# Enhanced Photovoltaic Properties of Dye-Sensitized Solar Cells through Ammonium Hydroxide-Modified (Nitrogen-Doped) Titania Photoanodes

Authors: Tharmakularasa Rajaramanan; Fatemeh Heidari Gourji; Dhayalan Velauthapillai; Punniamoorthy Ravirajan; Meena Senthilnanthanan
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Doping is a unique strategy to modulate the optical and electronic properties of semiconducting materials. This study reports a facile approach to fabricate nitrogen-doped TiO2 (N-doped TiO2) photoanode for DSSC application. A solid-state reaction was employed to synthesize a series of N-doped TiO2 nanoparticles with different volumetric ratios of nitrogen dopant and the TiO2 host. The NH4OH as a nitrogen dopant was combined with P25-TiO2 via grinding followed by calcination at 500°C. The synthesized nanoparticles were extensively characterized by XRD, XPS, EDX, SEM, and TEM techniques. XRD results suggested that the incorporation of nitrogen had not altered the structure of the TiO2 lattice, and the presence of nitrogen was confirmed through the XPS and EDX spectroscopies. SEM and TEM images, obtained before and after N doping, showed that N-doped TiO2 nanoparticles with low amounts of NH4OH (10 and 20 μL) had retained their spherical shapes and sizes while use of higher amounts of the N dopant (30 and 40 μL) had led to agglomeration of nanoparticles. BET and BJH analyses revealed that the optimized N-doped TiO2 with 20 μL of NH4OH (20N-TiO2) possesses the highest average pore diameter of 15.99 nm. Furthermore, the UV-visible spectroscopic analysis confirmed a red shift in the optical absorption edge on N doping and the corresponding bandgap reduced from 3.15 to 2.94 eV with increase in the amount of NH4OH from 0 to 40 μL. Eventually, DSSCs were fabricated using the prepared pure TiO2 and N-doped TiO2 photoanodes, N719 dye, I−/I3− electrolyte, and Pt counter electrode, followed by investigating their performance under simulated irradiation with 100 mW/cm2 intensity with AM 1.5 filter. The photoanode doped with 20 μL of NH4OH (20N-TiO2) exhibited the highest power conversion efficiency (PCE) of about 6.16%, which was 20% higher than that of the control device, with improved JSC. This enhancement in JSC could be predominantly attributed to higher dye uptake along with marginal contribution by reduced rate of recombination. Among the reported studies on DSSCs with N-doped P25-TiO2 photoanodes, our method gives the best efficiencies for the DSSCs.


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## Body

## 1. Introduction

Dye-sensitized solar cells (DSSCs) are being widely studied in the photovoltaic (PV) field for clean and sustainable energy supply. Due to its low-cost fabrication procedure, ecofriendliness, and working ability under low light intensity, it is considered as a more promising technology than other types of photovoltaic devices [1, 2]. Generally, a DSSC is comprised of a dye-sensitized semiconducting metal oxide as photoanode in combination with a redox electrolyte and a counter electrode [3]. Each component of the DSSC demonstrates its own significance in improving the power conversion efficiency (PCE). However, the photoanode plays a major role in the light harvest and charge transport processes [4]. To date, various semiconducting metal oxides have been studied in DSSCs as photoanode materials [5]. Among them, TiO2, ZnO, and SnO2 are widely explored [6]. For example, Wanninayake et al. have reported that SnO2 performs high election mobility as photoanode. However, the PV performance is considerably low for SnO2-based devices due to high electron recombination [7]. ZnO, as photoanode in DSSCs, shows several desired features such as relatively high electron mobility in compact form and high exciton binding energy [8–10]. However, ZnO-based devices are unable to compete with the TiO2-based cells in photovoltaic performance as ZnO possesses lower internal surface area than TiO2 which reduces its dye anchoring capacity compared to TiO2 [8, 9]. So far, TiO2 remains the most promising mesoporous metal oxide employed in DSSCs due to its outstanding properties such as high transparency under visible light, high refractive index, biocompatibility, thermal stability, nontoxicity, and highly porous nature resulting in large surface area for dye anchoring [11, 12]. However, slow charge transport and wide bandgap of the TiO2 limit further enhancement in PCE of TiO2-based DSSCs [13]. Doping is largely explored to address the aforementioned limitations of TiO2-based DSSCs [14].In general, a dopant modifies both the electronic band structures and surface states of TiO2 lattice, which in turn affect important properties such as the conduction band energy, charge transport, recombination, and collection [15]. Both metal and nonmetal ions are potential candidates for doping. However, employing a metal ion as dopant may block the anchoring of dye molecules on Ti4+ ions due to possible strong coupling between the metal dopant and dye molecules which in turn may reduce dye absorption [15]. The said limitation could be avoided by doping TiO2 with a nonmetal ion that may result in high dye absorption and efficient electron injection [16]. Moreover, nonmetals can gain more electrons due to their large ionization energies and high electronegativities [17]. Also, nonmetal ions are the only anionic dopants that strongly influence the valance band (VB) of TiO2. Various nonmetal ions have been investigated as dopants on TiO2 (N [18], F [19], C [20], S [21], B [22], and I [23]) to improve the PV performance of DSSCs. Among them, nitrogen is considered as the prominent nonmetal candidate. When TiO2 is doped with N, the hybridization between the 2p orbitals of nitrogen (N) and oxygen (O) results in bandgap narrowing that leads to visible light absorption [24]. Moreover, enhanced photocurrent due to retarded electron recombination has been observed in N-doped TiO2-based DSSCs [25]. It has also been reported that N doping reduces the oxygen deficiency of TiO2 and thereby enhances the lifetime of the device [26].Considering the advantages of N doping, several studies regarding N doping on TiO2 using different synthetic procedures and its’ influence on the performance of DSSCs have been prompted recently. Kang et al. synthesized a N-doped TiO2-based photoanode through the sol-gel method, and the corresponding fabricated device showed the maximum PCE of 4.86% compared to the control device (3.73%). This enhancement in PCE was mainly attributed to the increase in the near-visible absorbance due to nitrogen doping and partially to the morphological properties of the N-doped TiO2 film [18]. In another study carried out by Motlak et al., N-doped TiO2 nanofibers, prepared by electrospinning and hydrothermal treatment processes, exhibited PCE of 4.7% due to enhanced electron injection, long electron lifetime, and retarded charge recombination [27]. In a separate study, Guo et al. synthesized a hierarchical macro-mesoporous N-doped TiO2 structure with parallel macrochannels through the template-free wet method which exhibited PCE of 7.27% (device active area 0.16 cm-2) due to the improved dye uptake [28]. Moreover, a few studies have investigated the performance of N-doped TiO2-based device by varying the nitrogen dopant source [25, 29]. Since N-doped TiO2 photoelectrode is found to be a promising candidate for PV applications, a facile synthetic route along with a suitable nitrogen source is necessary for its’ large-scale synthesis and possible applications.Among the different methods used for the synthesis of N-doped TiO2 nanomaterials, doping with commercially available P25-TiO2 is more time saving and efficient way for large-scale synthesis. Though many sources are available for the synthesis of N-doped TiO2 by different methods (urea, thiourea, N2(g), NH3(g), etc.), NH4OH was chosen as the nitrogen source for this study because of its’ liquid nature, which enabled easy and uniform mixing of NH4OH with P25-TiO2 nanoparticles during the grinding process. Also, this avoids doping of impurities such as C and S with TiO2 during the doping process. Further, the application of N-doped TiO2 (utilizing NH4OH as a nitrogen source) in DSSC fabrication has not been reported previously to the best of our knowledge. Generally, the PV performance of doped TiO2-based DSSCs depends on the employed nitrogen source, doping method, and TiO2 properties.In this regard, we initially prepared N-doped TiO2 nanoparticles through a solid-state reaction by varying the volumetric ratio between P25-TiO2 and ammonium hydroxide (NH4OH). Then, the corresponding films were prepared from the said nanoparticles and sintered at 500°C. Subsequently, the prepared N-doped TiO2 nanoparticles were characterized structurally and optically followed by investigation on charge transport properties together with PV performance of the fabricated N-doped TiO2 photoanode-based DSSCs.


