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Performance of TiO2 / in (OH) ISJ / Pb (OH) xSy composite ETA solar cells prepared by nitrogen doped TiO2 thin film window layer

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  • Save International Journal of M aterials Engineering 2013, 3(2): 11-16 DOI: 10.5923/j.ijme.20130302.01 Performance of TiO2/In(OH)iSj/Pb(OH)xSy Composite ETA Solar Cell Fabricated from Nitrogen Doped TiO2 Thin Film Window Layer C. O. Ayieko1,*, R. J. Musembi1, S. M. Waita1, B. O. Aduda1, P. K. Jain2 1Department of Physics, University of Nairobi, P.O. Box 00100-30197, Nairobi, Kenya 2Department of Physics, University of Botswana, P/ Bag 0022, Gaborone, Botswana Abstract In this work, Titaniu m Dio xide (TiO2) thin films were prepared by spray pyrolysis and thermally annealed at 400℃. The films we re characterized as deposited (no annealing) as we ll as after annealing. Optical studies showed that the energy band gap of the films was lo wered fro m 3.25 eV to 2.90 eV on Nitrogen (N2) doping. The reduction in energy band gap was attributed to the introduction of N2 impurity states on the bands (conduction band and or valence band). The effect of N2 doping of Titaniu m Dio xide window layer on the efficiency of the ETA TiO2/In(OH)iSj/Pb(OH)xSy solar cell was investigated using a conventional current-voltage (I-V) technique. The photovoltaic conversion efficiency (η) increased fro m 1.06% for the solar cell with undoped films to 1.32% for the solar cell with N2-doped films. The increase in photovoltaic conversion efficiency on doping was attributed to increased light absorption due to the Nitrogen doping. Keywords Titaniu m Dio xide, Conversion efficiency, Doping 1. Introduction Meeting human energy requirements has been an elusive tas k even after the discovery of the immense energy from the atom when the phrase “too cheap to meter” was coined[1] and the energy cost has been soaring each passing day. The energy crisis in the 1970’s wh ich made the cost of petroleum almost quadruple necessitated intensive research and development on renewable energy sources[1]. Solar energy, one of the renewable sources, is environmentally friendly. It is also universal and versatile with the ability to be made availab le even to the remotest parts of the world. Solar energy systems do not involve heavy moving parts and hence present min imal wear and tear and therefore require very low maintenance once installed. The init ial cost of installat ion for solar energy systems is high just as for other energy sources but due to low maintenance costs, it guarantees free energy after a short payback period. This makes solar energy systems viable[2]. Solar cells convert solar energy into electrical energy. Different t echnolog ies have been developed to fabricate so lar cells , an d th e Ext remely Th in Abso rber (ETA ) technique is one of the promising methods. The concept of ETA solar cells involves using an extremely thin absorber * Corresponding author: (C. O. Ayieko) Published online at Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved layer of less than 50 nm. So me pro mising characteristics of ETA solar cells include: reduced charge carrier transport time through the solar cell due to thin absorber, enhanced photon absorption due to scattering resulting fro m the porous nature of the window layer[3]. A major challenge facing ETA solar cells is lo w photoelectric conversion efficiencies. TiO2 has been used as window layer for both all-solid state and dye sensitized solar cells. A porous TiO2 deposited by spray pyrolysis was used as a window layer in the fabrication of TiO2/In(OH)iSj/Pb(OH)xSy/PEDOT:PSS ETA solar cell and it was observed that the highly porous TiO2 ensured increased absorption of incident rad iation through s catterin g [4] . Studies on N2-doped TiO2 deposited by the pressing technique for dye sensitized solar cells (DSSC) application have been done[5]. However, the challenge encountered with the DSSC solar cell was leakages fro m the liquid electro lyte. Gartner et. al[6] have also done some studies on N2-doped TiO2 thin films and reported an increase in light harvesting towards the visible range. However, they never used the doped films in any specific applicat ion. Although some wo rk has been done for N2-doped TiO2, to the best of our knowledge, N2 doping of TiO2 has not been done for spray pyrolysis deposited TiO2 films and applied to the ETA solar cell. Th is work therefore investigates the performance of an all-solid state ETA solar cell with TiO2 thin films deposited by spray pyrolysis followed by N2 