The Synchronization of Electron Enricher and Electron Extractor as Ternary Composite Photoanode for Enhancement of DSSC Performance

Materials Technology Program, School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi (KMUTT), 126 Pracha Uthit Rd., Bang Mod Thung Khru, Bangkok 10140, Thailand Nanotec-KMUTT Center of Excellence on Hybrid Nanomaterials for Alternative Energy (HyNAE), School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, 126 Pracha Uthit Rd., Bang Mod Thung Khru, Bangkok 10140, Thailand Fuel Cells and Hydrogen Research and Engineering Center, King Mongkut’s University of Technology Thonburi, 126 Pracha Uthit Rd., Bang Mod Thung Khru, Bangkok 10140, Thailand National Nanotechnology Center, National Science and Technology Development Agency, Thailand Science Park, Pathum Thani 12120, Thailand


Introduction
Dye-sensitized solar cell (DSSC) has gained a considerable attention as a potential solar device. The performance of the DSSC could be improved by several strategies, including the development of more efficient dye sensitizers [1,2] electrolyte systems [3], counter electrodes [4,5], and photoanodes [6,7]. Among these, the photoanode is an integral part determining the overall efficiency of DSSC because it has strong influence on dye adsorption ability and charge transport properties [8,9]. However, the commonly used photoanode material such as titanium dioxide P25 still has some drawbacks, including large bandgap energy (BGE) and low electron (e − ) mobility, resulting in low power conversion efficiency (PCE) of devices [10]. In this regard, an inclusion of TiO 2 with exposed {001} facet (TiO 2{001} ) as a 2D material into the photoanode systems was reported that can significantly enhance charge transport [11,12]. In addition, the electronic properties of TiO 2{001} can be easily tuned by doping with nonmetallic atoms to improve light harvesting efficiency [13,14]. However, this might also lead to the formation of deep trap state, which induces the charge recombination [15]. To cope up with this problem, the use of nonmetallic atom-doped reduced graphene oxide (rGO) is of our interest because of its high conductivity to quickly extract the photoexcited e − before the recombination could take place [16][17][18].
In this work, we introduce an alternative approach to enhance DSSC performance using a novel ternary composite photoanode based on N-, S-doped TiO 2{001} , N-, S-doped rGO, and P25. First, the N-, S-doped TiO 2{001} and N-, S-doped rGO were synthesized and optimized. The effect of mixing ratios between N-, S-doped TiO 2{001} and/or N-, S-doped rGO in P25 on performance of the DSSC was also investigated. The mechanism behind a synergistic effect of the ternary composite photoanode was elucidated by using ultraviolet-visible (UV-vis) spectroscopy, photoluminescence spectroscopy (PL), external quantum efficiency (EQE), and electrochemical impedance spectroscopy (EIS).

Results and Discussion
TiO 2{001} was synthesized by hydrothermal technique. More details can be found in Supplementary Information (SI) S1.1 and S1.2. The morphology of various TiO 2{001} was observed by scanning electron microscope (SEM). It was found that when HF concentration was increased, the thickness of TiO 2{001} particles was decreased, while the lateral size of particles increased (Figures 1(a)−1(c)). This result is in agreement with our recent report [19]. In addition, the platelet TiO 2{001} particles were transformed to a hierarchical sheet-like structure and tended to aggregate, forming stacked hierarchical particles when HF/Ti ratio was reached to 3 and 4, respectively (Figures 1(c) and 1(d)). Figure 1(e) showed that the X-ray diffraction patterns of all TiO 2{001} samples were perfectly indexed as anatase (JCPDS card no. . Furthermore, as shown in Figure 1(g), the percentage of {001} facet (%S) calculated by Raman spectra in Figure S2 [20] increased with the increment of HF/Ti ratio from 1 to 3 (42.6% to 97.2%). However, %S of TiO 2{001} -4F dropped to 76.3%. This may be due to the stacking phenomenon of TiO 2{001} sheet and the collapse of TiO 2{001} particles at the side edges ( Figure 1(d)). In addition, it was found that the dye loading capacity of TiO 2{001} decreased with an increase of %S (Figure 1(g)). This result is in contrast to the conventional understanding that the increase of %S can lead to the higher dye absorption capacity [21]. In our work, grain size seems to be more important factor for increasing dye absorption ability. Thus, 20 wt.% of TiO 2{001} was selected to mix in P25 to further increase dye loading capacity of material.
