Simulation, Analysis, and Characterization of Calcium-Doped Zno Nanostructures for Dye-Sensitized Solar Cells

energies

Article Simulation, Analysis, and Characterization of Calcium-Doped ZnO Nanostructures for Dye-Sensitized Solar Cells

Shahzadi Tayyaba 1, Muhammad Waseem Ashraf 2,*, Muhammad Imran Tariq 3 , Maham Akhlaq 2 , Valentina Emilia Balas 4,*, Ning Wang 5 and Marius M. Balas 4

1 Department of Computer Engineering, The University of Lahore, Lahore 54000, Pakistan; [email protected] 2 Department of Physics (Electronics), GC University Lahore, Lahore 54000, Pakistan; [email protected] 3 Department of Computer Science, The Superior University Lahore, Lahore 54000, Pakistan; [email protected] 4 Automatics and Applied Software Department, “Aurel Vlaicu” University of Arad, 310130 Arad, Romania; [email protected] 5 Center for Green Innovation, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100069, China; [email protected] * Correspondence: [email protected] (M.W.A.); [email protected] (V.E.B.)

Received: 8 May 2020; Accepted: 11 September 2020; Published: 17 September 2020

Abstract: In this research article, the authors have discussed the simulation, analysis, and characterization of calcium-doped zinc oxide (Ca-doped-ZnO) nanostructures for advanced generation solar cells. A comparative study has been performed to envisage the effect of Ca-doped ZnO nanoparticles (NP), seeded Ca-doped ZnO nanorods (NR), and unseeded Ca-doped ZnO NR as photoanodes in dye-sensitized solar cells. Simulations were performed in MATLAB fuzzy logic controller to study the effect of various structures on the overall solar cell efficiency. The simulation results show an error of less than 1% in between the simulated and calculated values. This work shows that the diameter of the seeded Ca-doped ZnO NR is greater than that of the unseeded Ca-doped ZnO NR. The incorporation of Ca in the ZnO nanostructure is confirmed using XRD graphs and an EDX spectrum. The optical band gap of the seeded substrate is 3.18 eV, which is higher compared to those of unseeded Ca-doped ZnO NR and Ca-doped ZnO NP, which are 3.16 eV and 3.13 ev, respectively. The increase in optical band gap results in the improvement of the overall solar cell efficiency of the seeded Ca-doped ZnO NR to 1.55%. The incorporation of a seed layer with Ca-doped ZnO NR increases the fill factor and the overall efficiency of dye-sensitized solar cells (DSSC).

Keywords: DSSC; chemical bath deposition; Ca-doped ZnO; nanorods

1. Introduction Due to prevailing energy requirements, the necessity of a clean energy technology is substantial. The use of photovoltaic (PV) solar cells is considered as one of the most suitable ways for the generation of clean energy technology [1]. Mostly, PV solar cells are found in three major types including high-eﬃciency silicon solar cell, thin ﬁlm solar cell, and advanced generation solar cell. Among advanced generation solar cells, dye-sensitized solar cells (DSSC) are an emerging type of advanced generation solar cells. They are considered as an alternate to silicon solar cells due to their easy and low-cost fabrication [2]. DSSC comprises of a mesoporous dye-loaded photoanode, electrolyte, and a counter electrode. The photogeneration of an electron hole pair is carried out by the

Energies 2020, 13, 4863; doi:10.3390/en13184863 www.mdpi.com/journal/energies Energies 2020, 13, x FOR PEER REVIEW 2 of 15 Energies 2020, 13, 4863 2 of 14 electrolyte, and a counter electrode. The photogeneration of an electron hole pair is carried out by the dye. The dye molecules attached to the mesoporous photoanode absorb the photon to create an dye. The dye molecules attached to the mesoporous photoanode absorb the photon to create an excited excited electronic state. Furthermore, the injection of the electrons takes place from the dye molecule electronic state. Furthermore, the injection of the electrons takes place from the dye molecule to the to the conduction band of the semi‐conductor photoanode, keeping the dye in its oxidizing state, as conduction band of the semi-conductor photoanode, keeping the dye in its oxidizing state, as shown in shown in Figure 1. At electrolyte, the redox reaction takes place that reduces the dye back to its Figure1. At electrolyte, the redox reaction takes place that reduces the dye back to its neutral state [ 3]. neutral state [3]. The semi‐conductor photoanode transfers the electron to the counter electrode or The semi-conductor photoanode transfers the electron to the counter electrode or cathode via the cathode via the external circuit [4]. The cathode moves the electron to the electrolyte. At the external circuit [4]. The cathode moves the electron to the electrolyte. At the electrode, the reduction electrode, the reduction reaction takes place, and electrolyte restores to its neutral state by accepting reaction takes place, and electrolyte restores to its neutral state by accepting electrons from the external electrons from the external circuit [5]. circuit [5].

