Perforated BaSnO3 Nanorods ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Perforated BaSnO3 Nanorods Exhibiting Enhanced Efficiency in Dye Sensitized Solar Cells Anurag Roy,†,‡ Partha Pratim Das,†,§ Prabhakaran Selvaraj,‡ Senthilarasu Sundaram,*,‡ and Parukuttyamma Sujatha Devi*,† †

Sensor and Actuator Division, CSIR-Central Glass and Ceramic Research Institute, 196, Raja S.C. Mullick Road, Kolkata−700032, India ‡ Environment and Sustainability Institute, University of Exeter, Penryn, Cornwall TR10 9FE, United Kingdom S Supporting Information *

ABSTRACT: Here, we report the synthesis of phase pure perforated porous BaSnO3 (BSO) nanorods and their application as an alternative photoanode in dye sensitized solar cells (DSSCs). BaSnO3, synthesized using different amounts of dextran, has been characterized through various physicochemical techniques to understand the effect of dextran in controlling its morphology. The porous morphology of the rod facilitated enhanced N719 dye loading capability within a very short duration of 20 min. The dye adsorption behavior of the nanorod has been monitored through UV−vis absorption spectroscopy and contact angle measurements. Further, as an alternative photoanode, a DSSC of active area 0.2826 cm2 fabricated with the porous BaSnO3 exhibited a maximum efficiency of 4.31% with a significantly high VOC of 0.82 V whereas, after TiCl4 treatment, the same cell exhibited an enhanced efficiency of 6.86% under 1 sun AM 1.5. On the basis of our results, we are able to establish porosity as an important factor in reducing the time required for effective dye adsorption which will be highly beneficial for technology development. KEYWORDS: BaSnO3, Morphology, Dextran, Dye sensitization, Photovoltaic

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with an efficiency of only 1.1%.14 Recently, Rajamanickam et al. reported a much lower efficiency (0.71%) fill factor by BaSnO3.15 Very recently (2017), the same group reported an enhanced efficiency of 7.78% from 0.03% Fe doped BSO on treatment with TiCl4 compared to bare pristine BSO having 5.68% efficiency.16 Besides, Shin et al. reported a high efficiency of 4.5% for bare BSO and 6.2% with TiCl4 treatment.5 A maximum efficiency of 5.3% for ∼43 μm thick BSO film has also been reported by Kim et al.17 In most of the above reports, BSO has been synthesized by chemical routes to get nanoparticle or hierarchical nanostructures wherein the dye adsorption property has not been investigated in detail. Dye adsorption time can be improved by altering the porosity of the BSO which can in turn also enhance the efficiency. Therefore, our basic interest was to identify a technique to synthesize porous BSO to improve its dye adsorption capability for enhanced cell performance. In this work, porosity improvement in the BSO structure was achieved by using dextran as a template by following a report of Walsh et al. for metallic sponge synthesis.18 Instead of involving any conjugated polymers, surface directing agents, surfactant, heterocyclic compounds, high viscous solvent, etc., dextran

INTRODUCTION Dye sensitized solar cells (DSSCs) have attracted much attention due to their ecofriendly and inexpensive manufacturing. DSSCs are considered as alternative devices for energy harvesting in diffused sunlight.1,2 Lesser electron mobility, higher dye absorption time, metal ion dye complex formation, interfacial charge recombination between metal oxide and electrolyte, and overall limited synthesis strategies are issues that restrict the scope of applications in DSSCs. There is ample scope to introduce distinctive types of oxides to perform as photoanodes and also monitor the physical, chemical, and optical properties of the complex oxide by altering the compositions of the binary oxides. In order to overcome the limitations of the existing photoanode oxides such as TiO2 and ZnO, efforts have been made to replace TiO2 with other oxides with multiple components such as Zn2SnO4, BaTiO3, BaSnO3, CdSnO3, SrTiO3, and BiFeO3.3−9 In our earlier studies, we have carried out extensive work on Zn2SnO4 to understand its suitability as an alternative photoanode in DSSCs.4,10,11 As an alternative oxide, barium stannate (BaSnO3, BSO) also has been studied as a photoanode in DSSCs. BSO is an important n-type semiconductor (Eg ∼ 3.2 eV) with a perovskite structure exhibiting higher electron mobility and faster dye sensitizing capability at room temperature than the traditional binary oxides, TiO2 and ZnO.12,13 In an earlier study, Guo et al. reported BaSnO3 nanoparticles based photoanode for DSSC © XXXX American Chemical Society

Received: September 28, 2017 Revised: January 15, 2018

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DOI: 10.1021/acssuschemeng.7b03479 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

