- Email: [email protected]
The influence of ZnS buffer layer on the size dependent efficiency of CdTe quantum dot sensitized solar cell
Accepted Manuscript The influence of ZnS buffer layer on the size dependent efficiency of CdTe quantum dot sensitized solar cell
Nideep T. K, Ramya M, M. Kailasnath PII:
S0749-6036(19)30545-2
DOI:
10.1016/j.spmi.2019.04.034
Reference:
YSPMI 6099
To appear in:
Superlattices and Microstructures
Received Date:
17 March 2019
Accepted Date:
23 April 2019
Please cite this article as: Nideep T. K, Ramya M, M. Kailasnath, The influence of ZnS buffer layer on the size dependent efficiency of CdTe quantum dot sensitized solar cell, Superlattices and Microstructures (2019), doi: 10.1016/j.spmi.2019.04.034
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
The influence of ZnS buffer layer on the size dependent efficiency of CdTe quantum dot sensitized solar cell Nideep T K, Ramya M, M. Kailasnath International School of Photonics, Cochin University of Science and Technology, Kochi-22 E mail: [email protected] Abstract We report the investigations on the quantum dot size dependence of efficiency of a CdTe sensitized solar cell. With an additional layer of ZnS over the QD layer, the efficiency of solar cell was found to vary according to the size of the synthesized CdTe QDs. The presence of trap states due to the surface imperfections of the QDs as a result of the colloidal growth mechanism was also found to affect the overall efficiency of the solar cell. By coating a buffer layer of ZnS over the CdTe QD layer with the help of SILAR method, the efficiency enhanced up to 2.16% by decreasing the charge recombination rate between QDs and metal oxide layer. Keywords CdTe quantum dot; quantum dot sensitized solar cells; ZnS buffer layer; SILAR cycle.
Introduction Semiconductor quantum dots (QD) open up a new area in the field of solar cell technology known as quantum dot sensitized solar cells (QDSSC) where in the unique opto-electronic properties of semiconductor quantum dots make it a very good light absorbing material to construct third generation photovoltaic devices[1β5]. These properties include tunable band gap, slow electron cooling, high absorption and excitation coefficient, large dipole moment, low cost solution processability, auger recombination, possibility of multiple exciton generation (MEG), hot electron extraction possibilities expecting to break the Shockley-Queisser limit etc. The third generation solar cell has high impact on solving the energy crisis due to the low cost synthesis, high flexibility and transparency[6]. The semiconductor nanocrystals which are small enough to confine the motion of electrons in all the three dimensions are referred to as quantum dots[7]. In the quantum dot nanostructures, the radius of the particles is comparable with exciton bohr radius of the material. They usually consists of 100 to 10,000 number of atoms per particle[8]. So the quantum dots can be considered as artificial atoms and the addition or removal of a few numbers of atoms or molecules of a quantum dot can sometimes lead to change in the photo physical properties of the material[9].
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The stability and absorption coefficient of quantum dots is higher when compared even with the high performance dye molecules[10]. The phenomenon of producing more than one exciton by a single photon is called multiple exciton generation effect, which results in the increase of theoretical efficiency of QDSSCs more than any other solar cells[11]. Klimov et. al. claimed to have generated 7 excitons from a single photon which could yield a photon-to-exciton conversion efficiency up to 700% [12]. Even though QDs can possess this much charge carrier generation, the possibility of electron hole recombination is very much high in these nanostructures due to the presence of large number of trap states[13]. This reduces the practical photo conversion efficiency of QDSSCs to very low value in comparison with dye sensitized solar cells (DSSC). Efficiency of the QDSSCs can increase by using certain techniques such as passivation, successive ionic layer adsorption and reaction (SILAR) method etc[14]. It was Nicolau in 1980 who first introduced SILAR method[15]. The II-VI, IV-VI, III-V group semiconducting compounds are mainly used as the photosensitive layer for QDSSCs, of them CdTe is the most promising material due to ideal band gap of 1.56eV and conduction band edge of -3.70eV. So the injection of electrons from the QD layer to the metal oxide layer is easier compared to other quantum dots. But the efficiency of CdTe QDSSCs is limited due to the types of Te precursors used and the highly oxidizing nature of the CdTe [3]. Light photosensitive layer is the most important part of any QDSSC or DSSC. The energy conversion of the absorbed photons in the photosensitive layer generate charge carrier which results in the creation of external electricity. Higher the absorption property and photo conversion efficiency of the light absorbing material, higher will be the electricity generated. So the absorbing layer should have a high absorption coefficient and the ability to generate charge carriers up on incidence with photons. In the present work, we have studied the performance of QDSSCs fabricated by making use of CdTe colloidal quantum dots of varying sizes. The size dependent variation of the power conversion efficiency (PCE) of the solar cells has been investigated. The low efficiency of the QDSSC due to the charge recombination was overcome through an additional coating of ZnS by the method of SILAR cycle. To the best of our knowledge, this is the first study on the efficiency of CdTe QDSSCs incorporating ZnS buffer layer. In addition to the size of the QDs, the presence of trap states also found to have an important role in controlling the efficiency of the solar cell.
Experimentation Sample synthesis CdTe colloidal quantum dots were synthesized in water using mercapto succinic acid (MSA) as the capping agent. The detailed description of the method of preparation is given in our previous work[16]. Five samples of CdTe quantum dots with different sizes were synthesized by simple
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chemical method in which the initially prepared QD samples were subjected to heat treatment for different time intervals such as 30min, 60min 90min and 120min. The successive samples were named as C30, C60,C90 and C120 respectively. The initially synthesized sample without any heat treatment was named as C0. The synthesized QD samples were used for preparing the photosensitive layer in the fabrication of QDSSC. The absorption and emission spectra of the samples were recorded using UV-Vis absorption spectrophotometer JASCO-V570 and Cary Eclipse Spectrophotometer- VARIAN, respectively. The morphology of the samples were analyzed using HRTEM analysis model JEM 2100. Fabrication of solar cells Other than QDs, the main components used for the fabrication of solar cell in laboratory were Fluorine doped Tin Oxide (FTO) glass plate, TiO2 as the metal conducting oxide layer, Iodolyte solution and platisol. All of these components were purchased from SOLARONIX, Switzerland. Triton X-100 was purchased from Sigma Aldrich. Efficiency of QDSSCs were evaluated with and without the layer of ZnS. The processes involved in the fabrication of the solar cell are described below. (i)
(ii)
(iii)
(iv)
Deposition of TiO2 layer on FTO TiO2 powder was made as a paste by mixing it with Triton X-100 under ultrasonication for 1 hour. It was then coated over the FTO glass slide over the active layer using Doctorβs blade technique. The film was then dried inside a furnace at 4500C for 3 hours. Deposition of QDs over TiO2 TiO2 films were sensitized with pre-prepared mercapto succinic acid (MSA)-capped CdTe quantum dots with different sizes through self-assembling approach by dipping TiO2 film in to the aqueous solution of QDs. Deposition of ZnS layer ZnS layer was deposited over the CdTe coated photo anode by the most commonly used SILAR cycle. CdTe sensitized TiO2 was immersed in to 0.1M ZnCl2 and 0.1 M Na2S aqueous solution at a rate of 1min/dip alternatively for each cycle.
