Article pubs.acs.org/JPCC
Introducing an Intermediate Band into Dye-Sensitized Solar Cells by W6+ Doping into TiO2 Nanocrystalline Photoanodes Zhengfu Tong,† Tao Peng,† Weiwei Sun, Wei Liu,* Shishang Guo,* and Xing-Zhong Zhao* School of Physics and Technology and Key Laboratory of Artificial Micro- and Nano-structure of Ministry of Education, Wuhan University, Luojiasan, Wuhan 430072, China ABSTRACT: The novel concept of introducing intermediate band into the mesoporous TiO2 backbone of dye-sensitized solar cells (DSSCs) is proposed to take full advantage of the sunlight and enhance the power conversion efficiency. Nominal trace amount Wdoped TiO2 nanocrystralline films were prepared with the purpose of forming intermediate band in the bandgap of TiO2. A notable improvement of the device performance was obtained when N-type W-doped TiO2 films were applied as the photoanode of DSSCs. The short-circuit current density (Jsc) increased from 12.40 mA cm−2 to 15.10 mA cm−2, and the conversion efficiency increased from 6.64 to 7.42% when nominal 50 ppm (ppm) W-doped TiO2 was adopted.
1. INTRODUCTION Dye-sensitized solar cells (DSSCs) based on mesoporous nanocrystalline titanium dioxide (TiO2) film have drawn extensive attentions over the past two decades since Grätzel et al. reported their breakthrough work in 1991.1 The wideband-gap TiO2 nanocrystalline semiconductor photoanode is one of the main components and the backbone of DSSCs and plays a very important role in determining the performance of DSSCs. Just by changing the morphology of TiO2 photoanode from compact thin film to a mesoporous one, Grätzel et al realized their ground-breaking discovery two decades ago. Unfortunately, after development for 10 years, the study of DSSCs is now going through a bottleneck period in pursuing higher conversion efficiency.2,3 One important reason is that the infrared light that accounts for a large part of the solar spectrum still cannot be utilized.4 It has been proposed that a better utilization of the full solar spectrum could be achieved with one or more narrow intermediate bands properly located in the band gap of a wide-gap semiconductor.5−7 The intermediate bands act as stepping stones allowing low-energy photons to pump electrons from the valence band (VB) to the conduction band (CB). By introducing of intermediate bands into the band gap of the wide gap semiconductor, not only the high energy photon but also the infrared light can be utilized. As a result, a very high theoretical efficiency of 63.2% has been predicted for the intermediate band solar cell (IBSC).8 Unlike the IBSC, the function of light absorption and charge carrier transport is separated in a typical DSSC. To be specific, the photons are absorbed by dyes, while the wide-band-gap semiconductor TiO2 is just a transport channel for photoinduced electrons. Here a conjecture that TiO2 will absorb visible light and even infrared light and then generate electrons when an intermediate band properly locates in the band gap of © 2014 American Chemical Society
TiO2 is introduced. So, in addition to the electrons injected to the conduction band of TiO2 from the excited dye molecules, the electrons can also be pumped directly from the intermediate band to the conduction band or from the valence band to the intermediate band and then finally to the conduction band. In consequence, the photoelectron density in the conduction band of TiO2 may be enhanced greatly, which would result in an improvement of photocurrent and conversion efficiency of DSSCs. As we all know, doping is an important approach to the realization of intermediate band in wide-band-gap semiconductor.9 A series of nominal trace amount W-doped TiO2 nanocrystalline films were prepared with the attempt to form stable intermediate band between the VB and CB of TiO2. When the W-doped films were applied as photoanodes in DSSCs, both short-circuit current density (Jsc) and powerconversion efficiency (η) of DSSCs were improved significantly, which might be due to the formation of intermediate band in the band gap of TiO2 after nominal trace-amount doping of W6+ ions. The remarkable change in performance of DSSCs employing nominal trace amount of W-doped TiO2 films with different concentrations was obvious, which might be ascribed to successfully introducing an intermediate band photovoltaic device.
