocyanate (GuSCN) in 85/15 (v/v) acetonitrile and valeronitrile) by a laser engraved 60 µm Surlyn gasket under heat and pressure. The solar cell devices using different photo anodes treated by varied ammonia concentrations (0 mL, 2 mL, 4 mL and 6 mL) were also marked as N0, N2, N4 and N6, respectively. 2.3. Characterization The surface morphology was observed by a high-resolution scanning electron microscope (HR-SEM, S-4800, HITACHI, Japan) and a field emission transmission electron microscopy (FE-TEM, JEM-2100F, JEOL, Japan). The crystalline phase of these products was determined by X-ray powder diffraction (XRD, MXPAHF, Mark Corp., Japan), and the crystal size was calculated by the Scherrer equation. The specific surface area (BET) and pore size distribution (BJH) of the samples were investigated by nitrogen adsorption-desorption isotherms measured at 77 K on a BELSORP-mini instrument. UV–vis absorption spectra and diffused reflectance spectra of samples were measured by using a UV–vis spectrophotometer (SOLID 3700, Shimadzu Co. Ltd, Japan) equipped with an integrated sphere. The current density-voltage (J-V) curves, shielded by a mask with an aperture area of 0.16 cm2, were measured by using a Keithley 2420 Digital Source Meter under a xenon lamp (100 mW cm−2). Incident photon-to-current conversion efficiency (IPCE) spectra were confirmed as a function of wavelength from 300 to 800 nm (PV Measurements, Inc.). An electrochemical workstation (IM6e, Zahner, Germany) was employed to measure the intensity of the modulated photocurrent/ photovoltage spectroscopy (IMPS/IMVS) and the open-circuit voltage decay (OCVD). A light emitting diode (LED, λ = 610 nm) driven by Export (Zahner, Germany) were used to provide both dc and ac components of the illumination. Small amplitude is defined as 10% or less than that of the dc component provided by the LED, and the frequency range was from 300 mHz to 3 kHz. The open-circuit voltage decay (OCVD) was conducted by turning off the illumination on DSSC at a steady state and monitoring the subsequent decay of the open-circuit voltage. The electrochemical impedance spectra (EIS) were recorded by a computer controlled potentiostat (Autolab 320, Metrohm, Switzer Land) in a frequency range of 10 mHz–1000 kHz in dark. The impedance data were fitted to a transmission line mode using ZView software.
2. Experimental method 2.1. Synthesis of TiO2 microspheres Spherical mesoporous TiO2 microspheres were prepared by using a two-step method, which combines a sol-gel and a solvothermal process, as described elsewhere (Ding et al., 2015). Firstly, 25 mL of titanium isopropoxide (TTIP) were dropwise added into the sol-gel solution, composed of 3.5 mL of deionized water, 3.5 mL KI of aqueous solution (0.1 M), and 1.3 L of ethanol. The amorphous TiO2 microspheres were obtained by centrifuging at 5000 rpm for 30 min and washing three times with ethanol. Then, those amorphous TiO2 microspheres were dispersed into a mixture of ethanol (100 mL) and deionized water (50 mL). To prepare the symbiosis of TiO2 microspheres and nanoparticles, different contents of ammonia (0 mL, 2 mL, 4 mL and 6 mL) were added into the mixed solution under stirring for 5 min, sealed within a Ti autoclave (260 mL), and heated at 160 °C for 16 h. The asprepared samples were named as N0, N2, N4 and N6, corresponding to the amount of ammonia. Finally, the products were collected by filtration and washed with deionized water and ethanol. The resulting products were named as N0, N2, N4, and N6 throughout this report.
3. Results and discussion 3.1. Properties of symbiotic-structured TiO2 microspheres The morphology and microstructure of mesoporous TiO2 microspheres were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1 shows SEM images of TiO2 microspheres synthesized with different contents of ammonia. In the absence of ammonia, TiO2 microspheres (N0, Fig. 1a and b) exhibit smooth spherical morphology composed of tiny interconnected nanocrystals with a diameter of around 500 nm. With the increase of the ammonia content to 2 mL (N2, Fig. 1c and d), the spherical shape is maintained without breakage, and the nanocrystals at the surface of microspheres grow larger. When the content of ammonia is increased to 4 mL (N4, Fig. 1e and f), the resulting products are not necessarily fully spherical and bounded by relatively rough surfaces with some breaks in between. The symbiotic nanocrystals are fully filled into the adjacent TiO2 microspheres, and eventually a necessary necking was formed between microspheres. However, further increase of the ammonia content to 6 mL (N6) results in the dissociation of microspheres and formation of over-grown nanocrystals (Fig. 1g). As depicted in Fig. S1 (see Supporting Information), all of four samples show pure anatase crystallographic phase. And the average grain sizes are 9.30 nm, 13.24 nm, 15.90 nm, and 20.74 nm for N0, N2,
2.2. Preparation of symbiotic structured TiO2 microspheres and DSSCs assemble Homogenous screen-printing pastes were prepared by the following procedure. 5.0 g of these products (N0, N2, N4 and N6) were completely redispersed into ethanol suspension, containing α-terpineol (25.0 g) and ethyl cellulose (1.0 g) by stirring and ultrasonic dispersing. Afterwards, the TiO2 pastes were obtained by rotary evaporation to remove ethanol. The as-prepared pastes were screen-printed onto the Fluorine-doped tin oxide glass (FTO, 15 Ω sq−1) and then annealed in ambient air condition at 510 °C for 30 min to form TiO2 photoanodes. These photoanodes were treated by 0.1 M TiCl4 aqueous solution at 65 °C for 40 min. After rinsing with water, these photoanodes were sintered at 510 °C for 30 min again. These photoanodes were sensitized with 300 µM C101 dye with cheno-3a, 7a-dihydroxy-5b-cholic acid overnight. After being washed by acetonitrile and dried in air, the sensitized photoanodes were assembled with a Pt-modified counter electrode and electrolyte (30 mM I2, 50 mM LiI, 1.0 M 1,3588
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Fig. 1. SEM images of the TiO2 microspheres synthesized with (a, b) 0 mL, (c, d) 2 mL, (e, f) 4 mL, and (g, h) 6 mL of ammonia.
N4, and N6, respectively, as calculated by using Scherrer equation. The increased grain size with the increase of ammonia content is well consistent with the SEM results. Further, the increased diffraction peak intensity indicates that higher ammonia concentration is beneficial for the grain growth, at least in the range of no more than 6 mL ammonia addition. The main reason is that, due to the hydrogen-bond conjugation to hydroxide monomer, the addition of ammonia reduced the solubility of hydroxide precipitate in ethanol, which is beneficial for the grain growth of TiO2. (Ding et al., 2015). Ammonia has also been used as the nitrogen dopants to synthesize N-doped TiO2 microspheres. (Guo et al., 2011) As shown in Fig. S2, the XPS peaks for the Ti 2p and O 1s reveal meaningful change upon N incorporation. Compared with the binding energy of N0 powder (458.86 eV for Ti 2p and 530.11 eV for O 1s), the binding energy of Ti 2p (458.53 eV) and O 1s (529.78 eV) decreases after N doping, suggesting N doping into TiO2 lattice and substituting for oxygen. The N 1s binding energy further shows the absence of nitrogen in the N0, whereas for N4 a high and broad peak is observed at about 399.6 eV and 400.7 eV, indicating the presence of OeTieN structure. (Chen and Burda, 2004) Besides, the new absorption shoulder at 400–520 nm was found in the UV–vis absorbance spectra for N4 and N6 (Fig. S3), which is related to the presence of nitrogen, because there should not be absorption peak in this region for intrinsic TiO2 microsphere. (Guo et al., 2011) It has been found that the incorporation of N atoms produces partially occupied impurity levels above the valence band maximum of anatase TiO2, and a new mid-gap stat will be formed. (Li et al., 2015) In addition, the absorbance spectra show red shift after N doping and the conduction band minimum position will be decreased with the increased doping concentration. (Niu et al., 2015) The increase of the ammonia content will not only speed up the grain growth, but also raise the structural stress. In this case, most TiO2 microspheres are easily distorted and collapsed into isolated debris of nanocrystals, as revealed by the typical SEM in Fig. 1h. N4 seems to have ideal properties for the photoanode application in DSSCs because the microspheres in it not only possess good continuality in between but also maintain the integrity of the sphere shapes. Therefore, TEM was carried out to obtain further information of N4 and the results are shown in Fig. 2. Fig. 2a suggests that the N4 microspheres are constructed by nanoparticles. The increased ammonia concentration could produce symbiotic nanoparticles, which are inclined to melt together and form a closely linked network. As confirmed from high resolution TEM image in Fig. 2b, the morphology implies complete connection at the necks between adjacent microspheres, and the connection could offer shorter electron transport pathways in TiO2 photoanode of DSSCs. Continuous lattice fringes can be found throughout primary nanoparticles (Fig. 2c), indicating its single-crystal
Fig. 2. (a, b) TEM images, (c) high-resolution TEM image and (d) the corresponding fast Fourier transform (FFT) pattern of N4 sample.
nature. Measurement of the lattice fringes gives a d-spacing of 0.48 nm and 0.35 nm, which can be indexed to the (0 0 1) and (1 0 1) faces of anatase TiO2, respectively. In addition, the (1 0 1) face is interconnected through the (0 0 1) face, indicating that crystal growth occurs primarily by the oriented attachment mechanism. (Ke et al., 2012) According to the theory of crystal growth, the driving force for such oriented attachment process is to reduce the total surface energy by eliminating the surfaces at which the crystallites join. Because the surface energy of the (0 0 1) face is 1.4 times larger than that for the (1 0 1) faces, it is reasonable that the oriented attachment occurs in the [0 0 1] direction and makes a network structure. In this case, the (1 0 1) face is mainly exposed. The fast Fourier transform (FFT) pattern further demonstrates that individual nanoparticles inside a bead are single crystals (Fig. 2d). Thus, detailed analysis of the HRTEM results confirmed that the TiO2 microspheres synthesized with the more ammonia could produce nanoparticles with good crystallinity. Fig. 3 depicts specific surface areas and the corresponding pore size distributions of the TiO2 microspheres, measured by nitrogen adsorption-desorption isotherm and the related parameters are summarized in Table 1. Type IV isotherms with H1 hysteresis loop are observed for all the TiO2 microspheres, indicating the presence of mesopores in the inner microspheres. A hysteresis at high relative pressure was witnessed with the increased ammonia content and this trend turns to be much 589
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Fig. 3. (a) Nitrogen sorption isotherms and (b) the corresponding pore size distributions of the four microspheres (N0, N2, N4, and N6).
with the integrated Jsc from the IPCE spectra, as shown in Fig. 4(b).
