Synthesis of CdSe quantum dots for quantum dot sensitized solar cell Neetu Singh, Vinod Kumar, R. M. Mehra, and Avinashi Kapoor Citation: AIP Conference Proceedings 1591, 595 (2014); doi: 10.1063/1.4872686 View online: http://dx.doi.org/10.1063/1.4872686 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1591?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Green synthesis of highly efficient CdSe quantum dots for quantum-dots-sensitized solar cells J. Appl. Phys. 115, 193104 (2014); 10.1063/1.4876118 Efficiency improvement of CdS and CdSe quantum dot-sensitized solar cells by TiO2 surface treatment J. Renewable Sustainable Energy 6, 023107 (2014); 10.1063/1.4870996 Assembly of CdSe nanoparticles on graphene for low-temperature fabrication of quantum dot sensitized solar cell Appl. Phys. Lett. 98, 093112 (2011); 10.1063/1.3558732 An oleic acid-capped CdSe quantum-dot sensitized solar cell Appl. Phys. Lett. 94, 153115 (2009); 10.1063/1.3117221 Effect of ZnS coating on the photovoltaic properties of CdSe quantum dot-sensitized solar cells J. Appl. Phys. 103, 084304 (2008); 10.1063/1.2903059
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Synthesis of CdSe Quantum dots for Quantum Dot Sensitized Solar Cell Neetu Singh1,*, Vinod Kumar2, R. M. Mehra3, and Avinashi Kapoor1 1
Department of Electronic Science, University of Delhi South Campus, New Delhi-110 021, India 2 Department of Physics, University of the Free State, Bloemfontein, ZA9300, South Africa 3School of Engineering and Technology, Sharda University, Greater Noida-201 306, U.P., India *E-mail:
[email protected]
Abstract. CdSe Quantum Dots (QDs) of size 0.85 nm were synthesized using chemical route. ZnO based Quantum Dot Sensitized Solar Cell (QDSSC) was fabricated using CdSe QDs as sensitizer. The Pre-synthesized QDs were found to be successfully adsorbed on front ZnO electrode and had potential to replace organic dyes in Dye Sensitized Solar Cells (DSSCs). The efficiency of QDSSC was obtained to be 2.06 % at AM 1.5. Keywords: CdSe, QDs and QDSSC. PACS: 61.05.cp, 68.65.Hb and 68.37.Lp
resulting solution was cooled to room temperature. The CdSe QDs were obtained. ZnO based QDSSC was fabricated using synthesized CdSe QDs as sensitizer. The ZnO electrode (front electode) was dipped in QD solution in toluene for proper adsorption of QDs. Iodide/tri-iodide - redox couple (I /I3 ) was used as electrolyte. Platinum (Pt) was used as counter electrode. The details fabrication technique has been reported elsewhere [3]. XRD measurements were carried out using Bruker AXS – D8 discover diffractometer having CuKα incident beam (λ = 1.54Å). TEM image was obtained using TECNAI G2 T30, u-TWIN TEM. I–V characteristic of the cell was measured using Keithley sourcemeter (2400) and a M-91190 Newport class-A solar simulator. The I–V measurement was made at 1 sun illumination (100 mW/cm2, AM 1.5).
INTRODUCTION As the drive to seek alternative energy sources significantly increases across the world, there is a growing interest in low cost, easily manufactured and efficient energy sources [1]. QDSSCs are a promising low cost alternative to existing PV technologies [1]. A QDSSC make use of Quantum Dots (QDs) as light absorbing material [1]. The absorption spectrum of QDs can be tailored by controlling their size [2]. So, different sized QDs of varying wavelengths having absorption maximum peaks in the entire visible spectrum can be used in QDSSCs to harness solar energy efficiently. This could boost the power conversion efficiency above the Schokley Queisser limit of 32% for conventional Si based solar cells [2]. The present paper reports the fabrication and characterization of ZnO based QDSSC using CdSe QDs as sensitizer.