The photovoltaic performance of DSSCs assembled with various TiO 2{001} as a photoanode was also investigated. The cell configuration of DSSC and its fabrication method are described in Figures S1 and S1.4, respectively. The J-V curves of DSSCs are shown in Figure 2. It was found that the photoanode containing TiO 2{001} -2F showed the most promising short-circuit current density (J sc , 8.57 mA/cm 2 ), which results in an enhancement of PCE (4.63%), compared to that of pure P25 (J sc , 6.40 mA/cm 2 and PCE, 3.20%). The derived parameters, relating to photovoltaic performance of various DSSCs, were summarized in Table 1.
The enhancement of PCE was further supported by the results from photoluminescence spectroscopy (PL). The PL spectra (Figure 3(a)) of all TiO 2{001} samples show similar of broad emission peaks in visible light region. Since the red emission band (~600−650 nm), which is related to Ti 3+ species, was absent, the emission peaks are therefore associated with the defect states generated by oxygen vacancies, creating the shallow trap and deep trap state [22,23]. The amount of shallow trap and deep trap states plays a significant role in determining e − mobility of photoanode materials [24]. The shallow trap states, locating near conduction band edge (CB) of TiO 2{001} , can effectively capture photoexcited e − via the hopping mechanism. This can increase the amount of effective e − and eventually retard charge recombination. In contrast, the deep trap states located at deeper energy can permanently trap the photoexcited e − , promoting the e − −h + recombination. The PL spectra were deconvoluted at 460 nm and 510− 555 nm, corresponding to a shallow trap (blue emission) and a deep trap state (green emission), respectively [25] (SI Figure S3). Interestingly, the TiO 2{001} -2F has the highest areal emission ratio (A shall /A deep ) of 1.13, as shown in Figure 3(b). This result is also consistent with an increase of J sc value. In this study, it was believed that the presence of shallow trap sites may be caused by the unsaturated bonds (5c−Ti and 2c−O) [26]. However, the TiO 2{001} -3F, which has the highest %S value (97.24%), provided a lower J sc and fill factor (FF). This could be due to high F − concentration, which was incorporated into TiO 2 lattice, forming −Ti−F−Ti− bonds, leading to the formation of defect at deep in TiO 2{001} crystal [27].
The electrochemical impedance spectroscopy (EIS) was conducted to further gain insight into the charge transport dynamics of DSSCs fabricated with various TiO 2{001} . Because of low charge transfer resistance at the Pt electrode (R CE ) (SI Figure S4), the Nyquist plots ( Figure 3(c)) show only the 2 nd semicircle, which represents charge recombination resistance between photoanode and electrolyte interface (R rec ) [28]. The radius of the semicircle decreased with respect to the increase of HF content, resulting in a higher charge recombination rate. Moreover, the series resistances (R s ) of DSSCs [29] were increased with an increased HF concentration. This directly corresponds to the increased deep trap state in TiO 2{001} [30]. Interestingly, the DSSC fabricated by using TiO 2{001} -2F as a photoanode has the higher charge recombination resistance (200.51 Ω) and lower series resistance (15.62 Ω) compared to the other samples. Furthermore, Bode plots ( Figure 3(d)) and the parameters relating to the impedance properties (Table 2) suggest that the TiO 2{001} -2F can significantly enhance the electron lifetime (τ n = 1/2πω max , 1.599 ms) and improve the electron transport time (τ s = R s C μ , 0.125 ms). Consequently, the charge collection efficiency (η c = ð1 + τ s /τ n Þ −1 , 92.77%) of DSSC was significantly increased [31][32][33]. Based on the above results, the TiO 2{001} -2F was selected to further dope with nonmetallic atoms to enhance the light harvesting efficiency of material.

Journal of Nanomaterials
This suggests that the dopants can reduce the grain size of the TiO 2{001} [34], resulting an increase of dye loading capacity (blank red circle in Figure 1(g)). Furthermore, the successful incorporation of N and S atoms in TiO 2{001} lattice was also confirmed by the XPS spectra, as shown in Figures S5a and S5b. However, we found that the %S of N-, S-TiO 2{001} -2F decreased (blank black square in Figure 1(g)). This implies that the incorporation of N and S atoms in TiO 2{001} lattice might affect the vibrational signal of O−Ti−O (E g and A 1g bands), which was corresponded to the blue shifted and broader of E g band in Raman spectrum of N-, S-TiO 2{001} -2F (SI Figure S2, inset). This suggests that the local lattice was changed due to the oxygen vacancies, forming by the incorporation of nonmetallic atoms [35].