performed on a STR500 (Cornes Technologies system by using 514.5 nm Ar+ green laser with 50 mW power). Ethanol dispersion of the synthesized BSO has been used to measure the absorption spectrum on a UV−vis-NIR spectrophotometer (Shimadzu UV-3600). The morphology of the synthesized powder has been monitored on a Tecnai G2 30ST (FEI) high-resolution transmission electron microscope operating at 300 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI 5000 Versa probe II scanning XPS microprobe (ULVAC-PHI, U.S.). The measurements were performed at room temperature at a base pressure better than 6 × 10−10 mbar. All spectra were recorded with monochromatic Al Kα (hν = 1486.6 eV) radiation with a total resolution of about 0.7 eV and a beam size of 100 μm. Nitrogen physisorption measurements of all the samples were carried out by using a Quantachrome (iQ3) instrument after evacuation at 150 °C for 4 h. The specific surface area was calculated by the BET method, whereas desorption cumulative pore volume and pore size distribution were calculated by the BJH method. To optimize the duration for maximum loading the optical absorption spectra of the unadsorbed dye solutions have been recorded on a UV−vis-NIR spectrometer (Shimadzu UV-3600). The comparative surface wettability of the films was accomplished by measuring the successive water contact angles on a drop shape analyzer (Krüss DSA25) using Young’s equation (sessile drop method). The volume of each drop was fixed at 5 mL and the dosing rate was 500 mL/min. The instrument was equipped with a CCD camera for image capture. The diffuse reflectance (DR) spectra for before and after dye loading for 30 min were also recorded subsequently. The absorption spectra of the dyes in ethanol solution, ethanolic dispersion of BaSnO3, and dyes after adsorption onto BaSnO3 film was recorded to establish the comparative stability of the metal oxides. The cross-sectional microstructural FESEM images and elemental energy dispersive X-ray analysis (EDAX) of the coated films were checked on a field emission scanning electron microscope (Supra 35VP, Carl Zeiss). Zeta potential measurements have been carried out on a Horiba Nanoparticle Analyzer-SZ100. Fabrication of DSSC. The BSO films fabricated by the screen printing (120T mesh/inch, Mascoprint, UK) method consist of a circular area of 0.2826 cm2 on fluorine doped tin oxide (FTO) (7 Ω/ cm2) glass substrate using a homemade paste with ethyl cellulose and α-terpinol (Sigma Aldirch).26 We used three layers of BSO paste to fabricate the BSO photoanode followed by annealing under an ambient atmosphere following multiple heating steps (125 °C for 5 min, 375 °C for 10 min and 450 °C for 30 min.). For the TiCl4 treatment, the cleaned FTO glass substrates were immersed in a 0.04 M TiCl4 aqueous solution at 70 °C for 30 min and further used for BSO film fabrication. The films were sintered again at 450 °C for 30 min. Besides, the TiCl4 treated BSO films were again dipped in a 0.04 M TiCl4 aqueous solution at 70 °C for 10 min and sequentially fired at 450 °C for 30 min to prepare post TiCl4 treated BSO photoanode films. For the dye adsorption, the BSO films with (or without) the TiCl4 (Sigma-Aldrich) treatment were soaked in a N719 dye (0.5 mM, Solaronix) in absolute ethanol (Merck, Germany) at room temperature for a period of 5−30 min. It is interesting to note that the soaking period for the BSO films is much shorter than that has been reported for conventional materials (e.g., 24 h for TiO2, 4 h for ZnO, 12 h for Zn2SnO4, 120 h for BiFeO3 etc.).6,10,27,28 After the dye adsorption process, the films were thoroughly rinsed with absolute ethanol to remove the physically adsorbed excess dye molecules. Sandwich-type DSSCs were then assembled using the dye-adsorbed BSO film and a platinized FTO substrate (by drop casting) with a hotmelt film (∼25 μm, Surlyn, Dyesol) between them. For electrolyte preparation, 0.3 M 1-methylbenzimidazole (NMB) was thoroughly mixed with 1:1 volume ratio acetonitrile and 3-methoxypropionitrile (MPN) solution followed by the addition of 0.4 M LiI, 0.4 M tetrabutylammonium iodide (TBAI), and 0.04 M I2. The entire mixture was stirred overnight. All the chemicals used were from SigmaAldrich and used without any further purification. Finally, the prepared I3−/I− liquid electrolyte was infiltrated into the cell, and a sandwiched DSSC device was made. The active area of the dye-coated BSO film was 0.2826 cm2.

was introduced by Walsh et al. to synthesize highly porous material. Dextran as a soft template has several advantages like biocompatibility, less stringent use conditions, facile use as a gel in an appropriate mold to readily obtain a patterned material even up to macroscopic morphologies, etc.19−21 At the same time it is a water-soluble glucose-based polysaccharide having a unique structural engagement with a multiband orientation which attracted much attention for use as a soft sacrificial templating agent to synthesize single metal ions, functional metal oxides, porous sponges, zeolites, etc.18,22−24 The recent advances in utilizing organo-halide perovskite nanocrystals as light energy harvesters provide unique opportunities for the development of next generation perovskite solar cells (PSCs) due to their rapid progress toward high photovoltaic performance, potentially high efficiency, and simple assembly procedures. Accordingly, efforts to design new nanostructured architectures for the next generation solar cells including new strategies for solar-to-electric energy conversion have been a major area of research and development. The PSCs are an adaptation of the solid-state DSSCs. The role of the mesoporous layer is quite important to fabricate a PSC device. The commonalities and differences between the crystal structures of BSO and CH3NH3PbI3, which is a classic light harvester in PSCs, are important. BSO has the same perovskite structure as the light harvester, which might produce a more efficient interface. This can be further employed as an alternative electron transporting layer in PSCs.25 In this work, dextran has been utilized to synthesize porous nanorods of BaSnO3 (BSO) followed by detailed studies on the effect of the concentration of dextran in controlling the morphology. In order to control the morphology of the final products the amount of dextran used has been varied and optimized. The performance of synthesized BSO as a photoanode has also been established in this work.