Figure 1. Working of a typical dye-sensitized solar cell. Figure 1. Working of a typical dye‐sensitized solar cell. Initially, bulk semi-conductor materials were used as photoanode. However, these materials show Initially, bulk semi‐conductor materials were used as photoanode. However, these materials photocorrosion when exposed to sunlight, which highly affects the stability of the cell. To diminish show photocorrosion when exposed to sunlight, which highly affects the stability of the cell. To these concerns, wide band-gap nanostructured semi-conductors/metal oxides are used. These newly diminish these concerns, wide band‐gap nanostructured semi‐conductors/metal oxides are used. employed structures show resistance toward photocorrosion. Furthermore, these materials manifest These newly employed structures show resistance toward photocorrosion. Furthermore, these spectacular optical and electrical properties. Multiple metal oxide-based nanostructures including materials manifest spectacular optical and electrical properties. Multiple metal oxide‐based nanoparticles, nanospheres, nanorods, and nanofibers are used for the semi-conductor anode in nanostructures including nanoparticles, nanospheres, nanorods, and nanofibers are used for the DSSC. Metal oxides mainly comprise titanium dioxide (TiO2)[6–8], tin oxide (SnO2)[9], zinc oxide semi‐conductor anode in DSSC. Metal oxides mainly comprise titanium dioxide (TiO2) [6–8], tin (ZnO) [10,11], Niobium pentaoxide (Nb2O3)[12], and composite-based metal oxides. Among various oxide (SnO2) [9], zinc oxide (ZnO) [10,11], Niobium pentaoxide (Nb2O3) [12], and composite‐based metal oxide nanostructures, nanoparticles provide a better surface area to volume ratio. However, metal oxides. Among various metal oxide nanostructures, nanoparticles provide a better surface due to grain boundaries, the hindering of electron transfer happens in nanoparticles. In this area to volume ratio. However, due to grain boundaries, the hindering of electron transfer happens case, nanorods-based structures provide continuous charge transfer, which makes them suitable in nanoparticles. In this case, nanorods‐based structures provide continuous charge transfer, which for DSSC [13]. makes them suitable for DSSC [13]. Among various metal oxide materials, ZnO is considered as an advanced and efficient material Among various metal oxide materials, ZnO is considered as an advanced and efficient material due to its wider band gap, low cost, and easy fabrication. ZnO shows excellent piezo and pyro electric due to its wider band gap, low cost, and easy fabrication. ZnO shows excellent piezo and pyro properties. It has vast applications as an energy harvester, solar cell, in electronic devices, and in the electric properties. It has vast applications as an energy harvester, solar cell, in electronic devices, biomedical field [14–16]. ZnO is considered as an excellent material that can be converted into various and in the biomedical field [14–16]. ZnO is considered as an excellent material that can be converted nanostructures including nanoparticles, nanorods, nanowires, nanotubes, and nanoflowers with a into various nanostructures including nanoparticles, nanorods, nanowires, nanotubes, and large surface area to volume ratio [17,18]. Nanoparticles-based structures provide a large surface nanoflowers with a large surface area to volume ratio [17,18]. Nanoparticles‐based structures area to volume ratio; however, due to grain defects, these types of nanostructures do not provide provide a large surface area to volume ratio; however, due to grain defects, these types of efficient output. Among other nanostructures, ZnO nanorods-based structures are considered as an nanostructures do not provide efficient output. Among other nanostructures, ZnO nanorods‐based efficient structure that can be deployed in advanced generation solar cells mainly due to continuous structures are considered as an efficient structure that can be deployed in advanced generation solar charge transfer. These nanorods can be fabricated using various methods including hydrothermal cells mainly due to continuous charge transfer. These nanorods can be fabricated using various synthesis [19], electrochemical synthesis [20], chemical vapor deposition [21], pulsed laser ablation, methods including hydrothermal synthesis [19], electrochemical synthesis [20], chemical vapor physical vapor deposition [22], and chemical bath deposition [23]. Among these methods, chemical deposition [21], pulsed laser ablation, physical vapor deposition [22], and chemical bath deposition bath deposition is considered as an efficient and easy method for the smooth thin film fabrication [23]. Among these methods, chemical bath deposition is considered as an efficient and easy method of nanorods. The process parameters can easily be controlled using the chemical bath deposition for the smooth thin film fabrication of nanorods. The process parameters can easily be controlled

Energies 2020, 13, 4863 3 of 14 process, which make it an excellent method for the fabrication of nanorods. The doping of ZnO with a second group including magnesium, calcium, beryllium, and strontium elements has been considered as an effective technique to varying the structural, optical, and electrical properties of ZnO [24–26]. Majeed Gul [27] et al. reported the band-gap tuning of ZnO nanorods by the addition of Mg from 3.18 eV to 3.32 eV. Similarly, with strontium doping, the band gap can be reduced, and the solar cell efficiency of the DSSC can be enhanced. Similarly, calcium and beryllium can be used to enhance the particle size and optical properties of ZnO nanostructures. Various simulation tools are used for the parametric estimation and power conversion efficiency comparison of the prepared nanostructures including ANSYS, COMSOL, ABAQUS, and MATLAB fuzzy logic controller [28–30]. Among other methods, fuzzy logic controller is considered as an efficient and easy technique that is close to human thinking. It is considered as an efficient method to interpret the output values based on real-life conditions. In this work, a comparative study has been performed to study the overall solar cell efficiency of Ca-doped ZnO nanoparticles deposited using spin-coating and Ca-doped ZnO nanorods fabricated using chemical bath deposition. The parametric estimation is conducted on the basis of the fuzzy logic tool. The effect of the seed layer of ZnO has also been studied to see its effect on the total solar cell efficiency.

2. Fuzzy Analysis Fuzzy analysis of the Ca-doped ZnO nanoparticles (NP) and seeded and unseeded Ca-doped ZnO nanorods (NR)-based photoanodes have been performed using the MAMDAMI model. The fuzzy inference system (FIS) diagrams of the three samples are shown in Figure2. Figure2a shows the input of the first fuzzy analysis. The Ca-doped ZnO NP size and annealing temperature are taken as input. The effects of the two inputs are studied on the band gap and power conversion efficiency of the solar cell. Similarly, the effect of the diameter of the Ca-doped ZnO NR and annealing temperature are studied onEnergies the band2020, 13,gap x FOR and PEER powerREVIEW conversion efficiency for seeded and unseeded substrates.4 of 15

Figure 2. FigureFIS ﬁgure 2. FIS figurefor (a for) Ca-doped (a) Ca‐doped ZnO ZnO nanoparticles nanoparticles (NP (NP)) and and (b) Unseeded (b) Unseeded Ca‐doped Ca-doped ZnO ZnO nanorodsnanorods (NR) (c) ( SeededNR) (c) Seeded Ca-doped Ca‐doped ZnO ZnO NR. NR.

