Study of How Photoelectrodes Modiﬁed by TiO 2 / Ag Nanoﬁbers in Various Structures Enhance the E ﬃ ciency of Dye-Sensitized Solar Cells under Low Illumination

: Dye-sensitized solar cells (DSSCs) are low-cost solar cells belonging to the thin-ﬁlm photovoltaic cell type. In this study, we studied the photovoltaic performances of DSSCs based on titanium dioxide (TiO 2 ) nanoﬁbers (NFs) containing silver (Ag) nanoparticles (NPs) under low illumination. We used the sol-gel method with the electrospinning technique to prepare the TiO 2 NFs containing Ag NPs. Herein, we used two ways to add TiO 2 / Ag NFs to modify the photoelectrode successfully and enhance the performance of DSSCs. One way was that the TiO 2 / Ag NFs were mixed with pristine TiO 2 ; the other way was that the TiO 2 / Ag NFs were seeded beside the TiO 2 colloid layer as an additional layer on the photoelectrode of the DSSC. According to this experiment, the photovoltaic conversion e ﬃ ciency of the DSSC which had TiO 2 / Ag NF seeded as an additional layer on the photoelectrode (5.13%) was increased by 28% compared to the DSSC without the photoelectrode modiﬁcation (3.99%). This was due to the suppression of electron recombination and the more e ﬀ ective utilization of the light radiation by adding the TiO 2 / Ag NFs. Because of the good conductivity of Ag, the electrons were quickly transported and electron recombination was reduced. In addition, the photovoltaic conversion e ﬃ ciency of the DSSC which had TiO 2 / Ag NF seeded as an additional layer on the photoelectrode increased from 5.13% to 6.23% during the decrease in illumination from 100 mW / cm 2 to 30 mW / cm 2 ; however, its photovoltaic conversion e ﬃ ciency decreased to 5.31% when the illumination was lowered to 10 mW / cm 2 .


Introduction
In recent years, renewable energy has become critical to sustainable development due to restrictions on fossil fuels. Solar or photovoltaic cells have been extensively researched, especially silicon solar cells. However, silicon solar cells have some disadvantages such as high cost, complicated processes, and time consuming processes. Dye-sensitized solar cells (DSSCs) are low-cost solar cells belonging to Energies 2020, 13,2248 3 of 14 nanofibers by electrospinning. The nanofibers materials were kept in an ambient condition for one day. Finally, we annealed the nanofibers at 500 • C for one hour.

Preparation of the TiO 2 -Ag Composited Photoelectrode
We cut a 0.5 cm × 0.5 cm square working area insulating tape and pasted the insulating tape onto cleaned fluorine-doped tin oxide (FTO)glass to control the coating area of the TiO 2 slurry on the FTO glass. The TiO 2 slurry was deposited onto the FTO glass substrate by the spin coating method and the doctor blade method, and used as the photoanode (denoted as TiO 2 photoanode) of the dye-sensitized solar cell. As stated above, we used two ways to add TiO 2 /Ag NF to modify the photoelectrode. One was that TiO 2 /Ag NFs were mixed with the TiO 2 slurry, the other way was that the TiO 2 /Ag NFs were seeded beside the TiO 2 colloid layer as an additional layer on the photoelectrode of the DSSCs. In preparation for adding the TiO 2 /Ag NFs to modify the photoelectrode by mixing the TiO 2 /Ag NFs with the TiO 2 slurry, TiO 2 paste was coated on fluorine-doped tin oxide (FTO) glass by the doctor blade method. The blade colloid consisted of 0.75 g of TiO 2 powder (P25), 0.25 g of TiO 2 /Ag NF, 1.0 mL of deionized water (D.I. water), and 0.1 mL of anhydrous ethanol. The colloid was uniformly mixed by a magnetic stirrer for one day. The mixed colloid was then coated on the FTO glass. The photoelectrode was calcined at 450 • C for one hour, and then the photoelectrode was soaked in the N3 dye for 24 h. The composite photoelectrode, which was named TiO 2 / Ag NF mixed photoanode (TANP), was completed. Another type of photoelectrode was prepared by doctor blading the TiO 2 paste on the FTO glass. Then, the TiO 2 /Ag NFs were seeded beside the TiO 2 paste as an additional layer on the photoelectrode, which named TiO 2 / Ag NF additional layer photoanode (TANLP). We used stylus profilometry to measure the thickness of the photoanodes. The thickness of the TiO 2 photoanode, TANP photoanode, and TANLP photoanode were 15.07, 18.36, and 23.22 µm, respectively.

