Effects of the morphology and diameter of TiO2 nanofibers as light-scattering layers on the efficiency of dye-sensitized solar cells

In this study, TiO2 nanofibers was synthesized using electrospinning method with varying applied voltage (10 kV–20 kV) to obtain high surface-volume ratio and porous material. As the applied voltage increased, diameter of TiO2 nanofibers decreased and the presence of beads disappeared resulting in homogeneous nanofibers. At applied voltage higher than 16 kV, TiO2 nanofibers have diameter less than 100 nm. TiO2 nanofibers are deposited on top of TiO2 nanoparticles which act as a light-scattering layer. Based on the I–V characteristic, TiO2 nanofibers produced by applied voltage of 18 kV gives the highest efficiency of 2.38% with JSC 6.37 mA cm−2, VOC of 0.74 V and fill factor of 50.54%. Adding the TiO2 nanofibers as light-scattering layer improve and extend the path of light, thereby increasing the power conversion efficiency of dye-sensitized solar cells.


Introduction
Dye-sensitized solar cells (DSSC) has attracted interest in research as the next-generation solar cell due to its simple, low-cost fabrication process, and environmental friendliness [1].One of the key components that play a major role in DSSC's stability and efficiency is photoanode [2].The photoanode facilitates the charge transfer during charge carrier generation inside the cell.The choice of photoanode material needs to pay attention to the high electron mobility, which is required to reduce the electron-hole recombination and increase the efficiency [3].To facilitate electron transfer, the energy level of photoanode conduction band must be matched with the LUMO (Lowest Unoccupied Molecular Orbital) level of dye molecule [4].In addition, a high-porosity large surface area of photoanode is necessary for dye diffusion [5].For the DSSC to work efficiently, the photoanode material must also have the ability to transmit visible light and good adhesion to the conducting substrate [5].
Titanium dioxide (TiO 2 ), an oxide semiconductor material, is widely used in DSSC as a photoanode.TiO 2 anatase phase has a band gap of 3.2 eV, is non-toxic, has a large surface area, is abundantly available, inexpensive, and has good stability [6,7].Most importantly, TiO 2 shows good optical and electronic properties and thus has the potential to be used in optoelectronic applications [8].Among various forms of nanostructures, TiO 2 nanofibers have attracted much attention in research due to its advantages of having a large surface area and high porosity with good physical and mechanical properties [9,10].There are many applications of nanofiber such as photocatalyst [11], sensors [12], drug delivery [13], and many other different applications [14].Furthermore, with the advantages possessed by nanofibers, they have the potential to be used as photoanodes in DSSCs.Sabba, et al made TiO 2 nanofibers with a diameter of 150-200 nm as photoanodes on DSSC resulting in the highest efficiency of 1.42% with a V oc of 0.84 V and J sc of 3.93 mA cm −2 [15].Kadam et al synthesized TiO 2 nanofibers as photoanode using electrospinning with different parameter electrospinning, resulting in highest efficiency of 3.13% produced from diameter nanofibers of 66.83 nm with V oc of 0.7 V and J sc of 9.51 mA [16].The use of nanofibers as a photoanode in DSSC can be a promising solution to replace nanoparticle films, providing a direct pathway for the collection of the generated charge across the device [17].A light-scattering layer is introduced to improve the performance of DSSC devices by increasing light harvesting on the photoanode [18].The scattering layer usually consists of larger TiO 2 particles (>100 nm), which can effectively collect light [19].However, the scattering layer will reduce the capacity of the photoanode to load dye.The nanofibers structure has a high porosity and light-scattering effect so it can be used as a light-scattering layer.In order to make the lightscattering layer without reducing the ability of the photoanode to load the dye, a homogeneous nanofiber structure with an optimum diameter is required.Figure 1 shows a dye-sensitized solar cell with (a) and without (b) nanofibers serving as a light-scattering layer.The addition of TiO 2 nanofibers as a light-scattering layer increases light harvesting in DSSC.The scattering effect of the TiO 2 nanofibers layer reduces the loss of light during illumination.Consequently, a DSSC structure with a light-scattering layer generates more photons than one without.
The Easy and simple method to produce nanofibers is electrospinning.This technique is widely used because it does not require high pressure and temperature, or vacuum conditions, as well as low comparative costs, and high production levels [20].The working principle of electrospinning is the polymer solution is inserted into a syringe, and continuously sprayed at adjusted constant speed.The polymer exits through the spinneret hole (jet) and is then stretched by electrostatic energy from the applied DC electric voltage.The applied DC voltage affects the shape of the Taylor cone resulting nanofiber structure collected on the collector screen [21].Yarin and Reneker reported that process control of electrospinning resulted in fibers with nanoscale diameters [22].The morphology and diameter of the obtaining nanofibers depend on polymer properties and parameters processes, such as polymer molecular weight, solvent, applied voltage, solution viscosity, needle diameter, and needle tip distance to collector [23,24].The working principle of comprehensive electrospinning has been discussed in another paper [25].
Arshad, et al synthesized TiO 2 nanofibers using polyvinyl pyrollidone with an applied voltage of 20 kV, the distance between the needle tip to the collector was 16-20 cm, and a flow rate of 0.5 ml min −1 , resulting diameter in the nanoscale [26].Otieno, et al produced diameter TiO 2 nanofibers up to 140 nm with different electrospinning parameters i.e. polyvinyl pyrrolidone as matrix polymer, 25 kV applied voltage, and 1 ml h −1 flow rate [27].Someswararo, et al have investigated the smooth and randomly distributed nanofibers with diameters 293, 226, 189, and 175 nm by applying voltages 8, 9, 10 and 11 kV using polyvinyl pyrollidone as a polymer matrix [28].El-Lateef et al has succeeded in forming TiO 2 nanofibers with different matrix polymer i.e. polyvinyl acetate at an applied voltage of 12 kV, and the distance between the needle tip to the collector was 16 cm, resulting diameter of nanofibers around 100-250 nm [29].Based on these reseraches, the difference in applied voltage affects the diameters of nanofibers.Therefore, in this study, we varied the applied voltage in the electrospinning process to obtain TiO 2 nanofibers with diameters below 100 nm.The TiO 2 nanofibers were then used as a photoanode in DSSC with ruthenium as the dye molecule.We analyzed the effect of applied voltage on the morphology and diameter of TiO 2 nanofibers and correlated it to DSSC efficiency.

