A single layer deposition of Li-doped mesoporous TiO 2 beads for low-cost and efficient dye-sensitized solar cells

Herein, we report a new strategy for improving the efficiency and reducing the fabrication cost of dye-sensitized solar cells (DSCs) by elimination of the three-or four-fold layer deposition of TiO 2 . This is performed by replacing a single layer deposition of mesoporous TiO 2 beads, with sub-micrometer size, high surface area and tunable pore size, synthesized by a combination of sol-gel and solvothermal methods. Furthermore, superior electronic properties raised by a reduction in electronic trap states are achieved through doping of pristine TiO 2 beads with lithium. The beads show spherical shape with monodispersed texture consisting of anatase-TiO 2 nanocrystals and ultra-fine pores. The outstanding light scattering and harvesting characteristics of the beads are emerged from a combination of tailored morphology and crystal structure. These have resulted in 40% increase in a solar to electric power conversion efficiency, for a single spin-coated film without an additional scattering layer and pre-and post-treatment with TiCl 4 solution, compared to the reference nanoparticulate TiO 2 device.


Introduction
The urgency for development of clean electrical energy is appreciated today more than ever to reduce carbon dioxide gas emissions and global warming effect.Solar energy is considered as one of the most promising sources for the clean electrical energy, due mainly to its abundance and reliability.A solar cell is a photovoltaic device that converts the solar energy directly into electricity by the photovoltaic effect.Dye-sensitized solar cells (DSCs), proposed in 1991 by Gratzel and O'reagen 1 , have been extensively studied due to their low fabrication cost, nontoxicity of the materials involved 2, 3 and good potential in the form of flexible device 4,5 and improved stability. 67The main component of a DSC is a photoanode electrode carrying a nanostructured oxide semiconductor, typically mesoporous TiO2.Photogenerated electrons inject from dyesensitizer into the conduction band of crystalline TiO2 and then diffuse through the mesoporous network of photoanode.Light scattering of the incident light in DSC devices is a major challenge that has to be improved in terms of transport characteristics of charge carriers and light-harvesting.
In fact, incident light scattering can be achievable by large particles with diameters comparable to the wavelength of the incident light, i.e., submicron particles.Deposition of a single layer, composed of mixtures of nanoparticles and large particles, and double-layer, made of nanoparticulate under-layer as a light-absorber and a coarse top-layer as light scattering film, are two main approaches to encourage light scattering property of the photoanode electrode. 12Among the most prominent approaches taken over the years is the incorporation of a scattering layer in the form of double-layer electrode consisting of TiO2 with various potentially scattering morphologies such as metal titanates, 13 hollow spheres, 14 dandelion-like, 15 nanocubes, 16 corn-like nanowires, 17 octahedron-like particles, 18 nanotubes 19 and spherical York-shell-like 20 structures.Although these approaches are proper configuration for efficient utilization of the solar spectrum, they have some limitations such as enhanced internal resistance and lowered accessible surface for dye loading.In addition, the preparation procedure of double-layer films is complex, since different TiO2 precursors as well as different deposition techniques are required.An alternative approach to overcome these limitations has been to design hierarchical mesoporous TiO2 structures. 21The hierarchical structures not only have exceptionally high surface area due to abundance of mesopores, but also are capable of scattering light within the active layer due to their size resembling wavelength of the incident light.
Therefore, it is possible to simplify and reduce fabrication cost of the device in such a way that solely a deposition of a single layer TiO2 film would be sufficient to achieve a highly efficient photoactive layer. The results showed that the mesoporous beads can enhance the light harvesting within the photoanode without sacrificing the available surface for dye loading, thereby increasing the photon-to-current conversion efficiency.There are several methods such as decreasing bandgap of TiO2 by doping a foreign ion into the TiO2 structure to enhance its electron injection. 25The ion incorporates into the TiO2 lattice, creating subsidiary Fermi level boosts this phenomenon.Further improvement of photovoltaic characteristics of TiO2 beads by doping some impurities such as N2 26 and Fe 27 into the semiconductor TiO2 has been reported.Such enhancement is attributed either to passivation of electronic defect states 28 or enhanced electron trapping, 29 thereby lowering the recombination rate of electron-hole pairs.It has been demonstrated that perovskite solar cells prepared using Li-doped TiO2 electrodes produce substantially higher performances compared with undoped electrodes due to their superior electronic properties, by reducing electronic trap states enabling faster electron transport. 30So far, no work has been reported on the performance of Li-doped mesoporous TiO2 beads as light scattering sector in photoanode of DSC devices.Since Li + has a lower number of valence electrons than Ti 4+ , the excess holes may create an acceptor band near TiO2 valence band.
The migration of electrons from valence to acceptor band could happen easier due to the lower bandgap between the valence and acceptor.
In the present work, we have focused on synthesis of Li-doped mesoporous TiO2 beads with various atomic ratios by a two-step sol-gel and solvothermal procedure for solar cell applications.
Our aim was to simplify the preparation of DSC by a single deposition of TiO2 beads for a highly efficient photoanode electrode.The impact of Li introduction is studied on phase composition, morphology, optical properties, incident light scattering and photovoltaic performance of fabricated DSC devices.