The N-, S-doped reduced graphene oxide (rGO) was used to improve electron collection efficiency of DSSC because of its high electrical conductivity [36]. The graphene oxide (GO) used as precursor of rGO was successfully prepared by using modified Hummer's method under low temperature assisted with preexfoliation condition (rGO-(PreEx/LT)). The N-, S-doped rGO (N-, S-rGO-(PreEx/LT)) was then synthesized by the wet chemical reduction method. More details about their synthesis were described in SI S1.3. Figure 4(a) illustrates the schematic of the synthesized GOs together with the resulting morphologies of the materials characterized by SEM technique. It was found that GO-(PreEx/LT) (Figure 4(d)) has smooth surface, thin sheet, and large lateral size. In addition, from the XRD pattern ( Figure 4(f)), the GO-(PreEx/LT) has larger d-spacing along (001) direction (d 001 , 8.67 Å), larger average crystallite size (L 001 , 9.63 nm), larger average diameter of stacking layers (D A ,~24 nm), and higher number of layer of graphene sheets (N L , 12 layers) compared to other GOs [37] (see Table S2 for more calculated data). Moreover, to observe the quality of GOs, the structural changes, defects, and disorders of the GO samples were further investigated by Raman spectroscopy [38]. As shown in Figure 4(g), the 1 st -order scattering region [39] of various GOs exhibits D band (1354−1364 cm -1 ) and G band (1593−1603 cm -1 ), which represent the disordered of graphitic structure and in-plane sp 2 C=C stretching mode, respectively. The intensity ratio of I D /I G of GO-(PreEx/LT) (0.84) is lower than that of GO-(HT) (0.90), indicating the larger of sp 2 graphitic domain with less defects. These indicate that GO-(PreEx/LT) provides better quality of exfoliated graphene sheet.
The successful formation of oxidized GO can be confirmed by FTIR spectra (Figure 5(a)). Basically, four peaks of υ(O-H) (~3400 cm -1 ), υ(C=O) (1719− 1731 cm -1 ), C=C of unoxidized sp 2 graphitic domain (1564−1620 cm -1 ), and C-O (949-1405 cm -1 ) vibration are presented in all GO samples [40]. The FTIR data of GO products were fit to perform quantitative analysis of oxygen-containing groups, as shown in Equation S2 [41]. It was found that the overall oxygenrelated bond (ORB) of GO-(PreEx/LT) (82.5%) is greater than those of GO-(LT) (77.4%) and GO-(HT) (61.3%). This reveals that the GO synthesized by low temperature assisted with pre-exfoliation process gave higher oxidation degree. The peak position and calculated peak area of each functional groups of various GOs was summarized in Table S3.
Furthermore, UV-visible absorption spectra (UV-vis), as shown in Figure 5(b), present the peaks at 238−245 nm, corresponding to the π⟶π * electronic transitions of aromatic conjugated system of sp 2 domain (C=C) bonds, while the weak shoulders at around 300−311 nm assigned to the n⟶π * transitions present the oxygen containing groups, locating at the edge and basal plane of graphene sheet [42,43]. From these results, the relative normalized intensity of absorption peak at n⟶π * band of GO-(PreEx/LT) is relatively high and the position of π⟶π * band of GO-(Pre-Ex/LT) (238 nm) is blue shifted as compared to GO-(LT) (inset in Figure 5(b)). This means that the GO-(PreEx/LT) has high-oxidation degree, which is consistent with FTIR result [44]. Thus, GO-(PreEx/LT) was considered to be a suitable precursor for N-, S-codoped rGO.
