Pure anatase TiO2 nanoglue-An inorganic binding agent to improve nanoparticle for DSSCs

Pure anatase TiO2“nanoglue”:An inorganic binding agent to improve nanoparticle interconnections in the low-temperature sinteringof dye-sensitized solar cellsYuelong Li,1,2Wonjoo Lee,1Doh-Kwon Lee,1Kyungkon Kim,1Nam-Gyu Park,3andMin Jae Ko1,a͒1Solar Cell Center,Korea Institute of Science and Technology(KIST),Seoul136-791,Republic of Korea2International R&D Academy,University of Science and Technology,Daejeon305-333,Republic of Korea3School of Chemical Engineering,Sungkyunkwan University,Suwon440-746,Republic of Korea͑Received10November2010;accepted10February2011;published online8March2011͒An interparticle-binding agent,or“nanoglue,”was prepared for the fabrication ofwell-interconnected TiO2photoelectrodes in dye-sensitized solar cells͑DSSCs͒sintered at lowtemperature.This nanoglue consisted of pure anatase particles approximately5nm in diameterdispersed in alcoholic solvent.When mixed with TiO2particles21nm in diameter,the nanoglueinduced interparticle chemical bridging via the condensation of hydroxyl groups on surfaces of TiO2particles.The efficiency of nanoglue-based DSSCs was improved by approximately25%comparedto non-nanoglue-based DSSCs.This improvement was mainly ascribed to enhanced interparticleconnections due to the presence of the5nm TiO2particles in nanoglue.©2011American Instituteof Physics.͓doi:10.1063/1.3562030͔Recently,low-temperature fabrication processes for dye-sensitized solar cells͑DSSCs͒have attracted much attention because low temperature is critical for manufacturing light-weight,flexible,and low-cost DSSCs on a plastic substrate. The key to the technology for fabricating DSSCs on a plastic substrate is that all the processes should be performed at low temperature because the plastic substrate cannot withstand a process temperature above150°C.Therefore,well-interconnected and mechanically strong mesoporous TiO2 photoelectrodes should be constructed from a viscous TiO2 paste,without an organic binder.Several methods have been reported for the low-temperature fabrication of photoelec-trodes,such as chemical sintering,1,2mechanical pressing,3 hydrothermal crystallization,4–6electrophoretic deposition,7,8 microwave irradiation,9UV light irradiation,10,11andfilm transfer.12However,most of these require a complicated pro-cess or a post-treatment.Moreover,the incomplete necking of TiO2nanoparticles resulting from low-temperature sinter-ing processes limits the efficiency and practical application offlexible DSSCs.Therefore,the development of a simpler and more efficient method to improve the poor interconnec-tions between nanoparticles is required.In this work,we developed a“nanoglue,”an inorganic interparticle-binding agent composed of pure anatase TiO2 particles with a diameter of approximately5nm,to simulta-neously increase the viscosity of the TiO2paste and improve interparticle connections by the chemical bridging of nano-particles at low temperature when mixed with larger TiO2 particles͑P25;21nm,Degussa͒.A viscous alcoholic paste was prepared by mixing a P25 TiO2suspension with the sol-gel processed TiO2nanoglue gel.This paste was coated onto afluorine-doped tin oxide ͑FTO͒/glass substrate using the doctor blade method.TheTiO2films were sintered at150°C for1h,slowly cooled to 80°C,and then immersed into a0.5mM solution of N719in anhydrous ethanol for dye loading.A Pt on FTO/glass assembly was used as the counterelectrode.The liquid elec-trolyte was composed of0.7M PMII,0.03M I2,0.05M GuSCN,and0.5M tBP in a mixture of acetonitrile and vale-ronitrile͑85:15v/v͒.A solar simulator equipped with a1000 W xenon lamp was used to measure the photovoltaic prop-erties͑see Ref.13for full details of the experimental and characterization procedures͒.Scheme1demonstrates the mechanism of interparticle connections among the TiO2particles in the nanoglue-containing paste.During the paste fabrication process,nano-glue particles,with abundant surface hydroxyl groups,were well mixed with the P25particles.TiO2films were formed on a conducting substrate by coating and drying.During the heating of the TiO2film at150°C,hydroxyl groups on the surfaces of the nanoglue particles underwent a dehydration process with those on the P25particles,accompanied by thea͒Author to whom correspondence should be addressed.Tel.:ϩ82-2-958-5518.FAX:ϩ82-2-958-6649.Electronic