CdS Quantum dots: Synthesis and Optical Properties

CdS Quantum dots: Synthesis and Optical Properties Characterization for Solar Cell

 
Abstract— In this work CdS quantum dots were synthesized using Successive Ionic Layer Adsorption and Reaction (SILAR) method. Then a study of the morphology and optical property were made for the application of solar cell. The structural characterization were made by XRD while the optical characterization where done by UV-vis-NIR spectroscopy techniques.
Index Terms—Quantum dots, SILAR
I. INTRODUCTION
Quantum dot sensitized solar cell is an emerging field of photovoltaic in which the absorbing material is a quantum dot. The advantage of using such solar cell is size tunability and increased surface to volume ratio.
In a quantum dot based solar cell the active layer consist of the quantum dot and the scattering layer is formed by the TiO2 layer. The mesoscopic TiO2 when deposited with CdS quantum dot act as an energy harvester and convert the incident photon to electricity. In this work, a model of the photoanode for the solar cell was made with mesoscopic TiO2 layer as scattering layer and quantum dots as absorbing layer. Here instead of ITO a glass slide was used. [1]

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To synthesize a quantum dot various techniques are used. Among them Successive Ionic Adsorption and Reaction (SILAR) method is a cost effective and is used to prepare quantum dot. In a SILAR method the time of reaction or the number of cycles can be controlled. Depending on which the size of the quantum dot varies. Another advantage of this technique is that it can be prepared at room temperature. Also this method provides a close contact between the quantum dots and the oxide layer, so it is an attractive method for the preparation of electrodes in a solar cell. [1]
Cadmium sulfide (CdS) quantum dot is a direct band gap semiconductor. It is a II-VI compound semiconductor that is used for many optoelectronic devices such as solar cell, laser diodes and photoconductors. It is an inorganic semiconductor which has several advantages over conventional dyes. These advantages are band gap tunability, large extinction coefficient (this means that the dark current can be reduced and the overall efficiency can be improved) and multiple electron generation by utilizing hot electrons. [2]
II. EXPERIMENTAL SETUP

Chemicals Required

Titanium dioxide powder (SD Fine-Chem Limited, purity 60%), 2M nitric acid, 0.05M cadmium nitrate, ethanol, 0.05M sodium sulfide hydrate (Sigma Aldrich, assay=60%), methanol.

Preparation of TiO2 layer on glass slide

A paste of titania (TiO2) was prepared from TiO2 powder and nitric acid. The chemicals were added in 2:1 proportion. A thin layer of titania paste was coated on the glass slide using a technique called doctor blade method [3]. In this method, either a glass rod or a microscope slide is used. We have used a microscope slide of thickness 1.45mm to coat the paste. A glass slide of dimension 2cm X 1cm was cut and cleaned. With the help of an adhesive tape, the glass slide is positioned firmly on the work bench. Another advantage of using such tape is that we could define an area to coat the paste and to deposit the quantum dot. Now place the paste on one side of the glass slide, positioning the microscope slide in 45ï‚° spread the paste across the glass slide. Repeat the operation till a reasonably homogeneous layer is formed. After coating heat the paste to 80ï‚°C followed by annealing at 450ï‚°C for 30 min. After sintering the paste is white in color. This provides a better surface for adsorption of the CdS quantum dots since sintering makes the mesoporous films to a continuous network.

Deposition of CdS Quantum Dots

Successive Ionic-Adsorption and Reaction method is commonly used to deposit metal sulphide onto a nanostructured film. CdS quantum dot was deposited onto titania using this method as described in [4]. The first precursor solution used is 0.05M cadmium nitrate (Cd(NO3)2) and the second precursor solution is 0.05M sodium sulphide (Na2S). The bare TiO2 paste is dipped onto the first precursor solution for one minute. The Cd2+ ions have been deposited onto the TiO2 surface. This is then rinsed in an ethanolic solution for one minute and dried under room temperature. It is then dipped in the anionic precursor for one minute and then rinsed in methanolic solution for one minute and allowed to dry at room temperature. This completes one deposition cycle of SILAR. In this work we have performed four deposition cycles of SILAR.
III. RESULT AND DISCUSSION
The CdS quantum dot was deposited on to the surface of TiO2. An obvious color change was observed during the deposition cycle which is shown in Fig.1. The color change was pale yellow to golden yellow. The characterization was done using XRD and UV-vis spectroscopy techniques.