Synthesis of mesoporous TiO2 beads
Mesoporous anatase-TiO2 beads with diameter of 120-1900 nm were synthesized according to a two-step procedure previously reported by Chen et al. 31 In the first step, 0.5 g of hexadecylamine (HDA) with a purity of 98% (Aldrich), as a structure-directing agent, was dissolved in 25 mL ethanol, followed by addition of 0.27 mL deionised water and 0.2 mL of aqueous KCl (with a purity of 99.5%, Merck) solution (0.1 M).KCl controls the monodispersity of the sol-gel derived powders by adjusting the ionic strength of the solution.In a separate beaker, 1.15 mL of titanium tetraisopropoxide (TTIP) with a purity of 97% (Aldrich) were dissolved in 25 mL ethanol and immediately added to the prior solution under vigorous stirring.A milky suspension was instantly formed as a result of hydrolysis and condensation reactions of the sol-gel process.It was found that the reaction time was decreased with increasing water content.The white suspension was kept in the atmosphere for 18 h to deposit all sediments, separated with decanting, washed with ethanol and finally dried at room temperature.The diameter of beads was tailored by controlling the Ti:H2O molar ratio.This was carried out by further addition of 0.20, 0.27, 0.33, 0.40, 0.46 and 1.60 mL deionised water into the first solution, as listed in Table 1.For sample B27NK, no KCl solution was added in the first solution, just contained 0.20 mL deionized water.In the second step, the sol-gel derived powders were treated solvothermally to obtain the mesoporous TiO2 beads.0.4 g of the powder was dispersed in a mixture of 18 mL ethanol and 9 mL deionised water and heated in a Teflon-lined stainless-steel autoclave for 16 h at 160 °C, and subsequently cooled naturally to room temperature.The collected precipitates (i.e., mesoporous beads) were washed with ethanol, dried at room temperature and finally annealed at 500 °C for 2 h.

Li-doped mesoporous TiO2 beads
Lithium was doped into mesoporous TiO2 beads via in-situ exposing of samples to LiCO3 solution in the sol-gel process (i.e., the first step), while the above mentioned deionised water was partially replaced by different amounts of aqueous LiCO3 solution (0.05 M), maintaining Ti:H2O ratio = 1:7.Therefore, B7 sample was used as the reference (i.e., undoped TiO2 beads) and B7L5 and B7L20 were prepared by addition of 0.02 mL and 0.08 mL of the aqueous LiCO3 solution corresponding to Li:Ti molar ratio of 0.0005:1 and 0.002:1, respectively (See Table 1).

Synthesis of TiO2 nanoparticles
TiO2 nanoparticles were also synthesized by a combination of sol-gel and hydrothermal methods based on our previous work, 32 to provide an additional control sample.The performance of cells made of mesoporous beads were compared with that composed of anatase-TiO2 nanoparticles.
Briefly, TTIP was dissolved in 1-octanol with a purity of 99% (Merck) to form a 0.55 M precursor solution.Deionised water was added into the above solution, with a molar ratio of H2O:Ti = 3.3:1, to form the TiO2 sol.The sol was hydrothermally treated in a Teflon-lined stainless-steel autoclave at 180 °C for 4 h.The final products (i.e., TiO2 nanoparticles, NP) were washed with ethanol and deionised water and dried at room temperature.

