lar cells based on hierarchically structured porous TiO 2 fi lled with nanoparticles †

A new morphology of TiO2 photoanodes for N-719 dye-sensitized solar cells (DSCs) has been developed with enhanced power conversion performance. Strategies for the synthesis of hierarchically structured three-dimensionally ordered macroporous (HS-3DOM) TiO2 with controlled macropore sizes (ca. 85– 155 nm) by using well-arrayed polymethyl methacrylate with different diameters as well as two kinds of photoanode films based on hierarchically structured porous TiO2 filled with nanoparticles have been demonstrated. DSCs based on a special TiO2 photoanode with a macropore size of 105 nm exhibited a current density (Jsc) of 20.6 mA cm 2 and a high photo-to-electrical energy conversion efficiency (h) of 9.7%. This high power conversion efficiency is ascribed to the special morphology of the TiO2 photoanode with high dye adsorption due to its ordered and open structures, and also its light scattering and charge collection efficiency.


Introduction
The utilization of solar energy becomes increasingly important, as the fossil and mineral sources are limited and also the main sources of environmental pollution.Dye-sensitized solar cells (DSCs) based on nanostructured semiconductor lms as a type of photovoltaic device have been considered as promising alternatives to silicon-based solar cells due to their low cost of materials and manufacturing. 1 However, improving their relatively low power conversion efficiency remains a big challenge confronting us because of the highest efficiency of up to 13% compared with $20% of silicon-based solar cells. 2 A photoelectrode, when loaded with dyes, as a component of DSCs is important due to its role as a light harvester, charge generator, as well as an electron collector. 3The pore size, specic surface area, surface chemistry and defects, charge mobility, and charge recombination at the interface are all critical in achieving high power conversion efficiency.1d,4 Therefore, a lot of research including fabrication of semiconductors with desired and unique micro and nanostructures as photoelectrodes, most commonly TiO 2 and ZnO 2 , has been carried out to enhance the power conversion efficiency of DSCs.
In comparison with the bulk material, nanocrystalline photoelectrodes can adsorb more dye molecules due to their high specic surface area. 5However, the existence of numerous boundaries in nanocrystalline lms would unavoidably increase their interfacial charge recombination and reduce the charge mobility.Nanowires, nanorods, or nanotubes with one-dimensional structures in single crystal form would provide direct pathways for efficient electron transport to reduce charge recombination at the interface or grain boundaries; 3a,6 however, one dimensional nanostructures suffer from a relatively low specic surface area to ensure sufficient dye loading.Photoelectrodes with hierarchical structures exhibit many advantages in enhancing the power conversion efficiency of DCSs, 7 such as possessing a large specic surface area and effective light scattering, accommodating sufficient dye loading, facilitating electron transport and electrolyte diffusion due to the compact packing of the nanocrystallites and the relatively open structure. 4Hierarchical structures, which have been studied in the eld of DSCs, include spherical/nanorod/core-shell aggregates.7a,c,8 Three-dimensionally ordered macroporous (3DOM) TiO 2 has attracted particular attention in photocatalytic 9 and photovoltaic 10 applications due to its high surface area, novel optical and electronic abilities, interconnected porosity, extremely uniform size, and periodic distributions of pores.3DOM TiO 2 can be fabricated by using monodisperse latex spheres as a hard template. 11However, the overlayers on top of the 3DOM architecture destroy the quality of 3DOM TiO 2 due to the air-and moisture-sensitivity of titanium alkoxides.Therefore many synthetic methods such as a sandwich-vacuum method 12 and an opal inltration method 13 by using polystyrene as a template have been adopted.Herein, a simple method using selfassembled polymethyl methacrylate (PMMA) spheres as templates and tetraisopropyl titanate solution with moisture stability was used to fabricate high quality hieratically structured 3DOM TiO 2 under ambient conditions.Aer the removal of PMMA spheres and subsequent heat treatment, a kind of hierarchically structured porous TiO 2 lled with nanoparticles has been studied as the photoanode in DSCs and the impacts of such hierarchical structures on dye loading, light scattering and charge transport properties, and the consequent power conversion efficiency have been studied and discussed.

