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Investigations of rare earth doped CdTe QDs as sensitizers for quantum dots sensitized solar cells
Journal Pre-proof Investigations of rare earth doped CdTe QDs as sensitizers for quantum dots sensitized solar cells Ayyaswamy Arivarasan, Sambandam Bharathi, Sozhan Ezhilarasi, Surulinathan Arunpandiyan, M.S. Revathy, Ramasamy Jayavel PII:
S0022-2313(19)31208-6
DOI:
https://doi.org/10.1016/j.jlumin.2019.116881
Reference:
LUMIN 116881
To appear in:
Journal of Luminescence
Received Date: 16 June 2019 Revised Date:
20 October 2019
Accepted Date: 5 November 2019
Please cite this article as: A. Arivarasan, S. Bharathi, S. Ezhilarasi, S. Arunpandiyan, M.S. Revathy, R. Jayavel, Investigations of rare earth doped CdTe QDs as sensitizers for quantum dots sensitized solar cells, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.116881. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
INVESTIGATIONS OF RARE EARTH DOPED CdTe QDs AS SENSITIZERS FOR QUANTUM DOTS SENSITIZED SOLAR CELLS Ayyaswamy Arivarasana*, Sambandam Bharathib, Sozhan Ezhilarasia, Surulinathan Arunpandiyan, M.S. Revathya and Ramasamy Jayavelc a
Department of Physics, International Research Centre, Kalasalingam Academy of Research and Education, Krishanakoil - 626 126, Tamilnadu, India b
NGSeq Analytics LLC, San Diego, California, United States of America c
Crystal Growth Centre, Anna University, Chennai - 600 025, India
*Corresponding Author: [email protected] (A.Arivarasan)
Graphical Abstract
INVESTIGATIONS OF RARE EARTH DOPED CdTe QDs AS SENSITIZERS FOR QUANTUM DOTS SENSITIZED SOLAR CELLS Ayyaswamy Arivarasana*, Sambandam Bharathib, Sozhan Ezhilarasia, Surulinathan Arunpandiyan, M.S. Revathya and Ramasamy Jayavelc a
Department of Physics, International Research Centre, Kalasalingam Academy of Research and Education, Krishanakoil - 626 126, Tamilnadu, India b
NGSeq Analytics LLC, San Diego, California, United States of America c
Crystal Growth Centre, Anna University, Chennai - 600 025, India
*Corresponding Author: [email protected] (A.Arivarasan) Abstract Gd doped CdTe (Gd:CdTe) QDs sensitized working electrodes were fabricated for QDSSC applications. In this fabrication process, mercapto succinic acid capped Gd:CdTe QDs were synthesized by colloidal method and used as the sensitizer. Improved optical properties of the prepared QDs were examined by optical absorption and emission spectral analysis. Fluorescence quantum yield measurement reveals that 10% Gd:CdTe QDs shows the highest quantum yield of 67%. XRD analysis confirms the cubic zinc blende crystalline structure of the prepared QDs and the dopant concentration dependent cell parameters and crystallite sizes were revealed. Performance of capping molecules over the prepared QDs was analyzed by FT-IR studies. Enhanced photovoltaic performance of prepared pure and doped QDs was analyzed through J-V characteristic curves, which show better photovoltaic response with an efficiency of 2.24% for 10% Gd:CdTe QDs. Keywords: Quantum Dots; Gd:CdTe; working electrodes; QDSSC; photo-conversion efficiency.
1.
INTRODUCTION Recent advancement of dye sensitized solar cell (DSC) was quantum dots sensitized solar
cells (QDSSC). Sensitizers were the only physical difference between DSC and QDSSC. All the other active components were remains same. Mathew et al in 2014 reported the highest photoconversion efficiency of 13% for DSC solar cells [1]. Tremendous efforts were made to enhance the efficiency of DSC. Among these attempts, utilization of quantum dots (QDs) as the sensitizer was the important one [2, 3]. Since QDs were used as sensitizers, DSC can be named as QDSSC. QDSSC raise the probability of increasing solar cell efficiency at low cost. The size induced optical properties of QDs was used as a key factor in third generation solar cells [4, 5]. Along with this, QDs have some attractive characteristics like large intrinsic dipole moment [6], high absorption coefficient [7, 8], broad excitation spectra, narrow emission spectra, photostability and low cost, which enhances the efficiency of QDSSC. The multi-exciton generation behavior of QDs [9-11] raises the probability of overcoming the Schottky Queisser’s limit (32.9 %) of single junction solar cells [12]. It arises due to lack of photo-electrons generation, by the incident of lesser energy photons than the band gap energy. Recently, QDs have attracted a large attention due to their outstanding optoelectronic behavior by its size tunable optical properties. The absorption window of semiconductor QDs can be altered by altering the size and compositions of QDs. Even though QDs play a crucial role in solar cell applications, the photo-conversion efficiency of QDSSC was still low. The benchmark efficiency of cadmium based QDSSC was 7.24% reported for CdS/CdSe QDs sensitize solar cells [13]. But the photo-conversion efficiency of perovskite quantum dots based solar cells was far ahead than normal QDSSC. The highest efficiency of perovskite QDs based solar cells was 13.43% for CsPbI QDs arrays as reported by Sanihira et al in 2017 [14]. The
lacking of QDSSC efficiency resulted from the poor transfer and high recombination rate of charge carriers in between the various active components of the solar cell. Numerous attempts have been made to overcome these issues. Particularly, recent research works focused on the fabrication of doped QDs as sensitizers to enhance the charge transfer rate by reduces the recombination rate of charge carriers. Recently, fabrication of cadmium chalcogenide sensitizers (CdS, CdSe and CdTe) attracted a large attention in QDSSC. This was due to their simple fabrication procedures, optical tuneability by their size and compositions and the possibility of multi exciton generation [15]. Among these cadmium chalcogenides, CdTe QDs play a crucial role in optoelectronic applications. The absorption window of CdTe QDs can be tailored to absorb the major intensity radiations (visible and NIR radiations) of solar spectrum by tuning its size and compositions. During the past few years, large number of attempts was made to alter the physical and chemical properties of CdTe QDs, including core/shell CdTe QDs, alloyed CdTe QDs, doped CdTe QDs [16], etc. Recently, some post thermal treatments were also tested to enhance the photovoltaic response of CdTe QDs. But the overall efficiency of solar cells still remains low. This was mainly due to the poor charge carrier transfer rate between the QDs and TiO2 nanoparticles and it can be enhanced by the addition of dopant materials in QDs. By adding dopant materials, the conduction and valence energy levels of CdTe QDs were altered equivalent to the wide band gap semiconductors. But the number of articles, which explain the behavior of doped CdTe QDs and its photovoltaic performance, were very less. Here we have reported the improved photovoltaic performances of rare earth doped CdTe QDs. Rare earth elements were well known for their unique optical properties. Among the various rare earth elements, gadolinium (Gd) play a vital role in fluorescence applications [17,
18]. In general Gd (III) ions were used as a fluorescence sensitizer. In Gd (III) ions, the fluorescence was produced by the electronic transitions between 4f orbitals. Since Gd (III) ions were used as the optical enhancer, it was used to enrich the optical behavior of CdTe QDs. In this work, we have prepared Gd doped CdTe (Gd:CdTe) QDs sensitized TiO2 photoelectrodes and its photovoltaic performance was studied by the fabrication of sandwich type QDSSC. Thiol capped Gd:CdTe QDs were synthesized through colloidal route in normal atmosphere. The prepared Gd:CdTe QDs were analytically characterized to reveal its dopant concentration dependent structural and optical properties. QDSSC was fabricated using Gd:CdTe QDs sensitized TiO2 photo-electrode and sulfide/polysulfide electrolyte. The photovoltaic response of the prepared QDSSC was studied under standard illumination conditions (100 mW/ cm2). 2. Experimental Techniques 2.1. Precursors Cadmium chloride monohydrate (CdCl2.H2O), air stable potassium tellurite (K2TeO3), gadolinium nitrate and sodium borohydride (NaBH4) were used for the QDs synthesis and they were procured from Sigma Aldrich chemicals. The purchased chemical are in analytical grade and used without further purifications. All the solutions were prepared under normal atmosphere using Millipore water as a solvent. 2.2. Preparation of pure CdTe QDs Colloidal route was adopted to prepare thiol capped CdTe QDs [19]. 0.25 M CdCl2 aqueous solution was prepared in 100 ml of Millipore water and 0.402 g of mercapto succinic
