Accepted Manuscript Title: Enhanced efficiency of large-area dye-sensitized solar cells by light-scattering effect using multilayer TiO2 photoanodes Authors: Wei Wang, Huihui Yuan, Junjie Xie, Di Xu, Xinyu Chen, Yunlong He, Tao Zhang, Zongqi Chen, Yumei Zhang, Hujiang Shen PII: DOI: Reference:
S0025-5408(17)33491-8 https://doi.org/10.1016/j.materresbull.2017.12.032 MRB 9749
To appear in:
MRB
Received date: Revised date: Accepted date:
11-9-2017 22-11-2017 23-12-2017
Please cite this article as: Wang W, Yuan H, Xie J, Xu D, Chen X, He Y, Zhang T, Chen Z, Zhang Y, Shen H, Enhanced efficiency of large-area dye-sensitized solar cells by light-scattering effect using multilayer TiO2 photoanodes, Materials Research Bulletin (2010), https://doi.org/10.1016/j.materresbull.2017.12.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced efficiency of large-area dye-sensitized solar cells by light-scattering effect using multilayer TiO2 photoanodes
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Zhang1, Zongqi Chen1, Yumei Zhang1, and Hujiang Shen1, 2, *
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Wei Wang1, Huihui Yuan1, Junjie Xie1, Di Xu1, Xinyu Chen1, Yunlong He1, Tao
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of
Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Shanghai 201899,
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State Key Laboratory of High Performance Ceramics and Superfine Microstructure,
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2
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China.
Corresponding author:
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*
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Shanghai 200050, China
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Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road,
Dr. Hujiang Shen
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E-mail:
[email protected]; Tel.: +86-21-69906295;
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Fax: +86-21-69906700.
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Graphical abstract
Highlights
DSSCs with an active area of 100.6 cm2 and light-scattering layers are fabricated.
The light-scattering effect is shown to enhance the light absorption in the red part
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The light-scattering effect is more important than the dye adsorption capacity for
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of the solar spectrum.
the light harvesting property of TiO2 photoanodes with same thickness. The highest power conversion efficiency of 7.52% has been obtained by the
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Abstract
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DSSC with optimized conditions.
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In order to reduce the usage of photoactive dye and improve light harvesting by
the photoanode of dye-sensitized solar cells (DSSCs), TiO2 scatting layers were introduced. We have investigated the light-scattering effect in large-area DSSCs with
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an active area of 100.6 cm2 that contain diffusing or reflective layers. The result shows that the light-scattering effect is more important than the dye adsorption capacity for the light harvesting property of TiO2 photoanodes with same thickness, and the light-scattering effect is shown to enhance the light absorption in the red part of the solar spectrum. The highest power conversion efficiency of 7.52% has been obtained by the cell with optimized high dye adsorption layer and high light-scattering 2
layer, which is 9.5% higher than that of the cell without the light-scattering layer (6.87%).
KEYWORDS: A. multilayers; B. optical properties; C. X-ray diffraction; D. electrical
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properties;
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1. Introduction
Dye-sensitized solar cells (DSSCs) have attracted extensive attentions in recent years since they were first invented by Grätzel and co-workers [1]. The power
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conversion efficiency (PCE) of DSSC has been greatly improved to 13.0% during
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years’ development [2]. A traditional DSSC consists of a transparent conductive-oxide
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glass covered by dye-loaded mesoporous nanocrystalline film, electrolyte that
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contains a redox couple (usually I-/I3-), and a counter electrode [3]. Mesoporous TiO2 film is a good candidate due to its great chemical stability and resistance to the
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corrosion of I- in the electrolyte, and high electron conductivity. The total amount of dye adsorbed onto the TiO2 layer has great influence on the PCE, because electrons
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are obtained directly from the excited dye molecules by sunlight [4]. Therefore, many works have been focused on improving the dye adsorption capacity of TiO2
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photoanodes through fabricating nanoparticles [5-7], hierarchical [8, 9] or multi-wall structures [10-12] with high specific surface area, and increasing the film thickness [13, 14]. However, the improvement of PCE through enhancing dye loading is limited.
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The reason is that higher surface area is usually obtained with smaller particle sizes [15], and thicker porous film and smaller particles will lead to slower electron transport in the photoanode, which is not beneficial for a higher PCE. Enlarging the light scattering of TiO2 is another effective way to enhance the light harvesting of dye molecules and PCE. Wang et al. first applied light-scattering layer to DSSC [16], and enhanced light harvesting by increasing the optical path length in the photoanode 3
rather than increasing dye loading. In addition, the cost of DSSC would be reduced due to lower dye consumption, especially for DSSCs with large active areas. Many workers were committed to synthesis of a light-scattering layer (LSL) consisted of multiple nanocrystalline layers or films with special structures through various methods [17-20]. However, the complex fabrication routes in the works above are not
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suitable for industrialized scale production of DSSC. In this paper, we have investigated the influence of dye adsorption and light-scattering effect of the photoanode on the PCE of large-area DSSCs.