Results and discussion
Fig. 1(a-e) show the SEM images of the as-fabricated hierarchically structured three-dimensionally ordered macroporous TiO 2 aer the removal of PMMA templates.All of these samples exhibited excellent three-dimensionally ordered macroporous architectures.It indicates that TiO 2 with three-dimensionally ordered macroporous structures could be successfully fabricated with regularly packed PMMA microspheres, and their qualities were better than that of three-dimensionally quasiordered macropore TiO 2 prepared through a poly(vinyl alcohol) gelated crystalline colloidal array. 14We synthesized four samples with macropore diameters of ca.155, 115, 105, and 85 nm.In Fig. 1(f), the positions of XRD lines for all the samples were the same and could be well indexed to the standard anatase TiO 2 XRD pattern (JCPDS PDF no.78-2486).Therefore, it can be deduced that all these samples possessed an anatase crystal structure. 15It has been reported that the macropore diameters could be controlled by the size of the PMMA microspheres. 16The SEM images of hard templates shown in Fig. S1 of ESI † exhibited the well-arrayed PMMA microsphere templates with 280, 220, 200, and 150 nm diameter and their corresponding pore sizes in TiO 2 samples are 155, 115, 105, and 85 nm, respectively.The size of the macropore in 3DOM TiO 2 samples is 55% smaller than the diameter of PMMA microspheres.It can be easily understood that aer the removal of the PMMA microsphere through decomposition and oxidation during thermal treatment, loosely assembled TiO 2 nanoparticles would go through slight adjustment and partial sintering and densication leading to the shrinkage of void spaces.What is more, the surfaces of all these 3DOM TiO 2 samples were rough as observed from SEM images (Fig. 1(a-e)).Under TEM observation (Fig. 2), detailed morphological structures can be seen and that the frameworks of these TiO 2 samples with 3DOM architectures were comprised of nanoparticles of 7-35 nm.The high-resolution TEM (HR-TEM) image in Fig. 2(f) shows that the lattice space (d) of nanoparticles was 0.35 nm and matches nearly exactly to the theoretical value (3.5165 Å) of the (101) plane of crystalline anatase TiO 2 .
The nitrogen adsorption-desorption isotherms of the TiO 2 samples are shown in Fig. S2(A).† According to the IUPAC classication, the adsorption isotherms can be classied as type II isotherms. 17An adsorption-desorption hysteresis loop with a H3 type is observed in the relative pressure (p/p 0 ) range of 0.8-1.0,17a which does not clearly exhibit any adsorption plateau at relative pressures close to saturation, suggesting the fact of macropore existence, 17a which was identical to the macroporous structures observed from the SEM images.What is more, the low-pressure portion of the almost linear middle section of the isotherm, which is attributable to the unrestricted mono-or multilayer adsorption, indicates the existence of macropores. 18rthermore, there are initial jumps of adsorption isotherms observed from the low-pressure section in Fig. S2(C).† It represents the existence of micropores. 19The pore size distribution curves calculated by the BJH method are shown in Fig. S2(B).† All samples exhibited two pore size distributions: one broad peak centered at 29-45 nm and one narrow peak centered at 2-10 nm and, which can be due to the macropores generated by the removal of PMMA spheres and micropores which are the gaps between adjacent packed nanoparticles, respectively.This result was in good agreement with that of adsorption-desorption isotherms and the observation from TEM images.Table S1 † lists their BET surface areas and pore volumes.It was found that the surface areas and pore volumes increased in the range of 46.1-114.5 m 2 g À1 and 0.25-0.82cm 3 g À1 , respectively, with the decreasing macropore size of HS-3DOM TiO 2 samples from 155 nm to 85 nm.In contrast, a decline trend was observed in the values of the surface area/pore volume from 184 cm À1 to 140 cm À1 while the macropore sizes were decreased.
The pastes based on HS-3DOM TiO 2 were obtained by the treatments described in the Experimental section.The crosssectional SEM images of the photoanode based on Ti-155 are shown in Fig. 3(a), the thickness of the porous TiO 2 layer was about 9 mm and the compact layer, which was used as a holeblocking layer to prevent the direct contact between the FTO and electrolyte, was about 0.3 mm.A conguration of a full DSCdevice is illustrated in Fig. 3(b).From magnifying images of the Ti-155 layer shown in Fig. 3(c and d), it can be observed that HS-3DOM TiO 2 particles had been broken into small particles.However, they did not lose their three-dimensionally ordered skeletal structure completely.These splitting particles mixed together randomly to form a disordered status.From the images of the Ti-85 layer in Fig. 3(f and g), it can be found that many nanoparticles were covered on the surface of particles with 3DOM structures.The status was different from the former one.We deduced that small particles with 3DOM architectures and nanoparticles, which can be broken from their mother particles easily, could generate aer the same experimental treatments in all samples.As for the Ti-155 paste, some nanoparticles could be lled into macropores as described in Fig. 3(e).So, it is hard to nd nanoparticles from the SEM image of the Ti-155 layer in Fig. 3(d).Whereas, as for the Ti-85 pastes, macropores with a smaller size could not allow the nanoparticles to enter, they have to stay out of the pores as illustrated in Fig. 3(h).There are many factors for these two kinds of special morphology pastes that facilitate the performance of DSCs.For one thing, as depicted in the schemes (Fig. 3(e and h)), lots of nanocrystallites broken from larger particles can help contribute to enhancing their surface area which adsorbed N719 dye for light harvesting.Secondly, the ordered structure is benecial to photogenerated charge transportation as shown in Fig. 3(e).Lastly, the porous TiO 2 possesses a relatively open structure which allows the I À / I 3 À electrolyte to completely wet its surface and facilitate electrolyte diffusion.
A strategy for preparing HS-3DOM TiO 2 with tunable macropore sizes has been developed by using a crystalline colloidal array PMMA photonic crystal microsphere packed in the form of the face-centered-cubic structure as a template shown in Fig. 4(a) and tetraisopropyl titanate with PEG-400 and HNO 3 in EtOH solution as a Ti precursor.The rst step was dipping the well-arrayed PMMA powder (Fig. 4(a)) into the Ti precursor solution for four hours.Then, the residual solution has been removed and the remaining solution has been absorbed into the voids of the hard template by the force of capillary pressure as described in Fig. 4(b).EtOH or absorbing water from air can be removed through drying at ambient temperature and calcination at 160 C for 3 h, which was conrmed by the TGA/ DSC measurement (Fig. S3 in ESI †).Then, the micelle will rearrange during the removal of EtOH solution due to the density of the micro-emulsion reaching the critical density as shown in Fig. 4(c).In this state, the organic molecule (PEG-400, (CH 3 ) 2 CHO) becomes an inhibiter to wrap up Ti compounds, which was mainly TiO(OH) 2 as described in Fig. 4(c).Aer calcination at 550 C in an air atmosphere, organic molecules  including PEG-400, (CH 3 ) 2 CHO, and PMMA were removed thoroughly, and TiO(OH) 2 was decomposed and turned into anatase TiO 2 with HS-3DOM structure aggregated nanoparticles shown in Fig. 4(d).According to different macropore sizes of HS-3DOM TiO 2 , the hierarchically structured porous TiO 2 layers present two kinds of morphologies as shown in the HR-SEM images (Fig. 4(k and l)).Their big particles could break into many pieces when subjected to ultrasonication and still maintain 3DOM architectures (Fig. 4(f and i)).As for the Ti-155, Ti-115, and Ti-105 pastes with macropore sizes larger than 105 nm (Fig. 4(e)), big particles blend together randomly and split nanoparticles could get into the macropores of big pieces, which were large enough to let these nanoparticles fall into them (Fig. 4(g)).As for the Ti-85 pastes, most nanoparticles were blocked at the surface