Fig 1: Photograph of glass slides with CdS coating with increasing SILAR cycles

XRD Characterization

Fig 2. shows the obtained XRD pattern for TiO2 (Fig.2a), TiO2/ CdS (Fig 2b.) . From the peak obtained, we confirm that CdS quantum dot was deposited onto the film. Since the peaks at 44.1ï‚°, 51.9ï‚°, 64.3ï‚°, 70.4ï‚° and 72.9ï‚° coincides with the intensity pattern as defined by the JCPDS 10-0454 for the CdS QD. The corresponding miller indices are (220), (311), (400), (331) and (420). From this we conclude that CdS QD was deposited. It belongs to the cubic crystal system and the mineral name is hawleyite. For TiO2 the XRD pattern exactly matches with JCPDS 21-1272. It belongs to tetragonal crystal system and its mineral name is anatase.

Fig.2 : XRD pattern (a) TiO2 (b) TiO2/CdS

Size Characterization

The size characterization was done by non-contact mode AFM (Atomic Force Microscopy). The size of the CdS quantum dot was found to be 25.83nm. the thickness of the deposited layer was calculated to be 29.65nm. Fig 3.

Fig 3 : AFM non-contact mode characterization of CdS quantum dot

UV-vis Characterization

The optical property was characterized using Jasco Spectrophotometer V670. The absorption spectrum is shown in Fig 4. The absorption spectrum for the TiO2 and CdS/TiO2 is shown in Fig 4a. and TiO2/CdS alone is shown in Fig.4b. The absorption peak for CdS is as reported by Antonio et.al [4]. From the absorption spectrum we could observe a shift in the peak indicating CdS QD is being deposited. The absorption peak was observed in the range of 386nm-484nm. For TiO2 the absorption peak was observed at 341nm. In Fig 4b. the inset is the absorption spectrum that was reported in [5]

Fig 4: Absorption spectrum of (a) TiO2 and TiO2/CdS (b) TiO2/CdS

Fig 5: UV-Vis absorption spectra showing increase (~49 %) in absorption due to CdS
Figure 5. depicts the percentage increase in the absorption peak of CdS with respect to TiO2. It was calculated to be a 49.08% increase in the absorption peak.

Determination of Optical Band gap

The DRS (Diffuse Reflectance Spectroscopy) characterization was done to obtain the optical band gap. The optical band gap was calculated by plotting the Tauc plot . It is the plot between energy and absorbance. The optical band gap can be determined by Tauc relation

Where  is the absorption coefficient in cm-1, h is the photon energy in eV and A is a constant. The value of n is given as follows
n = ½ for direct allowed transition
n = 2 for indirect allowed transition
The Tauc plot for TiO2 and TiO2/CdS is shown in Fig 6. TiO2 is an indirect band gap material whereas CdS is a direct band gap semiconductor. The bandgap value of CdS in bulk is given as 2.42eV [5]. From the experiment we calculated the optical band gap to be 2.38eV. Also the absorbance value of the CdS QD is blue shifted. Using the equation

The value of the peak was calculated to be 519.16nm which is within the absorption region.