The ranges and membership functions are assigned to the respective inputs and outputs. The range for the input annealing temperature in all the three models is 400–500 °C. For Ca‐doped ZnO nanoparticles, the range is 120–140 nm. For unseeded Ca‐doped ZnO nanorods and seeded Ca‐doped ZnO nanorods, the range for the input is 100–200 nm. The range for band gap was 3.1 to 3.2 eV, and for power conversion efficiency, the range is 1% to 2%. Then, rules for the memberships functions are added to the fuzzy simulations. The numbers of rules is based on the number of input. Figure 3 shows the 3D graphs for the respective Ca‐doped ZnO nanostructure‐based photoanodes. Figure 3a,b shows the effect of Ca‐doped ZnO nanoparticles and annealing temperature on band gap and power conversion efficiency. Figure 3c,d shows the effect of unseeded Ca‐doped ZnO nanorods and annealing temperature on band gap and power conversion efficiency. Figure 3e,f shows the effect of seeded Ca‐doped ZnO nanorods and annealing temperature on band gap and power conversion efficiency. The increase in particle size and the nanorods diameter directly decreases the band gap. The effect predominantly appears due to the quantum confinement effect.

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The ranges and membership functions are assigned to the respective inputs and outputs. The range for the input annealing temperature in all the three models is 400–500 ◦C. For Ca-doped ZnO nanoparticles, the range is 120–140 nm. For unseeded Ca-doped ZnO nanorods and seeded Ca-doped ZnO nanorods, the range for the input is 100–200 nm. The range for band gap was 3.1 to 3.2 eV, and for power conversion efficiency, the range is 1% to 2%. Then, rules for the memberships functions are added to the fuzzy simulations. The numbers of rules is based on the number of input. Figure3 shows the 3D graphs for the respective Ca-doped ZnO nanostructure-based photoanodes. Figure3a,b shows the effect of Ca-doped ZnO nanoparticles and annealing temperature on band gap and power conversion efficiency. Figure3c,d shows the e ffect of unseeded Ca-doped ZnO nanorods and annealing temperature on band gap and power conversion efficiency. Figure3e,f shows the e ffect of seeded Ca-doped ZnO nanorods and annealing temperature on band gap and power conversion efficiency. The increase in particle size and the nanorods diameter directly decreases the band gap. The effect predominantlyEnergies 2020, 1 appears3, x FOR PEER due REVIEW to the quantum confinement effect. 5 of 15

FigureFigure 3. 3D 3. 3D graphs graphs of of band band gap gap and and power power conversionconversion efficiency eﬃciency and and its itseffect eﬀect on on(a,b ()a Annealing,b) Annealing temperaturetemperature and and Ca-doped Ca‐doped ZnO ZnO NP; NP (; c(,cd,d)) AnnealingAnnealing temperature temperature and and unseeded unseeded Ca‐ Ca-dopeddoped ZnO ZnONR; NR; (e,f)( Annealinge,f) Annealing temperature temperature and and seeded seeded Ca-doped Ca‐doped ZnO NR.

TheThe simulation simulation results results are are shown shown using using the the rule rule viewer. viewer. The The annealing annealing temperature temperature is setis set at 500at ◦C as used500 in°C the as experiment. used in the experiment. Figure4 shows Figure the 4 crisp shows input the values crisp input and thevalues corresponding and the corresponding output values basedoutput on the values rules. based on the rules.

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Figure 4. Rule Viewer: (a) Ca-doped ZnO NP; (b) Unseeded Ca-doped ZnO NR; (c) Seeded Ca-doped Figure 4. Rule Viewer: (a) Ca‐doped ZnO NP; (b) Unseeded Ca‐doped ZnO NR; (c) Seeded Ca‐doped ZnO NR. ZnO NR. The simulation results from the rule viewer are compared with the theoretical values using the The simulation results from the rule viewer are compared with the theoretical values using the MAMDANI model formula, MAMDANI model formula, MAMDANI Model = [ Σ (Mi Si)/Mi] 100 MAMDANI Model = [ ∑ ×(Mi × Si)/Mi]× × 100 where MiMi is is the the membership membership function function value value and Si and is the Si singleton is the singleton value. Table value.1 shows Table the 1 comparison shows the comparisonand error between and error the between simulated the and simulated calculated and theoretical calculated value. theoretical value.

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Table 1. Comparison between simulated and calculated values.

Simulated Value Calculated Value Error Band Gap (eV) PCE (%) Band Gap (eV) PCE (%) Band Gap (eV) PCE (%) Ca-doped ZnO NP 3.15 1.21 3.13 1.22 0.7 0.9 Unseeded Ca-doped ZnO NR 3.15 1.34 3.15 1.35 0 0.8 Seeded Ca-doped ZnO NR 3.19 1.5 3.2 1.51 0.4 0.7

The table shows the accuracy of the simulated and calculated value with an error less than 1% between the values.

3. Materials and Methods Zinc acetate di-hydrate, methoxyethanol, hexamethylenetetramine, and calcium acetate have been used to carry out the experimental work. Indium tin oxide glass was used as a substrate. N3 Ruthenium has been used as a dye for DSSC fabrication. 2-Propanol and deionized (DI) water were used as solvent for the synthesis of photoanode. The spin-coating method was used to fabricate the ZnO nanoparticles layer, while ZnO nanorods were fabricated using the chemical bath deposition method [31,32].

Photoanode Preparation The 2 cm 2 cm ITO glass was pre-cleaned using DI water, acetone, and ethanol. The glass was × initially cleaned with soap and furthermore sonicated in ethanol, DI water, and acetone for 20 min. For ZnO nanoparticle-based photoanodes, 20 mM solution of zinc acetate dehydrate was prepared in 2-propanol with 2% doping of Ca. The solution was stirred at 45 ◦C for 2 h. Then, 2–3 drops of methoxyethanol were added to get a transparent solution. The prepared solution was aged for 24 h before being spin-coated on the ITO glass. The solution was added dropwise on the substrate, which was furthermore spun at a rate of 2500 rpm for 20 s Then, the substrate was heated at 90 ◦C for the complete evaporation of the solvent. The process was repeated 10 times to get a clear thin film of ZnO nanoparticles. Then, the substrate was subjected to annealing at 500 ◦C for 2 h to get a thin film of ZnO nanoparticles. For the preparation of ZnO nanorods, 2 substrates were fabricated. One of the substrates is directly subjected to the growth of ZnO nanorods, and the other substrate is first seeded. For seeding, the substrate is subjected to the above stated spin-coating process without the addition of Ca doping. For nanorods growth, 20 mM zinc acetate di-hydrate and hexamethylenetetramine solution was made in DI water with 2% calcium doping. The solution was added to the double-wall beaker of the chemical bath deposition (CBD) setup. The seeded and unseeded substrate was inserted in the beaker using a Teflon holder. The CBD temperature was set at 90 ◦C with continuous stirring at 400 rpm for 4 h. Then, the substrate was washed with DI water and annealed at 500 ◦C with a 5 ◦C rise per minute. The Ca-doped ZnO nanoparticles and nanorods-based samples prepared are shown in Table2.

