CaCO 3 Coated SnO 2 -Based Dye-Sensitized Solar Cells with p-type CuI Solid Electrolyte

Transcription

1 CaCO 3 Coated SnO 2 -Based Dye-Sensitized Solar Cells with p-type CuI Solid Electrolyte W.M.N.M.B. Wanninayake 1,4, K. Premaratne 2,4, R.M.G. Rajapakse 3,4 1 Department of Civil Engineering, Faculty of Engineering, University of Peradeniya, Peradeniya, 20400, SRI LANKA. 2 Department of Physics, Faculty of Science, University of Peradeniya, Peradeniya, 20400, SRI LANKA. 3 Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya, 20400, SRI LANKA. 4 Postgraduate Institute of Science, University of Peradeniya,Peradeniya, 20400,SRI LANKA. mihira.wanninayake@gmail.com Abstract: Although TiO 2-based Dye-sensitized Solar Cells (DSCs) show the heights efficiency, TiO 2 material has some drawbacks due to its low electron mobility which leads to dark current and photocatalytic ability led to degraded of sensitizer molecules. Somehow, these problems can overcome using SnO 2 which has promising electronic properties, in place of TiO 2. The high recombination associated with the SnO 2 nanoparticle surface can be minimized through the use of very thin CaCO 3 layer which covers the recombination sites of SnO 2 nanoparticles. Even though liquid electrolytes based DSCs show promising performance, still they suffer from problems such as electrolyte leakage, solvent evaporation and sealing imperfections. In order to circumvent the above problems associated with the liquid electrolyte-based DSCs, one of the alternatives is used solid electrolyte in place of liquid electrolyte. One of the best p-type semiconductor as a solid electrolyte is CuI. In this study triethylamine hydrothiocyanate (THT) was used as crystal growth inhibitor as CuI prone to form crystals soon it insert into semiconductor network. The structure of FTO/SnO 2/CaCO 3 composite film/dye (D358)/CuI solid electrolyte/lightly platinized FTO counter electrode was used in solar cell device. A molar ratio of 0.12:1 of CaCO 3 to SnO 2 based solid state solar cell device gave efficiency of 1.84%. Even if it is quite low efficiency, this study will leading to build a sustainable future for energy demand. Keywords: Solid electrolyte, dye-sensitized solar cells, CuI, SnO 2, insulating coating layer 1. INTRODUCTION The dye-sensitized solar cell (DSC) is still continuing to draw the attention of the solar cell research (1991). Recent developments of solar cell enabled to achieve solar-to-electricity conversion efficiency of about 12% Yella et al. (2011). Generally, a solar cell comprises an active electrode composed of nanoporous interconnected TiO2 nanoparticle matrix covered by a light absorbing dye and a lightly platinized fluorine-doped tin oxide (FTO) counter electrode with a liquid electrolyte containing a redox couple sandwiched between the two electrodes. However, the charge separation ability of TiO2-based DSC is suppressed by its low electron mobility resulting in a higher dark current. Moreover, TiO2 shows a high photocatalytic effect and as a result, sensitizer which is attached onto the nanoporous TiO2 network prone to degrade rapidly. Therefore, in order to overcome these problems regarding TiO2-based DSCs, other high band gap semiconductors such as ZnO, SnO2, CdS, etc were investigated as possible alternative semiconductor materials. ZnO is similar to TiO2 as far as band gap and band positions are concerned (band gap 3.2 ev) but has a much lower effective electron mass ( 0.26me for ZnO and ~10me for TiO2) and much higher electron mobility ( cm 2 V s for bulk ZnO, cm 2 V s for TiO2) and much higher electron diffusion coefficient ( cm 2 s for nanoparticulate film of ZnO and 10 to 10 cm 2 s for TiO2) Zhang et al. (2009) and Premaratne et al. (2012). Given such favorable conditions, the DSC based on ZnO should perform superior to TiO2 based DSC though the practical situation is completely opposite as amphoteric ZnO is prone to dissolve in slightly acidic dye or electrolyte medium thus lowering the overall performance. To overcome some of the above mentioned drawbacks, high bandgap SnO2 semiconductor material Kwong et al. (2004) having a low effective electron mass, is the one possible alternative material in place of TiO2. The effective electron mass and the electron mobility values of SnO are 0.1 me and Bandara et al. (2008), ~250 cm 2 V -1 s