Preparation of photoanode electrodes
TiO2 pastes with excellent stability and rheology were prepared using the synthesized pristine and Li-doped TiO2 beads according to our previously reported procedure. 33The pastes were spin-coated on cleaned fluorine-doped tin oxide (FTO) substrates in two consecutive steps with coating speeds of 3500 rpm and 4500 rpm each for 15 sec, followed by annealing at 400 °C for 2 h.Our telic photoanodes were obtained by soaking the electrodes in a 0.5 mM ethanol solution of N719 dye (Ruthenium 535-bisTBA; Solaronix, Aubonne, Switzerland) for 18 h, followed by washing unloaded dyes by ethanol.It should be noted that the photoanode electrode was prepared by a single deposition of pristine and Li-doped TiO2 beads with neither pre-and post-treatment with TiCl4 solution, nor the anti-reflecting layer and the light scattering layer.

DSC assembly
The counter electrode was made by drop-coating of H2PtCl6 solution (2 mg Pt in 1 mL ethanol) on clean FTO substrate, carrying a small hole to allow the introduction of the liquid electrolyte using vacuum.The wet electrode was dried and then heat-treated at 400 °C for 30 min.
The dye-soaked photoanode and Pt-coated counter electrode were sandwiched using a thin thermoplastic frame (Surlyn, Solaronix) that melted at 125 °C.DSC devices were fabricated by vacuum injecting of a redox iodine-based electrolyte consisting of 0.5 mol/L 1-butyl-3methylimidazolium iodide, 0.1 mol/L lithium iodide, 0.05 mol/L iodine and 0.5 mol/L tertbutylpyridine in acetonitrile.

Characterisation and measurements
The crystal structure of pristine and Li-doped TiO2 beads was characterized by X-ray diffraction (XRD) using a X'pert Pro MPD diffractometer (PANalytical, Kassel-Waldau, Germany), operating at 30 kV, 10 mA at 2θ of 20−80° (Cu Kα, λ = 1.5406Å).The FT-IR spectra of samples were recorded by a spectrometer (SHIMADZU-IRsolution, 8400S series, Japan) in the wavenumber range between 400 and 4000 cm -1 .The microstructural graphs were imaged by a field