of macroporous particles due to their macropore size as observed in Fig. 4(i).Fig. 5 shows the current density-voltage (J-V) curves of DSCs based on hierarchically structured porous TiO 2 lled with nanoparticles.The power conversion efficiency of these devices increased from 7.08% to 9.74% along with the decreasing macropore size of porous TiO 2 layers from 155 nm to 105 nm as listed in the table inserted in Fig. 5.In particular, the solar cell device based on Ti-105 exhibited a short-circuit current density (J sc ) of 20.68 mA cm À2 and an energy conversion efficiency (h) of The amount of adsorbed dye on each cell was investigated; Table 1 shows that the amounts of adsorbed dye on hierarchically structured porous TiO 2 increased from 238 nmol cm À2 to 264 nmol cm À2 with reduced macropore sizes in Ti-155, Ti-115, and Ti-105 photoanodes, and dropped dramatically to 233 nmol cm À2 on the Ti-85 one.In order to investigate the amount of dye adsorption which was altered with the changing macropore sizes in our synthesized photoanode lms, the surface area and pore volume of all powders obtained from drying pastes, which were used to assemble as photoanodes, had been detected by nitrogen sorption isotherms and calculated by BET and BJH methods, respectively.Observed from the texture parameters listed in Table 1, the values of the surface area became larger than those of their corresponding samples before treatment (Table S1 †).The total area per square centimeter of photoanode A can be deduced by the value of the surface area (a) and pore volume (v p ).A given volume v of per gram porous material includes the volume of 1 g bulk material and the total one of pores in 1 g porous material, just as the pore volume (v p ), can be calculated as eqn (1): where r bulk ¼ 3.893 cm 3 g À1 is the density of bulk anatase TiO 2 .Therefore, the total surface area per square centimeter of photoanode A can be calculated by the following eqn (2).
where d is the thickness of the photoanode lm.It was about 9.5 mm in our photoanodes.All of the data are listed in Table 1.The resulting value of A increased in the range of Ti-155 < Ti-115 < Ti-105 from 1.076 Â 10 17 nm 2 to 1.143 Â 10 17 nm 2 and dropped to 1.045 Â 10 17 nm 2 in the Ti-85 sample.It has been reported that per 1 nm 2 TiO 2 can be lled with 1.95 N719 molecules.1a Therefore, we can calculate that the ideal amount of adsorbed dye in 1 cm 2 active area of devices varied from 338.6 to 358.8 nmol cm À2 and is listed in Table 1.Apparently, the amount of adsorbed dye was ca.70% of ideal ones and has the same sequence with their J sc values, which were higher than that (7-10 mA cm À2 ) of DSCs based on TiO 2 nanoparticles with the same dye coverage.1a Furthermore, the as-assembled solar cell based on Ti-105 with 264 nmmol cm À2 dye-loading performed much better than the one based on the three-dimensional array of TiO 2 nanotubes with nearly the same amount of adsorbed dye (260 nmmol cm À2 ). 21iffused reectance and transmittance of hierarchically structured porous TiO 2 layers with and without dye adsorption were measured using a UV-vis spectrophotometer to characterize their light scattering, which is an important factor inuencing the performance of DSCs.It is widely known that the ideal light scattering activities of the photoanode should have a higher diffuse reectance and lower diffuse transmittance.In Fig. 6(A and B), the diffuse reectance of the bare porous TiO 2 layer was in the sequence of Ti-155 < Ti-115 < Ti-105 in the range of 400-800 nm, corresponding to their transmission in the order of Ti-155 > Ti-115 > Ti-105.The increased reectance and suppressed transmission along with the decreasing macropore sizes in Ti-155, Ti-115, and Ti-105 lms may be ascribable to sufficient light scattering due to their porous architectures. 22owever, it is interesting that the reectance of Ti-85 did not continue to increase with decreasing macropore size and became weaker than that of Ti-155 in the range of 500-800 nm.This result may be ascribable to their morphologies.Furthermore, the diffused reectance and transmission of dye-adsorbed porous TiO 2 lms have been investigated and are shown in Fig. 6(C and D).Apparently, they have the same sequences in the reectance and transmission abilities in the dye-absorbed porous TiO 2 layer according to their corresponding layers without dye loading.
The transport and interfacial transfer of electrons in DSCs can be investigated using intensity modulated photocurrent and photovoltage spectroscopy (IMPS/IMVS).The effective electron transport (s t ) and electron lifetime (s r ) can be obtained from IMPS/IMVS measurements by adjusting the light intensity to determine the open-circuit voltage, respectively.The data can be calculated using eqn (3) and ( 4) shown below: where f t and f r are the characteristic frequency minima of the IMPS and IMVS imaginary components, respectively.Fig. 7(A  Obviously, the s t of all devices was decreasing slowly with the increasing voltage, which can be attributed to the trapping/ detrapping of electrons with shallower levels due to the deep traps lled by more photoelectrons at higher voltage.As for the s r , the decrease is ascribable to the increase in the electron recombination rate in higher voltage.Furthermore, the s t and s r of these DSCs were increasing slightly in the sequence of Ti-155 < Ti-115 < Ti-105 < Ti-85, suggesting that the electron transport rate and electron recombination rate decreased a little with the increasing macropore size in hierarchically structured porous TiO 2 .It is said that the electron recombination could enhance with the increasing surface area/pore volume due to the increasing surface defects that result in more charge trapping. 24owever, the electron recombination rate of DSCs based on Ti-85 which possessed the largest surface area/pore volume (140 cm À1 ) listed in Table S1 † was the lowest compared to other devices.This maybe due to the special geometry architecture of the photoelectrodes.Furthermore, the electron collection efficiency (h col ) can be estimated using s t and s r according to eqn (5)  given below: 25 where J inj is the electron-injection current density from the excited dye to TiO 2 and J r is the recombination-current density.Fig. 7(C) shows the h col plots as a function of voltage from 0.18 V to 0.90 V.The h col of the DSCs increased in the order of Ti-85 < Ti-155 < Ti-115< Ti-105 with the same range of J sc which is proportional to h col as depicted in eqn (5).
The electrochemical impedance spectroscopy (EIS) analysis was carried out to study the charge-transfer properties in the DSCs based on hierarchically structured porous TiO 2 as photoelectrodes; the Nyquist plots of the EIS results including various resistance elements for these DSC devices are shown in Fig. 7(D) and Table 2.The equivalent circuit can be interpreted by resistance-capacitance elements inserted in Fig. 7(D).The impedance plots of a DSC device have two semicircles in the Nyquist diagram as shown in Fig. 7(D).The rst semicircle at high frequencies (R t ) is generated by the redox reaction at the platinum counter electrode/electrolyte interface; another one at lower frequencies (R ct ) can be ascribable to the electron transfer between the oxide/electrolyte interface and is equivalent to the recombination resistance; 26 the ohmic resistance R s is caused by the sheet resistance of the FTO glass. 8It is obvious that R s and R t of the DSCs were almost 6.92 and 2.45 U cm 2 , respectively, and the extent of the numerical value variation was very small; the R ct of all DSCs based on hierarchically structured porous TiO 2 increased from 10.97 to 16.55 U cm 2 with the decreasing macropore size of photoelectrodes.It means that the ohmic resistance and the redox reaction resistance at the counter electrode of these DSCs were almost equal; the charge recombination capabilities of these DSCs reduced with the decreasing macropore size of hierarchically structured porous TiO 2 in the order of Ti-155 < Ti-115 < Ti-105 < Ti-85, whose changing trends were the same as the open-circuit voltage variations of these DSCs.These results are consistent with the result obtained by the J-V test (Fig. 5) and IMVS study (Fig. 7(B)).