## 2. Materials and Methods

### 2.1. Materials

All reagents and solvents were purchased from commercial sources: titanium dioxide (TiO2) nanopowder (21 nm primary particle size, ≥99.5% trace metal basis, Sigma-Aldrich), ammonium hydroxide (NH4OH) (~25% NH3 basis, Sigma-Aldrich), Triton TM X-100 (laboratory grade, Sigma-Aldrich), di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) dye (N-719) (95%, Sigma-Aldrich), acetonitrile (CH3CN) (≥99.9%, Sigma-Aldrich), tert-butyl alcohol (C4H10O) (≥99.7%, Sigma-Aldrich), and acetylacetone (C5H8O2) (≥99.5%, Fluka Analytical).


### 2.2. Preparation of N-Doped TiO2 Photoanodes and Fabrication of DSSCs

The fluorine-doped tin oxide- (FTO-) coated conducting glass sheets (sheet resistance 7.5Ω/cm2 and size 2×1cm2) were cleaned initially with soap water and subsequently with distilled water and ethanol using an ultrasonic bath. The undoped and N-doped (with systematically varied N contents) TiO2 films were prepared by grinding 100 mg of P25-TiO2 separately with 0, 10, 20, 30, and 40 μL of NH4OH as nitrogen source, DI water, 20 μL acetylacetone, and a drop of Triton TM X-100 into pastes followed by separately coating the resultant pastes via doctor blade method on the cleaned FTO glass sheets. The prepared undoped and N-doped TiO2 films were dried and calcinated at 500°C for 30 minutes. The resultant films were separately soaked in 0.3 mM N719 dye solution, prepared by dissolving N719 dye in a mixture of acetonitrile and tert-butyl alcohol (50% v/v), for 12 h. After the dye-sensitization process, the photoanodes were washed with acetonitrile to remove the unanchored dye molecules and dried. Then, the corresponding devices were assembled by employing N719 dye-coated undoped or N-doped TiO2 photoanode, I−/I3− redox couple, and Pt-coated glass sheet as dye-sensitized photoanode, electrolyte, and counter electrode, respectively. Hereafter, the undoped TiO2 shall be referred as pure TiO2 and the N-doped TiO2 with systematically varied N contents shall be referred as 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 with indication of the respective volumes of NH4OH employed.


### 2.3. Characterization and Performance Evaluation

The crystalline structures of the synthesized nanoparticles were analyzed using X-ray diffraction spectroscopy (XRD, PANalytical-AERIS, Almelo, Netherlands). The diffraction pattern was collected with Cu Kα radiation (λ=1.5408Å) under ambient temperature, through the following operational conditions: accelerated voltage of 40 kV, emission current of 44 mA, scan range (2θ) from 10° to 90° with a step size of 0.0027°, and a scan speed of 4°/min. The surface electron state of the N-doped TiO2 was determined by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD). The surface characterization and elemental composition of the nanoparticles were studied by transmission electron microscope with an accelerating voltage of 120 kV (TEM, Hitachi HT7800) and scanning electron microscope (FESEM, Jeol JSM-7400F) along with energy-dispersive X-ray spectrometer (EDX). The Brunauer-Emmett-Teller (BET) model (Micromeritics Tristar 3000, at 77 K) was used to calculate the specific surface area. Pore size distribution of the nanoparticle was obtained from the desorption branch of the nitrogen isotherm through the Barrett-Joyner-Halenda (BJH) model. Absorption spectra of nanoparticles were recorded using a JENWAY 6800 UV-visible spectrophotometer (OSA, UK) which was controlled by Flight Deck software. The photovoltaic performances of all the fabricated devices with an effective area of 0.25 cm2 were studied using Keithley-2400 source meter under simulated irradiation of 1 sun illumination intensity (100 mW/cm2) by 150 W Xe lamp with AM 1.5 filter (Peccell-PEC-L12, Kanagawa, Japan). Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements were performed using SP-150 electrochemical workstation (Biologic Science Instruments) through a two-electrode electrochemical setup.


## 2.1. Materials

All reagents and solvents were purchased from commercial sources: titanium dioxide (TiO2) nanopowder (21 nm primary particle size, ≥99.5% trace metal basis, Sigma-Aldrich), ammonium hydroxide (NH4OH) (~25% NH3 basis, Sigma-Aldrich), Triton TM X-100 (laboratory grade, Sigma-Aldrich), di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) dye (N-719) (95%, Sigma-Aldrich), acetonitrile (CH3CN) (≥99.9%, Sigma-Aldrich), tert-butyl alcohol (C4H10O) (≥99.7%, Sigma-Aldrich), and acetylacetone (C5H8O2) (≥99.5%, Fluka Analytical).


