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A unique, hierarchically structured, aggregated TiO2 nanowire (A-TiO2-nw) is prepared by solvothermal synthesis and used as a dual-functioning photoelectrode in dye-sensitized solar cells (DSSCs). The A-TiO2-nw shows improved light scattering compared to conventional TiO2 nanoparticles (TiO2-np) and dramatically enhanced dye adsorption compared to conventional scattering particles (CSP). The A-TiO2-nw is used as a scattering layer for bilayer photoelectrodes (TiO2-np/A-TiO2-nw) in DSSCs to compare the cell performance to that of devices using state-of-the-art photoelectrode architectures (TiO2-np/CSP). The DSSCs fabricated using bilayers of TiO2-np/A-TiO2-nw show improved power conversion efficiency (9.1%) and current density (14.88 mA cm−2) compared to those using single-layer TiO2-np (7.6% and 11.84 mA cm−2) or TiO2-np/CSP bilayer structures (8.7% and 13.81 mA cm−2). The unique contribution of the A-TiO2-nw layers to the device performance is confirmed by studying the incident photon-to-current efficiency. The enhanced external quantum efficiencies at approximately 520 nm and 650 nm clearly reveal the dual functionality of A-TiO2-nw. These unique properties of A-TiO2-nw may be applied in other devices utilizing light-scattering n-type semiconductor.
© 2015 Optical Society of America
1. Introduction
The dye-sensitized solar cell (DSSC) is a potential candidate for next-generation photovoltaic devices owing to its low-cost fabrication, light absorption tenability [1], and relatively high efficiency [2], A high efficiency of 12.3% was reported recently for the cell type, which may allow its commercialization [2], Typical DSSCs consist of dye-adsorbed n-type semiconductor-based photoelectrodes (often composed of TiO2 nanoparticles), electrolytes containing redox couples, and electrocatalytic counter electrodes. The structure of the photoelectrode is essential in the absorption of light-absorbing sensitizers and the collection of electrons from incident photons. To fabricate high-efficiency DSSCs, photoelectrodes with structures containing large active areas for enhanced dye absorption, light-trapping capabilities deep within their layers, or both are beneficial [3].
Common materials for photoelectrodes include >10-μm-thick layers of approximately 20-nm-diameter anatase-phased TiO2 nanoparticles, because they provide large active areas for dye adsorption within the electron diffusion limits. However, these nanoscale particles are transparent over many wavelengths that the dyes absorb, thus hampering the efficient utilization of incident light. For this reason, the application of structures that efficiently scatter incident light to prolong its time within photoelectrodes has been suggested. The employment of plasmonic materials [4], large void structures [5], and large-sized TiO2 particles in or over transparent TiO2 layers [6] have been investigated. However, the incorporation of larger particles as scattering centers often reduces the area for dye adsorption, thus requiring the judicious design of photoelectrode structuring. Bilayers, consisting of a transparent layer and a scattering layer, have been shown to be effective, permitting predominant absorption in the transparent layer while the scattering layer effectively scatters photons with lower dye adsorption ability [6].
Considering the surface area losses by scattering layers based on larger-size particles, researchers have worked to further enhance dye adsorption while maintaining light scattering. Huang et al. suggested the replacement of large TiO2 scattering particles with a mesoporous hierarchical TiO2 structure, which would improve the power conversion efficiency (PCE) by enhancing dye adsorption [7]. The PCE of the resulting DSSCs reached 8.84%, exceeding that of DSSCs using large TiO2 scattering particles (7.87%) [7]. Sun et al. demonstrated similar logic using a mesoporous TiO2 layer, achieving a PCE of 8.25% [8]. The use of 1D structures, such as TiO2 nanorods and nanowires, as scattering layers has also been reported [9]. 1D TiO2 structures, 20‒80 nm in diameter and 200‒400 nm in length, have been tested as scattering materials. To further characterize diverse structures of TiO2 as DSSC photoelectrodes, herein we demonstrate the use of unique aggregated TiO2 nanowires (A-TiO2-nw) as scattering materials.
In this work, we prepared A-TiO2-nw and used these structures as dual-function photoelectrodes. The A-TiO2-nw possessed a hierarchical nanostructure, in which wires of several hundred nanometers in length and tens of nanometers in diameter were entangled or bundled with one another. This unique hierarchical nanostructure offered a dual function, simultaneously adsorbing dye efficiently and scattering light. The anatase-phased A-TiO2-nw was prepared in a one-step solvothermal synthesis followed by thermal annealing. The dye-uptake properties, light-scattering properties, and performance as a scattering layer in DSSCs were investigated. These were compared to those of conventional TiO2 nanoparticles (TiO2-np) and commercial scattering particles (CSP). The A-TiO2-nw displayed light-scattering ability comparable to CSPs with enhanced dye adsorption. By introducing the A-TiO2-nw as a scattering layer, the performance of the DSSCs improved dramatically compared to that of TiO2-np-only devices and conventional bilayer (TiO2-np/CSP) devices. The TiO2-np/A-TiO2-nw bilayer-based DSSCs showed a PCE of 9.1%, with an open-circuit voltage (VOC) of 0.83 V, short-circuit current (JSC) of 14.88 mA cm−2, and fill factor (FF) of 0.74. In contrast, the PCEs of TiO2-np-only devices and TiO2-np/CSP bilayer devices were 8.2% and 8.7%, respectively.