Figure 1: Schematic diagram of deposition of ZnS layer over CdTe QD coated photo anode by SILAR cycle Preparation of counter electrode
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(v)
The platinum based counter electrode was prepared by spreading platisol paste followed by a thermal treatment at 4500C for 30minutes. Finally the photo anode and the counter electrode were combined together like a sandwich type cell. The Iodolyte solution, which is the electrolyte used was injected in to the active layer through a small hole provided in the counter electrode.
Ten parallel cells sensitized with CdTe QDs of different sizes with and without ZnS layer were fabricated using the above technique. Six cells with C60 sample as sensitizer as well as with different number of layers of ZnS were also prepared. Current-Voltage characteristics of the fabricated solar cells were recorded by using Keithely 2400 digital source meter by illuminating the cells with Newport Solar Simulator (AM 1.5, 100mW/cm2). Working principle of QDSSC Figure 2 depicts the schematic representation of the working of a typical QDSSC where the incident light absorbed by the QDs results in the creation of excitons. The QD injects the excited electron from its conduction band to the conduction band of TiO2. The electron percolates through the porous TiO2 and finally reaches the transparent conducting glass. This electron enter the counter electrode by travelling through the external circuit. The difference in the quasi-Fermi level of electron in the photoanode and the redox potential of electrolyte results in an output voltage. The QD with a hole in the valence band is reduced by the negative ions in the electrolyte and in turn oxidizes and diffuses to the back electrode.
Figure 2: Schematic diagram showing the working of QDSSC The PCE of the solar cell can be found out using the equation as follows ππππΌπ ππΉπΉ πππ
Γ 100%
Where πππis the open circuit voltage, πΌπ πis the closed circuit current and πΉπΉ is the fill factor which determines the quality of a solar cell and is given by
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πΉπΉ =
πππΌπ ππππΌπ π
Where ππ and πΌπ are the corresponding optimal voltage and current[17].
Results and discussion The UV-Visible absorption spectra of the CdTe QD samples shown in figure 3(a) indicates a peak shift according to the time of heat treatment given to each sample. This is an indication of the quantum confinement effect in the semiconductor due to the increase in size of the nanoparticles[5]. The emission spectra of the CdTe QD samples are shown in figure 3(b) where the emission intensity and peak wavelength vary for each quantum dot samples. The normalized fluorescence spectra in the inset of figure 3(b) clearly indicates the red shift in the emission peak. This is also another confirmation of the confinement effect of the QDs. With the increase in size, the energy gap between the conduction and valance band decreases. The TEM image of the particles with spherical shape depicted in figure 3 confirms the formation of zero dimensional quantum dot structures.
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Figure 3: (a) Absorption and (b) emission spectra of CdTe QD samples showing red shift in the peak intensity, (c) TEM image of the QD samples. Peng et. al proposed a theoretical model for calculating the approximate particle size of the quantum dots based on the relation between excitonic absorption and particle size of quantum dots[18,19]. Using the equation given below, one can find the particle size of the quantum dots π· = (9.8127 β 10 β7)π3 β (1.7147 β 10 β3)π2 + 1.0064 β π β 194.84 Here π· is the diameter of the quantum dot and π is the excitonic absorption wavelength. The
calculated particle size of the quantum dots were found to be ranging from 1.94nm to 2.98nm. In order to study the solar cell efficiency, three sets of efficiency measurements were performed. The first one was with solar cells sensitized with CdTe QDs of different sizes. The second measurement involved samples photosensitized with previously optimized QDs for maximum efficiency with additional ZnS layers to optimize the number of cycles of SILAR. The third one was the measurement to find out the efficiency of each of the quantum dot samples with the optimized number of SILAR cycle from the second measurement. Figure 4 shows the photovoltaic output of QDSSCs sensitized with the five samples of CdTe quantum dots. The voltage-current plot clearly shows that the photovoltaic parameters are varying with respect to the size of the QDs.
