DOI:10.30919/esmm5f790

Received: 13 Apr 2020
Accepted: 26 Aug 2020
Published online: 31 Aug 2020

The Influence of Polysulfide Solvent on the Performance of Cadmium Sulfide Sensitized Zirconium Dioxide-Based Quantum Dots

Ravi V. Ingle, Abhijit T. Supekar and Vikram P. Bhalekar*, Bikram Prasad and Habib M. Pathan*

 

Advanced Physics Laboratory, Department of Physics, Savitribai Phule Pune University, Pune-411 007, India.

*Corresponding Authors E-mail: pathan@physics.unipune.ac.in (H. Pathan),  bhalekarvp@gmail.com (V. Bhalekar)

 

Abstract

The effect of two different types of polysulfide solvents (i.e., distilled water and methanol) was investigated for zirconium dioxide (ZrO2) based quantum dots sensitized solar cells (QDSSCs). This was mainly depending upon how easily the injection of electrons in the conduction band of CdS particles to the ZrO2 photoanode. Compared to that with methanol solvent-based polysulfide, distilled water-based polysulfide (S2−/Sn2−) electrolytes have efficient electron transportation characteristics at the interface of ZrO2/CdS photoanode and carbon counter electrode. Solar cell efficiency using distilled water-based polysulfide for ZrO2/CdS reaches 1%. The catalytic reaction due to incorporation of polysulfide solvents positively affects the solar cell performance as evident from Nyquist plots. Distilled water-based polysulfide electrolyte has significant impacts on the overall performance of QDSSCs.

 

Keywords: Zirconium dioxide, Cadmium Sulfide, Electron Transport, Polysulfide


1. Introduction

Last two decades, several metal oxides and chalcogenide semiconductors such as titanium dioxide (TiO2),[1] zinc oxide,[2] tungsten oxide,[3] zinc sulfide[4] and cadmium sulphide[5] were applied for the solar cells, supercapacitors and quantum dots application perspective. Zirconium dioxide or zirconia (ZrO2) is one of the newly explored metal oxides used as photoanodes in dye-sensitized solar cells (DSSCs) and quantum dots (QD) SSCs.[6] However, ZrO2 shows prolonged stability for QD based solar cells compared to the dye-based SCs even at the high temperature of oxidation.[7]

 

The reported band gaps of ZrO2 can be stretched between 3.35 to 5.11 eV depending upon the synthesis process and phase of the crystal structure.[8] The reported porous structure and crystallinity nature of ZrO2 provide surface sites for the reactants to satisfy all requirements for the light-harvesting photoanodes.[9] On account of all these characteristics, ZrO2 becomes a new class of photoelectrode for photovoltaic applications.[10, 11] Due to wide bandgap nature, ZrO2 is capable of absorbing only ultraviolet (UV) range of photons; however, these are only a small fraction of the solar spectrum. For harvesting visible light, it is necessary to extend their absorption range in the visible regions. Thus, it needs to combine two semiconductors films, i.e., one for UV light absorber (ZrO2) and one with the visible light absorber or sensitizers (CdS). Among the sensitizers, CdS is one of the efficient QD sensitizers for the application of QDSSCs because of its suitable bandgap (2.25 to 2.42 eV depending upon the synthesis process and phase of the crystal structure) for harvesting broad visible range of photons from the solar spectrum.[12-14]

Although ZrO2 is a direct band gap metal oxide having an optical band gap around 5.11 eV, it is transparent for the visible range of photons, possesses high refractive index, good adhesion to the substrates and exhibits high thermal stability.[15-18] After the deposition of CdS QDs over ZrO2 photoanode, a boundary layer in between energy levels of CdS QDs and conduction band of ZrO2 is formed.[19] For easy transition or injection of electrons, the conduction band edge of photoanodes must be at the lower level than the conduction band edge of sensitizers or window materials. However, in this case of ZrO2/CdS, the conduction band edge of ZrO2 is almost at the same level of the conduction band of CdS, resulting in the injection of electrons or smooth transition of electrons from the conduction band of CdS into the conduction band edge of ZrO2.[20] Therefore, there is little probability for electrons to get transition smoothly from the conduction band of CdS into the conduction band edge of ZrO2.