Table 1 Physical properties of the four TiO2 microspheres (N0, N2, N4, and N6). Samples
Pore sizeα (nm)
Porosityβ (P)
SBET (m2∙g−1)
Crystal sizeγ (nm)
N0 N2 N4 N6
9.38 12.87 14.03 21.21
0.6341 0.6304 0.6235 0.6193
189.82 136.17 121.31 84.49
9.30 13.24 15.90 20.74
3.2.1. The mechanisms for the varied JSC The mechanisms of the improved JSC for N4 based device will be explained by incident photon-to-current conversion efficiency (IPCE) spectra (Fig. 4b). IPCE is defined as the number of electrons in the external circuit generated by an incident photon at a given wavelength, which is determined by light harvesting efficiency (LHE(λ)), electron injection efficiency (φinj) and electron collection efficiency (ηc), according to the following equation:
α Pore sizes are based on the adsorption average pore width (4 V/A by BET) calculated from the BJH model. Vp β Porosity is calculated from the BJH pore volume according to P = 1 + V . γ
p
Crystal size is calculated by the Scherrer equation.
more obvious in the corresponding isotherms with further increase of ammonia concentration, which resulted in a broadening in pore size distribution, as shown in Fig. 3b. As summarized in Table 1, the average pore sizes are 9.38 nm, 12.87 nm, 14.03 nm and 21.21 nm for N0, N2, N4 and N6, respectively. Whereas, the specific surface area decreases with increasing the contents of ammonia due to the growth of nanoparticles formed at the surface of microspheres. In addition, the SEM images (Fig. 1) demonstrate that the TiO2 microspheres with larger amount of ammonia could easily be broken and therefore allow for denser packing, leading to a lower porosity. (Heiniger et al., 2014).
Jsc = q × IPCE( ) × I0
(1)
IPCE( ) = LHE( )
(2)
inj c
where q is elementary charge, I0 is light intensity. In terms of efficient ruthenium dye (C101), the φinj is generally close to 100%, which can be ignored. While the LHE(λ) is mainly determined by Beer's law expressed by the equation, LHE(λ) = 1 −10 × ( ) , where is the surface coverage of the sensitizer, and ( ) is the molar cross section of C101 dye. The amount of dye loaded on the four TiO2 photoanodes is listed in Table 2. For the N0-based TiO2 photoanode, the adsorption capacity was 2.58 × 10−7 mol cm−2, while for samples N2, N4, and N6, values of 2.20 × 10−7 mol cm−2, 2.14 × 10−7 mol cm−2, and 1.86 × 10−7 mol cm−2 were obtained, respectively. This is attributed to the fact that the increased contents of ammonia could promote the crystallization process and thus decrease the specific surface area. Except for the dye adsorption ability, the light scattering ability is supposed to be one of the most critical issues that affect the performance of LHE(λ). The light scattering effect of the four photoanodes can be characterized by measuring the UV/Vis reflection spectra in the wavelength range from 400 nm to 800 nm. As shown in Fig. S2, the diffused reflectance in the spectral range from 400 nm to 800 nm decreases with the increased ammonia concentration. According to the Mie theory, the light scattering properties of particles are dependent on the size of the particle and the wavelength of the incident light. (Ferber and Luther, 1998) The increased ammonia concentration could destroy the integrity of the TiO2 microspheres and generate a large number of nanoparticles. The larger quantity of nanoparticles results in a more Rayleigh-type scattering of individual small nanoparticles, leading to the inferior light absorption, (Heiniger et al., 2014) which is consistent with the IPCE
3.2. Device performance of DSSCs To explore their potential utilization for solar energy conversion, TiO2 microspheres symbiosis with nanoparticles were used as the photoanode to fabricate DSSCs. Their photovoltaic properties were characterized, with the parameters summarized in Table 2. The typical current density-voltage (J-V) characteristics are shown in Fig. 4a. As shown in Fig. 4a and Table 1, N0 based DSSC demonstrates a shortcircuit current density (Jsc) of 16.65 mA·cm−2, open-circuit voltage (Voc) of 0.708 V, fill factor (FF) of 74.4%, and a power conversion efficiency (PCE) of 8.77%. Interestingly, except for the Voc and FF, the DSSCs assembled with the N2, N4 and N6 photoanodes all demonstrate a higher PCE than that of N0-based DSSC. The remarkable enhancement is mainly ascribed to the improvement of the Jsc, where N4 based cell shows the highest Jsc up to 18.71 mA·cm−2, compared to 18.48 and 17.32 mA·cm−2 for N2 and N6 based DSSCs. The tendency is consistent
Table 2 Comparison of the photovoltaic characteristics (average values of eight cells from same batch) measured under 1 sun illumination for TiO2 microspheres synthesized with different contents of ammonia. Device ID
TiO2 thickness (µm)
Voc (V)
Jsc (mA·cm−2)
FF (%)
η (%)
Absorbed dye (×10−7 mol·cm−2)
N0 N2 N4 N6
12.7 12.9 12.4 12.1
0.708 0.695 0.694 0.700
16.65 18.48 18.71 17.32
74.4 73.1 73.8 73.9
8.77 9.38 9.58 8.96
2.58 2.20 2.14 1.86
590
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Fig. 4. (a) Current density-voltage characteristics (J-V curves) and (b) incident photon-to-electron conversion spectra (IPCE) of representative devices based on TiO2 microspheres with different contents of ammonia.
spectra (Fig. 4b). For N4 based cell, the IPCE is appreciably higher than that of N2 based device in the wavelength range of 400–620 nm, while it is slightly lower in longer-wavelength regions, such as 620–800 nm. The lower IPCE in this long-wavelength region could be caused by the Rayleigh-type scattering of nanoparticles, which brings adverse effect on the LHE(λ). (Heiniger et al., 2014) Based on these results, it can be concluded that the ammonia treatment is not beneficial for enhancing the LHE(λ). For this reason, we may suspect that the real determinant factor may be related to electron collection efficiency (ηc).
would generate a hole in the N-doped TiO2, which might improve the conductivity of anatase TiO2. (Niu et al., 2015) Last reason is the formation of necessary necking between microspheres, which can provide continuous transport channel in the N4 photoanode. As the symbiotic nanoparticles intelligently fill into voids between TiO2 microspheres in the N4 photoanode, the seamless connection between microspheres and nanoparticles is established, which builds extra channels to boost electron transport and suppress charge recombination. And as a result, the prolonged τn will then be achieved. (Ding et al., 2015) Whereas, the N6 photoanode has numerous grain boundaries among the nanoparticles and therefore gives a higher transport resistance, thus prolonging the τd and shorting the τn. The combination of τd and τn allows us to estimate the electron collection efficiency, by using the equation ηc = 1 − τd/τn. The ηc of 96.8% and 95.6% are demonstrated in the N4 and N2 based cells at an irradiation intensity of 96 mW cm−2, respectively, while the values for N6 and N0 base devices show a limited ηc of less than 90%. Besides, when the film thickness is taken into consideration, electron diffusion length (Ln) could be estimated by using the formula Ln=(Dn·τn)1/2 and Dn = d2/(2.35τd) to make a fair comparison of intrinsic electron transport and recombination dynamics of DSSCs, where d is the film thickness. Normally, the Ln values suggest whether photoelectrons can transport to an external circuit prior to recombination, and thereby a long diffusion length is preferred for an ideal TiO2 photoanode. Fig. 5c shows the Ln values of the four photoanodes-based DSSCs, indicating a trend of Ln with an order of N4 > N2 > N6 > N0 at the same light intensity, which is basically in accordance with their ηc sequence. This may be ascribed to the fact that the N4 photoanode assembled from the TiO2 microspheres and symbiotic nanoparticles is favorable for electron diffusion through a longer distance with less grain boundaries and fewer trapping sites. As a result, the ammonia treatment in the preparation of TiO2 microspheres can simultaneously enhance the crystallization and generate the symbiotic nanoparticles, resulting in an
3.2.2. Carriers transport dynamics ηc is determined by the kinetic competition between electron transport and recombination. Intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) have been performed to provide quantitative information on electron transport and recombination behaviors in the TiO2 photoanode. Electron transport time (τd) and electron lifetime (τn) can be obtained according to the equations: τd = 1/ 2πfd and τn = 1/2πfn, where fd and fn are the characteristics minimum frequency of the IMPS and IMVS imaginary component, respectively. Fig. 5a and 5b show plots of the τd and τn as a function of light intensity from 10 mW cm−2 to 100 mW cm−2. Obviously, N2 based cell shows shorter τd and longer τn than N0 based device, indicating a faster electron transport rate and a slower electron recombination rate for the former. It is generally believed that electron transport in TiO2 photoanode is limited by numerous defects and grain boundaries via a series of trapping/detrapping processes. As compared with N0, after ammonia treatment, the better crystalline structure was formed in N2. In this case, a porous film with lower defect density should be achieved. The shorter τd and longer τn will then be obtained. In comparison to other counterparts, N4 based cell shows shortest τd and the longest τn. One reason is that the N4 photoanode has lower surface area and fewer surface trapping sites and defects, as compared to those of the N2 photoanode. Second reason is that the N atom substituting for O atom
Fig. 5. (a) Electron transport time (τd), (b) electron lifetime (τn) and (c) electron collection efficiency (ηc) as a function of light intensities for the DSSCs based on the N0, N2, N4 and N6 based photoanode. 591
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Fig. 6. (a) Electrochemical impedance spectra (EIS) and (b) open-circuit voltage decay (OCVD) curves of DSSCs fabricated using N0, N2, N4 and N6 based photoanode. The inset in (a) is the equivalent circuit applied to fit the resistance data.