acid was added as the capping agent. 2 M NaOH aqueous solution was used to adjust the pH of reaction to 7. 0.253g of K2TeO3 was added into the reaction mixture as the tellurium precursor. 0.1 g of NaBH4 was used as the reducing agent to produce CdTe monomers. Rate of CdTe monomers formation and the preparation of QDs from CdTe monomers were facilitated by the refluxing technique at 100 °C for 30 min. Finally, aliquots were collected at regular interval for optical characterizations and the resultant solution was precipitated with ethanol. Finally the precipitates were collected and air dried at 80 °C for 3 h. The reaction mechanism for the formation of CdTe QDs was given in equation 1. ܍܂ష
۱܌ା + ܘ ۯ܁ۻ۶ିૠ/℃ → ۱܍܂܌
(1)
2.3. Synthesis of Gd:CdTe QDs CdTe QDs synthesis was repeated with the appropriate amount of Gd to obtain Gd:CdTe QDs. Dopant concentration was varied between 1% and 20% (Cd1-xGdxTe, where x= 0.01, 0.03, 0.05, 0.1 and 0.2). The synthesis protocol was explained in Figure 1 and the resultant equation was given in equation 2. ܍܂ష
(۱܌ା ۵܌ା ) + ܘ ۯ܁ۻ۶ିૠ/℃ → ۵܌: ۱܍܂܌
(2)
2.4. Gd:CdTe QDs Sensitized Solar Cell Fabrication Sandwich type QDSSC was fabricated by assembling QDs sensitized working electrodes and graphite coated counter electrode. In this process, TiO2 nanocrystalline layer was coated over the ITO substrate uing doctor blade technique. Thickness of the TiO2 nanocrystalline layer was fixed as 50 µm using scotch tape. The pre-synthesized Gd:CdTe QDs by colloidal route was
deposited over TiO2 (~25 nm) surface using mercapto succinic acid as bi-linker molecules and the schematic representation of Gd:CdTe QDs sensitized TiO2 photoelectrode was shown in Figure 2. Prepared photoelectrodes and counter electrode were assembled together to form sandwich structured solar cells using hot melt tape as the spacer. Space between the electrodes was filled with sulfide/polysulfide electrolyte and it was acts as the regenerative redox couple [20]. 2.5. Characterization Techniques Rigaku Miniflex II-C X-ray diffractometer was used to reveal the crystalline structure of prepared QDs at the scanning rate of 3°/min. Absorption behavior of prepared QDs was measured using UV-WIN T90+ UV-vis spectrophotometer and the corresponding emission property was studied using Horiba Fluorolog-3 spectrofluorometer. The quantum yield (QE) measurement was carried out using Rhodamine 6G as the reference material, under standard excitation conditions (excitation wavelength λ=400 nm). Functional group analysis was carried out using Perkin Elmer Spectrum1 FT-IR spectrometer. TG-DTA: SII 6300 EXSTAR thermal analyzer was used to examine the thermal stability of prepared QDs at the scanning rate of 10°C/min. The solar cell performance was studied using a Keithley 2400 source meter (Oriel, Model: 91192) under standard illumination condition (100 mWcm-2). 3. RESULTS AND DISCUSSION 3.1.XRD Analysis XRD patterns of pure and Gd:CdTe QDs were shown in Figure 3(a). Diffraction pattern of pure CdTe QDs shows the appearance of diffraction peaks at (2θ) 24.4, 29.2, 40.3 and 47.4° related to (111), (200), (220) and (311) crystalline planes, which confirms the cubic zinc blende
crystalline structure (JCPDS No: 65-1046). This XRD pattern is in good agreement with the previous results reported by Maity et al, 2015 [21], Xiao et al, 2012 [22] and Silva et al, 2012 [23]. The XRD patterns of Gd:CdTe QDs shows an overlapped diffraction peaks of CdTe and GdTe QDs, due to the mutual interactions between the oxygen ions and Gd, Te ions. The major diffraction peak by the interaction between Gd and Te ions was observed at 29.1° corresponding to (200) crystalline plane. At lower dopant concentration (1 %), the diffraction intensity of (200) plane was minimum and it was gradually increased with dopant concentration as shown in figure 3 (a). At the same time, the diffraction intensity of (111) crystalline plane of CdTe QDs were suppressed by higher intensity (200) plane. It may be due to the increased molecular interactions between Gd and Te ions at higher dopant concentration. This overlapping of diffraction peaks was clearly identified by the Gaussian fittings as shown in Figure 3(b). The diffraction peak positions of Gd:CdTe QDs lies between the pure CdTe QDs and GdTe with cubic phase (JCPDS No: 65-5020) due to the mutual interactions of Gd, Cd ions with Te ions. It confirms that, the Gd3+ ions were substituted on CdTe lattices. Incorporation of Gd3+ ions in CdTe lattice reduces the crystalline nature of CdTe QDs, which was confirmed by the amorphous like XRD patterns of Gd:CdTe QDs. Similar diffraction patterns were observed in our previous report, for Zn and S co-doped CdTe QDs [16]. Along with this difference in peak positions, a clear visible shift in diffraction peak positions of pure CdTe QDs towards the lower diffraction angles depending on dopant concentration was also observed. This was due to the replacement of lower ionic radii Cd2+ (95 pm) ions by higher ionic radii Gd3+ (107 pm) ions. The increase in size and lattice parameters was clearly identified by the lower angle shift of diffraction peaks with doping concentration. The mean crystallite size was calculated by using Scherer’s formula
۲ = ૃܓ/ ܛܗ܋ી where
(3)
k - Scherer’s constant (k=0.9) D- Mean crystallite size λ- Wavelength of the X-ray radiation (CuKα=1.540Å) β- Full width at half maximum (FWHM) of the diffraction peaks θ- Angle of diffraction. The average crystalline size of pure CdTe QDs was found to be 2.1 nm and it was
increased with dopant concentration. Increase in crystallite size with dopant concentration was confirmed by the reduction in peak broadening of XRD pattern as shown in Figure 3(b). Variation in crystallite sizes with dopant concentrations were listed in Table 1. The crystallite size calculation reveals that the doping of Gd3+ ions could increase the size of CdTe QDs as shown in Figure 4(a). The lattice parameter and inter-planar distances were calculated from XRD pattern and listed in Table 1. Change in crystallite size with dopant concentration was shown in Figure 4(b). The inter-planar spacing of CdTe lattices was gradually increased with dopant concentration. The inter-planar spacing of pure CdTe QDs is 3.65 Å and it is increased to 3.96 Å when the dopant concentration was increased to 10%. This was due to the substitution of larger ionic radii Gd3+ ions (107 pm) in lower ionic radii Cd2+ sites (95 pm). 3.2. Morphological analysis - HRTEM Morphological properties of pure and 5% Gd doped CdTe QDs were examined by HRTEM analysis and the corresponding HRTEM images were shown in Figure 5. As prepared QDs were utilized for the HRTEM analysis. HRTEM images indicates that the pure CdTe QDs were predominantly spehrical in shape with well ordered grain bounderies. This was due to the lower aggreagtion level between CdTe QDs by effective capping of MSA molecules around the
CdTe QDs surface. HRTEM images also reveals the homogeneous distribution of pure CdTe QDs with particles sizes of around 2 nm. When compared to pure CdTe QDs, Gd:CdTe QDs showed larger sized paricles with small aggregations. Also the grain boundaries were not clearly visible, which indicates that the effective capping nature of thiols was reduced by dpoing process. The crystallinity of CdTe QDs was also reduced with addition of Gd ions. The size variation of CdTe QDs were clearly observed in HRTEM images by the addition of Gd ions. 3.3.UV-vis Absorption Studies Absorption behavior of QDs was analyzed by UV-vis absorption spectroscopy and the recorded absorption spectra were shown in Figure 6. Pure and Gd:CdTe QDs shows a well resolved absorption maximum at around 470 nm. But, there was a significant red shift observed in absorption peaks of Gd:CdTe QDs with different dopant concentrations, which reveals that Gd3+ ions were substituted in CdTe crystal lattices, resulting in the formation of CdGdTe alloys, which was confirmed by the XRD analysis. In other cases, Gd3+ ions were doped on the surface of the CdTe QDs and reacted with excessive Te2- ions, which results in the formation of CdTe:GdTe nanocomposites rather than CdGdTe alloys. In that case, the absorption spectra show two different excitonic absorption maxima at different wavelengths corresponding to CdTe and GdTe QDs. Size variation with increasing dopant concentration was clearly identified by the red shift in absorption maximum. This was in good agreement with XRD analysis. The band gap energy (Eg) of the prepared QDs was calculated by ۳ = ܋ܐ/ૃ where,