Fabrication of the Dye-Sensitized Solar Cell
The Pt counter electrode of the DSSCs was fabricated using radio frequency (RF) sputtering Pt on the FTO glass substrate. The prepared photoelectrode was coupled with the Pt counter electrode. An iodide electrolyte was introduced between the electrodes. After that, the photoelectrode, electrolyte, and Pt counter electrode were assembled in a sandwich structure. The structure of the DSSCs is shown in Figure 1.
Energies 2020, 13, x FOR PEER REVIEW 3 of 15 prepared to form nanofibers by electrospinning. The nanofibers materials were kept in an ambient condition for one day. Finally, we annealed the nanofibers at 500 °C for one hour.

Preparation of the TiO2-Ag Composited Photoelectrode
We cut a 0.5 cm × 0.5 cm square working area insulating tape and pasted the insulating tape onto cleaned fluorine-doped tin oxide (FTO)glass to control the coating area of the TiO2 slurry on the FTO glass. The TiO2 slurry was deposited onto the FTO glass substrate by the spin coating method and the doctor blade method, and used as the photoanode (denoted as TiO2 photoanode) of the dyesensitized solar cell. As stated above, we used two ways to add TiO2/Ag NF to modify the photoelectrode. One was that TiO2/Ag NFs were mixed with the TiO2 slurry, the other way was that the TiO2/Ag NFs were seeded beside the TiO2 colloid layer as an additional layer on the photoelectrode of the DSSCs. In preparation for adding the TiO2/Ag NFs to modify the photoelectrode by mixing the TiO2/Ag NFs with the TiO2 slurry, TiO2 paste was coated on fluorinedoped tin oxide (FTO) glass by the doctor blade method. The blade colloid consisted of 0.75 g of TiO2 powder (P25), 0.25 g of TiO2/Ag NF, 1.0 ml of deionized water (D.I. water), and 0.1 ml of anhydrous ethanol. The colloid was uniformly mixed by a magnetic stirrer for one day. The mixed colloid was then coated on the FTO glass. The photoelectrode was calcined at 450 °C for one hour, and then the photoelectrode was soaked in the N3 dye for 24 hours. The composite photoelectrode, which was named TiO2/ Ag NF mixed photoanode (TANP), was completed. Another type of photoelectrode was prepared by doctor blading the TiO2 paste on the FTO glass. Then, the TiO2/Ag NFs were seeded beside the TiO2 paste as an additional layer on the photoelectrode, which named TiO2/ Ag NF additional layer photoanode (TANLP). We used stylus profilometry to measure the thickness of the photoanodes. The thickness of the TiO2 photoanode, TANP photoanode, and TANLP photoanode were 15.07, 18.36, and 23.22 µm, respectively.

Fabrication of the Dye-Sensitized Solar Cell
The Pt counter electrode of the DSSCs was fabricated using radio frequency (RF) sputtering Pt on the FTO glass substrate. The prepared photoelectrode was coupled with the Pt counter electrode. An iodide electrolyte was introduced between the electrodes. After that, the photoelectrode, electrolyte, and Pt counter electrode were assembled in a sandwich structure. The structure of the DSSCs is shown in Figure 1.

Measurement System
The photovoltaic parameters of the DSSCs were measured by the solar simulator (MFS-PV-Basic-HMT, Taiwan) at a sunlight intensity of 100 mW/cm 2 . The Nyquist plot of interface impedance for the DSSCs was investigated by electrochemical impedance spectroscopy (BioLogic SP-150, France), and the frequency of measurement was set from 1 to 50 MHz. The morphology of the TiO2/Ag nanofibers was characterized by a field-emission scanning electron microscope (FE-SEM,

Measurement System
The photovoltaic parameters of the DSSCs were measured by the solar simulator (MFS-PV-Basic-HMT, Taiwan) at a sunlight intensity of 100 mW/cm 2 . The Nyquist plot of interface impedance for the DSSCs was investigated by electrochemical impedance spectroscopy (BioLogic SP-150, France), and the frequency of measurement was set from 1 to 50 MHz. The morphology of the TiO 2 /Ag nanofibers was characterized by a field-emission scanning electron microscope (FE-SEM, Hitachi S4800-l, Japan). We used XRD to characterize the Ag/TiO 2 nanofibers. The amount of absorbed dye molecules was determined by detaching the dye from the photoanode in 1M NaOH solution and measuring the absorption spectra of N3 solution on a UV-Vis spectrophotometer (Perkin Elmer precise Lambda 850, America). The UV-Vis spectrophotometer was also employed to measure the UV-vis absorption spectra of Ag/TiO 2 NF. It was measured for the dye-sensitized solar cells by Incident Photon-to-electron Conversion Efficiency (IPCE). The Solar Cell Spectral Response Measurement System (IPCE, QE-R) was procured from Enlitech, Taiwan.

