Characteristics of SnO2 nanofiber/TiO2 nanoparticle composite for dye-sensitized solar cells Jiawei Gong, Hui Qiao, Sudhan Sigdel, Hytham Elbohy, Nirmal Adhikari, Zhengping Zhou, K Sumathy, Qufu Wei, and Qiquan Qiao Citation: AIP Advances 5, 067134 (2015); doi: 10.1063/1.4922626 View online: http://dx.doi.org/10.1063/1.4922626 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/5/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhanced conversion efficiency of dye-sensitized solar cells using a CNT-incorporated TiO2 slurry-based photoanode AIP Advances 5, 027118 (2015); 10.1063/1.4908179 Transient photocurrent and photovoltage studies on charge transport in dye sensitized solar cells made from the composites of TiO nanofibers and nanoparticles Appl Phys Lett 98, 082114 (2011); 10.1063/1.3560057 The effect of annealing on the photoconductivity of carbon nanofiber/ TiO core-shell nanowires for use in dye-sensitized solar cells Appl Phys Lett 97, 043102 (2010); 10.1063/1.3464965 Electron transport in electrospun TiO nanofiber dye-sensitized solar cells Appl Phys Lett 95, 012101 (2009); 10.1063/1.3167298 Fabrication of dye-sensitized solar cells with Ti O photoelectrode prepared by sol-gel technique with low annealing temperature J Vac Sci Technol A 27, 1042 (2009); 10.1116/1.3116588 All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 134.117.10.200 On: Thu, 09 Jul 2015 09:46:36 AIP ADVANCES 5, 067134 (2015) Characteristics of SnO2 nanofiber/TiO2 nanoparticle composite for dye-sensitized solar cells Jiawei Gong,1,2 Hui Qiao,3,a Sudhan Sigdel,1 Hytham Elbohy,1 Nirmal Adhikari,1 Zhengping Zhou,1 K Sumathy,2 Qufu Wei,3,b and Qiquan Qiao1,c Center for Advanced Photovoltaics, Department of Electrical Engineering, South Dakota State University, Brookings, SD 57007, USA Department of Mechanical Engineering, North Dakota State University, Fargo, ND 58102, USA Key Laboratory of Eco-Textiles, Jiangnan University, Ministry of Education, Wuxi 214122, China (Received March 2015; accepted 26 May 2015; published online 18 June 2015) SnO2 nanofibers and their composites based photoanodes were fabricated and investigated in the application of dye-sensitized solar cells The photoanode made of SnO2/TiO2 composites yielded an over 2-fold improvement in overall conversion efficiency The microstructure of SnO2 nanofibers was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) A compact morphology of composites was observed using scanning electron microscopy (SEM) A long charge diffusion length (62.42 µm) in the composites was derived from time constant in transient photovoltage and photocurrent analysis These experimental results demonstrate that one-dimensional nanostructured SnO2/TiO2 composites have a great potential for application in solar cells C 2015 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4922626] I INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted tremendous research interest during the past several decades and their efficiency has recently been raised up to ∼ 15%.1–3 In the operation of DSSCs, wide band gap semiconductor photoanodes (typically TiO2) play a crucial role in dye sensitizer uptake and electron transport Although TiO2 nanoparticle based photoanodes provide large surface area for sufficient dye loading, numerous boundary defects at contacts between nanoparticles lead to the scattering of free electrons and reduced charge mobility Tin dioxide (SnO2) is a promising alternative to TiO2 due to its large band gap (3.6 eV), high electron mobility (100 − 200 cm2V −1 S −1), and low conduction band effective mass (0.1mo ).4 A large band gap can promote device stability by suppressing the generation of oxidative holes under ultraviolet light illumination.5 The oxidative holes can decompose organic compounds such as Ru dyes adsorbed on oxide surface In addition, the holes can oxidize I− to I3−; and any holes that oxidize the electrolyte irreversibly rather than regenerating I3− will lead to loss of I3− and decreasing of DSSC performance