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EXPERIMENTAL SECTION

BSO Nanorod Synthesis. In a typical synthesis, 10 mM each of BaCl2·2H2O (Emsure, Merck, India) and SnCl4·5H2O (98% pure, Loba Chemicals, India) were thoroughly stirred under 30% H2O2 (Emparta, Merck, India) medium for 30 min to form a homogeneous solution. A 25% NH4OH (Merck, India) solution was added dropwise to the mixed solution until the pH of the resultant solution reached 10. To this mixture, dextran (MW ca. 75 000, Alfa Aeasar, Ward Hill, MA) has been added in different weight ratios (2, 4, 6, and 8 g) to control the growth leading to different textures at room temperature. After the addition of dextran, the color of the mixture changed to yellowish white which was aged for 18 h for the formation of the gel. The gel was preheated to 130 °C followed by direct calcination at 1000 °C for 8 h to produce the BaSnO3 phase. The same synthesis process has been carried out without dextran to understand the role of dextran in controlling the morphology. The products formed with 0, 2, 4, 6, 8 g of dextran has been designated as BSO 0, 2, 4, 6, and 8, respectively, in this manuscript. Material Characterization. Thermogravimetric analysis (TGA) of the as-prepared gel was carried out from RT to 1000 °C at a heating rate of 10 °C/min (Shimadzu thermogravimetric analyzer TGA-50) to understand the thermal decomposition characteristics and confirm the phase formation temperature. Dried BSO powder has been characterized for structure analysis using X-ray diffraction (XRD) collected on a X’pert pro MPD XRD of PANanalytical with Cu Kα radiation (λ = 1.5406 Å). The Fourier transformed-infrared (FTIR) spectrum has been measured between 4000 and 400 cm−1 on a PerkinElmer, Spectrum two FTIR spectrometer with a resolution of 4 cm−1 using potassium bromide (FTIR grade ≥99%, Sigma-Aldrich). As received KBr was oven-dried overnight at ∼100 °C and then stored in a desiccator prior to use. Further, a Raman spectrum has been B


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Figure 1. Structural and optical properties of BSO6. (a) Experimental and FullProf refined X-ray diffraction pattern; (inset) crystal structure of pervoskite BaSnO3, green Ba, purple Sn, red O; (b) FTIR spectrum; (c) Raman spectrum; and (d) UV−vis spectrum of BSO6 (inset) Corresponding Taucs’ plot.

Figure 2. TEM bright field images (a) BSO0, (b) BSO2, (c) BSO4, (d) BSO6, and (d) BSO8 samples. to 100 kHz. All the devices were measured at the 0.70 V open circuit voltage of the devices. The experimental data were fitted with the Zview software (version 3.4d, Scribner Associates, Inc., USA) using appropriate equivalent circuits. Incident photon to current efficiency (IPCE) was carried out on a Bentham PVE300 Photovoltaic EQE (IPCE) and IQE solution under 300−800 nm wavelength using tungsten halogen lamp source. All the data presented are an average of measurements taken on three different devices for each sample.

The photovoltaic performances of the assembled devices were measured under 1000 W/m2 of light from a Wacom AAA continuous solar simulator (model WXS-210S-20, AM 1.5 G). The I−V characteristic of the devices was recorded using an EKO MP-160i I−V Tracer. Electrochemical impedence spectroscopy (EIS) measurements were carried out with an Autolab frequency analyzer setup equipped with an Autolab PGSTAT 10 and a frequency response analyzer (FRA) module. The measurements were performed under the same solar simulator condition with the frequency range from 0.1 Hz C


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Figure 3. (a−c) TEM bright field images at different magnifications; (d) HRTEM [(inset) corresponding fast fourier transform pattern]; (e) SAED pattern; and (f) BET surface area isotherm [(inset) BJH pore size distribution curve] of the BaSnO3-6 sample.

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RESULTS AND DISCUSSION Structural and Optical Properties. Thermogravimetric analysis (TGA) of the as prepared gel (Figure S1, SI) resulted in an overall weight loss of only 2.27% within the temperature range from room temperature to 1000 °C. A continuous change in weight terminating at around 110 °C is basically due to the elimination of adsorbed water. Two minor changes in weight ∼470° and 720 °C could be due to the decomposition of hydroxides. The significant asymptotic nature of the TG curve also indicates decomposition of dextran into different carbonaceous materials within 200−700 °C. In order to study the crystallinity and phase purity, the samples were characterized using XRD technique as shown in Figure S2, SI. According to the JCPDS card no. 74-1300, the phase pure BaSnO3 is reported to be a cubic perovskite with a lattice parameter a = 0.4117 nm. The XRD pattern for BSO-6 sample confirms the formation of phase pure BaSnO3 without bearing any SnO2 phase. Therefore, the sample prepared using 6 g dextran and calcined at 1000 °C (BSO6) has been selected for detailed phase analysis through the Reitveld program. Figure 1a shows the Reitveld analysis of BSO6 sample exhibiting a lattice parameter of a = 0.4115 ± 0.0005 nm thereby confirming the formation of a phase pure BaSnO3 pervoskite phase. The refinement factor of 9.06 and χ2 value of 2.19 confirms a good fitting of the collected data. Raman and FTIR spectroscopic studies have been carried out to monitor and confirm the decomposition of dextran followed by BaSnO3 formation. A strong stretching vibration mode of SnO6 octahedral is assigned at 624 cm−1 in the FTIR spectrum of BSO6, as shown in Figure 1b followed by 1458, 1633, and 3426 cm−1 bands which are responsible for the vibration mode of CO and different vibrational coupling of surface bound hydroxyl groups (OH−, OH2+), respectively.29 The surface adsorbed polar groups are preferential attractions for the polar entities of N719 dye for better sensitizing. In addition to FTIR, the Raman scattering measurement of the same is also important information for identifying different bond vibrations in the BaSnO3 lattice.