Eg - Band gap energy (eV) h - Plank’s constant (h=6.625*10-34 m2 Kg/s)
(4)
c - Velocity of light (c=3*108 m/s) λ - Absorption maxima of the prepared QDs The band gap energy for Gd:CdTe QDs varies between 2.64 eV and 2.52 eV with increasing dopant concentration. It reveals that the band gap energy decreases with increasing dopant concentration. The average particle size of the prepared QDs was estimated using the following relation [7] ࡰ = (ૢ. ૡૠૡ ∗ ିૠ )ࣅ − (. ૠૠ ∗ ି )ࣅ + . ࣅ − ૢ. ૡ
(5)
Where ‘D’ is particle size and ‘λ’ is the absorption maximum. The particle size varies between 1.8 nm and 3.9 nm with increasing dopant concentration. From UV-vis results, it was concluded that the QDs sizes were increased with dopant concentration and the measured values were listed in Table 2. 3.4.Fluorescence Spectroscopy Analysis Room temperature fluorescence spectra of as-prepared QDs were measured under the excitation wavelength of 420 nm and the recorded emission spectra were shown in Figure 7. By the absorption of photon, electron-hole pair was created in CdTe QDs and the excessive energy was released in the form of photon during electron-hole recombination process. It was clearly observed that pure and Gd:CdTe QDs possess a strong near band edge emission. The fluorescence spectra for both pure and Gd:CdTe QDs show a well resolved emission peak at around 517 nm. The narrow width of emission peaks clearly indicates the homogeneous distribution of prepared QDs. However, a significant red shift and intensity variations were
observed for Gd:CdTe QDs with dopant concentration. The red shift in emission maximum was attributed to the alloying effect and size variation of CdGdTe QDs by the incorporation of Gd3+ ions in CdTe crystal lattices. The improved fluorescent behavior of Gd:CdTe QDs was observed at lower doping concentration (up to 10%) [24], which was due to the influence of Gd3+ ions. In contrast, the fluorescence intensity was further decreased when the doping concentration exceeds more than 10% which may be due to quenching effect. The intensity variation in emission peaks clearly indicates the quantum yield (QE) difference between pure and Gd:CdTe QDs. The QEs for the QDs were calculated using the following relation [25,26]. ࢙ = ࢘ ∗ (࢘ /࢙ ) ∗ (ࡵ࢙ /ࡵ࢘ ) ∗ (ࣁ࢙ /ࣁ࢘ ) where,
(6)
Φ - Quantum yield (QE) A - Absorbance I - Integral area of the fluorescence spectrum Η - Refractive index
The suffix ‘r’ and ‘s’ stands for reference and sample respectively. The QE for pure and Gd:CdTe QDs were calculated with Rhodamine 6G as the reference material. QE of 32% was calculated for pure CdTe QDs at a refluxing time of 30 min and the QE get increases with dopant concentration and reaches the maximum of 67% for 10% Gd doping [24]. Beyond this concentration, QEs were decreased by the fluorescence quenching effect. The calculated values of QEs with different doping concentration were listed in Table 2.
3.5.EDX Analysis Energy dispersive X-ray spectroscopy (EDX) technique was adopted to examine the elemental composition of prepared QDs and the recorded EDX spectrum with quanitative measurements was shown in Figure 8(a & b). EDX spectrum shows the corresponding peaks for Cd, Gd, Te and S elements. The presence of S was due to capping of MSA on the QDs surface and the presence of Gd indicates the formation of Gd:CdTe QDs. The quantitative analysis results indicates that nearly 3% of Gd was replaced on CdTe QDs for 3% Gd:CdTe QDs. 3.6. FT-IR Analysis Presence of functional groups of the prepared QDs was identified by their molecular vibrations using FT-IR analysis and the recorded spectra were shown in Figure 9. In prepared QDs (pure and Gd:CdTe QDs), the asymmetric and symmetric stretching vibrations due to COOof MSA molecule was observed at 1566 cm-1 and 1388 cm-1, which confirms that the MSA molecules were coated over the CdTe QDs. The adsorption of water molecules by the prepared QDs was revealed by the presence of O-H stretching vibrations at around 3410 cm-1. C-S stretching vibration of the capping agent was observed at 1200 cm-1. Lower frequency shift in CH2 active modes of MSA molecules (2931 cm-1) was observed at 2916 and 2908 cm-1 for MSA capped CdTe QDs and Gd:CdTe QDs respectively. It reveals that the surface of MSA molecules was affected by van der Waals fields CdTe and Gd:CdTe QDs [27]. Disappearance of S-H vibrations for MSA molecules (2638 cm-1 and 2553 cm-1), in prepared QDs, confirms that the surface of MSA molecules was modified into Cd-SR bond by absorption of Cd ions. Observed functional groups were listed in Table 3.
3.7.Thermogravimetric analysis (TGA) Thermal stability of prepared QDs (pure CdTe QDs and Gd:CdTe QDs) was studied up to 1000°C by thermogravimetric analysis and the recorded thermograms were shown in Figure 10. Similar types of TGA curves were recorded for both the samples with three sequential weight losses. Evaporation of adsorbed water molecules by the prepared QDs, leads to the first weight loss at around 100 °C, as discussed by the FT-IR analysis. Second weight loss at around 300°C was attributed to the removal of chloride ions in the precursors [28]. Dissociation of Cd-MSA complexes leads to the final weight loss at around 700°C. TGA result confirms that the prepared QDs were thermally stable up to 800 °C and the thermal stability of CdTe QDs was significantly reduced when doping with Gd ions. 3.8. Photovoltaic Studies Prepared QDs were sensitized over TiO2 nanoparticles coated electrode and utilized as the working or active electrode for the fabrication of QDSSC and the performance of fabricated solar cells was studied by I-V characteristics under standard illumination condition (100 mW/cm2). I-V characteristic curves of fabricated working electrode were shown in Figure 11 and derived solar cell parameters, such as, conversion efficiency (η), fill factor (FF), open circuit voltage (Voc), and short circuit current (Isc) were listed in Table 4. The photovoltaic conversion efficiency (η) and the fill factor (FF) of Gd:CdTe QDs sensitized solar cells were calculated from the following relations [29] િ = ۸ ∗ ܋ܗ܄ ∗ ܋ܛ۴۴ ∗ ۯ/ܖܑ۾
(7)
۴۴ = (۸) ܕ܄ ∗ ܕ/(۸) ܋ܗ܄ ∗ ܋ܛ
(8)
where, A - area of solar cell (1 cm2), Jm - power maximum, Vm - voltage maximum and Pin intensity of incident radiation (100 mW/cm2) Poor photostability of CdTe QDs in polysulfide electrolyte leads to the very less photovoltaic conversion efficiency of η = 0.48% for pure CdTe QDs [30]. Photoconversion efficiency of TiO2 photoelectrodes increases with the sensitization of Gd:CdTe QDs. Addition of dopant ions in CdTe QDs create trap states between the highest occupied and lowest unoccupied energy levels of CdTe QDs. This trap states reduces the electron-hole recombination rate leads to the increase in charge transfer rate betweem QDs and TiO2 nanoparticles. Since, the charge transfer rate gets increased with dopant concentration, Voc and Isc of the fabricated solar cells also increased, which leads to the increase in solar cell conversion efficency also. The electron transfer mechanism of CdTe QDs between the components of QDSSC were given as follows [31] Generation of photo-electrons through the absorption of photon was represented by ۱ ܍܂܌+ = ૅܐ۱ ܍(܍܂܌+ )ܐ
(9)
Tranferring of photo-electrons towards the TiO2 photoelectrodes was represented by ۱ ܍(܍܂܌+ )ܐ+ ۽ܑ܂ = ۱ )ܐ(܍܂܌+ ۽ܑ܂ ()܍
(10)
Hole transfer between the QDs and the polysulfide electrolyte was given by ۱ )ܐ(܍܂܌+ ܁ି = ۱ ܍܂܌+ ܠ܁ି
(11)
The highest efficiency of η=2.24% was achieved for Gd:CdTe QDs sensitized solar cells with 10% of Gd substitution, it may be attributed to the superior quatum yield of prepared QDs. The conversion efficiency of Gd:CdTe QDs sensitized quantum dots was decreased beyond 10%
dopant concentration [32-34]. It may be due to the lattice dilocations or distrotions created by the dopant ions.