Figure 4: Current voltage characteristics of QDSSC solar cell with different QDs The photovoltaic parameters of the QDSSCs with bare CdTe as sensitizer are summarized in table 1. The open circuit voltage (Voc), Short circuit current (Isc) and the power conversion efficiency (PCE) are found to be varying with respect to the size of the quantum dots. The efficiency was found to increase with increase in size of the quantum dots up to an optimum value, above which it decreases. This can be explained based on the size dependent variation of the population of atoms and the concentration of trap states present in the QDs. With the increase
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in particle size, the number of atoms per particle increases. This leads to the increase in the possibility of formation of the excitons which increases the efficiency of the solar cell. The second factor is the trap state, which is mainly controlled by the dynamic growth process and Ostwald ripening mechanism during the synthesis of the quantum dots[19β21]. The formation and decay of the trap states can be explained based on the emission intensity of different QDs shown in Figure 2 (b). During the synthesis of the QDs, the presence of defect or trap states will be more at the initial state of the QD formation. This can be verified from the emission spectrum recorded for the initial sample (C0) where there is higher defect state emission due to the presence of large number of defects. Here the nanoparticles possess low emission intensity with higher non radiative decay. But as the growing progress, the number of defect states decreases due to dynamic growth process and Ostwald ripening mechanism. The number of defects states become minimum at an optimum point of the growth process, where the growth and dissolution are in equilibrium. The particles at this optimum growth point with minimum number of defects will possess higher emission intensity with very low non radiative decay. The further growth process of the nanoparticle lead to particles having rougher surface with increased number of defects resulting in the increased non-radiative decay, and less emission intensity. The higher numbers of defects decreases the injection of charge carriers to the conduction band of TiO2 from the conduction band of QDs. But the QDSSC with optimum QD size possess higher efficiency due to the increase in charge transportation by the decrease in defect states. From the above sets of measurements C60 was selected as the best sample for the efficiency measurement of QDSSC fabricated with different layers of ZnS coating. Table 1: Size dependent efficiency of QDSSC with different sized CdTe QD samples QDs C0 C30 C60 C90 C120
Size (nm) 1.94 2.25 2.55 2.79 2.98
Jsc (mA/cm2) 1.25 2.07 3.24 3.50 4.381
Voc (V) 0.422 0.403 0.371 0.341 0.190
FF (%) 0.303 0.323 0.439 0.368 0.360
PCE (%) 0.16 0.27 0.53 0.44 0.30
Figure 4 (b) shows the variation of open circuit voltage (Voc) and short circuit current (Isc) with respect to the change in particle size of the QDs. It is seen that, with increase in QD size, the Isc is increasing. This can be explained based on the relation between particle size and the population of atoms. With increase in particle size, the number of atoms increases. This leads to the absorption of more number of photons which in turn create more number of charge carriers enhancing the increase of current flow as well as Isc. On the other hand, Vos is found to decrease with increase in particle size which can be explained based on the band gap shift arising from the quantum confinement effect. As the particle size increase, the conduction band shifts downwards
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due to the weaker quantum confinement effect. This decreases the driving force for injection of charge carriers in to the TiO2 layer leading to a decrease in Voc[4,22].
Figure 5: (a) Current voltage characteristics of QDSSC solar cell with sample C60 as the absorbing layer, with different SILAR cycles of ZnS, (b) variation in the PCE for different SILAR cycles. Figure 5(a) shows the solar cell characteristics where the quantum dot sample C60 was used as the sensitizer along with different number of ZnS coatings by SILAR cycle. Initially with the increase in ZnS layer thickness, the efficiency of solar cell increases. From the measurements the sample corresponding to 4 SILAR cycles was found to be showing the highest efficiency of 2.16% beyond which it decreases. So the sample with 4 cycles of ZnS coating was chosen for further studies of efficiency enhancement in QDSSCs. Table 2: Optimization of SILAR cycle for the sample C60 ZnS Cycles 0 1 2 3 4 5 6
Jsc (mA/cm2) 3.24 4.09 4.98 6.21 6.92 5.99 5.48
Voc(V)
FF (%)
PCE (%)
0.371 0.485 0.494 0.498 0.502 0.479 0.422
0.439 0.449 0.476 0.527 0.622 0.606 0.585
0.53 0.89 1.17 1.63 2.16 1.74 1.35
The photovoltaic parameters of the QDSSCs with different SILAR cycles of ZnS coatings are given in Table2. The efficiency of the QDSSC is very low, 0.53% when the QD (C60) alone was incorporated in the solar cell. But with coating of ZnS layer, the efficiency was found to increase with each coating cycle up to four cycles of SILAR.
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A schematic representation on the working of QDSSC with ZnS layer is shown in figure 6. Here the ZnS coating could reduce the interfacial recombination at TiO2/QD and QD/electrolyte interfaces which can be explained based on two possibilities. The decrease of recombination at QD/electrolyte interface is one of them and reduction of back-electron from the TiO2 to the QD being the second. The reduction in Jsc, Voc and PCE values for more than 4 ZnS SILAR cycle is attributed to the difficulty in the electron injection from electrolyte to the QD and TiO2 due to the presence of a thick passivation layer of ZnS.
Figure 6: Schematic representation on the working of QDSSC with ZnS coating Figure 7(a) shows the solar cell I-V characteristics of QDSSC sensitized with QDs of different sizes along with 4 SILAR cycles. The QD size dependent variation of efficiency is shown in Table 3. Here all the QD samples were found to exhibit very high value of PCE compared to the corresponding values of QDSSC made up of bare CdTe QD samples.
Figure 7: (a) Current voltage characteristics of QDSSC solar cell with different QDs with 4 ZnS SILAR cycle, (b) Particle size dependent PCE of QDSSC (i) without SILAR, (ii) with SILAR. The photovoltaic parameters of the QDSSCs which were sensitized with different CdTe QD samples and coated with ZnS layer are tabulated in table 3. For the sample C60 the value of PCE was found to increase from 0.53% to 2.16 % after the ZnS coating.
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Table 3: Size dependent efficiency of QDSSC sensitized with various CdTe QD samples with C4 SILAR cycle QDs C0 C30 C60 C90 C120
Size (nm) 1.94 2.25 2.55 2.79 2.98
Jsc (mA/cm2) 2.67 4.91 6.92 7.54 7.81
Voc (V) 0.546 0.523 0.502 0.481 0.426
FF (%) 0.487 0.500 0.622 0.516 0.490
PCE (%) 0.71 1.29 2.16 1.88 1.63
Figure 7(b) shows the difference in power conversion efficiency of CdTe QD sensitized solar cells with and without ZnS coating. The reason for the variation of efficiencies of both sets of solar cells can be explained as follows. The main purpose of ZnS layer is the passivation of the surface states of CdTe QDs which helps in decreasing the surface trapping of carriers in the QDs. Thus the photo generated charge carriers can be effectively transferred to the conduction band of TiO2 which results in the increase of Jsc. The increase in Voc can be due to the quasi-Fermi level in the TiO2 which increases with increase in density of electrons injected from the QDs to conduction band of TiO2. Since the band gap of ZnS (3.8eV) is higher than that of CdTe QD, the additional layer of ZnS can also act as a potential barrier in between CdTe electrolyte-interface. Hence the leakage of electrons from the QD in to the electrolyte decreases. The increase of Jsc and Voc thus results in the increase in FF upon coating with ZnS. Hence the efficiency of a CdTe based QDSSC can be enhanced upon coating with ZnS layer using 4 cycles of SILAR. Conclusion The efficiency of CdTe QDSSC sensitized with quantum dots having different sizes have been investigated. The efficiency was found to be depending upon the QD size as well as the concentration of trap states present in the quantum dots. ZnS coating by SILAR cycle is found to be an effective method in enhancing the efficiency of CdTe QDSSC. Through proper selection of the QD size and the number of cycles for ZnS buffer layer, the overall efficiency of the QDSSC can be considerably improved. Acknowledgments The authors acknowledge Department of Chemistry, CUSAT for the solar cell characterization and E-Grantz, Kerala Govt. for the financial support. References 1.