Synthesize of TiO 2 nanofibers
The nanofibers film can be easily peeled off from the FTO conductive glass substrate due to shrinkage during calcination.To increase the adhesion between the film and the substrate, nanoparticle film was given first using the spin-coating method before coating the nanofibers using electrospinning [30].TiO 2 nanofibers synthesis was carried out using electrospinning technique using 1.5 ml titanium (IV) isopropoxide (TTIP) precursor, 0,8 ml Triton-X, 0,69 g polyvinyl acetate (PVAc), 4 ml N, N-dimethylformamide (DMF), and 0,5 ml of acetic acid which was stirred for 12 h to obtain a solution with a viscosity of 140 mPa.s.The solution is put into a syringe with a needle connected to a high-voltage power supply.The voltage applied in this experiment was varied by 10 kV, 12 kV, 14 kV, 16 kV, 18 kV, and 20 kV with a flow rate of 0.7 ml h −1 .The TTIP/PVAc composite nanofibers formed will be collected on the FTO that has been coated with TiO 2 nanoparticles using spin coating.The distance of the collector from the tip of the needle is 10 cm.The electrospinning process was carried out for 10 min.Then, the samples obtained were evaporated at a temperature of 80°C for 1 h.With a heating rate of 1 °C min −1 , the sample was calcined.After reaching a temperature of 450°C, the calcination was maintained at that temperature for 45 min and cooled to room temperature [31].

Characterization of TiO 2 nanofibers
The characteristics of TiO 2 nanofibers produced in this experiment were characterized using a Scanning Electron Microscope (SEM) to determine the morphology and x-ray Diffraction (XRD) to determine the phase and crystal structure using an x-ray diffractometer with Cu K α radiation (λ = 15.4056nm) between 0 and 90°.

Characterization of dye-sensitized solar cells
The prepared photoanode was immersed in a mixture of ethanol and Chenodeoxycholic acid (1:1, v/v) containing ruthenium (II) dye (N719, Solaronix) with a concentration of 0.7 mM for 18 h.Then, the photoanode was rinsed with acetonitrile to remove the unbound dye.DSSC was assembled by stacking a photoanode, a platinum-coated counter electrode, and a 60 μm sealing spacer (Surlyn).In the end, the electrolyte solution was injected between the TiO 2 nanofiber photoanode and platinum-coated counter electrode.The photocurrent-voltage (I-V) characteristics were measured under an illumination of 100 mW cm −2 .The active area of DSSC cells was fixed to 0.25 cm 2 .