Table 2. Samples prepared for Ca-doped ZnO NP and NR.

Sample Name ZnO Seeding Layer Ca-Doped ZnO NP Ca-Doped ZnO NR

Z1 X √ X

Z2 XX √

Z3 √ X √

For device fabrication, the prepared photoanodes of Ca-doped ZnO NP, seeded NR, and unseeded NR are dipped in dye solution. The anodes are dipped for 2 h in N3 ruthenium dye solution for better absorption of dye molecules. Gold is used as a counter electrode, while the substrate was ITO-coated. The anode and electrode are clipped together, and redox electrolyte is inserted. Three sets of each samples are produced, and the results are calculated on the basis of the average of the results. EnergiesEnergies2020 ,201320,, 4863 13, x FOR PEER REVIEW 8 of 157 of 14

inserted. Three sets of each samples are produced, and the results are calculated on the basis of the The preparedaverage of photoanodes the results. Theare characterizedprepared photoanode using scannings are characterized electron microscopy using scanning (SEM) electron and energy dispersivemicroscopy X-ray (SEM) spectroscopy and energy (EDX) dispersive to study X‐Ray their spectroscopy morphology (EDX and) to chemical study their composition. morphology Similarly, and UV-Vischemi spectrophotometrycal composition. Similarly is used,to UV analyze‐Vis spectrophotometry the optical properties is used of to theanalyze prepared the optical photoanode. properties For IV measurementsof the prepared including photoanode. open circuitFor IV voltagemeasurements and short including circuit open current, circuit a solarvoltage simulator and short is circuit used with lightcurrent, intensity a solar similar simulator to sunlight. is used The with DSSCs light photovoltaicintensity similar parameters to sunlight. including The DSSCs open-circuit photovoltaic voltage (Voc),parameters the short-circuit including current open‐circuit density voltage (Jsc), (Voc), the fillthe factorshort‐circuit (FF), andcurrent the density photoconversion (Jsc), the fill f eactorfficiency were(FF) studied, and through the photoconversion the I-V measurements. efficiency were The studied I-V measurements through the were I‐V measurements. conducted using The a I‐ sourceV measurements were conducted using a source measure unit (Keithley 2400) and a solar simulator measure unit (Keithley 2400) and a solar simulator (Keithlink) under a power intensity of 750 W/m2. (Keithlink) under a power intensity of 750 W/m2. 4. Results and Discussion 4. Results and Discussion The scanning electron microscopy of the Ca-doped ZnO nanoparticles is shown in Figure5. The scanning electron microscopy of the Ca‐doped ZnO nanoparticles is shown in Figure 5. The The figurefigure confirmconfirmss the the formation formation of of nanoparticles nanoparticles,, which which are are agglomerated agglomerated together together to form to form a large a large cluster.cluster. The The average average nanoparticles nanoparticles size size was was estimated estimated during during SEM, SEM, whichwhich isis inin aa rangerange of 120 120–140–140 nm. The particlesnm. The particles are joined are togetherjoined together to form to aform cluster. a cluster. The The particles particles have have a hexagonal a hexagonal geometry. geometry.

Figure 5. ZnO nanoparticles photoanode prepared using spin coating with magniﬁcation: (a) 5 µm and (b) 2 µm. Energies 2020, 13, x FOR PEER REVIEW 9 of 15 Energies 2020, 13, 4863 8 of 14 Figure 5. ZnO nanoparticles photoanode prepared using spin coating with magnification: (a) 5 µm and (b) 2 µm. Figure6 shows the nanorods on seeded and unseeded substrates. The images show the formation of nanorodsFigure on 6 the show ITOs the substrate. nanorods Figure on seeded3a shows and unseededthe unseeded substrate nanorods-baseds. The images photoanodes. show the The rodsformation are uneven of nanorods with a variableon the ITOdiameter substrate. and length Figure ranging 3a shows from the 100 unseeded to 200 nm. nan Theorods nanorods‐based are not thephotoanode same dimensions.s. The rods Theare uneven shape ofwith the a rods variable is similar diameter to hexagonaland length geometry.ranging from However, 100 to 200 the nm. seeded The nanorods are not the same dimensions. The shape of the rods is similar to hexagonal geometry. substrate with Ca-doped ZnO nanorods shows highly ordered, symmetric and hexagonal-shaped However, the seeded substrate with Ca‐doped ZnO nanorods shows highly ordered, symmetric and vertical nanorods. The seed layer provides better nanorods sticking and growth, which results in a hexagonal‐shaped vertical nanorods. The seed layer provides better nanorods sticking and growth, highlywhich smooth results and in a ordered highly smooth nanostructural and ordered growth. nanostructural The diameter growth. ofThe the diameter Ca-doped of the ZnO Ca‐doped nanorods depositedZnO nanorods on the seeded deposited substrate on the isseeded in the substrate range of is160–250 in the range nm of with 160 a–250 length nm with of 0.5–1.5 a lengthµm. of The0.5– rods are highly1.5 µm. dense The rods and are symmetric. highly dense and symmetric.

Figure 6. (a) Ca-doped ZnO nanorods on unseeded substrate; (b) Ca-doped ZnO nanorods on seeded substrate.

EDX analysis was also performed to study the chemical composition of the prepared photoanodes. Figure7 shows the presence of calcium, zinc, and oxygen in the prepared seeded Ca-doped ZnO Energies 2020, 13, x FOR PEER REVIEW 10 of 15

Figure 6. (a) Ca‐doped ZnO nanorods on unseeded substrate; (b) Ca‐doped ZnO nanorods on seeded substrate.

