Recent Advances in SnO2 Based Photo anode

Materials Science Forum Vol. 771 (2014) pp 25-38 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.771.25

Recent Advances in SnO2 Based Photo anode Materials for Third Generation Photovoltaics Baraneedharan Pari1, Siva Chidambaram 1, Nehru Kasi 2, Sivakumar Sivakumar1# 1

Division of Nanoscience and Technology, Bharathidasan Institute of Technology, Anna University, Tiruchirappalli – 620 024, India. 2 Department of Chemistry, Bharathidasan Institute of Technology, Anna University, Tiruchirappalli – 620 024, India. #

corresponding author. Fax:91-431-240 7999: e-mail address: [email protected]

Keywords: Photoanode, Hybrid, Energy gap, Metal oxide, Electron mobility.

Abstract Dye Sensitized Solar Cell (DSSC) based on metal oxide photo anode is of greater interest at the present scenario. The light harvesting capability of the photo anode is the most crucial factor in determining the efficiency of DSSC. Thus to decide on suitable photo anode to attain greater efficiency is critical confront. The wide band gap (3.6eV) and higher electron mobility (µe ~ 250 cm2 V-1 S-1) of SnO2 put together a promising material when compared to other photo electrode materials . Besides, its low sensitivity towards UV makes them more stable for a long time. This review will focus on recent progress in development of SnO2 and hybrid SnO2 based photo anode material and its allied key issues based on articles published in the last five years. A short introduction about the current energy scenario, DSSC principle and working will be presented followed by a brief description about the importance of photo anode in DSSC. Subsequently a complete review on SnO2 and hybrid SnO2 photo anode materials will be explained together with the recent year reports considering all the challenges and perspectives related to DSSC. Introduction The increased exhaustion ratio of fossil fuels progressively increased the demand for alternative fuel supplies. We are literally on the verge of standstill when it comes to consuming non-renewable energy resources. Day-by-day, as our technology grows and life style develops, our energy consumption rapidly pushes up the limits of the scale whereas the availability of the energy resources falls down drastically. To address this, switching the consumption of energy from fossil fuels to renewable resources is considered as a feasible option. Although it has not acquired a potential that equals conventional sources, renewable energy is still seen as the best answer to the energy crisis. Researches on increasing the efficiencies of alternative energy supplies like wind energy, geothermal energy, hydro energy, photovoltaic (PV) solar cells, bio fuels, hydrogen production and natural gas production is the hot topic in current research. One can talk about hydro and wind energy, but when compared on the basis of consistency, solar energy is more efficient than other forms of renewable resources. “More energy from sunlight strikes Earth in 1 hour than all of the energy consumed by humans in an entire year”. Many problems regarding energy and ecofriendly can be solved even if only a fraction of solar energy is harvested. Thus research activities on solar energy is well acknowledged in the past decade [1-2]. After the French scientist, Alexander Edmond Becquerel discovered the photovoltaic effect in 1839 [3], research continued in the field of material science for possible materials used in photovoltaic technology. Photo electrochemical cells (PECs) another extends of photovoltaics which utilize electrochemistry and photovoltaics effect simultaneously. A PEC is generally composed of a photoactive semiconductor working electrode, a metal counter electrode (semiconductors can also used as counter electrode) and electrolyte solution. The overall efficiency of such device depends on the performance of photoactive electrode [4-6]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 117.211.89.154, Anna University, Chennai, India-08/10/13,08:51:00)

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Potential Development in Dye-Sensitized Solar Cells for Renewable Energy

