SCIENCE CHINA BiVO4 semiconductor sensitized solar cells

SCIENCE CHINA Chemistry • ARTICLES •

September 2015 Vol.58 No.9: 1489–1493 doi: 10.1007/s11426-015-5348-3

BiVO4 semiconductor sensitized solar cells Yi Li1,2, Jun Zhu1*, Hui Chu1, Junfeng Wei1, Feng Liu1, Mei Lv1, Junwang Tang3*, Bing Zhang4,5, Jianxi Yao4,5, Zhipeng Huo1, Linhua Hu1 & Songyuan Dai1,4,5* 1

CAS Key Laboratory of Novel Thin Film Solar Cells; Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China 2 Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China 3 Department of Chemical Engineering, University College London, London WC1E 7JE, UK 4 State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources; North China Electric Power University, Beijing 102206, China 5 Beijing Key Lab of Novel Thin Film Solar Cells; North China Electric Power University, Beijing 102206, China Received October 17, 2014; accepted November 18, 2014; published online March 12, 2015

Semiconductor sensitized solar cells (SSSCs) are promising candidates for the third generation of cost-effective photovoltaic solar cells and it is important to develop a group of robust, environment-friendly and visible-light-responsive semiconductor sensitizers. In this paper, we first synthesized bismuth vanadate (BiVO4) quantum dots by employing facile successive ionic layer adsorption and reaction (SILAR) deposition technique, which we then used as a sensitizer for solar energy conversion. The preliminary optimised oxide SSSC showed an efficiency of 0.36%, nearly 2 orders of magnitude enhancement compared with bare TiO2, due to the narrow bandgap absorption of BiVO4 quantum dots and intimate contact with the oxide substrate. This result not only demonstrates a simple method to prepare BiVO4 quantum dots based solar cells, but also provides important insights into the low bandgap oxide SSSCs. solar cells, bismuth vanadate, successive ionic layer adsorption reaction deposition, sensitizer

1 Introduction Recently, semiconductor sensitized solar cells (SSSCs) have attracted a great deal of attention as promising candidates for the third generation of cost-effective photovoltaic solar cells [1–8]. The process of semiconductor sensitization offers potential advantages over the utilization of ruthenium or organic dyes, such as a high light-harvesting capability, a tunable band gap over a wide range, which can be tuned by controlling the semiconductor size, and a large intrinsic dipole moment. In addition, there have been a few reports of multiple exciton generation and hot carrier injection in some quantum dot sensitized systems, which demonstrate a great *Corresponding authors (email: [email protected]; [email protected]; [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2015

potential to exceed the Shockley-Queisser limit [9,10]. Sensitization of TiO2 using InP [11], InAs [12], CuInS2 [3], CdSe [4], PbS [13], Bi2S3 [14], and organic lead halide perovskite [2]. have been extensively reported. However, there are some concerns about toxic elements such as cadmium and lead included. In addition, there is an urgent need to improve the stability of the widely used sulphide and selenide semiconductor sensitizers. Therefore, it is important to develop a new group of robust, environment-friendly and visible-light-responsive semiconductor sensitizers. Oxides as the active materials of solar cells have the advantages of low cost, good stability, and abundance. Cu2O, CuO, and BiFeO3. as the active layer of solar cells have been investigated. Bismuth vanadate (BiVO4) has attracted considerable interests recently due to its composition of inexpensive and nontoxic elements. BiVO4 has been identichem.scichina.com

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fied as one of the most promising photoanode materials. In addition, BiVO4 is a direct bandgap semiconductor with a narrow band gap and can absorb visible light in the short wavelength region of the solar spectrum. In its common monoclinic phase, the band gap is approximately 2.4–2.5 eV [15]. BiVO4 has been widely used in the photocatalytic decomposition of organic pollutants and the photoelectrical catalytic evolution of O2 and H2 [16]. Unfortunately, there have been few researches on BiVO4 based solar cells. Liu et al. [17] investigated bilayer P3HT-BiVO4 hybrid solar cell with a power conversion efficiency (PCE) of 0.08%. Zhang et al. [18] reported a Mo-doped BiVO4 (~50 nm) liquidjunction photovoltaic solar cell in I−/I3− electrolytes. The BiVO4 photoelectrodes exhibit a porous microstructure with 50–200 nm wormlike particles. Compared to the state-of-art structure employed in dye sensitized solar cells or extremely thin absorber solar cells, there is much room for improvement in the active layer morphology to increase the light harvesting ability. In this work, we report a BiVO4 quantum dot sensitized solar cell grown by the successive ionic layer adsorption and reaction (SILAR) deposition technique. The microstructure was analyzed by electron microscopy and X-ray diffraction. The assembled BiVO4 solar cells yielded a power conversion efficiency of 0.36% and a short-circuit current density of 2.2 mA/cm2 under AM 1.5 illumination. This work not only demonstrates a simple method to prepare BiVO4 quantum dots based solar cells, but also provides important insights into the low bandgap oxide semiconductor sensitized solar cells.