For the ZrO2/CdS based QDSSC, two types of CdS QD or sensitizers were grown by chemical route over ZrO2 films. Apart from these, two different solvents were used for the preparation of polysulfide i.e., distilled water and methanol. In the current study, Nyquist, Bode plots and equivalent circuits were used to analyze the role of polysulfide electrolytes for the performance of solar devices and their electron transport properties and the mechanism of QDSSC. Polysulfide electrolyte[21] is a well-known electrolyte for QDSSC because its redox couple (S2-/Sn2-) can stabilize most QDs.

 

2. Experiments

Zirconia powder, ethyl cellulose, terpineol, acetyl acetone, cadmium sulphate, thiourea, ammonium hydroxide, aqueous ammonia, sulfur powder, and sodium sulfide were purchased from SRL Chemicals Ltd. India. Methanol and ethanol were purchased from C. H. Fine Chemicals Co. Ltd. and used as received without any further purification.

2.1 Synthesis of CdS nanoparticles

The synthesis technique of colloidal CdS particles, bulk CdS and CdS QDs is mentioned in our previous published article.[21] The three different types of CdS quantum dots are named as type- a, type-b and type-c, which contains 0.05 M cadmium sulfate, 0.05 M thiourea, and 20% ammonium hydroxide at room temperature. Excess aqueous ammonia (NH4OH) was added to the growth solution to attain pH.

2.2 Fabrication of ZrO2 photoanodes

The ZrO2 powder and ethyl cellulose were grinded in mortar pastel by adding Terpineol solution, the assembly was maintained in an ultrasonication bath for 3 hrs. The acetyl acetone was added during the ultrasonication at room temperature. The slurry formed was pasted on the fluorine doped tin oxide (FTO) substrate by the doctor blade technique. The as synthesized type-a, type-b and type-c CdS nanoparticles were deposited on ZrO2 photoelectrode using a chemical bath deposition technique.

2.3 Synthesis of polysulfide electrolyte

A non-aqueous polysulfide redox electrolyte composed of 0.98 g Na2S in 22.5 mL methanol and 0.08 g sulfur powder in 5 mL ethanol solution is a mixed and grinded for 15 min. Finally, polysulfide electrolytes were synthesized in two different types of solvents methanol and distilled water. In the methanol case, 0.98 g of sodium sulfide (Na2S) was crushed using mortar pestle and 22.5 mL of methanol was used to form a solution. 0.08 g of sulfur powder with 5 mL of methanol was added into the resultant solution of Na2S to form methanol based polysulphide electrolyte; whereas in the distilled water case, 0.78 g of Na2S was dissolved in the 25 mL of distilled water, pallets of sodium hydroxide (NaOH) and of sulfur powder, were crushed and added to the Na2S solution to form the distilled water based polysulphide electrolyte.

The X-ray diffraction (XRD) (model: XRD, Rigaku‘‘D/B max-2400’’, Cu Kα, λ = 1.54 Å) was used to determine the crystalline nature, phase, and crystallite size of ZrO2 films. and field emission scanning electron microscopy (FESEM) (Carl Zeiss, Merlin Compact) techniques were used for the structural and morphological properties of the samples, respectively. Optical absorption spectra is obained using UV–Vis spectrophotometer (JASCO V-670) in the wavelength range of  200–1000 nm and the Electrochemical Impedance Spectroscopy (EIS) studied by Potentiostat/Galvanostat (IVIUM Vertex model) , whereas J-V characteristics were obtained from the 2420 Kethley Source meter.