Till date the commercially available inorganic solar cell are of high cost and fabricated with in sophisticated instruments/environment like clean room. In addition solar cells made from organic materials proven to be advantageous in areas of cost, toxicity and natural abundance. Therefore low cost solar cells have been under intense research in the recent few years. The first report by O Regan & Gratzel in 1991 on dye sensitized solar cell (DSSC) using nanocrystalline TiO2 with conversion efficiency of 7.1-7.0% provided new way for DSSC research. First the focus was on amorphous semiconductors but dye sensitized solar cells emerged as a new class of low cost photovoltaic devices which is economically incredible and eco-friendly. They have simple synthesis and fabrication methodology and thus have low embodied energy with payback period less than one year [7]. By incorporating dye molecules in the solar cell structure, the charge generation is governed by semiconductor-dye interface and carrier transport by semiconductor. Thus, enhancing the properties of DSSC and its conversion efficiency [8]. A numerous of works have been reported on wide band gap semiconductor based dye materials for DSSC till date. The wide band gap (3.6eV) and higher electron mobility (µe ~ 100- 200 cm2 V_1 S_1) of SnO2 put together a promising material when compared to other photo electrode materials. Besides, its low sensitivity towards UV makes them more stable for a long time [9-10]. This review will focus on recent (from 2011-till date) progress in development of SnO2 and hybrid SnO2 based photo anode material. Also current energy scenario, principle and working of DSSC will be presented followed by a brief description about the importance of photo anode in DSSC. Subsequently a complete review on SnO2 and hybrid SnO2 photo anode materials will be explained together with the recent year reports considering all the challenges and perspectives related to DSSC. Operating Principle of Dye sensitized solar The operating principle of DSSC involves three stages i) absorption of photons, ii) generation of charge carriers and iii) charge transport [11-12]. When photons with energy close to the energy gap of dye molecule approach, it will be absorbed by the dye promoting one electron to an excited state. Absorption is the process of light absorption by sensitizing dye molecule (D0) and proceeds to excited sensitized state (D*). The excited electron is injected into the conduction band of semiconductors and the dye molecule gets oxidized (D+), which is the separation process. In the collection process, the electron from the electrolyte is utilized to reduce the dye molecule and make it stable again (D0). The following chemical equations are describing the oxidation and reduction reactions occurring at the sensitizing dye molecule (D). Absorption

Photon energy (h ) + D0

Separation

D*

Collection

D+ + e- (electrolyte)

D* (Excited dye)

D+ + e- (electron to semiconductor) D0 (retaining dye)

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Fig. 1: Schematic working of DSSC. (i) Absorption (ii) Collection and (iii) Separation Photoanode in Dye sensitized solar cell In earlier periods of PEC, photo electrode made from bulk semiconductors was used. These photo electrodes undergo photo corrosion when exposed to light, thus lowering the stability of photo electrochemical cell. Thus in order to enhance the stability, wide band gap semiconductor employed as photo anode because of its resistance to photo corrosion. Though it addresses the problem of corrosion it fails in enhancing the light to current conversion efficiency because of its inadequate surface area. Consequently it becomes a need to find a solution to enhance the light to current conversion efficiency in DSSC [13]. In photoanode design, nanostructured materials often given considerable attention when compared bulk counterparts owing to their remarkable optical and electrical properties. Its higher surface area gives way for sufficient absorption of sensitizer dye molecules thus enhancing the light harvesting efficiency. Further uniform semiconductor layer does not allow higher incorporation of dye molecules instead layers with larger pores allows the same. This porous nature in photo anode layer can be achieved with annealing at ambient temperatures [14-15]. Large number of wide band gap metal oxides like TiO2 [16-17], ZnO [18-19] given considerable attention owing to their particular functionalities and its hierarchical morphologies. Among them, SnO2 (Tin Oxide) with wide band gap of 3.8eV given considerable interest in DSSC phtoanode due to its high permittivity and higher transparency. Further the higher electron mobility (~100 – 200 cm2V-1S-1) when compared to TiO2 (~ 0.1 – 1.0 cm2V-1 S-1) implies the faster transport of photo induced electrons in them. Different morphology of SnO2 materials have been widely investigated and found it not only influences the light harvesting efficiency and change in collection efficiency in addition it enhances the electron transport related open circuit voltage (VOC) [20- 22]. More recently SnO2 hollow spheres and octahedron based photo anode exhibited higher photovoltaic performance. This higher photovoltaic performance is due to the SnO2 better electron transport properties and light scattering ability [23-24]. Several methods till date reported for the preparation of SnO2 based photo anode layer for DSSC which includes seed mediated growth, hydrothermal growth of hierarchical structures followed by non vacuum technique and nanocluster deposition.

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Seed mediated growth SnO2 seed layer is prepared by mixing potassium hydroxide to tin tetra chloride in aqueous medium under stirring with pH maintained at 7-8. The medium was aged at room temperature for 24hrs. The obtained gel spin coated on a substrate and heat treated at 800C for 12hrs thus obtaining a seed layer. Aqueous solution containing 0.01M of tin tetrachloride and 0.06M of ammonium fluoride serves as growth solution [25]. The preparation of hierarchical structures involves hydrothermal route which uses tin chloride precursor and sodium hydroxide/any reducing agent. The mixture is transferred to autoclave and usually maintained at a suitable temperature for about several hours. The prepared material coated using non vacuum technique by making a paste out of them and coated on substrates using screen printing or by simple doctor blade technique).

