Dye-sensitized Solar Cells Based on Nitrogen-doped Titania ...

Key Engineering Materials Vol. 451 (2011) pp 21-27 

Online available since 2010/Nov/11 at www.scientific.net 

© (2011) Trans Tech Publications, Switzerland 

doi:10.4028/www.scientific.net/KEM.451.21 

<strong>Dye</strong>-<strong>sensitized</strong> <strong>Solar</strong> <strong>Cells</strong> <strong>Based</strong> on Nitrogen-doped Titania Electrodes 

Wei Guo, Qingqing Miao, Gang Xin, Liqiong Wu, and Tingli Ma a 

State key laboratory of fine chemicals, School of Chemical Engineering, Dalian University of 

Technology, 116012, China 

a 

tinglima@dlut.edu.cn 

Keywords: nitrogen-doping; titania; dye-<strong>sensitized</strong> solar cell; stability; efficiency 

Abstract <strong>Dye</strong>-<strong>sensitized</strong> solar cell(DSC) is a new type of photovoltaic device. This paper mainly 

describes the research results of the development of a novel nitrogen-doped photoanode for DSC in 

our group. Highly efficient dye-<strong>sensitized</strong> solar cells (DSCs) of 7.6-10.1% were fabricated using 

nitrogen-doped titania electrodes. The photoelectrochemical properties of the nitrogen-doped titania 

powder, film, and solar cell were systemically investigated. We confirmed the substitution of 

oxygen sites and oxygen deficiency with nitrogen atoms in the titania structure by X-ray 

photoemission spectroscopy (XPS). The UV-Vis spectra of the nitrogen-doped powder and film 

showed visible light absorption in the wavelength range between 400 nm and 535 nm. The results 

of the stability test indicated that the DSCs fabricated by the nitrogen-doped titania exhibited great 

stability. 

Introduction 

Since Grätzel and coworkers developed a new type of solar cells based on the nanocrystalline TiO 2 

electrode,[1-3] dye-<strong>sensitized</strong> solar cells (DSCs) have attracted much attention for their high energy 

conversion efficiency as well as the possibility of becoming the a low-cost alternative to 

commercial solar cells based on silicon.[4-8] In order to successfully commercialize DSCs, 

however, it is necessary to further improve the energy conversion efficiency. Although many 

methods have been undertaken in the past to improve the conversion efficiency of the DSC, we 

have not seen significant improvement. Another problem is that the nanostructure TiO 2 electrode in 

DSCs has oxygen deficiency in TiO 2 crystal structure.[5-7] It is well known that the oxygen 

deficiency can create electron-hole pairs and the oxidizing holes can react with the dye or be 

scavenged by iodide ions.[8] Those deficiencies are able to cause shortening the lifetime of the 

DSCs. In order to solve the problems mentioned above, we have developed a DSC system based on 

N-doped titania. As a result, we have successfully achieved high energy conversion efficiency and 

high stability of DSCs using N-doped titania photoanodes. [9] 

Aiming to further understand the influence of the N-doped titania on the DSC system, we 

recently carried out detailed investigations such as preparing N-doped titania powders by several 

methods, and comparing the photoelectrochemical properties and electron lifetime and transport 

time of the DSCs through intensity-modulated spectroscopic analysis. We also investigated the 

effect of electron injection of the DSCs based on N-doped electrodes by surface photovlotage 

spectrum (SPS). In this review, we will summarize some of the results obtained by our group. 

Lately, Dai et al. also reported the results of retarded charge recombination in dye-<strong>sensitized</strong> 

nitrogen-doped TiO 2 solar sells.[10] They prepared the N-doped titian using sol-gel method and 

discussed the mechanism of charge recombination of DSCs based on nitrogen-doped TiO 2 

electrodes. They indicated that the enhanced electron lifetime for doped TiO 2 solar cells could be 

attributed to the formation of O-Ti-N in the TiO 2 electrode to retard the recombination reaction at 

the TiO 2 photoelectrode/electrolyte interface, as compared to the undoped TiO 2 solar cells. They 

also carried out the stability test under one sun light soaking and a high temperature condition (70 

°C) for more than 1000 hs, they observed that the DSC based on the N-doped TiO 2 photoanode is 

more stable than that of undoped TiO 2 .[10] These results are consistent with those of our group.[9] 

Yang et al. designed a device to generate TiN x O 2–x powders using NH 3 as a gaseous precursor. 

