Synthesis and Characterization of Cadmium Sulfide (CdS) Quantum Dots (QDS) for Quantum Dot Sensitized Solar Cell Applications

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1 Nano Vision, Vol. 5(7-9), , July-September 2015 (An International Research Journal of Nano Science & Technology), ISSN (Print) ISSN (Online) Synthesis and Characterization of Cadmium Sulfide (CdS) Quantum Dots (QDS) for Quantum Dot Sensitized Solar Cell Applications S. Sakthivel and V. Baskaran Thin Film Physics and Nano Science Laboratory, PG and Research Department of Physics, Rajah Serfoji Govt., College (Autonomous), Thanjavur, Tamilnadu, INDIA. Presented in Second National Conference on Thin Film Science and Nano Technology (SECOND-NCTFSANT-2015) March 2-3, 2015, Rajah Serfoji Govt. College, Thanjavur, T.N. (India). ABSTRACT The main objective of the work is to present a systematic study on the growth, physical and chemical characterization of Cadmium sulfide (CdS) quantum dots were prepared at various time durations and deposited by sol-gel and hydrothermal method. The structural, morphological, and optical properties studied using powder X-ray diffraction (PXRD), scanning electron microscope (SEM) and UV-Vis NIR analysis respectively. From the observed PXRD pattern, the particle size calculated using Debye Scherer s method. The cell parameters well agreed with standard JCPDS data. Using CdS QDs, quantum dot sensitized solar cells (QDSSC) fabricated on FTO substrates. Sol-gel method synthesized TiO 2 nanoparticles, CdS quantum dots, I3 - /I - electrolytes and Pt as counter electrode were used for cell fabrications. Keywords: Quantum Dots, hydrothermal method, solar energy, Sol-gel. 1. INTRODUCTION Among the II-VI semiconductors, Cadmium Sulphide (CdS) is one of the most important wide gap semiconductors with a Eg 2.5 ev ( 500 nm) for the bulk hexagonal wurtzite phase and Eg 3.53 ev ( 350 nm) for the bulk cubic zinc blende phase. Tuning of the emission wavelength by the size and shape of the CdS Qdots has been extensively studied 1-5. Qdots have synthesized by a variety of methods including the sol-gel, solvothermal, ion beam, and ultrasonic techniques. The size, shape, and crystalline structure of the CdS Qdots are important factors that control their optical properties. Violet band edge and green-light emission due to trap states are observed from CdS Qdots under UV excitation 6. The PL

2 238 S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) intensity depended on the Cd 2+ /S 2 ratio, and samples with an excess of S 2 ions (Cd 2+ /S 2 ratio less than 1: 2) were unstable as compared to those with an excess of Cd 2+ ions. The PL intensity increased very strongly for Cd 2+ /S 2 ratios from 1 to 1.5, but saturated at higher ratios. The most efficient blue PL was found for Cd 2+ /S 2 >1.5. In recent years, many quantum dot (QD) materials investigated and applied as the sensitizer replacing organic dyes in the dye-sensitized solar cell (DSSC). Some of the advantages of QDs are tunable energy gaps, ability of multiple exciton generation, photo stability, low cost and high absorption coefficient, which known to reduce the dark current and increase the overall efficiency of solar cells 7. QD materials can tune their band gap energy level and this characteristics offers new opportunity to improve the light harvest ability. In conventional solar cells, the photons from solar radiation collected in the p-n junctions 8. Their efficiency is constrained by the Shockley-Queisser limit 9, due to blackbody radiation 10, spectral loss, excessive energy of the absorbed photons, and recombination loss 11. Up to December 2014, the highest efficiency achieved by a single-layer solar cell, made of GaAs, is 28.8% 12, 13. Multi-junction solar cells 14, 15 and light-concentrated solar cells 16 have been proposed to break the efficiency limit. The efficiency has pushed to 37.9% with a threelayered solar cell, composed of InGaP/GaAs/InGaAs, and 44.4%, under 302 suns of concentration 17. In the present investigation, CdS QDs prepared by chemical bath deposition (CBD) technique. The prepared QDs characterized using PXRD, SEM, and UV- Vis NIR analysis to identify the structure, morphology and band gap energy respectively. The QDSSC fabricated using laboratorial prepared CdS QDs and commercially available TiO 2 nanoparticle, electrolyte and Pt. 2. EXPERIMENTAL PROCEDURES For a typical synthesis, CdS quantum dots have deposited in a glass substrate by using chemical bath deposition (CBD) method. The cationic precursor 0.1 M of Cadmium nitrate (Cd(NO 3 ) 2.4H 2 O) was dissolved in 20 ml of ethanol (C 2 H 6 O) solution. Then, the anionic precursor 0.1 M of thiourea (CH 4 N 2 S) dissolved in the same solution. The glass substrate immersed in the prepared solution for 1 minute. Finally, the CdS film dried in hot air oven for few minutes. 3. RESULTS AND DISCUSSIONS 3.1 Powder X-ray diffraction (PXRD) analysis Fig.1. X-ray diffraction pattern of the CdS Q dot film

