and FTO Interface by N-Doped Anatase TiO - ACS Publications

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Improved the Electron Transfer Between TiO and FTO Interface by N-Doped Anatase TiO Nanowires and Its Applications in Quantum Dot-Sensitized Solar Cells 2

Qingqing Qiu, Shuo Li, Jingjing Jiang, Dejun Wang, Yanhong Lin, and Tengfeng Xie J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07795 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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The Journal of Physical Chemistry

Improved the Electron Transfer Between TiO2 and FTO Interface by N-doped Anatase TiO2 Nanowires 27x14mm (300 x 300 DPI)

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Improved the Electron Transfer Between TiO2 and FTO Interface by N-doped Anatase TiO2 Nanowires and Its Applications in Quantum Dot-sensitized Solar Cells Qingqing Qiua, Shuo Lia, Jingjing Jiang,a Dejun Wanga,b, Yanhong Lina, Tengfeng Xie*a a

College of Chemistry, Jilin University, Changchun 130012, P. R. China

b

Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China

ABSTRACT: The growth of anatase TiO2 nanowires (NWs) on fluorine doped tin oxide (FTO) substrates through hydrothermal reaction has attracted wide attention and research, especially in the case of the solar cells. Actually, the built-in electric field at anatase TiO2 NWs/FTO interface leads the photoexcited holes transfer to FTO conductive substrates because the Fermi energy of anatase TiO2 NWs film is higher than that of FTO substrates. But efficient transport of photoexcited electron to the FTO conductive substrates is desirable. Hence, the built-in electric field at pure TiO2 NWs/FTO interface has prevented anatase TiO2 NWs based solar cells from achieving a higher photoelectric performance. In this work, we elaborately design and construct the N-doped anatase TiO2 NWs/FTO interface with the desirable orientations from FTO towards N-doped anatase TiO2 NWs, which favors the photoexcited electron transfer to the FTO conductive substrates. The surface photovoltage (SPV) and Kelvin Probe measurements demostrate that the N-doped anatase TiO2 NWs/FTO interface favors the photoexcited electron transfer to the FTO conductive substrates due to the


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fact that the orientation of built-in electric field at the N-doped TiO2 NWs/FTO interface is from FTO towards TiO2. The photoexcited charge transfer dynamics of CdS QD-sensitized TiO2 NWs and N-doped TiO2 NWs electrodes was investigated using the transient photovoltage (TPV) and transient photocurrent (TPC) technique. Benefiting from the desirable interface electric field, CdS based quantum dot-sensitized solar cells (QDSCs) with the optimal N doping amount exhibits a remarkable solar energy conversion efficiency of 2.75% under 1 sun illumination, which is 1.46 times enhancement compared with the undoped reference solar cells. The results reveal that the N-doped anatase TiO2 NWs electrodes have promising applications in solar cells.

1. Introduction Great attention and effort has been focused on the development of solar energy due to the growing problem of environment pollution problem and energy crisis.1-6 In this regard, solar cells are one of the effective methods to satisfy the requirement of clean energy in the future as the devices to convert sunlight into electricity directly.7-9 Quantum dot-sensitized solar cells (QDSCs) have captured much attention due to the fact that the merits of QDSCs are demonstrated in the architecture of QDSCs and the unique properties of the quantum dot (QD) sensitizers.10-12 TiO2 nanomaterials, as the wide bandgap semiconductor, has attracted extensive research because of its potential application in solar cells.11-17 Among



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the nanostructured materials, one dimensional (1D) nanomaterials have obtained extensive attention because of 1D channel for fast electron transfer and light scatter.18-32 Hence, the recent report about a facile method of hydrothermal growth anatase TiO2 NWs on FTO substrates has obtained extensive attention and research in solar cells, especially for dye-sensitized solar cells (DSSCs) and QDSCs.12,29,32-34 However, the final PCE reports are still far less than expectations due to the improvement of charge transfer for this type of anatase TiO2 NWs. Obviously, there are some key factors for the improvement of PCE in solar cells that photoexcited electron can quickly and efficiently separate and transfer to the FTO substrates. According to the previously reports, the built-in electric field at the TiO2/substrate interfaces plays a key role for the photoelectric performance of solar cells.35-38 Our recent report on the photoexcited charge transfer dynamics at anatase TiO2 NWs/FTO interface has shown that the built-in electric field at anatase TiO2 NWs/FTO interface leads the photoexcited holes transfer to FTO conductive substrates in both anatase TiO2 NWs and QD-sensitized anatase TiO2 NWs films because the Fermi energy of anatase TiO2 NWs film is higher than that of FTO conductive substrates, as shown in Fig. 1.39 The higher Fermi energy of anatase TiO2 NWs film (Fig. 1(a)) leads to the built-in electric field direction at TiO2/FTO interface from TiO2 towards FTO (Fig. 1(c)), therefore the photoexcited holes transfer to the FTO substrate at the anatase TiO2 NWAs film/FTO interface under the built-in electric field.


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The photoexcited electrons are injected into the anatase TiO2 NWAs film and transferred to the FTO substrate according to the band structure of CdS/CdSe QDs and anatase TiO2 NWAs (Fig. 1(b)). But inter-facial barrier at the anatase TiO2 NWAs film/FTO interface leads to the photoexcited holes transfer to the FTO substrate (Fig. 1(d)), which will be at a disadvantage for the photoelectric properties of solar cells. As a matter of fact, efficient transport of photoexcited electron to the FTO conductive substrates is desirable. Thus, photoexcited charge separation can be greatly improved through constructing the TiO2/FTO interface with the desirable orientations of the built-in electric field, which can enhance photoelectric performance of solar cells. However, there were few reports about the research on the built-in electric field at anatase TiO2 NWs/FTO interface.

