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. 2018 Oct 17;3(10):13433–13441. doi: 10.1021/acsomega.8b01631

Influence of CdS Morphology on the Efficiency of Dye-Sensitized Solar Cells

Entidhar Alkuam 1,*, Emad Badradeen 1, Grégory Guisbiers 1,*
PMCID: PMC6645277  PMID: 31458055

Abstract

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Cadmium sulfide (CdS) used in dye-sensitized solar cells (DSSCs) is currently mainly synthesized by chemical bath deposition, vacuum evaporation, spray deposition, chemical vapor deposition, electrochemical deposition, sol–gel, solvothermal, radio frequency sputtering, and hydrothermal process. In this paper, CdS was synthesized by hydrothermal process and used with a mixture of titanium dioxide anatase and rutile (TiO2(A+R)) to build the photoanode, whereas the counter electrode was made of nanocomposites of conductive polymer polyaniline (PANI) and multiwalled carbon nanotubes (MWCNTs) deposited on a fluorine-doped tin oxide substrate. Two morphologies of CdS have been obtained by using hydrothermal process: branched nanorods (CdSBR) and straight nanorods (CdSNR). The present work indicates that controlling the morphology of CdS is crucial to enhance the efficiency of DSSCs device. Indeed, the higher power conversion energy of 1.71% was achieved for a cell CdSBR–TiO2(A+R)/PANI–MWCNTs under 100 mW/cm2, whereas the power conversion energy of 0.97 and 0.83% for CdSNR–TiO2(A+R)/PANI–MWCNTs and TiO2(A+R)/PANI–MWCNTs, respectively. Therefore, by increasing the surface to volume ratio of CdS nanostructures and the crystallite size into those structures opens the way to low-cost chemical production of solar cells.

1. Introduction

The dye-sensitized solar cell (DSSC) has been extensively studied and appears as a powerful substitute to other thin-film solar cells, like silicon solar cells, largely because of the low cost of the materials used and its relatively simple fabrication process. In DSSCs, light absorption and charge-carrier transport occur in different steps (dissimilar to what happen in solar cells using inorganic p–n junctions).1−9 DSSCs comprise three key parts: a transparent conducting oxide dispersed on glass, a layer of mesoporous metal oxide semiconductor that is photosensitized by a monolayer of dye molecules fixed onto the oxide surface, and a counter electrode, which is most frequently a platinum-coated fluorine-doped tin oxide (FTO) substrate. The cell is filled by an electrolyte to ensure hole transport and close the circuit.10,11

By incorporating a photoanode made of titanium dioxide (TiO2) nanostructures using ruthenium-dye sensitizers, certain DSSCs demonstrated a power conversion efficiency greater than 12.5%.12 Currently, DSSCs based on zinc oxide (ZnO) continue to show low efficiency numbers, compared to TiO2-based DSSCs. The photocatalytic features of TiO2 are dependent on its crystalline phase, its pore structure, its specific surface area, and its crystallite size. TiO2 occurs in three crystalline phases: rutile (the thermodynamic stable phase), anatase and brookite (the two metastable phases). Numerous studies have established the anatase phase of TiO2 as a highly effective photocatalytic material for multiple uses.13 It is still poorly understood why the anatase phase exhibits higher photocatalytic activity than rutile phase. TiO2 acts as a catalyst in the ultraviolet region because of its wide band gap (∼3.2 eV) and this restricts its ability to harvest light. To overcome this drawback, sensitized composites of two or more semiconducting materials with offset band gaps and positions are used (Bi2S3, CuInS2, CdTe, CdS, CdSe, PbS are examples). These semiconductors capture light in the visible region and function as useful sensitizers due to their capacity to transfer electrons to larger band gap semiconductors, including TiO2.14 Cadmium sulfide (CdS) recently became a promising alternative material because of its 2.3 eV direct band gap absorbing into the visible region of the spectrum.15−18