Conclusions
Herein, a kind of TiO 2 photoanode with hierarchically structured pores lled with nanoparticles was prepared by using hierarchically structured three-dimensionally ordered macroporous (HS-3DOM) TiO 2 .In this paper, we reported the synthesis method of HS-3DOM TiO 2 with different macropore sizes (155, 115, 105, and 85 nm) by using well-arrayed PMMA microspheres.The results of their physical characterization showed that micropores and macropores coexisted in all the HS-3DOM TiO 2 samples with an anatase crystal structure.Hierarchically structured porous TiO 2 layers fabricated from HS-3DOM TiO 2 exhibited two kinds of morphologies, including the ones of nanoparticles lled into macropores and the ones of nanoparticles blocked on the surface of macropores.The highest energy conversion efficiency of these devices was achieved at 9.44% for the DSC device based on Ti-105 with the highest short-circuit current density (J sc ¼ 20.76 mA cm À2 ), which was the main factor inuencing the efficiency due to the little change in the open-circuit voltage V oc .The amount of dye adsorption (101.5-114.8mmol cm À2 ), which was affected by the surface area, pore volume, and thickness of the photoanode, affected the DSC short-circuit current density and was just in the same sequence of J sc (Ti-85 < Ti-155 < Ti-115 < Ti-105).What's more, the light scattering capability of the photoanode, another important factor inuencing the J sc , exhibited the same sequence, too.The IMPS/IMVS results showed that the effective electron transport s t and electron lifetime s r increased in the order of Ti-155 < Ti-115 < Ti-105 < Ti-85, which was proved by EIS.The efficiencies were also affected by the electron collection efficiency h col which was obtained with s t and s r .

