CdIn₂S₄ quantum dots: novel solvent-free synthesis, characterization and enhancement of dye-sensitized solar cells performance

Abstract

Herein, CdIn₂S₄ (CdIS) quantum dots were synthesized via a solid-state thermal decomposition approach for the first time. [Cd(en)₂]SO₄, In(NO₃)₃·5H₂O and thioacetamide were used as the cadmium, indium and sulfur sources. The as-synthesized products were characterized extensively by techniques such as XRD, EDS, SEM, TEM, AFM, FTIR and DRS. The effect of the cadmium source type, temperature and reaction atmosphere on the morphology and purity of the final products were studied. The results showed that choosing the appropriate temperature and cadmium source has a significant influence on CdIS–QDs synthesis. Moreover, the as-prepared CdIS–QDs were utilized as a barrier layer in dye sensitized solar cells (DSSCs) for the first time. The short-circuit photocurrent density (J_sc), open-circuit voltage (V_oc), fill factor (FF) and efficiency of the solar cell were studied. The power conversion efficiency of the DSSCs using a CdIS–QDs barrier layer was 8.17%, which is 32.2% higher than the cell without a novel barrier layer (6.18%).

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Introduction

Conventional dye-sensitized solar cells (DSSCs) have attracted extensive scientific attention and technical interest over the past two decades as one of the most encouraging third generation solar cells, due to the superior properties compared with silicon based solar cells such as low cost, relatively high efficiency, flexibility and easy fabrication.^1–3 In general, a DSSC consists of an n-type semiconductor with a large band-gap (also named photoanode), dye molecules as a sensitizer, a redox electrolyte and a counter electrode.^4,5 Titanium dioxide (TiO₂), a wide band-gap (3.0–3.2 eV) n-type semiconductor, is one of the most prominent oxide materials for performing various types of industrial applications such as photovoltaics,⁶ photocatalysis^7,8 and photochromic applications.⁹ When TiO₂ photoanodes are used in DSSCs, the electron transfer between the excited dyes to the conduction band of TiO₂ is ultra-fast (femto seconds), but the recombination of the injected electron with the electrolyte is high because of the low electron mobility of TiO₂. To decrease the recombination rate of the electron, some metal oxide materials, such as Al₂O₃,¹⁰ ZrO₂, SiO₂, and ZnO,¹¹ have been coated on the TiO₂ film as a barrier layer. However, barrier layer materials, such as Al₂O₃ and ZnO, not only decrease the electronic coupling between the dye and TiO₂, but also suppress the electron transport from the excited dyes to conduction band of TiO₂ because of the lower electron transport rate and modifying the TiO₂ electronic structure.^12,13 Recently, materials, such as CuInS₂,^13,14 AgInS₂ (ref. 15) and CuIn₂S₄,¹⁶ with long-term stability and high light-absorbing properties are utilized excellently in thin film solar cells. Moreover, the conduction band levels of these materials is higher than that of TiO₂, indicating that the electron can be injected from them into the TiO₂ conduction band rapidly and efficiently. Therefore, their quantum dots (QDs) can act as an energy barrier layer like the position of the ZnO barrier layer to block the recombination of photo injected electrons with redox ions in the electrolyte and hole formed in excited dyes. Cadmium indium sulfide (CdIn₂S₄) is the a semiconducting chalcogenide compound and owing to its excellent properties in solar cells and optoelectronic applications,^13,16 considerable attempts have been made to synthesize CdIn₂S₄ QDs with various morphologies, including spherical,¹⁶ rod-like,¹⁷ flower-like¹⁸ and sheet-like.¹⁹ To date, the CdIn₂S₄ nanostructure has been synthesized by methods such as hydrothermal,²⁰ solvothermal²¹ microwave²² and sol gel templating.²³ In most cases, these techniques require expensive solvents and precursors. To the best of our knowledge, the solid-state thermal decomposition approach for synthesizing CdIn₂S₄ (CdIS) QDs and other ternary compounds has not been reported yet and in this present study, we conducted the CdIS–QDs preparation using a simple, low-cost and solvent-less route by thermal decomposing a mixture of thioacetamide (TAA), [Cd(en)₂]SO₄ and In(NO₃)₃·5H₂O under argon atmosphere at 500 °C. Moreover, utilizing CdIS–QDs as a barrier layer has not been studied and for the first time we investigated the use of the as-prepared CdIS–QDs as a barrier layer in DSSCs. This method can be extended to the preparation of other ternary chalcogenide compounds.