A high electron mobility and low conduction band effective mass indicate fast electron transport, which contributes to long diffusion length and efficient charge collection A considerable amount of effort has been devoted to develop nanostructured SnO2 with diverse morphologies A variety of nanostructured SnO2 such as nanosheets,6 nanofibers,7 nanoflowers,8 hollow mircrospheres,9 and urchin-like structures10 have been synthesized and widely investigated in the appplication of DSSCs However, the application of SnO2-based DSSCs has been limited due a huiqiao@jiangnan.edu.cn b qfwei@jiangnan.edu.cn c qiquan.qiao@sdstate.edu; Tel.: 605-688-6965; Fax: 605-688-4401 2158-3226/2015/5(6)/067134/10 5, 067134-1 © Author(s) 2015 All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 134.117.10.200 On: Thu, 09 Jul 2015 09:46:36 067134-2 Gong et al AIP Advances 5, 067134 (2015) to poor photovoltaic performance caused by fast interfacial electron recombination and insufficient attachment of dye molecules Fast recombination dynamics for SnO2 electrode results from a lower trap density and a 300 mV positive shift of the SnO2 conduction band.11 The insufficient dye attachment is a result of less surface area and lower isoelectric point (pH 4-5), which limits the attachment of dye molecules with acidic carboxyl groups (e.g N719 dye, (Bu4N)2[Ru(Hdcbpy)2(NCS)2]) To minimize the interfacial charge recombination and increase dye upload, hybrid TiO2-SnO2 structures such as TiO2-coated SnO2 hollow microspheres,12 TiO2-coated SnO2 nanosheets,13 and TiO2@SnO2 core/shell nanoparticles14 have been previously attempted by others Our previous work demonstrated that incorporation of TiO2 nanoparticles into SnO2 nanofibers could significantly improve the device efficiency The excellent performance was expected due to alleviated band shift effect and increased dye loading In this study, detailed characterization was performed to analyze the microstructure, transient response and recombination dynamics of SnO2 naonofiber/TiO2 nanoparticle composites In addition, the nanorod/nanowire geometry can enhance the photogeneration process by inducing strong light scattering /trapping when the diameter of nanofibers is larger 200 nm.15 Thus, SnO2 nanofibers with a diameter 200 - 300 nm was used to induce light scattering/harvesting, and therefore enhance the photogeneration process by scattering/trapping light in the photoanode II EXPERIMENTAL DETAILS A Synthesis and characterization of SnO2 nanofibers SnO2 nanofibers were prepared by electrospinning and calcination from polyvinyl pyrrolidone/stannic chloride pentahydrate (PVP/SnCl4·5H2O) precursors Specifically, the electrospinning solution was prepared by adding SnCl4·5H2O into 10 wt% PVP in an ethanol/DMF mixed solvent (weight ratio 1:1) The weight ratio of SnCl4·5H2O to polymer intermediate (PVP) was fixed at 1:1 The solution was stirred by a magnetic bar at room temperature Subsequently, electrospinning was carried out with this solution The PVP/SnCl4·5H2O precursor was ejected from a stainless steel needle under a high voltage of 17 kV to form fibrous nonwoven mats on the collector The flow rate was kept at 1.0 ml/h, and the needle-to-collector distance was fixed at 21 cm The electrospun nanofiber mats were calcinated at 500 ◦C for h with a heating rate of 0.5 ◦C/min The structural analysis of SnO2 nanofibers was performed on a D8 Advance X-ray diffractometer (XRD, Bruker AXS, Germany) over the 2θ range from 10◦ to 80◦ The morphology of SnO2 nanofibers was observed by scanning electron microscope (SEM; Quanta-200, Netherlands) and transmission electron microscope (TEM) with selected-area electron diffraction (SAED) (TEM; JEM-2100HR, JEOL) For TEM measurements, precursor nanofibers were directly deposited on the copper grids during electronspinning, and SnO2 nanofibers were dispersed in ethanol by ultrasound and then transferred onto copper grids.16 B Fabrication of dye-sensitized solar cells To prepare SnO2/TiO2 composite based