Synthesis of highly ordered PMMA colloidal crystal microspheres
The PMMA colloidal crystal microspheres with different diameters were synthesized using an emulsier-free emulsion polymerization approach by adjusting the used amount of K 2 S 2 O 8 and MMA. 16A three-necked round-bottomed alaskite reactor lled with deionized water equipped with a magnetic stirrer was heated at 70 C.A water-cooling condenser with the upper end connected to a wash-bottle containing ethanol solution was xed to the reactor.A certain amount of methyl methacrylate monomer was poured into the reactor at 70 C. Aer stirring thoroughly with water, K 2 S 2 O 8 as a polymerization inhibitor was added into the reactor under stirring.Then, a milky white solution containing PMMA could be generated aer 40 min.The reaction could be stopped by dropwise addition of the solution into cool deionized water.The highly ordered PMMA arrays could be acquired by centrifugation of the PMMA-containing liquid.All of the experimental processes must be protected by a N 2 atmosphere.Herein, PMMA with 280, 220, and 200 nm macropore diameters could be fabricated by using 115 mL of MMA in 1300 mL of water with 0.4, 0.2, and 0.1 g of K 2 S 2 O 8 , respectively.PMMA with 150 nm diameter could be obtained by using 30 mL of MMA in 330 mL of water with 0.2 g of K 2 S 2 O 8 .