2 Experimental 2.1

Materials

Bi(NO3)3·5H2O (98 %) was purchased from Strem Chemicals, Inc. (USA). NH4VO3 (99%) was purchased from J&K Chemical Ltd. (China). Lithium iodide (LiI), iodine (I2) and 4-tert-butylpyridine (TBP) were purchased from SigmaAldrich (USA). All the other solvents and the chemicals are pure grade and used without further purification. 2.2

Fabrication of TiO2/BiVO4 electrode

Nanocrystalline TiO2 paste and film were prepared according to the method reported previously [19]. Fluorine-doped tin oxide conducting glass (FTO, 15 Ω/square) was cleaned in an ultrasonic bath with acetone, isopropanol, and ethanol for 20 min, followed by rinsing with ultrapure water and drying in a flow of air, subsequently sintered at 510 °C for 30 min. The nanocrystalline TiO2 paste was deposited on the FTO substrate by screen printing technique and annealed in air at 510 °C for 30 min. TiO2 particles and the porous size of the film are both about 20 nm. The thickness

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of the TiO2 film, determined by a surface profilometer (XP-2, AMBIOS Technology Inc., USA), was about 2 μm. The SILAR deposition of BiVO4 was carried out as follows: the TiO2 film was dipped into 0.01 mol/L Bi(NO3)3 aqueous solution for 30 s, washed with ultrapure water, then dipped into 0.01 mol/L NH4VO3 aqueous solution (pH 3) for 30 s, and again washed. The wash time of 30 s was sufficient to remove excess ions adsorbed by the TiO2 surface. The twostep procedure was termed as one cycle. The optimal SILAR cycle number was 5 for the photovoltaic performance and the samples investigated were also subjected to 5 SILAR cycles. After 5 SILAR cycles, the TiO2/BiVO4 film was sintered at 500 °C for 1 h in air. 2.3

Device fabrication

The counter-electrode was platinum set by the spraypyrolysis method. Iodide-based electrolyte was a solution of 0.6 mol/L 1-butyl-3-methylimidazolium iodide, 0.03 mol/L I2, 0.10 mol/L guanidinium thiocyanate, and 0.5 mol/L 4-tert-butyl pyridine in 3-methoxypropionitrile. The photovoltaic devices were fabricated in a sandwich structure FTO/TiO2/BiVO4/electrolyte/Pt. The active area of the photovoltaic device was 0.25 cm2. 2.4

Characterization

The crystallinity of the film was determined using X-ray powder diffraction analysis (XRD, TTR-III, Rigaku Corp., Japan) with Cu-K irradiation (=1.5406 Å). The surface morphology of the film was observed with a field emission scanning electron microscope (FE-SEM, sirion200, FEI Corp., Holland). The TEM images were observed with transmission electron microscopy (TEM, JEOL-2010, Japan). A typical EDS spectrum was performed with an energy dispersive spectrometer equipped on the TEM equipment. The UV-Vis spectrum of the films was obtained using a UV-Vis spectrophotometer (U-3900H, Hitachi, Japan). The incident-photon-to-electron conversion efficiency (IPCE) measurement was conducted using a QE/IPCE measurement kit (Newport Corporation, CA). The current-voltage characteristics (J-V curves) were measured with a Keithley model 2420 digital source meter (Keithley Instruments, Inc., OH, USA) under the illumination of 100 mW/cm2 (AM 1.5) provided by a solar simulator (solar AAA simulator, Oriel USA).

3 3.1

Results and discussion Characterization of TiO2 and TiO2/BiVO4 films

Figure 1 shows the XRD patterns of bare TiO2 and TiO2/ BiVO4 films. In comparison, all the diffraction peaks of the TiO2 film are indexed to the standard data of anatase TiO2

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Figure 2 film (b).

