improved dye-sensitized TiO2 solar cells

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Appl. Phys. Lett. 99, 042106 (2011); https://doi.org/10.1063/1.3617460 99, 042106

SrTiO3 modified TiO2 electrodes and

improved dye-sensitized TiO2 solar cells

Cite as: Appl. Phys. Lett. 99, 042106 (2011); https://doi.org/10.1063/1.3617460 Submitted: 07 May 2011 . Accepted: 09 July 2011 . Published Online: 28 July 2011 Sujuan Wu, Xingsen Gao, Minghui Qin, J.-M. Liu, and Shejun Hu

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SrTiO

₃

modified TiO

₂

electrodes and improved dye-sensitized TiO

₂

solar cells

Sujuan Wu,^1,a)Xingsen Gao,¹Minghui Qin,¹J.-M. Liu,^2,3,b)and Shejun Hu¹

1Laboratory for Advanced Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, China

2Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

3International Center for Materials Physics, Chinese Academy of Sciences, Shenyang, China (Received 7 May 2011; accepted 9 July 2011; published online 28 July 2011)

The SrTiO₃-coated TiO₂ (TiO₂/SrTiO₃) electrodes prepared by radio frequency magnetron sputtering are used to improve the performance of dye-sensitized TiO₂ solar cells by means of surface modification. The structural and performance characterizations reveal that the TiO₂/SrTiO₃ electrodes, in comparison with fresh TiO₂ electrodes, have low density of oxygen vacancies, passivated surface states, and suppressed interfacial recombination effect, thus resulting in improved performance parameters of the cells. An optimized coating of SrTiO₃layer on the TiO₂ film surface allows an enhancement of the power conversion efficiency from 4.78% to 5.91%.

V^C 2011 American Institute of Physics. [doi:10.1063/1.3617460]

Dye-sensitized solar cells (DSSCs) have been regarded as one of the most promising candidates to replace silicon cells ever since the milestone progress achieved in 1991.¹As the core component of the cell, the nanoporous electrode has high surface area which enables both efficient electron injection and light harvesting, and thus has been receiving contin- uous attentions. Unfortunately, the nanoporous TiO₂ electrode also introduces charge recombination at the electrode/dye/electrolyte interface,² which severely limits the cell efficiency. Substantial efforts by various approaches have been made to avoid the charge recombination, includ- ing dip coating,³spin coating,⁴chemical vapor deposition,⁵ electrochemical deposition,⁶and modifying or doping nano- particles by metallic salts in solution.^7,8 Nevertheless, it is still unable to remove these defects by these chemical approaches to modify the TiO₂surface. Therefore, minimiza- tion of trap states remains to be one of the central issues for reaching high efficiency.

Recently, physical methods have shown some promises in dealing with this issue. It was reported that the TiO2elec- trode treated with Ar plasma shows better optical and electri- cal activities.⁹Exposure of the TiO₂electrode to O₂plasma can reduce the oxygen vacancies (OVs) and enhance the dye absorption on the TiO2surface.^3,10These results suggest that Ar or O₂plasma processing for TiO₂electrode is feasible.

Another approach involves modifying TiO₂surface by sputtering other material layer. It has been shown that the cell performance can be improved by a sputtered ZnO layer,¹¹ however, ZnO itself can also exhibit relatively high density of OVs. Alternatively, coating of other layers may lead to higher cell performance, one of which is SrTiO₃layer. It was reported that SrTiO₃layer coated on TiO₂electrode makes a barrier of 200 mV, which can enhance the open-circuit pho- tovoltage (V_oc) and suppress the charge recombination.¹²In this work, we investigate the performance of DSSCs with the TiO2/SrTiO3 electrodes prepared by radio frequency (RF)

magnetron sputtering. The effect of the as-prepared SrTiO₃ layer on the microstructure, trap states, and interfacial electrochemical characteristic of TiO2 electrode will be addressed.

In our experiments, the preparation of porous TiO₂ film electrodes was reported previously.¹²The TiO2films of 6.0lm were used to deposit the SrTiO₃layers by RF magnetron sputtering of a commercial SrTiO3 ceramic target. It was found that the performance deteriorates with increasing temperature of TiO₂electrode, caused by heating in the sputtering process.

Therefore, the SrTiO3 layer was deposited at room temperature. Both the TiO2and TiO2/SrTiO3electrodes were dipped in a N719 solution (0.5 mM in dry ethanol solution) for 12 h to adsorb dye. The active area of the cells is 0.45 cm². A plati- nized conducting glass was used as the counter-electrode. For electrolyte, we used propylene carbonate with 0.1 M LiI, 0.05 M I2, 0.6 M 1,2-dimethyl-3-n-propylimidazolium, and 0.5 M 4-tert-butypyridine.