Q. Wu, H. Zhao, F. Huang, J. Hou, H. Cao, Z. Liu, S. Peng, and G. Cao, "Impacts of Reduced Graphene Oxide in CdS / CdSe Quantum Dots Co- sensitized Solar Cells," J. Phys. Chem. C 121, 18430β18438 (2017).
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2.
J. Yang, J. Wang, K. Zhao, T. Izuishi, Y. Li, Q. Shen, and X. Zhong, "CdSeTe / CdS Type-I Core / Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9 %," J. Phys. Chem. C 119, 28800β28808 (2015).
3.
C.-Y. S. Xiao-Yun Yu, Bing-Xin Lei, Dai-Bin Kuang, "Highly efficient CdTe / CdS quantum dot sensitized solar cells fabricated by a one-step linker assisted chemical bath deposition β ," chem. sci. 2, 1396β1400 (2011).
4.
X. Z. Junwei Yanga, "CdTe based quantum dot sensitized solar cells with efficiency exceeding 7% directly from quantum dots prepared in aqueous media," Mater. Chem. A 4, 16553β16561 (2016).
5.
T. K. Das and P. Ilaiyaraja, "Whispering Gallery Mode Enabled Efficiency Enhancementβ―: Defect and Size Controlled CdSe Quantum Dot Sensitized Whisperonic Solar Cells," Sci. Rep. 8(April), 1β12 (2018).
6.
A. A. F. Husain, W. Z. W. Hasan, and M. N. Hamidon, "A review of transparent solar photovoltaic technologies," Renew. Sustain. Energy Rev. 94(June), 779β791 (2018).
7.
U. Bockelmann, "Electron relaxation in quantum dots by means of Auger processes," Phys. Rev. B 46(23), 574β577 (1992).
8.
A. P. Alivisatos, "semiconductor clusters, nanocrystals and quantum dots," Science (80-. ). 271, 933β937 (1996).
9.
R C Ashoori, "electrons in artificial atoms," Nature 379, 413β419 (1996).
10.
K. E. Roelofs, S. M. Herron, and S. F. Bent, "Increased Quantum Dot Loading by pH Control Reduces Interfacial Recombination in Quantum-Dot- Sensitized Solar Cells," ACS Nano 9(8), 8321β8334 (2015).
11.
V. I. Klimov, "Spectral and Dynamical Properties of Multiexcitons in Semiconductor Nanocrystals," Annu. Rev. Phys. Chem. 58, 636β674 (2007).
12.
R. D. Schaller, M. Sykora, J. M. Pietryga, and V. I. Klimov, "Seven Excitons at a Cost of Oneβ―: Redefining the Limits for Conversion Efficiency of Photons into Charge Carriers," nano lett. 6(3), 424β429 (2006).
13.
Q. Dot, S. Cells, X. Lan, O. Voznyy, F. P. Garc, M. Liu, J. Xu, A. H. Proppe, G. Walters, F. Fan, H. Tan, M. Liu, Z. Yang, S. Hoogland, and E. H. Sargent, "10.6% Certified Colloidal Quantum Dot Solar Cells via Solvent- Polarity-Engineered Halide Passivation," nano lett. 16, 4630β4634 (2016).
14.
H. K. Jun, M. A. Careem, and A. K. Arof, "Quantum dot-sensitized solar cells β perspective and recent developmentsβ―: A review of Cd chalcogenide quantum dots as sensitizers," Renew. Sustain. Energy Rev. 22, 148β167 (2013).
15.
G. Cedex, "Solution deposition of thin solid compound films by a successive ionic-layer adsorption and reaction process," Appl. Surf. Sci. 23, 1061β1074 (1985).
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16.
T. K. Nideep, M. Ramya, V. P. N. Nampoori, and M. Kailasnath, "Optical limiting and optical properties of water soluble CdTe quantum dots prepared through a colloidal chemical route," Opt. - Int. J. Light Electron Opt. (September), 0β1 (2018).
17.
A. Badawi, N. Al-hosiny, and S. Abdallah, "The photovoltaic performance of CdS quantum dots sensitized solar cell using graphene / TiO 2 working electrode," SUPERLATTICES Microstruct. 81, 88β96 (2015).
18.
W. W. Yu, L. Qu, W. Guo, and X. Peng, "Experimental Determination of the Extinction Coefficient of CdTe , CdSe , and CdS Nanocrystals," chem. mater. 125(17), 2854β2860 (2003).
19.
C. Wang, Q. Ma, and X. Su, "Synthesis of CdTe Nanocrystals with Mercaptosuccinic Acid as Stabilizer," J. Nanosci. Nanotechnol. 8(9), 4408β4414 (2008).
20.
M. Gogoi, P. Deb, and A. Kostka, "Differential tunability effect on the optical properties of doped and undoped quantum dots," Phys. Status Solidi Appl. Mater. Sci. 209(8), 1543β 1551 (2012).
21.
H. L. Cao, X. F. Qian, Q. Gong, W. M. Du, X. D. Ma, and Z. K. Zhu, "Shape- and sizecontrolled synthesis of nanometre ZnO from a simple solution route at room temperature," Nanotechnology 17(15), 3632β3636 (2006).
22.
X. H. Zhong, "ChemComm Efficient CdSe quantum dot-sensitized solar cells prepared by a postsynthesis assembly approach," chem. commun. 48, 11235β11237 (2012).
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Highlights
ο· ο· ο· ο·
Investigated the size dependence of CdTe quantum dots on the efficiency of solar cells. Studied the influence of ZnS buffer layer on the efficiency of CdTe sensitized quantum dot solar cells. Optimized the number of ZnS SILAR cycles and estimated the size dependent efficiency of CdTe based quantum dot solar cells. Investigated the role of trap states on the efficiency of quantum dot sensitized solar cells.