improved ηc and adverse effect on the LHE(λ) of the DSSCs. We then concluded that there is a tradeoff between the LHE(λ) and ηc.
equilibrium between the photo-generated electron and electron recombination by the I3− ions in the electrolyte. Fig. 6b shows the photovoltage decay rate as a function of time upon switching off the light. In general, the photovoltage is related to the electron lifetime and will decay sharply along with the electron recombination. Obviously, the photovoltage decay rate of the devices based on the four types of TiO2 photoanodes is in an order of N0 > N6 > N2 > N4, implying a decline trend of recombination rate with the increased ammonia concentration. This result is in concurrence with the EIS analysis shown above. Although electron lifetime has a major influence on the Voc of DSSCs, a small Voc difference between the four DSSCs was observed in J-V curves. It is well-known that the Voc is mainly affected by the conduction band edge movement in DSSCs. As discussed above, there was a decrease in the dye loading of films from N0 to N6, resulting in that the conduction band edge shifts positively and reduce the Voc, which would counteract the increase of Voc in less electron recombination. Therefore, there is a tiny difference between the four types of DSSCs in final Voc.
3.2.3. Energy loss mechanisms in the devices Fig. 6a shows Nyquist plots of the four photoanodes based DSSCs obtained by electrochemical impedance spectroscopy (EIS) in dark conditions. In order to get a clear image of the electron recombination dynamics in photo anode, a negative potential with a value higher than the VOC (∼0.7 V) of the devices, −0.73 V, was applied while carrying out the EIS measurement. (Fabregat-Santiago et al., 2005) In general, two well-defined semicircles are observed in the medium frequency and high frequency range. The large semicircle at medium frequency represents the electron transfer at the dye-TiO2/electrolyte interface, while the small semicircle at high frequency denotes to the redox transfer at the Pt/FTO interface. According to the EIS model and equivalent circuit, the fitted data are listed in Table 3. From Table 3, one can notice that the value of series resistance (Rs) for four DSSCs is similar due to the utilization of the same electrolyte and Pt/FTO counter electrode assembled in the nearly same manner. Whereas, transfer resistance (Rct), which is generally considered to be primarily determined by the electron recombination resistance, shows significant discrepancy, i.e., 51.5 Ω, 64.9 Ω, 88.5 Ω, and 44.4 Ω for the four photoanodes based DSSC, respectively. Moreover, the electron lifetime (τn(EIS)) of the devices based on N0, N2, N4 and N6 is 39.1 ms, 41.5 ms, 42.5 ms, and 30.6 ms, respectively. These results demonstrate that N4 based device, derived from TiO2 microspheres and symbiotic nanoparticles, shows a slower recombination rate as compared with that of other counterparts. This suggests that the unique composite structure is beneficial for electron transport, and hence leads to a slower recapture of photoelectrons by I3− ions occurring in the recombination process, which corresponds well with the IMVS results. Under open-circuit conditions, the electron transfer kinetics can also be investigated by the evaluation of the photovoltage decay rate, which is generally proportional to the interfacial recombination rate. To conduct this measurement, all the DSSCs were illuminated under the simulated sun light to obtain a steady state voltage, indicating the
4. Conclusion In the present work, we provide a facile method for the fabrication of highly crystalline mesoporous TiO2 microspheres with tunable surface areas (84 ∼ 190 m2∙g−1), pore sizes (9 ∼ 21 nm) and particle sizes (9 ∼ 20 nm) by adjusting the ammonia concentration in the solvothermal process. This method can be used to improve the crystallinity of TiO2 nanoparticles and optimize the charge transportation capability. Additionally, the fabricated TiO2 photoanode from the symbiosis of TiO2 microspheres and nanoparticles (N4) provides an appropriate pore structure and good scattering effect. Interestingly, this symbiotic structure demonstrates a considerably higher Jsc of the related photovoltaic devices due to its better contact between adjacent microspheres for fast electron transport rate and slow electron recombination rate. Consequently, the highest η of 9.58% was obtained by using this symbiotic structure, compared with 8.77% of the welldefined TiO2 microspheres without ammonia treatment (N0). The improved photo-electrical properties of symbiotic TiO2 make it promising candidate for photoanode of efficient DSSCs and other optoelectrical applications.
Table 3 Characteristic parameters of the electrochemical impedance spectra (EIS) of the DSSCs based on the four photoanodes in the dark environment at bias of −0.73 V. Cell
Rs (Ω)
Rct (Ω)
Cµ (mF)
τn(EIS)(ms)α
N0 N2 N4 N6
2.32 2.41 2.42 2.39
51.5 64.9 88.5 44.4
0.76 0.64 0.48 0.69
39.1 41.5 42.5 30.6
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51702096, and U1705256), the Fundamental Research Funds for the Central Universities, China (No. 2017MS021 and 2018MS040).
α Rs, Rct, Cµ, and τn(EIS) denote the series resistance, transfer resistance, chemical capacitance, and electron lifetime, respectively.
592
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Appendix A. Supplementary material
B 107, 5709–5716. Marandi, M., Bayat, S., 2018. Facile fabrication of hyper-branched TiO2 hollow spheres for high efficiency dye-sensitized solar cells. Sol. Energy 174, 888–896. Marandi, M., Goudarzi, Z., Moradi, L., 2017. Synthesis of randomly directed inclined TiO2 nanorods on the nanocrystalline TiO2 layers and their optimized application in dye sensitized solar cells. J. Alloys Compd. 711, 603–610. Mohammadpour, F., Moradi, M., Lee, K., Cha, G., So, S., Kahnt, A., Schmuki, P., 2015. Enhanced performance of dye-sensitized solar cells based on TiO 2 nanotube membranes using an optimized annealing profile. Chem. Commun. 51, 1631–1634. Moniz, S.J.A., Shevlin, S.A., Martin, D.J., Guo, Z.X., Tang, J., 2015. Visible-light driven heterojunction photocatalysts for water splitting – a critical review. Energy Environ. Sci. 8 (3), 731–759. Niu, M., Cui, R., Wu, H., Cheng, D., Cao, D., 2015. Enhancement mechanism of the conversion efficiency of dye-sensitized solar cells based on nitrogen-, fluorine-, and iodine-doped TiO2 photoanodes. J. Phys. Chem. C 119, 13425–13432. Oregan, B.C., Gratzel, M., 1991. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737–740. Poudel, P., Qiao, Q., 2012. One dimensional nanostructure/nanoparticle composites as photoanodes for dye-sensitized solar cells. Nanoscale 4, 2826–2838. https://doi.org/ 10.1039/C2NR30347G. Ranjith, K.S., Uyar, T., 2017. Rational synthesis of Na and S co-catalyst TiO2-based nanofibers: presence of surface-layered TiS3 shell grains and sulfur-induced defects for efficient visible-light driven photocatalysis. J. Mater. Chem. 5, 14206–14219. Roy, P., Kim, D., Lee, K., Spiecker, E., Schmuki, P., 2010. TiO2 nanotubes and their application in dye-sensitized solar cells. Nanoscale 2, 45–59. Scalia, A., Bella, F., Lamberti, A., Bianco, S., Gerbaldi, C., Tresso, E., Pirri, C.F., 2017. A flexible and portable powerpack by solid-state supercapacitor and dye-sensitized solar cell integration. J. Power Sources 359, 311–321. Sheng, X., He, D., Yang, J., Zhu, K., Feng, X., 2014. Oriented assembled TiO2 hierarchical nanowire arrays with fast electron transport properties. Nano Lett. 14, 1848–1852. Simya, O., Selvam, M., Karthik, A., Rajendran, V., 2014. Dye-sensitized solar cells based on visible-light-active TiO2 heterojunction nanoparticles. Synth. Met. 188, 124–129. So, S., Hwang, I., Schmuki, P., 2015. Hierarchical DSSC structures based on “single walled” TiO 2 nanotube arrays reach a back-side illumination solar light conversion efficiency of 8%. Energy Environ. Sci. 8, 849–854. Song, T., Paik, U., 2016. TiO2 as an active or supplemental material for lithium batteries. J. Mater. Chem. 4, 14–31. Sun, Z., Kim, J.H., Zhao, Y., Attard, D., Dou, S.X., 2013. Morphology-controllable 1D–3D nanostructured TiO 2 bilayer photoanodes for dye-sensitized solar cells. Chem. Commun. 49, 966–968. Tang, J., Durrant, J.R., Klug, D.R., 2008. Mechanism of photocatalytic water splitting in TiO2. reaction of water with photoholes, importance of charge carrier dynamics, and evidence for four-hole chemistry. J. Am. Chem. Soc. 130, 13885–13891. Tao, X., Ruan, P., Zhang, X., Sun, H., Zhou, X., 2015. Microsphere assembly of TiO2 mesoporous nanosheets with highly exposed (101) facets and application in a lighttrapping quasi-solid-state dye-sensitized solar cell. Nanoscale 7, 3539–3547. Wang, M., Anghel, A.M., Marsan, B., Ha, N.C., Pootrakulchote, N., Zakeeruddin, S.M., Gratzel, M., 2009a. CoS supersedes Pt as efficient electrocatalyst for triiodide reduction in dye-sensitized solar cells. J. Am. Chem. Soc. 131, 15976–15977. Wang, X., Yang, Y., Jiang, Z., Fan, R., 2009b. Preparation of TiNxO2-x photoelectrodes with NH3 under controllable middle pressures for dye-sensitized solar cells. Eur. J. Inorg. Chem. 23, 3481–3487. Xi, J., Zhang, Q., Park, K., Sun, Y., Cao, G., 2011. Enhanced power conversion efficiency in dye-sensitized solar cells with TiO2 aggregates/nanocrystallites mixed photoelectrodes. Electrochim. Acta 565, 1960–1966. Zhang, J., Sun, Q., Zheng, J., Zhang, X., Cui, Y., Wang, P., Zhu, Y., 2011. The characterization of nitrogen doped TiO2 photoanodes and its application in the dye sensitized solar cells. J. Renew. Sust. Energy 3, 033108. Zhang, Q., Dandeneau, C.S., Zhou, X., Cao, G., 2009. ZnO Nanostructures for dye-sensitized solar cells. Adv. Mater. 