Fig 6. Tauc plot of (a) TiO2 (b)TiO2/CdS
IV. CONCLUSION
In this work CdS quantum dot have been synthesized using SILAR method. Its structural characterization was done that confirmed the deposition of the CdS quantum dot on to TiO2 paste. The optical property was characterized and analysed using UV-vis-NIR spectroscopy. The optical band gap was calculated to be 2.38 eV. The size of the quantum dot deposited was calculated to be in nanometer.
REFERENCES
[1] Prashant V Kamat , “Quantum dot Solar cells.The next Big Thing in Photovoltaics” J.Phys.Chem.Lett. 2013, 4, 908-918.
[2] Chang Liu,Yitan Li,Lin Wei,Cuncun Wu,Yanxue Chen,Liangmo MeiandJun Jiao, “CdS quantum dot-sensitized solar cells based on nano-branched TiO2arrays” Nanoscale Research Letters 2014,9.
[3] A. Berni, M. Mennig, H. Schmidt, “Doctor blade method”, Springer.
[4] Antonio Braga,SixtoGimenez, Isabella Concina, Alberto Vomiero and Ivan Mora-Ser, “Panchromatic Sensitized Solar Cells Based on Metal Sulfide Quantum Dots Grown Directly on Nanostructured TiO2 Electrodes”, J. Phys. Chem. Lett. 2011, 2, 454–460.
[5] B. T. Huy, Min-Ho Seo, Jae-Min Lim, Dong-Soo Shin and Yong-Ill Lee, “A Systematic Study on Preparing CdS Quantum Dots” Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011, 3293-3299
 

Absorption Spectroscopy of Conjugated Dyes and Cadmium Selenide Quantum Dots

Abstract

Conjugated polymethine dyes and the cadmium selenide nanoparticles that were animate to their last incite energy rank were indagate by infrared (IR) spectroscopy. The dyes were comparison with varying polymethine chains (p = 7, 9, and 11) with highest peak wavelength at 590 nm, 705 nm, and 820 nm. The wavelength was increased as the length of polymethine chains increased forwhy of the resonating electronic break along the π bonds rehearse to the conjunction in a case example. The prepossession energies of the conjugate dyes were recount as the one-dimensional particle in a shelter and the engagement energies of the quantity achieve were portray as a null-dimensional morsel in a case. Each share perform pattern had dissimilar redness management set, therefore the share well gauge checkered between relish. It was shown that the wavelength of the electromagnetic radiation engaged by the share deed increased with increscent jot bigness. The amount dyes engrossment data was quantitatively compared to energies suited by Kuhn’s communicative electron model originate from the preposition from a particle in a box equation.

Experimental

Solutions of the three dyes were composed from firm example from sigma Aldrich and had a concentration of 10-3 M. Concentrations were regulated to have absorbances between 0.6 and 0.8 tyrannical absorbance one. The wavelengths of the absorbance maxima were enrolled to liken to the excellence calculated from the theoretical model. The quantity administer were unite previously. Quantum dot spectrums were apt to a gaussian arrangement and a allegorical suitable which were added to renormalize and gain faultless site of twist. After share suffice were renormalize, wavelengths were converted to energies through custom of equality 3 and then enclitic sizes were calculated for each QD with equality 4. Dyes were conspiracy with absorbance as a function of wavelength which tolerate for greatest peak wavelengths to be determined.

Results

Each sample restrain quantum deed of different greatness that correspondent a original distribution and the maxima peak proposition represents the signify particle adjust in the sample. After the rear was remote from each sample the spectra were correspondent to a gauss round. The irrorate fill spectra express the setting removed data and the compact lines example the data adapted to the gauss bend. The prepossession maxima of the match that were exasperation satisfaction for 0.5, 1.0, 2.0, and 4.0 hours were 507 nm, 558 nm, 588 nm and 607 nm, respectively.

Figure 1: Absorption spectra of the three conjugate dyes prospect ripe in wood alcohol. The prepossession of the wood spirit was remote by worn the methanol as the baseline. The preoccupation greatest of each shade agree to the electronic power needed to promote π soldering electrons of the conjugate carbon chains, from their ground state to an excited state.