Fig. 1 Schematic band diagrams of the TiO2 NWs/FTO interface in both pure anatase TiO2 NWs and QD-sensitized anatase TiO2 NWs films.

Herein, we propose the concept of controlling interface electric-field at anatase TiO2 NWs/FTO interface to meet the above-mentioned desirable. Undoubtedly, modification of anatase TiO2 NWs film is the effective and direct



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way to regulate the Fermi level of anatase TiO2 NWs films. Therefore, CdS QD-sensitized N-doped anatase TiO2 NWs film was employed as the model photoanode and FTO as the conductive substrates in this study. We elaborately design and construct the N-doped anatase TiO2 NWs/FTO interface with the desirable orientations from FTO towards N-doped anatase TiO2 NWs, which favors the photoexcited electron transfer to the FTO conductive substrates. Surface photovoltage (SPV) results demonstrate that the built-in electric field at N-doped anatase TiO2 NWs/FTO interface favors the photoexcited electron transfer to the FTO conductive substrates. The Kelvin Probe technique was used to test the work function of anatase TiO2 NWs film, anatase TiO2 NWs film and FTO conductive substrates in order to confirm the orientation of built-in electric field. And the results further demonstrate that the built-in electric field direction at TiO2/FTO interface from TiO2 towards FTO turns into from FTO towards TiO2 through N-doped anatase TiO2 NWs. Mott-Schottky plots were tested to investigate electronic properties of TiO2 NWs and N-doped TiO2 NWs electrodes, and the results indicate that the Efb of N-TiO2 NWs electrodes shift positively and the Nd of N-doped TiO2 NWs electrodes decreases due to the fact that N as a P-type dopant in anatase TiO2 NWs plays a leading role. Photoluminescence (PL) spectroscopy, transient photovoltage (TPV) and transient photocurrent (TPC) techniques were applied to demonstrate that N-doped anatase TiO2 NWs film could enhance the separation efficiency of photoexcited charge. Therefore, the experimental results indicate


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that QDSCs based on N-doped anatase TiO2 NWs photoanode exhibits better photoelectric performance compared with QDSCs based on anatase TiO2 NWs photoanode. Furthermore, these findings demonstrate that modification of anatase TiO2 NWs could be a general strategy for constructing the interface with the desirable orientations of the built-in electric field.

2. Experimental 2.1. Preparation of anatase TiO2 NWs and N-doped anatase TiO2 NWs. The anatase TiO2 NWs film is directly growing on FTO conductive substrates under hydrothermal condition according to a previously report.12 In brief, 0.35 g titanium oxalate potassium (98+%, Aladdin) and the pre-calculated amount of urea (98+%, Xilong Chemical Co.Ltd) were added to a mixture containing 5.0 mL water and 15.0 mL diethylene glycol (98+%, Tianjin East China Chemical Reagent Factory). The mixture was stirred for 30 min at room temperature. Then, the mixture solution was transferred to a Teflon-lined autoclave, where the FTO was placed in the Teflon liner against the wall of the Teflon liner with the conductive side face down. The hydrothermal reaction was maintained at 180 ℃ for 9 h. Finally, the obtained anatase TiO2 NWs film was washed with double-distilled water and dried in ambient air. Then the dried anatase TiO2 NWs film was immersed into a 40 mM titanium tetrachloride aqueous solution at 70 ℃ for 35 min and washed with double-distilled water and dried in ambient air, and annealed at 500 ℃ for 40 min. (According to the adding quantity of urea, hereafter the N-doped anatase TiO2 NWs films are denoted as



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0.27% N-TiO2 NWs, 0.56% N-TiO2 NWs, 1.08% N-TiO2 NWs and 1.38% N-TiO2 NWs (N/TiO2, mass ratio).)

2.2. Solar cells fabrication. The TiO2 film electrodes were sensitized with CdS QDs by using the successive ionic layer adsorption and reaction (SILAR) method.40 Typically, the TiO2 films were first immersed in a 0.125 M cadmium nitrate tetrahydrate (99+%, Sinopharm Chemical Reagent) ethanol solution for 2 min, then the photoelectrodes rinsed with ethanol and dried in ambient air. After that the photoelectrodes were dipped into a 0.125 M sodium sulfide nonahydrate (98+%, Xilong Chemical Co.Ltd) methanol solution for another 2 min, then the photoelectrodes were rinsed with methanol and dried in ambient air. This procedure was repeated ten times to obtain the desired thickness of CdS QDs. The Cu2S counter electrode was prepared through a chemical bath deposition (CBD) method.41 In brief, 0.24 g copper sulfate pentahydrate (99%, Beijing Chemical Works) was dissolved in 60 mL double-distilled water, and N2 was buddled in the solution for 10 min. Then 0.37 g sodium thiosulfate pentahydrate (99+%, Beijing Chemical Works) was added to the solution and ultrasonic stirred. Afterwards, the cleaned FTO was immersed in the mixture solution with the conductive side face down, and the solution was maintained at 90 ℃ water bath for 1 h. Finally, the FTO with Cu2S was washed with double-distilled water and dried in ambient air. The working electrode and the counter electrode were assembled into a sandwiched type solar cell by epoxy resin. The polysulfide electrolyte was


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methanol-water (7:3 by volume) solution consisted of 2 M sodium sulfide nonahydrate (98+%, Xilong Chemical Co.Ltd), 2 M sublimate sulphur (99.5+%, Tianjin East China Chemical Reagent Factory) and 0.2 M potassium hydroxide (85+%, Beijing Chemical Works).