Nideep T. K, Ramya M, M. Kailasnath PII:
S0749-6036(19)30545-2
DOI:
10.1016/j.spmi.2019.04.034
Reference:
YSPMI 6099
To appear in:
Superlattices and Microstructures
Received Date:
17 March 2019
Accepted Date:
23 April 2019
Please cite this article as: Nideep T. K, Ramya M, M. Kailasnath, The influence of ZnS buffer layer on the size dependent efficiency of CdTe quantum dot sensitized solar cell, Superlattices and Microstructures (2019), doi: 10.1016/j.spmi.2019.04.034
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
The influence of ZnS buffer layer on the size dependent efficiency of CdTe quantum dot sensitized solar cell Nideep T K, Ramya M, M. Kailasnath International School of Photonics, Cochin University of Science and Technology, Kochi-22 E mail: [email protected] Abstract We report the investigations on the quantum dot size dependence of efficiency of a CdTe sensitized solar cell. With an additional layer of ZnS over the QD layer, the efficiency of solar cell was found to vary according to the size of the synthesized CdTe QDs. The presence of trap states due to the surface imperfections of the QDs as a result of the colloidal growth mechanism was also found to affect the overall efficiency of the solar cell. By coating a buffer layer of ZnS over the CdTe QD layer with the help of SILAR method, the efficiency enhanced up to 2.16% by decreasing the charge recombination rate between QDs and metal oxide layer. Keywords CdTe quantum dot; quantum dot sensitized solar cells; ZnS buffer layer; SILAR cycle.
Introduction Semiconductor quantum dots (QD) open up a new area in the field of solar cell technology known as quantum dot sensitized solar cells (QDSSC) where in the unique opto-electronic properties of semiconductor quantum dots make it a very good light absorbing material to construct third generation photovoltaic devices[1β5]. These properties include tunable band gap, slow electron cooling, high absorption and excitation coefficient, large dipole moment, low cost solution processability, auger recombination, possibility of multiple exciton generation (MEG), hot electron extraction possibilities expecting to break the Shockley-Queisser limit etc. The third generation solar cell has high impact on solving the energy crisis due to the low cost synthesis, high flexibility and transparency[6]. The semiconductor nanocrystals which are small enough to confine the motion of electrons in all the three dimensions are referred to as quantum dots[7]. In the quantum dot nanostructures, the radius of the particles is comparable with exciton bohr radius of the material. They usually consists of 100 to 10,000 number of atoms per particle[8]. So the quantum dots can be considered as artificial atoms and the addition or removal of a few numbers of atoms or molecules of a quantum dot can sometimes lead to change in the photo physical properties of the material[9].
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The stability and absorption coefficient of quantum dots is higher when compared even with the high performance dye molecules[10]. The phenomenon of producing more than one exciton by a single photon is called multiple exciton generation effect, which results in the increase of theoretical efficiency of QDSSCs more than any other solar cells[11]. Klimov et. al. claimed to have generated 7 excitons from a single photon which could yield a photon-to-exciton conversion efficiency up to 700% [12]. Even though QDs can possess this much charge carrier generation, the possibility of electron hole recombination is very much high in these nanostructures due to the presence of large number of trap states[13]. This reduces the practical photo conversion efficiency of QDSSCs to very low value in comparison with dye sensitized solar cells (DSSC). Efficiency of the QDSSCs can increase by using certain techniques such as passivation, successive ionic layer adsorption and reaction (SILAR) method etc[14]. It was Nicolau in 1980 who first introduced SILAR method[15]. The II-VI, IV-VI, III-V group semiconducting compounds are mainly used as the photosensitive layer for QDSSCs, of them CdTe is the most promising material due to ideal band gap of 1.56eV and conduction band edge of -3.70eV. So the injection of electrons from the QD layer to the metal oxide layer is easier compared to other quantum dots. But the efficiency of CdTe QDSSCs is limited due to the types of Te precursors used and the highly oxidizing nature of the CdTe [3]. Light photosensitive layer is the most important part of any QDSSC or DSSC. The energy conversion of the absorbed photons in the photosensitive layer generate charge carrier which results in the creation of external electricity. Higher the absorption property and photo conversion efficiency of the light absorbing material, higher will be the electricity generated. So the absorbing layer should have a high absorption coefficient and the ability to generate charge carriers up on incidence with photons. In the present work, we have studied the performance of QDSSCs fabricated by making use of CdTe colloidal quantum dots of varying sizes. The size dependent variation of the power conversion efficiency (PCE) of the solar cells has been investigated. The low efficiency of the QDSSC due to the charge recombination was overcome through an additional coating of ZnS by the method of SILAR cycle. To the best of our knowledge, this is the first study on the efficiency of CdTe QDSSCs incorporating ZnS buffer layer. In addition to the size of the QDs, the presence of trap states also found to have an important role in controlling the efficiency of the solar cell.
Experimentation Sample synthesis CdTe colloidal quantum dots were synthesized in water using mercapto succinic acid (MSA) as the capping agent. The detailed description of the method of preparation is given in our previous work[16]. Five samples of CdTe quantum dots with different sizes were synthesized by simple
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chemical method in which the initially prepared QD samples were subjected to heat treatment for different time intervals such as 30min, 60min 90min and 120min. The successive samples were named as C30, C60,C90 and C120 respectively. The initially synthesized sample without any heat treatment was named as C0. The synthesized QD samples were used for preparing the photosensitive layer in the fabrication of QDSSC. The absorption and emission spectra of the samples were recorded using UV-Vis absorption spectrophotometer JASCO-V570 and Cary Eclipse Spectrophotometer- VARIAN, respectively. The morphology of the samples were analyzed using HRTEM analysis model JEM 2100. Fabrication of solar cells Other than QDs, the main components used for the fabrication of solar cell in laboratory were Fluorine doped Tin Oxide (FTO) glass plate, TiO2 as the metal conducting oxide layer, Iodolyte solution and platisol. All of these components were purchased from SOLARONIX, Switzerland. Triton X-100 was purchased from Sigma Aldrich. Efficiency of QDSSCs were evaluated with and without the layer of ZnS. The processes involved in the fabrication of the solar cell are described below. (i)
(ii)
(iii)
(iv)
Deposition of TiO2 layer on FTO TiO2 powder was made as a paste by mixing it with Triton X-100 under ultrasonication for 1 hour. It was then coated over the FTO glass slide over the active layer using Doctorβs blade technique. The film was then dried inside a furnace at 4500C for 3 hours. Deposition of QDs over TiO2 TiO2 films were sensitized with pre-prepared mercapto succinic acid (MSA)-capped CdTe quantum dots with different sizes through self-assembling approach by dipping TiO2 film in to the aqueous solution of QDs. Deposition of ZnS layer ZnS layer was deposited over the CdTe coated photo anode by the most commonly used SILAR cycle. CdTe sensitized TiO2 was immersed in to 0.1M ZnCl2 and 0.1 M Na2S aqueous solution at a rate of 1min/dip alternatively for each cycle.