Experimental

Materials and physical measurements

All the chemicals used in this method were of analytical grade and used the as-received without any further purification. The X-ray diffraction (XRD) patterns were recorded on a Philips-X'PertPro, X-ray diffractometer using Ni-filtered Cu Kα radiation at a scan range of 10° < 2θ < 80°. Energy dispersive spectrometry (EDS) analysis was studied using an X-Max Oxford, Philips microscope. Scanning electron microscopy (SEM) images were obtained on a LEO-1455VP equipped with an energy dispersive X-ray spectroscopy apparatus. Transmission electron microscope (TEM) images were obtained on a Philips EM208S transmission electron microscope with an accelerating voltage of 100 kV. Atomic force spectroscopy was performed by ARA-AFM, an Iranian made AFM (manufactured by Ara-Research Company in Tehran, Iran). The Fourier transform infrared (FT-IR) spectra were obtained on Magna-IR, spectrometer 550 Nicolet with 0.125 cm⁻¹ resolution in KBr pellets in the range of 400–4000 cm⁻¹. The diffused reflectance UV-visible spectrum (DRS) of the sample was recorded by an Ava Spec-2048TEC spectrometer. A photocurrent density–voltage (J–V) curve was measured using computerized digital multimeters (Ivium-n-Stat Multichannel potentiostat) and a variable load. A 300 W metal xenon lamp (Luzchem) served as an assimilated sun light source and its light intensity (or radiant power) was adjusted to simulate AM 1.5 radiation at 100 mW cm⁻² with a filter for this purpose.

Preparation of [Cd(en)₂]SO₄

0.01 mol of Cd(NO₃)₂·4H₂O was dissolved in 20 mL of distilled water. 0.02 mol of ethylenediamine was dissolved in 20 mL of distilled water and was then added slowly to the obtained solution. The mixture was stirred and heated at 80 °C for 5 h. The obtained blue precipitate was centrifuged, washed several times with absolute ethanol and distilled water and dried at 50 °C.

Synthesis of CdIn₂S₄ QDs

The starting powders (0.08 g of [Cd(en)₂]SO₄, 0.2 g of In(NO₃)·5H₂O, and 0.08 g of TAA) were milled and mixed in a ball mill for 20 min using zirconia grinding media. Furthermore, the powder mixtures were calcined at 500 °C for 1.5 h under an argon atmosphere. Finally, it was washed several times with distilled water and absolute ethanol and dried at 70 °C for 10 h (sample 1). An experiment was performed with the same abovementioned procedure under atmospheric conditions (sample 2). To examine the temperature and cadmium source effect on the morphology and purity of the final products, two experiments were performed at 400 °C for 1.5 h under an argon atmosphere (sample 3) using Cd(NO₃)₂·4H₂O instead [Cd(en)₂]SO₄ (sample 4).

Fabrication of FTTiO₂

Electrophoresis deposition (EPD) was used to prepare the TiO₂ films. During EPD, the cleaned FTO glass remained at a positive potential (anode), while a pure steel mesh was used as the counter (cathode) electrode. The linear distance between the two electrodes was about 3 cm. Power was supplied by a Megatek Programmable DC power supply (MP-3005D). The applied voltage was 10 V. The deposition cycle was repeated 4 times, each time for 15 s and the temperature of the electrolyte solution was maintained constant at 25 °C. The coated substrates were air dried. The apparent area of the film was 1 × 1 cm². The resulting layer was annealed under an air flow at 500 °C for 30 min.

Fabrication of FTO/TiO₂/N719/Pt

For the fabrication of FTO/TiO₂/N719/Pt, the FTO/TiO₂ prepared by EPD technique was immersed into a N-719 (Dyesol) dye solution in ethanol (0.5 mM) and maintained at 40 °C for 24 h. The counter electrode was made from the deposition of a Pt solution on FTO glass. Subsequently, this electrode was placed over the TiO₂ electrode. Sealing was accomplished by pressing the two electrodes together on a double hot-plate at a temperature of about 110 °C. The redox electrolyte consisting of 0.05 M of LiI, 0.05 M of I₂ and 0.5 M of 4-tert-butylpyridine in acetonitrile as a solvent was introduced to the cell through one of two small holes drilled in the counter electrode. Finally, these two holes were sealed by a small square of sealing sheet and characterized by an I–V test.