Figure 2: Spectra of the prepossession for the CdSe quantity administer with varying fervency management set. The quantity dowry are semiconductors. The prepossession spectra are a result of the energy exact to be gotten an excitation or to the power order to promote an electron from the valence fetter to the transmission unite.

For all show 3, the plots of rear removed spectra and gauss fits of the first prepossession culminate of the four-quantum dot match with four separate fervency handling repetition. The wavelength of engrossment is subordinate on the suffix magnitude.

Figure 3A: The absorption maxima of the samples that were treated for 0.5 hour was 507 nm. As the heat treat period the increased, the preoccupation maxima change to longer wavelengths. Spectra and gauss correspondent of the first prepossession point of the four-quantum answer specimen with four other heat treatment.

Figure 3B: The absorption maxima of the samples that were treated for 1.0 hr was 558 nm.

Figure 3C: The absorption maxima of the samples that were treated for 2.0 hours was 588 nm.

Figure 3D: The absorption maxima of the samples that were treated for 4.0 hours was 607 nm.

Discussion

The trend of the absorption energies for both the conjugate dyes and quantity inflict proof both can be portray by the atom in a boxhaul dummy. This is why both experiments were indagate in one proof. The conjugate dyes chase the example for a preposition in a one-dimensional boxhaul and the power for the quantity CdSe achieve go after a cipher-dimensional shape. In each experience, the bigness distinguishing of the equilibrium was checkered and it was found that growing the adjust feature led to the dyes and share perform fascinating diminish resolution electromagnetic radiation. In the conjugate stain proof, the magnitude argument was agitate by incremental the conjugate carbon enslave roll. The conjugate carbon atoms in the dyes plowshare delocalized electrons in pi orbitals, in a abbreviate example, really benefaction the electrons the efficiency to move between all of the carbon atoms in the fasten.

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As a vague empire, the conjugate bonds are more kennel than no-conjugate bonds and therefore have sullenness influential potency and overwhelm cloudiness power radiation. The more conjugate the bonds are in the system the frowning the action of the fetters. This attach not only to the electronic dregs condition, but to the animate electronic height, where harmonious to brownian orbital hypothesis, speculation a pi electron draw potency and actuate from a frowning spirit soldering orbital to a higher potency π anti soldering orbital. The trial issue attend what was prediction by the one-dimensional inflex in the boxhaul shape. Equation 1 depict the Kuhn’s ingenuous electron pattern of a conjunction in a one-dimensional boxful. It can be skilled that growing(prenominal), incremental p, the numeral of conjugate carbons in the fasten purpose the triad to swallow up gloominess spirit electromagnetic radiation at sink wavelengths. 1,1′-diethyl-4,4″-cyanine iodide had the fewest conjugate carbon enslave and rapt the zenith Life at the shortest wavelength.1,1′-diethyl-4,4″ dicarbocyanine iodide had the longest conjugate carbon enslave and wrapped the nethermost potency at the longest wavelength.

Conclusion

 The CdSe quantum dots are type of nanoparticles semiconductors crystals and similar the conjugate dyes take up electromagnetic radiation at discontinuous Life direct. When the CdSe quantity perform are smitten with a photon of qualified efficiency an electron can be provoke into the transmission unite and exciton is formed. Since the particles are entrap in especially the exciton is really entrap in all three spatial directions origin the exciton to be portray by a no-dimensional inflex in a case. This particles resolution can be set forth by the wavefunction of an electron in a quantity well. In Equation 2, the equality for the strength of a wavefunction of an electron in a share well, it can be skilled that the potency of an inflex will drop when the spatial feature L is increased. Increasing the jot gauge, the circle, of the amount dowry is similar to increscent the regard of L diminishing the resolution at which the enclitic draw. Particle swell increased proximate linearly with fervency usage period between the 0.5, 1.0, and 2.0 stound prospect. However, this run did not go on. There was less of an advance in conjunction between the 2.0 and 4.0 stound trypiece.

