High performance dye-sensitized solar cells with record open circuit voltage using tin oxide nanoflowers developed by electrospinning

Abstract

Flower shaped nanostructures of an architypical transparent conducting oxide, SnO₂, have been synthesized by an electrospinning technique for the first time by precisely controlling the precursor concentration in a polymeric solution. The flowers were made up of nanofibrils of diameter ∼70–100 nm, which in turn consisted of linear arrays of single crystalline nanoparticles of size 20–30 nm. Mott–Schottkey analysis shows that flowers have an order of magnitude higher electron density compared with the fibers despite their chemical similarity. Dye-sensitized solar cells fabricated using the flowers showed a record open circuit voltage of ∼700 mV and have one of the highest photoconversion efficiencies so far achieved with SnO₂ and the iodide/triiodide electrolyte. The photoaction spectra and impedance spectroscopic measurements show that the flowers are characterized by a higher electron lifetime, owing to their enhanced crystallinity, compared to the conventional fibrous structure.

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Broader context

If larger amounts of metal precursors can be enclosed in a nanostructured polymeric architecture, then unconventional morphologies such as flowers of the corresponding inorganic solid could result upon annealing. This hypothesis has been proved by producing flowers of tin oxide, an architypical transparent conducting oxide for energy and environmental applications, from electrospun polyvinyl acetate nanofibers of increased tin concentration. The flowers are comprised of several rare combinations such as ‘high crystallinity and high specific surface area’, and ‘enhanced electron density, diffusivity, and mobility for materials of similar chemical identity’. The flowers, when employed as charge separation and transport medium in dye-sensitized solar cells, resulted in a record open circuit voltage of ∼700 mV and have the highest photoconversion efficiency so far achieved with dye-sensitized tin oxide and the iodide/triiodide electrolyte. The novel properties of this material combined with high electrical conductivity and optical transparency could be of wide interest in sensors, batteries, supercapacitors, solar cells, and other optoelectronic devices requiring a large surface area and high electron mobility. Moreover, the electrospinning technique used for producing the flowers is cost effective as well as feasible for large scale production of advanced materials and membranes for energy and environmental applications.

1. Introduction

Dye-sensitized solar cells (DSCs) represent a low-cost renewable energy device due to several advantages, including simple fabrication technique and high energy photovoltaic conversion efficiency (η).^1–5 The photovoltage in DSCs is proportional to the difference in Fermi levels of a dye-anchored wide band gap semiconductor, the photoanode, and a hole conductor, usually a redox electrolyte. The efficiency of DSCs depends on various parameters which include morphology and compactness of photoanode, chemical nature and purity of dye, electrolyte composition, counter electrode, and other related parameters.^6–12 The dye-loading and hence the number of photogenerated electrons in DSCs, charge transport, and a number of other efficiency determining factors depend on the photoanode and therefore, immense research is directed at optimizing suitable materials and morphology for this application.^6,9,13–18

Among the various photoanodes tested, mesoporous TiO₂ is the most successful material so far in DSCs for the charge separation and transport due to a number of reasons, such as (i) ease in getting mesoporous particles, (ii) band alignment with various organic and inorganic dyes, and (iii) stability. However, low electron mobility (μ_n) through mesoporous TiO₂ (∼0.1 cm² V⁻¹ s⁻¹) is a crucial issue and imposes severe limitations in enhancing the η of DSCs closer to the theoretical limits.¹⁹ One of the main hurdles due to the inferior μ_n is the electron recombination with the electrolyte if the photoanode layer thickness is larger than the diffusion length (L). L is the electron transit length above which they are lost via recombination. Tin oxide (SnO₂) nanostructures, on the other hand, are a well-known transparent conducting oxide for nanoelectronics due to high μ_n (10–125 cm² V⁻¹ s⁻¹) and a wider band gap (∼3.6 eV).^20–23 However, its conduction band minimum occurs at an energy lower than that of TiO₂²⁴ and therefore, DSCs with a SnO₂ electrode usually give low open circuit voltages (V_OC ≤ 600 mV).²⁵ Recently, V_OC up to ∼600 mV has been achieved by preparing the SnO₂ core–shell electrodes and/or making composite electrode with other wide band gap semiconductors.^16,26,27 To increase the η of SnO₂ DSCs, several approaches have been considered: (i) modifying the electrode surface²⁸ (ii) modifying electrolyte composition^28,29 (iii) combining with other metal oxide nanoparticles²⁷ and (iv) by using a core–shell configuration of suitable energy band matched metal oxides.^27,30,31