2.3. Characterization. The morphology of the samples was characterized by the Gemini 550 field-emission scanning electron microscope (Zeiss Company). Transmission electron microscopy (TEM) was conducted by the transition electron microscopy (TEM, TECNAIG2, FEI Company). X-ray photoelectron spectroscopy (XPS) measurement was performed on a VG Scientific ESCALAB 250 spectrometer with monochromatized Al Kα excitation. The crystal structure of the samples was characterized by X-ray diffraction (XRD) using a Rigaku D/Max-2250 X-ray diffractometer with Cu Kα radiation (λ=1.5418 Å; 50 KV, 200 mA) in the range of 20-80° (2θ) at a scanning rate of 5°/min. The adsorption spectra were obtained on a UV-vis spectrophotometer (UV3600, Shimadzu). The N elemental analysis was conducted using a CHNSO analyzer (vario EL cube). The surface photovoltage (SPV) spectra were measured by a lock-in-based surface

photovoltage

(SPV)

measurement

system

with

a

grating

monochromator (Omni-λ5007, Zolix), a lock-in amplifier (model SR830-DSP) and an optical chopper (model SR540) running at a frequency of 24 Hz. Transient photovoltage (TPV) measurements and Transient photocurrent (TPC) measurements were performed on a self-assembled instrument with a Nd:YAG laser (Q-smart 450, Quantl, Inc.).42 The intensity of the pulse was adjusted with



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a neutral grey filter and determined by a Joule meter (Starlite, Ophir, Inc.). The transient signal was registered by using a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix). All the SPV, TPV and TPC measurements were tested in ambient air.42 PL spectra were recorded on a FLS920 fluorescence spectrometer (Edinburgh Instruments) in ambient air. The work function (Φ) of the samples is tested by the Kelvin Probe technique (SKP5050, KP Technologies), which is indirectly provided by the contact potential difference (CPD) of samples and the ΦAu of Au probe. The width of the gold reference probe is 1.8 mm, whose work function is 5.12 eV.The work function of the samples is calculated by the following equation:43 Φsample = ΦAu + eCPD, (ΦAu = 5.12 eV) The contact potential difference (CPD) between the sample and the probe was measured in dark. The CPD of each sample was measured with the same probe distance from the sample, and the probe distance was controlled by the software of Kelvin probe system.44

2.4. Photoelectrochemical experimental. Photocurrent–voltage characteristics (J–V curves) of the QDSCs were obtained on an electrochemical analyzer (CHI 630B, made in China) in the dark and under illumination. The electron lifetime of the samples was acquired from open-circuit photovoltage decay (OCVD) measurements by using the electrochemical analyzer (CHI 630B, made in China). Electrochemical impedance spectroscopy (EIS) was obtained on an electrochemical analyzer (CHI 660E, made in China) under illumination. A 500W xenon lamp (CHFXQ500 W,


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Global Xenon Lamp Power) with appropriate filter was used for the light source of J–V curves, OCVD technique and EIS. The incident light intensity was 100 mW/cm2, which was measured by an irradiatometer (Photo-electronic Instrument Co., attached to Beijing Normal University, China). The effective area of the solar cell was 0.19625 cm2. The incident photo-to-current (IPCE) was measured by the electrochemical analyzer (CHI 660E, made in China) with a 300 W xenon lamp (FLS-SXE500, Trusttech) and a grating monochromator (Omni-λ5007, Zolix). The IPCE is given by the following equation: IPCE =

J ×1240 Pmono λ

Where J is the photocurrent density (mA/cm2), Pmono is monochromatic light intensity (mW/cm2), λ is the wavelength of exciting light (nm). The Mott-Schottky plots were obtained on an electrochemical analyzer (CHI 660E, made in China) in the dark. The carrier concentration (Nd) and the flat band potential (Vfd) can be acquired by the Mott-Schottky equation:45,46

1 / C 2 = (2 / eε oε N d )  (V − V fd ) − kT / e  Where C is the capacitance of the space charge region, ε0 is the vacuum permittivity, ε is the dielectric constant of TiO2 film (ε=55),47,48 e is the electron charge, V is the applied potential, KB is the Boltzmann constant, T is the temperature. The donor concentration is calculated with the equation:49-51

N d = (2 / eεε o )  d (1/ C 2 ) / dV −1  From the best fit of the linear part of the curve, it is possible to determine the Efb from 1/C2=0.



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3. Results and discussion 3.1. Characterization of anatase TiO2 NWs and N-doped anatase TiO2 NWs. Fig. S1 (a,b) and (c,d) show the typical top-view FESEM images of the pure anatase TiO2 NWs and the 1.38% N-doped anatase TiO2 NWs, respectively. Fig. 2 (a) and (b) show typical TEM images of the pure anatase TiO2 NWs and the 1.38% N-doped anatase TiO2 NWs, respectively. As can be seen, all the anatase TiO2 NWs show a similar morphology of nanowire branches, and the surface of nanowires is relatively smooth. In addition, the anatase TiO2 nanowire branches are about 10-20 nm in diameter and the length of anatase TiO2 nanowire are about 200-300 nm, but the growth of the nanowires is in disorder. It should be noted that no additional deposits or differences in morphology can be distinguished after N-doped the anatase TiO2 NWs. Fig. S2 shows the X-ray diffraction (XRD) patterns of undoped TiO2 NWs and N-doped TiO2 NWs with different proportion of N adding amount. The (101), (200), (105), (211) and (204) peaks observed in the Fig. S2 correspond to the anatase phase of TiO2 (JCPDS card no. 21-1272). The XRD patterns illustrate that no diffraction peaks of impurities are observed and no shift of diffraction peaks can be identified, which may be a reason of the low doping amount.


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Fig. 2 TEM images of (a) undoped TiO2 NWs, (b) 1.38% N-doped TiO2 NWs.