A huge effort has been focused on controlling the shape and size of CdS nanocrystals. As a result, several new methods have been published for synthesizing CdS nanostructures, especially CdS nanowires.19 Inducing the formation of branches on existing nanowires or nanorods results in enlarged surface areas that can boost charge-transportation channels and increase light absorption. These techniques may have significant industrial applications in optoelectronics.20

Numerous chemical and physical techniques have been employed, such as colloidal, solvothermal, vapor deposition, template, and laser ablation methods, to synthesize CdS micro/nanostructures.21 Yao et al. and Peng et al.  synthesized CdS nanowires by using laser ablation of a metal catalyst mixed target and CdS.22,23 Solution deposition protocols, with or without organic surfactants, chemical vapor deposition, and high-temperature thermal decomposition methods, were used to synthesize one-dimensional CdS nanostructures. These structures include nanobelts,22,24 nanowires,22,25 nanorods,22,26 and nanotubes.20,22,27 Chen et al. have developed a simple one-step, no template hydrothermal technique for fabricating CdS nanorod arrays on an indium tin oxide substrate.20,28 Hydrothermal process was defined by Byrappa and Yoshimura (2008) as any homogeneous (nanoparticles) or heterogeneous (bulk materials) reaction occurring at high temperature and pressure larger than 1 atm.29 Hydrothermal fabrication has numerous benefits compared to other conventional processes. The technique can produce homogeneous and pure-phase materials, as well as speeding up reaction rates by improving solid and fluid species interactions. Indeed, hydrothermal fluids have lower viscosity, higher diffusion rates, which consequently enable efficient mass transport. There are a number of practical and environmental benefits to this process: pollution free (reactions are completed within a closed system), low process temperatures (possible within a suitable solvent), overall low energy consumption, better dispersion, better shape regulation, improved control of nucleation, simplicity of the technique, and cost effectiveness.29 Hossain et al. presented CdS deposited by chemical bath deposition (CBD) on TiO2 to fabricate DSSCs, and the conversion efficiency was 1.13%.30 Abdulelah et al. reported an improvement in the conversion efficiency of DSSCs  (∼1.25%) by producing porous CdS via CBD.31 The incorporation of carbon nanotubes (CNTs) and polyaniline (PANI) enhances the DSSC performance. Benetti et al. were the first to fabricate CNTs–polymer composites by mixing epoxy resin with multiwalled carbon nanotubes (MWCNTs).10 Since then, many studies have studied various configurations and formulations of CNTs–polymer composites. PANI is extremely stable in air. In addition, its physical properties and electronic structure display extensive changes when in a protonated form. These properties make PANI an exceptionally useful contender for numerous applications.32−34 Conductive PANI-packed nanotubes might produce innovative materials with very useful optical, electronic, or thermal characteristics.32,35

In this study, TiO2(A+R) were deposited by spray-coating onto CdS nanorods, which were prepared by hydrothermal deposition technique on top of a commercial FTO transparent conductive glass. Branched cadmium sulfide (CdSBR) or straight cadmium sulfide nanorods (CdSNR) with TiO2(A+R) were used as a photoelectrode in DSSCs, whereas PANI–MWCNTs nanocomposites were used as a counter electrode. The DSSC fabrication and characterization are described in Sections 2 and 3, respectively.

2. Experimental Section

2.1. Materials and Chemicals

Ethylene glycol (CH2OH)2, ditetrabutylammonium cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato) ruthenium(II) (N719 dye) C58H86N8O8RuS2 95%, thiourea ≥99.0%, multiwalled carbon nanotubes >99%, titanium(IV) oxide rutile TiO2 99.99%, l-glutathione reduced ≥98.0%, and titanium(IV) oxide anatase TiO2 99.7% were all purchased from Sigma-Aldrich. Iodine I2 was purchased from Mallinckodi chemical work. Fluorine-doped tin oxide (FTO) glass substrate, with a resistivity of 12–17 Ω cm, was purchased from Nanocs. N,N-Dimethylformamide (DMF), acetone (C3H6O), cadmium nitrate Cd(NO3)2·4H2O, hydrochloric acid (HCl), potassium iodide (KI), and ethanol (C2H6O) were purchased from Fisher Scientific. Aniline C6H7N ≥ 99% and sulfuric acid H2SO4 were purchased from Alfa Aesar. All of the chemicals were used without any further purification.