Fabrication of FTO/TiO₂/CdIn₂S₄ QDs/N719/Pt

The chemical bath method was employed to fabricate FTO/TiO₂/CdIS–QDs/N719/Pt. For this purpose, the as-prepared QDs were sonicated at a power setting of 60 W in 40 mL of ethanol for 10 min. Subsequently, the prepared FTO/TiO₂ via the EPD method was immersed in the obtained suspension for 5 h to deposit the QDs on the TiO₂ surface. Finally, the obtained FTO/TiO₂/CdIS–QDs was washed with a few drops of absolute ethanol and immersed in a N179 (Dyesol) dye solution in ethanol (0.5 mM) and maintained at 40 °C for 24 h.

Results and discussion

Synthesis and characterization of nanostructures

The crystal structure and phase composition of the obtained products were determined by XRD. Fig. 1a and b show typical XRD patterns of the obtained products in the presence (sample 1) and absence of argon gas (sample 2), respectively.

Fig. 1 XRD patterns of the as-synthesized samples: (a) in presence of argon gas (sample 1) and (b) in absence of argon gas (sample 2) at 500 °C.

As shown in Fig. 1a, in the presence of argon gas, a pure cubic phase of CdIn₂S₄ with the space group, Fd3m, JCPDS no. 27-0060 and the cell parameter a = 10.8450 Å was obtained. Moreover, no crystalline impurity phases were observed in the product, indicating the relatively high purity of the CdIn₂S₄ nanostructures. When the experiment was performed in the absence of argon gas, pure cubic phase of In₂O₃ with the space group of Ia3̄, JCPDS no. 71–2194 and unit cell parameter of a = 10.1170 Å was synthesized (Fig. 1b). The crystallite sizes of the as-synthesized CdIn₂S₄ nanoparticles were estimated to be 7 nm by the Scherer formula shown in eqn (1).¹⁵

D(hkl) = kλ/β cos θ (1)

where D is the crystallite size, as calculated for the (hkl) reflection, k is the wavelength of Cu Kα radiation (0.154 nm), k is a constant related to the crystal shape (0.94), and β is the full width at half-maximum intensity (FWHM). The obtained XRD results show that for synthesizing pure CdIn₂S₄ QDs, the presence of argon gas is required.

Energy dispersive X-ray analysis (EDX) was used to determine the elemental composition and chemical purity of the obtained products. Fig. 2 shows a typical EDX spectrum of the as-synthesized product in the presence of argon gas (sample 1). As can be observed, only Cd, In and S peaks exist in the product, which shows that the CdIS–QDs were obtained without impurities. As expected, based on the calculation of the peak areas in Fig. 2, the ratio of Cd : In : S was found to be approximately 1 : 2 : 4. Therefore, both XRD and EDX analyses show that pure CdIn₂S₄ nanoparticles were synthesized successfully in the presence of argon gas via the mentioned synthetic route.

Fig. 2 EDX spectrum of the as-synthesized sample annealed at 500 °C in the presence of argon gas (sample 1).

To study the surface nature of the as-prepared CdIS–QDs (sample 1) and the valence states of elements in CdIn₂S₄ nanostructures, the sample was characterized by X-ray photoelectron spectroscopy (XPS). The core levels of Cd 3d, In 3d, and S 2p were examined. As shown in the survey spectrum (Fig. 3a) with peak identification, the oxygen peak at a binding energy of 532.5 eV was attributed to the presence of H₂O absorbed on the sample surface and no obvious impurities can be detected. The peak areas of these high-resolution scans were measured and used to calculate the elemental ratio of the nanocrystals. The quantification of the peaks obtains the ratio of Cd : In : S as 1 : 1.9 : 3.89, which closely agrees with that obtained from EDX analysis. From the elemental analyses (XPS) results, we can conclude that the composition of CdIn₂S₄ nanocrystals is close to the stoichiometric proportion of individual atoms. The Cd 3d core (Fig. 3b) splits into 3d_5/2 (405.6 eV) and 3d_3/2 (412.1 eV) peaks, which are all in good agreement with the reported values for Cd²⁺.

Fig. 3 XPS spectrum of the sample annealed at 500 °C in the presence of argon gas (sample 1) (a) survey spectrum; (b) Cd 3d; (c) In 3d; and (d) S 2p.