According to the adding quantity of urea, hereafter the N-doped anatase TiO2 NWs films are denoted as 0.27% N-TiO2 NWs, 0.56% N-TiO2 NWs, 1.08% N-TiO2 NWs and 1.38% N-TiO2 NWs. However, the N elemental analysis was performed to determine the practical N elemental content in N-doped anatase TiO2 NWs films, which are presented in Table 1. The results illustrate that the practical content of N element in N-doped anatase TiO2 NWs films was lower than those from proportion of adding amount. In addition, the practical content of N element in 0.27% N-TiO2 NWs is lower than the detection limit and the practical content of N element does not increase as expected with the increase of adding amount when the N proportion of adding amount reached 1.38%. Table 1 Elemental contents of N in anatase TiO2 NWs films Proportion of adding 0.27 0.56 1.08 amount 0.39 0.45 Determined (%) ≤ 0.1 (±0.02) (±0.01)

1.38 0.47 (±0.03)

The XPS was applied to further analyze the composition of the N-doped anatase TiO2 NWs. Fig. 3 (a) shows the XPS survey spectra of 0.17% N-TiO2 NWs, 0.56% N-TiO2 NWs, 1.08% N-TiO2 NWs and 1.38% N-TiO2 NWs films. There are strong signals of Ti and O for N-doped anatase TiO2 NWs films. In addition, the weak signals of N are observed for 0.17% N-TiO2 NWs, 0.56% N-TiO2 NWs, 1.08% N-TiO2 NWs and 1.38% N-TiO2 NWs. Fig. 3 (b) shows N 1s XPS spectra of 0.17% N-TiO2 NWs, 0.56% N-TiO2 NWs, 1.08% N-TiO2



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NWs and 1.38% N-TiO2 NWs films. According to the previous reports, the N 1s peak can be fitted in two peaks, marked as N1 at 399.4 eV and marked as N2 at 400.3 eV, and a relatively enhanced portion of the N1 peak over N2 was observed for these N-doped anatase TiO2 NWs films (Fig. 3 (b)). But the portion of the N2 peak for 0.17% N-TiO2 NWs film is smaller than other N-doped anatase TiO2 NWs films. According to the reported, N1 peak corresponds to Ti-N (or N-Ti-O) bond when N atom substitute O atom in the TiO2 lattice,52 which results in N element as a P-type dopant for anatase TiO2 NWs. And N2 peak is attributed to the anionic N in Ti-O-N, which is the interstitial N, indicating that N atoms lie into the lattice to form interstitial Ti-O-N bonds,53 which results in N as an N-type dopant for anatase TiO2 NWs. Thus, the P-type dopant and the N-type dopant coexist in N-doped anatase N-TiO2 NWs, but N as a P-type dopant for TiO2 NWs plays a leading role.

Fig. 3 (a) XPS survey spectra, (b) high-resolution N 1s XPS collected for 0.17% N-TiO2 NWs, 0.56% N-TiO2 NWs, 1.08% N-TiO2 NWs and 1.38% N-TiO2 NWs films.

2. Assessment of separation and transport of photoexcited charge carriers. The absorption spectra of pure TiO2 NWs and N-doped TiO2 NWs films are shown in Fig.


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S3 (a). The absorption band edge of N-TiO2 NWs films has no blue shift or red shift, which indicates that the band gap of TiO2 NWs film has no significant change after N-doped. And the absorption spectra of TiO2 NWs and N-doped TiO2 NWs films with CdS QDs sensitization are shown in Fig. S3 (b). After CdS QDs deposition, the absorption spectra illustrate that the loading amounts of CdS QD for N-TiO2 NWs films show no obvious change compared with TiO2 NWs film. The SPV technique is an effective method to study the process of photoexcited charge carriers separation and transport at the interface.11,54 Thus the SPV spectra were tested to research the photovoltaic properties of pure TiO2 NWs and N-TiO2 NWs films, as shown in Fig. 4 (a). Schematic diagram of the SPV measurement configuration under back side illumination is shown in Fig. 4 (b), as well as the transient photovoltage (TPV) measurement configuration. The top electrode is the samples, which is linked to the amplifier, and the negative spike implies that photoexcited electrons move to top electrode. The SPV spectra illustrate that the SPV response for both pure TiO2 NWs film and N-TiO2 NWs films is at the wavelength of 300-400 nm because of the band-band transitions of anatase TiO2 NWs under ultraviolet light. But the positive SPV signal is observed for pure TiO2 NWs film, which implies the photoexcited holes transfer to top electrode. And the negative SPV signal is observed for N-TiO2 NWs films, which implies the photoexcited electrons transfer to the top electrode. The results indicate that the driving force of photoexcited charge carriers separation and transfer for pure TiO2 NWs film is



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opposite compared with N-TiO2 NWs films, and the built-in electric field at N-doped anatase TiO2 NWs/FTO interface favors the photoexcited electron transfer to the FTO substrates.

Fig. 4 (a) Surface photovoltage spectra of the pure TiO2 NWs film and N-TiO2 NWs films under the back side illumination, (b) The Schematic diagram of the SPV and TPV measurement configuration under back side illumination.