photoanodes, SnO2 nanofibers and TiO2 nanoparticles (P25, Degussa) were mixed at an optimized weight ratio (1:1) with ethyl cellulose, α-terpineol and ethanol to form a paste through successive sonication and stirring Specifically, 0.125 g SnO2 and 0.125 g TiO2 were added to 4.9 ml ethanol and left for dispersion The solution was sonicated and mechanically stirred alternatively at 1-hour intervals for a total of hours Subsequently, 0.125 g ethyl cellulose as binder and 0.89 ml α-terpineol as solvent were added to the mixture for consecutive sonication and stirring until all grains disappeared and the solution became homogeneous The solution was heated in a vacuum oven at 80 ◦C to remove excess ethanol until it turned into a form of homogeneous slurry ready for doctor-blading A thin layer of the slurry was doctor-bladed onto fluorine doped tin oxide (FTO) glass substrate with an active area of 0.16 cm2 An optimal thickness (8-9 µm) was confirmed by a Dektak profilometer.17 This sample was annealed at 500 ◦C for 30 to form a mesoporous film On the top of the film, a scattering layer (Solaronix Ti-Nanoxide R/SP) was coated that enhanced light absorption of the underneath All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 134.117.10.200 On: Thu, 09 Jul 2015 09:46:36 067134-3 Gong et al AIP Advances 5, 067134 (2015) mesoporous active layer Then the sample was dipped in 40 mM TiCl4 solution at 80 ◦C for 30 min, followed by sintering at 500 ◦C to form a TiO2 blocking layer This layer can passivate the 3D interpenetrated nanofiber/nanoparticle network, and therefore can improve the electron transport This TiO2 blocking layer effectively prevents electron recombination This thin blocking layer was coated on all the samples using the same methods in order to exclude its effects on the performance enhancement for different samples including TiO2 nanoparticle sample The thickness of this blocking layer was typically tens of nm, which is quite smooth The resulting photoanodes were immersed in dye solution containing 0.5 mM Ruthenizer 535-bisTBA dye (Dyesol N-719) in acetonitrile/t-butanol (volume ratio: 1:1) for 24 hours In the final step, any excess dye molecules on the photoanode were rinsed in acetonitrile for several hours Counter electrode was prepared by sputtering 40 nm Pt onto FTO glass substrates In the end, the photoanode and counter electrode were sandwiched and sealed with 60 µm thick plastic and injected with I−/I3− electrolyte through reversed channels All devices were fabricated in the same procedure.18 C Dye desorption from photoanodes 10 mM NaOH solution in ethanol and DI water (volume raio 1:1) was used to desorb dyes from SnO2 nanofiber, and SnO2/TiO2 composite Each of the dye attached photoanodes were dipped FIG X-Ray powder diffraction pattern of SnO2 nanofibers and (b) transmission electron microscopy image of a single SnO2 nanofiber All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 134.117.10.200 On: Thu, 09 Jul 2015 09:46:36 067134-4 Gong et al AIP Advances 5, 067134 (2015) FIG Scanning electron micrographs of (a) electrospun nanofiber SnO2 network calcinated at 500◦C, (b) Short SnO2 nanofibers randomly packed on FTO glass substrate, and (c) SnO2 nanofiber/TiO2 nanoparticle composite (weight ratio: 1:1) in the 10 mM NaOH solution for 24 h at room temperature to desorb dye molecules The dye molecules were peeled off from the photoanode into solution by neutralizaiton reaction between acidic carboxylic group and basic solution.19 The volume of each solution was kept as mL for dye desorption Spectra of the dye molecules desorbed from different photoanodes were measured III RESULTS AND DISCUSSION The XRD pattern shown in Fig 1(a) indicates a high purity of SnO2 nanofibers annealed in air at 500 ◦C Peaks with 2θ values of 26.48, 33.87, 37.91, 51.72, 54.85, and 57.97 were observed, All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 134.117.10.200 On: Thu, 09 Jul 2015 09:46:36 067134-5 Gong et al AIP Advances 5, 067134 (2015) which