2. Results and discussion
Figure 1(a) shows a schematic for the preparation of A-TiO2-nw. The conventional one-step solvothermal synthesis of TiO2 yielded the A-TiO2-nw under particular conditions. These detailed synthetic conditions are described in the experimental section. The nanoscale bent-wire structure of TiO2 was obtained after the solvothermal synthesis. The scanning electron microscope (SEM) images in Fig. 1(b) revealed the entangled morphology of the as-synthesized TiO2 nanowires; individual wires have diameters of 20–50 nm and lengths varying from hundreds of nanometers to micrometers. After annealing, the A-TiO2-nw is shown to have a bundled structure. Notably, both the morphology and size of our A-TiO2-nw are different from those of frequently used 1D scattering materials, such as TiO2 nanowires and nanotubes, synthesized by anodizing methods and presented as individual objects [10,11]. The bundled structure of the A-TiO2-nw is expected to enhance the scattering of incident light, as efficient light back-scattering by bundled nanowires has been demonstrated in previous reports [12]. When the incident light is oriented perpendicular to the nanowire’s axis, the scattering induced by the entangled nanowires’ bent structures is much more effective than that induced by the diameter or length of a single nanowire. Furthermore, bundling can also prolong photons’ movements within the scattering layers [5]. Considering this effect, the photoelectrode layers consisting of our A-TiO2-nw can be expected to offer efficient light scattering. The anatase-phase dominancy of A-TiO2-nw is confirmed by X-Ray diffraction (XRD) characterization data (Fig. 1(d)); this phase favors electron transport over others [13]. The use of A-TiO2-nw as the scattering layer in DSSCs will be discussed in the following sections.
Fig. 1 (a) Scheme of synthesis route of TiO2 nanowires. SEM images of nanowires (b) after drying and (c) after annealing process. (d) XRD pattern of TiO2 nanowires (*might come from the trace impurity of brookite). Inset: TEM image.
To clearly investigate the dual functions of A-TiO2-nw, we fabricated four types of DSSCs containing various photoelectrodes, as presented in Fig. 2. Type-1 devices utilize single-layer TiO2-np as a scattering layer, with a thickness of approximately 11 μm and individual particle diameters of approximately 20 nm. Type-2 devices use a TiO2-np layer with thickness of 18 μm. Type-3 devices contain bilayer TiO2-np/CSP with the same 11 μm/7 μm thicknesses. The average particle diameters of the CSP was ~500 nm, as indicated in Fig. 3. Type-4 devices utilized a bilayer of TiO2-np/A-TiO2-nw, again with 11 μm/7 μm thicknesses. The 11 μm/7 μm dimensions of the bilayers were chosen based on the optimum performance results of Type-3 devices, which represent current state-of-the-art device architecture.
Fig. 2 (a) The architecture of the four types of photoelectrodes used in this study (b) reflectance data of various photoelectrodes used in this experiment (c) current density-voltage (J-V) characteristics and (d) incident photon-to-current efficiency (IPCE) results of DSSCs fabricated using various types of photoelectrodes. The layer thickness of the bottom TiO2-np layer was 11 μm.
The photocurrent density-voltage (J-V) characteristics of the four types of DSSCs are shown in Fig. 2(c), with resulting cell parameters summarized in Table 1. The PCE of DSSCs using the single-layer TiO2-np-based photoelectrodes (Type-1, 7.6%) is enhanced by using additional layers of TiO2 (Type-2, 3, and 4). Adding one layer of TiO2–np (Type-2) increases the JSC value to 13.90 mA cm−1, because of the enhanced amount of dye adsorbed by the photoelectrodes (2.71 × 10−7 mol cm−1, Table 1). However, the VOC and FF decrease because of the longer charge-traversing distance. Type-3 devices display improved PCEs compared to Type-2 devices. The JSC value of the Type-3 devices, 13.81 mA cm−1, is comparable to that of Type-2 devices, although the adsorbed dye volume is much less than that in Type-2 devices (Table 1). The additional light scattering by the CSP in Type-3 devices is attributed to the high current density, even with the reduced amount of adsorbed dyes.
Table 1. Summary of J-V analysis results in Fig. 2(c) and dye uptake of the four types of photoelectrodes.