Fig. 1 XRD patterns for (a) CdS sensitized ZrO2 photoanodes and (b) ZrO2 film on FTO substrate.

3. Results and Discussion

3.1 Structural properties

The XRD patterns of CdS, ZrO2, and ZrO2 based QDSSC sensitized by CdS are shown in Fig. 1(a). The observed diffraction peaks are corresponding to 24.2(011), 28.4(111), 31.7(220) and 34.4˚(221) for the monoclinic (m) phase of ZrO2 confirmed by JCPDS card no. 37-1484. But 24.2, 34.4, 40.9, 54.3 and 55.7˚ show doublet peaks with some other unknown peaks, which are the significant peaks assigned for m-ZrO2, as shown in Fig. 1(b). The reported phase for this peak is due to O2-deficient ZrO0.35 confirmed by JCPDS card no. 17-0385, space group P6322 having the hexagonal phase not shown in Fig. 1.[8] The characteristic peaks along (111), and (200) planes at 2θ values are at 26.7, 50.3 and 60.1˚ corresponding to CdS with the cubic crystal structure (JCPDS Card No. 27-0997). However, the characteristic peaks at 35.3, 50.1 and 59.8˚ having lattice planes (110), (200) and (211) respectively confirmed the tetragonal phase of ZrO2 (JCPDS card No. 05-0665). The crystal phase purity shows that monoclinic, tetragonal and cubic phases are 67%, 22%, and 10% respectively whereas the remaining 1% is due to O2-deficient ZrO0.35. The diffraction peaks for the m-ZrO2 show the highest intensities compare to c-ZrO2 and t-ZrO2 which represents m-ZrO2 which is more crystalline nature than c-ZrO2 and t-ZrO2 as shown in Fig. 1 (b).

3.2 Optical properties

The deposition of three types of colloidal CdS particles over ZrO2 photoanode is carried out using a chemical method. The band gap (Eg) for CdS type-a, b and c are 2.38, 2.49 and 2.61 eV respectively, as shown in Fig. 2(a). The Eg for ZrO2 photoanode is calculated as 5.11 eV, the corresponding Eg in the UV optical spectrum is at 240 nm as shown in Fig. 2(b). But after depositing Cds on ZrO2, the corresponding Eg for CdS-ZrO2 film shows red shifts towards longer wavelengths as shown in Fig. 2(a). The thickness (d) of the ZrO2 film is ~10 μm. Hence despite ZrO2 is a wide band gap metal oxide, after the deposition of colloidal CdS particles as sensitizers or window materials, the combined ZrO2 and CdS gives typical metal oxide based like solar devices having better efficiency (PCE), fill factor (FF), current density (Jsc) and open circuit voltage (Voc), which have not been reported yet in the literature.

        Fig. 2 UV-visible spectra for (a) colloidal CdS particles, it shows blue shifts for CdS type-b and c; and (b) CdS/ZrO2 film,it also shows blue shifts for CdS type-a/ ZrO2, CdS type-b/ZrO2 and CdS type-c/ZrO2.

Fig. 3 SEM images of CdS, ZrO2 and ZrO2/CdS films: a1-a3 are for bared film of ZrO2. Bulk and nano size CdS particles deposited

Fig. 3 shows the SEM images of the obtained films. The images were obtained at a various magnification of x3000, x10, 000 and x30, 000. SEM images for bared ZrO2 photoanode a1 to a3; whereas b1 to b3 for ZrO2/CdS type-a to type-c respectively show the comparable morphology.  ZrO2 film structure is a granular type having macroporous morphology (> 50 nm). After the deposition of CdS, type-b shows macroporous morphology having flakes as shown in (b2). However, these flake shapes of macroporosity almost disappear in the case of type-c, hence in the J-V measurement of ZrO2/CdS type-c based QDSSC, very low Jsc, Voc and hence low FF and efficiency are observed. The JV-measurement for ZrO2/CdS type-a gives an intermediate response between type-c and type-b, this could be verified from the intermediate porous morphology for ZrO2/CdS type-a film.