Figure 1: Schematic diagram of deposition of ZnS layer over CdTe QD coated photo anode by SILAR cycle Preparation of counter electrode
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(v)
The platinum based counter electrode was prepared by spreading platisol paste followed by a thermal treatment at 4500C for 30minutes. Finally the photo anode and the counter electrode were combined together like a sandwich type cell. The Iodolyte solution, which is the electrolyte used was injected in to the active layer through a small hole provided in the counter electrode.
Ten parallel cells sensitized with CdTe QDs of different sizes with and without ZnS layer were fabricated using the above technique. Six cells with C60 sample as sensitizer as well as with different number of layers of ZnS were also prepared. Current-Voltage characteristics of the fabricated solar cells were recorded by using Keithely 2400 digital source meter by illuminating the cells with Newport Solar Simulator (AM 1.5, 100mW/cm2). Working principle of QDSSC Figure 2 depicts the schematic representation of the working of a typical QDSSC where the incident light absorbed by the QDs results in the creation of excitons. The QD injects the excited electron from its conduction band to the conduction band of TiO2. The electron percolates through the porous TiO2 and finally reaches the transparent conducting glass. This electron enter the counter electrode by travelling through the external circuit. The difference in the quasi-Fermi level of electron in the photoanode and the redox potential of electrolyte results in an output voltage. The QD with a hole in the valence band is reduced by the negative ions in the electrolyte and in turn oxidizes and diffuses to the back electrode.
Figure 2: Schematic diagram showing the working of QDSSC The PCE of the solar cell can be found out using the equation as follows ππππΌπ ππΉπΉ πππ
Γ 100%
Where πππis the open circuit voltage, πΌπ πis the closed circuit current and πΉπΉ is the fill factor which determines the quality of a solar cell and is given by
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πΉπΉ =
πππΌπ ππππΌπ π
Where ππ and πΌπ are the corresponding optimal voltage and current[17].
Results and discussion The UV-Visible absorption spectra of the CdTe QD samples shown in figure 3(a) indicates a peak shift according to the time of heat treatment given to each sample. This is an indication of the quantum confinement effect in the semiconductor due to the increase in size of the nanoparticles[5]. The emission spectra of the CdTe QD samples are shown in figure 3(b) where the emission intensity and peak wavelength vary for each quantum dot samples. The normalized fluorescence spectra in the inset of figure 3(b) clearly indicates the red shift in the emission peak. This is also another confirmation of the confinement effect of the QDs. With the increase in size, the energy gap between the conduction and valance band decreases. The TEM image of the particles with spherical shape depicted in figure 3 confirms the formation of zero dimensional quantum dot structures.
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Figure 3: (a) Absorption and (b) emission spectra of CdTe QD samples showing red shift in the peak intensity, (c) TEM image of the QD samples. Peng et. al proposed a theoretical model for calculating the approximate particle size of the quantum dots based on the relation between excitonic absorption and particle size of quantum dots[18,19]. Using the equation given below, one can find the particle size of the quantum dots π· = (9.8127 β 10 β7)π3 β (1.7147 β 10 β3)π2 + 1.0064 β π β 194.84 Here π· is the diameter of the quantum dot and π is the excitonic absorption wavelength. The
calculated particle size of the quantum dots were found to be ranging from 1.94nm to 2.98nm. In order to study the solar cell efficiency, three sets of efficiency measurements were performed. The first one was with solar cells sensitized with CdTe QDs of different sizes. The second measurement involved samples photosensitized with previously optimized QDs for maximum efficiency with additional ZnS layers to optimize the number of cycles of SILAR. The third one was the measurement to find out the efficiency of each of the quantum dot samples with the optimized number of SILAR cycle from the second measurement. Figure 4 shows the photovoltaic output of QDSSCs sensitized with the five samples of CdTe quantum dots. The voltage-current plot clearly shows that the photovoltaic parameters are varying with respect to the size of the QDs.