Results and discussion
3.1.Scanning electron microscope (SEM) 3.1.1.Morphology of TiO 2 nanofibers Figure 2 shows the SEM results of TiO 2 nanofibers at various applied voltages in the range of 10 kV to 20 kV.Based on these results, it is clear that the presence of beads gradually disappeared with increasing applied voltage.Figures 2(a)-(c) shows the presence of TiO 2 beads on the nanofibers structure, but starting at an applied voltage of 16 kV the presence of beads completely disappeared as shown in figures 2(d)-(f).In the electrospinning process, it is necessary to consider the Coulomb, gravitational, and electrostatic forces which are responsible for the stretching of the charged jet.The Coulomb force and the electrostatic force together affect the elongation and thinning of the straight beam portion.The length and the behavior of the jet have a significant influence on the diameter of the nanofibers [25].An electric field is applied to the droplet of polymer solution at the end of the spinneret, and the surface of the solution becomes charged.When the electric field is high enough, the electrostatic forces overcome the surface tension to produce a straight and stable jet.Taylor cones and jets are affected by gravity, causing droplets and jets of fiber to be dragged to the bottom of the collector [32].In this study at a voltage of 10 kV a Taylor cone was formed but the emission that was formed was unstable and did not reach the collector.Under these conditions, the resulting nanofiber has many beads and the diameter is about 200 nm.The formation of the Taylor cone is governed by the electrostatic force of the surface charge created by an externally applied voltage.At low voltage, the jet becomes less charged even though the initial jet is formed but the electrostatic force cannot overcome the surface tension, so beads were formed.
At low applied voltage (10 kV-14 kV), the magnitude of the Coulomb force is not high when compared to the surface tension resulting in large diameter fibers and the presence of beads as shown in figure 2(a).At an applied voltage of 16 kV, the Coulomb force and the surface tension acting on the electrospinning process were balanced, resulting in a smaller fiber diameter and the beads disappeared as shown in figure 2(d).As the voltage increased higher than 16 kV, the Coulomb force was much greater than the surface tension, producing the diameter of the nanofibers less than 100 nm, and the beads completely disappeared as shown in figures 2(e)-(f).In this study, we found that the increasing applied voltage reduces the presence of beads.In contrast to the results of the research by Syed Bakar, et al an increase in voltage produces beads [33].Onozuka, et al used an applied voltage of 20 kV which produced a nanofibers diameter of around 170-700 nm [23].In this study, by using the same voltage, it was possible to produce nanofibers with good morphology and a diameter of less than 100 nm.The photoanode layer is approximately 11.68 ± 0.26 μm thick, while the TiO 2 nanoparticle layer is around 10.46 ± 0.15 μm thick.The electrospinning time was consistent for all applied voltages, so the thickness of the TiO2 nanofiber layer was assumed to be uniform, at approximately 1.07 ± 0.19 μm.

Diameter distribution
As the applied voltage increases, the nanofibers diameter distribution shifts to a smaller value as presented in figure 2. As shown in figures 2(a)-(c), the broad peak of diameter distribution indicates that the nanofiber has a wide diameter in the range of 60-300 nm and it is confirmed that the formed nanofibers were inhomogeneous.The diameter of 200-300 nm probably correlates to the diameter of beads that were produced at low applied voltage.At higher applied voltage, the diameter distribution showed a sharp peak as shown in figures 2(d)-(f) with a narrow diameter in the range of 50-180 nm indicating that the nanofibers have a more homogeneous morphology.
The average diameter of nanofibers decreases with increasing of applied voltage as shown in figure 3. The average diameter at an applied voltage of 10 kV-14 kV is around 180 nm, while at an applied voltage of 16 kV the average diameter is around 100 nm.At a higher applied voltage of 18 kV and 20 kV, we found that the diameter reached 94.6 nm.
Based on figure 3, at applied voltage of 18 kV shows the smallest standard deviation of the diameter that indicates the nanofibers are the most homogeneous compared to others.Based on the analysis of diameter nanofibers, we found that the applied voltage affects the morphology and diameter of nanofibers.High applied voltage produces higher electrostatic forces to stretch the jet resulting in the long and thin fiber.In the electrospinning process, the applied voltage must exceed the threshold voltage in order to provide sufficient charge for the solution to be pulled by electrostatic forces from the collector.In addition, the most important thing is that the applied voltage must also be high enough to produce nanofibers with a small diameter, homogeneous, and without beads.Therefore, by knowing the effect of the applied voltage on the electrospinning process, we can control the desired nanofibers diameter.

X-ray diffraction (XRD)
Figure 4 shows the XRD diffraction pattern of TiO 2 nanofibers at various applied voltages in the range of 10 kV − 20 kV.The diffraction pattern of the measurement results was compared with the diffraction pattern from the Crystallography Open Database (COD) no.data 96-500-0224.The highest intensity of all XRD patterns is around 2θ = 25°.TiO 2 nanofibers diffraction pattern for all voltage variations shows the anatase phase as the calcination process was done at 450°C.Anatase TiO 2 phase is suitable for the application of photoanode due to its photoactive properties compared to other TiO 2 phases.It is known that the anatase TiO 2 phase has low electron/hole pair recombination which matches with the photoanode criteria [6].