Fig. 2. FE-SEM images of the hydrothermal method grow SnO2 nanoflowers and SnO2 thin film by nanocluster deposition [9] [26]. T.T Duong et.al reported SnO2 photo anode thin film preparation using Nanocluster deposition. The starting precursor was tetra-n propyl tin with argon and oxygen as carrier and source gas. The flow rate of argon and oxygen maintained at 100sccm with tin source bubbler temperature maintained at 70-1200C [26]. Photoanode based on one dimensional SnO2 It is well known that one dimensional nanomaterial have higher interfacial charge transfer rate and larger surface area compared to spherical and other shaped SnO2 system. Considerable attention given to slower the interfacial recombination rate and speed up the electron transfer capability in SnO2 based photo anode design. The less success rate for SnO2 based DSSC arises due to positive shift of conduction band and lower isoelectric point compared to TiO2 nanostructures [28]. Several mechanisms carried out which includes core shell system of SnO2 –TiO2, surface modification of SnO2 with TiO2 layer, double layered SnO2 nanofiber /nanoparticle photo anode, surface properties of SnO2 nanowires and oriented attachment growth of SnO2 nanorods to meet out the disadvantages and have been given greater interest in SnO2 based photo anode research in recent five years. S.Gubbala et.al., [29] studied a comparative works on SnO2 nanowires and nanoparticles as photo anode material for DSSC. Their work shows nanowires exhibit higher open circuit voltage of 520560mV and faster electron transport when compared to nanoparticles. Further to study the dynamics behind such higher open circuit voltage in nanowires, photocurrent decay measurements performed. The measurement showed that the electron transport in nanowires was on order of magnitude faster than that of nanoparticles. The transport of electron between each trap within the material decides the transport of them. Longer waiting times are customized when the states with longer waiting time is filled, thus leading to electron transport through unfilled states closer to conduction band. Fig.3. shows light intensity Vs transport time, which illustrate that the time constant value obtained for


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SnO2 nanowires is 0.45 sec higher than that of SnO2 nanoparticles. The difference in distribution of trap states between the nanowires and nanoparticles makes such increase in open circuit voltage of SnO2 nanowires based DSSC.

Fig. 3 Photon flux plot Vs Transport time constants of SnO2 nanowire and nanoparticle [29]. C. Gao et.al., reported a simple method to prepare porous SnO2 nanotube-TiO2 core shell system based photo anode. The nano fibers / nanotubes were obtained using tin dichloride dihydrate and poly vinyl pyrrolidone in ethanol + dimethly formamide medium by electrospining by keeping applied voltage and the distance between the tip and collector were 13.5 kV and 15cm respectively. The obtained one dimensional SnO2 system annealed at 5000C for 2hrs to obtain the porous nature. Further to prepare SnO2-TiO2 core shell one dimensional system, SnO2 nano fibers/ nanotubes were ultrasonically dispersed in mixture containing acetic acid, deionized water and ethanol. The paste coated using drop drying method in FTO glass substrate and sintered at 5000C. In order to obtain TiO2 shell layer above SnO2 core it is dipped in TiCl4 aqueous solution for 2hrs at 85OC followed by air sintered at 500OC for 30min. Analysis made using high resolution transmission electron microscopy to confirm the formation of core-shell system (Fig.4a). SnO2 nano tube/nano fiber TiO2 illuminated under AM 1.5G, 100 mW cm-2 for performance of DSSC. The power conversion efficiency of performance of SnO2 nanotube/TiO2 is ~5.11% higher than that of SnO2 nano fibers. The short circuit and open circuit values for different combination shown in Fig.4b. The higher power conversion efficiency can be attributed to conduction band edge of SnO2 , which is 300mV more positive than that of TiO2. Thus leading to rapid interfacial electron recombination and lower trapping density. The coating of TiO2 on SnO2 give way to surface dipole layer towards SnO2,which causes the shift of conduction band to move towards negative value thus leading increased open circuit and short circuit voltage and current[29].

b

Fig. 4 (a) TEM images of SnO2 nanofibers and (b) current density Vs Voltage plot of photoelectrode films of SnO2 Nanotube/TiO2, SnO2 nanofiber/TiO2, SnO2 nanotube, SnO2 nanofiber, TiO2 nanoparticle [30].