21, 4087–4108. Zhang, X., Peng, T., Song, S., 2016. Recent advances in dye-sensitized semiconductor systems for photocatalytic hydrogen production. J. Mater. Chem. 4, 2365–2402.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.02.063. References: Adachi, M., Murata, Y., Takao, J., Jiu, J., Sakamoto, M., Wang, F., 2004. Highly efficient dye-sensitized solar cells with a titania thin-film electrode composed of a network structure of single-crystal-like TiO2 nanowires made by the “oriented attachment” mechanism. J. Am. Chem. Soc. 126, 14943–14949. Bisquert, J., Vikhrenko, V.S., 2004. Interpretation of the time constants measured by kinetic techniques in nanostructured semiconductor electrodes and dye-sensitized solar cells. J. Phys. Chem. B 108, 2313–2322. Chen, X., Burda, C., 2004. Photoelectron spectroscopic investigation of nitrogen-doped titania nanoparticles. J. Phys. Chem. B 108, 15446–15449. Ding, Y., Ma, Y., Tao, L., Hu, L., Li, G., Jiang, L., Dai, S., 2015a. Continuous electron transport pathways constructed in TiO 2 sub-microsphere films for high-performance dye-sensitized solar cells. RSC Adv. 5, 17493–17500. Ding, Y., Zhang, T., Liu, C., Yang, Y., Pan, J., Yao, J., Dai, S., 2017. Shape-controlled synthesis of single-crystalline anatase TiO 2 micro/nanoarchitectures for efficient dye-sensitized solar cells. Sust. Energy Fuels 1, 520–528. Ding, Y., Zhou, L., Mo, L.E., Jiang, L., Hu, L., Li, Z., Dai, S., 2015b. TiO2 microspheres with controllable surface area and porosity for enhanced light harvesting and electrolyte diffusion in dye‐sensitized solar cells. Adv. Funct. Mater. 25, 5946–5953. Fabregat-Santiago, F., Bisquert, J., Garcia-Belmonte, G., Boschloo, G., Hagfeldt, A., 2005. Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy. Sol. Energy Mater. Sol. Cells 87, 117–131. Ferber, J., Luther, J., 1998. Computer simulations of light scattering and absorption in dye-sensitized solar cells. Sol. Energy Mater. Sol. Cells 54, 265–275. Guo, W., Shen, Y., Boschloo, G., Hagfeldt, A., Ma, T., 2011. Influence of nitrogen dopants on N-doped TiO2 electrodes and their applications in dye-sensitized solar cells. Electrochim. Acta 56, 4611–4617. Han, G.S., Lee, S., Noh, J.H., Chung, H.S., Park, J.H., Swain, B.S., Jung, H.S., 2014. 3-D TiO2 nanoparticle/ITO nanowire nanocomposite antenna for efficient charge collection in solid state dye-sensitized solar cells. Nanoscale 6, 6127–6132. Heiniger, L.P., Giordano, F., Moehl, T., Grätzel, M., 2014. Mesoporous TiO2 beads offer improved mass transport for cobalt-based redox couples leading to high efficiency dye-sensitized solar cells. Adv. Energy Mater. 4. Heiniger, L.P., O'Brien, P.G., Soheilnia, N., Yang, Y., Kherani, N.P., Grätzel, M., Tétreault, N., 2013. See-through dye-sensitized solar cells: photonic reflectors for tandem and building integrated photovoltaics. Adv. Mater. 25, 5734–5741. Ito, S., Murakami, T.N., Comte, P., Liska, P., Gratzel, C., Nazeeruddin, M.K., Gratzel, M., 2008. Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10. Thin Solid Films 516, 4613–4619. Ito, S., Zakeeruddin, S.M., Humphrybaker, R., Liska, P., Charvet, R., Comte, P., Miura, H., 2006. High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2 electrode thickness. Adv. Mater. 18, 1202–1205. Ke, C., Chen, L., Ting, J., 2012. Photoanodes consisting of mesoporous anatase TiO2 beads with various sizes for high-efficiency flexible dye-sensitized solar cells. J. Phys. Chem. C 116, 2600–2607. Lee, H.M., Yoon, J.H., 2018. Power performance analysis of a transparent DSSC BIPV window based on 2 year measurement data in a full-scale mock-up. Appl. Energy 225, 1013–1021. Li, G., Li, J., Li, G., Jiang, G., 2015. N and Ti 3+ co-doped 3D anatase TiO 2 superstructures composed of ultrathin nanosheets with enhanced visible light photocatalytic activity. J. Mater. Chem. A 3, 22073–22080. Lindgren, T., Mwabora, J.M., Avendaño, E., Jonsson, J., Hoel, A., Granqvist, C.-G., Lindquist, S.-E., 2003. Photoelectrochemical and optical properties of nitrogen doped titanium dioxide films prepared by reactive DC magnetron sputtering. J. Phys. Chem.
593
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Controlled synthesis of symbiotic structured TiO2 microspheres to improve the performance of dye-sensitized solar cells Yong Dinga,b, Jianxi Yaoa, Linhua Hua, Songyuan Daia,b, a b
T
⁎
Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing 102206, PR China Key Laboratory of Novel Thin Film Solar Cells, Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, PR China
ARTICLE INFO
ABSTRACT
Keywords: TiO2 microsphere Dye-sensitized solar cell Electron transfer dynamic Electron lifetime
Due to the excellent light scattering and dye loading ability, mesoporous TiO2 microspheres are vastly utilized in dye-sensitized solar cells (DSSCs) to serve as photoanode. Despite of the good electron transport properties due to the unique geometry of mesoporous TiO2 microspheres, the existence of large holes between adjacent microspheres limits the formation of necking and deteriorates the device performance. In this work, we report a simple 2-step process to synthesize symbiotic structured TiO2 microspheres treated with different ammonia concentrations. Meanwhile, the nitrogen doping of TiO2 microspheres is also introduced while ammonia treatment is performed and as a result, the charge transport property is greatly improved. By using the symbiotic structured TiO2 to substitute pure TiO2 microspheres photo anode, the photovoltaic performance of DSSCs was greatly improved from 8.77% to 9.58%. It is demonstrated that the improvement is mainly due to the improved charge transportation property of symbiotic structured TiO2 photoanodes and the effective suppression of carrier recombination. The results give us a comprehensive understanding of electron transport and recombination mechanism in mesoporous TiO2 microspheres, which will provide significant information for the optimization of TiO2 structures for DSSC and other optoelectrical applications.
1. Introduction The superior properties of the dye-sensitized solar cell, such as lightweight, cost effective, light transmittance, flexibility and so on, make it attractive for applications in portable power banks, wearable devices, as well as in the building integrated photovoltaic windows. (Heiniger et al., 2013; LeeYoon, 2018; Scalia et al., 2017) At present, the main bottle neck of their practical application is the device performance. Since DSSCs were reported in 1991, earnest efforts have been made to achieve higher light harvesting and electron collection efficiency to pursue better performance. (Ito et al., 2008; 2006; Oregan and Gratzel, 1991; Wang et al., 2009; Zhang, et al., 2009) A generous portion of them was concentrated on novel photoanode materials and architectures. Generally, to achieve highly efficient DSSC, several essential properties of photoanode materials should be taken into consideration, such as high specific surface area, efficient light scattering and rapid electron transport capabilities, and so on. Therefore, for a TiO2 hierarchical architecture, it should possess good electron transport capability within the spheres as well as high light harvesting properties, which means a strong scattering ability. Whereas, most of the reported architectures composed of
⁎
nanoparticles have numerous grain boundaries, which will act as recombination paths for charge carriers. (Adachi et al., 2004; Bisquert and Vikhrenko, 2004) Further, the point-like connection between adjacent architectures would reduce the connectivity of photoanode based on TiO2 and therefore retard the electron transport across the photoanodes. One strategy for this problem is to mix the nanoparticles with the microspheres. In this case, fast electron transport and high dye loading of composite films were witnessed. (Xi et al., 2011) Another is to synthesize branched TiO2 architectures consisting of various shapes (e.g. nanorods, nanotubes, nanosheets and nanowires) to facilitate the electrons transportation. (Han et al., 2014; Marandi and Bayat, 2018; Marandi et al., 2017; Mohammadpour et al., 2015; Poudel and Qiao, 2012; Roy et al., 2010; Sheng et al., 2014; So et al., 2015; Sun et al., 2013; Tao et al., 2015) In particular, rod-shaped TiO2 nanocrystals based DSSC have considerable advantages over spherical nanoparticles, which is embodied in the features of highly decreased inter-crystalline contacts between grain boundaries and stretched grown structure with the specified directionality. This structure could accelerate the transport of electrons in one direction. (Ding et al., 2017) At the same time, it is difficult to provide sufficient active sites to adsorb dye molecules in the limited surface-to-volume-ratio.