2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2. TiO2 colloid was synthesized by hydrolysis of titanium isopropylate (TTIP), followed by Special Issue: Michael Grätzel Festschrift Received: January 14, 2014 Revised: March 20, 2014 Published: March 24, 2014 16892
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hydrothermal synthesis. Ten mL of TTIP was dispersed in 2.1 g of acetic acid and stirred for 10 min. Then, the solution was poured quickly in 50 mL of deionized water and stirred for 1 h to complete the hydrolysis process. 0.68 mL of 68% nitric acid was injected into the solution, and the solution was heated to 80 °C over 30 min and held at the temperature for 3 h. After cooling, the volume of the solution was adjusted to 63 mL by adding deionized water. Then, the solution was poured into a 100 mL autoclave and maintained at 220 °C for 12 h. After the hydrothermal synthesis, 0.4 mL of nitric acid was injected to the colloid; then, the mixture was stirred for 15 min, and the colloid was dispersed by a sonic machine. Then, the solution was concentrated to 25 mL in a rotary evaporator. 0.56 g of Carbowax 20000 and 0.5 mL of Triton X-100 were added to the solution and stirred for 3 h. For the case of W-doped TiO2 colloid, we adopted (NH4)10W12O41·xH2O as a W source. After the TTIP was hydrolyzed, several milliliters of (NH4)10W12O41 solution was added to the TiO2 sol when it just reached 80 °C. Except for the doping process, the preparation of the W-doped TiO2 colloid followed the procedure previously described. 2.2. Device Fabrication. The TiO2 colloid and doped TiO2 colloid were spread on FTO substrates by doctor-blading technique and followed by sintering at 500 °C for 30 min to obtain mesoporous nanocrystalline TiO2 film electrodes and W-microdoped TiO2 film electrodes. All of the films thus prepared were ∼8 μm thick. After the electrodes were cooled, they were immersed in 0.5 mM ethanolic N719 solution at 60 °C for 12 h. Then, the electrodes were washed with ethanol to remove the excessive dye molecules on the surface of nanocrystalline TiO2 to ensure that the film was covered only with monolayer of dye molecules. The DSSC configuration was fabricated by assembling the dye-loaded mesoporous TiO2 with platinum plate counter electrode. The electrolyte, composed of 1 M 1-propyl-3-methylimidazolium iodide (PMII), 0.03 M I2, 0.05 M LiI, 0.1 M GNCS, and 0.5 M 4-tert-butylpridine (TBP) in mixed solvent of propylene carbonate (PC) and acetonitrile, was injected into the cell. 2.3. Characterizations and Measurements. The content of W ions in TiO2 nanocrystalline was measured by inductively coupled plasma mass spectrometry (ICP-MS). Photovoltaic measurements were performed by a 500 W xenon light source giving an irradiance of 100 mW cm−2 (AM 1.5, global). The irradiated area of each cell was kept at 0.25 cm2 by using a lighttight metal mask. Impedance measurements and open-circuit voltage decay analysis were performed by an electrochemical workstation (CHI660C, CH Instruments).
Figure 1. J−V characteristics of DSSCs based on undoped and Wdoped TiO2 electrodes.
performance parameters are listed in Table 2. Obviously, as seen from the graph, the energy conversion efficiency went up Table 2. Performances of DSSCs Based on Undoped and WDoped TiO2 Electrodes
Table 1. Actual Content of W6+ Ions in Undoped and WDoped TiO2 Nanocrystallines Tested by ICP-MS 0 0
10 9.67
50 38.98
100 81.55
Voc (V)
Jsc (mA cm−2)
FF
η (%)
pure 10 ppm 50 ppm 100 ppm 500 ppm
0.730 0.730 0.730 0.705 0.695
12.40 14.67 15.10 14.00 13.24
0.72 0.67 0.67 0.67 0.68
6.55 7.27 7.42 6.63 6.26
with the increase in W6+ ions content, which was attributed to the enhancement of the Jsc. The Jsc of DSSCs based on 50 ppm W-doped TiO2 was 15.10 mA cm−2, which was much higher than that of undoped cells (12.40 mA cm−2). The effect on Voc and fill factor (ff) as a result of doping trace amount of W6+ ions in TiO2 was negligible. An energy conversion efficiency of 7.42% was achieved for cells based on 50 ppm W-doped TiO2 electrode, which was 13.2% higher than that of undoped cells. W doping into TiO2 is a typical N-type deep level doping. So the intermediate energy level or band within the bandgap may generate. As a result, not only the visible light can be utilized, but some infrared light can also be used. So, both the Jsc and energy conversion efficiency increased significantly. Electrochemical impedance spectroscopy (EIS) analysis is a useful tool for the