In order to confirm the change of built-in electric field, the Kelvin Probe technique was used to test the work function of anatase TiO2 NWs film, N-doped anatase TiO2 NWs films and FTO conductive substrates (100 points were tested, and the x-axis and the y-axis represent the test area). Fig. 5 shows the CPDs of the anatase TiO2 NWs film, N-doped anatase TiO2 NWs film and FTO substrates. The results indicate that the work function of FTO, pure TiO2 NWs film, 0.17% N-TiO2 NWs, 0.56% N-TiO2 NWs, 1.08% N-TiO2 NWs and 1.38% N-TiO2 NWs films are 4.90 eV, 4.70 eV, 4.95 eV, eV, 5.07 eV, and 5.13 eV, respectively. However, the CPDs of 1.38% N-TiO2 NWs film shows no significant change compared with that of 1.08% N-TiO2 NWs film, which may due to the fact that the practical content of N element do not


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increase as expected with the increase of adding amount when the N proportion of adding amount reached 1.38%. Based on the information of the work function, we are able to determine the built-in electric field exists at the interface, and the orientation of the built-in electric field at the TiO2 NWs/FTO and N-TiO2 NWs/FTO interfaces. Schematic energy band diagrams of the TiO2 NWs/FTO and N-TiO2 NWs/FTO interfaces are shown in Fig. 6. The orientation of built-in electric field at the pure TiO2 NWs/FTO interface is from TiO2 towards FTO because the work function of FTO is significantly larger compared with the anatase TiO2 NWs, which leads the photoexcited holes transfer to the FTO conductive substrates. However, the orientation of built-in electric field at the N-TiO2 NWs/FTO interface is from FTO towards TiO2 due to the increase of work function for N-doped anatase TiO2 NWs samples, which leads the photoexcited electrons transfer to the FTO substrates.

Fig. 5 CPDs of FTO, pure TiO2 NWs film, and N-TiO2 NWs film.



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Fig. 6 Schematic band diagrams of (a) the TiO2 NWs/FTO interface and (b) the N-TiO2 NWs/FTO interface.

In order to investigate the effect of N-doped TiO2 NWs on photoanode of solar cells, the SPV measurements and the PL measurements were carried out to study the process of photoexcited charge separation and transfer for TiO2 NWs and N-TiO2 NWs electrodes after CdS QDs sensitization, as shown in Fig. 7 (a) and (b). It is well known that for a similar composition of semiconductors, the lower the SPV response intensity is, the lower separation efficiency of photoexcited charge is; against that, the lower the PL response intensity is, the higher separation efficiency of photoexcited charge is.52 Fig. 7 (a) shows the SPV spectra of TiO2 NWs and N-TiO2 NWs after CdS QDs sensitization. The method of sample loading for SPV of CdS QDs-sensitized anatase TiO2 NWs and N-TiO2 NWs electrodes is the same as that for SPV of TiO2 NWs and N-TiO2 NWs electrodes, as shown in Fig. 4 (b). After CdS QDs deposition, the range of SPV response expands to the visible region. The negative spikes are observed for TiO2 NWs and N-TiO2 NWs electrodes with CdS QDs sensitization, which indicates that photoexcited electrons move to top electrode.


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However, the SPV response intensity of CdS QDs-sensitized TiO2 NWs electrode is weak. After N doping, the SPV response of CdS QDs-sensitized N-TiO2 NWs electrodes increases evidently. The results indicate separation efficiency of photoexcited charge increases significantly after N-doped, which is resulted from the built-in electric field at the N-TiO2 NWs/FTO interface favors the photoexcited electron transfer to FTO conductive substrates. And with the increase of N doping amount, the SPV response intensity of CdS QDs-sensitized N-TiO2 NWs electrodes increase. But there is little difference between the SPV response intensity of CdS QDs-sensitized 1.08% N-TiO2 NWs electrode and CdS QDs-sensitized 1.38% N-TiO2 NWs electrode, which may be due to the fact that that there is little difference for the separation and transfer of photoexcited electron between CdS QDs-sensitized 1.08% N-TiO2 NWs electrode and CdS QDs-sensitized 1.38% N-TiO2 NWs electrode. Similarly, the PL response intensity of CdS QDs-sensitized N-TiO2 NWs electrodes, as shown in Fig. 7 (b), are much lower compared with that of CdS QDs-sensitized TiO2 NWs electrode. These results demonstrate that the photoexcited charge separation efficiency can be evidently improved by N-doped TiO2 NWs film. Furthermore, the TPV was carried out to study the dynamical properties of photoexcited charge seperation and transfer process of TiO2 NWs and N-TiO2 NWs electrodes with CdS QDs sensitization.55 Device of the TPV measurement is illustrated in Fig. S4. The method of sample loading for TPV is the same as



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that for SPV, as shown in Fig. 4 (b). The TPV responses of TiO2 NWs and N-TiO2 NWs electrodes with CdS QDs sensitization are shown in the Fig. 7 (c). It is found a negative transient signal for TiO2 NWs and N-TiO2 NWs electrodes with CdS QDs sensitization. It implies that photoexcited electrons from QDs are quickly injected into TiO2 film and transferred to the FTO substrate, which is consistent with the observation from SPV responses of TiO2 NWs and N-TiO2 NWs electrodes with CdS QDs sensitization. Moreover, the fast photoexcited charge carriers separation process takes place within one nanoparticle and the TPV responses are normalized to the range of the fast TPV response.42,56 Thus the first peaks (1×10-7 - 1×10-6 s) of TPV responses provide the separation and injection process of photoexcited electrons, which is a fast process (see the P1 of Fig. 7 (c)). Fig. 7 (c) illustrates that the first peaks in TPV responses of all electrodes appear to be noteworthy shoulders at the fast process. The shoulders of TPV responses for TiO2 NWs and N-TiO2 NWs electrodes with CdS QDs sensitization are the same, which indicates that N-doped TiO2 NWs has no effect on the separation and injection process of photoexcited electrons. The second peak (1×10-6 – 0.03 s) provides the transfer process of photoexcited electron, which is a slow process (see the P2 of Fig. 7 (c)). The intensity of the TPV maximum for CdS QDs-sensitized N-TiO2 NWs electrodes is stronger compared with that for CdS QDs-sensitized TiO2 NWs electrodes. As the N doping amount increases, the TPV intensity increases accordingly.