Corresponding author at: Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing 102206, PR China. E-mail address: [email protected] (S. Dai).
https://doi.org/10.1016/j.solener.2019.02.063 Received 27 December 2018; Received in revised form 22 February 2019; Accepted 25 February 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Here, we demonstrated a 2-step process to synthesize mesoporous TiO2 microspheres with tunable surface area, good crystallinity and large pore size by combining sol-gel with a solvothermal treatment. Novel symbiotic structure of TiO2 microspheres and single crystalline nanoparticles were obtained in the process of solvothermal treatment by adjusting the concentration of ammonia. The crystal structure was optimized with the increase of ammonia concentration. And as a result, necessary necking between microspheres was formed and the charge recombination was significantly suppressed due to the improved electron transport capability of TiO2. In addition, ammonia could also serve as nitrogen dopant source in the synthesizing process of TiO2 microspheres. And it has been clarified that the N doping would result in the conduction band shift and therefore be beneficial for the electron transport. (Guo et al., 2011; Lindgren et al., 2003; Simyaet al., 2014; Wang et al., 2009; Zhang et al., 2011) This work accurately characterized the electron transport and recombination dynamics in the symbiotic structure of TiO2 microspheres and nanoparticles. The results give us a comprehensive understanding of electron transport and recombination mechanism in mesoporous TiO2 microspheres, which will provide significant information for the optimization of TiO2 nanostructures for DSSCs. Besides, based on the photocatalytic features of TiO2, (Li et al., 2015; Moniz et al., 2015; Ranjith and Uyar, 2017; Song and Paik, 2016; Tang et al., 2008; Zhang et al., 2016) the results of our present work could also offer important implications for other optoelectrical applications.
dimethylimidazolium iodide (DMII), 0.5 M tert-butylpyridine, and 0.1 M guanidinium thiocyanate (GuSCN) in 85/15 (v/v) acetonitrile and valeronitrile) by a laser engraved 60 µm Surlyn gasket under heat and pressure. The solar cell devices using different photo anodes treated by varied ammonia concentrations (0 mL, 2 mL, 4 mL and 6 mL) were also marked as N0, N2, N4 and N6, respectively. 2.3. Characterization The surface morphology was observed by a high-resolution scanning electron microscope (HR-SEM, S-4800, HITACHI, Japan) and a field emission transmission electron microscopy (FE-TEM, JEM-2100F, JEOL, Japan). The crystalline phase of these products was determined by X-ray powder diffraction (XRD, MXPAHF, Mark Corp., Japan), and the crystal size was calculated by the Scherrer equation. The specific surface area (BET) and pore size distribution (BJH) of the samples were investigated by nitrogen adsorption-desorption isotherms measured at 77 K on a BELSORP-mini instrument. UV–vis absorption spectra and diffused reflectance spectra of samples were measured by using a UV–vis spectrophotometer (SOLID 3700, Shimadzu Co. Ltd, Japan) equipped with an integrated sphere. The current density-voltage (J-V) curves, shielded by a mask with an aperture area of 0.16 cm2, were measured by using a Keithley 2420 Digital Source Meter under a xenon lamp (100 mW cm−2). Incident photon-to-current conversion efficiency (IPCE) spectra were confirmed as a function of wavelength from 300 to 800 nm (PV Measurements, Inc.). An electrochemical workstation (IM6e, Zahner, Germany) was employed to measure the intensity of the modulated photocurrent/ photovoltage spectroscopy (IMPS/IMVS) and the open-circuit voltage decay (OCVD). A light emitting diode (LED, λ = 610 nm) driven by Export (Zahner, Germany) were used to provide both dc and ac components of the illumination. Small amplitude is defined as 10% or less than that of the dc component provided by the LED, and the frequency range was from 300 mHz to 3 kHz. The open-circuit voltage decay (OCVD) was conducted by turning off the illumination on DSSC at a steady state and monitoring the subsequent decay of the open-circuit voltage. The electrochemical impedance spectra (EIS) were recorded by a computer controlled potentiostat (Autolab 320, Metrohm, Switzer Land) in a frequency range of 10 mHz–1000 kHz in dark. The impedance data were fitted to a transmission line mode using ZView software.
2. Experimental method 2.1. Synthesis of TiO2 microspheres Spherical mesoporous TiO2 microspheres were prepared by using a two-step method, which combines a sol-gel and a solvothermal process, as described elsewhere (Ding et al., 2015). Firstly, 25 mL of titanium isopropoxide (TTIP) were dropwise added into the sol-gel solution, composed of 3.5 mL of deionized water, 3.5 mL KI of aqueous solution (0.1 M), and 1.3 L of ethanol. The amorphous TiO2 microspheres were obtained by centrifuging at 5000 rpm for 30 min and washing three times with ethanol. Then, those amorphous TiO2 microspheres were dispersed into a mixture of ethanol (100 mL) and deionized water (50 mL). To prepare the symbiosis of TiO2 microspheres and nanoparticles, different contents of ammonia (0 mL, 2 mL, 4 mL and 6 mL) were added into the mixed solution under stirring for 5 min, sealed within a Ti autoclave (260 mL), and heated at 160 °C for 16 h. The asprepared samples were named as N0, N2, N4 and N6, corresponding to the amount of ammonia. Finally, the products were collected by filtration and washed with deionized water and ethanol. The resulting products were named as N0, N2, N4, and N6 throughout this report.
3. Results and discussion 3.1. Properties of symbiotic-structured TiO2 microspheres The morphology and microstructure of mesoporous TiO2 microspheres were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1 shows SEM images of TiO2 microspheres synthesized with different contents of ammonia. In the absence of ammonia, TiO2 microspheres (N0, Fig. 1a and b) exhibit smooth spherical morphology composed of tiny interconnected nanocrystals with a diameter of around 500 nm. With the increase of the ammonia content to 2 mL (N2, Fig. 1c and d), the spherical shape is maintained without breakage, and the nanocrystals at the surface of microspheres grow larger. When the content of ammonia is increased to 4 mL (N4, Fig. 1e and f), the resulting products are not necessarily fully spherical and bounded by relatively rough surfaces with some breaks in between. The symbiotic nanocrystals are fully filled into the adjacent TiO2 microspheres, and eventually a necessary necking was formed between microspheres. However, further increase of the ammonia content to 6 mL (N6) results in the dissociation of microspheres and formation of over-grown nanocrystals (Fig. 1g). As depicted in Fig. S1 (see Supporting Information), all of four samples show pure anatase crystallographic phase. And the average grain sizes are 9.30 nm, 13.24 nm, 15.90 nm, and 20.74 nm for N0, N2,
2.2. Preparation of symbiotic structured TiO2 microspheres and DSSCs assemble Homogenous screen-printing pastes were prepared by the following procedure. 5.0 g of these products (N0, N2, N4 and N6) were completely redispersed into ethanol suspension, containing α-terpineol (25.0 g) and ethyl cellulose (1.0 g) by stirring and ultrasonic dispersing. Afterwards, the TiO2 pastes were obtained by rotary evaporation to remove ethanol. The as-prepared pastes were screen-printed onto the Fluorine-doped tin oxide glass (FTO, 15 Ω sq−1) and then annealed in ambient air condition at 510 °C for 30 min to form TiO2 photoanodes. These photoanodes were treated by 0.1 M TiCl4 aqueous solution at 65 °C for 40 min. After rinsing with water, these photoanodes were sintered at 510 °C for 30 min again. These photoanodes were sensitized with 300 µM C101 dye with cheno-3a, 7a-dihydroxy-5b-cholic acid overnight. After being washed by acetonitrile and dried in air, the sensitized photoanodes were assembled with a Pt-modified counter electrode and electrolyte (30 mM I2, 50 mM LiI, 1.0 M 1,3588
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Fig. 1. SEM images of the TiO2 microspheres synthesized with (a, b) 0 mL, (c, d) 2 mL, (e, f) 4 mL, and (g, h) 6 mL of ammonia.