Synthesis of hierarchically structured three-dimensional macroporous TiO 2 pastes
The titanic precursor was tetraisopropyl titanate (Ti(OC 4 H 8 ) 4 ) solution (40 vol%) containing 2 mol L À1 concentrate nitric acid and 0.2 mol L À1 PEG-400 additives.The total amount of the precursor was 25 mL.The obtained well-arrayed PMMA templates were impregnated into the above precursor solution for 4 hours under room conditions.Aer ltering the excessive liquid, the obtained samples were dried in a desiccator.Then, the hierarchically structured three-dimensionally ordered macroporous (HS-3DOM) TiO 2 samples could be obtained by calcinating the precursor in air at a ramp of 1 C min À1 from room temperature to 200 C and 550 C and kept at these temperatures for 3 h.The macropore size of HS-3DOM TiO 2 depends on the diameter of the PMMA microsphere.Herein, the as-fabricated HS-3DOM TiO 2 samples with a macropore size of ca.155, 115, 105, and 85 nm were fabricated using PMMA microspheres with 280, 220, 200 and 115 nm diameters, respectively, and can be denoted as Ti-155, Ti-115, Ti-105, and Ti-85 according to their macropore sizes.The HS-3DOM TiO 2 pastes could be obtained via the following method: add 0.05 g of the as-obtained HS-3DOM TiO 2 powders into 0.4 mL of ethanol solution (50%).The mixtures were sequentially treated by ultrasonic treatment with 60 Hz for 1 min and mixing for 1 min.The resulting pastes could be obtained aer 5 cycles.