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This phenomenon corresponds with the SPV spectra of TiO2 NWs and N-TiO2 NWs electrodes with CdS QDs sensitization. The result is owing to the change of the orientation of the built-in electric field at the TiO2/FTO interface, as shown in Fig. 6. Thus, the built-in electric field at the N-TiO2 NWs/FTO interface favors the photoexcited electrons transfer to FTO conductive substrates, resulting in improved separation efficiency correspondingly. However, the TPV intensity shows no obvious change with the increase of N doping amount continuously, which may be due to the fact that there is little difference for the separation and transfer of photoexcited electron between CdS QDs-sensitized 1.08% N-TiO2 NWs electrode and CdS QDs-sensitized 1.38% N-TiO2 NWs electrode. In order to further investigate the separation and transfer dynamical properties of photoexcited charge, the TPC of TiO2 NWs and N-TiO2 NWs photoanodes was tested in the presence of the polysulfide electrolyte and Cu2S counter electrode, as shown in Fig. 7 (d). Device of the TPV measurement is illustrated in Fig. S4. Inset of Fig. 7 (d) is sketch map of the method of sample loading. The TPC responses of the first peak (1×10-8 – 1×10-6 s) arise from charge exchange of photoexcited charge and electrolyte (see the P1 of Fig. 7 (d). Fig. 7 (d) shows that the first spikes in TPC responses are small peaks, and little differences is observed, which corresponds with the TPV responses of TiO2 NWs and N-TiO2 NWs electrodes with CdS QDs sensitization. In addition, the



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plateau stage (5×10-7 – 1×10-5 s) is still the charge exchange process for all samples. The TPC responses of the second peak (1×10-5 – 0.05 s) arise from photoexcited electron transfer to form circuit (see the P2 of Fig. 7 (d)). The transfer process includes photoexcited electrons transferring to FTO conductive substrates, and forming circuit from the external circuit back to the Cu2S counter electrode. Thus the differences for the response intensity of second spike are mainly caused by transfer process of photoexcited electrons. As shown in Fig. 7 (d), the intensity of the TPC maximum for N-TiO2 NWs electrode is significantly enhanced compared with that for TiO2 NWs electrode, which indicates that the N-TiO2 NWs electrode is beneficial to improve the separation efficiency and transfer process of photoexcited charge. As the N doping amount increases, the TPC intensity increases. The results reflect the built-in electric field at the N-TiO2 NWs/FTO interface favors the photoexcited electron transfer to FTO conductive substrates. However, the TPC intensity shows no obvious change with the increase of N doping amount continuously, which corresponds with the TPV responses of TiO2 NWs and N-TiO2 NWs electrodes with CdS QDs sensitization.


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Fig. 7 (a) Surface photovoltage spectra of TiO2 NWs and N-TiO2 NWs after CdS QDs sensitization. (b) PL spectra of TiO2 NWs and N-TiO2 NWs after CdS QDs sensitization excited at 460 nm. (c) Transient photovoltage responses of TiO2 NWs and N-TiO2 NWs after CdS QDs sensitization. The wavelength and the intensity of the laser are 532 nm and 20 µJ, respectively. (d) The TPC responses of QDSCs fabricated from TiO2 NWs and N-TiO2 NWs photoanodes under illumination. The wavelength and the intensity of the laser are 532 nm and 40 µJ, respectively.

3. Photoelectrochemical performance of N-doped TiO2 NWs photoanodes based QDSCs. With the goal of understanding the electronic properties of TiO2 NWs and N doped TiO2 NWs electrodes, the Mott-Schottky plots of TiO2 NWs and N-TiO2 NWs electrodes were tested, as shown in Fig. 8. The Efb values of TiO2 NWs and N-TiO2 NWs electrodes measured from Fig. 8 are listed in Table 2. Compared with the pure anatase TiO2 NWs electrode, the Efb of N-TiO2 NWs electrodes shift positively, as shown in Fig. 8. The positively shift of the Efb indicate the reducing Fermi energy



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level of N-doped TiO2 NWs electrodes, which is consistent with the results of work function of TiO2 NWs and N-TiO2 NWs electrodes. The carrier concentration (Nd) was calculated from the slope of linear portion of the Mott-Schottky plots. As shown in Table 2, the Nd of N-doped TiO2 NWs electrodes is lower than that of pure anatase TiO2 NWs electrodes, which may be due to the fact that N as a P-type dopant for TiO2 NWs plays a leading role. However, as the doping amount of N increases continuously, the Nd of N-doped TiO2 NWs electrodes show no obvious change. According to the results of XPS, the P-type dopant and the N-type dopant coexists in N-doped TiO2 NWs film. Thus, as the doping amount of N increases continuously, the P-type dopant and the N-type dopant increase at the same time. The results of the Mott-Schottky plots further confirm that N-doped leads the decline of the Fermi energy level of N-doped TiO2 NWs film and the change of orientation of built-in electric field at the TiO2 NWs/FTO interface. In addition, the spatial range of the electric field (w) is formed between the TiO2 NWs and FTO interface. The width of the space charge region has been calculated by the formula: w=(2εε0Vfb/qND)1/2, as shown in Table 2.57 The result indicates that the band bending exists evidently between the TiO2 NWs and FTO interface and the width of the space charge region slightly decreases with the N-doped the TiO2 NWs film. However, for the pure anatase TiO2 NWs electrodes, the band bending is upward at the interface. After N-doped TiO2 NWs electrodes, the band bending is downward at the interface, which is


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beneficial to the photoexcited electron transfer to FTO conductive substrates at the TiO2 NWs/FTO interface.