2 Santiago et al. (2009) and Wanninayake et al. (2016), respectively, at room temperature. Even if SnO2 has high electron mobility and lower effective electron mass, it shows inferior performance due to the recombination of injected electrons with excited dye molecules and redox species of the electrolyte. One of the possible ways to reduce the electrons reaching the SnO2 surface traps is by introduction of a surface passivation layer on top of the semiconductor surface using high band gap insulator materials. Then it will help reduce the back tunneling of the injected electrons. One of our previous studies was determined the effect of coating material on the performance of SnO2-based DSCs and reducing the recombination effect through the use of different insulating material as a passivation layer Wanninayake et al.(2016). Though the support of similar studies and based on our study, a thin CaCO3 coating layer was introduced in order to suppress surface recombination occurred in SnO2-based DSCs. Even though DSCs show superior energy conversion efficiencies with the liquid electrolyte, the presence of a liquid in a solar cell limits its practical applications due to the ability for the evaporation, leakage and sealing imperfections. In order to address these drawbacks, the liquid electrolyte has been replaced by a hole conducting material such as poly(3-hexylthiophene), poly(ethylene oxide) or 2,2-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9-spirofluorene (spiro-ometad) but conversion efficiencies obtained were rather low. The use of a p-type semiconductor with matching band positions is yet another approach to assemble dye-sensitized solid-state solar cells. In this regard, Tennakone et al. have reported the use of CuI as a p-type semiconductor with dye-coated TiO2 active electrode Thennakon et al. (1998) and conversion efficiencies around 3% have been reported. The use of CuSCN in such TiO2-based DSSCs has also been reported Premalal et al.(2010) and Premalal et al. (2012). A conversion efficiency of 4.7% is obtained for magnesium oxide-coated nanocrystalline TiO2 active electrode with N719 dye and p-type CuI hole conductor Kumara et al. (2004). An ultrathin layer of MgO around the interconnected TiO2 particles allows the electron injection from the excited dye molecules while preventing the recombination of injected electrons with the holes in the valence band of CuI or the oxidized dye molecules. A breakthrough in dye-sensitized solid-state solar cells (DSSCs) has been reported very recently in which n-type TiO2 nanoparticles were sensitized by N719 dye and a p-type semiconductor CsSnI3 was used as the hole collector. A conversion efficiency of around 10% has been obtained Chung et al. (2012). However, more work needs to be carried out to address questions such as the stability of CsSnI3 under illumination and exposed to high temperatures. In spite of a large number of publications available for the DSSCs with TiO2 electrodes, only a very few reports exist in the literature for the use of p-type inorganic semiconductors in combination with n-type SnO2 active electrodes in DSSCs. Reported in this manuscript are the fabrication and the performance of DSSCs based on nanoporous composite SnO2/CaCO3 nanoparticles sensitized by D-358 dye and p- type CuI hole collector. 2. EXPERIMENTAL 2.1. Preparation of Working Electrode First, SnO2 colloidal solution (Loba Chemie, Mumbai, India, 3.00 cm 3 ), acetic acid (10 drops, Wako Chemicals, Japan), g of CaCO3, absolute ethanol (40.00 cm 3, Sigma Aldrich) and 5 drops of Triton X 100 were ground well. Then, the mixture was ultrasonicated for 20 min. Next, the FTO glass plates were cleaned using ethanolic solution by sonication for 10 min. Then the plates were heated at 200 o C. After that, above prepared solution was added drop wise on to the FTO plates (Drops casting) and unbound crust were blow off. This method was repeated several times and samples were sintered as wiped off by cotton wool. Then the same procedure was repeated nearly 20 times. Next, the plates were further sintered at Application of the Dye Here D358 was used as the photosensitizer. The electrodes prepared as above were dipped in freshly prepared dye in a mixed solvent composed of acetonitrile (Wako Chemicals, Japan) and tertiary-butyl alcohol (Wako Chemicals, Japan) in 1:1 volume ratio. The electrodes were allowed to dye for 12 h. Subsequently, they were taken out, washed with acetonitrile to remove any excess dye molecules and allowed to dry in air. 276

3 2.3. Application of CuI The CuI solution was prepared by dissolving 1.0 g of CuI (Sigma Aldrich, 99%) in 30 ml of acetonitrile. 15 mg of triethylammonium thiocyanate (THT) was then added and thoroughly mixed. CuI was applied on to the dyeevaporate. This process was continued until the CuI is filled just above the dye. Finally, the I V characteristics were measured using PECCELL (PEC-L01) AM 1.5 Solar Simulator. 3. RESULTS AND DISCUSSION Figure 01 shows the molecular structure of D358 dye which is employed to sensitized the nanoporous SnO2/CaCO3 network. D358 dye consists of a rhodanine dimer which acts as the acceptor and the carboxylic acid acts as the anchoring group. D358 dye is an amphiphilic derivative of D131 dye in which the cyano-acrylic acid group is replaced by C17N2S3O8H26 group which can limit the dye aggregation. D358 dye shows an absorption peak at about 532 nm by producing higher light harvesting and chelating ability. Figure 1 Molecular structures of indoline D358 dye In order to examine the panchromatic effect of D358 dye system, UV-Visible studies were conducted. The absorption spectra of the adsorbed dye and of the individual dyes are shown in figure 02. The absorption spectra for a dyed film with coating materials show significant broadening due to the better anchorage of dye molecules onto the composite surface. Also, absorption peak corresponds to the dyed film shows red shifted due to the J aggregation of the dye molecules on the semiconductor surface. Figure 2 UV-visible absorption spectra of indoline D358 dye in 1:1 volume ratio of acetonitrile/tert-butyl alcohol and system consisting of dye/caco 3/SnO 2 film on the FTO substrate. 277

