Article pubs.acs.org/Langmuir
Multi-Functionality of Macroporous TiO2 Spheres in Dye-Sensitized and Hybrid Heterojunction Solar Cells Ganapathy Veerappan,† Dae-Woong Jung,‡ Jeong Kwon,‡ Jeong Mo Choi,† Nansra Heo,‡ Gi-Ra Yi,*,‡ and Jong Hyeok Park*,†,‡ †
SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Republic of Korea Department of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
‡
S Supporting Information *
ABSTRACT: Micron-sized macroporous TiO2 spheres (MAC-TiO2) were synthesized using a colloidal templating process inside emulsions, which were then coated on a nanocrystalline TiO2 light absorption film to prepare a bilayered photoanode for liquid-based dye-sensitized solar cells (DSSC) and hybrid heterojunction solid-state solar cells. MAC-TiO2 layers can enhance light scattering as well as absorption, because their pore size and periodicity are comparable to light wavelength for unique multiple scattering and a porous surface can load dye more. Moreover, due to the bicontinuous nature of macropores and TiO2 walls, electrolyte could be transported much faster in between the TiO2 spheres rather than within the small TiO2 nonporous architectures. Electron transport was also facilitated along the interconnected TiO2 walls. In DSSCs with these MAC-TiO2 scattering layers, efficiency was higher than conventional DSSCs incorporating a commercial scattering layer. The unique geometry of MAC-TiO2 results in strong improvements in light scattering and infiltration of hole-transporting materials, thereby the MAC-TiO2-based solid-state device showed comparatively higher efficiency than the device with conventional nanocrystalline TiO2.
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INTRODUCTION
could further improve the cell efficiency, though this has not yet been thoroughly or intensively explored.13,14 Recently, the use of mesoporous TiO2 microspheres for photoelectrodes in DSSC has been reported,12,16,18−20 as dye can also be loaded inside mesopores. However, the fabrication of these microspheres requires multiple complicated steps and long processing times, which must be performed at high temperatures. Therefore, production yield is relatively low, which has been a major barrier to practical application and further development.11−15,17−19 Furthermore, nanocrystalline TiO2 are less effective in dye or quantum dot sensitized solidstate solar cells due to the disconnected pore structures for hole-transporting material (HTM) penetration.21,22 In this study, we report that films of macroporous TiO2 spheres (MAC-TiO2) can be used as photoelectrodes in liquid DSSCs and perovskite-sensitized all-solid-state mesoscopic heterojunction solar cells. Porous spheres were prepared in bulk using a process of colloidal templating in emulsions and were then bar-coated on substrates with conventional binder. Due to many macropores and the high specific surface area of individual MAC-TiO2 spheres, the photoelectrodes of MACTiO2 films could represent strong light scattering ability and good dye loading ability, respectively. In viewpoint of light
Over the past decade, silicon-based solar cells have dominated photovoltaics both in commercial markets and academic research. However, high fabrication costs and vacuum processing have led to the development of alternative lowcost photovoltaic devices such as dye-sensitized solar cells (DSSCs), organic solar cells, and others.1−3 In particular, DSSCs can be fabricated easily at low cost, and their efficiency has continued to improve.4−10 A key issue in improving cell performance is the structural design of the photoelectrode, which requires a high surface area, fast electron transport, and strong light scattering. However, it is very difficult to realize such performance using single-component materials.11,12 Particulate films of nanocrystalline TiO2 (Nano-TiO2) have been widely used as the photoelectrode material in DSSCs. However, the scattering intensity of light is relatively weak. Therefore, the integration of a light-scattering layer in the photoelectrode may improve cell efficiency. Indeed, scattering layers of submicrometer-sized particles have been introduced in electrodes in which back-scattered light enhanced the cell efficiency of DSSCs.11−19 However, since the surface areas of these electrodes are not much higher than those of conventional electrodes and since the pores are not well interconnected, relatively less dye is loaded, and charges may be easily recombined, thus limiting solar cell performance. Therefore, the introduction of pores in light-scattering particles © 2014 American Chemical Society
Received: December 18, 2013 Revised: February 25, 2014 Published: February 26, 2014 3010
dx.doi.org/10.1021/la404841h | Langmuir 2014, 30, 3010−3018
Langmuir
Article