Results and discussion
Figure 1 illustrates the impact of Ti:H2O molar ratio on microstructure of prepared mesoporous TiO2 beads by a combination of sol-gel and solvothermal methods.The spherical beads, composed of fine nanocrystals, had tunable overall dimeter ranging from 100 nm to 2 μm, as presented in Table 1.An intense reduction in diameter of the beads, from 1700 ± 80 nm to 350 ± 40 nm, was achieved by decreasing Ti:H2O ratio down to 1:8 (Figures 1 (a-c)), followed by slightly reduction down to 200 ± 20 nm with further decrease in Ti:H2O ratio to 1:27 (Figures 1   (d-f)).B7 would be an ideal candidate for light scattering in photoanode of the cell since it showed monodisperse beads with a diameter around 900 nm (i.e., comparative particle size to the optical wavelengths).The particle size distribution became more polydisperse (200 ± 60 nm) and more dimers were found when the synthesis process is conducted without KCl solution (Figure 1g).The beads with large particle size give the film the ability to scatter light.To realize high-efficiency DSCs, a double layer film containing a scattering layer made of 400 nm particles is coated on top of an active layer composed of the nanocrystalline TiO2 layer.However, such configuration makes the fabrication process more complex and potentially increases the cost of manufacture.Moreover, the mesoporous beads provide sufficient surface area (see Figure 5) for dye loading, resulting in enhanced light harvesting.
The influence of solvothermal treatment on microstructure of TiO2 beads is shown in Figure 2. It is evident that the spherical TiO2 beads synthesized by sol-gel route had a smooth surface without obvious granular features (Figure 2), while their surfaces were roughly textured, composed of nanoparticles, with around 20% shrinkage in the diameter by subsequent solvothermal treatment.Therefore, a combination of sol-gel and solvothermal processes afforded individual, monodisperse beads with rough surfaces and high surface area.Both pristine and Li-doped beads highly conform to full-anatase pattern, indicating no changes in the crystal structure upon addition of Li ion.Since the Li + radius is greater than that of Ti 4+ (0.76 nm vs. 0.60 nm), the substitution may induce the lattice expansion, resulting in a shift of anatase peak to the lower angles.Although for the replacement of Ti 4+ by Li + ions some Ti-O bonds are broken, which leads to the formation of oxygen vacancies, the contraction of lattice caused by oxygen deficiency is eliminated through lattice expansion induced by the entrance of slightly larger lithium ions.The average crystallite size of both pristine and Li-doped TiO2 beads was calculated to be around 11 nm, using Scherrer equation.The band located around 1633 cm -1 is attributed to bending modes of Ti-OH, due to atomic absorbed water.Two sets of peaks at 1383 cm -1 and 658 cm -1 are related to bending and stretching vibrations modes of Ti-O bands, respectively. 34FTIR spectrum of Li-doped beads shows a significant shoulder-type peak at 498 cm -1 , which can be assigned to Li-O symmetrical stretching vibrations. 29This further confirms the presence of lithium cations in the crystalline network of mesoporous TiO2 beads.BET isotherms obtained from N2 gas adsorption-desorption into pristine and Li-doped mesoporous TiO2 beads and the corresponding pore size distribution curves are shown in Figure 5.The resulted type IV isotherms prove that when relative pressures are too low, only monolayer formation is occurring and as the relative pressures increases higher than 0.5, N2 gas condenses in the tiny capillary pores of the porous beads up to saturation pressure.The Li-doped beads exhibited a greater adsorbed N2 volume before saturation, associated with a slightly higher SBET of 78.1 m 2 /g, in comparison to 75.4 m 2 /g for pure TiO2 beads.In addition, the inset of Figure 5 shows a similar pore size distribution curve for both beads, although the maximum peak was appeared at a smaller pore size for lithium containing sample.Such a porous characteristic of Li-doped beads promotes a more perfect skeletal medium for adsorption of dye molecules.As a comparison, the diffuse reflection spectra (DRS) and corresponding optical bandgap energy of pure and Li-doped mesoporous TiO2 beads were measured (Figure 6).Both beads presented high diffuse reflection capabilities in the wavelengths higher than 400 nm.Such a high DRS property in the visible and near infrared regions indicates that the incident light would be efficiently scattered within the photoanode made up of mesoporous TiO2 beads due to their comparable size to the wavelength of visible light. 13It is noteworthy that Li-doped mesoporous TiO2 beads exhibit a slightly higher light scattering property than pristine TiO2 beads.This can be related to the lattice expansion, due to greater ionic radius of Li compared to that of Ti, and an increase in surface area inducing rougher surface by introduction of Li + into the TiO2 lattice.The higher diffuse reflectance properties of Li-doped mesoporous TiO2 beads under visible light irradiation is expected to improve the utilization of the solar spectrum in the DSCs.In addition, the substitution of Li + with three valence state less than that of the Ti 4+ host atom, results in the formation of positively charged oxygen vacancy, which can act as an acceptor and decrease the donor density.The amount of the adsorbed dye greatly influences the short-circuit current and power conversion efficiency of the cells.Therefore, the impact of Li-doping on N719-dye adsorption of the photoanodes made up of mesoporous TiO2 beads is studied by UV-vis absorption spectra, as illustrated in We can observe a lower dye loading on the photoanode when lithium ions are introduced, which can be explained by two facts.First, this phenomenon can be attributed to different interactions of Ti 4+ and Li + with N719-dye molecules, raised by their different bond strengths.
Considering the bonding energy of Ti-O (i.e., 666.5 ± 5.6 kJ/mol), which is almost two times greater than that of Li-O (i.e., 340.5 ± 6.3 kJ/mol), dye absorption capability of Li-doped TiO2 beads would be less than that of pristine TiO2 beads.Second, the adsorption of the dye molecules by the chemisorption on the thermodynamically stable Ti 4+ sites is greater than that on Ti 3+ surface defects produced by the doping process.It is also observed that higher level doping of Li into TiO2 matrix leads to further decrease in dye absorption due to the above-mentioned facts.
The surface morphology and cross-sectional image of the photoanode composed of mesoporous TiO2 beads is shown in Figure 8. Keeping in mind microstructure and morphology of synthesized beads (Figure 1), it is evident that the preparation process and heat treatment of photoanode using a homemade paste had no influence on morphology and average size of TiO2 beads.The electrode showed uniform, homogeneous and porous structure as a result of removal of the additives.For a high efficiency DSC photoanodes needs to have high surface area with optimum porosity.The photoanode with thickness around 21 µm contained submicrometer-sized beads.
In order to improve the photovoltaic performance of the solar cells, the electron transport mechanism by reducing electronic trap states through Li doping has been reported. 30We present the impact of this mechanism on the photocurrent density-potential (J-V) characteristics of the DSC devices made of Li-doped mesoporous TiO2 beads, as shown in Figure 9.In addition, the corresponding photovoltaic parameters such as short-circuit current (Jsc), open-circuit potential (Voc), fill factor (FF) and power conversion efficiency (PCE) are summarized in Table 2.