investigation of electron transport in DSSCs. To investigate the change of the performance of DSSCs after doping trace amount W in the TiO2 electrode in detail, we employed EIS analysis. EIS was scanned from 0.1 to 105 Hz in the illumination at the applied bias of Voc. The small semicircle (Z1) in the high-frequency range of 103 to 105Hz accounted for the sum of charge transfer resistance (R1), which is related to the charge transfer at the interfaces of the electrolyte/Pt counter electrode and FTO/TiO2 interface. The large semicircle in the frequency range of 1 to 103 Hz accounted for a transport resistance (R2) is not only related to the charge recombination across the TiO2/electrolyte but also related to the accumulation10,11/transport10 of the electrons in TiO2 films. EIS spectra of DSSCs using W-doped TiO2 electrodes are shown in Figure 2. As can be seen in Figure 2, the Z2 semicircle got smaller with the increase in W6+ ions in TiO2
3. RESULTS AND DISCUSSION The actual content of W in TiO2 nanocrystalline was listed in Table 1. The result conformed the existence of trace amount W ions, as expected. It is obvious that the actual content of W ions is similar to the nominal content. The series of W-doped films were applied as DSSCs photoanodes. The photocurrent density versus voltage characteristics of DSSCs based on W-doped film photoanode are shown in Figure 1. Details of the photovoltaic
nominal content (ppm) actual content (ppm)
DSSCs
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dx.doi.org/10.1021/jp500412e | J. Phys. Chem. C 2014, 118, 16892−16895
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was calculated and is graphically shown in Figure 4. Obviously, the electron lifetime was extended after W-doping TiO2 films.
Figure 2. EIS of DSSCs based on undoped and W-doped TiO2 electrodes measured in the illumination at the applied bias of Voc.
films. This change reflected the acceleration of electron transport process in TiO 2 photoanode, 12 which made contributions to the charge-collection efficiency and thus also increased Jsc. The current density improvement was also associated with the increase in electron lifetime (τ). Open-circuit voltage decay (OCVD) technique, which monitors the subsequent decay of photovoltage after turning off the illumination in a steady state, is a wonderful tool to measure electron lifetime in DSSCs. The OCVD data are shown in Figure 3. Obviously, the velocity of
Figure 4. Electron lifetime as a function of open-circuit voltage for of DSSCs employing undoped and W-doped TiO2 electrodes.
The improvement of electron lifetime means the recombination is slowed after doping. However, the excess W6+ causes electron scattering and trap electrons, which lead to an increase in dark current. As a result, the properties of DSSCs based on 500 ppm W-doped TiO2 electrode produce worse performance than those of DSSCs based on 50 ppm W-doped TiO2 electrode.
4. CONCLUSIONS The concept of introducing intermediate band into DSSCs was proposed. The preliminary experiments of introducing intermediate band into the TiO2-based photoanodes of DSSCs by doping nominal trace amount W6+ ions have been done. The enhancement of Jsc and conversion efficiency might partially be attributed to the formation of intermediate band in the bandgap of TiO2. In addition, the electron transport and electron lifetime were improved after doping W6+ in TiO2, which also benefited the enhancement of Jsc and conversion efficiency.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (W.L.). *E-mail:
[email protected] (S.G.). *E-mail:
[email protected] (X.-Z.Z.). Author Contributions †
Z.T. and T.P. contributed equally to the work.
Figure 3. Open-circuit voltage decay for DSSCs based on undoped and W-doped TiO2 electrodes.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of this work by the National Basic Research Program (no. 2011CB933300) of China and the National Science Fund for Talent Training in Basic Science (grant no J0830310).
open-circuit voltage decay of cells based on doped and undoped films was related to the number of W6+ ions in the films. With more W doping, the open-circuit voltage decayed slower. The decay of photovoltage reflected the decrease in the electron density in the TiO2 films, which was caused by recombination. The electron lifetime, given by the formula13 kBT ⎛ dVoc ⎞ ⎜ ⎟ e ⎝ dt ⎠
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−1
τ=−
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