This alattice mismatches reduces the diffusion of electrons towards TiO2
nanocrystalline layers resulting in reduction of solar cell efficiency. According to J-V measurements, CdTe QDs with 10% of Gd doping was used as the potential sensitizer in QDSSC. The solar cell efficiency of prepared QDs still low when compared with theoretical effciency of QDSSC. It may be due to less active sites of TiO2 nanoparticles, which limits the QDs sensitization process. To increase the active sites of TiO2 nanoparticles and amount of QDs sensitization, one dimensional TiO2 nanostructures e.g. nanorods, nanotubes, etc, can be used. The solar cell efficiency of fabricated QDSSC can also be enhanced by the substitution of graphene layers instead of TiO2 nanoparticles [35-38]. 4. CONCLUSION Gd:CdTe QDSSC with polysulfide electrolyte was fabricated and its photovoltaic performance was analyzed. Pure and Gd:CdTe QDs were successfully synthesized in aqueous phase. Structural and optical behaviors of the prepared QDs were studied under different dopant concentrations. Powder XRD analysis confirms the cubic zinc blende crystalline structure of prepared QDs. It also confirms that the mean crystallite size was increased with dopant concentration in the range between 2.1 nm and 4.2 nm. Band gap energy variation with dopant concentration was estimated by UV-vis absorption spectral analysis. Fluorescence QE result reveals that the doping concentration favors fluorescence behavior of CdTe QDs. Highest QE of 67% was achieved for 10% Gd doping. The weak fluorescence behavior at higher doping concentration was due to fluorescence quenching. The variations in COO- stretching vibrations
of MSA molecules confirm the effective capping of MSA molecules over the prepared QDs surface. TGA result reveals that the thermal stability of prepared QDs was significantly increased by Gd doping. The photovoltaic response of Gd:CdTe QDs was studied and the higher solar cell efficiency of about 2.24% was achieved for 10% of Gd doping. From the overall observations, we can conclude that the CdTe QDs with 10% of Gd doping will be used as the potential sensitizer for QDSSC. Declarations of interest: None Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References 1.
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List of Tables Table 1. Crystallite size, lattice parameter and inter-planar distance of Gd:CdTe QDs for different doping concentrations Table 2. QEs for pure and Gd:CdTe QDs for different doping concentrations with the corresponding emission and absorption maximum and band gap energy Table 3. Functional groups of MSA capped CdTe and Gd:CdTe QDs Table 4.
Photovoltaic variables of Gd:CdTe QDs sensitized solars cells for different Gd
doping concentrations
Table 1 XRD analysis Gd concentration Lattice parameters
Inter-planar spacing Mean crystallite size D
X (Å)
d (Å)
(nm)
Pure CdTe
6.33
3.65
2.1
0.01
6.35
3.67
2.3
0.03
6.39
3.76
2.5
0.05
6.42
3.87
3.2
0.1
6.48
3.96
3.6
0.2
6.52
4.07
4.2
Table 2 Doping
Absorption
Emission
Quantum
Band gap
Particle size
concentration
maximum
maximum
yield (%)
energy (eV)
from UV
(X)
(nm)
(nm)
Pure CdTe
470
517
32
2.64
1.8
0.01
474
521
35
2.61
2.1
0.03
478
526
42
2.59
2.3
0.05
482
531
44
2.57
2.9
0.1
487
536
67
2.54
3.1
0.2
492
542
40
2.52
3.9
(D, nm)
Table 3 Frequency (cm-1) Functional Groups MSA
CdTe
Gd:CdTe
-----
3440
3430
OH stretching vibrations
2931
2916
2908
CH2 vibrations
2638&2553
-----
-----
S-H vibrations
1697
-----
-----
C=O vibrations
-----
1566
1558
COO- assymmetric vibrations
1419
1388
1388
COO- symmetric vibrations
1311
-----
-----
C-O stretching vibrations
1180
-----
-----
C-O stretch
933
995
995
C-H bend
678
671
671
C-S vibrations
Table 4 Sample Name
Doping concentration
Voc (V)
Jsc (mA)
FF
η (%)
TiO2/CdTe
0
0.68
1.05
0.56
0.48
TiO2/Gd:CdTe-1
1%
0.79
1.49
0.44
0.62
TiO2/Gd:CdTe-2
3%
0.79
2.47
0.44
1.03
TiO2/Gd:CdTe-3
5%
0.78
3.02
0.52
1.48
TiO2/Gd:CdTe-4
10 %
0.78
4.00
0.59
2.24
TiO2/Gd:CdTe-5
20%
0.78
4.05
0.29
1.11
List of Figures Figure 1. Schematic representation of the synthesis protocol for (a) MSA capped CdTe QDs and (b) Gd:CdTe QDs Figure 2.
Schematic representation of Gd:CdTe QDs sensitized TiO2 photoelectrodes
Figure 3. XRD patterns of (a) pure and Gd:CdTe QDs for different concentrations (x= 0.01, 0.03, 0.05, 0.1 and 0.2) and (b) Gaussian fit for CdTe QDs with 5% Gd doping Figure 4. (a) Lattice parameter and interplanar distance of pure and Gd:CdTe QDs with different doping concentrations (b) Crystallite size variations of pure and Gd:CdTe QDs with different doping concentrations Figure 5.
HRTEM images of (a) MSA capped pure and (b) 5% Gd doped CdTe QDs
Figure 6. UV-vis absorption spectra of pure and Gd:CdTe QDs for different doping concentrations Figure 7. Fluorescence spectra of pure and Gd:CdTe QDs for different doping concentrations at an excitation wavelength of 420 nm Figure 8.
(a) EDX spectrum of Gd:CdTe QDs and (b) Quantitative results
Figure 9. FT-IR spectra of MSA, MSA capped CdTe and Gd:CdTe QDs Figure 10. TGA curves of MSA capped CdTe QDs and Gd:CdTe QDs with the scanning rate of 10°C/min Figure 11. Current - Voltage characteristics of Gd:CdTe QDs sensitized solar cells for different doping concentrations
Figure 1
(a)
(b)
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
INVESTIGATIONS OF RARE EARTH DOPED CdTe QDs AS SENSITIZERS FOR QUANTUM DOTS SENSITIZED SOLAR CELLS Ayyaswamy Arivarasana*, Sambandam Bharathib, Sozhan Ezhilarasia, Surulinathan Arunpandiyan, M.S. Revathya and Ramasamy Jayavelc a
Department of Physics, International Research Centre, Kalasalingam Academy of Research and Education, Krishanakoil - 626 126, Tamilnadu, India b
NGSeq Analytics LLC, San Diego, California, United States of America c
Crystal Growth Centre, Anna University, Chennai - 600 025, India
*Corresponding Author: [email protected] (A.Arivarasan)
Highlights
Gadolinium doped CdTe QDs (Gd:CdTe) were successfully synthesized
Dopant concentration dependent structural and optical properties were investigated
Incorporation of Gd ions gradually increase the optical response of the CdTe QDs
Gd:CdTe QDs sensitized solar cells were fabricated with TiO2 photo anode
Highest solar cell efficiency of 2.24 % were achieved for 10% Gd doped CdTe QDs
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Nil
S0022-2313(19)31208-6
DOI:
https://doi.org/10.1016/j.jlumin.2019.116881
Reference:
LUMIN 116881
To appear in:
Journal of Luminescence
Received Date: 16 June 2019 Revised Date:
20 October 2019
Accepted Date: 5 November 2019
Please cite this article as: A. Arivarasan, S. Bharathi, S. Ezhilarasi, S. Arunpandiyan, M.S. Revathy, R. Jayavel, Investigations of rare earth doped CdTe QDs as sensitizers for quantum dots sensitized solar cells, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.116881. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.