Assembly of dye-sensitized solar cells
The FTO-coated glass was used as the substrate aer washing subsequently with diluted cleaning detergent solution, deionized water, acetone, and ethanol.A liquid admixture containing 1 mL tetraisopropyl titanate, 0.5 mL ethanolamine, and 5 mL ethylene glycol monomethyl ether was spin-coated onto the clean substrates at 3000 rpm.A compact TiO 2 layer was formed aer drying at 160 C for 1 h and sintering at 500 C for 1 h.It was used to prevent the direct contact between the substrate and the electrolyte.The doctor-blade technique was used to prepare the porous TiO 2 layer onto the compact layer.Aer thermal treatment under 125 C for 1 h and 450 C for 2 h, the resulting photoanode was immersed in 0.3 mM N719 absolute ethanol solution for 72 h, followed by cleaning with absolute ethanol.The chemically platinized silicon wafer (Co.Ltd.PV-Tech) was used as a counter electrode.The electrolyte (OPV-AN-I, Co. Ltd.PV-Tech) was sandwiched by a sensitized TiO 2 electrode and a counter electrode with two clips.

Characterization
The morphology, crystal structure, and surface area of the asfabricated HS-3DOM TiO 2 samples were characterized by Scanning Electron Microscopy (SEM, Gemini Zeiss Supra 55), High-Resolution Transmission Electron Microscopy (HR-TEM, JEOL-2010), X-Ray Diffraction (Bruker/AXS D8 Advance), and nitrogen sorption isotherms (Micrometeritic 2020), respectively.Current-voltage behaviors were characterized under simulated AM 1.5 sunlight with an output power of 100 mW cm À2 , which was calibrated using a NREL-calibrated Si solar cell, generated using a solar simulator (91192, 1 kW Xe lamp, Oriel).The solar cells were masked with a metal aperture to dene an active area of 0.126 AE 0.002 cm 2 .A UV-vis-NIR spectrophotometer (Shimadzu, UV-3600) was used to analyze the diffuse reectance and diffuse transmittance of the TiO 2 lms.Intensity-modulated photovoltage spectra (IMPS) at a short circuit and intensity-modulated photocurrent spectra (IMVS) at an open circuit were measured on an electrochemical workstation (Zahner, Zenium) with a frequency response analyzer under blue light emitting diodes (457 nm) driven by a Zahner (PP211) source supply as a modulated light source.Electrochemical impedance spectra (EIS) were measured at an open circuit under dark conditions.Impedance data were tted using Zview 3.3e soware (Scribner Associates, Inc.).

Fig. 3
Fig. 3 (a) Cross-sectional SEM image of the Ti-155 film without dye loading, (b) configuration of a full DSC-device based on HS-3DOM TiO 2 in this study, high-resolution SEM images of (c and d) T-155, (f and g) Ti-85 layers and their structural illustrations of T-155 (e) and Ti-85 (h) layers.

Fig. 4
Fig. 4 Schematic illustration of the paste formation process based on (a) Ti-155, Ti-155, and Ti-105 samples and the ones based on (c) Ti-85 sample, as well as HR-SEM pictures of photoanode films based on (k) Ti-105 and (l) Ti-85 samples.

9 .
74%.Compared to the DSC device based on the Ti-105 lm, devices based on Ti-155 and Ti-115 had J sc and h values of 16.35 mA cm À2 , 7.07%, and 19.92 mA cm À2 , 8.17%, respectively, with the lowest power conversion efficiency of 6.15% found in DSCs based on the Ti-85 lm.Their open-circuit voltages V oc (718 AE 11 mV) determined by the difference between the Fermi levels of the oxide semiconductor and the redox potential of the electrolyte were almost the same in all of the hierarchically structured porous TiO 2 devices. 20Their ll factors FF, which were imitated with the geometry structure and the transport mechanism of carriers in each individual cell, 20 varied from 57.9% to 64.9%.

Fig. 5 J
Fig. 5 J-V curves of DSC devices based on HS-3DOM TiO 2 with different macropore sizes and parameters of the photovoltaic performance of DSCs devices based on three-dimensionally ordered hierarchically porous TiO 2 (inset).

Fig. 6 (
Fig. 6 (A and C) Diffuse reflectance and (B and D) transmittance of porous TiO 2 layers based on HS-3DOM TiO 2 (A and B) without and (C and D) with N-719 sensitization.

Table 1
Texture parameters of the as-fabricated photoanodes and their amount of adsorbed dye

Table 2
Resistances of N719 dye-sensitized solar cells based on HR-3DOM TiO 2 Films R s /U cm 2 Error/% R t /U cm 2 Error/% R ct /U cm 2 Error/% This journal is © The Royal Society of Chemistry 2015