2.2. Fabrication of the Photoanode

FTO-coated glass with dimensions of 2.5 × 2.5 cm2 was used as substrate. Prior to the deposition, the substrate was cleaned ultrasonically within acetone, methanol, and deionized water (DI water, 18.20 MΩ cm) for 5 min each. CdSNR and CdSBR were synthesized to fabricate the photoanode of the DSSC. Initially, CdSNR was synthesized by using an hydrothermal process,20 specifically an aqueous solution with thiourea, cadmium nitrate, and glutathione was prepared according to the following molar ratio (1:1:0.6) and stirred for a few minutes. Then, the solution was transferred into a Teflon-lined stainless steel autoclave (20 cm3 capacity). Finally, the FTO substrate was placed into the autoclave and then loaded into the oven at 200 °C for 3.5 h. The sample was rinsed with DI water after being cooled down naturally to room temperature. Then, the sample was naturally dried in air at room temperature.

To functionalize CdSBR, the resultant CdSNR was rinsed in dilute HCl with a ratio (1:9) for 30 s and then cleaned with DI water. The product was then placed into a fresh solution, and the hydrothermal approach was repeated for 3.45 h at 200 °C to get the branched CdS structure (CdSBR). A mass of 3 g from each titanium dioxide phase (anatase and rutile, TiO2(A+R)) was mixed with acetic acid and DI water (10:90%) to create a suspension. After stirring it for nearly 35 min, the suspension was deposited by spray deposition onto CdSBR and CdSNR. The deposition occurred on a hot plate at 180 °C to produce a porous layer of TiO2(A+R) nanoparticles on FTO/CdSBR and FTO/CdSNR. The photoanode was then annealed in an oven at 450 °C for 1 h. Finally, the FTO/CdSBR–TiO2(A+R) and FTO/CdSNR–TiO2(A+R) photoanodes were sensitized under dark conditions using an ethanol solution containing the N719 dye (0.01 g of N719 in 20 mL of ethanol) for 24 h.

2.3. Fabrication of the Counter Electrode

PANI was synthesized by electrochemical polymerization.34,36 Specifically, aniline monomer (2 M) was dissolved into sulfuric acid (1 M), and the solution was stirred for a few minutes until a clear solution was obtained. The polymerization process of aniline was performed on a clean FTO substrate at room temperature for 3 min at 2 V, then the dark green produced electrode was rinsed with DI water to remove the remaining aniline. MWCNTs were deposited on PANI to improve the electrical conductivity and chemical stability of PANI. The purified MWCNTs were initially dispersed into DMF (0.3 g/mL) and sonicated for 3 h.36 The resultant suspension is then simultaneously deposited by spray deposition onto FTO/PANI. The deposition occurred at 130 °C on a hot plate. A liquid electrolyte containing the I3–/I– redox couple (RE) of potassium iodide (0.83 g) and iodine (0.127 g) was dissolved into 10 mL of ethylene glycol. The electrolyte was then added between the photoanode and the counter electrode (FTO PANI–MWCNTs). The experimental steps are shown in Figure 1.

Figure 1.

Figure 1

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General procedure used to build the DSSCs structure.

2.4. Solar Cell Fabrication

The DSSC consists of FTO/CdSBR or FTO/CdSNR as photoelectrode, whereas FTO/PANI–MWCNTs was the counter electrode. The dimensions of the DSSC are 2.5 × 2.5 cm2 for the outer area and 0.9 × 0.9 cm2 the inner area, which is filled with the electrolyte. The schematic diagram of FTO/CdSBR–TiO2(A+R)/PANI–MWNT/FTO device is shown in Figure 2.