EnergiesEDX2020 , analysis13, 4863 was also performed to study the chemical composition of the prepared9 of 14 photoanodes. Figure 7 shows the presence of calcium, zinc, and oxygen in the prepared seeded Ca‐doped ZnO nanorods‐based photoanodes. The presence of these three chemicals shows the nanorods-based photoanodes. The presence of these three chemicals shows the purity of the prepared purity of the prepared photoanodes without the incorporation of any impurities. The EDX graphs photoanodes without the incorporation of any impurities. The EDX graphs show the proper doping of show the proper doping of calcium in the ZnO nanostructure. calcium in the ZnO nanostructure.

Figure 7. EDX spectrum of the seeded Ca-doped ZnO nanorods. Figure 7. EDX spectrum of the seeded Ca‐doped ZnO nanorods. Table3 shows the atomic percentage of the above stated elements. The table shows that the Ca elementsTable are 3 shows completely the atomic incorporated percentage in the of ZnOthe above nanostructure. stated elements. The prepared The table anodes shows are that pure the and Ca containelements no are impurity. completely incorporated in the ZnO nanostructure. The prepared anodes are pure and contain no impurity. Table 3. Atomic percentage of Zn, O, and Ca present in the prepared photoanodes. Table 3. Atomic percentage of Zn, O, and Ca present in the prepared photoanodes. Detail Atomic % Detail Atomic % Elements Zinc Oxygen Calcium Elements Zinc Oxygen Calcium Ca-doped ZnO nanoparticles 45.32 2.59 52.09 Ca‐doped ZnO nanoparticles 45.32 2.59 52.09 Unseeded Ca-dopedUnseeded ZnO nanorods Ca‐doped ZnO nanorods 48.9 48.9 2.3 48.8 48.8 Seeded Ca-dopedSeeded ZnO nanorods Ca‐doped ZnO nanorods 46.55 46.55 1.99 51.46 51.46

The XRDXRD graphgraph for for the the Ca-doped Ca‐doped ZnO ZnO nanoparticles nanoparticles photoanodes photoanode is showns is shown in Figure in Figure8. The graph8. The showsgraph shows the presence the presence of zinc of oxide zinc andoxide calcium. and calcium. The low-intensity The low‐intensity peaks peak at 2θs= at29.51, 2ϴ = 29.51, 43.3, and43.3 45.51, and indicate45.51 indicate the presence the presen of calciumce of calcium in the prepared in the prepared Ca-doped Ca ZnO‐doped nanoparticle ZnO nanoparticle photoanodes. photoanode The majors. peaksThe major at 2θ peaks= 31.7, at 34.5,2ϴ = 47.58, 31.7, 34.5, 54.59, 47.58, and 54.59,69.88 show and 69.88 the peakshow of the ZnO peak with of a ZnO wurtizle with hexagonala wurtizle structurehexagonal of structZnO.u There high-intensity of ZnO. The peak high of‐intensity ZnO shows peak that of the ZnO photoanode shows that is commonly the photoanode composed is ofcommonly ZnO. However, composed the small of ZnO. intensity However peaks, ofthe calcium small confirmintensity the peaks incorporation of calcium of Ca confirm in the ZnO the nanostructureincorporation of and Ca the in formationthe ZnO nanostructure of Ca-doped ZnOand the nanoparticles. formation of Ca‐doped ZnO nanoparticles. Figure9 9shows shows the the XRD XRD graph graph of unseeded of unseeded and seeded and seeded Ca-doped Ca ZnO‐doped nanorods ZnO nanorods using chemical using bathchemi deposition.cal bath deposition. The peaks The of ZnO peaks and of Ca ZnO are inand accordance Ca are in with accordance the Ca-doped with the ZnO Ca nanoparticles.‐doped ZnO Thenanoparticles. intensity of The the intensity most dominant of the dimostffraction dominant peaks diffraction of ZnO (002), peaks (101), of andZnO (102) (002), shows (101) an, and increase (102) inshows seeded an substrate. increase in This seeded effect sub iss duetrate. to This the formation effect is due of more to the aligned formation and ofsymmetric more aligned nanorods. and Thesymmetric incorporation nanorods. of CaThe in incorporation the ZnO wurtizle of Ca hexagonalin the ZnO structure wurtizle canhexagonal also be structure confirmed can using also the be XRDconfirmed graph. using the XRD graph.

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FigureFigure 8. XRD8. XRD spectra spectra of of Ca-doped Ca‐doped ZnO ZnO nanoparticles. nanoparticles. Figure 8. XRD spectra of Ca‐doped ZnO nanoparticles.