mail:mjko@kist.re.kr.SCHEME1.Mechanism of TiO2interparticle connection by nanoglue.APPLIED PHYSICS LETTERS98,103301͑2011͒0003-6951/2011/98͑10͒/103301/3/$30.00©2011American Institute of Physics98,103301-1release of water molecules.This dehydration process en-hanced interparticle connection,resulting in facilitated elec-tron transport in the film.A similar type of dehydration be-tween hydroxyl groups on the surfaces of TiO 2particles by treatment with an aqueous hydrochloric acid solution was suggested by Miyasaka and co-workers.2Figure 1͑a ͒shows the highly viscous gel obtained by dissolving 5wt %nanoglue in ethanol attached to the bottom of a glass bottle.As shown in Fig.1͑b ͒,the viscous gel was readily fluidized by hand shaking and flowed down to the bottom.This thixotropic behavior of the TiO 2gel is essential in the construction of DSSC photoelectrodes to enable coat-ing formation without organic binders in the paste.High-resolution transmission electron microscopy ͑HR-TEM ͒images of the primary TiO 2particles in the as-prepared nanoglue and a single primary particle therein ͑the inset image ͒are presented in Fig.1͑c ͒.We observed that the nanoglue particles were well dispersed without significant agglomeration.The inset HR-TEM image clearly indicates that the size of the single nanoglue particle was approxi-mately 5nm and displayed the lattice plane of anatase ͑101͒,with an interlayer spacing of 0.352nm.Figure 1͑d ͒shows the powder X-ray diffraction ͑XRD ͒results for the TiO 2par-ticles in the nanoglue.The characteristic ͑101͒peak indicates the highly crystalline and pure anatase phase of the TiO 2particles in the nanoglue.The particle size of 5.2nm was confirmed by the ͑101͒peak by using the Scherrer equation,which is consistent with the HR-TEM observations.Figures 2͑a ͒and 2͑b ͒reveal the microstructures of the nanoglue-based TiO 2film containing P25and the primary particles.It was observed that all the TiO 2particles were uniformly dispersed in the mixture of nanoglue and P25,which is necessary to induce interconnections between the P25particles.As shown in the enlarged image of Fig.2͑b ͒,the primary particles from the nanoglue were located in be-tween the larger P25particles,thus serving as an inorganic binder to bridge and connect the P25particles.With further dehydration during the heating process,better interconnec-tion could be achieved between the nanoglue and P25par-ticles and hence faster electron transport could be achieved between particles in DSSCs.To investigate the effect of the nanoglue on the perfor-mance of low-temperature-fabrication DSSCs,TiO 2elec-trodes were prepared with and without added nanoglue.Based on the results of our preliminary experiments for DSSC optimization,TiO 2particles from the P25suspension and nanoglue were mixed at a weight ratio of 7:1.Figure 3͑a ͒compares the photovoltaic performance of cells fabri-cated with and without nanoglue.The efficiency of the solar cells with nanoglue was enhanced by 25%over those with-out nanoglue.The cells without nanoglue displayed a photo-voltage ͑V OC ͒of 763mV ,a photocurrent density ͑J SC ͒of 5.50mA /cm 2,a fill factor ͑FF ͒of 71.12%and an overall conversion efficiency of 2.98%;the nanoglue-based cells ex-hibited a V OC of 763mV ,a J SC of 6.87mA /cm 2,a FF of 70.93%and an overall conversion efficiency of 3.71%.By inspection,it was recognized that the efficiency enhancement was mainly ascribed to the increased J SC rather than the changes FF and V OC ,likely resulting from improved inter-particle connection.Photocurrent transient measurement was performed to elucidate the origin of the higher J SC of the nanoglue-based DSSCs.In this measurement,total photocurrent is generally limited by charge carriers with lower mobility.Therefore theFIG.1.͑Color online ͒Thixotropic behavior of a nanoglue suspension ͑a ͒before and ͑b ͒after shaking.͑c ͒HR-TEM images of the nanoglue particles;the inset shows a single nanoglue TiO 2crystal.͑d ͒Powder XRD spectrum of TiO 2particles in the nanoglue.FIG.2.͑a ͒HR-TEM images of the microstructure of the interparticle con-nections between the P25particles and the smaller nanoglue particles.͑b ͒Enlarged image of nanoglue TiO 2particles located in between the P25par-ticles;the arrows indicate the boundaries of the nanoglue and P25particles.FIG.3.͑Color online ͒͑a ͒Photovoltage-current curves of DSSCs based on TiO 2films fabricated with P25only and with a mixture of P25and nanoglue particles.