Figure 1c represents identified several longitudinal (LO) and transverse (TO) optical phonon modes at 131, 248, and 412 cm−1 responsible for LO1, TO2, and LO2, respectively, along with TO1 + LO2, LO3, and LO2 phonon modes with high intense overtones assigned at 552, 664, 723, and 832 cm−1, respectively.30 UV−vis absorption spectra of the BSO6 show a steady absorption which is predominately increasing from 380 nm, as given in Figure 1d. The calculated band gap of the synthesized BSO6 sample is ∼3.36 eV from the derived wellknown Taucs’ plot (shown in the inset of Figure 1d). Transmission Electron Microscopic (TEM) Analysis. BaSnO3 exhibiting various morphologies formed as a result of the change in the dextran concentration in the initial stage are shown in Figure 2a−e. In all cases, dextran assists to render the porous nature throughout the surface of the material. Interestingly, in the absence of dextran, a highly agglomerated particle linkage has been formed, as shown in Figure 2a. It is highly interesting to find that the porous rods have been formed only at a fixed dextran amount of 6 g. A lower amount of dextran resulted in the formation of a spindlelike morphology, and a higher amount of dextran lead to the formation of BSO with irregular geometry, as shown in Figure 2b−e. Figure 3a−c represent TEM bright field images of the synthesized BaSnO3 at different magnifications. For the first time, a porous nanorod based morphology has been formed for the BaSnO3 phase with a range of diameters of 30−70 nm. The 6 g dextran used BaSnO3 powder exhibited highly porous long rods having well-defined perforated structure as evident in Figure 3b. An average diameter of the synthesized rod was found to be around 55 nm, as calculated from the histogram given in Figure 3b (inset) . The formed rods appeared to be very long (>1 μm), and the perforated porous structure has been found to be highly crystalline. The HR-TEM image in Figure 3d shows good crystallinity as evident from the interlayer spacing of the most intense (110) peak as 0.291 nm. The inset of Figure 3d shows the corresponding fast fourier transform (FFT) pattern of the D


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Figure 4. (a) XPS survey and (b−d) core level spectra of Ba-3d, Sn-3d, and O-1s of the BaSnO3-6 sample, respectively.

Figure 5. (a) Adsorption spectra of remaining N719 dye after adsorption on BSO4, 6, and 8 films at their corresponding initial and final stage of sensitization; (b) concentration of N719 dye as a function of sensitization time for BSO4, 6, and 8 films; (c) contact angle measurement with respect to dye sensitization time; and (d) DR spectra of the bare BSO6 and N719 dye adsorbed BSO6 film (inset) chemical structure of the N719 dye molecule.

and high dextran (BSO8) concentrations has been further measured. Comparatively a high surface area of ∼27.48 m2/g was exhibited by BET multipoint isotherm of BSO6 as reported in Figure 3f. The average pore size was ∼3.8 nm for the same sample, as shown in the BJH pore size distribution plot in Figure 3f (inset). The measured zeta potentials of our samples at 22 °C were 14.2, 19.9, and 7.3 mV for BSO4, 6, and 8

HR plane. The selected area electron diffraction (SAED) patterns exhibited the very clear signature of polycrystalline nature with the formation of (111), (110), (200), and (211) crystalline planes as shown in Figure 3e. In order to ascertain the effect of dextran in the formation of BSO rods, the surface area and porosity of three different BSO samples as low dextran (BSO4), optimized dextarn (BSO6), E


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Figure 6. (a) FESEM microstructural cross sectional view of dye loaded BSO6 film (inset) corresponding surface view; (b) elemental mapping of the same film; (c) FTIR spectra of BSO4, 6, and 8 films loaded with N719 dye; and (d) absorption spectra of N719 dye in ethanolic dispersion of BSO6 nanorods at various concentrations of 0.05 × 10−2, 1 × 10−2, 2.5 × 10−2, and 5 × 10−2 M at room temperature.

samples, respectively. A positive surface charge of zeta potential indicates high possibility of the presence of OH2+ group in the sample as observed from FTIR. Table S1 presents a comparison of surface area, pore size, and zeta potential values. X-ray Photoemission Spectroscopic Analysis. The Xray photoelectron spectroscopy (XPS) analysis was performed to understand the chemical and binding states of the elements present in BaSnO3. The survey spectra of the BSO6 sample is shown in Figure 4a. Two peaks observed at 795.5 and 779.9 eV have been assigned to Ba 3d3/2 and 3d5/2, respectively, along with Sn 3d3/2 and 3d5/2 states appearing at 494.2 and 485.3 eV in Figure 4b and c. The symmetric peak centered at 530.2 eV shown in Figure 3d can be assigned to the O 1s spectra. The dominant and nearly symmetric nature of the O 1s spectra has been considered as the characteristics of O2− ions in the Ba−O− Sn metal oxide lattice framework. The XPS data confirms that the three basic elements Ba, Sn, and O present in BaSnO3 possesses only the appropriate as well as most stable valence state of +II, +IV, and −II, respectively, to form the cubic pervoskite BaSnO3 preventing the possibility of the existence of any other metal−oxygen composites. N719 Dye Adsorption Evaluation of BaSnO3 Rods. In order to examine the effect of porous structure on the dye loading capability, we have investigated the dye adsorption capability of the BSO samples. The dye loading performance was evaluated through adsorption analysis of N719 dye on BSO4, 6, and 8 based films as shown in Figure 5a and b. The brown colored curve shows the initial spectrum of N719 dye, whereas after treatment with BSO photoanode the intensity of the dye reduced extensively. The highest adsorption capability was exhibited by BSO6 film. Desorption of the same dye was also carried out after 30 min and monitored as above and BSO6 based photoanode showed enhanced adsorption as recorded in Figure 5a. The difference between the initial N719 dye absorption and after 30 min treatment of BSO was remarkably higher for BSO6 as compared to other BSO films. Figure 5b