doping, that is, N:TiO2/In(OH)iSj/Pb(OH)xSy solar cell. 12 C. O. Ayieko et al.: Performance of TiO2/In(OH)iSj/Pb(OH)xSy Composite ETA Solar Cell Fabricated from Nitrogen Doped TiO2Thin Film Window Layer 2. Experimental Fluorine-doped tin o xide coated glass substrates (FTO) were cleaned in an u ltrasonic bath filled with d istilled water for 2 minutes before deposition. The substrates were then rinsed in acetone and finally rinsed in distilled water. Deposition of TiO2 by spray pyrolysis was done by mixing 120 µl of co mmercially available Titaniu m (iv) Isopropoxide iC12H28O4) (purity 99.7%) and 200 ml Isopropanol (C3H8O) (purity 99.7%) fro m Fluka Ltd. The mixture was heated to 50 ℃ using an electric hot plate and the temperature maintained constant while stirring with a magnetic stirrer for about 15 minutes. The mixture fo rmed the precursor which was used for spray pyrolysis. Nitrogen gas was used as the carrier gas. The spray nozzle-to-substrate distance was kept at about 15.0 cm. The substrate temperature (Ts) was varied fro m 100℃ to 200℃ for d ifferent samp les. Coating was done by spraying in pulses. A pulse consisted of 5 seconds of spraying and 30 seconds of pause. Ten pulses were done for every sample at a precursor flow rate of 2.6 ml/ min. The chemical reaction resulting in the format ion of amorphous TiO2 thin film was as follows: TiC12H28O4 (aq) + 17O2 (g) + 2C3H8O (aq)(100℃- 200℃) TiO2(s)(a morphous) + 8CO2(g) + 22H2O(g). (1) TiO2(s)(amorphous) annealing (30 min at 400℃) TiO2 (s )(cry s tallin e) . (2) The film thicknesses were determined using a computerized KLA-Tencor Alpha –Step IQ surface profiler. Doping of the TiO2 films was done using a pyrex g lass tube placed in a programmab le horizontal tube furnace (ThermoScientific Lindberg/Blue M ini-Mite) in Nit rogen gas atmosphere flowing at a controlled at rate of 20 cm3/sec. The tube furnace was fitted with a digital themo meter which was used to measure the temperature of the doping chamber. The films were heated gradually fro m roo m temperature to about 450℃ and the temperature maintained constant for about 30 minutes after which, the films were allo wed to cool to room temperature without cutting off the Nitrogen flow. The crystal structure of the films was determined using a computer controlled Ph illips PW 3710 X-ray d iffraction (XRD). Scanning Electron M icroscope (SEM ) images of the films were carried out using a Carl ZeissTM LEO 1530 SEM model while Energy Dispersive X-ray (EDX) studies were carried out using an EDX unit attached to the SEM. Total transmittance and reflectance measurements were made at near normal incidence in the wavelength range of 300 -1200 n m using a UV-VIS-NEA R INFRA RED computerized double beam solid-spec 3700 DUV Shimad zu Spectrophotometer equipped with a Bariu m Su lphate (BaSO4) integrating sphere. The reflectance and transmittance data were used for calculation of optical properties: energy band gap and absorption coefficient as in equation (3)[7] and (4)[8] respectively; αhv = a(hv − Eg )r (3) k = αλ / 4π (4) 2.1. Deposition of Indium Hydroxy Sul phi de (In(OH)iSj) buffer Layer Nitrogen-doped and undoped TiO2 thin films were coated with Indiu m Hydro xy Su lphide buffer layer by chemical bath deposition (CBD) in a solution prepared using commercially obtained reagents from Sig ma A ldrich Ltd and prepared as follows: 20000 ± 1µl of d istilled water was mixed with 1250 ± 1µl of 0.005M of Hydrochloric acid (HCL) which was then mixed with 1250 ± 1µl of 0.005M Indiu m (III) Chloride (In2Cl3) and 2500 ± 1µl of 0.1M Thioacetamide (CH3CSNH2). The resulting solution was kept in a water bath that was maintained at a temperature of 70 ± 1℃. The TiO2 films were dipped into the solution prepared above in turns, each dip lasting 30 minutes. The films were then rinsed in distilled water to clean off unwanted chemical remnants. The procedure was repeated three times for each film. Finally the films were annealed at a temperature of 300℃ in air. 2.2. Deposition of Lead Hydroxy Sul phi de (Pb(OH)xSy) Coati ng Lead Hydro xy Sulphide (Pb(OH)xSy) coating was deposited by successive ion layer adsorption and reaction (SILA R). In this process, saturated Lead Acetate (Pb(CH3OOH)2) solution (97% pure) and Sodium Su lfide (Na2S) solution (97% pure) both commercially obtained fro m Sig ma Aldrich Ltd. were used. To deposit Lead Hydro xy Sulphide (Pb(OH)xSy) coating, the following four-stage process was followed: First the films were submerged in lead acetate solution for 10 seconds for the metal ions to be adsorbed then rinsed in distilled water Secondly, they were