Current density-voltage characterization (J-V)
Figure 5 shows the current density-voltage characteristic curve (J-V) of DSSC with TiO 2 nanofibers as a photoanode layer obtained at various applied voltage of the electrospinning process.DSSC's parameters such as short circuit current density (J sc ), open circuit voltage (V oc ) fill factor (FF) and conversion efficiency (η) were derived from the J-V curve as shown in table 1.
J sc increases with increasing applied voltage to the electrospinning process to produce TiO 2 nanofibers.If correlated with the nanofibers morphology, as shown in figure 1, the higher applied voltage results in a more homogeneous nanofibers structure without beads and smaller diameter.Nanofibers with these characteristics provide a continuous path that facilitates electron transport therefore it can increase efficiency compared to using nanoparticles as photoanodes.The nanofiber structure effectively facilitates the transport of electrons from one nanofiber to another which minimizes the recombination of electrons in the photoanode.The highest J sc (6.37 mA cm −2 ) came from the photoanode with TiO 2 nanofibers with a diameter of 96.81 nm (produced by the applied voltage of 18 kV).TiO 2 nanofibers produced by the applied voltage of more than 14 kV showed higher J sc and efficiency than TiO 2 nanoparticles.This is due to the scattering ability of the TiO 2 nanofibers which can act as a light-scattering layer.Nurhoffish et al stated that the presence of TiO 2 nanofibers acts as a light-scattering layer which can increase light harvesting in DSSC [34].Due to the scattering effect of TiO 2 nanofibers, it can reduce the amount of light that can be lost during the DSSC process.As a result, more photons can be generated than are produced by TiO 2 nanoparticles.The nanofibers produced at an applied voltage of 20 kV have a smaller diameter compared to the applied voltage of 18 kV, but it cannot load more dye.Therefore, the current generated on the TiO 2 nanofibers photoanode is also lower because it uses no more sunlight than the TiO 2 nanofibers photoanode produced at a voltage of 18 kV.Likewise, the low J sc value produced by nanoparticles is due to the denser morphology of TiO 2 nanoparticles compared to TiO 2 nanofibers so the absorbed dye molecules are not optimum.However, if TiO 2 nanoparticles were compared with TiO 2 nanofibers produced by the applied voltage of 10 kV and 12 kV, the J sc value and efficiency of the nanoparticles would be greater.This is because the photoanode has a large diameter and many beads have large gaps between the nanofibers that stack on top of each other.This condition causes a lot of trapping surface areas that can inhibit electron transport to move along the nanofibers, causing a low J sc value.The V oc of DSSC based on nanoparticle TiO 2 and nanofibers TiO 2 is generated from the layer interface in the cell.Good contact between nanofibers film and FTO substrate, the absence of surface defects and structure defects inhibit the electron recombination process resulting high value of V oc .In this study, DSSC with TiO 2 nanofibers as a photoanode gave V oc at around 0.74-0.80Volt.The smaller nanofibers diameter causes an increase in surface area which may lead to more electron recombination sites and consequently decreasing V oc .The smallest V oc (0.74 Volt) was obtained from the photoanode with the smallest nanofibers diameter of 94.62 nm which was produced by the electrospinning process at an applied voltage of 20 kV.The highest V OC voltage (0.80 Volts) was obtained from TiO 2 nanoparticles and TiO 2 nanofibers with the largest diameter of 192.2 nm in the electrospinning process with a voltage of 10 kV.However, the morphology of the nanofibers at 10 kV has many beads and was not homogeneous, while the TiO 2 nanoparticles have a dense morphology, resulting in the lowest J sc and efficiency, even though it has a high V oc value.In this study, we found that the highest efficiency of DSSC-based TiO 2 nanofibers photoanode was 2.38% resulting from the electrospinning process at a voltage of 18 kV.The efficiency produced by TiO 2 nanofibers is greater than TiO 2 nanoparticles because nanofibers diameters of less than ∼100 nm with no beads can provide electron transport.After obtaining the optimum conditions of the nanofiber, to improve the efficiency of DSSC, nanofiber composites of oxide semiconductor materials have been made to enhance electron mobility [35].In addition, it is also possible modify the morphology and surface of the nanofibers can optimize dye absorption [36].

Conclusion
TiO 2 nanofibers have been successfully prepared using the electrospinning method.The SEM results show that at higher voltages the nanofibers diameters are smaller and the beads are reduced.TiO 2 nanofibers as a photoanode were employed as an effective scattering layer to extend the light path in the DSSC device.TiO 2 nanofibers produced by applying a voltage of 18 kV showed the highest efficiency.At this given voltage, TiO 2 nanofibers display a long homogeneous one-dimensional structure with no beads and a diameter of less than 100 nm, resulting in a high pore morphology to maximize dye loading and achieve the highest efficiency.

Figure 3 .
Figure 3.Effect of the applied voltage on average fiber diameter.

Figure 5 .
Figure 5. Current density-voltage characteristics of DSSC consisting of nanofibrous TiO 2 with varying applied voltage.

Table 1 .
Photoelectric parameters of TiO 2 nanofibers photoanode with various applied voltage.