presents the concentration of the unadsorbed N719 dye solutions as a function of time. Here, it is evident that the concentration of the remaining dye solution exponentially decreases with time in the order BSO6 > 4 > 8, with BSO6 showing a maximum decrease in the absorbance, which is an indirect evidence for a higher amount of dye adsorbed for BSO6 sample. It could be clearly observed that the process of dye loading was rapid in the initial stage and on preceding the adsorption process. Interestingly, a saturation of adsorption was noticed for all the films within 20 min. It may be due to the highly porous nature of the BSO6 films which are capable of faster intake of dye in a much shorter time than TiO2 and ZnO based photoanodes.31,32 In our previous work with Zn2SnO4, we have projected contact angle (CA) measurement as an indirect tool to understand the dye loading behavior of the photoanode film.10 To further examine the intense dye adsorption behavior of the films we have carried out a chronological dye CA measurement as shown in Figure 5c. Bare nanorods exhibited a CA ∼ 72.3°, indicating the hydrophobic nature, and after dye adsorption of 30 min, the surface became hydrophilic with a CA of ∼17.2°. The change from a hydrophobic to hydrophilic surface as evident from a significantly decreased CA of ∼75% strongly confirms the faster dye loading characteristics of such nanorods. The room temperature diffuse reflectance spectra of the BSO6 film was measured to check the incident light reflectivity. We have reported earlier that incident light reflection is more for rod structures than the particle due to its rough structure.29,33,34 Figure 5d presents the diffuse reflectance (DR) spectra of bare BSO film and N719 dye−BSO6 film. After sensitization with N719 dye, the corresponding reflectance spectra get red-shifted and an additional intense broad band appears in the ∼400−650 nm region owing to the electronic excitation of the adsorbed N719 dye.35 We have closely monitored the dye adsorption experiment to quantify the amount of dye adsorbed. To measure the dye loading capacity, we compared the difference in the concentration of dye solution before and after dye F


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Figure 7. J−V characteristic plot of current density (mA/cm2) versus voltage (V) of (a) BSO4, 6, and 8 based tested devices and (b) BSO6A and BSO6B based tested devices.

Table 1. Photovoltaic and Impedance Performance Parameters of Different BSO Based Devices with Pt Counter Electrode sample

JSC (mA/cm2) ± 0.1

VOC (V)

FF

efficiency (%) ± 0.2

RS (Ω·cm−2)

RCT2 (Ω·cm−2)

BSO8 BSO4 BSO6 BSO6A BSO6B

4.92 8.32 8.11 14.75 16.81

0.701 0.703 0.824 0.765 0.752

0.54 0.56 0.64 0.55 0.54

1.86 3.21 4.31 5.99 6.86

21.65 20.78 20.12 19.35 18.25

42.86 38.11 33.35 31.65 28.85

is no dye/metal ion complex formation even at higher concentration of the dye with BSO. This is in contrast to the behavior of ZnO with dye molecules leading to the aggregation of dye molecules.10 Performance of the DSSCs with BaSnO3. The bare films fabricated with BSO4, 6, and 8 photoanodes followed by sensitization with N719 dye was further characterized by J−V measurements as shown in Figure 7a. A maximum efficiency of 4.31% was achieved for the BSO6 nanorod based films which is comparatively higher than the other irregular rodlike morphologies. The device exhibited a high fill factor (FF) ∼ 64% with a high open circuit voltage (VOC) of 0.82 V due to its high dye loading capacity. The details of the device testing parameters for three individual tested cells are shown in Table 1. It has been observed that BSO6 nanorods have quite an impressive dye loading capacity ∼72.92 μM/cm2 compared to other morphologies of synthesized BSO. The BSO4 and 8 samples exhibited 3.21% and 1.86% efficiencies, respectively. Also the VOC and FF recorded for BSO4 was 0.70 V and 0.56%, respectively, whereas for the BSO8 sample, it was 0.70 V and 0.54% which is quite lower than the BSO6 film. Lower efficiency arising from a lower current exhibited by BSO4 could be due to the unsymmetrical and dissimilar orientation of rods resulting in less dye loading. On the other side, due to too much growth, BSO8 had thicker rods along with nucleation of SnO2 as an extra phase; these could have resulted in the lower efficiency of BSO8. Interestingly, the porous structure of BSO6 nanorods has resulted in an exceptionally high open circuit voltage of 0.82 V and a higher efficiency than other samples. This porous BSO could be projected as a potential candidate for further improving the performance of DSSCs for various applications. Due to the enhanced efficiency of BSO6 nanorods, these have been chosen for TiCl4 treatment to explore the possibility of improving the cell efficiency. It has been reported that TiCl4 treatment on the surface of a thick film photoanode such as TiO2 enhances the bonding strength between the FTO glass and TiO2 layer and reduces the recombination between