To further elucidate the origin of photocurrent in the DSSCs, analysis of the incident photon-to-current efficiency (IPCE) was performed. The external quantum efficiency (EQE) spectra of all device types over the range of visible light are shown in Fig. 2(d). The EQE spectra reveal the origin of photocurrent to be a function of the photons absorbed at different wavelengths [14]. As depicted in Fig. 2(d), the EQE spectrum of each device type shows a different pattern. The Type-3 and Type-4 devices exhibit a shoulder at approximately 650 nm. The EQE values at this wavelength for Type-1, 2, and 3 devices are approximately 26%, 38%, and 48%, respectively. On the other hand, the EQE values at approximately 520 nm for Type-1, 2, and 3 devices are 67%, 75%, and 68%, respectively. The higher EQE values of Type-2 at the shorter wavelength stem from its enhanced dye adsorption (Table 1), because the N719 dye used in this study has strong absorption at 520 nm (Fig. 3(b)) [15]. However, the improved EQE of Type-3 device at approximately 650 nm, where N719 is weak in absorption, must not result from enhanced dye adsorption but instead from improved light scattering. The diffuse reflectance of the photoelectrodes in Fig. 2(b) confirms the enhanced light scattering of Type-3 devices throughout the visible spectrum. This accounts for the improved EQE at 650 nm, resulting in the enhanced JSC in Type-3 devices. Among the four types of devices, Type-4 exhibits the highest PCE of 9.1% with outstanding current density. The 14.88 mA cm−1 JSC value of the Type-4 devices exceeds that of Type-1 and Type 3 by approximately 26% and 8%, respectively (Table 1). Notably, the dye uptake value of Type-4 devices is higher than that of Type-3 but lower than that in Type-2.
Fig. 2(d) shows that the EQE spectrum of the Type-4 devices shows the highest values throughout the visible wavelength range among the spectra of all devices. The EQE values of the Type-4 devices exceed those of Type-3 at 520 nm (74%), indicating enhanced dye adsorption, and also exceed those of Type-2 at 650 nm (48%), indicating improved light scattering. This is attributed to the dual functionality of A-TiO2-nw, where sufficient dye adsorption and light scattering occur simultaneously.
The change in dye uptake by additional layering is summarized in Table 1. The uptake increases by approximately 4% with the addition of a 4-μm CSP layer, and by approximately 12% with an A-TiO2-nw layer of identical thickness. The reflectance values of Type-4 are comparable to those of Type-3, indicating improved light scattering by the addition of A-TiO2-nw to match the efficiency of CSP (Fig. 2(b)). The dye uptake and reflectance results in Fig. 2(b) and Table 1 confirm the dual function of our A-TiO2-nw as both adsorption and scattering layers, which achieves a significantly enhanced EQE in the DSSC.
To further examine the dual-function effects of A-TiO2-nw, we also fabricated four types of DSSCs containing double layers of different thickness combinations. Figure 4 shows the J-V characteristics of the four types of DSSCs containing thinner bottom TiO2-np layers. The PCE and JSC value of the Type-3 devices are much lower than those of Type-4 devices. This is because the A-TiO2-nw can adsorb sufficient dyes while scattering light, whereas the CSP can only scatter light with negligible contribution to the dye adsorption. The JSC values of Type-2 and Type-4 are comparable, indicating that the dye adsorption by A-TiO2-nw is sufficient to offer the highest current density among the cell types. Notably, the PCE values of the two Type-4 devices containing TiO2-np/A-TiO2-nw with thicknesses of either 11 μm/7 μm or 5 μm/13 μm are similar, which confirms the unique dual-function properties of A-TiO2-nw. The cell parameters were summarized in Table 2.
Fig. 4 Current density-voltage (J-V) characteristics of DSSCs fabricated using various types of photoelectrodes. The layer thickness of the bottom TiO2-np layer was 5 μm.
Table 2. Summary of J-V results in Fig. 4.
3. Experimental
3.1 Preparation of A-TiO2-nw
A-TiO2-nw was synthesized using a one-step solvothermal process (Fig. 1(a)). Briefly, 0.2 g lithium acetate dihydrate (Sigma Aldrich) was dissolved in 10 mL mixed solvent containing N,N dimethylformamide (Alfa Aesar, 99%) and acetic acid (Merck, 100%). After the addition of 2 mL titanium(IV) butoxide (Fluka, ≥97%), the solution was transferred into a Teflon-lined stainless steel autoclave and heated in an oven at 180 °C for 20 h. The 1D TiO2 nanowires were collected and washed thoroughly with ethanol several times and dried in an oven at 60 °C overnight. The as-synthesized TiO2 nanowires were further annealed in ambient air at 350 °C for 2 hours to yield the A-TiO2-nw structure.