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H.Song et.al., [31] reported SnO2 nanorods via a novel tin oxalate as tin source for the growth of nanorods on various substrates as photo anode for DSSC. The easy dissociation of tin oxalate at low temperature provides larger tin sources for the growth of nanorods. The growth of oriented attachment SnO2 nanorods stars from the dissociation of tin oxalate in water. The dissociation becomes supersaturated with Sn2+. The Sn2+ undergo hydration to generate Sn(OH)2 nuclei, leading to formation of SnO2 on the substrate and serves as seeding layer. Hexamethylenetetramine used as growth solution for nonpolar faces (110) of SnO2 thus leading to formation of nanorods on the substrate which serve as photo anode for DSSC. The SnO2 nanorod based substrate served as photoanode with platinum as counter electrode in the presence N719 dye molecules the performance evaluated. The higher transparency in SnO2 nanorods that is attributed due to single crystalline nature will benefit the faster electron transport. In addition the lower isoelectric point of SnO2 lowers the dye absorption capacity of SnO2 thus affecting the efficiency of the system. Hence TiO2 embedded SnO2 core shell system designed to enhance the power conversion efficiency, since the conduction band of TiO2 is ~0.3 eV higher than that of SnO2 thus enhancing the light harvesting capacity.

Fig. 5. Photocurrent Vs Voltage plot TiO2 photoanode and SnO2 nanorods entrenched TiO2 photoanode. Inset shows the schematic view of SnO2 nanorod embedded TiO2 DSSC [31]. The modified photoanode produces a higher conversion efficiency of about 8.61% which is 1.07% higher that of unmodified and their respective figure shown in Fig. 5. The intrinsic electron mobility difference between TiO2 and SnO2 attribute to the same. G. Shang et.al., [32] reported TiCl4 treated SnO2 nanorods as photoanode in DSSC in the presence of N719 dye molecule with platinized FTO as counter electrode. Their comparative studies on SnO2 nanoparticles and nanorods as photoanode gave a clear explanation for higher current density. The results shows higher open circuit voltage, short circuit current and fill factor for nanorods when compared to nanoparticles is shown in Fig. 6.


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Fig.6. Current density Vs Voltage plot of DSSC photoanode based on SnO2 nanorods and nanoparticles [32]. To further understand the reason for such increase roughness factor and dye absorption value compared for both. The results shows that SnO2 nanorod based photo anode possess a lower surface area and inherent surface roughness factor, thus leading to poorer dye loading capacity. On the other hand if dye loading capability affect the efficiency of the system, only SnO2 nanoparticle based photanode DSSC system would have resulted in higher efficiency rather than SnO2 nanorod based DSSC. Thus it is implicit that the one dimensional nature of SnO2 with higher surface and interfacial charge transfer rate assist superior charge transport rate when compared to nanoparticle of SnO2. Apart from core-shell and nanorod based SnO2 DDSC photoanode, formation of double layered photoanode emerged to overcome the disadvantage of lower dye loading capacity of SnO2 nanorods. Though it does not have higher impact as stated by G.Shang et.al., it could lead to higher performance of DSSC if addressed. Y.F Wang et.al., [33] reported the formation of SnO2 nanofibers consisting of nanoparticles for higher efficient light harvesting. The bilayered photoanode consist of SnO2 nanoparticle as bottom layer to increase the dye loading capacity and SnO2 nanofibers as top layer for to boost the electron transport. The connectivity between the top and bottom layer too is good enough for efficient transport of electron without any defects and voids as shown in Fig. 7a-b. The DDSC power conversion efficiency is 1.14 % is higher for bilayered DSSC when compared to single layered cell. This increase is attributed to higher dye absorption capacity of SnO2 nanoparticle layer (15.65 X 10-8mol cm-2) and higher light scattering capability of SnO2 nano fibers.