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Commercial TiO2 powder was used to fabricate the LSL. Three kinds of TiO2 layer (include LSL) with different dye adsorption capacity were fabricated separately or layer-by-layer. The light-scattering effect was investigated by using diffused
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reflectance spectra of TiO2 layers and incident photon-to-current efficiency (IPCE) of
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the corresponding DSSCs. The photovoltaic properties of DSSCs with or without the
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LSL and with different thickness of TiO2 layers were investigated by photoelectric
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2. Experimental
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measurements.
Fluorine-doped tin oxide (FTO) glass (TEC-8, NSG) and Ti foil (TA1, thickness
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of 0.1 mm) were used as the substrates for the photoanodes and counter electrodes, respectively. The substrates were first ultrasonically cleaned in acetone, ethanol and
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deionized water for 30 min, respectively. A TiO2 blocking layer was then deposited on cleaned FTO glass by spin coating. Three kinds of TiO2 pastes (all made in our lab) were used to fabricate the TiO2 photoanodes by screen printing. The paste used for
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transparent active layer (T layer) only contained TiO2 powder with average particle sizes of about 20 nm. To fabricate diffusing active layer (D layer), 20 wt% of TiO2 powder for T layer was replaced by powder with average particle sizes of about 200 nm (Wako Japan). These powders were also used to prepare the paste of reflective add-on (R layer). The R layer was a traditional LSL as mentioned in other literature 4
[21]. Commercial silver paste (PC-Ag-5310, Sino-Platinum Metals) was used to fabricate Ag grids, which helped to collect and transport photoelectrons. A cover layer (3035B UV-resin, ThreeBond) was screen printed onto Ag grids to prevent them from corrosion by I-. The thickness of the TiO2 layer was controlled by the screen mesh and repeat times of printing process. Each printing process was followed by drying at
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120 °C for 5 min. Finally, the photoanodes were sintered at 500 °C for 15 min to remove the additives in the pastes such as terpineol and ethylcellulose. Carbon
material paste containing KB and VGCF (weight ratio 1:1) was screen printed on Ti
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foils, followed by sintering at 450 °C for 15 min to fabricate the counter electrodes. The thickness of the carbon counter electrode in this work was fixed at ~25 μm.
Dye loading was performed by immersing the prepared TiO2 photoanodes in 0.3
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mM TG6 [22] solution (dye: TG6, solvent: t-butanol and acetonitrile mixture with a
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volume ratio of 1:1) for 24 h at room temperature. Then, the photoanodes were
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washed with acetonitrile for removing the unanchored dyes. Finally, the solar cells
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were assembled in a typical sandwich-type cell by placing the carbon counter electrode on the dye-sensitized photoanode separated by a hot-melting film
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(Bynel4033, DuPont, thickness of 120 μm) and sealed them together by a hot-pressing method. The electrolyte was injected through the holes which were pre-etched on Ti
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foils by laser ablation. The electrolyte was MPN solution containing MPII (1 mM), NBB (0.25 mM) and I2 (0.1 mM).
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The crystal structure and particle size of TiO2 layer on the photoanode were
characterized by X-ray diffraction system (XRD, Ultima IV Rigaku, using a Cu Kα1 radiation, λ=1.5406 Å, scanning speed: 5o/min) and field emission scanning electron
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microscope (FE-SEM, Magellan 400, FEI). The BET surface areas and BJH pore size distributions of TiO2 particles were determined from the adsorption isotherms of nitrogen (ASAP 2020, Micromeritics). The thickness of TiO2 layer was measured by a stylus profiler (P16+, KLA-Tencor). The dye loading of the TiO2 layers was analyzed by UV-vis spectrophotometer (U-2800, Hitachi) and the absorption spectra of these 5
layers were recorded using the same UV-vis spectrophotometer. The UV-vis diffused reflectance spectra of the TiO2 layers were recorded in a spectrophotometer (U-4100, Hitachi) using a 60 mm integrating sphere. The current density-voltage (J-V) curves were measured using a current source/monitor under one sun illumination (AM 1.5G, 1000 W/m2) with a solar light simulator (YSS-150, Yamashita Denso). The active cell
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area is 100.6 cm2. The incident photon-to-current efficiency (IPCE) spectra were recorded using a spectral response measurement system (CEP-1500, Bunkoh-Keiki).