INVESTIGATIONS OF RARE EARTH DOPED CdTe QDs AS SENSITIZERS FOR QUANTUM DOTS SENSITIZED SOLAR CELLS Ayyaswamy Arivarasana*, Sambandam Bharathib, Sozhan Ezhilarasia, Surulinathan Arunpandiyan, M.S. Revathya and Ramasamy Jayavelc a
Department of Physics, International Research Centre, Kalasalingam Academy of Research and Education, Krishanakoil - 626 126, Tamilnadu, India b
NGSeq Analytics LLC, San Diego, California, United States of America c
Crystal Growth Centre, Anna University, Chennai - 600 025, India
*Corresponding Author: [email protected] (A.Arivarasan)
Graphical Abstract
INVESTIGATIONS OF RARE EARTH DOPED CdTe QDs AS SENSITIZERS FOR QUANTUM DOTS SENSITIZED SOLAR CELLS Ayyaswamy Arivarasana*, Sambandam Bharathib, Sozhan Ezhilarasia, Surulinathan Arunpandiyan, M.S. Revathya and Ramasamy Jayavelc a
Department of Physics, International Research Centre, Kalasalingam Academy of Research and Education, Krishanakoil - 626 126, Tamilnadu, India b
NGSeq Analytics LLC, San Diego, California, United States of America c
Crystal Growth Centre, Anna University, Chennai - 600 025, India
*Corresponding Author: [email protected] (A.Arivarasan) Abstract Gd doped CdTe (Gd:CdTe) QDs sensitized working electrodes were fabricated for QDSSC applications. In this fabrication process, mercapto succinic acid capped Gd:CdTe QDs were synthesized by colloidal method and used as the sensitizer. Improved optical properties of the prepared QDs were examined by optical absorption and emission spectral analysis. Fluorescence quantum yield measurement reveals that 10% Gd:CdTe QDs shows the highest quantum yield of 67%. XRD analysis confirms the cubic zinc blende crystalline structure of the prepared QDs and the dopant concentration dependent cell parameters and crystallite sizes were revealed. Performance of capping molecules over the prepared QDs was analyzed by FT-IR studies. Enhanced photovoltaic performance of prepared pure and doped QDs was analyzed through J-V characteristic curves, which show better photovoltaic response with an efficiency of 2.24% for 10% Gd:CdTe QDs. Keywords: Quantum Dots; Gd:CdTe; working electrodes; QDSSC; photo-conversion efficiency.
1.
INTRODUCTION Recent advancement of dye sensitized solar cell (DSC) was quantum dots sensitized solar
cells (QDSSC). Sensitizers were the only physical difference between DSC and QDSSC. All the other active components were remains same. Mathew et al in 2014 reported the highest photoconversion efficiency of 13% for DSC solar cells [1]. Tremendous efforts were made to enhance the efficiency of DSC. Among these attempts, utilization of quantum dots (QDs) as the sensitizer was the important one [2, 3]. Since QDs were used as sensitizers, DSC can be named as QDSSC. QDSSC raise the probability of increasing solar cell efficiency at low cost. The size induced optical properties of QDs was used as a key factor in third generation solar cells [4, 5]. Along with this, QDs have some attractive characteristics like large intrinsic dipole moment [6], high absorption coefficient [7, 8], broad excitation spectra, narrow emission spectra, photostability and low cost, which enhances the efficiency of QDSSC. The multi-exciton generation behavior of QDs [9-11] raises the probability of overcoming the Schottky Queisser’s limit (32.9 %) of single junction solar cells [12]. It arises due to lack of photo-electrons generation, by the incident of lesser energy photons than the band gap energy. Recently, QDs have attracted a large attention due to their outstanding optoelectronic behavior by its size tunable optical properties. The absorption window of semiconductor QDs can be altered by altering the size and compositions of QDs. Even though QDs play a crucial role in solar cell applications, the photo-conversion efficiency of QDSSC was still low. The benchmark efficiency of cadmium based QDSSC was 7.24% reported for CdS/CdSe QDs sensitize solar cells [13]. But the photo-conversion efficiency of perovskite quantum dots based solar cells was far ahead than normal QDSSC. The highest efficiency of perovskite QDs based solar cells was 13.43% for CsPbI QDs arrays as reported by Sanihira et al in 2017 [14]. The
lacking of QDSSC efficiency resulted from the poor transfer and high recombination rate of charge carriers in between the various active components of the solar cell. Numerous attempts have been made to overcome these issues. Particularly, recent research works focused on the fabrication of doped QDs as sensitizers to enhance the charge transfer rate by reduces the recombination rate of charge carriers. Recently, fabrication of cadmium chalcogenide sensitizers (CdS, CdSe and CdTe) attracted a large attention in QDSSC. This was due to their simple fabrication procedures, optical tuneability by their size and compositions and the possibility of multi exciton generation [15]. Among these cadmium chalcogenides, CdTe QDs play a crucial role in optoelectronic applications. The absorption window of CdTe QDs can be tailored to absorb the major intensity radiations (visible and NIR radiations) of solar spectrum by tuning its size and compositions. During the past few years, large number of attempts was made to alter the physical and chemical properties of CdTe QDs, including core/shell CdTe QDs, alloyed CdTe QDs, doped CdTe QDs [16], etc. Recently, some post thermal treatments were also tested to enhance the photovoltaic response of CdTe QDs. But the overall efficiency of solar cells still remains low. This was mainly due to the poor charge carrier transfer rate between the QDs and TiO2 nanoparticles and it can be enhanced by the addition of dopant materials in QDs. By adding dopant materials, the conduction and valence energy levels of CdTe QDs were altered equivalent to the wide band gap semiconductors. But the number of articles, which explain the behavior of doped CdTe QDs and its photovoltaic performance, were very less. Here we have reported the improved photovoltaic performances of rare earth doped CdTe QDs. Rare earth elements were well known for their unique optical properties. Among the various rare earth elements, gadolinium (Gd) play a vital role in fluorescence applications [17,
18]. In general Gd (III) ions were used as a fluorescence sensitizer. In Gd (III) ions, the fluorescence was produced by the electronic transitions between 4f orbitals. Since Gd (III) ions were used as the optical enhancer, it was used to enrich the optical behavior of CdTe QDs. In this work, we have prepared Gd doped CdTe (Gd:CdTe) QDs sensitized TiO2 photoelectrodes and its photovoltaic performance was studied by the fabrication of sandwich type QDSSC. Thiol capped Gd:CdTe QDs were synthesized through colloidal route in normal atmosphere. The prepared Gd:CdTe QDs were analytically characterized to reveal its dopant concentration dependent structural and optical properties. QDSSC was fabricated using Gd:CdTe QDs sensitized TiO2 photo-electrode and sulfide/polysulfide electrolyte. The photovoltaic response of the prepared QDSSC was studied under standard illumination conditions (100 mW/ cm2). 2. Experimental Techniques 2.1. Precursors Cadmium chloride monohydrate (CdCl2.H2O), air stable potassium tellurite (K2TeO3), gadolinium nitrate and sodium borohydride (NaBH4) were used for the QDs synthesis and they were procured from Sigma Aldrich chemicals. The purchased chemical are in analytical grade and used without further purifications. All the solutions were prepared under normal atmosphere using Millipore water as a solvent. 2.2. Preparation of pure CdTe QDs Colloidal route was adopted to prepare thiol capped CdTe QDs [19]. 0.25 M CdCl2 aqueous solution was prepared in 100 ml of Millipore water and 0.402 g of mercapto succinic