Morphology and Characterization of Ag/ TiO 2 Nanofiber
Figure 2a, which we have characterized from our previous work [21], shows an image of the top view of the TiO 2 layer. Figure 2b shows an image of the TiO 2 /Ag NF using field emission scanning electron microscopy (FE-SEM). From Figure 2b, the average diameter of the TiO 2 /Ag NF is about 168 nm. The connected nanofiber structure provides better charge transport paths and prohibits the recombination of electrons. Therefore, it can provide a better passageway for electron transmission and high charge mobility [22]. Hitachi S4800-l, Japan). We used XRD to characterize the Ag/TiO2 nanofibers. The amount of absorbed dye molecules was determined by detaching the dye from the photoanode in 1M NaOH solution and measuring the absorption spectra of N3 solution on a UV-Vis spectrophotometer (Perkin Elmer precise Lambda 850, America). The UV-Vis spectrophotometer was also employed to measure the UV-vis absorption spectra of Ag/TiO2 NF. It was measured for the dye-sensitized solar cells by Incident Photon-to-electron Conversion Efficiency (IPCE). The Solar Cell Spectral Response Measurement System (IPCE, QE-R) was procured from Enlitech, Taiwan.

Results and Discussion
3.1. Morphology and Characterization of Ag/ TiO2 Nanofiber Figure 2a, which we have characterized from our previous work [21], shows an image of the top view of the TiO2 layer. Figure 2b shows an image of the TiO2/Ag NF using field emission scanning electron microscopy (FE-SEM). From Figure 2b, the average diameter of the TiO2/Ag NF is about 168 nm. The connected nanofiber structure provides better charge transport paths and prohibits the recombination of electrons. Therefore, it can provide a better passageway for electron transmission and high charge mobility [22].   1 1). Because the content of Ag is low, the diffraction peak of Ag is not obvious [25,26].

Ultraviolet-Visible Spectroscopy
The UV-vis absorption spectra of the Ag/TiO2 NFs is shown in Figure 4. The maximum absorption wavelength corresponding to the tangent line is 466.09 nm. Using the formula Eg = 1240/λ, the energy gap of Ag/TiO2 NF is about 2.66 eV [27]. The band gap of TiO2 is about 3 eV [28].  Figure 5 shows the UV-visible spectrum of the absorbed dye, which was determined by detaching the dye from the photoanode. The addition of TiO2/Ag NF in a photoelectrode (TANLP) increases the amount of dye absorbed in the photoelectrode and the addition of Ag nanoparticles (NP) can increase the absorption rate of the dye. According to the Mie theory [29], the increase in metal nanoparticles can be expected to lead to stronger absorption. In Figure 5, the dye solution obtained from TANLP shows highest absorbance, which suggests the largest amount of dye molecules incorporated into the photoanode in the DSSC. The Ag NP can distribute uniformly over the TiO2 surface. 20 30 Absorbance (a.u.)

Ultraviolet-Visible Spectroscopy
The UV-vis absorption spectra of the Ag/TiO 2 NFs is shown in Figure 4. The maximum absorption wavelength corresponding to the tangent line is 466.09 nm. Using the formula Eg = 1240/λ, the energy gap of Ag/TiO 2 NF is about 2.66 eV [27]. The band gap of TiO 2 is about 3 eV [28].