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3. Results and discussion
Fig. 1a shows the XRD patterns of the T, D and R layers fabricated on FTO glass. Bare FTO glass was tested to distinguish the substrate’s peaks in the results. Strong
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anatase-TiO2 diffraction peaks appear in all TiO2 layers and no rutile-TiO2 diffraction
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peak is detected, indicating that the TiO2 layers consist of pure anatase phase. This is
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important for adsorbing more dye molecules, as anatase-TiO2 has a higher surface
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area per unit volume than rutile-TiO2 [23]. A detailed comparison of the XRD patterns of the T and R layer samples shows that the full width at half maximum (FWHM) of
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the strongest diffraction peak (2θ=25.36o) of the R layer is smaller than that of the T layer, because of the larger TiO2 particles in the R layer. The average grain size is
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images.
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calculated to be 18.7 nm for the T layer, which would be confirmed by FE-SEM
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Fig. 1 (a) XRD patterns, (b) N2 adsorption-desorption isotherm of different TiO2 particles in the T, D and R layers. The inside of (b) is the corresponding pore-size
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distributions.
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Fig. 1b shows the N2 adsorption-desorption isotherms and the corresponding
Barrett-Joyner-Halenda (BJH) pore-size distribution curves of the TiO2 particles in the T, D and R layers. The specific Brunauer-Emmett-Teller (BET) surface areas and BJH
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pore sizes are summarized in Table 1. According to the IUPAC classification, both the T and D layer samples exhibit a type IV isotherm and H3 hysteresis loop, indicating the typical mesoporous nature of both materials and the presence of slit-like mesopores (2-50 nm) formed by the aggregation of TiO2 particles. The TiO2 particles in the R layer sample exhibit a type III isotherm, which is the typical feature of 7
non-porous materials, leading to the small surface area and low adsorption capability of the R layer [24]. The corresponding BJH pore sizes of the TiO2 particles in the T and D layer obtained from the adsorption curves are 18 nm and 23 nm, respectively. The increment of pore size is caused by the different constituent of the T and D layer (20 wt% of small particles replaced by large ones), which also results in a reduced
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surface area. Fig. 2a-2c show the surface FE-SEM images of the T, D and R layers with the same magnification. The images show that the T layer consists of a uniform
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mesoporous structure with the particle size of ~20 nm, which is close to the calculated result from XRD and other publications [25-27]. The D layer has some large particles embedded in small ones. In contrast, the R layer consists of only large TiO2 particles,
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surrounded by some nanocrystals. Since the size of larger particles in the D and R
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layers (200-400 nm) is comparable to the wavelength of visible light and the particles
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are large enough to act as light-scattering centers, one may expect stronger scattering
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effects from these layers compared to the T layer [28]. Fig. 2d shows the cross-sectional FE-SEM image of a TDR photoanode. The
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interface between the T and D layer is not distinct. Thus, the thicknesses of the T, D and R layers are determined through the surface profiler measurement after the
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corresponding printing-sintering process. The thicknesses for one printing-sintering
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process are presented in Table 1.
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Fig. 2 Surface FE-SEM images of the (a) T layer, (b) D layer and (c) R layer.
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Cross-sectional FE-SEM images of (d) TDR multilayer photoanode.
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The performance of DSSC is strictly related to the amount of dye adsorbed onto
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the TiO2 layer. The dye adsorption strongly depends on the BET surface area, crystalline size, and the phase of TiO2 [29]. The amount of dye loading can be calculated using the following equation:
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(C0 C1 ) V w
(1)
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q
where q (mol/g) is the amount of dye adsorbed onto a unit mass of the TiO2 layer; C0 and C1 (mol/L) are the concentration of the dye solution before and after dye
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adsorption, respectively; V (L) is the volume of dye solution; and w (g) is the weight of TiO2 particles in the T (or D/R) layer. The concentration of dye solution is determined by the spectrophotometer method based on Beer-Lambert’s law. The weight of TiO2 particles is calculated by multiplying the weight of paste printed per layer by the corresponding solid content. The q, w and the amount of dye adsorbed onto a unit area of the TiO2 layer (another indicator of dye adsorption in other 9
literatures) are summarized in Table 1. It can be seen that the q decreases with the decrease of specific surface area. The q of the T layer is about two times of that of the D layer, although the surface area only decreases by 19.5%. The phenomenon above indicates adding large particles in the D layer strongly deteriorates the adsorption capacity. No dye molecule is adsorbed onto the R layer, indicating the R layer only
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acts as LSL and has no photoelectric conversion characteristics. The amount of dye adsorbed onto a unit mass (or area) of the TiO2 layer is comparable to those reported
in other literatures [9, 30-32], which means that the amount of dye adsorbed onto the
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T or D layer has reached the ideal value.