Moreover, Fig. 3c shows that the peaks centered at 443.8 and 451.7 eV are in good accordance with In 3d_5/2 and In 3d_3/2. The single S 2p peak (Fig. 3d) at 161 eV is indicative of sulfur present as a sulfur ion.¹⁹

Fig. 4a–c show FTIR spectra of the samples before annealing, after annealing at 400 °C in Ar gas (sample 3) and after annealing at 500 °C in Ar gas (sample 1), respectively. As shown, before annealing (Fig. 4a), due to the presence of ethylenediamine in the precursors, all the absorption bands are related to ethylenediamine, including two absorption bands located in 3315 and 3364 cm⁻¹ related to the N–H stretching of primary amine (–NH₂) and the two bands at 2935 and 2844 cm⁻¹ correspond to the –CH₂ asymmetric and symmetric stretching modes.²⁴

Fig. 4 FTIR spectra of samples: (a) before annealing, (b) annealed at 400 °C (sample 3) and (c) annealed at 500 °C in argon gas (sample 1).

The absorption band at 1604 cm⁻¹ was assigned to N–H bending band of –NH₂ and the band at 1145 cm⁻¹ is related to the C–N stretching mode.²⁵ Fig. 4b (annealed at 400 °C in Ar gas, sample 3) shows that the ethylenediamine were not removed completely and there are trace amounts of organic materials in the final product. Finally, after annealing at 500 °C in Ar gas (sample 1), all the organic molecules were well removed and no significant absorption bands could be observed in the FTIR spectrum of the as-obtained product (Fig. 4c).

Fig. 5a shows a SEM image of the as-synthesized CdIS–QDs (sample 1). As shown, uniform nanoparticles with particles sizes in the range of 7–10 nm were obtained. To examine the particle size and morphology in more detail, the CdIS–QDs (sample 1) were characterized by TEM (Fig. 5b). As presented, the highly monodisperse nanoparticles with an approximate size of 8 nm were synthesized, concurring with the SEM image. Fig. 5c shows high resolution TEM (HRTEM) image of the as-prepared CdIS–QDs (sample 1) and the lattice fringes of the CdIn₂S₄ nanostructures reveal the highly crystalline nature of the particles. The spacing between the two adjacent lattice planes was 0.324 nm, which corresponds to the (311) plane of cubic CdIn₂S₄. To determine the particle size distribution, we performed DLS analysis, which is based on the scattering of the laser light in an aqueous solution. Fig. 6 exhibits the DLS results of CdIS–QDs (sample 1). The mean particle size for nanoparticles was 10 nm. In this technique, the particles are illuminated with a laser. Furthermore, the hydrodynamic diameter of the particles was calculated using the intensity fluctuations of scattered light arising from Brownian motion and the Stokes–Einstein equation.

Fig. 5 (a) SEM image, (b) TEM and (c) HR-TEM images of the as-synthesized sample annealed at 500 °C in argon gas (sample 1).

Fig. 6 Size distribution of CdIS–QDs (sample 1).

The diameter obtained by this technique is sphere-like and has the same translational diffusion coefficient as the particle being measured. The translational diffusion coefficient depends not only on the size of the particle (core), but also on the surface structure, as well as the concentration and type of ions in the liquid medium. This means that the size of the sample could be larger than when it is measured by electron microscopy or any other microscopic technique.

The XRD pattern of the sample obtained at 400 °C (sample 3) is presented in Fig. 7a. As it can be observed, conducting the experiment at 400 °C results in impurity formation and CdS as an undesired product is formed. Fig. 7b shows a SEM image of sample prepared after annealing at 400 °C (sample 3). As can be observed, the nanoparticles were obtained but in some parts, aggregation was observed and the nanoparticles were not uniform. This can be related to presence of a trace amount of ethylenediamine on the nanoparticles surface, which can promote aggregation. These results show that the SEM image and FTIR spectra are in good agreement with each other.

Fig. 7 (a) XRD pattern and (b) SEM image of the as-synthesized sample annealed at 400 °C in argon gas (sample 3).