Electrospinning provides a simple and versatile technique for producing continuous fibers of nanostructured advanced materials as well as membranes for many engineering applications to be enabled, e.g. mechanical reinforcement, water filtration, textile, tissue engineering, energy and environment, and drug release.^32–38 Usually electrospinning produces ultrafine polymeric fibers with diameters ranging from nanometers to sub-micrometers and possess high surface area to volume or mass ratios. If the polymeric solution contains precursors for formation of an inorganic solid, then appropriate annealing produces its continuous nanofibers in three steps, viz. (i) nucleation, (ii) growth, and (iii) directional mass transport. In this paper, we report a flower-like morphology of SnO₂ by electrospinning for the first time by controlling the Sn precursor concentration. The flowers thus obtained were characterised by several rare combinations such as ‘high crystallinity and high specific surface area’, and ‘enhanced electron density, diffusivity, and mobility for materials of similar chemical identity’. The DSCs fabricated by the flowers gave the highest V_OC (∼700 mV) and one of the highest photoconversion efficiency (η ∼ 3.0%) so far achieved using SnO₂.

2. Experimental details

2.1 Synthesis of wire and flower-shaped SnO₂ nanostructures by electrospinning

Anhydrous tin(II) chloride (SnCl₂), dimethylformamide (DMF), ethanol and polyvinyl acetate (PVAc) were the starting materials. Two sets of syntheses were done by simply changing the precursor concentration. In a typical synthesis, 0.4 g of SnCl₂ was dissolved in a mixture of 12.5 mL of DMF and 4 g of ethanol and was stirred for ∼2 h. To the above solution, 1.45 g of PVAc was added and the stirring was continued for ∼4 h until the solution became clear. The above procedure produced 2.5 mM of SnCl₂ solution (Sample S1). Similarly 5.5 mM solution of SnCl₂ was produced (Sample S2). The clear solutions thus obtained were electrospun using a 27 G1/2 needle under an applied voltage of 25 kV with a solution injection rate of ∼1 mL h⁻¹. The solid fibers were collected at distance of 10 cm below the spinneret. The composite fibers were then peeled off from the collector and calcined at 500 °C for ∼3 h to allow crystallization of the SnO₂ phase and removal of the polymer.

2.2 Characterization

The electrospun SnO₂ nanostructures were characterized by BET surface area measurements (NOVA 4200E surface area and pore size analyzer, Quantachrome, USA), powder X-ray diffraction (XRD, Bruker-AXS D8 ADVANCE powder X-ray diffractometer), field emission scanning electron microscopy (FE-SEM, Quanta 200 FEG System, FEI Company, USA, operated at 10 kV), energy dispersive X-ray spectroscopy (EDS, JEOL JSM-6701F microscope operated at 30 kV), and high resolution transmission electron microscopy (HR-TEM, JEOL 3010 operated at 300 kV). X-ray photoelectron spectra (XPS) of the SnO₂ flowers and fibers were obtained using a VG Scientific ESCA MK II spectrometer using monochromatic Mg Kα radiation (1253.6 eV). The survey spectra were recorded in the range 0–1099 eV with constant pass energy of 50 eV; the high resolution spectra were recorded with a smaller constant pass energy of 20 eV. Charge referencing was carried out against adventitious carbon C, assuming its binding energy at 284.6 eV. Analysis of the XPS spectra was done using XPS Peak-fit software. A Shirley-type background was subtracted from the recorded spectra and curve fitting was carried out with a Gaussian–Lorentzian (ratio 60 : 40) curve. The derived binding energies (BE) have accuracy better than ±0.1 eV.