Figure 9. XRD graph of (a) Seeded Ca-doped ZnO NR and (b) Unseeded Ca-doped ZnO NR. Figure 9. XRD graph of (a) Seeded Ca‐doped ZnO NR and (b) Unseeded Ca‐doped ZnO NR. TheFigure UV-Vis 9. spectroscopy XRD graph of results(a) Seeded show Ca the‐doped maximum ZnO NR absorbance and (b) Unseeded based onCa‐ thedoped absorbance ZnO NR. onset on a specificThe wavelength. UV‐Vis spectroscopy Using this absorbanceresults show onset the maximum value, the bandabsorbance gap of based the sample on the can absorbance be calculated onset usingon The a the specific UV Tauc‐Vis plot. wavelength. spectroscopy Figure 10 Using resultsshows this theshow absorbance UV-Vis the maximum spectroscopy onset absorbance value results, the bandbased of the gapon as-prepared the of absorbance the sample Ca-doped onset can be ZnOoncalculated a nanoparticles, specific using wavelength. the as well Tauc as Using plot. the seededFigure this absorbance and10 shows unseeded onsetthe UV Ca-doped value‐Vis ,spectroscopy the ZnO band nanorods. gap results of The the of samplespectra the as‐ prepared consist can be ofcalculatedCa three‐doped portions. using ZnO the Thenanoparticles, T firstauc plot. part isFigure as a slopewell 10 indicatingasshows the seededthe an UV absorption ‐ andVis spectroscopyunseeded edge. Ca The ‐resultsdoped second ofZnO partthe nanorods.as is‐ aprepared plateau The regionCaspectra‐doped where consist ZnO absorption nanoparticles, of three portions. seems as constant well The asfirst the or part showsseeded is a a slope and gradual unseededindicating change Ca an with‐ dopedabsorption increasing ZnO edge.nanorods. wavelength. The s econdThe Thespectrapart third is consist a partplateau isof the threeregion second portions. where absorption abso Therption first edge. part seems The is a constant wavelengthsslope indicating or show of eachs an a gradualabsorption sample change are edge. obtained with The increasing s byecond the tangentialpartwavelength. is a plateau fitting T regionhe of the third linearwhere part portion abso is therption of second the seems spectrum, absorption constant which or edge. show is called Thes a gradual thewavelengths cut-o changeff wavelength, of with each increasing sample and this are giveswavelength.obtained the onset by T he wavelengththe ttangentialhird part of is lightfitting the that secondof isthe absorbed. linear absorption portion The edge.band of the gap The spectrum, of wavelengths the prepared which of films is each called are sample calculated the cut are‐off n usingobtained the Taucby the plot tangential by using fittingαhv = ofA(hv the linearEg) , whereportion A isof thethe absorption spectrum, co-ewhichfficient. is called Using the the cut Tauc‐off wavelength, and this gives the onset− wavelength of light that is absorbed. The band gap of the plotwavelength,prepared of the prepared filmsand this are Ca-doped gives calculated the ZnO onset using NP, wavelength the seeded Tauc Ca-doped plot of light by ZnO using that NR, is αhv absorbed. and = unseeded A(hv The−Eg )n band, Ca-doped where gap A of ZnO is the the NR,preparedabsorption the band films co gap ‐ areefficient. of calculatedthe Ca-doped Using usingthe ZnO T aucthe NP plot Tauc is equalof plotthe to prepared by 3.13 using eV, C the αhva‐doped unseeded = A(hv ZnO− Ca-dopedE NP,g)n, wseededhere ZnO ACa NRis‐doped the is 3.15absorptionZnO eV, NR and, coand the‐efficient. seededunseeded Ca-doped Using Ca‐ dopedthe ZnO Tauc ZnO NR plot isNR, 3.18of thethe eV. band prepared The increasegap ofC athe‐ indoped Ca band‐doped ZnO gap can NP,ZnO be seeded NP attributed is equalCa‐doped to to the 3.13 increaseZnOeV, NR the in, unseededand the unseeded nanorods Ca‐doped Ca diameter.‐doped ZnO ZnO Furthermore,NR isNR, 3.15 the eV band the, and increase gap the ofseeded the in theCa Ca‐ banddoped‐doped gap ZnO ZnO of theNP NR photoanodeis isequal 3.18 to eV. 3.13 ofThe DSSCeV,increase the will unseeded increasein band Ca thegap‐doped open can becircuitZnO attributed NR voltage, is 3.15 to which theeV , increaseand will the result seededin the in thenanorods Ca enhancement‐doped diameter. ZnO NR of the Furthermore,is overall3.18 eV. solar The the cellincreaseincrease efficiency. in i nband the Similarly, bandgap cangap there beof theattributed is a photoanode decrease to inthe of the increase DSSC band will gap in theincrease of thenanorods prepared the open diameter. Ca-doped circuit Furthermore,voltage ZnO nanorods, which the will increase in the band gap of the photoanode of DSSC will increase the open circuit voltage, which will

Energies 2020, 13, x FOR PEER REVIEW 12 of 15

Energies 2020, 13, x FOR PEER REVIEW 12 of 15 result in the enhancement of the overall solar cell efficiency. Similarly, there is a decrease in the band Energiesgapresult of 2020in the the, 13 prepared enhancement, 4863 Ca‐doped of the ZnOoverall nanorods solar cell from efficiency. the already Similarly, reported there bandis a decrease‐gap value in the of11 band ZnO of 14 nanorods.gap of the This prepared effect Ca is‐doped mainly ZnO due nanorods to the from addition the already of a highly reported conductive band‐gap seco valuend of group ZnO elementnanorods.—i.e This., Ca in effect a ZnO is lattice. mainly due to the addition of a highly conductive second group from the already reported band-gap value of ZnO nanorods. This eﬀect is mainly due to the addition element—i.e., Ca in a ZnO lattice. of a highly conductive second group element—i.e., Ca in a ZnO lattice.

Figure 10. UV‐Vis absorption graph of Ca‐doped ZnO nanoparticles, unseeded nanorods, and Figure 10. UV-Vis absorption graph of Ca-doped ZnO nanoparticles, unseeded nanorods, seededFigure nanorods. 10. UV‐Vis absorption graph of Ca‐doped ZnO nanoparticles, unseeded nanorods, and and seeded nanorods. seeded nanorods. The effect of using seeded and unseeded NR as a photoanode on the overall performance of the The effect of using seeded and unseeded NR as a photoanode on the overall performance of DSSC is also studied. The open circuit voltage, short circuit current, and fill factor are estimated the DSSCThe effect is also of studied. using seeded The open and circuitunseeded voltage, NR as short a photoanode circuit current, on the and overall fill factor performance are estimated of the using a solar simulator. The overall efficiency of the cell is calculated using the above quantities and usingDSSC a is solar also simulator. studied. The The open overall circuit efficiency voltage, of the short cell circuit is calculated curren usingt, and the fill above factor quantities are estimated and Equation (1), Equationusing a solar (1), simulator. The overall efficiency of the cell is calculated using the above quantities and Equation (1), Ƞ =Voc Isc FF (1) ŋ = (1) Pin Ƞ = (1) wherewhere PinPinis is the the input input power. power. Figure Figure 11 11shows shows the the short short circuit circuit current curre densitynt density for the for respective the respective open circuitopenwhere circuit voltage.Pin is voltage. the input power. Figure 11 shows the short circuit current density for the respective open circuit voltage.