layer surface area pore size w
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Table 1 Physical properties and the amount of dye loading of the T, D and R layers q
dye adsorbed Thickness
(nm)
(g)
T
88.80
18
0.143 0.031
43.5
6.231
D
71.49
23
0.121 0.018
22
6.657
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6.38
/
0.09
0
3.187
0
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(mmol/g)
(nmol/cm2)
(μm)
(m2/g)
Fig. 3 shows the absorption spectra of the T (or D/R) monolayer, TD bilayer and
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TDR multilayers before and after dye adsorption. The absorption spectrum of TG6
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dye solution is also shown to indicate its intrinsic absorption peaks. It can be seen that TG6 has two absorption peaks, which are located at 390 nm and 550 nm. For the T layer without light-scattering effect, the absorbance around 390 nm or 550 nm
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increase obviously, and a high shoulder appears at 550 nm after dye loading. Distinct absorption increasing is observed around the absorption band between 480 nm to 550 nm after the dye adsorption onto the D layer, as is shown in Fig. 3b. The absorption enhancement around 480 nm may be from the red-shifted peak of 390 nm due to the increased optical path length by light-scattering effect of large particles [33]. The absorption spectra of the R layer before and after the dyes adsorption have no obvious 10
change, confirming no dye molecule adsorbed onto it. This result is in well accordance with the q of R layer. As can be seen in Fig.3d, because the absorption of the T layer is much smaller than that of the D layer, the absorption spectrum of the TD bilayer is similar to that of the D layer only with a slightly increase of absorption peak at 550 nm in the dye-adsorbed spectrum. The absorption band between 550 nm to 750
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nm changes from a convex-function like to a concave-function like after the R layer coated on the TD bilayer, indicating that the light-scattering effect is further enhanced.
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The photoanode can harvest more light in this region.
Fig. 3 UV-vis absorption spectra of the (a) T layer, (b) D layer and (c) R layer, and (d) TD bilayer and TDR multilayers before and after dye adsorption. The absorption
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spectrum of TG6 solution (solid line in (a)-(c)).
The diffused reflectance spectra of the TiO2 layers before dye adsorption are used to evaluate the light-scattering effect and shown in Fig. 4. The diffused reflectance spectrum of bare FTO glass is also shown as a reference. The incident monochromatic light first passed through FTO glass then TiO2 during the test. It can 11
be seen that the reflectance of R layer is much stronger than that of T layer, which supports the conjecture that larger TiO2 particles have a relatively stronger scattering effect. When these large particles are embedded in the small particles, the scattering effect will be enhanced, such as that of the D layer. Moreover, the diffused reflectance of TDR multilayers is almost the same as that of R layer in the visible light range,
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indicating that the strong scattering effect of the multilayers is mainly derived from the R layer. No evidence shows the interface between two layers has a great impact on the scattering effect, as the contact between T and D (or D and R) layers is tight and
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without obvious cracks (see Fig. 2d).
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Fig. 4 UV-vis diffused reflectance spectra of different TiO2 layers. The diffused
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reflectance spectrum of FTO glass (solid line).
Fig. 5a-5c show the J-V curves of DSSCs with different TiO2 photoanodes. Fig.
5d shows the picture of the DSSC with TDR multilayer photoanode. The active cell
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area is 100.6 cm2. The D* in Fig. 5c represents D layer with thickness of 9 μm rather than D layer with 6 μm. The details of the photovoltaic characteristics are summarized in Table 2. It can be seen that the PCE are first increased and then decreased in the DSSCs with single T or D layers. The improvement of PCE is due to enhanced light harvesting efficiency by increased dye loading, using thicker TiO2. Since the electrode has large thickness, solar cells made of dye-adsorbed nanostructured TiO2 can 12
drastically increase effective light absorption, resulting in the increased short-circuit current (Jsc) of the DSSCs with single T or D layers [34]. However, thick film may increase charge recombination between injected electrons and holes of the dye molecule arising from low drift mobility of electrons in the film, which limits the PCE [35]. The result is in consistent with the decrease of fill factor (FF) when the thickness
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is increased from 2T (or 2D, ~12 μm) to 3T (or 3D, ~18 μm). This also suggests that the optimal thickness of the cells with TiO2 single layer is in the range of 12 to 18 μm. The efficiencies of DSSCs with D layers are higher than those of DSSCs with same
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thickness T layers, indicating that the light-scattering effect is more important than the dye adsorption capacity in enhancing light harvesting. The light harvesting efficiency of T layer is lower than that of D layer though the amount of dye loading of former is
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much larger than that of the latter. Consequently, the 2D cell shows the highest PCE
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of 7.23%. When the first D layer in 2D cell is replaced to the T type and the second D
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layer is increased to 9 μm, the introduction of T layer and enhancement of
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light-scattering in TD* cell slightly improve the efficiency. If the exterior 3 μm of the D* layer is replaced by the R layer, the stronger light-scattering effect of the R layer
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would further improve the light harvesting efficiency and the highest PCE of 7.52% is obtained, which is increased by 9.5% compared to the 2T cell (Fig. 5c). The TDR
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structure consists of three distinct layers of TiO2: a 6 μm-thick transparent layer covered with a 6 μm-thick diffusing layer and a 3 μm-thick reflective layer. The total
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thickness of TDR structure is ~15 μm, which is in the range of the optimal thickness, and the result is also agreed with the optimum structure of photoanode for DSSC [36]. In addition, the influence of series resistance (Rs) on PCE also plays an important role.