2.3 Electrochemical impedance spectra and Mott–Schottky analysis

Samples for EIS were prepared by sintering SnO₂ flowers and fibers separately onto fluorine doped tin oxide (FTO) coated glass plates and sealed using a 50 μm spacer. Acetonitrile containing 0.1 M lithium iodide, 0.03 M iodine, 0.5 M 4-tert-butylpyridine and 0.6 M 1-propyl-2,3-dimethylimidazolium iodide was used as the electrolyte. A platinum coated glass plate was used as the counter electrode. The Mott–Schottkey plots were generated from electrochemical impedance spectroscopy (EIS) measurements in the dark for bias voltages from 0.4 to 0.7 V and fitting the observed EIS spectra to a simple RC circuit.³⁹

2.4 Cyclic voltammetry studies

Cyclic voltammetry measurements were carried out on a CHI 760 electrochemical workstation with three-electrode cell in a solution of Bu₄PF₆ (0.10 M) in acetonitrile at a scanning rate of 100 mV s⁻¹. A glassy carbon disk was used as working electrode and a silver wire as reference electrode. Ferrocene was used for potential calibration.

2.5 Fabrication of solar cells and evaluation of photovoltaic properties

The two polymeric solutions were electrospun separately onto FTO glass substrates (1.5 cm × 1 cm; 25 Ω/□, Asahi Glass Co. Ltd., Japan). Thickness of the electrode was controlled by placing the FTO substrates under the spinneret for different durations of time. The samples were wrapped in aluminium foil and hot-pressed (Stahls' Hotronix 6 × 6 press, USA) at ∼200 °C with a pressure of 0.4 MPa for 15 min to improve the adhesion of the fibers and the flowers to the FTO substrate. The hot-pressed films were then annealed at 500 °C for 3 h to completely remove PVAc and allow nucleation and growth of SnO₂. The DSCs were prepared by soaking a 0.28 cm²SnO₂ electrode in a dye solution containing a 1 : 1 volume mixture of acetonitrile and tert-butanol and a ruthenium-based dye [RuL₂(NCS)₂·2H₂O; L = 2, 2′-bipyridyl-4,4′-dicarboxylic acid (0.5 mM), N3 Solaronix)] for 24 h at room temperature. The dye-sensitized samples were then washed in ethanol to remove unanchored dye and dried in air. The dye-anchored electrodes were sealed using a 50 μm spacer. Acetonitrile containing 0.1 M lithium iodide, 0.03 M iodine, 0.5 M 4-tert-butylpyridine and 0.6 M 1-propyl-2,3-dimethyl imidazolium iodide was used as the electrolyte. A Pt sputtered FTO glass was used as the counter electrode. Photocurrent measurements were carried out by a XES-151 S solar simulator (San Ei, Japan) under AM1.5G conditions. The level of standard irradiance (100 mW cm⁻²) was set with a calibrated c-Si reference solar cell. Data acquisition was facilitated by an Autolab PGSTAT30 (Eco Chemie B.V., The Netherlands) integrated with a potentiostat. IPCE measurements were done using the standard kit from Oriel Instruments (model QE-PV-SI, Silicon detector, Newport Corporation, US). The AC responses of the cells were studied by electrochemical impedance spectroscopy using the Autolab PGSTAT30.

3. Results and discussion

3.1 Proposed growth model of nanoflowers

Inorganic fibers from the electrospun polymeric fibers form in three steps; viz. nucleation, growth of the inorganic phase and directional mass transport within the polymeric template.³³ As the nucleation and growth of the particles takes place in a supercooled liquid, i.e., in polymer, the evolution of ensemble of nanoparticles can be expressed in terms of the Gibbs–Thompson equation:

In the above expression, C(r) and are the solubility of the particle with radius r and of the bulk material, respectively; γ is the surface tension; V_m is the molar volume of the solid. The supersaturation achieved by heating the fibers results in nucleation of the SnO₂ nanocrystals thereby consuming a large fraction of the precursors. The nanocrystals then grow in the diffusion controlled regime with the leftover precursor concentration. Flux of the precursor (J^diff) reaching a growing particle is given by the Fick's law:

Here, D is the diffusion coefficient of the precursors; x is the distance from the centre of the particles and [M] is the precursor concentration. At high processing temperatures, x → r; and eqn (2) rewrites into:

Eqn (3) qualitatively predicts that [M] influences (i) activation of the surface of the growing nanoparticle, (ii) the effective growth rate, , and (iii) diameter of the final inorganic nanofibers. Li and Xia experimentally demonstrated that final diameter of the inorganic fibers can be reduced by decreasing the precursor concentration in the electrospun polymeric fibers.⁴⁰ In the event of enhancement in activation energy with precursor concentration, an optimal threshold concentration of precursors is required for the formation of continuous fibers. On the other hand, if the concentration of the precursors exceeds beyond the optimal concentration for fibrous morphology, a larger number of nuclei are formed upon annealing. Subsequent growth results in a non-equilibrium state such that all the crystallites cannot be accommodated within the given fibrous volume. The fibrous structure therefore undergoes disruption in the presence of the polymer and leads to non-equilibrium morphologies upon further annealing as shown schematically in Fig. 1.