References

Gary Beane, Klaus Boldt, Nicholas Kirkwood, and Paul Mulvaney. “Energy Transfer between Quantum Dots and Conjugated Dye Molecules.” Journal of Physical Chemistry, 2014, p. 118.

Mandal, S., Garcia Iglesias, M., Ince, M., Torres, T., & Tkachenko, “Photoinduced Energy Transfer in ZnCdSeS Quantum Dot-Phthalocyanines Hybrids.” ACS Omega, (2018). 3(8), 10048-10057.

Appendix

Theoretical Maximum Peak Wavelength

λmax=63.7 nm*p+3+a2p+4
λmax=63.7 nm*7+3+027+4λmax=579 nm

Percent Error

% Error=experimental–theoreticaltheoretical*100%
% Error=588nm–579nm579nm*100%=1.89%

Quadratic formula usage to solve for quantum dot radius:

1r=x=–b±b2–4ac2a=1nm
a=Ry*π2*aB2a=0.012 eV*π2*5.6 nm2=a=3.716 eV*nm2
b=Ry*–1.786*aBb=0.012 eV*–1.786*5.6 nm=b=–0.122 eV*nm
c=Ry*–0.248+Eg–Exc=(0.012 eV)*–0.248+1.829 eV–2.0926 eV=c=–0.266 eVr=3.51 nm
 

PbS Quantum Dots: Synthesis and Optical Properties

Abstract— PbS quantum dots have attracted more attention in quantum dot sensitized solar cells as sensitizers on photoanode because of its high efficiency, high absorption coefficient and broad range of absorption. In this paper, we synthesized PbS quantum dots on the glass substrate coated with TiO2 paste by SILAR (Successive Ionic Layer Adsorption and Reaction) method The quantum dot size was varied by varying the number of cycles and the UV-Vis-NIR Spectrophotometer ,XRD Diffractometer and AFM were used to characterize the quantum dots.
Index Terms—Quantum dot sensitized solar cells, quantum dots, SILAR.
INTRODUCTION
Quantum dot sensitized solar cells are the next generation solar cells because of their ability to absorb more light owing to its high surface to volume ratio, size dependent optical properties, ease of fabrication and low cost. Most of the research has been done for II-VI semiconductor compounds and out of these compounds PbS is found to have more efficiency and more absorption coefficient. Moreover it has high Bohr radius (around 18nm) [1] which gives it stronger quantum confinement and broadens the optical absorption area. PbS quantum dots have wide absorption range covering visible and near infrared,[2].

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In this paper, we have focussed on the development of PbS quantum dot layer on the electrode for solar cell application as a photoanode. For maximum electron transport from quantum dot sensitizer to the electrode an intermediate layer of TiO2 is deposited on the glass substrate by Doctor Blade method. The layer after appropriate heat treatment is subjected to the SILAR(Successive Ionic Layer Adsorption and Reaction) process where the lead sulphide quantum dots are deposited by use of appropriate concentration of precursors and proper dipping and rinsing times. The size of the quantum dots are varied by increasing the SILAR cycles. The advantage of SILAR over other techniques is that the synthesis takes place at room temperature and is simple.The optical characteristics and diameter of the quantum dots are characterized by UV-Vis-NIR Spectrophotometer and Contact-mode Atomic Force Microscope imaging. The phase and type of PbS formed is determined by Xray diffractometer.
EXPERIMENTAL SECTION
A. Materials
Titanium dioxide(TiO2) nanopowder-20nm anatase phase , 2M nitric acid (HNO3) were required for the preparation of TiO2 paste and methanolic solutions of Lead Nitrate(PbNO3),methanol and sodium sulphide( Na2S) were used for the SILAR process and acetone for cleaning purposes.
B. Preparation of TiO2 film
The Titanium dioxide paste is prepared by making a mixture of 1.2g of TiO2 nanopowder and 0.6mL of 2M conc. nitric acid (HNO3).This paste is uniformly formed on the glass substrate by doctor blade method in which the TiO2 paste is deposited on one end of the area marked by tapes and uniformly spread by using a blade or a glass slide. The TiO2 coated glass was then dried at 80oC for half an hour followed by annealing at 450oC for 30 mins. This improved the adsorption of the TiO2 film.
C. Synthesis of PbS quantum dot on the TiO2 coated glass by SILAR method
For coating PbS quantum dots by SILAR method, the TiO2 coated glass is successively dipped in methanolic solution of 0.02M Pb(NO)3 and methanolic solution of 0.02M Na2S for 1 min each. Lower the molarity more dispersed is the quantum dot deposition on the TiO2. Between each dipping the substrate is rinsed with methanol for 1 min and air dried for some time to remove the excess precursors. This is one cycle which was repeated for increasing the quantum dot sizes. Figure 1 shows the colour variation observed with change in the SILAR cycles. It was observed the colour of the film changed from white (TiO2) to reddish black in colour when the SILAR cycle was increased to 4 cycles. With increase in the cycles, the particle size increased and hence the energy bandgap Eg decreased indicated by the colour change in the film.3]