Figure 2.

Figure 2

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Schematic diagram of FTO/CdSBR/PANI–MWNT/FTO device.

The photoelectrode (anode) of the DSSC was made of CdSNR or CdSBR covered by a thin layer of nonporous TiO2(A+R) used on top of a conducting substrate (FTO). PANI coated by MWNTs was used as the counter electrode (cathode). The high surface area of CdS nanorods with nonporous TiO2(A+R) enable FTO/CdS–TiO2(A+R) to adsorb dye molecules (N719). Iodide electrolyte containing I–/I3– redox filled the space between the dye-sensitized n-type (CdS with nonporous TiO2(A+R)) and p-type (PANI–MWNTs). When the DSSC was exposed to light, the photoexcitation occurred in the dye molecules, then the electron transferred from the highest occupied molecular orbital to the lowest molecular orbital states, as illustrated in Figure 3. An electron transfers to the conduction band (CB) of TiO2(A+R) results by the oxidation of the dye molecules, which consequently transfers the electron into the CB of CdS. The oxidized dye molecules were regenerated by electrons coming from the reduced state of the redox couple (RE), which consequently oxidized to I3– (OX). Finally, the I3– was regenerated by accepting electron from the counter electrode.

Figure 3.

Figure 3

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Operating principle of CdSNR or CdSBR/TiO2 DSSC.

3. Characterization

The morphology and the structure of the CdS nanorods were observed within a scanning electron microscope JEOL JSM7000F with energy dispersive X-ray analysis (EDS). The crystallinity of the products was studied by X-ray diffraction (XRD) and carried out on a Rigaku Miniflex 600 X-ray diffractometer using Cu Kα radiation with a 1.54056 Å wavelength. Raman spectroscopy was also used to characterize the CdSBR and CdSNR. Optical absorption of the samples and substrate were recorded by acquiring the spectra with a UV–vis spectrometer from 300 to 900 nm. Current–voltage (I–V) characteristics were evaluated and measured using a Keithley 2400 source meter in the dark and under illumination at AM 1.5. The solar light simulator had an intensity of 100 mW/cm2.

4. Results and Discussion

The CdS nanorods (CdSNR) and branched nanorods (CdSBR) have been characterized by scanning electron microscopy (SEM, Figure 4). CdSNR and CdSBR were grown by using a hydrothermal process to cover uniformly the FTO substrate. As shown in Figure 4a,b, the dimensions of the vertically ordered CdSNR are ∼100 nm in diameter, ∼230 nm in length, which are consistent with the dimensions reported by Chen et al.37 The hexagonal cross-section of the CdSNR was clearly observed in Figure 4a. Furthermore, the EDS analysis exhibited the presence of Cd and S elements with a ratio ∼1:1, whereas the compositions for Cd and S in the compound are 48.55 and 51.45%, respectively, as shown in Figure 4e,f. EDS analysis of the CdSBR revealed that the composition of Cd = 51.09% and S = 48.91%. Figure 4c,d shows the CdSBR morphology at low and high magnifications, which indicate a uniform coverage of CdSBR onto the surface of the FTO substrate. CdSBR consists of straight CdSNR backbones with length of ∼250 nm and hexagonal branches dimensions of ∼200 nm length and ∼100 nm diameter. As expected, the reaction time for hydrothermal process has an effect on the dimensions of CdSNR and CdSBR. The presence of thin branches along the CdSNR backbones, which form the CdSBR structure, improves the performance of DSSCs due to a decrease in recombination.

Figure 4.

Figure 4

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Top-view and cross-sectional SEM images of as-prepared CdSNR at different magnifications (a, b), top view and cross-section of CdSBR grown by two steps hydrothermal process on FTO (c, d), EDS spectra for CdSNR and CdSBR (e, f).