RESULTS AND DISCUSSION
EXPERIMENTAL DETAILS
The XRD pattern of CdSe QDs is shown in Figure 1. The wurtzite crystalline structure of CdSe o corresponding to (002) plane at 25 is observed [4]. The broad diffraction peak in XRD pattern indicates about the extremely small size of the QDs. The single peak in (002) plane indicates the preferential growth of crystallites in this particular plane showing that QDs formed are single crystalline in nature. Debye Scherrer’s formula (below) was used to calculate the the QD size (D) [3].
For Se source, 30 mg of Selenium (Se), 0.4 ml trioctylphosphine (TOP) and 5 ml octadecene were added in a flask and stirred on a magnetic stirrer at ᵒ 80 C. In another flask 13 mg of CdO, 0.6 mL oleic acid and 10 ml octadecene were mixed using magnetic o stirrer. This solution was heated to 225 C. 1mL of Se solution was then added to this CdO solution. The
Solid State Physics AIP Conf. Proc. 1591, 595-596 (2014); doi: 10.1063/1.4872686 © 2014 AIP Publishing LLC 978-0-7354-1225-5/$30.00
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D=
Kλ β cos θ
and excitons are generated. These excitons (bound electron–hole pair) are separated by the ZnO/QD/electrolyte interface due to energetic and kinetic reasons. For efficient charge separation and collection the excited state of QDs should always lie above the conduction band edge of wide bandgap semiconductor (ZnO) and similarly, the valence band of QDs should lie below the redox potential of electrolyte; both presenting energetic and driving force for charge separation.
(1)
where, K is the particle shape factor which depends on the shape of the particles and its value is 0.94, β is FWHM of the selected diffraction peak and θ is the Bragg’s angle. The size of the QDs is obtained to be 0.85 nm. 002 2500
5
4
1500
Current (mA)
Intensity (a.u.)
2000
1000
500
0
3
2
1
20
30
40
50
60
2 θ (Deg)
0 0.0
FIGURE 1. XRD spectrum of CdSe QDs
0.2
0.4
0.6
0.8
Voltage (V)
FIGURE 3. I-V characteristic of QDSSC
TEM image of CdSe QDs is shown in Figure 2. The uniformly distributed spherical shaped QDs can be seen in the image. Size of the CdSe QDs as seen from TEM is in correspondence with the XRD results.
CONCLUSIONS In conclusion, CdSe QDs of size 0.85 nm were synthesized using chemical route. ZnO based QDSSC was then fabricated using synthesized CdSe QDs as sensitizer. The power conversion efficiency of was obtained to be 2.06% at AM 1.5.
ACKNOWLEDGEMENTS This work is done under DST-JSPS sponsored project DST/INT/JSPS/PROJ/10. The authors are also thankful to University Science Instrumentation Centre (USIC) for providing XRD and TEM facilities.
REFERENCES FIGURE 2. TEM image of CdSe QDs
1. O. Niitsoo, S. K. Sarkar, C. Pejoux, S. Ruhle, D. Cahen, G. Hodes, J Photochem Photobiol A 181, 306-313 (2006). 2. W. Schokley, H. J. Queisser, J. Appl. Phys. 32, 510-519 (1961). 3. N. Singh, R. M. Mehra, A. Kapoor, T. Soga, J. of Renew. and Sustain. Energy 4, 013110-013119 (2012). 4. S. Neeleshwar, C. L. Chen, C. B. Tsai, Y. Y. Chen, Phys Rev B 71, 201307-201310 (2005).
The I-V characteristic of QDSSC was taken under 1 sun illumination and is shown in Figure 3. The values of short circuit current (Isc), open circuit voltage (Voc), maximum power (Pm), fill factor (FF) and efficiency (η) are 4.99 mA, 0.71 V, 2.06 mW, 0.6 and 2.06 % respectively. In QDSSC, when sunlight falls on front electrode, then photons are absorbed by the QDs
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