The structure and morphology of the as-prepared TiO₂ and TiO₂/SrTiO₃electrodes were characterized using x-ray diffraction patterns (XRDs) with Cu Karadiation and atomic force microscopy (AFM, SPM-9500J3). Cyclic voltammetry (CV) measurements were performed on an electrochemical workstation (CHI660A, CH Instruments) in combination with a conventional three-electrode structure in the dark, with a potential ranging from1.2 to 0.6 V versus standard calomel electrode (SCE) in 0.2 M LiClO4/PC solution (pH¼2) at a scan rate of 100 mV/s. The scan direction is from the negative potential to the positive one. The active area is approximately 1 cm². Impedance measurements were performed with a potentiostat (Parstat 2273, USA), using a signal of 10 mV over a frequency range of 0.01-100 kHz.

For the cell performance, the current-voltage (I-V) character- istics were measured using a Keithley 2400 source meter under an illumination of 50 mWcm² (Newport 91192, USA).

Figures1(a)and1(b)show the AFM micrographs of the TiO2 and TiO₂/SrTiO₃ films, respectively (the sputtering time for SrTiO3istSTO¼30 min). The evaluated root mean

a)Electronic mail: [email protected].

b)Electronic mail: [email protected].

0003-6951/2011/99(4)/042106/3/$30.00 99, 042106-1 VC2011 American Institute of Physics APPLIED PHYSICS LETTERS99, 042106 (2011)

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squared roughness of the TiO₂ and TiO₂/SrTiO₃ films is 75.5 nm and 49.3 nm, respectively. The average size of SrTiO₃particles is slightly bigger than that of TiO₂grains.

As shown in Fig. 1(c), the XRD spectrum of TiO₂/SrTiO₃ film (tSTO¼30 min) confirms the formation of SrTiO₃phase.

Similar structural features are identified for other samples with differenttSTO.

The effect oftSTOas a substantial parameter on performance of the DSSCs is investigated. I-V curves are shown in Fig.2withtSTOlabeled numerically. The parameters of the DSSCs with differenttSTOare summarized in Table I. It is seen that the short-circuit photocurrent (Jsc) increases from 4.81 mA/cm² at tSTO¼0 to 5.12 mA/cm² at tSTO¼9 min, and Voc is also enhanced, leading to an increment of efficiency (g) from 4.78% to 5.17%. These parameters are further enhanced at tSTO¼12 min, at which g is raised up to 5.91%. However, further increasing tSTO results in gradual damage of the cell performance andgfalls down to 0.81%.

These results demonstrate that a proper thickness of the sputtered SrTiO₃layer on TiO₂electrode surface can remarkably enhance efficiency.

To understand the underlying physics of these effects, we need to distinguish the different effects introduced by the complicated sputtering process. On one hand, a proper O₂ plasma treatment during the sputtering may reduce OVs and

thus, favor the dye adsorption on the porous TiO₂film,^3,10as confirmed by the significantly increased Jsc. On the other hand, the SrTiO₃on the TiO₂surface may negatively shift the conduction band, as illustrated by the remarkable increase of Voc,¹² although the Ar ambient available in the sputtering may be unfavored for Voc.¹⁰ A long sputtering time (here, 18 min and beyond) leads to thick SrTiO₃layer which has two consequences: on the one hand, it blocks the photo-generated electron injection. On the other hand, it decreases the surface area by filling the pores of porous TiO₂ electrode as shown in Fig.1(b), resulting in hindering the ion motion inside the porous TiO₂ electrode. In consequence, drastic drops of both JscandVocyield, and the drop ofgis also observed.

For details of these possible mechanisms, one may char- acterize the trap states on the sample surface by CV measurements. In Fig.3, we plot the typical CV curves for TiO₂and TiO₂/SrTiO₃electrodes. For a perfectn-type semiconductor- electrolyte junction, the charge injection will commence once the quasi-Fermi level reaches the lower edge of the conduction band. However, if OVs on TiO₂ surface are available, some electronic levels exist at the energy levels below the conduction band edge.^13,14These surface states may trap those electrons injected under the forward bias, giving rise to a gradual onset of the capacitive current in the forward scan.

This mechanism seems applicable for the present case. As shown in Fig. 3, a scan of the potentials of TiO₂and TiO₂/ SrTiO₃ electrodes in the negative direction yields a large featureless cathodic current. The reversal scan, however, yields an anodic current in both the bare TiO₂ and TiO₂/ SrTiO₃electrode withtSTO¼12 min, indicating that the electron charging/discharge occurs in the anodic TiO₂/electrolyte interface, i.e., the Faradic currents in the electrolyte.