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SEM images of bare TiO2 film (a) and BiVO4 sensitized TiO2

Figure 1 XRD patterns on bare TiO2 film (a) and TiO2/BiVO4 film (b). Standard XRD pattern of anatase TiO2 (JCPDS, No. 21-1272) (c) and tetragonal zircon BiVO4 (JCPDS, No. 75-2481) (d).

(JCPDS, No. 21-1272) (Figure 1(c)), indicating that the TiO2 nanoparticle is pure anatase. Figure 1(b) shows the XRD pattern of the TiO2/BiVO4 film prepared by the SILAR method with equal stoichiometric ratio of vanadium to bismuth in solution and then sintered at 500 °C for 1 h. BiVO4 is known to exist in three crystalline phases: the tetragonal sheelite type, the tetragonal zircon type and the monoclinic sheelite type [20–22]. The crystalline phases of BiVO4 depend on the synthetic condition used. The monoclinical and tetragonal phases could be selectively obtained by changing the stoichiometric ratio of vanadium to bismuth in solution [20]. The phase transition from the tetragonal sheelite type to the monoclinic sheelite type irreversibly occurs at 397–497 °C [22]. The monoclinical sheelite phase can be distinguished from the tetragonal zircon phase of BiVO4 by the existence of a 2θ peak at 15° and splitting of the peaks at 18.5°, 35°, and 46° [21]. We did not find a peak at 15° and splitting of peaks at 18.5°, 35°, and 46° in XRD pattern of the TiO2/BiVO4 film as can be seen in Figure 1(b). In addition, XRD pattern of the TiO2/BiVO4 film was in good agreement with the standard data of anatase TiO2 and tetragonal zircon BiVO4 (JCPDS No. 75-2481) (Figure 1(c, d)). No diffraction peaks of other phases are detected. Therefore, it is concluded that the BiVO4 deposited on the TiO2 film by SILAR method can be confirmed as tetragonal zircon phase. The BiVO4 peaks are distinct and comparable with those of TiO2, similar to the case of liquid CH3NH3PbI3 sensitized solar cells [23]. Figure 2 (a, b) presents typical SEM images of the bare TiO2 film and the BiVO4 sensitized TiO2 film. The particle size slightly increased after the BiVO4 deposition and the BiVO4 sensitizers covered TiO2 particles nearly conformably. Furthermore, the porous structure of the TiO2 film remained after the BiVO4 deposition so that the electrolyte solution can easily penetrate the pores of the TiO2/BiVO4 film, which is important for the hole transport. TEM was performed to further investigate the microstructure of the BiVO4 sensitized TiO2 film. Figure 3(a)

Figure 3 TEM (a) and HRTEM (b) images of the TiO2/BiVO4 and EDS pattern of TiO2/BiVO4 (c).

shows a typical TEM image of the TiO2/BiVO4 film, where we can observe that BiVO4 quantum dots were highly dispersed on the TiO2 surface. Figure 3(b) provides a highresolution TEM image of TiO2/BiVO4 particles, which shows that 3–5 nm BiVO4 quantum dots are grown on the TiO2 film and the interface exhibits intimate contact between TiO2 and BiVO4 with a fairly large contact area. Therefore, TEM images clearly reveal that BiVO4 dots have been grown on the surface of TiO2 nanoparticles. The elemental composition of the TiO2 and TiO2/BiVO4 samples were analyzed by EDS. The EDS of TiO2/BiVO4 (Figure 3(c)) reveals that titanium, bismuth, vanadium, and oxygen signals exist, and the approximate ratio of vanadium to bismuth is 1:1, while the Cu signals arise from the copper grid used for analysis. 3.2

Optical properties of TiO2 and TiO2/BiVO4 films

To study the optical absorption properties of the TiO2/ BiVO4 heterostructure, UV-Vis absorption spectra were monitored and the results are shown in Figure 4. For the bare TiO2 film, it can be found that the threshold of the

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Figure 4 UV-Vis absorption spectra of bare TiO2 film and BiVO4 sensitized TiO2 film.