Fig. 1 Schematics showing the evolution of flower morphology in electrospun inorganic nanostructures when there is a difference in precursor concentration. (a1) Polymeric fibers with lower precursor concentration. (b1) Growth of SnO₂ grains contained within the fiber boundary resulting in fiber morphology. (a2) Polymeric fibers with higher precursor concentration. (b2) Highly populated growth of SnO₂ grains outstrips the fiber boundary and loses one dimensionality giving rise to flower morphology.

3.2 Morphological characterization of fibers and flowers

The samples S1 and S2 produced mats composed of fibers of similar diameter (∼700 nm) upon electrospinning. The mats were annealed at 500 °C for ∼3 h to allow crystallization of the SnO₂ phase and removal of the polymer. Fig. 2 shows the differences in the morphology of the SnO₂ developed from the two mats. The morphologies are clearly different, confirming the growth model described in Section 3.1. Sample S1, with a lower precursor concentration, gave conventional electrospun fibrous morphology (diameter ∼400 nm), whereas the one with higher precursor concentration, i.e., sample S2, yielded flower-shaped particles (size ∼400 nm). Although fibers and flowers were obtained by annealing the as-spun polymeric fiber at a similar temperature, i.e., 500 °C, the degree of crystallinity, judged from the discreteness of the diffraction spots and the extended periodicity in the lattice image, was notably different in both the cases. The flowers showed single crystalline diffraction pattern and lattice images (Fig. 2). On the other hand, diffraction pattern of the fibers consisted of rings – typical of polycrystalline materials having random orientation of crystallites. A closer examination showed that the flower structure is made up of fibrils of diameter ∼80–100 nm, which in turn are made up of particles of size ∼20–30 nm. Owing to the high porosity of the flowers due to polymer permeation during growth, they gave higher BET surface area (40 m² g⁻¹) compared with that of the fibers (19 m² g⁻¹).

Fig. 2 SEM images (A & B) and HRTEM images (C & D) of fibers and flowers, respectively. Insets: (B) magnified SEM images of the flower morphology; (C) selected area electron diffraction (SAED) pattern showing polycrystalline rings; and (D) SAED pattern of flowers showing single crystalline spotty patterns.

3.3 Structural characterization of the flowers and fibers

Fig. 3(A) shows the X-ray diffraction (XRD) patterns of the fibers and flowers. Positions of all the diffraction peaks were similar for both morphologies; however, it was noted that relative intensity of the (211) plane of the flower was slightly lower than the other. All the peaks in the XRD patterns were indexed for the cassiterite phase (space groupP4₂/mnm) of SnO₂ (JCPDS file 41-1445). The lattice parameters calculated from the XRD patterns were a = 4.7380 Å and c = 3.1865 Å. No impurity or secondary phase could be detected in the XRD pattern of both morphologies. The energy dispersive X-ray (EDX) spectra of the SnO₂ flower also confirmed the absence of impurities. Fig. 3 (B) and (C) combine the core-level XPS spectra of both morphologies. The Sn-3d level binding energies (BE) of both morphologies were 487.2 eV and 496 eV, which are assigned to Sn-3d_5/2 and Sn-3d_3/2 core levels of Sn⁴⁺, respectively. The O-1s level BE of oxygen were 531.0 eV and 532 eV, which are assigned to the oxygen (O²⁻) anion in Sn–O bonds and surface oxygen, respectively. The BEs of Sn⁴⁺ ions are in close agreement with the values reported for the cassiterite phase of SnO₂.⁴¹ All these characteristics lead to the conclusion that the flowers are chemically similar to the fibers.

Fig. 3 (A) XRD of flowers and fibers and XPS spectra showing core-level of (B) Sn and (C) O spectra.