The fractal geometry characterizes the scaling structure of a surface by a number Dfractal called the fractal dimension that varies between 2 (when the surface is flat) and 3. Considering the growth of thin films, the surface contains lakes within islands and islands within lakes, which we will refer to simply as islands. When the islands are characterized by a surface with a fractal dimension Dfractal, the coastlines formed by the islands sectioning by a plane are with a dimension D′fractal = Dfractal – 1. The surface of the island A is linked to its radius R by the relationship A ∝ R2. And the perimeter of the island P is linked to the fractal dimension of the thin-film D′fractal with the relation P ∝ RD′fractal. So, the relationship linking the perimeter with the surface is a power law given by P = μAαfractal, where μ is the proportionality factor between the perimeter and the surface, αfractal = D′fractal/2.38,39 The atomic force microscopy (AFM) measurements are carried out with a Bruker Dimension-Icon AFM (peak force) in tapping mode (f = 300 kHz). The tip was a silicon AFM probe Tap300 Al-G with a constant force of 40 N/m. AFM images are shown in Figure 5. The fractal dimension was extracted with the WSxM software.40 The measured fractal dimensions of straight and branched CdS nanorods are 1.670 ± 0.049 and 1.705 ± 0.045, respectively. Further analysis showed that the typical densities of CdSBR and CdSNR are ∼57 and ∼46 nanorods/μm2, respectively.

Figure 5.

Figure 5

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AFM image of straight CdS nanorods onto the FTO substrate (a), AFM of branched CdS nanorods onto the FTO substrate (b).

Figure 6a illustrates the absorbance spectra from (300–900) nm of CdSNR, CdSBR, and FTO substrate. The CdSBR structure absorbs mainly in the visible region ranging from 326 to 573 nm, whereas the CdSNR structure absorbs in the region ranging from 320 to 520 nm. Since, both structures absorb mainly the violet and blue wavelengths, the remaining transmitted light will exhibit the complementary color to the wavelengths absorbed, i.e., yellow–orange. Apparently, the absorbance peak for the CdSBR structure is higher than that of CdSNR structure, which is attributed to the high surface area covered by CdSBR compared to CdSNR, and thus absorbing and harvesting more photons. The band gaps of CdSNR and CdSBR were established to be 2.37 and 2.28 eV, respectively, as shown in Figure 6b, which indicates that the absorption edge of CdSNR is identified to be at lower wavelength than that of the CdSBR. On Figure 6c, the XRD patterns of as-prepared CdSNR, CdSBR, and FTO are presented. The hexagonal wurtzite phase of CdS is confirmed and the intensity peak (002) at 2θ = 24.6° referring the formation of the CdSBR grown on FTO with better crystallization than CdSNR. The mean crystallite sizes of CdSNR and CdSBR have been determined by using the Debye–Scherrer’s equation, D = Kλ/β cos θ, where λ is the wavelength of the X-ray radiation in nanometer (nm), D is the crystallite size, β is the full width at the half-maximum of the peak, θ the diffraction angle, and K is a constant, usually taken as 0.94.34,41−43 The measurements indicate that the mean crystallite sizes were 42.97 ± 4.2 and 49.37 ± 1.5 nm for CdSNR, CdSBR, respectively. On Figure 6d, the Raman spectra of the as-prepared CdSNR and CdSBR are shown at room temperature. The longitudinal optical phonon modes 1LO and 2LO of CdS were measured at 297 cm–1 and at 600 cm–1 for both structures. These values are in agreement with the literature.44−46 It can be clearly seen that there is no difference in the peak position between the Raman spectra of CdSNR and CdSBR because the same growth technique was used. However, there is a difference in the shape of the peak, which indicates a better crystalline structure for CdSBR compared to CdSNR.34 This enhanced crystalline structure is confirmed by XRD in Figure 6c.

Figure 6.

Figure 6

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UV–vis absorption spectra of CdSNR, CdSBR, and FTO (a), band gaps of CdSNR and CdSBR (b), XRD patterns of CdSNR, CdSBR, and FTO (c), Raman spectra of CdSNR and CdSBR (d).