[11] They observed that the photoelectrochemical property of the solar cells appeared to be highly 

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22 A New Sight towards <strong>Dye</strong>-<strong>sensitized</strong> <strong>Solar</strong> <strong>Cells</strong>: Material and Theoretical 

sensitive to the concentration of N-doped in the TiO 2 . [11] Thus, they concluded that the research 

has offered proof of the relationships among the concentration, the band gap, the nanoparticle size, 

and the photoelectric conversion efficiency of the N-doped TiO 2 photoelectrodes.[11] These results 

indicate that we can find an effective way to adjust the relationship between the concentration and 

the band gap of the N-doped TiO 2 photoelectrodes. [11] 

Experimental 

Preparation of -doped and undoped titania powders. Commercial titania powders of ST-01 

(Ishihara Sangyo, Ltd.) was used as the starting materials for preparing N-doped titania through dry 

and wet methods developed by our group. 17 Preparation of the N-doped titania powder was 

attempted using the hydrolysis of titanium tetraisopropoxide (TTIP) and triethylamine or urea, the 

obtained powders were denoted as N-doped T and N-doped U. The undoped titania was prepared by 

the same process without notrogen source and they were denoted as undoped T and undoped U. 

Preparation of -doped and undoped titania photoanodes. The pastes of the N-doped and 

undoped titania were prepared according to the procedure developed by our group, only adding 

polyethylene glycol (PEG) 600 as a dispersion medium without any other binder.[12] The prepared 

titania paste was printed on the fluorine-doped tin oxide conducting glass (FTO glass, Asahi Glass; 

sheet resistance: 10 ohm/square) by screen-printing technique and sintered at 500 o C for 30 mins in 

the air. 

Fabrication of the DSCs based on the -doped and undoped titania photoanodes. The 

sandwich-type solar cell consisted of the titania electrode adsorbed a Ru dye (<strong>Solar</strong>onix, N719), and 

the counter electrodes of Pt coated FTO-glass. The thickness and the area of the titania electrode 

were ca. 18 µm and 0.20 cm 2 respectively. The electrolyte solution was composed of 0.1 M LiI, 0.3 

M 1,2-dimethyl-3-propylimidazolium iodine, 0.05 M I 2 and 0.5 M tert-butylpyridine in 3- 

methoxypropionitrile. 

Photovoltaic measurement of DSCs. The UV-Vis spectra were taken on a JASCO V-550 using 

an integrating sphere setup. The current-voltage measurements of the N-doped and undoped titania 

were performed using a 300 W solar simulator as the light source. Intensity-modulated photocurrent 

and photovoltage spectroscopy (IMPS and IMVS) were performed using a white light-emitting 

diode (Lumiled Luxeon Star 1W) as the light source at room temperature. 

Fig.1. X-ray diffractogram for N-doped titania powder of T and U.

Key Engineering Materials Vol. 451 23 

Fig.2. XPS spectra of N-doped titania powders, (a.b) N-doped ST-01; (c) N-doped T; (d) N-doped U. 

Results and Discussion 

Structural characterizations of the titania powders were performed by X-ray diffraction patterns 

(XRD). Fig.1 displays the data for the N-doped titania prepared by the wet method. The results 

indicated that the crystal phase of the N-doped powder was anatase, and no crystal phase of the 

rutile was observed after annealing at 400ºC. At temperatures over 700 ºC, however, the crystal 

phase of the rutile was observed both in the case of N-doped tintania of T and U. 

The substitution of the oxygen sites and oxygen deficiency site with nitrogen atoms in the titania 

structure was confirmed by X-ray photoemission spectroscopy (XPS), as shown in Fig.2. Three 

binding energy peaks were observed at 396.2, 398.3, and 400.4 eV in the N 1s region. Concerning 

the assignment of the peak feature in XPS for the nitrogen-doped titania, some controversy still 

exists in the literature. A detailed discussion was performed in our previous publication. [9]. 

We considered that the signal at 396.2 eV is attributed to a chemically bound N - species and the 

weak peak around 398.3 eV is derived from the presence of the O-Ti-N linkages in the crystalline 

TiO 2 lattice (Fig. 2a). The formation of the Ti-N and O-Ti-N structures is suggested to proceed 

during the substitution doping process. It has been demonstrated that the N substitute O site or 

oxygen deficiency site in the crystalline TiO 2 lattice can stabilize the DSC system. [9, 10] On the 

other hand, the signal around 400.4 eV is considered to be a molecularly adsorbed nitrogen species, 

which absorbs onto the surface and into the interstitial sites of the titania lattice (Fig. 2b). The XPS 

results of the N-doped titania T (Fig. 2c) and N-doped titania U (Fig. 2d) can be also assigned as 

discussing above. These results are consistent with those described in previous literature [13-15]. 

The UV-Vis absorption spectra were measured using an integrating sphere setup for the N-doped 

powders and films with adsorbed dye, as shown in Fig.3. For comparison, the spectra of the N-


doped titania T powder is also illustrated (Fig. 3a). No absorption peak for the undoped T was 

observed above the wavelength of 400 nm. On the other hand, the N-doped titania T powders 

exhibited a new absorption peak in the visible light region between 400 nm and 550 nm (Fig. 3b). 

The visible light activity is possibly due to the N-doping in the titania crystalline structure, which 

was caused by the fact that the nitrogen doping induced a new state lying close to the valence band 

edge.[16] From Fig. 3, we also found that the amount of dye adsorption of N-doped T (Fig. 3d) was 

more than that of N-doped U (Fig. 3c). 