3.4 Mott–Schottkey analysis and evaluation of electron density

To understand the difference in electrical properties of the two morphologies, Mott–Schottkey (MS) plots were developed from the EIS measurements in the dark.^39,42Fig. 4 shows the MS plots determined from EIS plots for the two morphologies sintered onto fluorine doped SnO₂ substrates in the presence of acetonitrile containing 0.1 M lithium iodide, 0.03 M iodine, 0.5 M 4-tert-butylpyridine and 0.6 M 1-propyl-2,3-dimethylimidazolium iodide electrolyte. Sigmoidal plots were observed with an overall shape consistent with the n-type semiconductors with similar flat band potentials,⁴³i.e., the potential corresponding to the situation in which there is no charge accumulation in the semiconductor such that there is no band bending, which could be obtained from the x-intercept of the linear portion. The electron density (n_D) obtained from the slope of the 1/C²vs. V curve of the flowers (1.054 × 10²¹ cm⁻³) was an order of magnitude higher for than that of the fibers (0.868 × 10²⁰ cm⁻³). The enhanced crystallinity achieved by manipulating the precursor concentration is thought to be the reason for enhanced n_D of the flower structure.

Fig. 4 Mott–Schottky plot of flowers and fibers.

High n_D of the flower structure was also confirmed with the cyclic voltammetry measurements, which is displayed in the area of the cyclic voltammograms (CV).⁴⁴Fig. 5 shows the CV of the two morphologies, which clearly show that flowers have superior electrochemical properties than the other. The specific capacitance calculated from the area CV curves of the flowers was more than twice that of the fibers, which is expected to arise from the enhanced crystallinity of the flowers.

Fig. 5 Cyclic voltammetry measurements of flower and fiber morphology.

3.5 Performance of the flowers and fibers in DSCs

Fig. 6 shows the current density (J_SC) vs. voltage (V) plots as a function of cell thickness fabricated using both fibers and flowers. For a cell of 15 μm thickness, flower-based DSC showed a record V_OC ∼ 700 mV, J_SC ∼ 7.30 mA cm⁻², fill factor (FF) ∼ 59.6% and η of ∼3.0%. For a cell of similar thickness, the fiber based DSC gave V_OC ∼ 600 mV, J_SC of ∼3.0 mA cm⁻², FF ∼ 38.3 and η ∼ 0.71%, i.e., the flower based device show ∼140% increased J_SC, ∼16% increased V_OC and ∼55% increased FF than the fiber based device.

Fig. 6 Current–voltage characteristics of the solar cells made using fibers and flowers. Thicknesses of the samples are a: 15 μm, b: 12 μ; and c: 10 μm for flower and d: 15 μm, e: 11 μm and f: 9 μm for fibers, respectively.

The difference in electrical property of the two types of cells was determined from the shunt (R_SH) and series (R_S) resistances from the inverse of slops at J_SC and V_OC of the I–V curves. The R_SH is an estimation of internal currents in the cell and is a measure of back electron transfer and other charge recombination processes in DSCs whereas R_S denotes the resistance for charge collection.^18,45 A high R_SH (>1000 Ω) and low R_S (<0.01 Ω) are therefore preferred for high fill factor and efficiency of the solar cells. The best performing flower-based cell gave R_SH ∼ 1600 and R_S ∼ 0.001 Ω, which indicates that the flower-based cells were of high quality with lower charge recombination. On the other hand, the cells made using the fibers showed inferior performances. The R_SH and R_S of the fiber-based devices were ∼600 and 143 Ω, respectively, thereby accounting for its inferior performance.

3.5.1 Origin of high J_SC in the flower based device. The photoelectric conversion efficiency of DSCs is generally expressed as

η = α Φ_inη_c, (4)

where α is the strength of absorption, Φ_in is the injection efficiency, and η_C is the charge collection efficiency. As there is no difference in the flat band potential of flowers and fibers, Φ_in is assumed to be similar. The two types of DSCs were fabricated using similar dyes so their absorption cross-section does not differ. However, dye-loading calculated from desorption test for flowers and fibers were 7.8 and 5.6 mmoles, respectively, i.e., the dye-loading of the device based on the flower structure is only 1.4 times higher than that based on the fibers. If we assume that one photon absorbed by the dye injects one electron to SnO₂, then the dye-loading measurements gives the number of electrons received by flowers and fibers to be 7.8 × 10¹⁶ and 5.7 × 10¹⁶ per cycle, respectively. Despite this small variation in the received electrons, the J_SC of the devices using the flowers were found to be higher by a factor of ∼2.5 than that of the devices using the fibers, which means that the observed enhanced J_SC results from the enhanced charge collection efficiency, η_c, of the flower based device.