acid was added as the capping agent. 2 M NaOH aqueous solution was used to adjust the pH of reaction to 7. 0.253g of K2TeO3 was added into the reaction mixture as the tellurium precursor. 0.1 g of NaBH4 was used as the reducing agent to produce CdTe monomers. Rate of CdTe monomers formation and the preparation of QDs from CdTe monomers were facilitated by the refluxing technique at 100 °C for 30 min. Finally, aliquots were collected at regular interval for optical characterizations and the resultant solution was precipitated with ethanol. Finally the precipitates were collected and air dried at 80 °C for 3 h. The reaction mechanism for the formation of CdTe QDs was given in equation 1. ܍܂ష
۱܌ା + ܘ ۯ܁ۻ۶ିૠ/℃ → ۱܍܂܌
(1)
2.3. Synthesis of Gd:CdTe QDs CdTe QDs synthesis was repeated with the appropriate amount of Gd to obtain Gd:CdTe QDs. Dopant concentration was varied between 1% and 20% (Cd1-xGdxTe, where x= 0.01, 0.03, 0.05, 0.1 and 0.2). The synthesis protocol was explained in Figure 1 and the resultant equation was given in equation 2. ܍܂ష
(۱܌ା ۵܌ା ) + ܘ ۯ܁ۻ۶ିૠ/℃ → ۵܌: ۱܍܂܌
(2)
2.4. Gd:CdTe QDs Sensitized Solar Cell Fabrication Sandwich type QDSSC was fabricated by assembling QDs sensitized working electrodes and graphite coated counter electrode. In this process, TiO2 nanocrystalline layer was coated over the ITO substrate uing doctor blade technique. Thickness of the TiO2 nanocrystalline layer was fixed as 50 µm using scotch tape. The pre-synthesized Gd:CdTe QDs by colloidal route was
deposited over TiO2 (~25 nm) surface using mercapto succinic acid as bi-linker molecules and the schematic representation of Gd:CdTe QDs sensitized TiO2 photoelectrode was shown in Figure 2. Prepared photoelectrodes and counter electrode were assembled together to form sandwich structured solar cells using hot melt tape as the spacer. Space between the electrodes was filled with sulfide/polysulfide electrolyte and it was acts as the regenerative redox couple [20]. 2.5. Characterization Techniques Rigaku Miniflex II-C X-ray diffractometer was used to reveal the crystalline structure of prepared QDs at the scanning rate of 3°/min. Absorption behavior of prepared QDs was measured using UV-WIN T90+ UV-vis spectrophotometer and the corresponding emission property was studied using Horiba Fluorolog-3 spectrofluorometer. The quantum yield (QE) measurement was carried out using Rhodamine 6G as the reference material, under standard excitation conditions (excitation wavelength λ=400 nm). Functional group analysis was carried out using Perkin Elmer Spectrum1 FT-IR spectrometer. TG-DTA: SII 6300 EXSTAR thermal analyzer was used to examine the thermal stability of prepared QDs at the scanning rate of 10°C/min. The solar cell performance was studied using a Keithley 2400 source meter (Oriel, Model: 91192) under standard illumination condition (100 mWcm-2). 3. RESULTS AND DISCUSSION 3.1.XRD Analysis XRD patterns of pure and Gd:CdTe QDs were shown in Figure 3(a). Diffraction pattern of pure CdTe QDs shows the appearance of diffraction peaks at (2θ) 24.4, 29.2, 40.3 and 47.4° related to (111), (200), (220) and (311) crystalline planes, which confirms the cubic zinc blende
crystalline structure (JCPDS No: 65-1046). This XRD pattern is in good agreement with the previous results reported by Maity et al, 2015 [21], Xiao et al, 2012 [22] and Silva et al, 2012 [23]. The XRD patterns of Gd:CdTe QDs shows an overlapped diffraction peaks of CdTe and GdTe QDs, due to the mutual interactions between the oxygen ions and Gd, Te ions. The major diffraction peak by the interaction between Gd and Te ions was observed at 29.1° corresponding to (200) crystalline plane. At lower dopant concentration (1 %), the diffraction intensity of (200) plane was minimum and it was gradually increased with dopant concentration as shown in figure 3 (a). At the same time, the diffraction intensity of (111) crystalline plane of CdTe QDs were suppressed by higher intensity (200) plane. It may be due to the increased molecular interactions between Gd and Te ions at higher dopant concentration. This overlapping of diffraction peaks was clearly identified by the Gaussian fittings as shown in Figure 3(b). The diffraction peak positions of Gd:CdTe QDs lies between the pure CdTe QDs and GdTe with cubic phase (JCPDS No: 65-5020) due to the mutual interactions of Gd, Cd ions with Te ions. It confirms that, the Gd3+ ions were substituted on CdTe lattices. Incorporation of Gd3+ ions in CdTe lattice reduces the crystalline nature of CdTe QDs, which was confirmed by the amorphous like XRD patterns of Gd:CdTe QDs. Similar diffraction patterns were observed in our previous report, for Zn and S co-doped CdTe QDs [16]. Along with this difference in peak positions, a clear visible shift in diffraction peak positions of pure CdTe QDs towards the lower diffraction angles depending on dopant concentration was also observed. This was due to the replacement of lower ionic radii Cd2+ (95 pm) ions by higher ionic radii Gd3+ (107 pm) ions. The increase in size and lattice parameters was clearly identified by the lower angle shift of diffraction peaks with doping concentration. The mean crystallite size was calculated by using Scherer’s formula
۲ = ૃܓ/ ܛܗ܋ી where
(3)
k - Scherer’s constant (k=0.9) D- Mean crystallite size λ- Wavelength of the X-ray radiation (CuKα=1.540Å) β- Full width at half maximum (FWHM) of the diffraction peaks θ- Angle of diffraction. The average crystalline size of pure CdTe QDs was found to be 2.1 nm and it was
increased with dopant concentration. Increase in crystallite size with dopant concentration was confirmed by the reduction in peak broadening of XRD pattern as shown in Figure 3(b). Variation in crystallite sizes with dopant concentrations were listed in Table 1. The crystallite size calculation reveals that the doping of Gd3+ ions could increase the size of CdTe QDs as shown in Figure 4(a). The lattice parameter and inter-planar distances were calculated from XRD pattern and listed in Table 1. Change in crystallite size with dopant concentration was shown in Figure 4(b). The inter-planar spacing of CdTe lattices was gradually increased with dopant concentration. The inter-planar spacing of pure CdTe QDs is 3.65 Å and it is increased to 3.96 Å when the dopant concentration was increased to 10%. This was due to the substitution of larger ionic radii Gd3+ ions (107 pm) in lower ionic radii Cd2+ sites (95 pm). 3.2. Morphological analysis - HRTEM Morphological properties of pure and 5% Gd doped CdTe QDs were examined by HRTEM analysis and the corresponding HRTEM images were shown in Figure 5. As prepared QDs were utilized for the HRTEM analysis. HRTEM images indicates that the pure CdTe QDs were predominantly spehrical in shape with well ordered grain bounderies. This was due to the lower aggreagtion level between CdTe QDs by effective capping of MSA molecules around the
CdTe QDs surface. HRTEM images also reveals the homogeneous distribution of pure CdTe QDs with particles sizes of around 2 nm. When compared to pure CdTe QDs, Gd:CdTe QDs showed larger sized paricles with small aggregations. Also the grain boundaries were not clearly visible, which indicates that the effective capping nature of thiols was reduced by dpoing process. The crystallinity of CdTe QDs was also reduced with addition of Gd ions. The size variation of CdTe QDs were clearly observed in HRTEM images by the addition of Gd ions. 3.3.UV-vis Absorption Studies Absorption behavior of QDs was analyzed by UV-vis absorption spectroscopy and the recorded absorption spectra were shown in Figure 6. Pure and Gd:CdTe QDs shows a well resolved absorption maximum at around 470 nm. But, there was a significant red shift observed in absorption peaks of Gd:CdTe QDs with different dopant concentrations, which reveals that Gd3+ ions were substituted in CdTe crystal lattices, resulting in the formation of CdGdTe alloys, which was confirmed by the XRD analysis. In other cases, Gd3+ ions were doped on the surface of the CdTe QDs and reacted with excessive Te2- ions, which results in the formation of CdTe:GdTe nanocomposites rather than CdGdTe alloys. In that case, the absorption spectra show two different excitonic absorption maxima at different wavelengths corresponding to CdTe and GdTe QDs. Size variation with increasing dopant concentration was clearly identified by the red shift in absorption maximum. This was in good agreement with XRD analysis. The band gap energy (Eg) of the prepared QDs was calculated by ۳ = ܋ܐ/ૃ where,