correspond to SnO2 crystal planes of (110), (101), (200), (211), (220), and (002), respectively No obvious impurity peaks (e.g., unreacted Sn metal and other tin oxides) were observed, indicating the high purity of the rutile SnO2 nanofibers These signature diffraction peaks indicate a tetragonal rutile structure of SnO2 with lattice constants of a, b = 4.74 Å and c = 3.18 Å that agree well with documented values for the SnO2 crystals (JCPDS card, No 41-1445) Scherrer’s equation was adopted to estimate the size of SnO2 crystals in the form of powder It is stated that the average crystallite size D = 0.89λ/ β cos θ, where λ is the wavelength for the Cu Kα (= 1.54056 Å), β is the line broadening at half the maximum intensity (FWHM) expressed in radian, and θ is Bragg’s angle The average crystallite size was calculated to be c.a nm for SnO2 nanofibers based on the (211) peak.20,21 The TEM image shown in Fig 1(b) reveals a structure of single SnO2 nanofiber which retains an intact and uniform fibrous morphology Figure 2(a) shows scanning electron micrographs (SEM) of the as-spun PVP/SnCl4·5H2O precursor nanofibers It is seen that SnO2 nanofibers exhibit fibrous morphology, good rigidity and are separated from each other Due to weak adhesion between the original SnO2 nanofiber sheets and FTO substrate, the nanofiber sheets were dispersed into short fibers by sonication The short fibers were made into a paste from using the procudure discribed in Section II The paste was doctor-bladed onto FTO glass and anealed to form photoanode, which has the topology shown in Fig 2(b) Due to the fact that continuous SnO2 naonfibers were crushed into randomly packed short fragments, significant amount of voids were formed in the film which reduced the surface area of photoanode To minimize the number of voids and increase the surface area, closely-packed TiO2 nanoparticles were introduced into SnO2 nanofibers to form a compact film morphology as shown in Fig 2(c) Three samples of each type of devices were tested under AM 1.5 illumination at a light intensity of 100 mW/cm2 Figure shows the comparison of J-V characteristics of DSSCs based on SnO2 nanofiber, and SnO2/TiO2 composite The photovoltaic parameters are listed in Table I The SnO2 nanofiber based device shows poor performance with open-circuit voltage (Voc) of 0.7 V , short-circuit current density (Jsc) of 5.9 m Acm−2, and an overall conversion efficiency (η) FIG Comparison of current density versus voltage (J-V) curves of SnO2 nanofiber, and SnO2/TiO2 composite based DSSCs All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 134.117.10.200 On: Thu, 09 Jul 2015 09:46:36 067134-6 Gong et al AIP Advances 5, 067134 (2015) TABLE I PHOTOVOLTAIC PARAMETERS OF DSSCs BASED ON SnO2 NANOFIBER, AND SnO2/TiO2 COMPOSITE PHOTOANODE Samples SnO2 nanofiber SnO2/TiO2 composite Voc (V ) Jsc (m A c m −2) FF (%) η (%) L (µm) 0.70±0.003 0.79±0.04 5.9±0.05 10.1±0.07 41±0.1 57±0.3 1.68±0.05 4.54±0.1 55.68±0.93 62.42±1.25 FIG (a) UV-Vis absorbance spectra from the solutions of dyes that were desorbed from SnO2 nanofiber, and SnO2/TiO2 composite based photoanodes (b) Transmittance spectra before dye soaking of 1.68% A relatively low open circuit voltage (0.7 V ) was expected because of a more positive conduction-band edge of SnO2 with respect to nanocrystalline TiO2.15,22,23 The low current density can be mainly ascribed to insufficient dye attachment This was confirmed by UV-Vis absorbance spectra, shown in Fig 4(a), that dye solution derived from SnO2 nanofiber photoanode has a lower absorbance, which implies the least amount of dye molecules attached onto photoanodes in DSSCs A comparatively low fill factor (FF) of 0.41 was observed, which was caused by high charge resistance in porous SnO2 film due to randomly packed short fiberous morphology For each device, All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 134.117.10.200 On: Thu, 09 Jul 2015 09:46:36 067134-7 Gong et al AIP Advances 5, 067134 (2015) FIG (a) Nyquist plots