Figure 4: Current voltage characteristics of QDSSC solar cell with different QDs The photovoltaic parameters of the QDSSCs with bare CdTe as sensitizer are summarized in table 1. The open circuit voltage (Voc), Short circuit current (Isc) and the power conversion efficiency (PCE) are found to be varying with respect to the size of the quantum dots. The efficiency was found to increase with increase in size of the quantum dots up to an optimum value, above which it decreases. This can be explained based on the size dependent variation of the population of atoms and the concentration of trap states present in the QDs. With the increase
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in particle size, the number of atoms per particle increases. This leads to the increase in the possibility of formation of the excitons which increases the efficiency of the solar cell. The second factor is the trap state, which is mainly controlled by the dynamic growth process and Ostwald ripening mechanism during the synthesis of the quantum dots[19β21]. The formation and decay of the trap states can be explained based on the emission intensity of different QDs shown in Figure 2 (b). During the synthesis of the QDs, the presence of defect or trap states will be more at the initial state of the QD formation. This can be verified from the emission spectrum recorded for the initial sample (C0) where there is higher defect state emission due to the presence of large number of defects. Here the nanoparticles possess low emission intensity with higher non radiative decay. But as the growing progress, the number of defect states decreases due to dynamic growth process and Ostwald ripening mechanism. The number of defects states become minimum at an optimum point of the growth process, where the growth and dissolution are in equilibrium. The particles at this optimum growth point with minimum number of defects will possess higher emission intensity with very low non radiative decay. The further growth process of the nanoparticle lead to particles having rougher surface with increased number of defects resulting in the increased non-radiative decay, and less emission intensity. The higher numbers of defects decreases the injection of charge carriers to the conduction band of TiO2 from the conduction band of QDs. But the QDSSC with optimum QD size possess higher efficiency due to the increase in charge transportation by the decrease in defect states. From the above sets of measurements C60 was selected as the best sample for the efficiency measurement of QDSSC fabricated with different layers of ZnS coating. Table 1: Size dependent efficiency of QDSSC with different sized CdTe QD samples QDs C0 C30 C60 C90 C120
Size (nm) 1.94 2.25 2.55 2.79 2.98
Jsc (mA/cm2) 1.25 2.07 3.24 3.50 4.381
Voc (V) 0.422 0.403 0.371 0.341 0.190
FF (%) 0.303 0.323 0.439 0.368 0.360
PCE (%) 0.16 0.27 0.53 0.44 0.30
Figure 4 (b) shows the variation of open circuit voltage (Voc) and short circuit current (Isc) with respect to the change in particle size of the QDs. It is seen that, with increase in QD size, the Isc is increasing. This can be explained based on the relation between particle size and the population of atoms. With increase in particle size, the number of atoms increases. This leads to the absorption of more number of photons which in turn create more number of charge carriers enhancing the increase of current flow as well as Isc. On the other hand, Vos is found to decrease with increase in particle size which can be explained based on the band gap shift arising from the quantum confinement effect. As the particle size increase, the conduction band shifts downwards
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due to the weaker quantum confinement effect. This decreases the driving force for injection of charge carriers in to the TiO2 layer leading to a decrease in Voc[4,22].
Figure 5: (a) Current voltage characteristics of QDSSC solar cell with sample C60 as the absorbing layer, with different SILAR cycles of ZnS, (b) variation in the PCE for different SILAR cycles. Figure 5(a) shows the solar cell characteristics where the quantum dot sample C60 was used as the sensitizer along with different number of ZnS coatings by SILAR cycle. Initially with the increase in ZnS layer thickness, the efficiency of solar cell increases. From the measurements the sample corresponding to 4 SILAR cycles was found to be showing the highest efficiency of 2.16% beyond which it decreases. So the sample with 4 cycles of ZnS coating was chosen for further studies of efficiency enhancement in QDSSCs. Table 2: Optimization of SILAR cycle for the sample C60 ZnS Cycles 0 1 2 3 4 5 6
Jsc (mA/cm2) 3.24 4.09 4.98 6.21 6.92 5.99 5.48
Voc(V)
FF (%)
PCE (%)
0.371 0.485 0.494 0.498 0.502 0.479 0.422
0.439 0.449 0.476 0.527 0.622 0.606 0.585
0.53 0.89 1.17 1.63 2.16 1.74 1.35
The photovoltaic parameters of the QDSSCs with different SILAR cycles of ZnS coatings are given in Table2. The efficiency of the QDSSC is very low, 0.53% when the QD (C60) alone was incorporated in the solar cell. But with coating of ZnS layer, the efficiency was found to increase with each coating cycle up to four cycles of SILAR.
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A schematic representation on the working of QDSSC with ZnS layer is shown in figure 6. Here the ZnS coating could reduce the interfacial recombination at TiO2/QD and QD/electrolyte interfaces which can be explained based on two possibilities. The decrease of recombination at QD/electrolyte interface is one of them and reduction of back-electron from the TiO2 to the QD being the second. The reduction in Jsc, Voc and PCE values for more than 4 ZnS SILAR cycle is attributed to the difficulty in the electron injection from electrolyte to the QD and TiO2 due to the presence of a thick passivation layer of ZnS.
Figure 6: Schematic representation on the working of QDSSC with ZnS coating Figure 7(a) shows the solar cell I-V characteristics of QDSSC sensitized with QDs of different sizes along with 4 SILAR cycles. The QD size dependent variation of efficiency is shown in Table 3. Here all the QD samples were found to exhibit very high value of PCE compared to the corresponding values of QDSSC made up of bare CdTe QD samples.
Figure 7: (a) Current voltage characteristics of QDSSC solar cell with different QDs with 4 ZnS SILAR cycle, (b) Particle size dependent PCE of QDSSC (i) without SILAR, (ii) with SILAR. The photovoltaic parameters of the QDSSCs which were sensitized with different CdTe QD samples and coated with ZnS layer are tabulated in table 3. For the sample C60 the value of PCE was found to increase from 0.53% to 2.16 % after the ZnS coating.
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Table 3: Size dependent efficiency of QDSSC sensitized with various CdTe QD samples with C4 SILAR cycle QDs C0 C30 C60 C90 C120
Size (nm) 1.94 2.25 2.55 2.79 2.98
Jsc (mA/cm2) 2.67 4.91 6.92 7.54 7.81
Voc (V) 0.546 0.523 0.502 0.481 0.426
FF (%) 0.487 0.500 0.622 0.516 0.490
PCE (%) 0.71 1.29 2.16 1.88 1.63
Figure 7(b) shows the difference in power conversion efficiency of CdTe QD sensitized solar cells with and without ZnS coating. The reason for the variation of efficiencies of both sets of solar cells can be explained as follows. The main purpose of ZnS layer is the passivation of the surface states of CdTe QDs which helps in decreasing the surface trapping of carriers in the QDs. Thus the photo generated charge carriers can be effectively transferred to the conduction band of TiO2 which results in the increase of Jsc. The increase in Voc can be due to the quasi-Fermi level in the TiO2 which increases with increase in density of electrons injected from the QDs to conduction band of TiO2. Since the band gap of ZnS (3.8eV) is higher than that of CdTe QD, the additional layer of ZnS can also act as a potential barrier in between CdTe electrolyte-interface. Hence the leakage of electrons from the QD in to the electrolyte decreases. The increase of Jsc and Voc thus results in the increase in FF upon coating with ZnS. Hence the efficiency of a CdTe based QDSSC can be enhanced upon coating with ZnS layer using 4 cycles of SILAR. Conclusion The efficiency of CdTe QDSSC sensitized with quantum dots having different sizes have been investigated. The efficiency was found to be depending upon the QD size as well as the concentration of trap states present in the quantum dots. ZnS coating by SILAR cycle is found to be an effective method in enhancing the efficiency of CdTe QDSSC. Through proper selection of the QD size and the number of cycles for ZnS buffer layer, the overall efficiency of the QDSSC can be considerably improved. Acknowledgments The authors acknowledge Department of Chemistry, CUSAT for the solar cell characterization and E-Grantz, Kerala Govt. for the financial support. References 1.