Figure 1. (a) Schematic illustration of the macroporous TiO2 sphere synthesis procedures. (b) Schematic of electron transfer and light reflection in a commercial TiO2 scattering layer and (c) a macroporous TiO2 scattering layer. Preparation of TiO2 Paste and Device Fabrication. Fluorinedoped tin oxide (FTO) glass substrates were cleaned with a detergent solution, deionized (DI) water, ethanol, and acetone and were then dried using nitrogen gas. Doctor blading of the MAC-TiO2 paste was performed by mixing ethyl cellulose and α-terpineol with the MACTiO2 particles. The remaining solvent was then evaporated using a rotary evaporator to obtain a highly viscous paste.25 Different TiO2 electrodes were prepared in this work, and all the electrodes were prepared using the doctor blade method. First, a transparent ∼11 μm film of Nano-TiO2 (Figure S2b, Supporting Information) was prepared on the previously cleaned FTO substrates and was sintered at 500 °C for 30 min, and then the second and third electrodes, composed of commercial 400-nm TiO2 particles (EnB Korea, 400 nm) and MAC-TiO2, were deposited on top of the nanocrystalline layer in order to study the light-scattering effect. Again, all samples were sintered at 500 °C for 30 min to remove the organic binder used in the paste preparation. Once cooled to 80 °C, the sintered electrodes were immediately immersed in a 0.3 mM solution of N719 dye in a mixture of acetonitrile/tert-butyl alcohol for 24 h. Simultaneously, Pt counter electrodes were prepared on a FTO substrate by drop casting a H2PtCl6 solution, followed by sintering at 400 °C for 15 min. Sensitized TiO2 electrodes were removed from the dye solution, rinsed in ethanol, and dried using a nitrogen stream. Liquid electrolyte was added to the sandwiched cells through holes made in the counter electrodes. The electrolyte was composed of 0.5 M 1-butyl-3methylimidazolium iodide (BMII), 0.06 M iodine (I2), and 0.5 M tert-butylpyridine (tBP) in acetonitrile solvent.26 To fabricate solidstate solar cells, a dense TiO2 blocking layer was coated on the etched and cleaned FTO substrate with 20 mM titanium diisopropoxide bis(acetylacetonate) in ethanol by the spin-coating method. The above procedure was repeated several times to obtain the optimum thickness, and then the films were sintered at 500 °C for 30 min. Next, MACTiO2 and Nano-TiO2 pastes were further diluted with ethyl cellulose and teripneol to get a low-viscosity paste, which gives 700-nm-thick TiO2 films on top of the dense TiO2 blocking layer after thermal calcination. The sintered TiO2 electrodes were immersed in a 20 mM aqueous TiCl4 solution at 70 °C for 30 min and then sintered again at 500 °C for 30 min. Perovskite precursor (CH3NH3PbI3) was synthesized from the already reported literature.27−30 The prepared TiO2 electrodes were spin-coated with the perovskite precursor solution, followed by drying at 100 °C for 15 min. The HTM was
scattering, MAC-TiO2 has several scattering centers due to the unique MAC-TiO2 nanostructure. First, macropores with a diameter of several hundred nanometers presented in the MAC-TiO2 can lead to multiple light scattering, which is similar to conventional light-scattering materials. In addition, the individual spheres can also act as another light-scattering center. Furthermore, their three-dimensional interconnectivity could be beneficial in terms of fast electrolyte diffusion, and they therefore may be suitable for use in highly viscous electrolyte or solid HTM.11−14,18,19,23 As a result, liquid DSSCs made with MAC-TiO2 yielded an 8.1% power conversion efficiency (η), which is superior to those of the standard Nano-TiO2 (7.2%) and a commercial scattering layer electrode (7.8%). The optimized bilayer structure (Nano-TiO2/MAC-TiO2 electrode) exhibited an overall conversion efficiency of 9.6%. Additionally, MAC-TiO2 was also utilized for perovskite-sensitized solid-state solar cells and yielded promising power conversion efficiency, incidentally, which is also showing the superior property of MAC-TiO2 over Nano-TiO2.
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EXPERIMENTAL SECTION
Preparation of Macroporous TiO2 Spheres. A synthetic scheme for the macroporous TiO2 spheres (MAC-TiO2) is shown in Figure1a.24 Cross-linked polystyrene (PS) particles (300 nm, 2 g) were dispersed in 23 mL of toluene, and 2 mL of titanium(IV) butoxide (97%, Aldrich) was then added to the 23 mL PS suspension in toluene (10 wt %). The toluene with PS and titanium(IV) butoxide (20 mL) was emulsified in formamide (50 mL, 99.5%, Aldrich) with 0.567 g of a triblock copolymer (EO20PO70EO20, P123, Mw = 5800 g/ mol, Aldrich) using a homogenizer (IKA) for 1 min at 10 000 rpm. Subsequent heat treatment at 80 °C for 24 h selectively removed toluene, resulting in composite particles of PS and titania. The emulsion was stirred at low rpm (