submerged in Sodiu m Su lfide solution for 10 seconds to allow Su lphur ions to react with metal ions and finally were rinsed in distilled water. The procedure constituted one cycle giving a particular thickness. Load Copper conductors Silver paste back contact Pb(OH)xSy layer In(OH)iSj layer N:T iO2 coating FT O glass substrate Figure 1. Schematic cross-section of the N:TiO2/In(OH)iSj/Pb(OH)xSy composite (ETA) solar cell A thin silver coating was applied on the Pb(OH)xSy to make an electrical contact. Figure 1 below is a schemat ic International Journal of M aterials Engineering 2013, 3(2): 11-16 13 display of the assembled ETA solar cell. 2.3. I-V Characterization Current-voltage (I-V) characterization was done using a solar simulator equipped with a 0.5kW Xenon arc lamp and a 1.5 AM Global A ir Mass Filter. Illu mination was maintained at 1 sun (1000 W/ m2). The solar cell a rea was 0.1256 c m2. undoped and unannealed in the visible range-400-800 n m and this can be attributed to the presence of the dopant in the film since all the other conditions are the same. It can therefore be concluded that doping also contributes to the low film trans mittance (10-45% in the visible range-400-800 nm) in the doped and annealed film. 80 3. Results and Discussion 70 3.1. Structural Analysis 60 Transmittance (%) Figure 2 shows a SEM micrograph of Nitrogen-doped 50 TiO2 and Indiu m Hydro xy Sulphide (Pb (OH)xSy) layer. The TiO2 is porous with nano pore sizes of appro ximately 500 n m. 40 This morphology is favourable for light scattering effect and increases the light optical path, thus enhancing light 30 absorption which makes the films appropriate for solar cell ap p licatio n s . 20 undoped and annealed, 400 oC doped and unannealed undoped and unannealed doped and annealed 400 oC Reflectance (%) Nano-pore TiO2 matrix In(OH)iSj 10 10% transmittance line 0 300 400 500 600 700 800 900 1000 1100 1200 1300 Wavelength (nm) Figure 3. Dependence reflectance and transmittance on wavelength of nitrogen doped and undoped T io2 thin films of approximate thickness of 400 nm deposited on fluorine doped tin oxide (FTO) glass slides 2.0x107 1.5x107 undoped and annealed, 400 oC doped and unannealed undoped and unannealed doped and annealed,400 oC Absorption Coefficient (α )(m-1) 1.0x107 Figure 2. SEM micrograph of nitrogen-doped T iO2 (N:TiO2) coating on FT O glass substrates and annealed at 400 oC with indium hydroxyl sulphide (In(OH)iSj) deposited on top of porous nitrogen-doped T iO2 3.2. Optical Characterizati on 5.0x106 3.2.1. Reflectance, Transmittance and Absorption Figure 3 shows transmittance and reflectance characteristics for undoped and Nitrogen-doped TiO2 films (both annealed and unannealed). Except the doped and annealed film (which is thicker co mpared to all other films), the other three films are of co mparable thickness as seen fro m the transmittance and reflectance spectra. Generally, thicker films tend to have lower transmittance than thin films. The lower transmittance observed can therefore be attributed partly to the film’s thickness. To find out whether doping plays any role, we compare the undoped and unannealed and the doped and unannealed films since they are of similar thickness. It is observed that the doped and unannealed has a lower transmittance (22-55%) co mpared to (34-60%) for the 0.0 300 400 500 600 700 800 900 1000 1100 1200 1300 Wavelength (nm) Figure 4. Dependence of absorption co-efficient on wavelength of Nitrogen doped and undopedT iO2 thin films deposited on fluorine dopedtin oxide (FTO) glass slides Figure 4 shows the graph of absorption coefficient, α, versus wavelength of the Nitrogen-doped and undoped TiO2 for both annealed and unannealed thin films. Clearly, there is more absorption for the N2-doped film co mpared to both undoped films within the visible range 350-800 n m. The observed increase in doping can be explained in terms of the presence of the nitrogen dopant in the film. Th is is in agreement with the observation on transmittance. The doped 14 C. O. Ayieko et al.: Performance of TiO2/In(OH)iSj/Pb(OH)xSy Composite ETA Solar Cell Fabricated from Nitrogen Doped TiO2Thin Film Window Layer and annealed film stands out clearly with the highest absorption in all wavelengths. Doping as well as the film thickness both contribute to the high absorption observed. 