͑b ͒Electron-diffusion coefficient D e derived from the electron collection time using the equation D e =L 2/͑2.35␶c ͒,where L is the thickness of the TiO 2film and ␶c is the collection time constant.electron current mainly contributes to the measured photo-current because hole current through electrolyte is faster thanthe electron current through low temperature sintered photo-electrode.The data were obtained by applying a varyingbias light intensity and the time constant was obtained byfitting the decay of the photocurrent transient to the functionexp͑−t/␶c͒,where t and␶c are time and the collection-time constant,respectively.In this measurement,the time constantfor electron collection is related to the electron-diffusion co-efficient͑D e͒by the expression14D e=L2/͑2.35␶c͒,where L is thefilm thickness.The calculated diffusion coefficients of cells with and without nanoglue are compared in Fig.3͑b͒. The diffusion coefficient of the nanoglue-based cell was ap-proximately twice that of the cell without nanoglue,irrespec-tive of the bias light intensity,unambiguously indicating faster electron transport in the nanoglue-based DSSC.This result also suggested that the improved electron transport originated from an enhancement of the interparticle connec-tion in the nanoglue-based DSSCs,as shown in Fig.2͑b͒. Several research groups have reported results similar to ours, where the photocurrent enhancement is related to improved electron transport in the TiO2film due to better interconnec-tion between TiO2particles after the hydrothermal treatment of titanium tetraisopropoxide5or titanium tetrachloride.15,16 Although the improvement of particle interconnections was thought to play a major role in the efficiency enhance-ment of the nanoglue-based solar cell,an increased dye ad-sorption due to the introduction of nanoglue particles may have also contributed to the performance enhancement.This hypothesis was confirmed by the results of a dye desorption measurement,in which the amount of dye desorbed from the TiO2film including nanoglue particles was approximately 10%greater than that without nanoglue.However,this in-creased amount of dye adsorption could not have been solely responsible for the25%increase in photocurrent.Therefore, we concluded that the enhanced cell performance mainly re-sulted from the improved interparticle connection due to the nanoglue,leading to improved charge collection efficiency.In their attempts to enhance the interconnection of TiO2particles,most other researchers have employed Ti precur-sors or brookite particles.2,4–6In the case of the Ti-precursortreatment,a postprocessing such as hydrothermal treatmentin a teflon-lined autoclave for as long as12h is required tocrystallize the Ti precursor into TiO2particles.4–6Moreover,Ti-precursor-based paste is generally not stable due to itsacute sensitivity to humidity and temperature.It is wellknown that brookite-phase TiO2particles are not suitable foruse as efficient photoelectrode materials in DSSCs.17How-ever,for the pure anatase TiO2-based nanoglue,the post-treatment after coating is a simple drying at150°C for1h,which can be easily performed under normal conditions. With respect to stability,we did not observe any noticeable changes in its composition or morphology after one month of storage.In conclusion,a nanoglue gel with5nm anatase TiO2 particles was synthesized by a sol-gel process and served as an inorganic binding agent to facilitate the preparation of a well-interconnected TiO2film at low temperature.The pho-tocurrent of a nanoglue-based solar cell was improved by 25%compared to a cell without nanoglue.Improved inter-particle connection,resulting in faster electron transport,was confirmed by photocurrent transient measurements.The ap-plication of this nanoglue to high-efficiency plastic-based flexible solar cells is under way.This research was supported by a grant͑Grant No. 2010K000335͒from“Center for Nanostructured Materials Technology”under“21st Century Frontier R&D Programs”of the Ministry of Education,Science,and Technology, Korea and the New and Renewable Energy Program of the Korea Institute of Energy Technology Evaluation and Plan-ning͑KETEP͒grant͑Grant No.2010T100100651͒funded by the Korea Government Ministry of Knowledge Economy. 1N.G.Park,K.M.Kim,M.G.Kang,K.S.Ryu,S.H.Chang,and Y.J. 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