Ultraviolet-Visible Spectroscopy
The UV-vis absorption spectra of the Ag/TiO2 NFs is shown in Figure 4. The maximum absorption wavelength corresponding to the tangent line is 466.09 nm. Using the formula Eg = 1240/λ, the energy gap of Ag/TiO2 NF is about 2.66 eV [27]. The band gap of TiO2 is about 3 eV [28].  Figure 5 shows the UV-visible spectrum of the absorbed dye, which was determined by detaching the dye from the photoanode. The addition of TiO2/Ag NF in a photoelectrode (TANLP) increases the amount of dye absorbed in the photoelectrode and the addition of Ag nanoparticles (NP) can increase the absorption rate of the dye. According to the Mie theory [29], the increase in metal nanoparticles can be expected to lead to stronger absorption. In Figure 5, the dye solution obtained from TANLP shows highest absorbance, which suggests the largest amount of dye molecules incorporated into the photoanode in the DSSC. The Ag NP can distribute uniformly over the TiO2 surface. 20 30   Figure 5 shows the UV-visible spectrum of the absorbed dye, which was determined by detaching the dye from the photoanode. The addition of TiO 2 /Ag NF in a photoelectrode (TANLP) increases the amount of dye absorbed in the photoelectrode and the addition of Ag nanoparticles (NP) can increase the absorption rate of the dye. According to the Mie theory [29], the increase in metal nanoparticles can be expected to lead to stronger absorption. In Figure 5, the dye solution obtained from TANLP shows highest absorbance, which suggests the largest amount of dye molecules incorporated into the photoanode in the DSSC. The Ag NP can distribute uniformly over the TiO 2 surface. We use the Beer-Lambert law to calculate the dye-loading on photoanode.