3 S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) 239 Figure 1, shows the X-ray diffraction patterns of the CdS Q dot film. The diffraction peaks observed at 24.65, , , , and are attributed to the (100), (002), (101), (102), (103), (420) and (104) planes respectively, of hexagonal phase structure with a = 4.14 Å and c = Å, as can be seen in comparison with the JCPDS card no The films are polycrystalline in nature and highly oriented along (110) plane. The average crystallite size found from the XRD peaks to be between 6.29 nm. The average grain size was 4.8 nm, calculated from the broadening of the (101) line by Scherrer s formula. 3.2 Scanning electron microscopy (SEM) analysis The SEM micrographs presented in Fig. 2 (a-b). The grain sizes are spherical in shape and the distribution was closely packed giving rise to little porosity and voids. The averaged crystallite sizes visualized by SEM are 4.8nm for CdS film. The value higher than the grain sizes determined from FWHM of powder diffraction patterns using the Debye- Scherrer equation. Thus, in the former case, agglomerates visualized. Figure 2: The SEM image of the CdS Q dot thin film Fig.3&4. Optical spectrum of CdS Q dot film

4 240 S. Sakthivel, et al., Nano Vision, Vol.5 (7-9), (2015) 3.3 Optical absorption analysis Figure. 3 shows that the optical absorption spectrum of CdS quantum dots deposited using chemical bath deposition method. In semiconductors, the relation connecting the absorption coefficient α, the incident photon energy hν and optical band gap E g takes the form αhν = k(hν - E g ) m (1) where k is a constant related to the effective masses associated with the bands and m = 1/2 for a direct-gap material, 2 for an indirect-gap material and 3/2 for a forbidden-direct energy gap. From the optical absorption spectrum, it is clearly seen that the absorption edge of CdS quantum dots is being located at around nm. Figure.4 shows that the optical band gap energy figure of the CdS QDs. The obtained optical band gap energy (E g ) of CdS QDs calculated using the equation (1). The estimated E g value in the range of 2.31 ev, and it is in very good agreement with the earlier literature reports. 4. CONCLUSIONS CdS quantum dots were successfully synthesized using chemical bath deposition (CBD) technique. The results of PXRD analysis particle size calculated from Debye-Scherer s formula. The particle size found to be 4.8 nm. SEM images confirm the spherical like morphology observed and the morphologies agglomerated. The optical band gap energy (E g ) of CdS QDs was 2.31 ev. The X-ray diffraction patterns of the CdS Q dot film. The diffraction peaks observed at 24.65, , , , and are attributed to the (100), (002), (101), (102), (103), (420) and (104) planes respectively, of hexagonal phase structure with a = 4.14 Å and c = Å, as can be seen in comparison with the JCPDS card no ACKNOWLEDGMENT The authors would like to express their thanks to the University Grants Commission (UGC), New Delhi, India for sanctioning the financial assistance [F. No /2012(SR) Dated: JULY 2012] to carry out the present research work. REFERENCES 1. Darugar, Qian, W. and El-Sayed, M.A. Observation of optical gain in solutions of CdS quantum dots at room temperature in the blue region, App. Phys. Lett. 88(26) (2006). 2. Cao, H.Q., et al. Growth and optical properties of wurtzite-type CdS nanocrystals, Inorg. Chem. 45(13): (2006). 3. Kim, D., et al. Strong enhancement of band-edge photoluminescence in CdS quantum dots prepared by a reverse-micelle method, J. App. Phys. 98(8) (2005).

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