Fig. 8 Mott-Schottky plots of TiO2 NWs and N-TiO2 NWs electrodes. Table 2 Values of the plat-band potential, carrier concentration for TiO2 NWs and N-TiO2 NWs electrodes. Photoelectrodes

Efb (V)

Nd (cm-3)

w (nm)

TiO2 NWs

-0.950

6.88 × 1019

9.5

-0.906

19

9.2

19

0.17% N-TiO2 NWs

6.37 × 10

0.56% N-TiO2 NWs

-0.881

6.26 × 10

9.2

1.08% N-TiO2 NWs

-0.854

6.11 × 1019

9.2

-0.832

19

9.2

1.38% N-TiO2 NWs

5.97 × 10

In order to further explore the photoelectric performance of typical QDSCs fabricated

with

pure

TiO2

NWs

and

N-TiO2

NWs

photoanodes.

Photocurrent–voltage characteristics (J–V) curves of these QDSCs under illumination were recorded. Fig. 9 (a) and (b) show the current density-voltage (J-V) and power output curves of typical QDSCs fabricated with TiO2 NWs and N-TiO2 NWs photoanodes under illumination. Table 3 shows the detailed parameters of photoelectric performance for the typical QDSCs fabricated with pure TiO2 NWs and N-TiO2 NWs photoanodes.



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The evidently improved device performance of QDSCs fabricated with N-TiO2 NWs photoanodes is clearly observed. An enhanced power conversion efficiency (PCE) of 2.32% was obtained based on 0.27% N-TiO2 NWs photoanodes, compared to PCE=1.88% basing on pure anatase TiO2 NWs photoanode. As the N doping amount increases, the PCE and Jsc value are further enhanced and the highest PCE and Jsc values were achieved from 1.38% N-TiO2 NWs photoanodes. The key to high performance QDSCs depends on the sunlight utilization and separation efficiency. Thus the improvement of separation efficiency is crucial due to these QDSCs based on the same sensitizer. The orientation of built-in electric field at the TiO2 NWs/FTO interface is from TiO2 towards FTO, which leads the photoexcited holes transfer to the FTO conductive substrates. After N-doped, the orientation of built-in electric field at the N-TiO2 NWs/FTO interface is from FTO towards TiO2, which leads the photoexcited electrons transfer to the FTO conductive substrates. Therefore N-TiO2 NWs photoanodes favors the transfer process of photoexcited electron at the N-TiO2 NWs/FTO interface, which leads the improvement of separation efficiency and the increase of photoelectric properties

correspondingly.

But

as

the

N

doping

amount

increases

continuously, photoelectric properties of QDSCs based on 1.38% N-TiO2 NWs photoanode exhibits no significant increase compared with that of QDSCs based on 1.08% N-TiO2 NWs photoanode, which may be the reason that the practical content of N element in 1.38% N-doped anatase TiO2 NWs films does


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not increase as expected compared with that in 1.08% N-doped anatase TiO2 NWs films. Therefore, the CPDs of 1.38% N-TiO2 NWs film shows no significant change compared with that of 1.08% N-TiO2 NWs film, which leads to the result that there is little difference for the separation and transfer of photoexcited electron between the QDSCs based on 1.08% N-TiO2 NWs photoanode and the QDSCs based on 1.38% N-TiO2 NWs photoanode. In order to confirm the results, the photoelectric performance of typical QDSCs fabricated with N-TiO2 NWs photoanodes with more adding amount were tested, as shown in Fig. S5. And the practical N elemental content in these N-doped anatase TiO2 NWs films is presented in Table S1. Fig. S5 shows that the QDSCs based on 1.08% N-TiO2 NWs, 1.38% N-TiO2 NWs and 1.68% N-TiO2 NWs photoanode show the similar PCE value and other detailed photovoltaic parameters. And the practical content of N element in these N-doped anatase TiO2 NWs films is similar, as shown in Table S1. The Voc is corresponding to the quasi-Fermi level of the semiconductor (EFn) with respect to the electrolyte redox energy (Eredox).58 Therefore, the Voc can be acquired by the equation:

V o c = ( E F n − E F o ) / e = ( K B T / e ) In ( n / n o ) Where, the KBT is the thermal energy, e is the positive elementary charge, n is the free electron density in the semiconductor nanostructure and n0 is the free density in the dark. And the n is affected by two main factors which is photoexcited electrons and the recombination of the photoexcited charge. So



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the recombination rate has a major impact on the open-voltage.58,59 The quasi-Fermi level of the oxide semiconductor (EFn) could become lower through N doped, but the improvement of separation efficiencey could reduce the recombination of the photoexcited charge, which leads to the slightly increase of Voc. In order to further investigate the charge recombination of QDSCs based on TiO2 NWs and N-TiO2 NWs photoanodes, the J-V curves of QDSCs fabricated with TiO2 NWs and N-TiO2 NWs photoanodes in dark was tested. Fig. 9 (c) shows the dark current of QDSCs based on N-TiO2 NWs electrodes decreases evidently compared with that of QDSCs based on TiO2 NWs electrode, which illustrates that the charge recombination of QDSCs based on N-TiO2 NWs photoanodes decreases compared with that of QDSCs based on TiO2 NWs photoanode. Thus Voc of QDSCs based on N-TiO2 NWs electrodes is higher than that of QDSCs based on TiO2 NWs electrode. This phenomenon may be attributed to the favorable photoexcited charge transfer at the N-TiO2 NWs/FTO interface. The incidental photo-to-current efficiency (IPCE) spectra of QDSCs fabricated with TiO2 NWs and N-TiO2 NWs photoanodes were recorded. Fig. 9 (d) illustrates that the IPCE spectra of QDSCs based on N-TiO2 NWs photoanodes have the higher IPCE value compared with that of QDSCs based on TiO2 NWs photoanode. The IPCE is determined by the light harvesting efficiency of the photoanode (LHE), electron injection efficiency of the


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photoexcited electron into the TiO2 (φINJ), and the collection efficiency of the injected electron (φCOLL):60

IPCE = LHE × Φ INJ × Φ COLL φCOLL is related to the ratio of electron transport through TiO2 and charge recombination:

Φ COLL = K t / ( K t + K r ) Where, the Kt and Kr are the rate constants for transport and recombination, respectively.60,61 N-TiO2 NWs photoanodes favor the transfer process of photoexcited electron at the N-TiO2 NWs/FTO interface and reduce the charge recombination. In addition, LHE and φINJ for QDSCs based on TiO2 NWs and N-TiO2 NWs photoanodes have the same value due to these QDSCs based on the same sensitizer. Evidently, the higher IPCE response for the QDSCs based on N-TiO2 NWs photoanodes is due to the favorable photoexcited charge transfer and the reducing of charge recombination.