Nevertheless, for bare TiO₂electrode, the anodic current starts at a potential of0.175 V, which is much higher than

FIG. 1. (Color online) AFM micrographs of the TiO2electrode (a) and the TiO2/SrTiO3 electrode with tSTO¼30 min (b). The XRD pattern of the TiO2/SrTiO3electrode with tSTO¼30 min deposited on FTO substrate is shown in (c).

FIG. 2. (Color online) Measured I–V curves for the cells with the bare TiO2

electrode and TiO2/SrTiO3electrodes.

TABLE I. Performance of the TiO2and TiO2/SrTiO3cells.

tSTO(hTO¼6.0lm) Jsc(mA/cm²) Voc(mV) FF g(%)

0 4.81 680 0.729 4.78

9 min 5.12 693 0.729 5.17

12 min 5.81 696 0.731 5.91

18 min 4.36 665 0.726 4.21

30 min 1.02 636 0.625 0.81

FIG. 3. (Color online) Cyclic voltammograms of the bare TiO2(tSTO¼0) and TiO2/SrTiO3electrodes.

042106-2 Wuet al. Appl. Phys. Lett.99, 042106 (2011)

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the potential of 0.285 V for the TiO2/SrTiO₃ electrode with tSTO¼12 min. This anodic current vanishes for the TiO₂/SrTiO₃ electrode with tSTO¼30 min. The presented data reveal that the edge of the conduction band for the TiO₂/SrTiO₃electrode moves negatively toward the vacuum level with respect to the bare TiO₂ electrode,¹⁵ obviously due to the better passivation and reduction of the trap states on the TiO₂ surface.¹⁶ The reason lies in that the porous TiO₂electrode is not fully covered by dye molecules, allow- ing some surface area directly in contact with the electrolyte.

The surface passivation and reduction of the trap states take effect, and thus, Jsc and performance of the DSSCs are improved.

Subsequently, one is also allowed to understand the effect of sputtered SrTiO₃ on the porous TiO₂ surface by investigating the kinetics of electrochemical process in the DSSCs, using the electrochemical impedance spectroscopy (EIS). The Nyquist plots of the bare TiO₂and TiO₂/SrTiO₃ cells, measured under a forward bias of0.75 V in the dark, are shown in Fig.4. For both types of cells, the Nyquist plot consists of three semicircles. The detected responses within the three frequency regions of 10⁵–10³Hz, 10³–1.0 Hz, and 1.0–0.01 Hz, are respectively assigned to the charge-transfer processes occurring at the Pt/electrolyte interface, the TiO₂/ dye/electrolyte interface, and the Nernstian diffusion of I₃/I within the electrolyte.¹⁷ It is reasonable to argue that the SrTiO₃layer enhances the resistance of the TiO₂/dye/electrolyte interface. A properly modulated interface resistance allows a delay of the electron back transfer from the TiO₂/ SrTiO₃ electrode to electrolyte via the TiO₂/dye/electrolyte interface and thus minimizes the charge recombination.¹⁸ Therefore, the photo-generated electrons can be extracted in a more efficient way, resulting in greatly improved photocurrent and enhanced conversion efficiency.¹⁹ It seems that the optimized resistance is obtained at tSTO¼12 min. It is no doubt that the TiO₂/SrTiO₃ cell with tSTO¼18 min and beyond sets in an over-high interface resistance barrier,

making the photoelectron-injection into the TiO₂less efficient.

This leads to a serious reduction in the photocurrent and conversion efficiency. Furthermore, it is noted that for the TiO₂/ SrTiO₃cell withtSTO¼30 min, the semicircle in the low frequency region, which corresponds to the Nernstian diffusion within the electrolyte, is larger than that of others. This confirms that the interface resistance is indeed high when the deposited SrTiO₃is over-thick.

In summary, the SrTiO₃-modified TiO₂electrodes have been fabricated by RF magnetron sputtering. The performance of the modified DSSCs is dependent of the sputtering time. The efficiency of the DSSCs is improved from 4.78%

to 5.91% after the sputtered SrTiO₃modification at the opti- mum fabrication conditions, attributed to the decrease of the trap states, and suppression of the charge recombination at the TiO₂/dye/electrolyte interface. These results indicate that an optimization of the TiO₂/dye/electrolyte interface is substantial for improving the performance of DSSCs.

We acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 51003035 and 50832002) and the help of Professor Xingzhong Zhao’s group with the School of Physical Science and Technology, Wuhan University.

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FIG. 4. (Color online) Impedance spectra for the bare TiO2 and TiO2/ SrTiO3cells.

042106-3 Wuet al. Appl. Phys. Lett.99, 042106 (2011)