loading. The amounts of dye adsorbed per BSO active area were spectroscopically estimated. Color change was observed on the film after 30 min of dye adsorption for BSO4, 6, and 8 films as shown in Figure S3, SI. The most intense N719 sensitized color was visibly observed for the BSO6 film as expected from previous physicochemical studies. The positive zeta value indicates the presence of surface OH2+ groups which is basically governing the interaction with −CN and COO− functional groups of N719 dyes for a faster dye adsorption. The dye loading characteristics data are tabulated in Table S1, SI, where we have mentioned the dye loading capacity, number of dye molecules attached with the BSO oxide surface with respect to their surface area, pore size, and surface charge density. Microstructural and Optical Measurement of BaSnO3 Film. The microstructural image in Figure 6a manifest the sensitization of N719 dye over the BSO6 fabricated film and revealing a thickness of ∼8.51 μm. The microstructure image also indicates undisturbed rod shape even after interaction with N719 dye. The elemental mapping further confirms the homogeneous distribution of Ba, Sn, and O throughout the surface, as evident in Figure 6b. Further, FTIR analysis of the N719 dye loaded films revealed predominant and intense signatures of the CN and COO− anchoring groups of N719 dyes as shown in Figure 6c. A sharp band at ∼2100 cm−1 and multiple bands at ∼3000 cm−1 due to the presence of −CN and −COOH groups of the N719 dye moiety also exist in the IR. The respective intensities get reduced after interaction with the BSO surface. The maximum intense peak of the anchoring unit of N719 was also found in the BSO6 photoanode after dye adsorption which also supports the better dye sensitization behavior of BSO6. Only a negligible change in the absorption spectra of N719 dye was noticed which also support stable interaction of BSO-dye without any aggregation complex between them, as shown in Figure. 6d. The absorption λmax remained the same even after varying the concentration of the dye from 0.5 to 2 mM. This result indirectly confirms that there G


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Figure 8. (a) Nyquist plots of N719 dye sensitized BSO6, 6A, and 6B photoanode devices for the I3−/I− electrolyte and Pt counterelectrode (inset) equivalent circuit used for fitting; (b) IPCE spectroscopic measurement plot for the BSO6, 6A, and 6B devices.

certain thickness of the film and then decreases as shown in Table S2, SI. An average thickness of 2.23, 5.56, 8.51, and 11.22 μm was measured using a cross-sectional FESEM technique for the same devices. As shown from the Table S2, SI, the overall energy conversion efficiencies for DSSCs using the BSO6 photoanode drastically increased as the film thickness increased from one to three layers. Further, there was insignificant change observed in the efficiency from three to four layers. Besides, a linear increase of VOC was recorded as the thickness of the BSO6 films increased. This indicates that the uptake of the N719 dye in the mesoporous BSO layer, which acts as an absorber of the incident light, increases with the thickness and hence efficiency. This improvement in energy conversion efficiency from one to three layers may be due to the adsorption of more dye with the increase in film thickness. However, a thicker film also causes higher resistance for the transportation of electronic charge leading to the recombination of electrons with I3− and becomes less transparent which decreases the solar cell efficiency, which may be causing the inconsequential results in the case of four layers. Thus, in contrast to the recorded photovoltaic parameters of the BSO6 devices, the highest efficiency was exhibited for the three layer device, and hence, this was selected for further study. The thickness and efficiency can be further tuned and optimized using advanced screen printing methods or any other available DSSC film fabrication techniques and deposition methods. Impedance Spectroscopy and Incident Photon-toCurrent Efficiency Analysis. EIS measurement was carried out to investigate the interfacial electron transfer for different BSO samples. The impedance spectra (Nyquist plot) and equivalent circuit diagram (inset) of cells for different BSO samples recorded from 10−2 to 106 Hz are shown in Figure 8a, and their corresponding different resistance values are summarized in Table 1. The high frequency intercepts on the real axis represent the series resistance (RS), which is mainly composed of the bulk resistance of counterelectrode material, resistance of FTO glass substrates, contact resistance, etc. All the cells exhibit similar semicircles in the higher frequency region, emphasizing a similar charge-transfer resistance (RCT1) for the counterelectrode. The high frequency semicircle indicates that the charge transfer resistance is attributed to the different BSO/dye/electrolyte interface (RCT2). As the counter (Pt) and electrolyte (I−/I3−) used are the same in all the cases, we are interested in the BSO/dye/electrolyte interface for the sake of comparison. Among the different fabricated devices, BSO6 exhibited lowest RS and RCT2 values such as 20.12 and 33.35 Ω·cm−2, respectively. In case of the BSO4 device, measured RS and RCT2 values were 20.78 and