of SnO2 nanofiber, and SnO2/TiO2 composite based DSSCs in dark at bias of Voc from 0.1 Hz to 100 Hz with an amplitude of 10 mV, and (b) Equivalent circuit model for full cell TABLE II FITTED VALUE Of RS, RCT, AND RCR FOR SnO2 NANOFIBER AND TiO2 NANOPARTICLE BASED DSSCs Samples R s (Ω) R CT (Ω) R C R (Ω) SnO2 nanofiber SnO2/TiO2 composite 20.58 17.45 16.81 13.54 303.7 217 charge diffusion length (L) was calculated based on transient photovoltage and photocurrent anlysis, shown in Fig A long diffusion length (62.42 µm) of SnO2/TiO2 composites indicates a small magnitude of recombination dynamics In contrast, SnO2/TiO2 composite showed an over 2-fold improvement compared to SnO2 nanofiber with a Voc of 0.79 V , a Jsc of 10.1 m Acm−2, a FF of 0.57, and an overall efficiency η of 4.54% The increase in Voc from 0.7 V to 0.79 V can be attributed to the alleviated shift of conduction band of SnO2 nanofiber by incorporating TiO2 nanoparticles The improvement in short circuit current density can be ascribed to the increase in dye attachment as shown in Fig 4(a) The enhanced fill factor is a result of low electron recombination rate due to the formation of TiO2 blocking layer, which is confirmed by transient analysis and electrochemical impedance spectroscopy (EIS).24 The UV-Vis absorbance spectrum of each dye solution is shown in Figure 4(a) It can be seen that SnO2/TiO2 composites absorbed ∼30% more of dye molecules than the SnO2 nanofibers, whereas it generated over 2-fold conversion efficiency Such significant improvement is a result of increased surface area and more compact morphology introduced by TiO2 nanoparticles It has All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 134.117.10.200 On: Thu, 09 Jul 2015 09:46:36 067134-8 Gong et al AIP Advances 5, 067134 (2015) FIG Normalized transient (a) photovoltage and (b) photocurrent decay of SnO2 nanofiber, SnO2/TiO2 composite based DSSCs been found that when the diameters of nanofibers increased to 200 nm or beyond, the light scattering becomes substantially stronger.15 Since the diameters of the electrospun SnO2 nanofibers were ca 200–300 nm, these nanofibers were expected to induce strong light scattering and thereby significantly enhance the light harvesting This prediction was confirmed by comparing transimittance spectra of SnO2 nanofiber, and SnO2/TiO2 composite based photoanodes in Fig 4(b) The SnO2 nanofiber based cells showed a broad light absorption property from 400 nm to 800 nm with a minimum tranmittance of 14% at green light (530 nm) This light harvesting capability can be attributed to multiple light scattering in large SnO2 agglomerates scattered in the electrode SnO2/TiO2 composite based photoanode has a higher transmittance compared to SnO2 nanofiber photoanode To further understand the interfacial charge transfer, electrochemical impedance spectroscopy (EIS) analysis was carried out Figure 5(a) shows EIS spectra of SnO2 nanofiber, and SnO2/TiO2 composite based DSSCs Two semicircles were clearly observed: one on the left side (high frequency region) that represents the charge transfer at the electrolyte/counter electrode interface; and the other on the right (low frequency region) that represents the back charge transfer from the photoanode to electrolyte.25,26 Equivalent circuit used to analyze the EIS spectra is presented in Fig 5(b) Each of the two interfaces was modeled by a parallel combination of a resistance and a capacitor RS represents the total series resistance of a device, RCT is the charge transfer resistance at the electrolyte/counter electrode interface, and RCR is the charge recombination resistance at All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license See: http://creativecommons.org/licenses/by/3.0/ Downloaded to IP: 134.117.10.200 On: Thu, 09 Jul 2015 09:46:36 067134-9 Gong et al AIP Advances 5, 067134 (2015) the photoanode/electrolyte interface Values of RS, RCT and RCR are extracted by fitting equivalent circuit and presented in Table II A large value