## 2.2. Preparation of N-Doped TiO2 Photoanodes and Fabrication of DSSCs

The fluorine-doped tin oxide- (FTO-) coated conducting glass sheets (sheet resistance 7.5Ω/cm2 and size 2×1cm2) were cleaned initially with soap water and subsequently with distilled water and ethanol using an ultrasonic bath. The undoped and N-doped (with systematically varied N contents) TiO2 films were prepared by grinding 100 mg of P25-TiO2 separately with 0, 10, 20, 30, and 40 μL of NH4OH as nitrogen source, DI water, 20 μL acetylacetone, and a drop of Triton TM X-100 into pastes followed by separately coating the resultant pastes via doctor blade method on the cleaned FTO glass sheets. The prepared undoped and N-doped TiO2 films were dried and calcinated at 500°C for 30 minutes. The resultant films were separately soaked in 0.3 mM N719 dye solution, prepared by dissolving N719 dye in a mixture of acetonitrile and tert-butyl alcohol (50% v/v), for 12 h. After the dye-sensitization process, the photoanodes were washed with acetonitrile to remove the unanchored dye molecules and dried. Then, the corresponding devices were assembled by employing N719 dye-coated undoped or N-doped TiO2 photoanode, I−/I3− redox couple, and Pt-coated glass sheet as dye-sensitized photoanode, electrolyte, and counter electrode, respectively. Hereafter, the undoped TiO2 shall be referred as pure TiO2 and the N-doped TiO2 with systematically varied N contents shall be referred as 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 with indication of the respective volumes of NH4OH employed.


## 2.3. Characterization and Performance Evaluation

The crystalline structures of the synthesized nanoparticles were analyzed using X-ray diffraction spectroscopy (XRD, PANalytical-AERIS, Almelo, Netherlands). The diffraction pattern was collected with Cu Kα radiation (λ=1.5408Å) under ambient temperature, through the following operational conditions: accelerated voltage of 40 kV, emission current of 44 mA, scan range (2θ) from 10° to 90° with a step size of 0.0027°, and a scan speed of 4°/min. The surface electron state of the N-doped TiO2 was determined by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD). The surface characterization and elemental composition of the nanoparticles were studied by transmission electron microscope with an accelerating voltage of 120 kV (TEM, Hitachi HT7800) and scanning electron microscope (FESEM, Jeol JSM-7400F) along with energy-dispersive X-ray spectrometer (EDX). The Brunauer-Emmett-Teller (BET) model (Micromeritics Tristar 3000, at 77 K) was used to calculate the specific surface area. Pore size distribution of the nanoparticle was obtained from the desorption branch of the nitrogen isotherm through the Barrett-Joyner-Halenda (BJH) model. Absorption spectra of nanoparticles were recorded using a JENWAY 6800 UV-visible spectrophotometer (OSA, UK) which was controlled by Flight Deck software. The photovoltaic performances of all the fabricated devices with an effective area of 0.25 cm2 were studied using Keithley-2400 source meter under simulated irradiation of 1 sun illumination intensity (100 mW/cm2) by 150 W Xe lamp with AM 1.5 filter (Peccell-PEC-L12, Kanagawa, Japan). Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements were performed using SP-150 electrochemical workstation (Biologic Science Instruments) through a two-electrode electrochemical setup.


## 3. Results and Discussion

### 3.1. Structural and Morphological Characterization of N-Doped TiO2

The crystal structures of the synthesized pure and N-doped TiO2 nanoparticles were analyzed by XRD, and the corresponding XRD patterns are shown in Figure 1. The XRD patterns of all nanoparticles exhibit peaks at 25.20°, 37.60°, 48.20°, 53.70°, 55.00°, 62.50°, 68.50°, 70.20°, 74.89°, and 82.53° corresponding to the reflection planes of (101), (004), (200), (105), (211), (204), (116), (220), (215), and (224) which are consistent with the well-crystallized pure anatase TiO2 phase (Anatase XRD JCPDS Card No. 21-1272) while peaks at 27.39°, 36.07°, and 41.2° correspond to the reflection planes of (110), (101), and (111) for rutile TiO2 phase (Rutile JCPDS Card No. 21-1276).Figure 1
XRD patterns of pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles.The above observation confirms that no phase transition in TiO2 had occurred due to N doping as both pure and N-doped TiO2 nanoparticles exhibit similar anatase and rutile peaks. The crystallite sizes (d) of the nanoparticles were calculated by the Scherrer equation using the predominant anatase plane of (101) [13, 14]. The estimated crystallite sizes of the pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles were found to be 18.26, 17.60, 17.55, 17.50, and 17.12 nm, respectively. Trivial reduction observed in the crystallite size with increase in N content of the N-doped TiO2 is in good agreement with the TEM images. Also, the observed insignificant variation in the crystallite size due to N doping on TiO2 is consistent with the results reported by Kang et al. [18]. Further, the absence of crystallite peaks representing nitrogen of N-doped TiO2 in the XRD spectra may be attributed to the low N dopant quantities [30, 31]. Hence, XPS and EDX spectroscopic studies were performed to confirm the presence of nitrogen in the N-doped TiO2 nanoparticles.The surface chemical composition and oxidation states of 20N-TiO2, as optimized sample, were studied and the findings are illustrated in Figure 2. The XPS survey reveals that the observed binding energies at 459.56 and 465.03 eV belong to the Ti 2p3/2 and Ti 2p1/2 spin-orbit splitting photoelectrons, respectively, which are consistent with the reported values for Ti4+ ion [32]. The binding energy at 529.6 eV of O 1s photoelectron confirms the presence of O2− state in the nanoparticles. The literature on N 1s XPS peak analysis states that N doping on TiO2 can occur in three different ways, namely, substitutional doping (replacement of O atom in the TiO2 by N), interstitial doping (addition of N atom into the TiO2 lattice), and atomic adsorption of N species [33, 34]. It is noteworthy to mention that though N doping on TiO2 is reported in many literature, the exact assignment of N 1s XPS peak is still debatable. In addition, the binding energy of N 1s photoelectron highly depends on the method employed to synthesize the N-doped TiO2 nanoparticles [35]. In the present study, N 1 s peak observed at 400.7 eV could be attributed to the interstitial N of the N-doped TiO2 with an O−Ti−N linkage [36] which is also in consistent with the XPS results of N-doped TiO2 reported by Liu et al. [37]. Moreover, the presence of this nitrogen in the N-doped TiO2 has led to reduction of TiO2 bandgap as well. Further, the EDX spectra (Figure S1 and Table S1) of pure TiO2 and 20N-TiO2 nanoparticles obtained in the binding energy region of 0.0-15.0 keV confirm the presence of N in the N-doped TiO2 nanoparticles, and the EDX mapping of 20N-TiO2 indicates that N ions are extensively and perfectly distributed on the surface of TiO2.Figure 2
(a) XPS spectrum of 20N-TiO2 and (b, c) high-resolution XPS spectra of Ti 2p, O 1s, and N 1s.
(a)(b)(c)(d)The morphology and surface nature of the synthesized nanoparticles were studied using TEM and SEM techniques. As displayed in Figures3(a)–3(c), the TEM images of pure TiO2, 20N-TiO2, and 40N-TiO2 nanoparticles reveal that the synthesized nanoparticles are of spherical shape with the estimated particle size in the range of 18-20 nm. The said particle size values are in good agreement with the results obtained from the XRD measurements. Furthermore, formation of aggregates was observed in the structure of N-doped TiO2 nanoparticles with high N content (40N-TiO2). The high-resolution transmission electron microscopic (HRTEM) images of pure TiO2 and 20N-TiO2 (Figures 3(d) and 3(e)) show one set of lattice fringes with a space of 0.35 nm that could be indexed to the (101) plane of anatase phase. These results are consistent with the findings of the XRD analysis. However, significant fringe lattice associated with the nitrogen of N-doped TiO2 is not observed due to the small quantity of the N dopant.Figure 3
TEM images of (a) pure TiO2, (b) 20N-TiO2, and (c) 40N-TiO2 nanoparticles; HRTEM images of (d) pure TiO2 and (e) 20N-TiO2.
(a)(b)(c)(d)(e)The SEM images of pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles, shown in Figure S2 (a-e), reveal that all nanoparticles are in uniform spherical shape and N doping had not caused any morphological changes. Similar observation is reported by Lin et al. for N-doped TiO2 using ammonium hydroxide as nitrogen source [38]. Further, Figure S2 (d and e) clearly exhibits that particles start to agglomerate when higher quantities of N dopant are used and consequently the surface area and pore size distribution of the nanoparticles get affected. The Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) measurements were carried out to investigate the surface area and pore size distribution of the nanoparticles, respectively.Figure4 shows the nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves of the pure TiO2, 20N-TiO2, and 40N-TiO2 nanoparticles. As it is observed in Figure 4(a), all three nanoparticles exhibit a IV type isotherm curve according to the IUPAC classification [39]. As per the IUPAC nomenclature, the pore size of a mesoporous material varies between 2 and 50 nm. The wide distribution of pore size/width in the range of 10–30 nm observed for the synthesized nanomaterial is depicted in Figure 4(b), which indicates its’ mesoporous structure [40]. The BET surface areas of pure TiO2, 20N-TiO2, and 40N-TiO2 nanoparticles were found to be 57.92, 53.28, and 51.97 m2g-1, respectively. Hence, the BET analysis reveals that the surface area of the synthesized nanoparticles is mainly controlled by the size of TiO2 though trivial changes occur due to N doping as already confirmed by the XRD and TEM measurements [41]. The average adsorption pore diameters of pure TiO2, 20N-TiO2, and 40N-TiO2 nanoparticles were found to be 10.65, 15.99, and 13.52 nm, respectively. The observed optimum value for 20N-TiO2 confirms that N doping, to a certain extent, not only modifies the surface area but also extremely enhances the pore size of the nanoparticles. However, further increase in the N dopant (e.g., 40N-TiO2) leads to reduction in both surface area and average adsorption pore diameter due to agglomeration of particles as observed in the corresponding TEM and SEM images. It is well-known that pore size improvement without sacrificing the surface area is beneficial as it allows a higher number of dye and electrolyte molecules to fill the pores of TiO2 film which in turn would enhance the short circuit current density of the device [42].Figure 4
(a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution curves of pure TiO2, 20N-TiO2, and 40N-TiO2.
(a)(b)