3.3 Fabrication of solar cells

The fabricated and chemical bath deposited CdS photoanodes (CdS/ZrO2) are used for the fabrication of solar cells, the carbon suites is used as counter electrode, which is fabricated on the FTO substrate.

3.4 Performance measurement

Table 1. Solar cell parameters for ZrO2 based QDSSC where CdS type-a and type-b used for the sensitization (methanol based electrolyte) (30 mW/cm2).

ZrO2/CdS based QDSSCs

Open circuit voltage

Voc

(V)

Current density,

Jsc (mA/cm2)

Fill factor, FF

Metastable state,

τe (ms)

PCE (%)

For type-a

0.38

0.31

0.15

0.79

0.058

For type-b

0.55

0.55

0.21

4.37

0.212

For type-c

0.055

0.03

0.23

0.05

0.006

 

The methanol-based polysulfide electrolyte shows that the current densities (Jsc) for ZrO2-CdS type-a, b and c are 0.31, 0.55 and 0.03 mA/cm2, whereas open circuit voltage (Voc) are 0.38, 0.55 and 0.06 V respectively. PCE for CdS-type-b coated ZrO2 solar device is 0.21% which is greater among the CdS type-a and c. Only CdS type-b is compatible with mesoporous structure of ZrO2. Excited states of CdS type-a (Bulk) are not at the same level with conduction band of ZrO2. For type-c due to large band gap, only few electrons are diffused. Hence mesoporous ZrO2 is compatible with CdS type-b only.

Thus, the PCE for CdS type-a and type-c are 0.06 and 0.006% respectively. Thus ZrO2-CdS type-c solar device shows a negligible PCE. However, the FF for CdS type-c is 0.23 compare to CdS type-a (0.15) and type-c (0.21). Current densities for CdS type-a and b are noticeable, 0.31 and 0.55 mA/cm2 respectively. The solar cell parameters for methanol-based electrolyte are shown in Fig. 4 and are summarized in Table 1. It is observed that the Jsc, Voc, and η initially increased and then decreased with decreasing the size of colloidal CdS particles. For distilled water-based polysulfide electrolyte, the JSC is higher for ZrO2-CdS type-b compared to CdS type-a and CdS type-c. It is also found that the FF for type-b (0.34) also shows a better performance than that for CdS type-a (0.30) and CdS type-c (0.24). Since not only the solar cell parameters but the stability for distilled water-based electrolyte are able to retain prolonged electron life time compared to the methanol based electrolyte.

Table 2. Solar cell parameters for ZrO2 based QDSSC where CdS type-a and type-b used for the sensitization (dw electrolyte) (30 mW/cm2).

ZrO2/CdS based QDSSCs

Open circuit voltage

Voc

(V)

Current density,

Jsc (mA/cm2)

Fill Factor, FF

Metastable state,

τe

(ms)

PCE (%)

For type-a

0.77

0.89

0.30

13

0.68

For type-b

0.61

1.44

0.34

9.55

1

For type-c

0.44

1.78

0.24

1.66

0.62

 

The solar cell parameters for distilled water based polysulfide electrolyte are summarized in Table 2. It is observed that the Jsc, Voc, η initially increased and then decreased with decreasing the particle size of CdS from  type-a to type-c. The maximum PCE for CdS type-b is 1.0%, which is higher than other two CdS type-a and type-c. The result of PCE and their variation clearly indicates that Polysulphide electrolyte gives a stable performance when used in solar cell devices.

Fig. 4 J-V characteristic of CdS sensitized ZrO2 photoanodes using (a) methanol based (b) DW based polysulfide electrolytes.