As a result, the annealing temperature has a remarkable influence on the final product morphology and purity and 500 °C is a preferable temperature to prepare CdIS–QDs via mentioned synthetic method. To evaluate the precursor effect, an experiment was carried out using Cd(NO₃)₂·4H₂O (sample 4) as a direct cadmium precursor without further modification, instead of [Cd(en)₂]SO₄. The XRD pattern of obtained product using Cd(NO₃)₂·4H₂O (sample 4) is shown in Fig. 8a. As can be observed, the use of Cd(NO₃)₂·4H₂O led to the formation of impurities and In₂O₃ and CdS along with CdIn₂S₄ were produced. Moreover, the morphology of the sample prepared using Cd(NO₃)₂·4H₂O was characterized by SEM and the obtained image is presented in Fig. 8b. As can be observed, the nanoparticles and microparticles were obtained and the nanoparticles are aggregated together. These findings show that precursor type play crucial role on CdIS–QDs synthesis and choosing an appropriate precursor is essential to obtain a preferable product.

Fig. 8 (a) XRD pattern and (b) SEM image of as synthesized sample using Cd(NO₃)₂·4H₂O as cadmium source (sample 4).

Solar cell section

EDX analysis was used to identify the elemental composition of the FTO/TiO₂/CdIn₂S₄ structure, as shown in Fig. 9.

Fig. 9 EDX spectrum of the as-prepared FTO/TiO₂/CdIn₂S₄ electrode.

The presence of Ti, O and Sn shows that TiO₂ was successfully deposited on the glass substrate. Moreover, presence of Cd, In and S indicates that CdIn₂S₄ was successfully deposited on the TiO₂ film.

The morphology and cross-section of the fabricated film (FTO/TiO₂) were investigated by SEM. As shown in Fig. 10a, the TiO₂ film consist spherical nanoparticles that were deposited uniformly on the glass substrate. Fig. 10b shows a SEM image of a cross-section of the TiO₂ nanoparticles electrode, which shows that the thickness of the fabricated P25 electrodes with 4 cycles for 15 s electrophoresis was 9.3 μm. For more investigation of the surface morphology and topography, the bare TiO₂ and TiO₂/CdIS–QDs electrodes were characterized by AFM and the 2D and 3D AFM images are presented in Fig. 11a and b. As can be observed, the bare TiO₂ electrode shows a uniform surface with hierarchical structure (Fig. 11a). In addition, its porosity is high, which can improve the CdIn₂S₄ absorption on the surface and increase the solar cell efficiency. Fig. 11b shows an AFM image of the TiO₂/CdIn₂S₄ electrode. As shown in Fig. 11b, the smoothness of the surface was improved. In other words, the ups and downs of the TiO₂ surface was well filled with CdIn₂S₄ and the CdIS–QDs were successfully loaded in the pores of TiO₂.

Fig. 10 (a) SEM image of TiO₂ film and (b) SEM micrograph of cross-section of the TiO₂ nanoparticles electrode.

Fig. 11 AFM images of (a) bare TiO₂ electrode, and (b) TiO₂/CdIn₂S₄ electrode.

The UV-vis absorption spectra of CdIS–TiO₂ and bare TiO₂ are shown in Fig. 12, which exhibit absorption edges at around 460 nm for CdIS–TiO₂ and no significant absorption for the bare TiO₂.

Fig. 12 UV-vis absorption spectrum of the FTO/TiO₂ and FTO/TiO₂/CdIn₂S₄ electrodes.

These results show that by depositing the CdIS–QDs on the TiO₂ surface, light harvesting in the visible light region is enhanced and the solar cell efficiency is improved. For performance comparison between the DSSCs fabricated with and without using CdIS–QDs, current density–voltage (J–V) curves were obtained and are presented in Fig. 13.

Fig. 13 J–V characterization of FTO/TiO₂/N719 and FTO/TiO₂/CdIS/N719.

As the results show, by depositing CdIS–QDs between TiO₂ and N719, all J_sc, V_oc and FF parameters and consequently the final efficiency (eqn (2)) showed remarkable enhancement from, 18.72 mA, 0.57 V, 0.58 and 6.18% to 20.96 mA, 0.65 V, 0.6 and 8.17%, respectively. CdIS–QDs as a barrier layer form intermediate levels (Fig. 14), and as a result, the excitation lifetime of the electron will be increased significantly and so the improvement of J_sc is expected.¹³ In addition, by depositing quantum dots between the TiO₂ and dyes, the Fermi levels increase, leading to an increase in V_oc. Moreover, as noted earlier, quantum dots fill well the TiO₂ surface hills and valleys and consequently improve the FF value.²⁶ Finally, enhancement in the J_sc, V_oc and FF parameters results in an improvement in efficiency from 6.18% to 8.17%.