### 3.2. Optical Characterization of N-Doped TiO2

An optical study, utilizing UV-visible spectrophotometer, was performed to measure the light absorption capacities and estimate bandgaps of pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles (Figures 5(a) and 5(b)).Figure 5
(a) UV-visible absorption spectra of pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles and (b) Tauc plot of the corresponding nanoparticles.
(a)(b)The N-doped TiO2 nanoparticles showed a gradual shift in the UV-visible region to longer wavelengths (red shift) with an increase in the amount of N dopant compared to pure TiO2 (Figure 5(a)). This observation is in consistent with the previously reported studies [18, 38, 43]. Further, the estimated bandgaps of the said nanoparticles, calculated by Tauc plot as reported in our previous study [14], were found to be 3.15, 3.10, 3.03, 2.99, and 2.94 eV for pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles, respectively (Figure 5(b)). This reduction in the bandgap as a result of N doping could have been caused by either formation of isolated narrow bands above the valance band of TiO2 by mixing the 2p states of N and O in the dopant and TiO2, respectively (interstitial doping) [44] or replacement of oxygen-deficient sites by the nitrogen (substitutional doping) [43]. However, no peak for substitutional nitrogen was detected in the XPS study. Hence, the reduction in the bandgap could be attributed to the impurity levels formed above the valance band of TiO2 due to the interstitial nitrogen doping. Also, it has been reported that interstitial doping by nitrogen extensively influences and reduces the bandgap of TiO2 compared to substitutional N doping [45].Further, the effect of N doping on dye adsorption as well as desorption was studied. Initially, pure TiO2-, 10N-TiO2-, 20N-TiO2-, 30N-TiO2-, and 40N-TiO2-coated films were soaked in the N719 dye overnight, and the resultant dye-coated films were analyzed by UV-visible spectroscopy (Figure 6(a)). Then, the same photoanodes were dipped in 5 mL of 1 N NaOH solution, and the desorbed dye in NaOH solution was subjected to UV-visible spectroscopic analysis (Figure 6(b)).Figure 6
UV-visible absorption spectra of (a) dye-coated pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 films and (b) desorbed dye from the same films.
(a)(b)As shown in Figure6, enhanced light absorption was observed for dye-coated N-doped TiO2 films, and the maximum light absorption was obtained for the dye-coated 20N-TiO2 film. The improved light absorption of dye-coated N-doped TiO2 films could be attributed to the increased pore size for dye adsorption as a result of N doping which was confirmed by the BET and BJH studies. The greater dye adsorption on N-doped TiO2 film due to the increased porosity of TiO2 by N doping has been reported earlier as well [46]. However, a further increase in the amount of N dopant reduces light absorption due to agglomeration of the particles giving rise to a reduction in dye adsorption. These observations are in good agreement with the JSC values of the corresponding devices fabricated from the above dye-coated N-doped TiO2 films.