Q. Wu, H. Zhao, F. Huang, J. Hou, H. Cao, Z. Liu, S. Peng, and G. Cao, "Impacts of Reduced Graphene Oxide in CdS / CdSe Quantum Dots Co- sensitized Solar Cells," J. Phys. Chem. C 121, 18430β18438 (2017).
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2.
J. Yang, J. Wang, K. Zhao, T. Izuishi, Y. Li, Q. Shen, and X. Zhong, "CdSeTe / CdS Type-I Core / Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9 %," J. Phys. Chem. C 119, 28800β28808 (2015).
3.
C.-Y. S. Xiao-Yun Yu, Bing-Xin Lei, Dai-Bin Kuang, "Highly efficient CdTe / CdS quantum dot sensitized solar cells fabricated by a one-step linker assisted chemical bath deposition β ," chem. sci. 2, 1396β1400 (2011).
4.
X. Z. Junwei Yanga, "CdTe based quantum dot sensitized solar cells with efficiency exceeding 7% directly from quantum dots prepared in aqueous media," Mater. Chem. A 4, 16553β16561 (2016).
5.
T. K. Das and P. Ilaiyaraja, "Whispering Gallery Mode Enabled Efficiency Enhancementβ―: Defect and Size Controlled CdSe Quantum Dot Sensitized Whisperonic Solar Cells," Sci. Rep. 8(April), 1β12 (2018).
6.
A. A. F. Husain, W. Z. W. Hasan, and M. N. Hamidon, "A review of transparent solar photovoltaic technologies," Renew. Sustain. Energy Rev. 94(June), 779β791 (2018).
7.
U. Bockelmann, "Electron relaxation in quantum dots by means of Auger processes," Phys. Rev. B 46(23), 574β577 (1992).
8.
A. P. Alivisatos, "semiconductor clusters, nanocrystals and quantum dots," Science (80-. ). 271, 933β937 (1996).
9.
R C Ashoori, "electrons in artificial atoms," Nature 379, 413β419 (1996).
10.
K. E. Roelofs, S. M. Herron, and S. F. Bent, "Increased Quantum Dot Loading by pH Control Reduces Interfacial Recombination in Quantum-Dot- Sensitized Solar Cells," ACS Nano 9(8), 8321β8334 (2015).
11.
V. I. Klimov, "Spectral and Dynamical Properties of Multiexcitons in Semiconductor Nanocrystals," Annu. Rev. Phys. Chem. 58, 636β674 (2007).
12.
R. D. Schaller, M. Sykora, J. M. Pietryga, and V. I. Klimov, "Seven Excitons at a Cost of Oneβ―: Redefining the Limits for Conversion Efficiency of Photons into Charge Carriers," nano lett. 6(3), 424β429 (2006).
13.
Q. Dot, S. Cells, X. Lan, O. Voznyy, F. P. Garc, M. Liu, J. Xu, A. H. Proppe, G. Walters, F. Fan, H. Tan, M. Liu, Z. Yang, S. Hoogland, and E. H. Sargent, "10.6% Certified Colloidal Quantum Dot Solar Cells via Solvent- Polarity-Engineered Halide Passivation," nano lett. 16, 4630β4634 (2016).
14.
H. K. Jun, M. A. Careem, and A. K. Arof, "Quantum dot-sensitized solar cells β perspective and recent developmentsβ―: A review of Cd chalcogenide quantum dots as sensitizers," Renew. Sustain. Energy Rev. 22, 148β167 (2013).
15.
G. Cedex, "Solution deposition of thin solid compound films by a successive ionic-layer adsorption and reaction process," Appl. Surf. Sci. 23, 1061β1074 (1985).
ACCEPTED MANUSCRIPT
16.
T. K. Nideep, M. Ramya, V. P. N. Nampoori, and M. Kailasnath, "Optical limiting and optical properties of water soluble CdTe quantum dots prepared through a colloidal chemical route," Opt. - Int. J. Light Electron Opt. (September), 0β1 (2018).
17.
A. Badawi, N. Al-hosiny, and S. Abdallah, "The photovoltaic performance of CdS quantum dots sensitized solar cell using graphene / TiO 2 working electrode," SUPERLATTICES Microstruct. 81, 88β96 (2015).
18.
W. W. Yu, L. Qu, W. Guo, and X. Peng, "Experimental Determination of the Extinction Coefficient of CdTe , CdSe , and CdS Nanocrystals," chem. mater. 125(17), 2854β2860 (2003).
19.
C. Wang, Q. Ma, and X. Su, "Synthesis of CdTe Nanocrystals with Mercaptosuccinic Acid as Stabilizer," J. Nanosci. Nanotechnol. 8(9), 4408β4414 (2008).
20.
M. Gogoi, P. Deb, and A. Kostka, "Differential tunability effect on the optical properties of doped and undoped quantum dots," Phys. Status Solidi Appl. Mater. Sci. 209(8), 1543β 1551 (2012).
21.
H. L. Cao, X. F. Qian, Q. Gong, W. M. Du, X. D. Ma, and Z. K. Zhu, "Shape- and sizecontrolled synthesis of nanometre ZnO from a simple solution route at room temperature," Nanotechnology 17(15), 3632β3636 (2006).
22.
X. H. Zhong, "ChemComm Efficient CdSe quantum dot-sensitized solar cells prepared by a postsynthesis assembly approach," chem. commun. 48, 11235β11237 (2012).
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Highlights
ο· ο· ο· ο·
Investigated the size dependence of CdTe quantum dots on the efficiency of solar cells. Studied the influence of ZnS buffer layer on the efficiency of CdTe sensitized quantum dot solar cells. Optimized the number of ZnS SILAR cycles and estimated the size dependent efficiency of CdTe based quantum dot solar cells. Investigated the role of trap states on the efficiency of quantum dot sensitized solar cells.