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Fig.7 a-b. FE-SEM SnO2 nanoparticles and SnO2 nanoparticle/nanofilm which shows good connectivity between top and bottom layer without any defects [33]. Further the SnO2 nano fibers provide direct linear path for transport/ collection electron and SnO2 nanoparticle with larger size and higher crystalline nature minimize the grain boundary favoring lower recombination rate in DSSC. These proposed investigations till date on one dimensional SnO2 system extent its possibility to make one dimensional SnO2 with higher aspect ratio and higher light harvesting efficiency as an alternative to other photo anode material. Hierarchical structured SnO2 based photo anode The oriented attachment of one dimensional SnO2 nanostructures though promotes faster electron transport and slower recombination rate is inferior to hierarchical structured photo anode. To achieve higher photon to current conversion efficiency in DSSC necessitates not only high surface area for dye loading capacity instead it needs a highlight harvesting capability. Recently several hierarchical morphology has been studied which includes porous/mesoporous/ hollow spheres, nanosheets nanotubes, hierarchical double layered structures and Sn based hybrid nanocatus etc,.

Fig.8. Schematic representation of ordered mesoporous SnO2 particles synthesized using hard template method. TiO2 or Al2O3 layer deposited using above SnO2 structures [37]. The hierarchical structures have compactly packed microstructures and poly dispersed aggregates with nanosized crystallites. These internal structures facilitate the superior light scattering capability thus enhancing the internal surface area for the effective dye loading capacity [34-36]. E.Ramasamy et.al., [37] prepared SnO2 ordered mesoporous photoanodes synthesized using KIT- 6 silica hard template method.The preparation route is schematically represented in Fig.8. Three different


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photoanodes with different composition as i) SnO2 alone ii) SnO2/TiO2 and iii) SnO2/Al2O3 in a core shell manner were prepared. The device efficiency of SnO2/TiO2 is about 2.7% higher than SnO2 photo anode DSSC. In order to confirm the reason for such increase a device with Al2O3 shell prepared and its performance analyzed which is about 0.2% lower than SnO2/TiO2 system. This modification of SnO2 with TiO2/Al2O3 slows the recombination of electron via passivation of sub band edge surface states. Similarly J.Chen et.al.,[38] ,H.Wang et.al., [39], G.Shang et.al., [40] and C.Ma et.al., [41] made an attempts with the use of microsphered SnO2 as photoanode in DSSC synthesized via hydrothermal and chemical route. Fig. 9a-e shows hydrothermally prepared SnO2 microsphere morphology and its photovoltaic performance. E

Fig. 9 (a-c) SEM images of SnO2 hollow spheres by hydrothermal reaction at different scale, (d) Schematic image of hollow sphere and (e) Current Vs Voltage plot of prepared SnO2 hollow spheres with and without TiCl4 treatment [39]. The maximum power conversion efficiency of 4.15%, 6.02%,4.97% and2.26% achieved in the different microsphere structured SnO2 photoanode. The advantages of hollow SnO2 micro/nano spheres is that It enhances the light harvesting capability due to multiple reflections and scattering in DSSC Larger dye absorption capability due to enhanced surface area Enhanced charge transport with less diffusive difficulty Decreased number of contact barrier between primary particles Possibility in tuning the structure and chain length of microsphere to alter the dye filling ability The presence of single crystallite SnO2 nanoparticles within the hollow sphere allows larger dye loading capacity and the hollow nature allows higher light harvesting capability with lower reflection ability in DSSC system. Novel SnO2 nanocactus (Zn-Sn-O) shown in Fig. 10 along with its photovoltaic performance, nanoechinus (Zn-SnO2) and octahedra (SnO2) structures reported by X.Dou et.al., [42] , Z.Li et.al., [43] and Y.F.Wang et.al., [44] for high performance DSSC.

C

Fig. 10 (a-b) Novel Zn-Sn-O nanocatus and its distinct view of single nanocactus and © Photovoltaic performance of nanocactus/TiCl4 treated nanocactus.