Figure 11. Photovoltaic characteristic plot of Ca-doped ZnO nanoparticles, unseeded nanorods, andFigure seeded 11. Photovoltaic nanorods. characteristic plot of Ca‐doped ZnO nanoparticles, unseeded nanorods, and seededFigure nanorods11. Photovoltaic. characteristic plot of Ca‐doped ZnO nanoparticles, unseeded nanorods, and Tableseeded4 nanorodsshows the. open circuit voltage, short circuit current, ﬁll factor, and e ﬃciency of the prepared Ca-doped ZnO NP and Ca-doped ZnO NR when used as a photoanode in DSSC.

Energies 2020, 13, 4863 12 of 14

Table 4. Comparison of photovoltaic parameters for the prepared photoanodes.

2 Sample Voc (V) Jsc (mAcm− ) Fill Factor Eﬃciency η (%) Ca-doped ZnO NP 0.63 5.3 0.36 1.20 Unseeded Ca-doped ZnO NR 0.65 5.4 0.39 1.36 Seeded Ca-doped ZnO NR 0.67 5.5 0.42 1.55

The solar cell efficiency of the cell is the highest for seeded Ca-doped ZnO NR. The efficiency of 1.55% of seeded Ca-doped ZnO NR is mainly due to less series resistance. A thin layer of ZnO deposited before the Ca-doped ZnO NR provide enhanced charge transform due to the even growth of nanorods. The small efficiency of Ca-doped ZnO NP is mainly due to the higher grain boundaries effect in nanoparticles, which results in a small charge transfer. Small Voc can be attributed to large amount of electron–hole pair recombination.

5. Conclusions In this work, a comparative study has been performed to envisage the structural and optical properties of Ca-doped ZnO NP with seeded and unseeded Ca-doped ZnO NR. Simulation has been performed in fuzzy logic controller. The simulation depicts that based on the rules, the simulated and calculated values show an error of less than 1%. The incorporation of Ca in ZnO nanocrystal is conﬁrmed using XRD and EDX spectrum. A SEM image shows the formation of nanoparticles and nanorods in the prepared photoanodes. The band gap as well as the diameter of the seeded Ca-doped ZnO NR is greater than that of unseeded Ca-doped ZnO NR. The photovoltaic parameters of the prepared photoanodes were also studied for dye-sensitized solar cells. The results depict that the seeded Ca-doped ZnO NR shows the highest solar cell power conversion eﬃciency of 1.55%. The increase can be attributed to the decrease in series resistance due to the more aligned nanostructure.

Author Contributions: S.T., M.W.A. and V.E.B. have contributed in Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software; S.T., M.W.A. and M.A. contributed the Writing—original draft; M.I.T., M.A., N.W. and M.M.B., contributed in Formal analysis, Supervision, Validation, Visualization, Writing—review & editing. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: Authors would like to acknowledge Managing Editor, Energies MDPI, Helen Li, for providing authors discount voucher to manage the publication fee of this manuscript. The authors are also thankful to Nano-electronics Research Lab, GC University Lahore, for providing facilities during this research. Conﬂicts of Interest: The authors have no conﬂicts of interest.

References

1. Selopal, G.S.; Zhao, H.; Tong, X.; Benetti, D.; Navarro-Pardo, F.; Zhou, Y.; Barba, D.; Vidal, F.; Wang, Z.M.; Rosei, F. Highly Stable Colloidal “Giant” Quantum Dots Sensitized Solar Cells. Adv. Funct. Mater. 2017, 27, 1701468. [CrossRef] 2. Mehmood, U.; Rahman, S.-U.; Harrabi, K.; Hussein, I.; Reddy, B.V.S. Recent Advances in Dye Sensitized Solar Cells. Adv. Mater. Sci. Eng. 2014, 2014, 1–12. [CrossRef]

3. O’Regan, B.; Grätzel, M.; Gr, M. A low-cost, high-eﬃciency solar cell based on dye-sensitized colloidal TiO2 ﬁlms. Nature 1991, 353, 737–740. [CrossRef] 4. Ince, M.; Yum, J.-H.; Kim, Y.; Mathew, S.; Grätzel, M.; Torres, T.; Nazeeruddin, M.K. Molecular Engineering of Phthalocyanine Sensitizers for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2014, 118, 17166–17170. [CrossRef] 5. Ardo, S.; Meyer, G.J. Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2semiconductor surfaces. Chem. Soc. Rev. 2009, 38, 115–164. [CrossRef][PubMed] Energies 2020, 13, 4863 13 of 14

6. Suhaimi, S.; Mukhzeer, M.S.; Alahmed, Z.; Chyský, J.; Reshak, A.H. Materials for Enhanced Dye-sensitized Solar Cell Performance: Electrochemical Application. Int. J. Electrochem. Sci. 2015, 10, 2859–2871.

7. Akhlaq, M.; Khan, Z.S. Synthesis and characterization of electro-spun TiO2 and TiO2-SnO2 composite nano-ﬁbers for application in advance generation solar cells. Mater. Res. Express 2020, 7, 015523. [CrossRef] 8. Dembele, K.T.; Selopal, G.S.; Soldano, C.; Nechache, R.; Rimada, J.C.; Concina, I.; Sberveglieri, G.; Rosei, F.;

Vomiero, A. Hybrid Carbon Nanotubes–TiO2 Photoanodes for High Eﬃciency Dye-Sensitized Solar Cells. J. Phys. Chem. C 2013, 117, 14510–14517. [CrossRef] 9. Hossain, M.K.; Al Mortuza, A.; Sen, S.K.; Basher, M.; Ashraf, M.; Tayyaba, S.; Mia, M.; Uddin, M.J.