A = εcl
Among them, A is the absorbance, ε is the molar extinction coefficient, c is the concentration of solution, and l is the length of optical path [30]. The literature shows that the molar absorption coefficient of the N3 dye is about 14,000 M −1 cm −1 at 538 nm [31]. We use the Beer-Lambert law to estimate the dye-loading as shown in Table 1.   We use the Beer-Lambert law to calculate the dye-loading on photoanode.
Among them, A is the absorbance, ε is the molar extinction coefficient, c is the concentration of solution, and l is the length of optical path [30]. The literature shows that the molar absorption coefficient of the N3 dye is about 14,000 M −1 cm −1 at 538 nm [31]. We use the Beer-Lambert law to estimate the dye-loading as shown in Table 1. 3.3. Measurement for the Photovoltaic Parameters of the DSSC Figure 6 shows the I-V curve of the DSSCs under dark conditions. Figure 7 shows the current density-voltage (J-V) curves for DSSCs with different photoelectrodes. The TANLP revealed an optimal open-circuit voltage (V OC ), short-circuit current density (J SC ), fill factor (FF), and photovoltaic conversion efficiency (η), which are 0.75 V, 11.35 mA/cm 2 , 61.28%, and 5.03%, respectively. The enhanced photovoltaic conversion efficiency was obvious for DSSC based on TANLP because Ag NPs could exhibit a strong scattering effect to improve light harvesting [32], which causes a higher short-circuit current density. The photoelectric properties of the DSSC based on TANP are significantly less than TANLP. That could lead to the silver nanoparticles being distributed uniformly over the surface, which suppresses recombination and more effective utilization of the visible-light radiation [33]. Figure 8a shows the equivalent circuit for DSSC in our research. The R S is the wire resistance. In addition, the first semicircle at high frequency is R 1 , which is the resistance of the interface between the electrolyte and counter electrode, and the second semicircle at intermediate frequency is R 2, which is the resistance of interface between electrolyte and photoelectrode [34,35]. Moreover, we measure the DSSCs at direct current, which can neglect the capacitance. Figure 8b is the Nyquist plots of DSSCs with different photoelectrodes. We compare DSSCs with different photoelectrodes. The highest R 2 is from TANLP at 46 ohms. The high R 2 indicates the reduced probability of charge transfer capacity, which was discussed in our previous study [21].
Energies 2020, 13, x FOR PEER REVIEW 7 of 15 Figure 6. The I-V characteristics of TiO2, TANP, and TANLP in dark conditions. Figure 7 shows the current density-voltage (J-V) curves for DSSCs with different photoelectrodes. The TANLP revealed an optimal open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and photovoltaic conversion efficiency (η), which are 0.75 V, 11.35 mA/cm 2 , 61.28%, and 5.03%, respectively. The enhanced photovoltaic conversion efficiency was obvious for DSSC based on TANLP because Ag NPs could exhibit a strong scattering effect to improve light harvesting [32], which causes a higher short-circuit current density. The photoelectric properties of the DSSC based on TANP are significantly less than TANLP. That could lead to the silver nanoparticles being distributed uniformly over the surface, which suppresses recombination and more effective utilization of the visible-light radiation [33].  Figure 8a shows the equivalent circuit for DSSC in our research. The RS is the wire resistance. In addition, the first semicircle at high frequency is R1, which is the resistance of the interface between the electrolyte and counter electrode, and the second semicircle at intermediate frequency is R2, which is the resistance of interface between electrolyte and photoelectrode [34,35]. Moreover, we measure the DSSCs at direct current, which can neglect the capacitance. Figure 8b is the Nyquist plots of DSSCs   , short-circuit current density (JSC), fill factor (FF), and photovoltaic conversion efficiency (η), which are 0.75 V, 11.35 mA/cm 2 , 61.28%, and 5.03%, respectively. The enhanced photovoltaic conversion efficiency was obvious for DSSC based on TANLP because Ag NPs could exhibit a strong scattering effect to improve light harvesting [32], which causes a higher short-circuit current density. The photoelectric properties of the DSSC based on TANP are significantly less than TANLP. That could lead to the silver nanoparticles being distributed uniformly over the surface, which suppresses recombination and more e  Figure 8a shows the equivalent circuit for DSSC in our research. The RS is the wire resistance. In addition, the first semicircle at high frequency is R1, which is the resistance of the interface between the electrolyte and counter electrode, and the second semicircle at intermediate frequency is R2, which is the resistance of interface between electrolyte and photoelectrode [34,35]. Moreover, we measure the DSSCs at direct current, which can neglect the capacitance. Figure 8b is the Nyquist plots of DSSCs  The Incident Photon-to-electron Conversion Efficiency (IPCE) is the ratio of the number of electrons collected by a solar cell to the number of photons of a given energy incident on the solar cell. Figure 9 shows the IPCE spectrum of DSSCs with different photoelectrodes with wavelengths from 400 to 800 nm. TANLP has a significant improvement in IPCE. with different photoelectrodes. We compare DSSCs with different photoelectrodes. The highest R2 is from TANLP at 46 ohms. The high R2 indicates the reduced probability of charge transfer capacity, which was discussed in our previous study [21]. The Incident Photon-to-electron Conversion Efficiency (IPCE) is the ratio of the number of electrons collected by a solar cell to the number of photons of a given energy incident on the solar cell. Figure 9 shows the IPCE spectrum of DSSCs with different photoelectrodes with wavelengths from 400 to 800 nm. TANLP has a significant improvement in IPCE. from TANLP at 46 ohms. The high R2 indicates the reduced probability of charge transfer capacity, which was discussed in our previous study [21]. The Incident Photon-to-electron Conversion Efficiency (IPCE) is the ratio of the number of electrons collected by a solar cell to the number of photons of a given energy incident on the solar cell. Figure 9 shows the IPCE spectrum of DSSCs with different photoelectrodes with wavelengths from 400 to 800 nm. TANLP has a significant improvement in IPCE.   illuminations. As the light intensity is decreased from 100 to 10 mW/cm 2 , the J SC and V OC are gradually weakened; in particular, the J SC decline is more obvious. When the light intensity is decreased from 100 mW/cm 2 to 30 mW/cm 2 , the optimal photovoltaic conversion efficiency (η) of the DSSC is under 30 mW/cm 2 . From Table 4 and Figure 12, when the light intensity is 30 mW/cm 2 , the photovoltaic parameters of DSSC based on TANLP are: 0.70 V in V OC , 3.67 in J SC , 72.66% in FF and 6.23% in η.      