absorption around 375 nm agreed with the fundamental absorption edge of TiO2 and no significant absorbance for visible light was seen because of its large energy gap (3.2 eV). Optical density in the range of 350–500 nm in the UV-Vis spectra obviously increased for the TiO2/BiVO4 electrode due to BiVO4 absorption. Furthermore, BiVO4 absorption edge was 480 nm, equal to 2.6 eV. It is larger than the reported bulk absorption (2.4–2.5 eV) due to quantum size effect. 3.3 Photovoltaic performance of the liquid junction solar cells based on bare TiO2 photoanode and TiO2/ BiVO4 photoanode The architecture of BiVO4 semiconductor sensitized solar cells is similar to that of a classical dye-sensitized solar cell. The mesoporous TiO2 electrode serves as an electron collector for the BiVO4, which has poor electron transport performance. The J-V characteristics of the liquid junction solar cells based on bare TiO2 photoanode and TiO2/BiVO4 photoanode are shown in Figure 5 and the derived photovoltaic parameters are summarized in Table 1. The BiVO4 sensitized photoanode shows great enhancements in both Jsc and Voc. The Jsc significantly increased from 0.07 to 2.2 mA/cm2 under AM 1.5, 100 mW/cm2, which reveals that there must be some photoelectrons generated in BiVO4 being transferred to TiO2. Due to the more negative conduction band edge of BiVO4 than TiO2, efficient electron injection is seemingly difficult [18]. However, the flat band potential of metal oxides is influenced by the structure of the material and the reported flab band potential for BiVO4 is in a large range of values, from approximately 0.7 to 0.3 V (vs. Ag/AgCl, pH 5.8–7) [24–26]. In addition, an efficient electron injection has also been observed in CdS/CdSe sensitized ZrO2 solar cells despite the much higher conduction band edge of ZrO2 [27]. The photoelectrons accumulation in the quantum dots is the only reasonable interpretation. The contribution of Jsc mainly arises from the light absorption of BiVO4 in the visible region. It is noted that there is still


Figure 5 Current density-voltage (J-V) characteristics of the liquid junction solar cells based on bare TiO2 photoanode and TiO2/BiVO4 photoanode.

Table 1 Photovoltaic parameters of the liquid junction solar cells based on bare TiO2 photoanode and TiO2/BiVO4 photoanode Photoanode Bare TiO2 TiO2/BiVO4

Voc (V) 0.43 0.56

Jsc (mA/cm2) 0.07 2.2

FF 0.52 0.29

 (%) 0.02 0.36

much room for further improvement of Jsc if referring to Cu2O-ZnO solar cells where the light absorber has a similar band gap of 2.0 eV [28]. A Voc of up to 0.56 V, the solar energy conversion efficiency is dramatically increased from 0.02% to 0.36 % after BiVO4 deposition. The redox couple that transport holes in the electrolyte is I3/I and the TiO2/BiVO4 photoelectrodes immediately become dark when immersed in solution with polysulfide redox. To further understand the influence of BiVO4 sensitizer on the photovoltaic performance, the IPCE of the BiVO4sensitized solar cell is shown in Figure 6. The IPCE spectrum exhibits relatively high values, that is, larger than 30% is in the wavelength range of 350–450 nm. Assuming a 15% optical loss of the conducting glass, the internal quantum

Figure 6 Incident photon to current conversion efficiency (IPCE) for the liquid junction solar cells based on bare TiO2 photoanode and TiO2/BiVO4 photoanode.

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efficiency should be at least 40%, which is much higher compared to bare TiO2 photoanode. The IPCE spectra correspond well to the UV-Vis absorption spectra shown in Figure 4. One thought is that the light harvesting of the device could be improved by increasing the BiVO4 deposition amount. However, the pores of TiO2 will be blocked if we increase the SILAR cycles beyond 5. To seek the optimal oxide film morphology for the BiVO4 deposition is under way.


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4 Conclusions 14

In conclusion, we have fabricated a BiVO4 quantum dot sensitized solar cell by employing the facile SILAR method. The BiVO4 sensitizers cover TiO2 nanoparticles conformally, resulting in more than 30 times increase in Jsc and 30% increase in Voc compared to bare TiO2 based solar cell. Further optimization of the BiVO4 quantum dot sensitized solar cell is under way by manipulating the dispersion of BiVO4 quantum dots on TiO2 particle and selecting optimal electrolytes that have more positive redox potential for higher Voc.

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18 This work was financially supported by the National Basic Research Program of China (2011CBA00700), the National High Technology Research and Development Program of China (2011AA050527) and the National Natural Science Foundation of China (21403247, 21173228, 21103197). 1

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