A comparative study on the inﬂuence of pure anatase and Degussa-P25 TiO2 nanomaterials on the structural and optical properties of dye sensitized solar cell (DSSC) photoanode. Optik 2018, 171, 507–516. [CrossRef]

10. Rani, R.; Sharma, S. Preparation and Characterization of SnO2 Nanofibers via Electrospinning. Adv. Nanoparticles 2016, 5, 53–59. [CrossRef] 11. Martinson, A.B.F.; Elam, J.W.; Hupp, J.T.; Pellin, M. ZnO Nanotube Based Dye-Sensitized Solar Cells. Nano Lett. 2007, 7, 2183–2187. [CrossRef][PubMed] 12. Gong, J.; Liang, J.; Krishnan, S. Review on dye-sensitized solar cells (DSSCs): Fundamental concepts and novel materials. Renew. Sustain. Energy Rev. 2012, 16, 5848–5860. [CrossRef] 13. Liu, X.; Yuan, R.; Liu, Y.; Zhu, S.; Lin, J.; Chen, X. Niobium pentoxide nanotube powder for efficient dye-sensitized solar cells. New J. Chem. 2016, 40, 6276–6280. [CrossRef] 14. Kim, K.H.; Utashiro, K.; Abe, Y.; Kawamura, M. Structural Properties of Zinc Oxide Nanorods Grown on Al-Doped Zinc Oxide Seed Layer and Their Applications in Dye-Sensitized Solar Cells. Materials 2014, 7, 2522–2533. [CrossRef] 15. Ali, B.; Ashraf, M.W.; Tayyaba, S. Simulation, Fuzzy Analysis and Development of ZnO Nanostructure-based Piezoelectric MEMS Energy Harvester. Energies 2019, 12, 807. [CrossRef] 16. Basher, M.K.; Khalid Hossain, M.; Afaz, R.; Tayyaba, S.; Akand, M.A.R.; Rahman, M.T.; Eman, N.M. Study and investigation of phosphorus doping time on emitter region for contact resistance optimization of monocrystalline silicon solar cell. Results Phys. 2018, 10, 205–211. [CrossRef] 17. Yangyang, Z.; Manoj, K.R.; Elias, K.S.; Yogi, D.G. Synthesis, Characterization, and Applications of ZnO Nanowires. J. Nanomater. 2012, 2012, 22. 18. Umar, A.; Singh, P.; Al-Ghamdi, A.A.; Al-Heniti, S. Direct growth of ZnO nanosheets on FTO substrate for dye-sensitized solar cells applications. J. Nanosci. Nanotechnol. 2010, 10, 6666–6671. [CrossRef] 19. Wahyuono, R.A.; Schmidt, C.; Dellith, A.; Dellith, J.; Schulz, M.; Seyring, M.; Rettenmayr, M.; Plentz, J.; Dietzek, B. ZnO nanoflowers-based photoanodes: Aqueous chemical synthesis, microstructure and optical properties. Open Chem. 2016, 14, 158–169. [CrossRef] 20. Sunandan, B.; Joydeep, D. Hydrothermal growth of ZnO nanostructures. Sci. Technol. Adv. Mater. 2009, 10, 13001. 21. Gupta, A.K.; Kashyap, V.; Gupta, B.K.; Nandi, S.P.; Saxena, K.; Khare, N. Synthesis of ZnO Nanorods by Electrochemical Deposition Method and Its Antibacterial Activity. J. Nanoeng. Nanomanufact. 2013, 3, 348–352. [CrossRef] 22. Wu, J.-J.; Liu, S.-C. Low-Temperature Growth of Well-Aligned ZnO Nanorods by Chemical Vapor Deposition. Adv. Mater. 2002, 14, 215–218. [CrossRef] 23. Seung, C.L.; Ye, Z.; Cheol, J.L.; Hyun, R.; Hwack, J.L. Low-Temperature Growth of ZnO Nanowire Array by a Simple Physical Vapor-Deposition Method. Chem. Mater. 2003, 15, 3294–3299. 24. Mahshid, P.; Pirooz, M.; Davoud, H.F.; Mohammadreza, K.E. Synthesis of ZnO nanorods via chemical bath deposition method: The effects of physicochemical factors. Ceram. Int. 2016, 42, 173–184. 25. Polat, I.; Yılmaz, S.; Tomakin, M.; Bacaksız, E. Role of Mg doping in the structural, optical, and electrical characteristics of ZnO-based DSSCs. Turk. J. Phys. 2017, 41, 160–170. [CrossRef] 26. Istrate, A.-I.; Nastase, F.; Mihalache, I.; Comanescu, F.; Gavrila, R.; Tutunaru, O.; Romanitan, C.; Tucureanu, V.; Nedelcu, M.; Müller, R. Synthesis and characterization of Ca doped ZnO thin films by sol–gel method. J. Sol-Gel Sci. Technol. 2019, 92, 585–597. [CrossRef] 27. Mahdhi, H.; Djessas, K.; Ben Ayadi, Z. Synthesis and characteristics of Ca-doped ZnO thin films by rf magnetron sputtering at low temperature. Mater. Lett. 2018, 214, 10–14. [CrossRef] 28. Gul, M.; Amin, D.; Abbas, M.; Ilyas, S.; Shah, N. Synthesis and characterization of magnesium doped ZnO

nanostructures: Methane (CH4) detection. J. Mater. Sci. Mater. Electron. 2019, 30, 5257–5265. [CrossRef] Energies 2020, 13, 4863 14 of 14

29. Sarwar, G.; Ashraf, M.W. Parametric estimation of Group II element doped zinc oxide nanostructures using fuzzy logic. J. Intell. Fuzzy Syst. 2020, 38, 5865–5875. [CrossRef] 30. Das, N.; Wongsodihardjo, H.; Islam, S. Modeling of multi-junction photovoltaic cell using MATLAB/Simulink to improve the conversion eﬃciency. Renew. Energy 2015, 74, 917–924. [CrossRef] 31. Wasim, M.F.; Tayyaba, S.; Ashraf, M.W.; Ahmad, Z. Modeling and Piezoelectric Analysis of Nano Energy Harvesters. Sensors 2020, 20, 3931. [CrossRef][PubMed] 32. Wasim, M.F.; Ashraf, M.W.; Tayyaba, S.; Nazir, A.S. Simulation and synthesis of ZnO nanorods on AAO nano porous template for use in a MEMS devices. Dig. J. Nanomater. Biostruct. 2019, 14, 559–567.

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