Eg - Band gap energy (eV) h - Plank’s constant (h=6.625*10-34 m2 Kg/s)
(4)
c - Velocity of light (c=3*108 m/s) λ - Absorption maxima of the prepared QDs The band gap energy for Gd:CdTe QDs varies between 2.64 eV and 2.52 eV with increasing dopant concentration. It reveals that the band gap energy decreases with increasing dopant concentration. The average particle size of the prepared QDs was estimated using the following relation [7] ࡰ = (ૢ. ૡૠૡ ∗ ିૠ )ࣅ − (. ૠૠ ∗ ି )ࣅ + . ࣅ − ૢ. ૡ
(5)
Where ‘D’ is particle size and ‘λ’ is the absorption maximum. The particle size varies between 1.8 nm and 3.9 nm with increasing dopant concentration. From UV-vis results, it was concluded that the QDs sizes were increased with dopant concentration and the measured values were listed in Table 2. 3.4.Fluorescence Spectroscopy Analysis Room temperature fluorescence spectra of as-prepared QDs were measured under the excitation wavelength of 420 nm and the recorded emission spectra were shown in Figure 7. By the absorption of photon, electron-hole pair was created in CdTe QDs and the excessive energy was released in the form of photon during electron-hole recombination process. It was clearly observed that pure and Gd:CdTe QDs possess a strong near band edge emission. The fluorescence spectra for both pure and Gd:CdTe QDs show a well resolved emission peak at around 517 nm. The narrow width of emission peaks clearly indicates the homogeneous distribution of prepared QDs. However, a significant red shift and intensity variations were
observed for Gd:CdTe QDs with dopant concentration. The red shift in emission maximum was attributed to the alloying effect and size variation of CdGdTe QDs by the incorporation of Gd3+ ions in CdTe crystal lattices. The improved fluorescent behavior of Gd:CdTe QDs was observed at lower doping concentration (up to 10%) [24], which was due to the influence of Gd3+ ions. In contrast, the fluorescence intensity was further decreased when the doping concentration exceeds more than 10% which may be due to quenching effect. The intensity variation in emission peaks clearly indicates the quantum yield (QE) difference between pure and Gd:CdTe QDs. The QEs for the QDs were calculated using the following relation [25,26]. ࢙ = ࢘ ∗ (࢘ /࢙ ) ∗ (ࡵ࢙ /ࡵ࢘ ) ∗ (ࣁ࢙ /ࣁ࢘ ) where,
(6)
Φ - Quantum yield (QE) A - Absorbance I - Integral area of the fluorescence spectrum Η - Refractive index
The suffix ‘r’ and ‘s’ stands for reference and sample respectively. The QE for pure and Gd:CdTe QDs were calculated with Rhodamine 6G as the reference material. QE of 32% was calculated for pure CdTe QDs at a refluxing time of 30 min and the QE get increases with dopant concentration and reaches the maximum of 67% for 10% Gd doping [24]. Beyond this concentration, QEs were decreased by the fluorescence quenching effect. The calculated values of QEs with different doping concentration were listed in Table 2.
3.5.EDX Analysis Energy dispersive X-ray spectroscopy (EDX) technique was adopted to examine the elemental composition of prepared QDs and the recorded EDX spectrum with quanitative measurements was shown in Figure 8(a & b). EDX spectrum shows the corresponding peaks for Cd, Gd, Te and S elements. The presence of S was due to capping of MSA on the QDs surface and the presence of Gd indicates the formation of Gd:CdTe QDs. The quantitative analysis results indicates that nearly 3% of Gd was replaced on CdTe QDs for 3% Gd:CdTe QDs. 3.6. FT-IR Analysis Presence of functional groups of the prepared QDs was identified by their molecular vibrations using FT-IR analysis and the recorded spectra were shown in Figure 9. In prepared QDs (pure and Gd:CdTe QDs), the asymmetric and symmetric stretching vibrations due to COOof MSA molecule was observed at 1566 cm-1 and 1388 cm-1, which confirms that the MSA molecules were coated over the CdTe QDs. The adsorption of water molecules by the prepared QDs was revealed by the presence of O-H stretching vibrations at around 3410 cm-1. C-S stretching vibration of the capping agent was observed at 1200 cm-1. Lower frequency shift in CH2 active modes of MSA molecules (2931 cm-1) was observed at 2916 and 2908 cm-1 for MSA capped CdTe QDs and Gd:CdTe QDs respectively. It reveals that the surface of MSA molecules was affected by van der Waals fields CdTe and Gd:CdTe QDs [27]. Disappearance of S-H vibrations for MSA molecules (2638 cm-1 and 2553 cm-1), in prepared QDs, confirms that the surface of MSA molecules was modified into Cd-SR bond by absorption of Cd ions. Observed functional groups were listed in Table 3.
3.7.Thermogravimetric analysis (TGA) Thermal stability of prepared QDs (pure CdTe QDs and Gd:CdTe QDs) was studied up to 1000°C by thermogravimetric analysis and the recorded thermograms were shown in Figure 10. Similar types of TGA curves were recorded for both the samples with three sequential weight losses. Evaporation of adsorbed water molecules by the prepared QDs, leads to the first weight loss at around 100 °C, as discussed by the FT-IR analysis. Second weight loss at around 300°C was attributed to the removal of chloride ions in the precursors [28]. Dissociation of Cd-MSA complexes leads to the final weight loss at around 700°C. TGA result confirms that the prepared QDs were thermally stable up to 800 °C and the thermal stability of CdTe QDs was significantly reduced when doping with Gd ions. 3.8. Photovoltaic Studies Prepared QDs were sensitized over TiO2 nanoparticles coated electrode and utilized as the working or active electrode for the fabrication of QDSSC and the performance of fabricated solar cells was studied by I-V characteristics under standard illumination condition (100 mW/cm2). I-V characteristic curves of fabricated working electrode were shown in Figure 11 and derived solar cell parameters, such as, conversion efficiency (η), fill factor (FF), open circuit voltage (Voc), and short circuit current (Isc) were listed in Table 4. The photovoltaic conversion efficiency (η) and the fill factor (FF) of Gd:CdTe QDs sensitized solar cells were calculated from the following relations [29] િ = ۸ ∗ ܋ܗ܄ ∗ ܋ܛ۴۴ ∗ ۯ/ܖܑ۾
(7)
۴۴ = (۸) ܕ܄ ∗ ܕ/(۸) ܋ܗ܄ ∗ ܋ܛ
(8)
where, A - area of solar cell (1 cm2), Jm - power maximum, Vm - voltage maximum and Pin intensity of incident radiation (100 mW/cm2) Poor photostability of CdTe QDs in polysulfide electrolyte leads to the very less photovoltaic conversion efficiency of η = 0.48% for pure CdTe QDs [30]. Photoconversion efficiency of TiO2 photoelectrodes increases with the sensitization of Gd:CdTe QDs. Addition of dopant ions in CdTe QDs create trap states between the highest occupied and lowest unoccupied energy levels of CdTe QDs. This trap states reduces the electron-hole recombination rate leads to the increase in charge transfer rate betweem QDs and TiO2 nanoparticles. Since, the charge transfer rate gets increased with dopant concentration, Voc and Isc of the fabricated solar cells also increased, which leads to the increase in solar cell conversion efficency also. The electron transfer mechanism of CdTe QDs between the components of QDSSC were given as follows [31] Generation of photo-electrons through the absorption of photon was represented by ۱ ܍܂܌+ = ૅܐ۱ ܍(܍܂܌+ )ܐ
(9)
Tranferring of photo-electrons towards the TiO2 photoelectrodes was represented by ۱ ܍(܍܂܌+ )ܐ+ ۽ܑ܂ = ۱ )ܐ(܍܂܌+ ۽ܑ܂ ()܍
(10)
Hole transfer between the QDs and the polysulfide electrolyte was given by ۱ )ܐ(܍܂܌+ ܁ି = ۱ ܍܂܌+ ܠ܁ି
(11)
The highest efficiency of η=2.24% was achieved for Gd:CdTe QDs sensitized solar cells with 10% of Gd substitution, it may be attributed to the superior quatum yield of prepared QDs. The conversion efficiency of Gd:CdTe QDs sensitized quantum dots was decreased beyond 10%
dopant concentration [32-34]. It may be due to the lattice dilocations or distrotions created by the dopant ions.