of Rs (20.58 Ω) in SnO2 nanofiber compared to SnO2/TiO2 composite (17.45 Ω) indicates a slower charge transport in SnO2 due to a slow interparticle electron motion in the porous SnO2 film.27 This high total series resistance was in accordance with the low fill factor in SnO2 nanofiber based devices The RCT values in all devices were consistent as the electrolyte and counter electrode were kept the same A significantly large charge recombination resistance (303.7 Ω) of SnO2 nanofibers was observed This implies a slow recombination on the SnO2 nanofibers, and a fast charge transport and collection in SnO2 nanofiber based photoanode Transient photovoltage is one of the major characterization techniques to probe recombination dynamics in solar cells Figure 6(a) shows the normalized transient photovoltage decay of SnO2 nanofiber, and SnO2/TiO2 composite Exponential decay of transient photovoltage was fitted in Origin® to derive the value of time constant (τe ) The competition between recombination and charge transport determines the diffusion length (L) of electrons, which measures the average distance that the electrons can travel in the photoanode without recombination Diffusion length, L, is calculated using the equation: w 2τe L = Dn τe = (1) 2.35τtrans where w is the thickness of photoanode, τe is the electron recombination lifetime, τtrans is the electron transport lifetime, and Dn is the effective diffusion coefficient.28 Electron recombination time of SnO2/TiO2 composite is comparable to SnO2 nanoiber (21.8 ms); and both showed a long recombination time The long transient photovoltage decay in the composite is attributed to slow recombination dynamics due to TiCl4 post treatment It has been found that an energy barrier of aproximately 300 mV created by TiO2 layer prevents back charge transfer from SnO2 to the electrolyte or dye.10 It can be seen, in Figure 6(b), that incorprating TiO2 nanoparticles in SnO2 nanofibers reduces the decay time constant τ from 0.2 ms to 0.15 ms, which indicates an improvement in charge transport and collection A relatively slow charge collection in SnO2 nanofiber based devices can be attributed to lower bulk electron mobility that is caused by boundaries and defects of SnO2 nanofibers in the film IV CONCLUSIONS In summary, incorporation of TiO2 nanoparticles into SnO2 nanofibers shows an over 2-fold efficiency improvement over SnO2 nanofibers as photoanode in DSSCs Such improved cell performance can be attributed to a compact morpholgy of SnO2/TiO2 composites, a large surface area introduced by the TiO2 nanoparicles, and a reduced interfacial charge recombination resulting from TiCl4 treatment In addition, a thin film (8-9 µm) photoanode would mitigate charge recombination and reduce series resistance, which is desirable in cell efficiency ACKNOWLEDGEMENT This work was partially supported by US-Pakistan joint Science and Technology through National Academy of Science, National Natural Science Foundation of China (21201083), and Cooperative Innovation Fund-Prospective Project of Jiangsu Province (BY2014023-29 and BY201402323) J Burschka, N Pellet, S.-J Moon, R Humphry-Baker, P Gao, M K Nazeeruddin, and M Graetzel, Nature 499, 316 (2013) J Gong, K Sumathy, and J Liang, Renewable Energy 39, 419 (2012) J Gong, K Sumathy, and J Liang, International Journal of Sustainable Energy, (2014) Q Wali, A Fakharuddin, I Ahmed, M H Ab Rahim, J Ismail, and R Jose, Journal of Materials Chemistry A 2, 17427 (2014) A Kay and M Gratzel, 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cells Jiawei Gong,1,2 Hui Qiao,3,a Sudhan Sigdel,1 Hytham... application of dye- sensitized solar cells The photoanode made of SnO2/ TiO2 composites yielded an over 2-fold improvement in overall conversion efficiency The microstructure of SnO2 nanofibers was... transferred onto copper grids.16 B Fabrication of dye- sensitized solar cells To prepare SnO2/ TiO2 composite based photoanodes, SnO2 nanofibers and TiO2 nanoparticles (P25, Degussa) were mixed at