3.2 Photoelectrode preparation
For the preparation of A-TiO2-nw paste, 0.75 g A-TiO2-nw was mixed with 0.225 g ethyl cellulose, 0.075 g lauric acid (Sigma Aldrich, ≥98%), and 4.5 g α-terpineol (Sigma Aldrich, ≥96%). Then, 2 mL ethanol was added to adjust the viscosity of the paste. The mixture was then sonicated and stirred for 5 h. To produce type 4 bilayer DSSC photoelectrodes (Fig. 2(a)), commercial TiO2-np paste (ENB Korea, 20 nm particle size) was used for the bottom layers. The TiO2-np paste was deposited onto a clean FTO/glass substrate by the doctor blade technique, followed by heating at 125 °C on a hot plate for 10 min. For the top layers, A-TiO2-nw paste was deposited onto the TiO2-np layer. After the deposition, the bilayer photoelectrodes were reheated at 125 °C for 5 min followed by thermal annealing at 500 °C for 1 h. The photoelectrodes were imersed into a dye solution containing 0.3 mM N719 dye (Solaronix) in 1:1 v/v acetonitrile and tert-butyl alcohol for 16 h, followed by rinsing in ethanol and drying in air. For the other types of photoelectrodes (Type-1, 2 and 3), the bottom layers were fabricated similarly, and the top layers were prepared by similar methods using the corresponding materials (TiO2-np or CSP). The CSP paste was purchased from ENB Korea with an average particle size of 500 nm.
3.2 DSSC fabrication
The dye-sensitized photoelectrodes were assembled together with the counter electrodes and fabricated by the thermal deposition of H2PtCl6 on FTO glass (annealed at 400 °C for 20 min) using thermal adhesive films (Surlyn, Dupont 1702, 25 µm thickness) to produce a sandwich-cell configuration. The organic electrolyte, containing 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.1 M guanidium thiocyanate (GuSCN), 0.03 M I2, and 0.5 M 4-tert-butylpyridine in acetonitrile/valeronitrile (85:15, v/v), was introduced into the cell via a drilled hole. Finally, the hole was sealed with thermal adhesive films and a cover glass.
3.3 Characterization
The structural phase compositions were obtained through X-ray diffraction (XRD, Bruker) analysis with Cu Kα radiation (λ = 0.15418 nm). Scanning electron microscopy (SEM) measurements were conducted on a LEO-1530 microscope (Germany). The current density–voltage (J–V) characteristics of the DSSCs were extracted under 1 sun illumination, in which the light intensity was adjusted using a Si solar cell equipped with a BK-5 filter to approximate AM 1.5G 100 mW cm−2 light radiation intensity, using a Newport (USA) solar simulator (450 W Xe source) and a Keithley 2400 source meter. The active area of the DSSCs was set to be 0.175 cm2 using a black metal mask. The EQE was measured as a function of incident wavelength from 300 nm to 800 nm using an EQE system specially designed for DSSCs (PV Measurement, Inc). A 75 W Xe lamp was used to generate a monochromatic beam. EQE values were collected at a low chopping speed of 4 Hz. The amount of dye adsorbed was determined by measuring the dye desorption after the immersion of the dye-adsorbed TiO2 film into a solution of 0.1 M NaOH in a solvent containing equal parts water and ethanol by volume. The concentration of the desorbed dye was analyzed using a UV-Vis spectrophotometer (Scinco, S-3100). The reflectance of the photoelectrodes was determined by measuring the reflectance of the double-layer-structured photoelectrodes consisted of TiO2 nanoparticle layers and TiO2 1D nanowire or CSP, in which the incident light was introduced from the FTO side.
4. Conclusions
The uniquely structured A-TiO2-nw was prepared by a one-step solvothermal synthesis followed by thermal annealing. The A- TiO2-nw displayed dual functions as a simultaneous light absorption layer and scattering layer. The A-TiO2-nw showed dramatically improved light-scattering properties compared to conventional TiO2-np and far enhanced dye absorption compared to CSP. The DSSCs using A-TiO2-nw as a scattering layer displayed a higher PCE (9.1%) than the devices using state-of-the-art CSP as a scattering layer (8.7%) because of the improved dye adsorption and light scattering. The photon-to-current efficiency studies clearly demonstrated the effects of the dual-function photoelectrode. The performance of DSSCs using the bilayer photoelectrodes of different thicknesses also confirmed the unique behaviour of A-TiO2-nw. These unique properties of A-TiO2-nw may be applied in other devices requiring light-scattering n-type semiconductors.
Acknowledgements
The authors gratefully acknowledge support from the Basic Science Research Program through the National Research Foundation of Korea (NRF, 2012045675), NRF by the Korea government (MSIP (Ministry of Science, ICT&Future Planning)) (No. 2011-0017449), New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20133030000210), and Global Scholarship Program for Foreign Graduate Students at Kookmin University in Korea.
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