Table 2 to Table 4, respectively. From the electrochemical impedence spectroscopy (EIS) measurements, we can find that the value of R2 gets larger with decreasing intensity. The R2 of the DSSC based on TiO2 increases from 36.91 Ω to 218.70 Ω, and the R2 of the DSSC based on TANLP increases from 43.40 Ω to 265.13 Ω. Because the amount of photo-generated electrons is decreased, the probability of recombination between the photo-generated electron and holes in the electrolyte can be reduced, which results in an increase in R2. Similarly, the value of R1 gets larger with decreasing intensity, which is due to the reduction in the amount of photo-generated electrons. The decrease in the amount of photo-generated electrons means the probability of recombination is diminished. In conclusion, the R1 and R2 are the interface impedances that represent the photogenerated electron degree of difficulty to recombine with the holes in the electrolyte. The suppression of electron recombination greatly reduces the electronic loss of DSSC [8,37], and this was the reason for the increase in fill factor, and thus the photovoltaic efficiency can be enhanced [38]. The highest efficiency is observed under an intensity of 30 mW/cm 2 . When the intensity keeps reducing to 10 mW/cm 2 , the value of R2 becomes several times larger than that with 30 mW/cm 2 . Under such light illumination, the number of photoelectrons generated within the DSSC is very low, resulting in a lower photovoltaic efficiency.   Table 2 to Table 4, respectively. From the electrochemical impedence spectroscopy (EIS) measurements, we can find that the value of R2 gets larger with decreasing intensity. The R2 of the DSSC based on TiO2 increases from 36.91 Ω to 218.70 Ω, and the R2 of the DSSC based on TANLP increases from 43.40 Ω to 265.13 Ω. Because the amount of photo-generated electrons is decreased, the probability of recombination between the photo-generated electron and holes in the electrolyte can be reduced, which results in an increase in R2. Similarly, the value of R1 gets larger with decreasing intensity, which is due to the reduction in the amount of photo-generated electrons. The decrease in the amount of photo-generated electrons means the probability of recombination is diminished. In conclusion, the R1 and R2 are the interface impedances that represent the photogenerated electron degree of difficulty to recombine with the holes in the electrolyte. The suppression of electron recombination greatly reduces the electronic loss of DSSC [8,37], and this was the reason for the increase in fill factor, and thus the photovoltaic efficiency can be enhanced [38]. The highest efficiency is observed under an intensity of 30 mW/cm 2 . When the intensity keeps reducing to 10 mW/cm 2 , the value of R2 becomes several times larger than that with 30 mW/cm 2 . Under such light illumination, the number of photoelectrons generated within the DSSC is very low, resulting in a lower photovoltaic efficiency. The photovoltaic parameters of the DSSCs based on TANLP have a very clear improvement in photovoltaic parameters. As mentioned earlier, the photoelectric properties of the DSSC based on TANP were significantly less than TANLP-based DSSC. Because the Ag NPs distribute uniformly over the surface, which suppressed recombination and caused a more effective utilization of the visible-light radiation [33]. From Tables 2-4, both J SC and V OC are reduced because the light intensity was reduced from 100 mW/cm 2 to 10 mW/cm 2 , which means that the output power is reduced. In other words, we can improve the utilization of optical power under low illumination. The photon amount is decreased as the light intensity decreases, which causes the decrease in the dye molecules being excited. Therefore, the J SC is significantly reduced. The lower J SC could be attributed to the reduction in the electron recombination rate due to the decrease in the concentration of photogenerated electrons [36]. The DSSCs could decrease recombination reactions to enhance η under low illumination. The reduction in V OC is not significant, and is correlated with material properties. Figures 13-15 show Nyquist plots of DSSCs based on TiO 2 , TANP, and TANLP photoelectrodes, respectively, under different illumination. The corresponding electrochemical impedance parameters are listed in Tables 2-4, respectively. From the electrochemical impedence spectroscopy (EIS) measurements, we can find that the value of R 2 gets larger with decreasing intensity. The R 2 of the DSSC based on TiO 2 increases from 36.91 Ω to 218.70 Ω, and the R 2 of the DSSC based on TANLP increases from 43.40 Ω to 265.13 Ω. Because the amount of photo-generated electrons is decreased, the probability of recombination between the photo-generated electron and holes in the electrolyte can be reduced, which results in an increase in R 2 . Similarly, the value of R 1 gets larger with decreasing intensity, which is due to the reduction in the amount of photo-generated electrons. The decrease in the amount of photo-generated electrons means the probability of recombination is diminished. In conclusion, the R 1 and R 2 are the interface impedances that represent the photo-generated electron degree of difficulty to recombine with the holes in the electrolyte. The suppression of electron recombination greatly reduces the electronic loss of DSSC [8,37], and this was the reason for the increase in fill factor, and thus the photovoltaic efficiency can be enhanced [38]. The highest efficiency is observed under an intensity of 30 mW/cm 2 . When the intensity keeps reducing to 10 mW/cm 2 , the value of R 2 becomes several times larger than that with 30 mW/cm 2 . Under such light illumination, the number of photoelectrons generated within the DSSC is very low, resulting in a lower photovoltaic efficiency.       The electron lifetime is calculated by the equation (τ) = R 2 × C 2 [39]. The electronic lifetime of the DSSCs with various photoanaodes under various illuminations is shown in Table 5. When the light intensity decreases, the electron lifetime gradually increases.

Conclusions
In summary, the properties of DSSCs with two structures using nanofibers under low illumination have been investigated. The TANLP-based DSSC has the best photovoltaic conversion efficiency of 5.13%. The experimental results suggest that the TANLP-based DSSC has better photovoltaic performances than TNAP-based DSSC. This results in a suppressed recombination and a more effective utilization of the visible-light radiation. From this study, the highest efficiencies are observed under the intensity of 30 mW/cm 2 for TANLP-based DSSC, and the maximum efficiency of 6.23% is achieved. The superiority of the dye-sensitized solar cell (DSSC) is utilized in low light illumination, while DSSCs are promising in indoor applications.