This alattice mismatches reduces the diffusion of electrons towards TiO2
nanocrystalline layers resulting in reduction of solar cell efficiency. According to J-V measurements, CdTe QDs with 10% of Gd doping was used as the potential sensitizer in QDSSC. The solar cell efficiency of prepared QDs still low when compared with theoretical effciency of QDSSC. It may be due to less active sites of TiO2 nanoparticles, which limits the QDs sensitization process. To increase the active sites of TiO2 nanoparticles and amount of QDs sensitization, one dimensional TiO2 nanostructures e.g. nanorods, nanotubes, etc, can be used. The solar cell efficiency of fabricated QDSSC can also be enhanced by the substitution of graphene layers instead of TiO2 nanoparticles [35-38]. 4. CONCLUSION Gd:CdTe QDSSC with polysulfide electrolyte was fabricated and its photovoltaic performance was analyzed. Pure and Gd:CdTe QDs were successfully synthesized in aqueous phase. Structural and optical behaviors of the prepared QDs were studied under different dopant concentrations. Powder XRD analysis confirms the cubic zinc blende crystalline structure of prepared QDs. It also confirms that the mean crystallite size was increased with dopant concentration in the range between 2.1 nm and 4.2 nm. Band gap energy variation with dopant concentration was estimated by UV-vis absorption spectral analysis. Fluorescence QE result reveals that the doping concentration favors fluorescence behavior of CdTe QDs. Highest QE of 67% was achieved for 10% Gd doping. The weak fluorescence behavior at higher doping concentration was due to fluorescence quenching. The variations in COO- stretching vibrations
of MSA molecules confirm the effective capping of MSA molecules over the prepared QDs surface. TGA result reveals that the thermal stability of prepared QDs was significantly increased by Gd doping. The photovoltaic response of Gd:CdTe QDs was studied and the higher solar cell efficiency of about 2.24% was achieved for 10% of Gd doping. From the overall observations, we can conclude that the CdTe QDs with 10% of Gd doping will be used as the potential sensitizer for QDSSC. Declarations of interest: None Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References 1.
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List of Tables Table 1. Crystallite size, lattice parameter and inter-planar distance of Gd:CdTe QDs for different doping concentrations Table 2. QEs for pure and Gd:CdTe QDs for different doping concentrations with the corresponding emission and absorption maximum and band gap energy Table 3. Functional groups of MSA capped CdTe and Gd:CdTe QDs Table 4.
Photovoltaic variables of Gd:CdTe QDs sensitized solars cells for different Gd
doping concentrations
Table 1 XRD analysis Gd concentration Lattice parameters
Inter-planar spacing Mean crystallite size D
X (Å)
d (Å)
(nm)
Pure CdTe
6.33
3.65
2.1
0.01
6.35
3.67
2.3
0.03
6.39
3.76
2.5
0.05
6.42
3.87
3.2
0.1
6.48
3.96
3.6
0.2
6.52
4.07
4.2
Table 2 Doping
Absorption
Emission
Quantum
Band gap
Particle size
concentration
maximum
maximum
yield (%)
energy (eV)
from UV
(X)
(nm)
(nm)
Pure CdTe
470
517
32
2.64
1.8
0.01
474
521
35
2.61
2.1
0.03
478
526
42
2.59
2.3
0.05
482
531
44
2.57
2.9
0.1
487
536
67
2.54
3.1
0.2
492
542
40
2.52
3.9
(D, nm)
Table 3 Frequency (cm-1) Functional Groups MSA
CdTe
Gd:CdTe
-----
3440
3430
OH stretching vibrations
2931
2916
2908
CH2 vibrations
2638&2553
-----
-----
S-H vibrations
1697
-----
-----
C=O vibrations
-----
1566
1558
COO- assymmetric vibrations
1419
1388
1388
COO- symmetric vibrations
1311
-----
-----
C-O stretching vibrations
1180
-----
-----
C-O stretch
933
995
995
C-H bend
678
671
671
C-S vibrations
Table 4 Sample Name
Doping concentration
Voc (V)
Jsc (mA)
FF
η (%)
TiO2/CdTe
0
0.68
1.05
0.56
0.48
TiO2/Gd:CdTe-1
1%
0.79
1.49
0.44
0.62
TiO2/Gd:CdTe-2
3%
0.79
2.47
0.44
1.03
TiO2/Gd:CdTe-3
5%
0.78
3.02
0.52
1.48
TiO2/Gd:CdTe-4
10 %
0.78
4.00
0.59
2.24
TiO2/Gd:CdTe-5
20%
0.78
4.05
0.29
1.11
List of Figures Figure 1. Schematic representation of the synthesis protocol for (a) MSA capped CdTe QDs and (b) Gd:CdTe QDs Figure 2.
Schematic representation of Gd:CdTe QDs sensitized TiO2 photoelectrodes
Figure 3. XRD patterns of (a) pure and Gd:CdTe QDs for different concentrations (x= 0.01, 0.03, 0.05, 0.1 and 0.2) and (b) Gaussian fit for CdTe QDs with 5% Gd doping Figure 4. (a) Lattice parameter and interplanar distance of pure and Gd:CdTe QDs with different doping concentrations (b) Crystallite size variations of pure and Gd:CdTe QDs with different doping concentrations Figure 5.
HRTEM images of (a) MSA capped pure and (b) 5% Gd doped CdTe QDs
Figure 6. UV-vis absorption spectra of pure and Gd:CdTe QDs for different doping concentrations Figure 7. Fluorescence spectra of pure and Gd:CdTe QDs for different doping concentrations at an excitation wavelength of 420 nm Figure 8.
(a) EDX spectrum of Gd:CdTe QDs and (b) Quantitative results
Figure 9. FT-IR spectra of MSA, MSA capped CdTe and Gd:CdTe QDs Figure 10. TGA curves of MSA capped CdTe QDs and Gd:CdTe QDs with the scanning rate of 10°C/min Figure 11. Current - Voltage characteristics of Gd:CdTe QDs sensitized solar cells for different doping concentrations
Figure 1
(a)
(b)
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
INVESTIGATIONS OF RARE EARTH DOPED CdTe QDs AS SENSITIZERS FOR QUANTUM DOTS SENSITIZED SOLAR CELLS Ayyaswamy Arivarasana*, Sambandam Bharathib, Sozhan Ezhilarasia, Surulinathan Arunpandiyan, M.S. Revathya and Ramasamy Jayavelc a
Department of Physics, International Research Centre, Kalasalingam Academy of Research and Education, Krishanakoil - 626 126, Tamilnadu, India b
NGSeq Analytics LLC, San Diego, California, United States of America c
Crystal Growth Centre, Anna University, Chennai - 600 025, India
*Corresponding Author: [email protected] (A.Arivarasan)
Highlights
Gadolinium doped CdTe QDs (Gd:CdTe) were successfully synthesized
Dopant concentration dependent structural and optical properties were investigated
Incorporation of Gd ions gradually increase the optical response of the CdTe QDs
Gd:CdTe QDs sensitized solar cells were fabricated with TiO2 photo anode
Highest solar cell efficiency of 2.24 % were achieved for 10% Gd doped CdTe QDs
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Nil