The morphology of the CdSBR/TiO2(A+R) is presented in Figure 7a, it is seen obviously that the surface of CdSBR was perfectly attached to the TiO2(A+R) nanoparticles. Figure 7b displays the typical SEM image tilted view at low and high magnifications of MWCNTs deposited on FTO. The good coverage of nanotube network onto the FTO substrate is visible in the Figure 7b, due to the dissolution of the MWCNTs in the DMF. Figure 7c shows the SEM image of PANI network after polymerization onto the FTO substrate. Figure 7d demonstrates a good interface between PANI–MWCNTs due to the diffusion of MWCNTs between the porous PANI film.

Figure 7.

Figure 7

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Tilted-view SEM images of as-prepared CdSBR–TiO2(A+R) at high magnification (a), tilted-view of MWCNTs (b), top-view of PANI (c), top-view of PANI–MWCNTs nanocomposite (d).

The UV–vis light absorption of rutile is more efficient compared to anatase, however, the reflectivity of anatase is higher than rutile in the UV–vis region.47 Therefore, a mixture of anatase and rutile provides better optical properties for the photoanode. On Figure 8a, the UV–vis absorption spectra of dye-sensitized CdSBR + TiO2(A+R), dye-sensitized CdSNR + TiO2(A+R), TiO2(A+R), and dye were recorded. It is noticed, as expected, that the absorbance of sensitized CdSBR + TiO2(A+R) is higher than CdSNR + TiO2(A+R) in the 450–500 and 575–675 nm regions due to increasing the surface area of CdSBR + TiO2(A+R) in comparison to CdSNR + TiO2(A+R). Although the absorption peak of TiO2(A+R) shows up near 540 and 700 nm, it shows that the presence of CdS enhances light absorption edge in the visible light regions at around 400–540 and 550–700 nm by harvesting more photons.48Figure 8a shows a typical XRD patterns of TiO2(A+R) before and after annealing. The largest intensity of XRD peaks of TiO2(A+R) after annealing as compared to product before annealing indicates a better crystallization due to the removal of DI water from the nanoparticles.

Figure 8.

Figure 8

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UV–vis spectra of CdSBR + TiO2(A+R) + dye, CdSNR + TiO2(A+R) + dye, TiO2(A+R), and dye (a), XRD patterns of TiO2(A+R) before and after annealing (b).

The current density versus voltage (J–V curves) of the DSSCs are shown at room temperature under illumination and dark conditions in Figure 9a,b respectively. The device parameters for each DSSC, i.e., the photoelectric conversion efficiency (PCE), current short circuit (JSC), open-circuit voltage (VOC), fill factor (FF), shunt resistance (Rsh), and series resistance (Rs) are enlisted in Table 1.

Figure 9.

Figure 9

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Photocurrent–voltage curves of DSSCs devices under illumination 100 mW/cm2 sunlight (a) and in dark conditions (b).

Table 1. Photovoltaic Parameters of DSSCs Devices.

devices JSC (mA/cm2) VOC (V) FF (%) Rsh (Ω cm2) Rs (Ω cm2) PCE (%)
CdSBR/TiO2(A+R)/PANI–MWCNTs 12.16 0.46 30 5.09 2.09 1.71
CdSNR/TiO2(A+R)/PANI–MWCNTs 6.73 0.42 34 8.84 3.42 0.97
TiO2(A+R)/PANI–MWCNTs 6.87 0.41 29 7.25 3.84 0.83
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Among all of the DSSCs devices, CdSBR–TiO2(A+R)/PANI–MWNT shows a significantly higher PCE (1.71%) than CdSNR–TiO2(A+R)/PANI–MWNT (0.98%) and TiO2(A+R)/PANI–MWNT (0.83%). The incorporation of CdSBR into TiO2(A+R) as photoanode improves the charge transport and reduces the recombination of charges due to the larger surface area, which results in a continuous path for electrons and more dye-binding sites. Moreover, the branched structure of CdS (CdSBR) absorbs more light than the straight structure of CdS (CdSNR), which consequently exhibits strong higher power conversion efficiency.49 Thus, it is reasonable to say that the PCE is enhanced by the morphology of CdSBR due to larger crystallite size for CdSBR49 37 ± 1.5 nm in comparison to CdSNR 42.97 ± 4.2 nm, meaning less grain boundaries, which consequently increase the electrical conductivity, and therefore the efficiency of solar cell is improved.