### 3.3. Performance Evaluation of DSSCs

The photovoltaic performance of the N719 dye-coated pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 photoanodes was studied using I−/I3− electrolyte and Pt counter electrode under the simulated irradiation with 100 mW cm-2 and AM 1.5 filter, and the results are depicted in Figure 7(a) and the PV parameters are summarized in Table 1.Figure 7
(a) Current-voltage (J-V) characteristics of the DSSCs assembled with pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 photoanodes tested under simulated irradiation of intensity 100 mW cm-2 with AM 1.5 filter and (b) variation in JSC and efficiency with different amounts of N dopant.
(a)(b)Table 1
Summary of photovoltaic parameters of the DSSCs assembled with pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 photoanodes.
PhotoanodeJSC (mA/cm2)VOC (V)FFPCE,η (%)Pure TiO211.900.650.655.1510N-TiO212.900.670.675.7920N-TiO213.700.670.666.1630N-TiO212.600.690.665.7440N-TiO211.300.650.664.86The J-V studies revealed that the PCE of N-doped TiO2-based DSSCs slightly improved because of marginal increase in FF (fill factor) and VOC (open circuit voltage) and considerable increase in JSC (short circuit current). It was observed that when the amount of N dopant had been increased from 0 to 20 μL, the JSC enhanced significantly from 11.90 to 13.70 mA/cm2; but, a further increase in the N dopant amount resulted in a downward trend with respect to JSC (11.30 mA/cm2 for 40N-TiO2). Figure 7(b) indicates the influence of different amounts of N dopant on JSC and η. The variation in JSC values could be attributed to the modified surface properties of N-doped TiO2 as explained earlier. The observed reduction in JSC and PCE beyond 20N-TiO2 could have been caused by decreased dye adsorption due to the particle agglomeration. In the present study, the device fabricated with 20N-TiO2 electrode showed the best η of 6.16%, which was 20% higher than that of the pure TiO2-based DSSC (η=5.15%).Table2 compares the PCE of the N-doped P25-TiO2 photoanode-based devices. Although the experimental conditions and other factors vary, based on the summary of the table, our proposed synthesized N-doped P25-TiO2 nanomaterial is shown to be more efficient for the DSSC application.Table 2
Comparison of the PCEs of N-doped P25-TiO2 photoanode-based devices.
Doping method and N sourceDevice active area (cm-2)DyeAdditional modificationCounter electrodePCE,η (%)Ref.Grinding P25-TiO2 and urea for 30 min in a mortar and annealing at 400°C for 1 hr0.50N719—Pt2.93[47]Grinding P25-TiO2 and thiourea for 30 min in a mortar and annealing at 400°C for 1 hr0.50N719(i) N codoped with SPt3.35[48]Sol-gel process with electrospinning0.25N719(i) N-doped TiO2 nanofiber photoanodePt4.70[27]Sol-gel process0.25—Pt4.86[18]Ball milling method (urea as N source)0.16N3—Pt6.71[49]Wet method0.16N719(i) Use of screen-printing technique to fabricate TiO2 film(ii) TiCl4 treatment(iii) Nanostructured photoanodePt7.27[28]Wet method0.16N719(i) Use of screen-printing technique to fabricate TiO2 film(ii) TiCl4 treatmentPt8.32[25]Sintering P25-TiO2 at 500°C for 3 h under dry N2 and NH3 gas flow0.16N719(i) A screen-printing technique was used to fabricate the TiO2 films(ii) TiCl4 treatment(iii) Modified I−/I3− electrolyte usedPt8.00[50]Solid-state reaction method (NH4OH as N source)0.25N719—Pt6.16This workMoreover, charge recombination of the injected electrons in the CB of TiO2 with either the oxidized dye or the oxidized form of the electrolyte species is another limiting factor in the DSSCs’ performance. To investigate the impact of N doping on the recombination mechanism of DSSCs, linear sweep voltammetry (LSV) and electrochemical impedance spectroscopic (EIS) measurements were carried out in dark.Figure8 illustrates the LSV measurements of the pure TiO2 and N-doped TiO2-based devices on application of a negative potential between the photoanode and the counter electrode. When a negative potential is applied between the photoanode and the counter electrode, the electrons get transferred under the electric field while undergoing different recombination pathways. As such, the higher recombination rate causes large dark current in the TiO2/dye/electrolyte interface while the smaller recombination rate gives rise to lower dark current. In the present study, reduced dark current values were found for the devices assembled with 10N-TiO2 and 20N-TiO2 photoanodes. This could be attributed to the formation of O-Ti-N bond which would have suppressed the reduction of I3− on the TiO2 electrode [51]. But, further increase in the amount of N dopant up to 40 μL enhanced the dark current which may be due to the recombination reaction caused by excessive insertion of nitrogen dopant in the TiO2 lattice.Figure 8
LSV measurements of the DSSCs assembled with pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 photoanodes under the dark condition.Another technique, which is widely employed to investigate the charge transport resistance at each interface in the DSSCs, is electrochemical impedance spectroscopy (EIS). In the present study, the measurements were carried out at a frequency range from 10-2 to 106 Hz, as well as keeping VOC as the bias voltage in dark. A typical electrochemical impedance spectrum consists of three semicircles intersecting at the X axis of which the smallest semicircle is known as series resistance (Rs) and related to the sheet resistance on the transparent conductive oxide (TCO) substrate and the contact resistance between TCO and TiO2; the semicircle in the higher frequency region is responsible for the charge transfer resistance at the electrolyte/Pt electrode interface (Rct); and the largest semicircle corresponds to the electron transfer at the TiO2/dye/electrolyte interface and is referred as charge recombination resistance in dark condition (Rrec) [13, 14, 52]. Figure 9(a) shows the Nyquist plots of the DSSCs assembled with pure TiO2, 20N-TiO2, and 40N-TiO2 photoanodes, and the respective EIS parameters are summarized in Table 3.Figure 9
(a) Nyquist plots of the DSSCs assembled with pure TiO2, 20N-TiO2, and 40N-TiO2 photoanodes tested under the dark condition and (b) bode plots of the same devices.
(a)(b)Table 3
Summary of EIS parameters of the DSSCs assembled with pure TiO2, 20N-TiO2, and 40N-TiO2 photoanodes.
PhotoanodeRsRctRrecfmax (Hz)Ƭe (ms)Pure TiO29.011.7839.393.2548.9720N-TiO29.361.5656.843.1550.5240N-TiO29.901.5752.033.2049.73As reported in Table3, charge recombination resistances (Rrec) of pure TiO2, 20N-TiO2, and 40N-TiO2 photoanode-based DSSCs were found to be 39.39, 56.84, and 52.03 Ω, respectively. The DSSC assembled with 20N-TiO2 exhibited the highest charge recombination resistance (Rrec) indicating that recombination rate is retarded when appropriate amount of nitrogen is doped into TiO2. However, further insertion of nitrogen into TiO2 (40N-TiO2) lowered the Rrec value due to enhanced recombination. Figure 9(b) illustrates the bode plots of the pure TiO2, 20N-TiO2, and 40N-TiO2 photoanode-based devices, and the electron life time (Ƭe), which is also referred as the charge recombination time under the dark condition, was estimated using the following equation:
(1)Ƭe=12πfmax,where fmax is the maximum frequency corresponding to the peak [53]. In the present study, slight variations in the charge recombination time were observed among the tested devices and the one with 20N-TiO2 photoanode exhibited the highest Ƭe value reconfirming that recombination rate is retarded. Also, excessive insertion of nitrogen dopant into the TiO2 decreased charge recombination time and thereby facilitated the recombination reaction. The said results are consistent with the LSV findings. Further, it has been reported in the literature that incorporation of huge numbers of nitrogen atom on the TiO2 lattice or existence of inactive nitrogen atoms can generate trap states around the Fermi levels and assist the recombination process [18, 34]. The results of this study reveal that the performance of N-doped DSSCs is significantly influenced by the dye uptake ability rather than the charge transport resistance of the said devices.


