Enhancement of Perovskite Solar Cells Efficiency using N-Doped TiO2 Nanorod Arrays as Electron Transfer Layer
© The Author(s). 2017
Received: 30 September 2016
Accepted: 26 December 2016
Published: 17 January 2017
In this paper, N-doped TiO2 (N-TiO2) nanorod arrays were synthesized with hydrothermal method, and perovskite solar cells were fabricated using them as electron transfer layer. The solar cell performance was optimized by changing the N doping contents. The power conversion efficiency of solar cells based on N-TiO2 with the N doping content of 1% (N/Ti, atomic ratio) has been achieved 11.1%, which was 14.7% higher than that of solar cells based on un-doped TiO2. To get an insight into the improvement, some investigations were performed. The structure was examined with X-ray powder diffraction (XRD), and morphology was examined by scanning electron microscopy (SEM). Energy dispersive spectrometer (EDS) and Tauc plot spectra indicated the incorporation of N in TiO2 nanorods. Absorption spectra showed higher absorption of visible light for N-TiO2 than un-doped TiO2. The N doping reduced the energy band gap from 3.03 to 2.74 eV. The photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra displayed the faster electron transfer from perovskite layer to N-TiO2 than to un-doped TiO2. Electrochemical impedance spectroscopy (EIS) showed the smaller resistance of device based on N-TiO2 than that on un-doped TiO2.
In recent years, extensive studies are focused on perovskite solar cells (PSCs) due to their outstanding properties, such as large absorption coefficient, electron-hole diffusion length, and high charge carrier mobility [1–5]. The power conversion efficiency (PCE) of perovskite solar cells has been over 22% . Conventionally, perovskite solar cells consist of a perovskite layer sandwiched between an electron transfer material (ETM) layer and a hole transfer material (HTM) layer. Mesoporous TiO2 has been used as the ETM in most of the perovskite solar cells [7, 8]. Compared with the mesoporous structure, one dimensional (1D) nanostructure has some advantages, such as easy pore filling of active layer or HTM, better electron transfer, and lower charge recombination [9, 10]. Therefore, TiO2 nanorods (NRs) have been widely applied to perovskite solar cells [11, 12]. However, there are a mass of oxygen vacancies defects exist in pristine TiO2 nanorods, which reduces the efficiency and stability of the perovskite solar cell .
In order to solve the issues, some methods have been adopted, such as metal doping [14, 15] and nonmetal doping . It has been reported that N-doped TiO2 as a photoanode of dye-sensitized solar cells (DSSCs) can improve the energy conversion efficiency due to the change of properties of TiO2, such as electron lifetime prolongation, charge transfer resistance reduction, and visible light absorption extension [17, 18].
We wondered about the effect of N-doped TiO2 on the performance of perovskite solar cells. Hence, in the present study, we synthesized N-doped TiO2 (N-TiO2) nanorod arrays with hydrothermal method and fabricated perovskite solar cells using them as electron transfer layer. The solar cell performance was optimized by changing the N doping contents. The PCE of solar cells based on N-TiO2 with the N doping content of 1% (N/Ti, atomic ratio) has been achieved 11.1%, which was 14.7% higher than that of solar cells based on un-doped TiO2. The possible mechanisms of enhancement were discussed based on some investigations.
Growth of TiO2 Nanorod Arrays
Patterned fluorine-doped tin oxide (FTO)-coated glass substrate was cleaned by sonication for 20 min in detergent, acetone, 2-propanol, and ethanol, respectively. A TiO2 compact layer was deposited by dipping the substrate in a 0.2 M TiCl4 aqueous solution at 70 °C for 30 min. TiO2 NRs were grown on the treated FTO substrate by a hydrothermal method in our previous report . A 0.7 mL of titanium(IV) n-butoxide was added to a mixture of hydrochloric acid and deionized water. Subsequently, the pre-calculated amount of CO(NH2)2 was added to the solution (the nominal N/Ti atomic ratio, 0.5, 1, 2, and 3%) and stirred until it was completely dissolved. The FTO substrate was put into the solution and sealed in an autoclave. The autoclave was heated to 170 °C for several hours. The obtained TiO2 nanorods film was rinsed and annealed at 500 °C for 60 min.
Methylammonium iodide (CH3NH3I) was synthesized with a reported method . The precursor solution for perovskite film formation was obtained by mixing PbCl2 and CH3NH3I in anhydrous N,N-dimethylformamide (DMF) at a 1:3 molar ratio at 60 °C overnight.
Solar Cell Fabrication
The perovskite film was formed by spin coating at 2000 rpm for 60 s in a glove box, and drying on a hotplate at 110 °C for 60 min. The HTM layer was obtained by spin coating a spiro-OMeTAD solution at 2000 rpm for 60 s. Finally, a gold layer was deposited on the top of the device by thermal evaporation.
X-ray diffraction (XRD) patterns were measured using a diffractometer (DX-2700). Photocurrent–voltage (I–V) curves were carried out with a Keithley 2440 Source meter under AM 1.5 G illumination from a Newport Oriel Solar Simulator with an intensity of 100 mW/cm2. A shadow mask was used to determine the active area of 0.1 cm2. Morphologies and microstructures were performed using a scanning electron microscope (SEM, JEM-7001F, JEOL) equipped with an energy dispersive spectrometer (EDS). Absorption spectra were obtained with a UV–Vis spectrophotometer (Varian Cary 5000). Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra were collected with a fluorometer (FLS 980E, Edinburgh Photonics). An electrochemical workstation (CHI660e, Shanghai CHI Co., Ltd.) was used to collect the electrochemical impedance spectroscopy (EIS) with a bias of 0.6 V.
Results and Discussion
Photovoltaic parameters of best performance solar cells based on the TiO2 and N-TiO2 NRs
V oc (V)
I sc (mA/cm2)
0.80 ± 0.02
0.82 ± 0.01
19.2 ± 0.0.6
20.5 ± 0.7
0.62 ± 0.03
0.65 ± 0.02
9.5 ± 0.3
10.9 ± 0.2
Parameters of the TRPL spectra
% of τ1
% of τ2
TiO2 /MAPbI3−x Cl x
1% N-TiO2 /MAPbI3−x Cl x
Fitting parameters of EIS date
6.3 × 10−6
5.9 × 10−6
In the present study, N-TiO2 NRs were synthesized with hydrothermal method, and perovskite solar cells based on them were fabricated. Compared with the solar cells based on un-doped TiO2, solar cells based on N-TiO2 present an enhanced performance. The solar cell performance was optimized by changing the N doping contents. The PCE of solar cells based on N-TiO2 with the N doping content of 1% (N/Ti, atomic ratio) has been achieved 11.1%, which was 14.7% higher than that of un-doped TiO2-based solar cells. To explain this phenomenon, some investigations were performed. The results indicate that the larger Voc could be due to the larger conduction band offset resulting from the smaller energy band gap for N-TiO2, and the enlarged Isc could be attributed to the faster electron transfer and reduced recombination rate for N-TiO2 NRs. These induce the enhanced performance of the solar cells based on N-TiO2 NRs.
This work is supported by the Science and Technology Development Project of Henan Province (No.142102210389), National Science Research Project of Education Department of Henan Province (No.17A140005), and Program for Innovative Research Team (in Science and Technology) in University of Henan Province (No. 13IRTSTHN017), and NSFC-Henan Province Joint Fund (U1604144).
Z-LZ, J-FL, and Y-LM carried out the main part of experiment and drafted the manuscript. The other authors provided assistance with the experimental measurements, data analysis, and the manuscript writing. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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