electrons on the surface of the glass with the electrolyte that contain holes. This process increases the electron hole recombination and produces enhanced current in the cell. The thin layer of TiO2 nanoparticles also can increase the conductivity of the interface between TiO2 layer and FTO and subsequently faster transition of electrons to the external load.36,37 We have performed both pre- and post-treatment of TiCl4 with the BSO6 photoanode. The FTO substrates treated with TiCl4 solution (pre-treatment) were used as substrates to deposit the BSO6 sample designated as BSO6A in the manuscript. The same film was further treated with TiCl4 solution for improvement and labeled as BSO6B (posttreatment) in the manuscript. Both of the films were further characterized by J−V measurements as shown in Figure 7b. The post-treated BSO6 film, i.e., BSO6B, shows an enhanced efficiency of 6.86% and pre-treated BSO6 film, BSO6A, exhibited a maximum efficiency of 5.99%. It is also noticeable that in the treated films FF was lower than the untreated films. The FF was 0.55 for BSO6A and 0.54 for BSO6B samples with not much change in the VOC. The recorded efficiency of BSO6B film was higher than the 6.20% efficiency reported by Shin et al. for BSO nanoparticles which is probably the highest efficiency recorded for BSO material until now without any doping or use of any scattering layer.5,15 BSO6B exhibited better efficiency than TiO2 itself and is used as a suitable replacement for TiO2. Quick and effective dye adsorption of porous BSO perforated nanorods plays a decisive role for the enhanced efficiency. In the case of DSSCs, the VOC is usually governed by the energy difference between the CB edge of the dye and metal oxide along with the redox potential of the ion conductor. In the case of dye-sensitization, the conduction band (CB) edge of the synthesized BSO is closer to the dye molecule to enable effective electron transfer from the molecule’s excited state to the semiconductor’s CB. On the other hand, a homoleptic dye like N719 having three carboxylate groups to the BSO surface leads to an increased electron donating effect by the additional binding group and a possible closer packing of the dye molecule at the surface, reducing dark current and therefore recombination.38−41 The thickness of the film is quite important indeed for effective dye loading and the consequent performances of DSSCs. The efficiency of DSSCs can be improved by (a) increasing adsorption of sensitizing dye molecules within the semiconducting film, (b) transporting the electronic charge through the film, and (c) reducing recombination of photoexcited electrons injected into semiconductor. The thickness of the film is an important parameter to control such properties. It was observed that the efficiency of the cell increases up to a H


Research Article


Figure 9. Schematic representation of (a) dextran template chemical synthesis of various nanostructures of BaSnO3 and (b) BSO6 samples with bare, pre-treatment TiCl4 (BSO6A), and post-treatment of TiCl4 (BSO6B) fabricated film.

Table 2. Reported Performances of N719 Dye Sensitized BSO Based Devices morphology

sensitizing time

nanoparticle nanoparticle

24 h 1h

nanoparticle and nanorods

1h

nanoparticle

24 h

nanoparticle

24 h

nanoparticles

12 h

porous perforated nanorod

20 min

VOC (V)

surface area (SA)/porosity SA: 5.47 m2/g SA: 7.5 m2/g

0.68 0.83 0.78

SA not mentioned pore size: 50−100 nm

0.62 0.62

SA: ∼46.6 m2/g pore size: ∼19.8 nm SA: ∼27 m2/g

0.70 0.82

pore size: ∼4 nm

efficiency (%) 1.10 bare: 0.71 with TiCl4 and TiO2 NPs: 5.68 bare: 5.68 0.03% Fe doping + TiCl4 treatment + TiO2 scattering layer: 7.78 bare: 4.50 with TiCl4: 6.20 7.6 μm: 2.9 43 μm: 5.20 bare: 3.5 with TiCl4: 5.18 bare: 4.31

ref 14 15 16

5 17 49 present work

with TiCl4: 6.86

38.11 Ω·cm−2, respectively, whereas for the BSO8 device they were 21.65 and 42.86 Ω·cm−2, respectively. BSO8 exhibited a comparatively higher RCT2 value which may be due to the existence of SnO2 as an additional phase with BaSnO3 which may reduce charge mobilization. In our case, the perforated nature of the BSO6 sample could facilitate faster and more dye adsorption leading to lesser RCT2 which may allow enhanced VOC (>0.8 V). The RCT2 becomes lower after TiCl4 treatment as seen from BSO6A and BSO6B. BSO6B exhibited the lowest RS and RCT2, 18.25 and 28.85 Ω·cm−2, respectively, compared to values for BSO6A (19.35 and 31.65 Ω·cm−2). These results reveal a better charge transfer in the cell after surface treatment with TiCl4. RS greatly affects the FF and JSC of the solar cell, with higher FF and JSC values resulting from the smaller RS value, which is consistent with the results of photovoltaic performance.4 In this work, it was observed that FF becomes lower after TiCl4 treatment which simultaneously reduced both the RS and RCT values. Thus, at this point we may state that the effect of the TiCl4 treatment does result in a distinctive increase in dye adsorption. Presumably, the BSO surface after the TiCl4 treatment provides more specific binding sites. Potentially, the TiCl4 treatment also reduces the fraction of the TiO2 surface area that is inaccessible for the dye due to steric

constraints. The rapid electron conduction of the BaSnO3 photoanode is clearly beneficial for efficient DSSC development. Further, we have performed IPCE measurements as a function of wavelength by comparing the devices BSO6, BSO6A, and BSO6B as shown in Figure 8b. IPCE resembles the external quantum efficiency of the DSSC device including the effects of optical losses caused by transmission and reflection.42 The IPCE curve for all the BSO cells shows a broad peak over the range of 300−800 nm with a maximum value of ∼82% at 530 nm for the highest efficiency recorded cell, BSO6B which is almost double that of the bare BSO6 based device (∼40%). The calculated JSC values for the same devices as obtained by the IPCE integration analysis were compared in Table S3, SI. The IPCE results strongly show the advantage of TiCl4 treatment with the BSO6 sample by improving the incident photon to current conversion greatly to reduce electron recombination, entrapment within surface trap states, or loss within the electrolyte. Effect of Dextran and Advantage of Porous Rods. As, we have used highly soluble Ba(OH)2 and Sn(OH)2 as precursors which could participate in strong hydrogen bonding with the soluble dextran preferably at the labile 4,3-glycoside I