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High-efficiency cells usually have small series resistance among the DSSCs with same active area. The Rs of the solar cells with T layers are larger than those of the cells with D layers. Electron transport in small particles occurs by a series of hopping events between trap states on neighboring particles [37, 38]. Therefore, smaller particles in T layer would decay the transportation of the injected electrons, resulting 13
in a large Rs. We believe that PCE could be further improved through proper control
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of the ratio of the T layer to the D layer in the first 12 μm TiO2 photoanode.
Fig. 5 (a)-(c) J-V curves of the DSSCs with TiO2 single layer, bilayer or multilayer
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photoanode.
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photoanode under one sun illumination. (d) Picture of the DSSC with TDR multilayer
Table 2 Summary of performance of the DSSCs fabricated with TiO2 single layer,
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bilayer or multilayers Cell
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
Rs (Ω)
T
0.750
10.12
69.11
5.24
0.107
2T
0.722
13.47
70.65
6.87
0.088
3T
0.710
13.86
66.51
6.55
0.099
D
0.758
11.57
72.09
6.33
0.092
2D
0.734
14.02
70.24
7.23
0.089
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3D
0.720
14.22
68.35
7.00
0.093
TD*
0.740
14.51
67.85
7.29
0.094
TDR
0.732
15.17
67.75
7.52
0.083
The IPCE spectra of the DSSCs are shown in Fig. 6a. It can be found that the DSSC with TDR photoanode exhibits the highest IPCE values. The light-scattering
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effect is further compared by using normalized IPCE (n-IPCE) spectra, calculated by dividing the IPCE results with the maximum IPCE value at 550 nm (Fig. 6b). In these DSSCs, the enhanced IPCE below 500 nm are due to the increased dye loading of
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photoanodes, whereas that above 575 nm result from reflection of unabsorbed light
back into the dye molecules (which is the light-scattering effect) [31]. The solar cell
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without any T layer (2D cell) shows the smallest n-IPCE below 500 nm, while the solar cell with the R layer showed the highest n-IPCE above 575 nm. The result is in
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consistent with the absorption spectra of the corresponding photoanodes. The TDR
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multilayers show higher absorbance than the TD bilayer in the absorption band
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between 550 nm to 750 nm. Meanwhile, the large particles in the D layer block the short-wavelength light to irradiate the dye molecules, resulting in the reduction
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n-IPCE below 500 nm in the 2D cell.
Fig. 6 (a) IPCE and (b) normalized IPCE spectra of the DSSCs fabricated with 2T, 2D TD* bilayer or TDR multilayers. 15
4. Conclusion The light-scattering effect in large-area DSSCs that contain the D or R layer with different intensity of light scattering was investigated. The result shows that the light-scattering effect is more important than the dye-loading capacity for the TiO2
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photoanodes with the same thickness. The PCE is improved with the increased light scattering by TiO2 multilayers. The enhanced PCE is caused by better light harvesting of the red part of the solar spectrum, which is due to the diffusing and reflection of
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unabsorbed light back into the dye molecules. The optimum condition is the TDR
structure containing both high dye adsorption layer and high light-scattering layer.
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Acknowledgements
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The authors gratefully acknowledge the financial support of this study from the
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National High Technology Research and Development Program of China (863
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Program) (No. 2014AA052002), Science and Technology Service Network Initiative (No. KFJ-SW-STS-152), and the Science and Technology Commission of Shanghai
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Municipality (No. 15DZ2281200).
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