The performance of the displayed CdSBR in DSSCs could be compared with previous studies based on CdS prepared by CBD. Table 2 shows highest power conversion efficiency for the presented work, which in terms refers that the current design of CdS synthesized via hydrothermal process is very effective for enhancing the performance of DSSCs.

Table 2. Comparison between Performances of DSSCs Having CdS with This Work.

technique to prepare CdS PCE (%) refs
CBD 1.13 (30)
CBD 1.04 (50)
CBD 1.25 (31)
CBD 0.73 (41)
hydrothermal process 0.90 (9)
hydrothermal process 2.44 previous work (34)
hydrothermal process 1.71 this work
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FF module depends on both Rsh and Rs. The series resistances Rs (should be low) under illumination for CdSBR–TiO2(A+R)/PANI–MWNT, CdSNR–TiO2(A+R)/PANI–MWNT, and TiO2(A+R)/PANI–MWNT were 2.09, 3.42, and 3.84 Ω cm2, respectively. This means that the lowest Rs based on the cell containing CdSBR gives the largest current, Isc = 12.16 mA, and high Rs reduces FF of the module and Isc. Although shunt resistance (should be high) is one of the factors that effects Voc, low shunt resistance resulting from cracks, pin holes, or impurities in and near the junction leads to an increasing shunt current, and then low Rsh decreases both of Pmax and FF of the module,51,52 and hence the performance of the DSSCs, as shown in Table 1. Another reason to explain the high efficiency of CdSBR–TiO2(A+R)/PANI–MWNT is  the large surface area of TiO2(A+R) nanoparticles. It should be mentioned that if CdS is deposited on top of TiO2 then the PCE will be higher than having TiO2 deposited on top of CdS, due to the difference between conduction bands of CdS and TiO2. However, this situation actually depends on the nature of deposition, since the CdS deposited here is produced via hydrothermal process, the growth should be performed on the top of the conduction substrate and cannot be on top of TiO2. The PCE (η) of DSSCs was calculated by the equation η = FF × JSC × VOC/Pin where FF is the fill factor, JSC the short-circuit current, VOC is the open-circuit voltage, and Pin is the power of incident light. The fill factor (FF) was calculated by equation FF = JmVm/JSCVOC where Jm is the current density at the maximum power point and Vm is the voltage at the maximum power point.2,34,53,54

5. Conclusions

In this paper, the effect of CdS nanorods on the performance of DSSCs has been studied. The low cost and simple solution deposition technique work reliably for high performance of DSSCs. CdSNR and CdSBR produced via hydrothermal technology and two-step hydrothermal process, respectively, allowed  higher absorbance in the visible region, larger crystallite size, better junctions, and uniform morphology. To design DSSCs devices, three cells were fabricated with one hybrid counter electrode PANI–MWCNTs, and the most efficient device was CdSBR–TiO2(A+R)/PANI–MWCNTs due to the high surface area of the hybrid working electrode compared to CdSNR–TiO2(A+R) and TiO2(A+R) devices. Therefore, CdSBR  morphology is an interesting nanostructure to improve the efficiency of  DSSCs.

Acknowledgments

The authors would like to thank the Center for Integrative Nanotechnology Sciences at the University of Arkansas at Little Rock for utilizing the SEM as well as Dr. Wissam Alobaidi for his assistance.

The authors declare no competing financial interest.

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