η = J_sc × V_oc × FF/I₀ × 100 (2)

Fig. 14 Successive adsorption of CdIn₂S₄ QDs along with dye on the FTO/TiO₂.

UV-vis spectroscopy was used to evaluate the dye molecules absorbed on the working electrodes. An alkaline solution was prepared by dissolving 0.05 g NaOH in 100 mL distilled water and the electrodes were then immersed in 10 mL of the solution for 24 h separately.²⁷ Subsequently, the desorbed dye solution was characterized by UV-vis spectroscopy (Fig. 15). The results are presented in Fig. 15. Higher absorbance coefficient in the spectra shows the higher dye adsorption on the film. As can be observed from the Fig. 15, the cell reference absorbs a higher amount of dye, while cell, including CdIS–QDs, show a better PCE. Based on these results, we can conclude that the performance enhancement in DSSCs is due to the light harvesting property of CdIS–QDs.

Fig. 15 UV-vis of the dye solution desorbed from different cells.

Electrochemical impedance spectroscopy (EIS) was used to elucidate the interfacial charge transfer processes in the devices.²⁷ Fig. 16 demonstrates typical Nyquist plots of the DSSCs with bare TiO₂ and cells containing CdIS–QDs. With increasing frequency, two semicircles are observed in the Nyquist plots. The first semicircle was assigned to the charge transfer resistance between the platinum counter electrode and redox couple (R_CE). In the middle frequency region, the second semicircle reflecting the charge transfer resistance at the dyed-TiO₂/electrolyte interface is related to the charge recombination process (R_CT). The impedance spectra of these devices with the best energy conversion efficiency were investigated. The impedance spectra were fitted to a simplified version of the equivalent circuit. Some important parameters of the working electrodes in these devices were determined from the second semicircles and are summarized in Fig. 16 and Table 1. The effective diffusion coefficient of electrons (D_eff) can be determined using eqn (2):

D_eff = [R_CT/R_CE]L²k_eff

where, k_eff is the reaction rate constant for electron recombination with triiodide and L is the film thickness.²⁷

Fig. 16 Typical Nyquist plots of the DSSCs with bare TiO₂ and cells containing QDs.

Table 1 Electrochemical parameters obtained by fitting the impedance spectra of DSSCs

DSCs	τ (s)	keff (S^−1)	RCE (Ω cm^2)	RCT (Ω cm^2)	Deff (cm^2 s^−1)
Reff	0.042	23.80	13	72	12.7 × 10^−5
QDs/GNPs/GNRs	0.075	13.33	55	156	3.78 × 10^−5

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There is small difference between these devices for the R_CE at the counter electrode/electrolyte interface. These results show that the CdIS–QDs do not make the over potential at the counter electrode. It can be noted that the fitted value of R_CT for the cell containing the CdIS–QDs was ∼156 Ω cm², while the corresponding value for reference cell was ∼72 Ω cm². This significant increase in R_CT shows that the CdIS–QDs are more favorable to suppressing the charge recombination process that arises from electrons in the TiO₂ film with I⁻ in electrolyte solution, which leads to an increase in J_sc. On the other hand, the adsorption of CdIS–QDs on the TiO₂ surface causes an increase in electron density in the conduction band of TiO₂.

In the CdIS QDs-based cell, the middle-frequency peak in the Nyquist plot shifts to a lower frequency compared to reference device, exhibiting a longer electron lifetime for these cells.

Conclusion

In this study, CdIn₂S₄(CdIS) quantum dots were prepared via a solid-state thermal decomposition route for the first time. [Cd(en)₂]SO₄, In(NO₃)₃·5H₂O and thioacetamide were used as precursors. The crystal structure, morphology, composition and optical properties of the as-synthesized products were characterized extensively by XRD, EDS, SEM, TEM, AFM, FTIR and DRS. The effect of precursor type, temperature and reaction atmosphere on the morphology and purity of final products were studied. Furthermore, for the first time, CdIS–QDs were employed as a barrier layer in DSSCs and their performance in solar cells was evaluated. The results showed that the CdIS–QDs with unique properties, such as high excitation lifetime, well pores filling and having intermediate levels, can enhance the FF, J_sc and V_oc parameters and as a result, improve the final cell efficiency significantly from 6.18% to 8.17%.

Acknowledgements

The authors are grateful to the council of Iran National Science Foundation (91053846) and University of Kashan for supporting this study by Grant No. (159271/179).

Notes and references

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