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Zn-Sn-O nanocactus structure composed of Zn2SnO4 plate and Zn doped SnO2 nanothrons both of which posses higher electron mobility. This increase is attributed to the incorporation of Zn into the SnO2 lattice system. This compound system is coated with thin layer of TiO2 to enhance the charge injection rate thereby directly affecting the photocurrent efficiency. The TiO2 coating enhances the thickness of Zn-Sn-O layer with increased roughness leading to higher dye absorption efficiency by 9% .The conversion efficiency is 4.41% higher for TiO2 coated Zn-Sn-O when compared to untreated one. Likewise size edge length tuned (0.5, 0.8, 1.0, 1.5, and 1.8 um) SnO2 octahedra have different power conversion efficiency (5.57%, 5.82%, 6.40%, 6.45% and 6.80%) when used as photo anode. The growth rate varies at different crystallographic planes thus gives different morphology. The higher open circuit voltage at large sized octahedra attribute to lower interparticle boundaries and surface defects in the crystal system leading to longer life time of electron. The higher light scattering capability with slower recombination rate in large sized SnO2 octahedra when compared smaller dimension in responsible for higher conversion efficiency. In Zn incorporated SnO2 nanoechinus , the Zn doping induce shift in flat band potential with increase in isoelectric point. This increase in isoelectric point helps in larger incorporation of dye molecule, thereby enhancing the efficiency of the DSSC. Further the multiple light reflections from closely packed nanorods on the surface favour the efficient transport with reduced recombination leading 4.15% conversion efficiency. A.Birkel et.al., [45] investigated tunable band edge effect of SnO2 nanocrystals synthesized via microwave synthesis route as photoanode in DSSC. Different reductant used for the formation of varied morphology of SnO2. The NaOH (sodium hydroxide) employed synthesis yield homogeneous bundles of nanorods, KOH (potassium hydroxide) employed route yield nanocrystals with lesser anisotropic, TMAH (tetramethylammonium hydroxide) used route yield and NH4OH (ammonium hydroxide) involved route yield ~8nm SnO2 nanocrystals. Further the different morphology SnO2 used as photoanode it directly affect the conversion efficiency of DSSC. The higher recombination rate with poor interconnectivity in NH4OH-SnO2 photoanode results in lower conversion efficiency (1.60%) whereas the NaOH-SnO2 and TMAH-SnO2 based structures photoanode results in higher conversion efficiency of (3.14%, 3.16%) respectively. This variation is caused by the shift in flat band potential which is associated with different SnO2 morphology and varied crystallographic orientation. The use of SnO2 nanosheet as interfacial layer on fluorine doped indium tin oxide substrate reported by C.F.Sei et.al.,[46] The presence of this SnO2 layer between TiO2/dye molecule suppress the recombination rate and promote higher electron transport pathway when compared interfacial layer lacking TiO2 photoanode . The conversion efficiency is increased from 3.89% to 4.62% with the introduction of SnO2 nanosheet interfacial layer. C

Fig. 11 (a-b) Low magnification SEM images of hydrothermally treated SnO2 and its cross sectional view and (c) current Vs voltage characteristics of SnO2 photoanode with/without TiCl4 treatment [25]. In addition self assembled double layered SnO2 film with hierarchical structures (shown in Fig.11. a-c along with its photovoltaic performance) grown directly on fluorine doped tin oxide reported by M.Liu et.al., [25]. The microspheres with superior light scattering capacity (top layer) and nanosheets with higher electron transport property (bottom layer) attribute to higher conversion efficiency of about 1.52% with TiO2 shell layer shown in Fig. 11a-b and corresponding its Current voltage plot shown in Fig.11c. The higher surface area with increased charge separation ability of


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SnO2 based hierarchical structures still has a potential to overcome the other photo anode contender in DSSC. If design of DSSC carried out with suitable care on concerning band edge potential and ISO electric point, it is sure SnO2, SnO2 composite, hierarchical SnO2 and its composites will be a potential material in DSSC device. Conclusion In summary, several new approaches developed in recent years to make SnO2 as photo anode material in DSSC is presented here. The majority of the methods presented were laborious and does not need any sophisticated tools and environment. The various strategies adopted in SnO2 based DSSC phtoanode design presented comprising the dimensional aspects and hierarchical structures of SnO2 is described. Lack of control over the flat band potential and band edge engineering makes SnO2 a less suited material for DSSC. If realization on such concerns made on SnO2 based DSSC it is possible for higher solar spectrum harvesting capability, thereby increasing the conversion efficiency. The authors desire that this review will make the researcher to rouse their minds towards innovative research methods for making SnO2 a promising material in photoanode research. References [1] K.E. Jasim, Dye sensitized solar cells – working principle, challenges and opportunities, solar cells - dye-sensitized devices, Intechopen Europe, 2011, pp. 171-204. [2]

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