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Fig. 9 (a) J-V and (b) power output curves of QDSCs fabricated with TiO2 NWs and N-TiO2 NWs photoanodes. (c) J-V curves of QDSCs fabricated with TiO2 NWs and N-TiO2 NWs photoanodes in dark. (d) IPCE curves of QDSCs fabricated with TiO2 NWs and N-TiO2 NWs photoanodes. Table 3. Detailed photovoltaic parameters of typical QDSCs fabricated with TiO2 NWs and N-TiO2 NWs photoanodes. Photoelectrode

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

TiO2

10.37

0.60

0.302

1.88 (±0.02)

0.17% N-TiO2

11.31

0.60

0.342

2.32 (±0.05)

0.56% N-TO2

12.22

0.61

0.334

2.49 (±0.03)

1.08% N-TiO2

13.33

0.62

0.329

2.72 (±0.02)

1.38% N-TiO2

13.77

0.63

0.317

2.75 (±0.03)

The open-circuit voltage-decay (OCVD) technique was applied to study the interfacial recombination kinetics of photoexcited charge in the QDSCs. The OCVD technique is an effective method to study the kinetics of recombination for the cells from the illuminated quasi-equilibrium to the dark equilibrium.58,59 Therefore, the electron lifetime can be calculated by τn=-(KT/e)(dVoc/dt), where the dVoc/dt is the first time derivative of Voc. Fig. 10 (a) illustrates the electron lifetime versus voltage curves of the typical QDSCs fabricated with TiO2 NWs and N-TiO2 NWs photoanodes. It is found that τn of QDSCs fabricated with N-TiO2 NWs photoanodes is obviously larger than that of QDSCs fabricated with TiO2 NWs photoanode. The reduced recombination of photoexcited charge can be owing to the favorable photoexcited charge transfer at the N-TiO2 NWs/FTO interface, thus resulting in the extension of the electron lifetime.


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Nyquist plots were tested to explore the interfacial resistance to further illustrate the interfacial dynamics of the charge transfer in the QDSCs, as shown in Fig. 10 (b). Fig. 10 (b) shows two obvious semicircles in the Nyquist plots, which related to high frequency region resistance (R1) at the Cu2S∣electrolyte interface, intermediate frequency region resistance (R2) at the photoanode∣electrolyte interface, respectively.62,63 According to the Nyquist plots under illumination, the electron interface resistance (R2) of QDSCs fabricated with N-TiO2 NWs photoanodes is significantly smaller than that of QDSCs fabricated with TiO2 NWs photoanodes. It is obvious that the smaller interface resistance relates to faster interfacial charge transfer. Thus the results may be due to the fact that N-TiO2 NWs photoanodes favor the transfer processes of photoexicited electrons from TiO2 NWs film to FTO conductive substrates, which is favorable for improving PCE.64

Fig. 10 (a) The electron lifetime versus voltage curves of QDSCs fabricated with TiO2 NWs and N-TiO2 NWs photoanodes. (b) Nyquist plots of QDSCs fabricated from TiO2 NWs and N-TiO2 NWs photoanodes under illumination.

Conclusions



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In summary, N-doped anatase TiO2 nanowires (NWs) films on FTO were fabricated using a facile method of hydrothermal growth. The built-in electric field at N-doped anatase TiO2 NWs/FTO interface was studied by the SPV and Kelvin Probe technique. And the results indicate orientation of the built-in electric field at TiO2/FTO interface from TiO2 towards FTO turns into from FTO towards TiO2 through N-doped anatase TiO2 NWs, which favors the photoexcited electron transfer to the FTO conductive substrates. The dynamics process of separation and transfer of photoexcited charge at anatase TiO2 NWs/FTO interfaces for CdS QDs sensitized TiO2 NWs and N-TiO2 NWs films was investigated by using the SPV, TPV and TPC techniques. And the results indicate that the built-in electric field is beneficial to photoelectric properties of solar cells due to the fact that the N-TiO2 NWs/FTO interface favors the photoexcited electrons transfer to the FTO substrate. The QDSCs based on N-doped TiO2 NWs electrodes exhibit higher short-circuit current density (Jsc) and power conversion efficiency (PCE) compared with the undoped reference solar cells, which is considered to be mainly attributable to the favorable photoexcited charge transfer. Our results confirm that the N-doped anatase TiO2 NWs electrodes have promising applications in solar cells and modification of anatase TiO2 NWs film is the effective way to regulate the Fermi level of anatase TiO2 NWs films.

ACKNOWLEDGEMENTS


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We are grateful to the National Natural Science Foundation of China (No.51572106), and the National Basic Research Program of China (973 Program) (2013CB632403), and the Science and Technology Developing Funding of Jilin Province (No.20150203009GX).

ASSOCIATED CONTENT Supporting Information

Figures S1-S5, Tables S1. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail:[email protected]

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and FTO Interface by N-Doped Anatase TiO - ACS Publications

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