## 3.1. Structural and Morphological Characterization of N-Doped TiO2

The crystal structures of the synthesized pure and N-doped TiO2 nanoparticles were analyzed by XRD, and the corresponding XRD patterns are shown in Figure 1. The XRD patterns of all nanoparticles exhibit peaks at 25.20°, 37.60°, 48.20°, 53.70°, 55.00°, 62.50°, 68.50°, 70.20°, 74.89°, and 82.53° corresponding to the reflection planes of (101), (004), (200), (105), (211), (204), (116), (220), (215), and (224) which are consistent with the well-crystallized pure anatase TiO2 phase (Anatase XRD JCPDS Card No. 21-1272) while peaks at 27.39°, 36.07°, and 41.2° correspond to the reflection planes of (110), (101), and (111) for rutile TiO2 phase (Rutile JCPDS Card No. 21-1276).Figure 1
XRD patterns of pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles.The above observation confirms that no phase transition in TiO2 had occurred due to N doping as both pure and N-doped TiO2 nanoparticles exhibit similar anatase and rutile peaks. The crystallite sizes (d) of the nanoparticles were calculated by the Scherrer equation using the predominant anatase plane of (101) [13, 14]. The estimated crystallite sizes of the pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles were found to be 18.26, 17.60, 17.55, 17.50, and 17.12 nm, respectively. Trivial reduction observed in the crystallite size with increase in N content of the N-doped TiO2 is in good agreement with the TEM images. Also, the observed insignificant variation in the crystallite size due to N doping on TiO2 is consistent with the results reported by Kang et al. [18]. Further, the absence of crystallite peaks representing nitrogen of N-doped TiO2 in the XRD spectra may be attributed to the low N dopant quantities [30, 31]. Hence, XPS and EDX spectroscopic studies were performed to confirm the presence of nitrogen in the N-doped TiO2 nanoparticles.The surface chemical composition and oxidation states of 20N-TiO2, as optimized sample, were studied and the findings are illustrated in Figure 2. The XPS survey reveals that the observed binding energies at 459.56 and 465.03 eV belong to the Ti 2p3/2 and Ti 2p1/2 spin-orbit splitting photoelectrons, respectively, which are consistent with the reported values for Ti4+ ion [32]. The binding energy at 529.6 eV of O 1s photoelectron confirms the presence of O2− state in the nanoparticles. The literature on N 1s XPS peak analysis states that N doping on TiO2 can occur in three different ways, namely, substitutional doping (replacement of O atom in the TiO2 by N), interstitial doping (addition of N atom into the TiO2 lattice), and atomic adsorption of N species [33, 34]. It is noteworthy to mention that though N doping on TiO2 is reported in many literature, the exact assignment of N 1s XPS peak is still debatable. In addition, the binding energy of N 1s photoelectron highly depends on the method employed to synthesize the N-doped TiO2 nanoparticles [35]. In the present study, N 1 s peak observed at 400.7 eV could be attributed to the interstitial N of the N-doped TiO2 with an O−Ti−N linkage [36] which is also in consistent with the XPS results of N-doped TiO2 reported by Liu et al. [37]. Moreover, the presence of this nitrogen in the N-doped TiO2 has led to reduction of TiO2 bandgap as well. Further, the EDX spectra (Figure S1 and Table S1) of pure TiO2 and 20N-TiO2 nanoparticles obtained in the binding energy region of 0.0-15.0 keV confirm the presence of N in the N-doped TiO2 nanoparticles, and the EDX mapping of 20N-TiO2 indicates that N ions are extensively and perfectly distributed on the surface of TiO2.Figure 2
(a) XPS spectrum of 20N-TiO2 and (b, c) high-resolution XPS spectra of Ti 2p, O 1s, and N 1s.
(a)(b)(c)(d)The morphology and surface nature of the synthesized nanoparticles were studied using TEM and SEM techniques. As displayed in Figures3(a)–3(c), the TEM images of pure TiO2, 20N-TiO2, and 40N-TiO2 nanoparticles reveal that the synthesized nanoparticles are of spherical shape with the estimated particle size in the range of 18-20 nm. The said particle size values are in good agreement with the results obtained from the XRD measurements. Furthermore, formation of aggregates was observed in the structure of N-doped TiO2 nanoparticles with high N content (40N-TiO2). The high-resolution transmission electron microscopic (HRTEM) images of pure TiO2 and 20N-TiO2 (Figures 3(d) and 3(e)) show one set of lattice fringes with a space of 0.35 nm that could be indexed to the (101) plane of anatase phase. These results are consistent with the findings of the XRD analysis. However, significant fringe lattice associated with the nitrogen of N-doped TiO2 is not observed due to the small quantity of the N dopant.Figure 3
TEM images of (a) pure TiO2, (b) 20N-TiO2, and (c) 40N-TiO2 nanoparticles; HRTEM images of (d) pure TiO2 and (e) 20N-TiO2.
(a)(b)(c)(d)(e)The SEM images of pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles, shown in Figure S2 (a-e), reveal that all nanoparticles are in uniform spherical shape and N doping had not caused any morphological changes. Similar observation is reported by Lin et al. for N-doped TiO2 using ammonium hydroxide as nitrogen source [38]. Further, Figure S2 (d and e) clearly exhibits that particles start to agglomerate when higher quantities of N dopant are used and consequently the surface area and pore size distribution of the nanoparticles get affected. The Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) measurements were carried out to investigate the surface area and pore size distribution of the nanoparticles, respectively.Figure4 shows the nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves of the pure TiO2, 20N-TiO2, and 40N-TiO2 nanoparticles. As it is observed in Figure 4(a), all three nanoparticles exhibit a IV type isotherm curve according to the IUPAC classification [39]. As per the IUPAC nomenclature, the pore size of a mesoporous material varies between 2 and 50 nm. The wide distribution of pore size/width in the range of 10–30 nm observed for the synthesized nanomaterial is depicted in Figure 4(b), which indicates its’ mesoporous structure [40]. The BET surface areas of pure TiO2, 20N-TiO2, and 40N-TiO2 nanoparticles were found to be 57.92, 53.28, and 51.97 m2g-1, respectively. Hence, the BET analysis reveals that the surface area of the synthesized nanoparticles is mainly controlled by the size of TiO2 though trivial changes occur due to N doping as already confirmed by the XRD and TEM measurements [41]. The average adsorption pore diameters of pure TiO2, 20N-TiO2, and 40N-TiO2 nanoparticles were found to be 10.65, 15.99, and 13.52 nm, respectively. The observed optimum value for 20N-TiO2 confirms that N doping, to a certain extent, not only modifies the surface area but also extremely enhances the pore size of the nanoparticles. However, further increase in the N dopant (e.g., 40N-TiO2) leads to reduction in both surface area and average adsorption pore diameter due to agglomeration of particles as observed in the corresponding TEM and SEM images. It is well-known that pore size improvement without sacrificing the surface area is beneficial as it allows a higher number of dye and electrolyte molecules to fill the pores of TiO2 film which in turn would enhance the short circuit current density of the device [42].Figure 4
(a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution curves of pure TiO2, 20N-TiO2, and 40N-TiO2.
(a)(b)