N4, and N6, respectively, as calculated by using Scherrer equation. The increased grain size with the increase of ammonia content is well consistent with the SEM results. Further, the increased diffraction peak intensity indicates that higher ammonia concentration is beneficial for the grain growth, at least in the range of no more than 6 mL ammonia addition. The main reason is that, due to the hydrogen-bond conjugation to hydroxide monomer, the addition of ammonia reduced the solubility of hydroxide precipitate in ethanol, which is beneficial for the grain growth of TiO2. (Ding et al., 2015). Ammonia has also been used as the nitrogen dopants to synthesize N-doped TiO2 microspheres. (Guo et al., 2011) As shown in Fig. S2, the XPS peaks for the Ti 2p and O 1s reveal meaningful change upon N incorporation. Compared with the binding energy of N0 powder (458.86 eV for Ti 2p and 530.11 eV for O 1s), the binding energy of Ti 2p (458.53 eV) and O 1s (529.78 eV) decreases after N doping, suggesting N doping into TiO2 lattice and substituting for oxygen. The N 1s binding energy further shows the absence of nitrogen in the N0, whereas for N4 a high and broad peak is observed at about 399.6 eV and 400.7 eV, indicating the presence of OeTieN structure. (Chen and Burda, 2004) Besides, the new absorption shoulder at 400–520 nm was found in the UV–vis absorbance spectra for N4 and N6 (Fig. S3), which is related to the presence of nitrogen, because there should not be absorption peak in this region for intrinsic TiO2 microsphere. (Guo et al., 2011) It has been found that the incorporation of N atoms produces partially occupied impurity levels above the valence band maximum of anatase TiO2, and a new mid-gap stat will be formed. (Li et al., 2015) In addition, the absorbance spectra show red shift after N doping and the conduction band minimum position will be decreased with the increased doping concentration. (Niu et al., 2015) The increase of the ammonia content will not only speed up the grain growth, but also raise the structural stress. In this case, most TiO2 microspheres are easily distorted and collapsed into isolated debris of nanocrystals, as revealed by the typical SEM in Fig. 1h. N4 seems to have ideal properties for the photoanode application in DSSCs because the microspheres in it not only possess good continuality in between but also maintain the integrity of the sphere shapes. Therefore, TEM was carried out to obtain further information of N4 and the results are shown in Fig. 2. Fig. 2a suggests that the N4 microspheres are constructed by nanoparticles. The increased ammonia concentration could produce symbiotic nanoparticles, which are inclined to melt together and form a closely linked network. As confirmed from high resolution TEM image in Fig. 2b, the morphology implies complete connection at the necks between adjacent microspheres, and the connection could offer shorter electron transport pathways in TiO2 photoanode of DSSCs. Continuous lattice fringes can be found throughout primary nanoparticles (Fig. 2c), indicating its single-crystal
Fig. 2. (a, b) TEM images, (c) high-resolution TEM image and (d) the corresponding fast Fourier transform (FFT) pattern of N4 sample.
nature. Measurement of the lattice fringes gives a d-spacing of 0.48 nm and 0.35 nm, which can be indexed to the (0 0 1) and (1 0 1) faces of anatase TiO2, respectively. In addition, the (1 0 1) face is interconnected through the (0 0 1) face, indicating that crystal growth occurs primarily by the oriented attachment mechanism. (Ke et al., 2012) According to the theory of crystal growth, the driving force for such oriented attachment process is to reduce the total surface energy by eliminating the surfaces at which the crystallites join. Because the surface energy of the (0 0 1) face is 1.4 times larger than that for the (1 0 1) faces, it is reasonable that the oriented attachment occurs in the [0 0 1] direction and makes a network structure. In this case, the (1 0 1) face is mainly exposed. The fast Fourier transform (FFT) pattern further demonstrates that individual nanoparticles inside a bead are single crystals (Fig. 2d). Thus, detailed analysis of the HRTEM results confirmed that the TiO2 microspheres synthesized with the more ammonia could produce nanoparticles with good crystallinity. Fig. 3 depicts specific surface areas and the corresponding pore size distributions of the TiO2 microspheres, measured by nitrogen adsorption-desorption isotherm and the related parameters are summarized in Table 1. Type IV isotherms with H1 hysteresis loop are observed for all the TiO2 microspheres, indicating the presence of mesopores in the inner microspheres. A hysteresis at high relative pressure was witnessed with the increased ammonia content and this trend turns to be much 589
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Fig. 3. (a) Nitrogen sorption isotherms and (b) the corresponding pore size distributions of the four microspheres (N0, N2, N4, and N6).
with the integrated Jsc from the IPCE spectra, as shown in Fig. 4(b).
Table 1 Physical properties of the four TiO2 microspheres (N0, N2, N4, and N6). Samples
Pore sizeα (nm)
Porosityβ (P)
SBET (m2∙g−1)
Crystal sizeγ (nm)
N0 N2 N4 N6
9.38 12.87 14.03 21.21
0.6341 0.6304 0.6235 0.6193
189.82 136.17 121.31 84.49
9.30 13.24 15.90 20.74
3.2.1. The mechanisms for the varied JSC The mechanisms of the improved JSC for N4 based device will be explained by incident photon-to-current conversion efficiency (IPCE) spectra (Fig. 4b). IPCE is defined as the number of electrons in the external circuit generated by an incident photon at a given wavelength, which is determined by light harvesting efficiency (LHE(λ)), electron injection efficiency (φinj) and electron collection efficiency (ηc), according to the following equation:
α Pore sizes are based on the adsorption average pore width (4 V/A by BET) calculated from the BJH model. Vp β Porosity is calculated from the BJH pore volume according to P = 1 + V . γ
p
Crystal size is calculated by the Scherrer equation.
more obvious in the corresponding isotherms with further increase of ammonia concentration, which resulted in a broadening in pore size distribution, as shown in Fig. 3b. As summarized in Table 1, the average pore sizes are 9.38 nm, 12.87 nm, 14.03 nm and 21.21 nm for N0, N2, N4 and N6, respectively. Whereas, the specific surface area decreases with increasing the contents of ammonia due to the growth of nanoparticles formed at the surface of microspheres. In addition, the SEM images (Fig. 1) demonstrate that the TiO2 microspheres with larger amount of ammonia could easily be broken and therefore allow for denser packing, leading to a lower porosity. (Heiniger et al., 2014).
Jsc = q × IPCE( ) × I0
(1)
IPCE( ) = LHE( )
(2)
inj c
where q is elementary charge, I0 is light intensity. In terms of efficient ruthenium dye (C101), the φinj is generally close to 100%, which can be ignored. While the LHE(λ) is mainly determined by Beer's law expressed by the equation, LHE(λ) = 1 −10 × ( ) , where is the surface coverage of the sensitizer, and ( ) is the molar cross section of C101 dye. The amount of dye loaded on the four TiO2 photoanodes is listed in Table 2. For the N0-based TiO2 photoanode, the adsorption capacity was 2.58 × 10−7 mol cm−2, while for samples N2, N4, and N6, values of 2.20 × 10−7 mol cm−2, 2.14 × 10−7 mol cm−2, and 1.86 × 10−7 mol cm−2 were obtained, respectively. This is attributed to the fact that the increased contents of ammonia could promote the crystallization process and thus decrease the specific surface area. Except for the dye adsorption ability, the light scattering ability is supposed to be one of the most critical issues that affect the performance of LHE(λ). The light scattering effect of the four photoanodes can be characterized by measuring the UV/Vis reflection spectra in the wavelength range from 400 nm to 800 nm. As shown in Fig. S2, the diffused reflectance in the spectral range from 400 nm to 800 nm decreases with the increased ammonia concentration. According to the Mie theory, the light scattering properties of particles are dependent on the size of the particle and the wavelength of the incident light. (Ferber and Luther, 1998) The increased ammonia concentration could destroy the integrity of the TiO2 microspheres and generate a large number of nanoparticles. The larger quantity of nanoparticles results in a more Rayleigh-type scattering of individual small nanoparticles, leading to the inferior light absorption, (Heiniger et al., 2014) which is consistent with the IPCE
3.2. Device performance of DSSCs To explore their potential utilization for solar energy conversion, TiO2 microspheres symbiosis with nanoparticles were used as the photoanode to fabricate DSSCs. Their photovoltaic properties were characterized, with the parameters summarized in Table 2. The typical current density-voltage (J-V) characteristics are shown in Fig. 4a. As shown in Fig. 4a and Table 1, N0 based DSSC demonstrates a shortcircuit current density (Jsc) of 16.65 mA·cm−2, open-circuit voltage (Voc) of 0.708 V, fill factor (FF) of 74.4%, and a power conversion efficiency (PCE) of 8.77%. Interestingly, except for the Voc and FF, the DSSCs assembled with the N2, N4 and N6 photoanodes all demonstrate a higher PCE than that of N0-based DSSC. The remarkable enhancement is mainly ascribed to the improvement of the Jsc, where N4 based cell shows the highest Jsc up to 18.71 mA·cm−2, compared to 18.48 and 17.32 mA·cm−2 for N2 and N6 based DSSCs. The tendency is consistent
Table 2 Comparison of the photovoltaic characteristics (average values of eight cells from same batch) measured under 1 sun illumination for TiO2 microspheres synthesized with different contents of ammonia. Device ID
TiO2 thickness (µm)
Voc (V)
Jsc (mA·cm−2)
FF (%)
η (%)
Absorbed dye (×10−7 mol·cm−2)
N0 N2 N4 N6
12.7 12.9 12.4 12.1
0.708 0.695 0.694 0.700
16.65 18.48 18.71 17.32
74.4 73.1 73.8 73.9
8.77 9.38 9.58 8.96
2.58 2.20 2.14 1.86
590
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Fig. 4. (a) Current density-voltage characteristics (J-V curves) and (b) incident photon-to-electron conversion spectra (IPCE) of representative devices based on TiO2 microspheres with different contents of ammonia.