Fig.3. UV-Vis spectra of the N-doped and undoped titania powder and film, (a) undoped T powder; 

(b) N-doped T powder; (c) N-doped U film / N719 dye; (d) N-doped T film/ N719 dye. 

Fig.4. UV-Vis spectra of the N-doped and undoped titania powder after sintering at different 

temperature.


In order to test the thermal stability of the N-doped titania prepared in this work, we measured 

the UV-Vis apectra after sintering at the range of 450-600˚C in the air. As shown in Fig. 4, the 

adsorption peaks were still observed after sintering at 600 ˚C for 30 mins, although the intensity of 

these peaks decreased as the sintering temperature increased. This result indicates that the nitrogendoped 

titania has remarkable thermal stability. 

Table 1 shows the current-voltage data of the open cells based on the N-doped T and U, and the 

undoped titania photoelectrodes. We observed a pronounced increase in the photocurrent and 

photovoltage for the DSCs based on the N-doped titania. High energy conversion efficiencies of 

7.9% and 7.6% were achieved, and the enhancement of ca. 10% and 5% were observed due to the 

introduction of the N-dped. A high efficiency of 10.1% for the DSC based on N-doped titania ST- 

01 was reached after the optimization. The details will be published in an another paper. 

Table 1. Performance of the DSCs based on the N-doped and undoped titania electrods 

Samples V OC (mV) J SC (mA) FF η(%) 

N-doped T 681 16.3 0.72 7.9 

N-doped U 780 15.8 0.62 7.6 

undoped TiO 2 747 14.8 0.65 7.2 

There was a concern that the visible-light-active titania can possibly accelerate the deterioration 

of the dye and the electrolyte in the DSC system. Therefore, we performed studies on the stability 

of the DSCs based on the N-doped T (pink line) and U (blue line) electrodes during irradiation for 

1000 hs under the white light (100 mW/cm 2 ) at 25 ºC(Fig.5). The stability of the DSCs based on the 

N-doped ST-01 (black line) was also tested for 2000 hs under the same condition. As shown in 

Fig.5, no photodegradation was observed for the cell involving the nitrogen-incorporated titania 

structure. We also carried out an outdoor stability test of the N719 dye/N-doped titania stored in the 

air for four years and complete solar cells exposed under one sunlight for 109 days at ambient 

temperature in Dalian, China. The result implies that the deterioration of the dye did not happen 

during the test period. Recently, Dai et el. also carried out the stability test of the DSCs based on the 

N-doped titania. They observed that the DSC based on the N-doped titania photoanode was more 

stable under one sun light soaking and a high temperature condition (70 o C) for more than 1000 hs. 

These results reveal that the dye-<strong>sensitized</strong> solar cell fabricated with the N-doped titania electrode 

possesses an excellent stability. 

Furthermore, the electron transport time, electron lifetime, and electron injection, in the dye<strong>sensitized</strong> 

N-doped and undoped solar cells were studied by intensity-modulated spectroscopic 

analysis and the surface photovlotage spectrum (SPS). The detailed data will be summarized in an 

another paper. 

Summary 

In conclusion, the N-doped titania nanocrystalline materials were successfully synthesized by the 

dry and the wet methods. Three binding energy peaks were shown in the N 1s region of the XPS. 

The signals at 396.2 eV and 398.3 eV were attributed to a chemically bound N - species and the O- 

Ti-N linkages within the crystalline TiO 2 lattice, respectively. A new absorption was observed for 

the UV-Vis spectrum of the N-doped titania in the visible light region. The high energy conversion 

efficiencies of 7.6% - 10.1% were achieved for the DSCs based on the N-doped titania electrodes. 

The results of the stability test demonstrated that the introduction of nitrogen into the titania 

photoelectrode of the DSC can stabilize the DSC system. Further detailed studies are in progress to 

improve the efficiency through the optimization of the amount of nitrogen doping, and the 

mechanism to improve the efficiency in N-doped DSC system.


Fig. 5. Stability for the DSC based on N-doped T (pink line), U (blue line) and N-doped ST- 

01(black line) titania electrodes over 1000 hs and 2000 hs of continuous illumination with white 

light of 100 mW/cm 2 intensity.


Acknowledgment 

This research was supported by the National Natural Science Foundation of China (Grant No. 

50773008). This work was also supported by the National High Technology Research and 

Development Program for Advanced Materials of China (Grant No. 2009AA03Z220) and State Key 

Laboratory of New Ceramic and Fine Processing Tsinghua University. 

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A New Sight towards <strong>Dye</strong>-<strong>sensitized</strong> <strong>Solar</strong> <strong>Cells</strong>: Material and Theoretical 

doi:10.4028/www.scientific.net/KEM.451 

<strong>Dye</strong>-Sensitized <strong>Solar</strong> <strong>Cells</strong> <strong>Based</strong> on Nitrogen-Doped Titania Electrodes 

doi:10.4028/www.scientific.net/KEM.451.21

Dye-sensitized Solar Cells Based on Nitrogen-doped Titania ...

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