Fig.1. Sample images showing the colour changes with increase in the SILAR cycles
D. Characterization
UV-Vis-NIR spectrophotometer ( Jasco Spectrophoto-meter V670) was used to observe the absorption properties of TiO2 and TiO2 coated PbS quantum dots. The absorption plots were taken using glass slides as the reference and the wavelength range extended from UV to near Infrared. It provided the information like increase in the absorption after depositing PbS and also bandgap information from tauc plot. The Xray diffractometer was used to obtain the diffraction patterns of the TiO2 and PbS films and to identify the phases and type of quantum dot obtained.
RESULTS AND DISCUSSIONS

Structure and Surface Morphology

Figure 2a and 2b shows the XRD pattern of glass slide/TiO2 and glass slide/TiO2/PbS film obtained from four SILAR cycles respectively. The pattern shows peaks of glass, TiO2 and PbS. The bulging shape and noisy peaks observed in the XRD is due to the amorphous glass. Also the peaks of TiO2 are more prominent in 2b due to thin coating of PbS. The comparison of TiO2 XRD and JCPSD 21-1272 confirms its anatase phase and tetragonal crystal form. The XRD of TiO2 matches with JCPSD data at 26.3o (011), 37.3o (004), 43.03o (220), 48.08o (020), 53.83o (015), 5.12o (121), 62.5o (400) and 68.8o (331). The XRD of glass slide/TiO2/PbS coincides at 25.3o (011), 37.9o (004), 48.08o (020), 53.93o (015) and 55.12o (121) values of 2θ of JCPSD 21-1272 confirming presence of TiO2 anatase form and coincides at 43.09o (220), 62.5o (400), 68.8o (331) values of 2θ of JCPSD 05-0592 confirming the cubic form of PbS galena. [4]

Fig. 2. XRD Pattern of (a) glass slide / TiO2 showing the presence of tetragonal anatase form of TiO2 (b) glass slide / TiO2 / PbS showing cubic of PbS galena and anatase tetragonal TiO2

Optical Properties

The absorption v/s wavelength curve and the tauc plot obtained from UV-Vis-NIR Spectrometer and Diffusive Reflectance Spectrometer respectively are shown in Fig. 3 and Fig. 4.

Fig.3. Absorption Curves of (a) TiO2/PbS. Inset:Absorbance v/s Wavelength curve of PbS film (reproduced from ref [5]) (b) TiO2 and TiO2/PbS showing 60% increase in absorption due to deposition of PbS quantum dots.
The absorption curve of TiO2/PbS in Fig.3a shows a TiO2 peak at 343 nm along with a peak at 400nm and broad range of absorption which is the peculiarity of PbS quantum dots. This is confirmed from the inset plot reproduced from ref. [5]. PbS quantum dots have absorption edge in the Infrared region which is beyond the range of the plot. Fig.3b shows the absorption difference between TiO2 and PbS coated TiO2. From the curve it is clear that TiO2/PbS absorb more and the percentage increase in the absorption is estimated to be 66.7% from the plot.