## 3.2. Optical Characterization of N-Doped TiO2

An optical study, utilizing UV-visible spectrophotometer, was performed to measure the light absorption capacities and estimate bandgaps of pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles (Figures 5(a) and 5(b)).Figure 5
(a) UV-visible absorption spectra of pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles and (b) Tauc plot of the corresponding nanoparticles.
(a)(b)The N-doped TiO2 nanoparticles showed a gradual shift in the UV-visible region to longer wavelengths (red shift) with an increase in the amount of N dopant compared to pure TiO2 (Figure 5(a)). This observation is in consistent with the previously reported studies [18, 38, 43]. Further, the estimated bandgaps of the said nanoparticles, calculated by Tauc plot as reported in our previous study [14], were found to be 3.15, 3.10, 3.03, 2.99, and 2.94 eV for pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 nanoparticles, respectively (Figure 5(b)). This reduction in the bandgap as a result of N doping could have been caused by either formation of isolated narrow bands above the valance band of TiO2 by mixing the 2p states of N and O in the dopant and TiO2, respectively (interstitial doping) [44] or replacement of oxygen-deficient sites by the nitrogen (substitutional doping) [43]. However, no peak for substitutional nitrogen was detected in the XPS study. Hence, the reduction in the bandgap could be attributed to the impurity levels formed above the valance band of TiO2 due to the interstitial nitrogen doping. Also, it has been reported that interstitial doping by nitrogen extensively influences and reduces the bandgap of TiO2 compared to substitutional N doping [45].Further, the effect of N doping on dye adsorption as well as desorption was studied. Initially, pure TiO2-, 10N-TiO2-, 20N-TiO2-, 30N-TiO2-, and 40N-TiO2-coated films were soaked in the N719 dye overnight, and the resultant dye-coated films were analyzed by UV-visible spectroscopy (Figure 6(a)). Then, the same photoanodes were dipped in 5 mL of 1 N NaOH solution, and the desorbed dye in NaOH solution was subjected to UV-visible spectroscopic analysis (Figure 6(b)).Figure 6
UV-visible absorption spectra of (a) dye-coated pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 films and (b) desorbed dye from the same films.
(a)(b)As shown in Figure6, enhanced light absorption was observed for dye-coated N-doped TiO2 films, and the maximum light absorption was obtained for the dye-coated 20N-TiO2 film. The improved light absorption of dye-coated N-doped TiO2 films could be attributed to the increased pore size for dye adsorption as a result of N doping which was confirmed by the BET and BJH studies. The greater dye adsorption on N-doped TiO2 film due to the increased porosity of TiO2 by N doping has been reported earlier as well [46]. However, a further increase in the amount of N dopant reduces light absorption due to agglomeration of the particles giving rise to a reduction in dye adsorption. These observations are in good agreement with the JSC values of the corresponding devices fabricated from the above dye-coated N-doped TiO2 films.