3.5 Equivalent circuit Analysis

The potentiostat/galvanostat is used to measure the photocurrent and photovoltage responses with respect to time and electrochemical impedance spectra. The solar devices were measured at -1.0 V bias voltages under the dark condition having frequencies from 10-1 to 105 Hz at 25 °C. EIS measurement invokes the investigation of charge transport properties for ZrO2 based QDSSCs. For CdS type-b/ZrO2 based QDSSCs, there is a semicircle observed in the Nyquist plot. Equivalent circuit is fitted for this semicircle. Resistance is due to (1) electron transport, S2- loses its electrons to the hole (h2+) in valence or lowest unoccupied molecular orbital (LUMO) level of ZrO2/CdS QDs to become S, this sulfur then takes electrons from sulfide ion (Sn-12n) whereas (2) at the counter electrode (CE), a reduction of Sn-12n occurs with the gain of electrons from Sn-12n to the Sn2-. Thus, since redox reactions for regeneration and diffusions occur at the two interfaces, not all the electrons contributed to the redox process, some of them lose due to recombination at ZrO2/CdS and leakage at CE, these losses of electrons created the Resistance (i.e., Rtr or R1) due to electron transport process.

For ZrO2/CdS type-b, the R1 = 618 Ω is connected in the series with the charge transfer resistance, Rct=R2= 9.56 kΩ which is in parallel capacitor C1= 0.57 μf. R1 is due to the recombination process after the electrons are injected from CdS QDs to ZrO2 while C1 acts as capacitive elements (constituent of reactance elements such as interface at conduction band and surface states of ZrO2 photoanodes).[22] Because of the recombination process, all the electrons do not contributed to current densities, some of the electrons recombined with the holes. These recombined electrons are responsible for capacitive elements in the porous structure of ZrO2 photoanode.[23, 24]

Semicircle in the lower frequency region shows a series combination of R2 with R1//C1 whereas the first semicircle represents the charge transfer at electrolyte/CE-FTO interface under higher frequency region shown in Fig. 6(a). This charge transfer also suffers from some electron losses at the interface of electrolyte/CE-FTO, which results in another charge transfer resistance, Rct= R3 = 650 Ω for ZrO2-CdS type-b. Counter electrodes are the carbon electrodes. Since the capacitive element C1 with R1 is attributed to the recombination process at the interface of ZrO2/CdS and the electrolytes.

Higher C1, in this case, indicates a higher related resistance to the recombination process. Another capacitance is observed at the interface of electrolyte and counter electrode, C2 = 1.63 μF which is less than C1. C2 is due to a reduction at electrolyte/CE-FTO interface. The details for ZrO2-CdS type-c are summarized in Table 3. Whereas details for distilled water-based polysulfide are summarized in Table 4. Equivalent circuits fitted on Nyquist plots are shown in Fig. 7.

 

The metastable state for methanol-based electrolyte: ZrO2/CdS type-b has a longer electron lifetime (0.7 ms) compared to 0.12 and 0.008 ms for ZrO2/CdS type-a and type-c based QDSSC respectively. Similar results were obtained for distilled water based electrolytes, but in this case Jsc, Voc and η are better than methanol-based polysulfide as shown in Fig. 5(b)and 6(b). Thus, the transition of electrons in ZrO2/CdS type-b for both the cases of methanol and distilled water-based electrolytes is in the lower frequency region compared to ZrO2/CdS type-a and type-c as explained in Scheme 1.

Higher capacitance (C1) in the range of micro Farad is due to electron recombination and regeneration process during the oxidation of polysulfide, holes in CdS QDs were replaced by electron from polysulfide whereas lesser capacitance (C2) is due to reduction at the interface of electrolyte and counter electrode as shown in the Scheme 1.[25] It is not necessary higher is the electron life time, but a better PCE.[26] For both the cases of  distilled water and methanol based polysulfide, electron life time in Bode phase plot is higher for ZrO2/CdS type-a as shown in of Fig. 4(b) and 5(b) but the PCE associated with CdS type-a is lower.