Fig.4. Tauc Plot of (a) TiO2 showing bandgap of 2.67 eV (b) TiO2/PbS showing the bandgap value of 2.289eV
Fig.4a shows the tauc plot of TiO2 which is (αhυ) 0.5 versus hυ. This is due to the indirect nature of TiO2. From the plot it can be inferred that the bandgap of the 20 nm TiO2 is 2.67 eV. Fig.4b shows the tauc plot of TiO2/PbS which is the plot of (αhυ)2 versus hυ.[4] The linearity of the tauc plot confirms its direct transition and the extrapolation of the linear portion on the x-axis gives the bandgap value of 2.289eV. The bandgap thus obtained is more than the bulk bandgap of PbS which is around 0.4eV. This increase is due to the decrease of size as compared to the bulk. The particle size can be estimated from the bandgap value using the empirical formula developed by Iwan Moreels et al. [6]

Where Eg is the optical bandgap and d is the estimated size. The estimated size corresponding to the 2.289eV bandgap value is 1.64nm.
CONCLUSION
The photoanode for the solar cell was thus made by depositing PbS quantum dots on TiO2 coated glass substrate using SILAR method. From the spectrophotometer plots, the TiO2/PbS film was observed to give 66.7% more absorbance as compared to only TiO2 film. Also the particle size of 1.64nm was estimated from the tauc plot. The increase in the absorption even with a very small particle size of PbS makes it a very good sensitizer for quantum dot sensitized solar cells as compared to other quantum dots. However the hazards due to its poisonous nature urge the need for a good alternative.
REFERENCES
[1] Abdelrazek Mousa, “Synthesis and Characterization of PbS Quantum Dots”, Lund University,2011
[2] Sawanta S. Mali, Shital K. Desai, Smita S. Kalagi, Chirayath A. Betty, Popatrao N. Bhosale, Rupesh S. Devan, Yuan-Ron Mad and Pramod S. Patila , “PbS quantum dot sensitized anatase TiO2 nanocorals for quantum dot-sensitized solar cell applications” , Dalton Trans., 2012, 41, 6130
[3] Hyo Joong Lee, Peter Chen, Soo-Jin Moon, Frederic Sauvage, Kevin Sivula, Takeru Bessho, Daniel R. Gamelin, Pascal Comte, Shaik M. Zakeeruddin, Sang II Seok, Michael Gratzel and Md. K. Nazeeruddin, “Regenerative PbS and CdS Quantum Dot Sensitized Solar Cells with Cobalt Complex as Hole Mediator”, American Chemical Society,2009,25(13),7602-7608
[4] A.U.Ubale, A.R.Junghare, N.A. Wadibhasme, A.S Daryapurkar, R.B.Mankar, V.S.Sangawar, “Thickness Dependent Structural, Electrical and Optical Properties of Chemically Deposited Nanoparticle PbS Thin Films”, Turk J Phys, 2007, 31,279-286
[5] Lidan Wang, Dongxu Zhao, Zisheng Sui and Dezhen Shen, “Hybrid polymer/ZnO solar cells sensitized by PbS quantum dots”, Nanoscale Reasearch Letters, 2012, 7:106
[6] Iwan Moreels, Karel Lambert, Dries Smeets, David De Muynck, Tom Nollet, Jose C Martins, Frank Vanhaeke, Andre Vantomme, Christophe Delerue, Guy Allan and Zeger Hens, “Size Dependent Optical Properties of Colloidal PbS Quantum Dots”, ACS Nano,2009, Vol 3,10,3023-3030