The difference in collection efficiencies of the two types of DSCs were studied from the photocurrent action spectra of the devices fabricated using the two types of cells. Fig. 7 shows the action spectra of the two types of devices. The spectra display significant enhancement in the incident photon to current conversion efficiency (IPCE) of the devices based on flowers. The highest IPCE for the devices based on the fibers and flowers were ∼12% and 42%, respectively. The improvement can be attributed to the enhanced electron density and charge transport efficiency as well as a slightly higher amount of the dye-loaded onto the flower based device. However, it is reported that an increase in the dye-loading of 1.4 times results only in an increase in IPCE of ∼3.5%.^46,47 In the present case IPCE of the flower based devices increased by ∼30% compared to that of the fibers thereby indicating the enhanced charge transport kinetics in the flower structure. To check the consistency of the results between the photo action spectra and I–V measurements, the J_SC of the devices were calculated from their IPCE spectra using the relation.

Fig. 7 Photoaction spectra (IPCE) of the DSCs using flowers and fibers.

The integrated IPCE over the entire wavelength (λ) was used to calculate the J_SC. The calculated J_SC of the flower and fiber based devices were 7.57 and 2.97 mA cm⁻², respectively. These values closely match with their measured J_SC from the I–V measurements, i.e., 7.3 and 3 mA cm⁻², respectively.

To determine the electron lifetime, which resulted in enhanced η_c, the EIS spectra of the DSCs were recorded in the dark under forward bias of 0.61 V (Fig. 8). The Nyquist plots (Fig. 8(A)) show that radius of the right semi-circle of the flower based device is larger compared to that made using the fibers. The larger semi-circle indicates an increase in the charge transfer resistance and therefore, an increased electron lifetime. The lifetime τ_n can be calculated from the Bode plots (Fig. 8(B)) using the equation τ_n = 1/2πf_c, where f_c is the peak frequency, which for the flowers and fibers were 0.84 and 4.22 Hz at 0.61 V, respectively. These frequencies correspond to a τ_n of 0.189 s and 0.0377 s, respectively, i.e., the τ_n of the flowers is an order of magnitude longer than that of the fibers. The longer τ_n of the device fabricated using the flower-like morphology indicates more effective suppression of the back reaction between electrons in its conduction band and ions in the electrolyte. The increased τ_n is thought to be the source of high J_SC and η of the device fabricated using the flower-like morphology.

Fig. 8 Impedance spectra of the flowers and fibers (A) Nyquist and (B) Bode plots.

3.5.2 Origin of high V_OC in the flower based device. To the best of our knowledge, the V_OC obtained using the flower-morphology (∼700 mV) is the highest so far achieved using SnO₂. The origin of high V_OC in flower based DSCs could be understood from the kinetic equation of DSCs under steady state condition, i.e., when the electron injection from the dye to SnO₂ is equal to the electron transfer from the counter electrode to the electrolyte, given by

In the above equation J_in is the flux of the injected electrons, k_rec[] is the rate constant of iodide reduction and n_cb is the conduction band electron density in the dark. Therefore, the large difference in the V_OC would partially result from the difference in the iodide reduction, which occurs in two pathways. The first pathway is the reduction by the counter electrode, which is same for similar device geometry. The second is the recombination pathway, i.e., the electron in the SnO₂ directly reduces the iodide ions, which account for the majority of the carrier loss. High internal resistance (R_H ∼ 1600 Ω) for this back electron transfer from SnO₂ to electrolyte measured for the flower based device, as well as high n_D, accounts for the observed high V_OC and FF. Remember that τ_n through the flowers was much higher than that through the fibers, which accounts for the faster charge collection before recombination and resulted in higher V_OC.

Conclusions

In conclusion, tin oxide nanoflowers were developed by a scalable nanofabrication process, electrospinning, by manipulating the precursor concentration for the first time. The flowers thus synthesized have an order of magnitude higher electron density compared with the conventional fibers synthesized using a normal concentration of Sn precursor. The dye-sensitized solar cells fabricated using the flowers showed a record open circuit voltage and photoconversion efficiency. The electrospun tin oxide nanoflowers thus provide promising opportunities as a superior transparent conducting oxide for fabrication of nanoelectronic devices.

Acknowledgements

RJ and MMY acknowledge the UMP internal grant RDU 100337.

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