The GO-(PreEx/LT) was further used to prepare N-, S-doped reduced graphene oxide (N-, S-rGO-(PreEx/LT)) to improve electrical conductivity. The morphology of N-, S-rGO-(PreEx/LT) is shown in Figure 4(e). The presence of many wrinkled, rippled, and scrolled graphene sheets were observed. Moreover, the XRD pattern of N-, S-rGO-(Pre-Ex/LT) (Figure 4(f)) clearly shows a broad diffraction peaks at~24°, corresponding to the (002) diffraction plane of rGO (JCPDS Card No.75−1621), which shifted to higher diffraction angle compared with (001) diffraction angle of GO-(PreEx/LT). This can be confirmed that the GO was successfully reduced, resulting in lower d-spacing.  To assure the successful reduction, changes in chemical structure of N-, S-rGO-(PreEx/LT) after reduction process were monitored by using FTIR. The absence of oxygencontaining groups in FTIR spectra ( Figure 5(a)) and the red shifted of π⟶π * band referred to aromatic conjugated sys-tem of C=C in UV-vis spectra ( Figure 5(b)) were noted. These indicate that the electronic conjugation within graphene sheet (sp 2 ) is restored after doping and reduction [45]. In addition, the quality of synthesized N-, S-rGO-(Pre-Ex/LT) was examined by using Raman spectroscopy    Journal of Nanomaterials ( Figure 4(g)). I D /I G values of the N-, S-rGO-(PreEx/LT) (0.98) was greater than that of GO-(PreEx/LT) (0.84), indicating that N and S atoms can cause some defects or disorders in the graphene structure. In this part, the N-, S-TiO 2{001} -2F and N-, S-rGO-(Pre-Ex/LT) with different ratios in P25 were utilized as a composite photoanode. The J-V curves, as seen in Figures S7a and  S7b, showed the significant enhancement in the J sc up to 13.10 mA/cm 2 (6.55% PCE), and 13.77 mA/cm 2 (6.09% PCE) can be found in two devices containing 0.6 wt.% of N-, S-TiO 2{001} -2F in P25 and 0.4 wt.% of N-, S-rGO-(PreEx/LT) in P25, respectively (the photovoltaic parameters of DSSCs fabricated by various photoanodes can be found in Table S4). From these optimum contents, weight ratio between N-, S-TiO 2{001} -2F and N-, S-rGO-(PreEx/LT) was fixed at 3 : 2 and then this component was mixed with P25 in various contents to make the ternary composite photoanode (Figure 6(a)).
Interestingly, as seen in Figure S7c, 1.0 wt.% of the mixture of N-, S-TiO 2{001} -2F and N-, S-rGO-(PreEx/LT) (3 : 2 by wt.) in P25 based ternary composite photoanode boosted the PCE up to 8.62%, which was about 2.7 times enhancement compared to the photoanode made of pure P25 (3.20%). In addition, the large improvement of J sc up to 19.47 mA/cm 2 , which was around 3 times enhancement compared to P25 (6.40 mA/cm 2 ) was noticed. This suggests the synergistic cooperation of both N-, S-rGO-(PreEx/LT) and N-, S-TiO 2{001} -2F in P25. The J-V results and photovoltaic parameters of DSSCs fabricated by various photoanodes were summarized in Figure 6(b) and Table 3, respectively.   Table 4. The results show that R s of DSSC was significantly reduced by applying the N-, S-rGO-(Pre-Ex/LT) as photoanode (R s = 15:54Ω). This indicates that the N-, S-rGO-(PreEx/LT) can effectively improve electrical conductivity of DSSC cell, resulting in a lower R rec (335.60Ω), a faster e − transport time (τ s ) (0.12 ms), and a higher e − lifetime (τ n ) (2.61 ms) as compared to the other photoanodes. Surprisingly, for ternary composite photoanodes, the lowest R s (12.34 Ω) and the highest R rec (520.95 Ω) values were achieved (Table 4). Moreover, the frequency at maximum ω max in Bode phase plot of DSSC fabricated by using the ternary composite photoanode was shifted to the lower frequency region. Therefore, from these results, the ternary composite photoanode not only shows faster e − transport time (τ s = 0:10 ms) but also exhibited longer e − lifetime (τ n = 4:29 ms) as compared to all samples, resulting in the highest charge collection efficiency (η c ) (97.69%).
Finally, the proposed charges transfer mechanism of DSSC fabricated using ternary composite photoanode could be described in terms of cascade energy diagram as shown in Figure 8. When dye molecule absorbs the light, the e − is excited (1) and transferred to the CB of P25 (2). The e − at this state can move to CB of N-, S-TiO 2{001} -2F (3) via e − hopping mechanism, and it can be further extracted by N-, S-rGO-(PreEx-LT), which acts as e − extractor, through the external circuit performing work (4). Moreover, the N-, S-TiO 2{001} -2F as the e − enricher can be easily excited by visible light (5) (pink arrow in the first circle), resulting in high density of photoexcited e − . However, photoexcited e − might be deactivated and recombined (red arrow in all circles). Thus, the combination of e − extractor can help to quickly extract the photoexcited e − before the recombination could take place.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare no conflicts of interest.

Acknowledgments
This work has been supported by the Nanotechnology Center (NANOTEC), NSTDA, and Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network. The financial support provided by King Mongkut's University of Technology Thonburi is through the "KMUTT 55th Anniversary Commemorative Fund" and "the Petchra Pra Jom Klao Doctoral Scholarship." Financial support from NANOTEC (grant number P1652084) is also appreciated.