spectra (Fig. 4b). For N4 based cell, the IPCE is appreciably higher than that of N2 based device in the wavelength range of 400–620 nm, while it is slightly lower in longer-wavelength regions, such as 620–800 nm. The lower IPCE in this long-wavelength region could be caused by the Rayleigh-type scattering of nanoparticles, which brings adverse effect on the LHE(λ). (Heiniger et al., 2014) Based on these results, it can be concluded that the ammonia treatment is not beneficial for enhancing the LHE(λ). For this reason, we may suspect that the real determinant factor may be related to electron collection efficiency (ηc).
would generate a hole in the N-doped TiO2, which might improve the conductivity of anatase TiO2. (Niu et al., 2015) Last reason is the formation of necessary necking between microspheres, which can provide continuous transport channel in the N4 photoanode. As the symbiotic nanoparticles intelligently fill into voids between TiO2 microspheres in the N4 photoanode, the seamless connection between microspheres and nanoparticles is established, which builds extra channels to boost electron transport and suppress charge recombination. And as a result, the prolonged τn will then be achieved. (Ding et al., 2015) Whereas, the N6 photoanode has numerous grain boundaries among the nanoparticles and therefore gives a higher transport resistance, thus prolonging the τd and shorting the τn. The combination of τd and τn allows us to estimate the electron collection efficiency, by using the equation ηc = 1 − τd/τn. The ηc of 96.8% and 95.6% are demonstrated in the N4 and N2 based cells at an irradiation intensity of 96 mW cm−2, respectively, while the values for N6 and N0 base devices show a limited ηc of less than 90%. Besides, when the film thickness is taken into consideration, electron diffusion length (Ln) could be estimated by using the formula Ln=(Dn·τn)1/2 and Dn = d2/(2.35τd) to make a fair comparison of intrinsic electron transport and recombination dynamics of DSSCs, where d is the film thickness. Normally, the Ln values suggest whether photoelectrons can transport to an external circuit prior to recombination, and thereby a long diffusion length is preferred for an ideal TiO2 photoanode. Fig. 5c shows the Ln values of the four photoanodes-based DSSCs, indicating a trend of Ln with an order of N4 > N2 > N6 > N0 at the same light intensity, which is basically in accordance with their ηc sequence. This may be ascribed to the fact that the N4 photoanode assembled from the TiO2 microspheres and symbiotic nanoparticles is favorable for electron diffusion through a longer distance with less grain boundaries and fewer trapping sites. As a result, the ammonia treatment in the preparation of TiO2 microspheres can simultaneously enhance the crystallization and generate the symbiotic nanoparticles, resulting in an
3.2.2. Carriers transport dynamics ηc is determined by the kinetic competition between electron transport and recombination. Intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) have been performed to provide quantitative information on electron transport and recombination behaviors in the TiO2 photoanode. Electron transport time (τd) and electron lifetime (τn) can be obtained according to the equations: τd = 1/ 2πfd and τn = 1/2πfn, where fd and fn are the characteristics minimum frequency of the IMPS and IMVS imaginary component, respectively. Fig. 5a and 5b show plots of the τd and τn as a function of light intensity from 10 mW cm−2 to 100 mW cm−2. Obviously, N2 based cell shows shorter τd and longer τn than N0 based device, indicating a faster electron transport rate and a slower electron recombination rate for the former. It is generally believed that electron transport in TiO2 photoanode is limited by numerous defects and grain boundaries via a series of trapping/detrapping processes. As compared with N0, after ammonia treatment, the better crystalline structure was formed in N2. In this case, a porous film with lower defect density should be achieved. The shorter τd and longer τn will then be obtained. In comparison to other counterparts, N4 based cell shows shortest τd and the longest τn. One reason is that the N4 photoanode has lower surface area and fewer surface trapping sites and defects, as compared to those of the N2 photoanode. Second reason is that the N atom substituting for O atom
Fig. 5. (a) Electron transport time (τd), (b) electron lifetime (τn) and (c) electron collection efficiency (ηc) as a function of light intensities for the DSSCs based on the N0, N2, N4 and N6 based photoanode. 591
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Fig. 6. (a) Electrochemical impedance spectra (EIS) and (b) open-circuit voltage decay (OCVD) curves of DSSCs fabricated using N0, N2, N4 and N6 based photoanode. The inset in (a) is the equivalent circuit applied to fit the resistance data.
improved ηc and adverse effect on the LHE(λ) of the DSSCs. We then concluded that there is a tradeoff between the LHE(λ) and ηc.
equilibrium between the photo-generated electron and electron recombination by the I3− ions in the electrolyte. Fig. 6b shows the photovoltage decay rate as a function of time upon switching off the light. In general, the photovoltage is related to the electron lifetime and will decay sharply along with the electron recombination. Obviously, the photovoltage decay rate of the devices based on the four types of TiO2 photoanodes is in an order of N0 > N6 > N2 > N4, implying a decline trend of recombination rate with the increased ammonia concentration. This result is in concurrence with the EIS analysis shown above. Although electron lifetime has a major influence on the Voc of DSSCs, a small Voc difference between the four DSSCs was observed in J-V curves. It is well-known that the Voc is mainly affected by the conduction band edge movement in DSSCs. As discussed above, there was a decrease in the dye loading of films from N0 to N6, resulting in that the conduction band edge shifts positively and reduce the Voc, which would counteract the increase of Voc in less electron recombination. Therefore, there is a tiny difference between the four types of DSSCs in final Voc.
3.2.3. Energy loss mechanisms in the devices Fig. 6a shows Nyquist plots of the four photoanodes based DSSCs obtained by electrochemical impedance spectroscopy (EIS) in dark conditions. In order to get a clear image of the electron recombination dynamics in photo anode, a negative potential with a value higher than the VOC (∼0.7 V) of the devices, −0.73 V, was applied while carrying out the EIS measurement. (Fabregat-Santiago et al., 2005) In general, two well-defined semicircles are observed in the medium frequency and high frequency range. The large semicircle at medium frequency represents the electron transfer at the dye-TiO2/electrolyte interface, while the small semicircle at high frequency denotes to the redox transfer at the Pt/FTO interface. According to the EIS model and equivalent circuit, the fitted data are listed in Table 3. From Table 3, one can notice that the value of series resistance (Rs) for four DSSCs is similar due to the utilization of the same electrolyte and Pt/FTO counter electrode assembled in the nearly same manner. Whereas, transfer resistance (Rct), which is generally considered to be primarily determined by the electron recombination resistance, shows significant discrepancy, i.e., 51.5 Ω, 64.9 Ω, 88.5 Ω, and 44.4 Ω for the four photoanodes based DSSC, respectively. Moreover, the electron lifetime (τn(EIS)) of the devices based on N0, N2, N4 and N6 is 39.1 ms, 41.5 ms, 42.5 ms, and 30.6 ms, respectively. These results demonstrate that N4 based device, derived from TiO2 microspheres and symbiotic nanoparticles, shows a slower recombination rate as compared with that of other counterparts. This suggests that the unique composite structure is beneficial for electron transport, and hence leads to a slower recapture of photoelectrons by I3− ions occurring in the recombination process, which corresponds well with the IMVS results. Under open-circuit conditions, the electron transfer kinetics can also be investigated by the evaluation of the photovoltage decay rate, which is generally proportional to the interfacial recombination rate. To conduct this measurement, all the DSSCs were illuminated under the simulated sun light to obtain a steady state voltage, indicating the
4. Conclusion In the present work, we provide a facile method for the fabrication of highly crystalline mesoporous TiO2 microspheres with tunable surface areas (84 ∼ 190 m2∙g−1), pore sizes (9 ∼ 21 nm) and particle sizes (9 ∼ 20 nm) by adjusting the ammonia concentration in the solvothermal process. This method can be used to improve the crystallinity of TiO2 nanoparticles and optimize the charge transportation capability. Additionally, the fabricated TiO2 photoanode from the symbiosis of TiO2 microspheres and nanoparticles (N4) provides an appropriate pore structure and good scattering effect. Interestingly, this symbiotic structure demonstrates a considerably higher Jsc of the related photovoltaic devices due to its better contact between adjacent microspheres for fast electron transport rate and slow electron recombination rate. Consequently, the highest η of 9.58% was obtained by using this symbiotic structure, compared with 8.77% of the welldefined TiO2 microspheres without ammonia treatment (N0). The improved photo-electrical properties of symbiotic TiO2 make it promising candidate for photoanode of efficient DSSCs and other optoelectrical applications.
Table 3 Characteristic parameters of the electrochemical impedance spectra (EIS) of the DSSCs based on the four photoanodes in the dark environment at bias of −0.73 V. Cell
Rs (Ω)
Rct (Ω)
Cµ (mF)
τn(EIS)(ms)α
N0 N2 N4 N6
2.32 2.41 2.42 2.39
51.5 64.9 88.5 44.4
0.76 0.64 0.48 0.69
39.1 41.5 42.5 30.6
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51702096, and U1705256), the Fundamental Research Funds for the Central Universities, China (No. 2017MS021 and 2018MS040).
α Rs, Rct, Cµ, and τn(EIS) denote the series resistance, transfer resistance, chemical capacitance, and electron lifetime, respectively.