4 4.0 D-358 dye with SnO 2 / CaCO 3 film D-358 dye Wave Length Figure 3 FTIR spectra of D-358 dye and D-358 dye attached composite film FTIR study was also conducted to analyse the surface modification due to the anchorage of dye molecules to the composite CaCO3/SnO2 surface. According to figure 03, there is no significant change of in FTIR spectrum due to the formation of novel bonds. Figure 4 IV curves of SnO 2 and CaCO 3 coated SnO 2-based DSSCs Table 01 shows the solar cell parameters measured for the SnO2 and SnO2/CaCO3-based DSSCs. The active area of the working electrode is 0.25 cm 2 and thickness of the device is of about 15 µm. It has been observed that the DSSC prepared in an identical manner except without the CaCO3 coating layer showed the relatively lower open circuit voltage (VOC) and short circuit current density (JSC). According to the figure 04, solar cell device fabricated with composite CaCO3/SnO2 shows significant enhancement of the performance compared to the SnO2 alone. Especially, current density shows an increase of about 157%. Reduction of the recombination and increase of electron harvesting due to 278

5 increase of adsorbed dye molecules to the CaCO3 surface might be the possible reasons for this higher jump of current density. Table 1 IV parameters of DSSCs fabricated using SnO 2 with CuI electrolyte This is attributed to the recombination of injected electrons by CuI particles touching the SnO2 nanoparticles at the FTO/ SnO2 interface. The presence of a coating layer of CaCO3 prevents the recombination occurred through the surface traps of SnO2. Heat treatment and drop casting method will help formation of the dense layer. Also, this dense layer formation will prevent approach of CuI particles on to the FTO surface. There are positive and negative effects of use of drop casting method. Formation of the dense layer increases the sheet resistance and reduce the fill factor of the device thus lowering the performance of the solar cells. However, higher dense layer thickness increases the conversion efficiency by facilitating to more dye molecules to harvest more electrons. 4. CONCLUSIONS The preparation and characterization of FTO/CaCO3 coated SnO2 nanoporous layer/d-358 Dye/CuI hole collector/lightly platinized FTO dye-sensitized solid-state solar cells have been described. A molar ratio of 0.12:1 of CaCO3 to SnO2 based solid state solar cell device gave efficiency of 1.84%. The best efficiency is obtained when the thickness of the composite film is of about 15 µm. 5. ACKNOWLEDGEMENT This work is supported by the National Research Council of Sri Lanka (NRC: 08-17) 6. REFERENCES 1991, A low cost, high efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature, 353, 737. A. Yella, H. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W. Diau, C.Yeh, S.M. Zakeeruddin, M. Grätzel, Porphyrin-sensitized solar cells with cobalt, (II/III)-based redox electrolyte exceed 12% efficiency Science, 334, 629. Q. Zhang, C.S. Dandeneau, X. Zhou, G. Cao, nanostructures for dyesensitized solar cells, Advanced Materials C.Y. Kwong, W.C.H. Choy, A.B. Djurisi P.C. Chui, K.W. Cheng, W.K. Chan, hexylthiophene):tio2 nanocomposites for solar cell applications, Nanotechnology, 15, K. Premaratne, G.R.A. Kumara, R.M.G. Rajapakse, M.L. Karunarathne, Highly efficient, optically semitransparent, ZnO-based dye-sensitized solar cells with Indoline D-358 as the dye, Journal of Photochemistry and Photobiology A: Chemistry, 229, 29. Poly(3- T.M.W.J. Bandara, M.A.K.L. Dissanayake, O.A. Ileperuma, K. Varaprathan, K. Vignarooban, B.-E. Mellander, Polyethyleneoxide (PEO)-based, anion conducting solid polymer electrolyte for PEC solar cells, Journal of Solid State Electrochemistry, 12, 913. F. Fabregat-Santiago, J. Bisquert, L. Cevey, P. Chen, M. Wang, S.M. Zakeeruddin, M. Grätzel, 2009, Electron transport and recombination in solid-state dye solar cell with Spiro-OMeTAD as hole conductor, Journal of the American Chemical Society, 131,

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