Research Article

ACS Sustainable Chemistry & Engineering sites, the growth became anisotropic thereby favoring the formation of rod shaped BaSnO3 nanostructures. The spindle turned rod-oriented structure may also follow a similar mechanism determined by the use of dextran. Further, the chainlike structure of the structure-directing agent dextran preferentially combines with the hydroxide nanocrystals anisotropically as building blocks thereby able to create patterned slits which also facilitates the etching of the sites without the adsorption of dextran. This may cause the perforated morphology of BSO allowing higher surface energy. As the amount of dextran addition was increased, more nanorods were formed, it resulted in denser microrods. Addition of a higher amount of dextran may lead to increased viscosity of the gel that restricted the Brownian motion of the metal ions in the gel and formation of large dextran−metal oxide composite open framework foam during heating resulting in such denser micro rods. Continued addition of dextran further resulted in larger and denser rods.43−47 The overall synthesis procedure has been schematically illustrated in Figure 9a. The rods due to their anisotropic one-dimensional structure and lesser grain boundaries are expected to favor a direct conduction pathway for rapid electron transport through them.33,34 In comparison to the reported results on the performance of BSO based photoanodes (Table 2), the results obtained in this work are quite competitive and interesting to execute dextran template BSO as an alternative photoanode for the DSSC. Onedimensional (1D) rod structures of BSO are expected to improve the electron diffusion length in the photoanode films significantly by providing direct conduction pathways for electron transport and lesser grain boundaries than other morphologies.48 JSC of DSSC device is mainly influenced by the light harvesting and charge recombination at the photoanode.28 Since the rod structured photoanodes are having many advantages, we focused our work on synthesizing BSO nanorods for evaluating its performance for DSSC applications. Further, for faster electron conduction as well as to reduce the loss of electron recombination, entrapment, or passivation, we allowed pre- and post-treatment of TiCl4 using BSO6A and BSO6B devices, as schematically presented in Figure 9b. Our objective was to explore these additional steps to enhance the device performance of BSO6 via the TiCl4 treatment which provides a blocking layer of TiO2 for reducing the loss of electron recombination. The enhancement of JSC indicates direct evidence of improvement of electron conduction rate into the device which ultimately provides enhanced efficiency. After posttreatment of TiCl4, BSO6B exhibits significant enhancement in JSC more than double that of bare BSO, ultimately leading to higher efficiency. The reported performance of the BSO photoanode in DSSC has been highlighted in Table 2. Interestingly, a general trend of improvement in photovoltaic performance has been noticed for the BSO nanorods compared to other nanoforms by a comparatively facile synthesis with higher surface area and VOC.

nanorods having well-defined perforated structure which was very effective in exhibiting extraordinarily fast dye loading of only 20 min. A maximum solar to energy conversion efficiency of 4.31% along with an exceptionally high VOC of 0.82 V has been observed for the same in comparison to other morphologies subjected to study. A typical post TiCl 4 treatment further enhanced the efficiency to 6.86% which is the highest efficiency reported so far as per the performance of the BSO based alternative photoanode is concerned.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03479. TG curve of as prepared dextran gel to prepare BaSnO3, X-ray diffraction pattern of BaSnO3 upon variation of dextran addition along with the standard JCPDS data, digital image of the N719 dye sensitized tested films of BSO8, 4, and 6, comparative study of surface characteristics and dye loading properties for different BSO morphologies in a tabular form, photovoltaic performance of thickness depended BSO6 film in a tabular form, comparative results of short circuit current density (JSC) and corresponding efficiency recorded from IPCE and J‑V measurement techniques by keeping the same values of FF and VOC as recorded for J‑V measurement for all the devices in a tabular form (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +91-33-2483-8082. E-mail: [email protected], [email protected] (P.S.D.). *Tel.: +44 (0) 1326 259486. E-mail: [email protected] (S.S.). ORCID

Parukuttyamma Sujatha Devi: 0000-0002-6224-7821 Present Address §

Crystallographic Lab, Department of Earth System Sciences, Yonsei University, Yonseiro 50, Seoul 03722, Korea (P.P.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.R. gratefully acknowledges the INSPIRE program of Department of Science and Technology (DST), Govt. of India, for the PhD fellowship and Newton−Bhabha Fellowship Program 2016−2017 funded by DST, Govt. of India, and British Council. P.S.D. acknowledges the TAPSUN program for funding the DSSC program under MNRE, GAP 0339.

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REFERENCES

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CONCLUSION In conclusion, the dextran templated process has been successfully introduced in synthesizing perovskite BaSnO3 where the amount of dextran, the structure directing surfactant, influences the evolution of spindle to rodlike microstructures exhibiting high surface area and porosity. A specific quantity of dextran lead to the formation of phase pure porous BaSnO3 J


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