592
Solar Energy 183 (2019) 587–593
Y. Ding, et al.
Appendix A. Supplementary material
B 107, 5709–5716. Marandi, M., Bayat, S., 2018. Facile fabrication of hyper-branched TiO2 hollow spheres for high efficiency dye-sensitized solar cells. Sol. Energy 174, 888–896. Marandi, M., Goudarzi, Z., Moradi, L., 2017. Synthesis of randomly directed inclined TiO2 nanorods on the nanocrystalline TiO2 layers and their optimized application in dye sensitized solar cells. J. Alloys Compd. 711, 603–610. Mohammadpour, F., Moradi, M., Lee, K., Cha, G., So, S., Kahnt, A., Schmuki, P., 2015. Enhanced performance of dye-sensitized solar cells based on TiO 2 nanotube membranes using an optimized annealing profile. Chem. Commun. 51, 1631–1634. Moniz, S.J.A., Shevlin, S.A., Martin, D.J., Guo, Z.X., Tang, J., 2015. Visible-light driven heterojunction photocatalysts for water splitting – a critical review. Energy Environ. Sci. 8 (3), 731–759. Niu, M., Cui, R., Wu, H., Cheng, D., Cao, D., 2015. Enhancement mechanism of the conversion efficiency of dye-sensitized solar cells based on nitrogen-, fluorine-, and iodine-doped TiO2 photoanodes. J. Phys. Chem. C 119, 13425–13432. Oregan, B.C., Gratzel, M., 1991. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737–740. Poudel, P., Qiao, Q., 2012. One dimensional nanostructure/nanoparticle composites as photoanodes for dye-sensitized solar cells. Nanoscale 4, 2826–2838. https://doi.org/ 10.1039/C2NR30347G. Ranjith, K.S., Uyar, T., 2017. Rational synthesis of Na and S co-catalyst TiO2-based nanofibers: presence of surface-layered TiS3 shell grains and sulfur-induced defects for efficient visible-light driven photocatalysis. J. Mater. Chem. 5, 14206–14219. Roy, P., Kim, D., Lee, K., Spiecker, E., Schmuki, P., 2010. TiO2 nanotubes and their application in dye-sensitized solar cells. Nanoscale 2, 45–59. Scalia, A., Bella, F., Lamberti, A., Bianco, S., Gerbaldi, C., Tresso, E., Pirri, C.F., 2017. A flexible and portable powerpack by solid-state supercapacitor and dye-sensitized solar cell integration. J. Power Sources 359, 311–321. Sheng, X., He, D., Yang, J., Zhu, K., Feng, X., 2014. Oriented assembled TiO2 hierarchical nanowire arrays with fast electron transport properties. Nano Lett. 14, 1848–1852. Simya, O., Selvam, M., Karthik, A., Rajendran, V., 2014. Dye-sensitized solar cells based on visible-light-active TiO2 heterojunction nanoparticles. Synth. Met. 188, 124–129. So, S., Hwang, I., Schmuki, P., 2015. Hierarchical DSSC structures based on “single walled” TiO 2 nanotube arrays reach a back-side illumination solar light conversion efficiency of 8%. Energy Environ. Sci. 8, 849–854. Song, T., Paik, U., 2016. TiO2 as an active or supplemental material for lithium batteries. J. Mater. Chem. 4, 14–31. Sun, Z., Kim, J.H., Zhao, Y., Attard, D., Dou, S.X., 2013. Morphology-controllable 1D–3D nanostructured TiO 2 bilayer photoanodes for dye-sensitized solar cells. Chem. Commun. 49, 966–968. Tang, J., Durrant, J.R., Klug, D.R., 2008. Mechanism of photocatalytic water splitting in TiO2. reaction of water with photoholes, importance of charge carrier dynamics, and evidence for four-hole chemistry. J. Am. Chem. Soc. 130, 13885–13891. Tao, X., Ruan, P., Zhang, X., Sun, H., Zhou, X., 2015. Microsphere assembly of TiO2 mesoporous nanosheets with highly exposed (101) facets and application in a lighttrapping quasi-solid-state dye-sensitized solar cell. Nanoscale 7, 3539–3547. Wang, M., Anghel, A.M., Marsan, B., Ha, N.C., Pootrakulchote, N., Zakeeruddin, S.M., Gratzel, M., 2009a. CoS supersedes Pt as efficient electrocatalyst for triiodide reduction in dye-sensitized solar cells. J. Am. Chem. Soc. 131, 15976–15977. Wang, X., Yang, Y., Jiang, Z., Fan, R., 2009b. Preparation of TiNxO2-x photoelectrodes with NH3 under controllable middle pressures for dye-sensitized solar cells. Eur. J. Inorg. Chem. 23, 3481–3487. Xi, J., Zhang, Q., Park, K., Sun, Y., Cao, G., 2011. Enhanced power conversion efficiency in dye-sensitized solar cells with TiO2 aggregates/nanocrystallites mixed photoelectrodes. Electrochim. Acta 565, 1960–1966. Zhang, J., Sun, Q., Zheng, J., Zhang, X., Cui, Y., Wang, P., Zhu, Y., 2011. The characterization of nitrogen doped TiO2 photoanodes and its application in the dye sensitized solar cells. J. Renew. Sust. Energy 3, 033108. Zhang, Q., Dandeneau, C.S., Zhou, X., Cao, G., 2009. ZnO Nanostructures for dye-sensitized solar cells. Adv. Mater. 21, 4087–4108. Zhang, X., Peng, T., Song, S., 2016. Recent advances in dye-sensitized semiconductor systems for photocatalytic hydrogen production. J. Mater. Chem. 4, 2365–2402.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.02.063. References: Adachi, M., Murata, Y., Takao, J., Jiu, J., Sakamoto, M., Wang, F., 2004. Highly efficient dye-sensitized solar cells with a titania thin-film electrode composed of a network structure of single-crystal-like TiO2 nanowires made by the “oriented attachment” mechanism. J. Am. Chem. Soc. 126, 14943–14949. Bisquert, J., Vikhrenko, V.S., 2004. Interpretation of the time constants measured by kinetic techniques in nanostructured semiconductor electrodes and dye-sensitized solar cells. J. Phys. Chem. B 108, 2313–2322. Chen, X., Burda, C., 2004. Photoelectron spectroscopic investigation of nitrogen-doped titania nanoparticles. J. Phys. Chem. B 108, 15446–15449. Ding, Y., Ma, Y., Tao, L., Hu, L., Li, G., Jiang, L., Dai, S., 2015a. Continuous electron transport pathways constructed in TiO 2 sub-microsphere films for high-performance dye-sensitized solar cells. RSC Adv. 5, 17493–17500. Ding, Y., Zhang, T., Liu, C., Yang, Y., Pan, J., Yao, J., Dai, S., 2017. Shape-controlled synthesis of single-crystalline anatase TiO 2 micro/nanoarchitectures for efficient dye-sensitized solar cells. Sust. Energy Fuels 1, 520–528. Ding, Y., Zhou, L., Mo, L.E., Jiang, L., Hu, L., Li, Z., Dai, S., 2015b. TiO2 microspheres with controllable surface area and porosity for enhanced light harvesting and electrolyte diffusion in dye‐sensitized solar cells. Adv. Funct. Mater. 25, 5946–5953. Fabregat-Santiago, F., Bisquert, J., Garcia-Belmonte, G., Boschloo, G., Hagfeldt, A., 2005. Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy. Sol. Energy Mater. Sol. Cells 87, 117–131. Ferber, J., Luther, J., 1998. Computer simulations of light scattering and absorption in dye-sensitized solar cells. Sol. Energy Mater. Sol. Cells 54, 265–275. Guo, W., Shen, Y., Boschloo, G., Hagfeldt, A., Ma, T., 2011. Influence of nitrogen dopants on N-doped TiO2 electrodes and their applications in dye-sensitized solar cells. Electrochim. Acta 56, 4611–4617. Han, G.S., Lee, S., Noh, J.H., Chung, H.S., Park, J.H., Swain, B.S., Jung, H.S., 2014. 3-D TiO2 nanoparticle/ITO nanowire nanocomposite antenna for efficient charge collection in solid state dye-sensitized solar cells. Nanoscale 6, 6127–6132. Heiniger, L.P., Giordano, F., Moehl, T., Grätzel, M., 2014. Mesoporous TiO2 beads offer improved mass transport for cobalt-based redox couples leading to high efficiency dye-sensitized solar cells. Adv. Energy Mater. 4. Heiniger, L.P., O'Brien, P.G., Soheilnia, N., Yang, Y., Kherani, N.P., Grätzel, M., Tétreault, N., 2013. See-through dye-sensitized solar cells: photonic reflectors for tandem and building integrated photovoltaics. Adv. Mater. 25, 5734–5741. Ito, S., Murakami, T.N., Comte, P., Liska, P., Gratzel, C., Nazeeruddin, M.K., Gratzel, M., 2008. Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10. Thin Solid Films 516, 4613–4619. Ito, S., Zakeeruddin, S.M., Humphrybaker, R., Liska, P., Charvet, R., Comte, P., Miura, H., 2006. High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2 electrode thickness. Adv. Mater. 18, 1202–1205. Ke, C., Chen, L., Ting, J., 2012. Photoanodes consisting of mesoporous anatase TiO2 beads with various sizes for high-efficiency flexible dye-sensitized solar cells. J. Phys. Chem. C 116, 2600–2607. Lee, H.M., Yoon, J.H., 2018. Power performance analysis of a transparent DSSC BIPV window based on 2 year measurement data in a full-scale mock-up. Appl. Energy 225, 1013–1021. Li, G., Li, J., Li, G., Jiang, G., 2015. N and Ti 3+ co-doped 3D anatase TiO 2 superstructures composed of ultrathin nanosheets with enhanced visible light photocatalytic activity. J. Mater. Chem. A 3, 22073–22080. Lindgren, T., Mwabora, J.M., Avendaño, E., Jonsson, J., Hoel, A., Granqvist, C.-G., Lindquist, S.-E., 2003. Photoelectrochemical and optical properties of nitrogen doped titanium dioxide films prepared by reactive DC magnetron sputtering. J. Phys. Chem.
593