## 3.3. Performance Evaluation of DSSCs

The photovoltaic performance of the N719 dye-coated pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 photoanodes was studied using I−/I3− electrolyte and Pt counter electrode under the simulated irradiation with 100 mW cm-2 and AM 1.5 filter, and the results are depicted in Figure 7(a) and the PV parameters are summarized in Table 1.Figure 7
(a) Current-voltage (J-V) characteristics of the DSSCs assembled with pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 photoanodes tested under simulated irradiation of intensity 100 mW cm-2 with AM 1.5 filter and (b) variation in JSC and efficiency with different amounts of N dopant.
(a)(b)Table 1
Summary of photovoltaic parameters of the DSSCs assembled with pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 photoanodes.
PhotoanodeJSC (mA/cm2)VOC (V)FFPCE,η (%)Pure TiO211.900.650.655.1510N-TiO212.900.670.675.7920N-TiO213.700.670.666.1630N-TiO212.600.690.665.7440N-TiO211.300.650.664.86The J-V studies revealed that the PCE of N-doped TiO2-based DSSCs slightly improved because of marginal increase in FF (fill factor) and VOC (open circuit voltage) and considerable increase in JSC (short circuit current). It was observed that when the amount of N dopant had been increased from 0 to 20 μL, the JSC enhanced significantly from 11.90 to 13.70 mA/cm2; but, a further increase in the N dopant amount resulted in a downward trend with respect to JSC (11.30 mA/cm2 for 40N-TiO2). Figure 7(b) indicates the influence of different amounts of N dopant on JSC and η. The variation in JSC values could be attributed to the modified surface properties of N-doped TiO2 as explained earlier. The observed reduction in JSC and PCE beyond 20N-TiO2 could have been caused by decreased dye adsorption due to the particle agglomeration. In the present study, the device fabricated with 20N-TiO2 electrode showed the best η of 6.16%, which was 20% higher than that of the pure TiO2-based DSSC (η=5.15%).Table2 compares the PCE of the N-doped P25-TiO2 photoanode-based devices. Although the experimental conditions and other factors vary, based on the summary of the table, our proposed synthesized N-doped P25-TiO2 nanomaterial is shown to be more efficient for the DSSC application.Table 2
Comparison of the PCEs of N-doped P25-TiO2 photoanode-based devices.
Doping method and N sourceDevice active area (cm-2)DyeAdditional modificationCounter electrodePCE,η (%)Ref.Grinding P25-TiO2 and urea for 30 min in a mortar and annealing at 400°C for 1 hr0.50N719—Pt2.93[47]Grinding P25-TiO2 and thiourea for 30 min in a mortar and annealing at 400°C for 1 hr0.50N719(i) N codoped with SPt3.35[48]Sol-gel process with electrospinning0.25N719(i) N-doped TiO2 nanofiber photoanodePt4.70[27]Sol-gel process0.25—Pt4.86[18]Ball milling method (urea as N source)0.16N3—Pt6.71[49]Wet method0.16N719(i) Use of screen-printing technique to fabricate TiO2 film(ii) TiCl4 treatment(iii) Nanostructured photoanodePt7.27[28]Wet method0.16N719(i) Use of screen-printing technique to fabricate TiO2 film(ii) TiCl4 treatmentPt8.32[25]Sintering P25-TiO2 at 500°C for 3 h under dry N2 and NH3 gas flow0.16N719(i) A screen-printing technique was used to fabricate the TiO2 films(ii) TiCl4 treatment(iii) Modified I−/I3− electrolyte usedPt8.00[50]Solid-state reaction method (NH4OH as N source)0.25N719—Pt6.16This workMoreover, charge recombination of the injected electrons in the CB of TiO2 with either the oxidized dye or the oxidized form of the electrolyte species is another limiting factor in the DSSCs’ performance. To investigate the impact of N doping on the recombination mechanism of DSSCs, linear sweep voltammetry (LSV) and electrochemical impedance spectroscopic (EIS) measurements were carried out in dark.Figure8 illustrates the LSV measurements of the pure TiO2 and N-doped TiO2-based devices on application of a negative potential between the photoanode and the counter electrode. When a negative potential is applied between the photoanode and the counter electrode, the electrons get transferred under the electric field while undergoing different recombination pathways. As such, the higher recombination rate causes large dark current in the TiO2/dye/electrolyte interface while the smaller recombination rate gives rise to lower dark current. In the present study, reduced dark current values were found for the devices assembled with 10N-TiO2 and 20N-TiO2 photoanodes. This could be attributed to the formation of O-Ti-N bond which would have suppressed the reduction of I3− on the TiO2 electrode [51]. But, further increase in the amount of N dopant up to 40 μL enhanced the dark current which may be due to the recombination reaction caused by excessive insertion of nitrogen dopant in the TiO2 lattice.Figure 8
LSV measurements of the DSSCs assembled with pure TiO2, 10N-TiO2, 20N-TiO2, 30N-TiO2, and 40N-TiO2 photoanodes under the dark condition.Another technique, which is widely employed to investigate the charge transport resistance at each interface in the DSSCs, is electrochemical impedance spectroscopy (EIS). In the present study, the measurements were carried out at a frequency range from 10-2 to 106 Hz, as well as keeping VOC as the bias voltage in dark. A typical electrochemical impedance spectrum consists of three semicircles intersecting at the X axis of which the smallest semicircle is known as series resistance (Rs) and related to the sheet resistance on the transparent conductive oxide (TCO) substrate and the contact resistance between TCO and TiO2; the semicircle in the higher frequency region is responsible for the charge transfer resistance at the electrolyte/Pt electrode interface (Rct); and the largest semicircle corresponds to the electron transfer at the TiO2/dye/electrolyte interface and is referred as charge recombination resistance in dark condition (Rrec) [13, 14, 52]. Figure 9(a) shows the Nyquist plots of the DSSCs assembled with pure TiO2, 20N-TiO2, and 40N-TiO2 photoanodes, and the respective EIS parameters are summarized in Table 3.Figure 9
(a) Nyquist plots of the DSSCs assembled with pure TiO2, 20N-TiO2, and 40N-TiO2 photoanodes tested under the dark condition and (b) bode plots of the same devices.
(a)(b)Table 3
Summary of EIS parameters of the DSSCs assembled with pure TiO2, 20N-TiO2, and 40N-TiO2 photoanodes.
PhotoanodeRsRctRrecfmax (Hz)Ƭe (ms)Pure TiO29.011.7839.393.2548.9720N-TiO29.361.5656.843.1550.5240N-TiO29.901.5752.033.2049.73As reported in Table3, charge recombination resistances (Rrec) of pure TiO2, 20N-TiO2, and 40N-TiO2 photoanode-based DSSCs were found to be 39.39, 56.84, and 52.03 Ω, respectively. The DSSC assembled with 20N-TiO2 exhibited the highest charge recombination resistance (Rrec) indicating that recombination rate is retarded when appropriate amount of nitrogen is doped into TiO2. However, further insertion of nitrogen into TiO2 (40N-TiO2) lowered the Rrec value due to enhanced recombination. Figure 9(b) illustrates the bode plots of the pure TiO2, 20N-TiO2, and 40N-TiO2 photoanode-based devices, and the electron life time (Ƭe), which is also referred as the charge recombination time under the dark condition, was estimated using the following equation:
(1)Ƭe=12πfmax,where fmax is the maximum frequency corresponding to the peak [53]. In the present study, slight variations in the charge recombination time were observed among the tested devices and the one with 20N-TiO2 photoanode exhibited the highest Ƭe value reconfirming that recombination rate is retarded. Also, excessive insertion of nitrogen dopant into the TiO2 decreased charge recombination time and thereby facilitated the recombination reaction. The said results are consistent with the LSV findings. Further, it has been reported in the literature that incorporation of huge numbers of nitrogen atom on the TiO2 lattice or existence of inactive nitrogen atoms can generate trap states around the Fermi levels and assist the recombination process [18, 34]. The results of this study reveal that the performance of N-doped DSSCs is significantly influenced by the dye uptake ability rather than the charge transport resistance of the said devices.


## 4. Conclusion

In the present study, N-doped TiO2 nanoparticles with systematically varied amounts of NH4OH were successfully synthesized by a facile solid-state reaction, characterized and effectively integrated in the fabrication of DSSCs. XRD patterns of the synthesized nanoparticles suggested that N doping had not altered the TiO2 anatase and rutile crystal structures. The presence of nitrogen in the N-doped TiO2 nanoparticles was confirmed by XPS and EDX analyses. SEM and TEM images showed that high amount of N dopant had resulted in the agglomeration of nanoparticles. BET, BJH, and UV-visible spectroscopic results revealed that optimum amount of N doping on TiO2 increases the pore diameter of TiO2 particles which leads to higher dye adsorption. Through the J-V measurement, it was found that the device assembled with 20N-TiO2 photoanode possessed 20% higher PCE (η=6.16%) compared to the control device (η=5.15%) mainly due to enhanced JSC. This improvement in JSC value of the optimised device could be attributed predominately to the enhanced visible light harvest as a result of increased dye uptake along with a trivial contribution from the reduced charge recombination process. Among the reported studies on DSSCs based on the N-doped P25-TiO2 photoanodes, our method gives the best PCEs for the DSSCs.



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*Source: 1090174-2023-02-03.xml*