3.2.2. Effect of Nitrogen-doping on the band gap of TiO2 Coated on Fluorine Doped Tin Oxide (FTO) Glass Slides Figure 5 shows the estimated band gap of the films coated on Fluorine doped Tin Oxide (FTO) substrate and annealed at 400 ℃. The band gap decreases from 3.25 eV for the undoped film to 2.90 eV fo r Nitrogen-doped. Our results are in agreement with those obtained by Baoshun[9] who also observed a decrease in the band gap TiO2 prepared by sputtering from 3.2 eV to 2.7 eV upon Nitrogen doping. This reduction is as a result of nitrogen impurity states which introduce tail energy levels either in the conduction band or valence. 3.3. TiO2/In(OH)iSj/Pb(OH)xSy ETA Sol ar Cell Perfor mance 3.3.1. J-V characterizat ion The current density versus voltage graph for the fabricated solar cells is shown in Figure 6. The ETA solar cell fabricated fro m the doped TiO2 film shows superior performance. The current density increases by a factor of 1.65 wh ile the efficiency increases by a factor of 1.24. The imp roved performance of the ETA solar cell on doping can be attributed to the increased light absorption due to Nitrogen doping. 1x104 9x103 FTO Ts= 200oC (Undoped Eg= 3.25 eV) FTO Ts= 200oC (Doped Eg= 2.90 eV) 8x103 7x103 (αhν)2 (eV/m)2 6x103 5x103 4x103 3x103 2x103 1x103 0 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 Energy, hν, (eV) Figure 5. Graph showing the effect of doping TiO2 thin film coated on conducting flourine doped tin oxide (FTO) glass slides with Nitrogen (N2) and annealed at 400℃ International Journal of M aterials Engineering 2013, 3(2): 11-16 15 Figure 6. Current density –Voltage curves for Nitrogen-doped and undoped T iO2 window layer in T iO2/In(OH)iSj/Pb(OH)xSy ET A solar cell taken under illumination of 1000 W/m2 at 23℃ with a cell area= 0.1256 cm2 4. Conclusions Programme in the Physical Sciences (IPPS), Uppsala University (Sweden) for financia l support. Doped and undoped TiO2 thin films have been prepared by spray pyrolysis. X-ray diffraction studies shows that the as prepared films are transformed fro m amo rphous to crystalline after annealing at 400 o C. The films were confirmed to be porous through SEM images. Optical studies showed that Nitrogen doping increases the light absorption of the films by lowering the energy band gap fro m 3.25 eV to 2.90 eV. ETA solar cells fabricated fro m doped TiO2 films had better photovoltaic performance than those from undoped films. The current density increases by a factor of 1.65 wh ile the efficiency increases by a factor of 1.24. The imp roved performance of the ETA solar cell on doping can be attributed to the increased light absorption due to energy band gap lowering through Nitrogen doping. It has been established that spray pyrolysis and the doping technique (heat treatment) used in this paper are viable options for the preparation of Nitrogen doped TiO2 films. ACKNOWLEDGEMENTS The authors thank African Materials Science and Engineering Network (AMSEN) and International REFERENCES [1] Quashing, V. (2005). Understanding renewable systems, Earthscan, London, 1-130. [2] Hankins, M . (1995), Solar electric systems for Africa, M otif Creative Arts, Ltd. Nairobi,Kenya. [3] Nanu, M ., J. Schooman, A. Goosens (2004), Inorganic Nano-composite Semiconductors; a new type of 3D solar cell, Advanced Materials 16, (5) 453-455. [4] M usembi, R. (2009), Fabrication and characterization of In(OH)iSj modified highly structured TiO2/ Pb(OH)xSy/ PEDOT :PSS eta Solar cell, and study of its transport mechanisms, PhD Thesis, Department of Physics, University of Nairobi, 88-91. [5] Wafula, B., J. Simiyu, S. Waita, B. Aduda, J. M wabora (2007), Effect of nitration on pressed TiO2 Photoelectrodes for Dye-Sensitized solar cells, African Journal of Science and Technology (AJST), Science and Engineering Series 8, (2) 63-71. [6] Gartner, M ., P. Osiceanu, M . Anastasescu, T. Stoica, T.F. Stoica, C. Trapalis, T. Giannakopoulou, N. Todorova, A. Lagoyannis (2008), Investigation on the nitrogen doping of 16 C. O. Ayieko et al.: Performance of TiO2/In(OH)iSj/Pb(OH)xSy Composite ETA Solar Cell Fabricated from Nitrogen Doped TiO2Thin Film Window Layer multilayered, porous TiO2 thin films, Thin solid Films, 516, [8] Aksay, S. and B. Altiokka (2007), Effect of substrate 8184-8189. temperature on some of the optical parameters of CuInS2 [7] M ardare, D., M . Tasca, M . Delibas, G. Sugu (2000), films, Phys. Sta. Sol. (c) 4, (2) 585-588. Structural Properties and Optical Transmittance of TiO2 R.F [9] Baoshun, L., W. Liping, Z. Xiujian (2007). The structure and Sputtered Thin Films, Applied Surface Science.156 GL Asahi, photocatalytic studies of N-doped TiO2 films prepared by Technical University, Iasi R-6600 Romania, 203-205. radio frequency reactive magnetron sputtering. Solar Energy Materials and Solar Cells, 92, 1-10.

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