Conclusions
To summarize, we demonstrated cost-effective DSC with high efficiency raised by two approaches.In the first approach, the photoanode electrode was simplified by a single spin-coated titania film composed of mesoporous beads without an additional scattering layer and pre-and post-treatment with TiCl4 solution.The diffuse reflectance of the beads was enhanced across the 500-800 nm wavelengths by tuning the size of the beads, their pores and their interconnections, resulted in a substantial enhancement in efficiency up to 7.07% when compared to the control device based on nanoparticulate titania film with PCE of 5.31%.Such enhancement was related to higher photocurrent and the improved charge collection efficiency in the spherical bead photoanodes.Our next approach was to improve the electron transport of the mesoporous TiO2 beads aided by substitutional lithium dopant.Li doping of TiO2 can enable faster electron injection and transport in the device by lowering the conduction band edge of TiO2, reducing the concentration of sub-bandgap states and inducing a partial reduction of Ti 4+ to Ti 3+ .We showed that a DSC prepared on Li-doped mesoporous TiO2 beads with optimum Li concentration of 0.0005 at.% achieved substantially higher performance of 7.48%, compared with the device based on pristine beads.
emission scanning electron microscope (FE-SEM), Mira-3 microscope (TESCAN, Kohoutovice, Czech Republic).The specific surface area and pore size distribution of synthesized beads were determined using a Micromeritics TriStar 3000 analyzer (Micromeritics, Aachen, Germany) by Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) equations at 77.35 K. Diffuse reflectance spectroscopy (DRS) of pristine and Li-doped TiO2 beads and their corresponding optical bandgap were estimated using a UV-visible spectrometer, AVASPEC-2048-TEC, Netherland, carrying a BaSO4 disc as a reflectance standard.The amount adsorbed N719-dye molecules on photoanodes was spectroscopically determined by a UV-vis spectrophotometer (6705 JENWAY, Staffordshire, UK) in a forced dye desorption into an aqueous solution of 0.1 molar NaOH.The photoelectrical metrics of DSC devices were investigated using a Zahner CIMPA-pcs solar simulator (Zahner, Kronach, Germany) under standard test conditions at one Sun (irradiance of 100 mW/cm 2 , AM 1.5) with a scan rate of 50 mV/s.It should be noted that each batch included five devices and the average values were reported.

Figure 1 .
Figure 1.FESEM images of synthesized mesoporous TiO2 beads by a combination of sol-gel and

Figure 3
Figure3shows the XRD patterns of synthesized pristine and Li-doped mesoporous TiO2

Figure 6b exhibits the
Figure 6b exhibits the optical bandgap energy variation of TiO2 beads in the presence of

Figure 8 .
Figure 8. FE-SEM images of the photoanode made of mesoporous TiO2 beads: (a) surface morphology

Figure 9 .
Figure 9. Impact of Li doping on the photocurrent density-potential curves of DSCs made of a single

Table 1 .
The impact of Ti: H2O molar ratio on diameter of synthesized TiO2 beads by the sol-gel process, measured by SEM images in

Table 2 .
Photovoltaic characteristics of fabricated DSCs by a single layer deposition of pristine and Li-doped mesoporous TiO2 beads compared to that made of TiO2 nanoparticles.