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Nanostructured TiO2 films with double-layered structure are prepared by a facile one-step soaking method. We have investigated the morphology of nanostructured TiO2 films by the reaction time of the soaking method, which has an effect on the thickness and layered structure of the TiO2 films. The TiO2 films prepared by this method have a unique double-layered structure, which is composed of a dense TiO2 bottom layer and TiO2 particulates on the bottom layer. By manipulating the reaction time of the soaking method, control of TiO2 particulate formation on the surface of the dense TiO2 bottom layer is possible. The double-layered structure of nanostructured TiO2 films is effective for achieving sufficient adsorption of Sb2S3 sensitizer and light scattering effect of photoelectrodes for inorganic sensitized solar cells, which induces the enhancement of short circuit current of solar cell devices. Our solar cell device, using a double-layered TiO2 film with particulate structure as a photoelectrode, exhibited JSC, VOC, FF, and η values of 12.94 mA/cm2, 498 mV, 57.0%, and 3.67%, respectively.
© 2014 Optical Society of America
Introduction
TiO2 is a very attractive and important material for photoelectrodes of dye-sensitized solar cells (DSSC) and is also widely applied to inorganic-sensitized solar cells (ISSC) because TiO2 possesses a wide band gap and relatively high chemical stability. One of the research strategies of TiO2 photoelectrodes for high efficiency solar cells focuses on searching for novel nanostructured TiO2 photoelectrodes with large surface area and reduced light loss by transmittance of light. The morphology of nanostructured TiO2 films is closely related to the surface area and light scattering characteristics of TiO2 photoelectrodes, which determines the photovoltaic properties of sensitized solar cells. Therefore, tailoring of the TiO2 nanostructure is a crucial aspect of improving the performance of sensitized solar cells. Methods for preparing the TiO2 photoelectrodes with various nanostructures are widely studied, such as nanoparticles, nanotubes, nanorods, nanoflowers, porous spheres, and hollow spheres [1–5]. Among these various nanostructures, the spherical structure of TiO2 films is generally used for light harvesting because of its large surface area and high surface permeability [6]. A large surface area is essential for TiO2 photoelectrodes to load large amounts of organic or inorganic sensitizer molecules that will generate electrons by absorbing sunlight [7]. In addition, TiO2 spheres are well known to be effective for obtaining light scattering effects for absorbed sunlight [8–10]. Therefore, most sensitized solar cell devices are normally based on nanoparticle-based TiO2 photoelectrodes.
Recently, research on ISSC is being actively studied by many researchers to overcome the drawbacks of conventional DSSC. Especially, interest in all-solid-state ISSC devices has greatly increased in recent years to avoid using liquid components in solar cell devices because of leakage problems [11–13]. The use of alternative sensitizers to dyes, such as inorganic semiconductors, is also one of the attractive topics in these technologies. Typically, inorganic semiconductors such as CdSe, CdS, Cu2-XS, In2S3, or Sb2S3 that are based on the II-VI or V-VI families are used as sensitizers for solar cells [14,15]. Among these materials, the Sb2S3 semiconductor is an excellent material due to its high absorption coefficient (α = 1.8 × 105 cm−1 at 450 nm) and energy band gap of 1.7 ~1.9 eV [15,16]. However, in the case of ISSC, the recombination reaction of electrons between the hole transport material and substrate is rapid and thus the main loss of efficiency comes from the charge recombination [17]. Therefore an additional blocking layer, such as a compact TiO2 thin film under 100 nm, must be placed between the mesoporous TiO2 photoelectrode and the glass substrate [18].
In this study, novel double-layered nanostructured TiO2 films were applied to the photoelectrode for ISSC based on the inorganic semiconductor Sb2S3. Double-layered TiO2 films composed of a dense TiO2 bottom layer and a particulate TiO2 top layer were easily fabricated by a simple one-step soaking method. The dense TiO2 bottom layer plays the role of the blocking layer and the particulate TiO2 top layer takes charge of the large surface area and light scattering effect of photoelectrodes. We investigated the growth behavior of double-layered TiO2 films based on the reaction time of the soaking method, which was closely related to the thickness and morphology of the TiO2 films. To determine the effect of the nanostructure of double-layered TiO2 films on the photovoltaic characteristics of ISSC, solar cell devices were fabricated by deposition of Sb2S3 on the TiO2 photoelectrode using atomic layer deposition (ALD) and device characteristics were investigated.
2. Experimental details
2.1 Preparation of double-layered TiO2 films
TiO2 films were deposited onto fluorine-doped SnO2 (FTO, 7 ohm/square) transparent conducting glass substrates. The substrates were ultrasonically cleaned with deionized (DI) water and acetone for 10 min each, followed by rinsing with isopropyl alcohol for 10 min. The substrates were then dried with nitrogen gas before the deposition process. A precursor aqueous solution for the TiO2 films was prepared by dissolving 0.3 M titanium tetrachloride (TiCl4) in DI water. For the deposition of TiO2 onto the FTO substrate, the substrate was soaked in the precursor solution at 70 °C statically without stirring. To investigate the growth rate of TiO2 films by the soaking method, several reaction times were used: 30, 60, 90, and 120 min. After the deposition process, the substrates were rinsed with DI water and dried with nitrogen gas. The annealing process of the TiO2 films was conducted at 450 °C for 30 min in an air furnace. To maximize the surface area of TiO2, the prepared TiO2 films were dipped in 0.5 M TiCl4 aqueous solution with stirring at 70 °C for 60 min. The last heating treatment of the TiO2 films proceeded at 500 °C for 15 min in an air furnace.
2.2 Fabrication of solar cells
Double-layered TiO2 films were used as the photoelectrode of solar cells. These films were approximately 1 μm thickness and were deposited by the soaking method on FTO (7 ohm) glass substrates. The atomic layer deposition (ALD) of amorphous antimony sulfide (Sb2S3) to form an inorganic sensitizer was performed at 120 °C from Sb(NMe3) and H2S gas. A 15 mg / ml solution of poly-3-hexylthiophene (P3HT) was prepared in 1,2-dichlorobenzene and deposited by spin coating on TiO2/Sb2S3 at 2500 rpm for 60 s. The TiO2/Sb2S3/P3HT layer was then annealed at 90 °C for 30 min in a vacuum oven. The back contact was finished by thermal evaporation of gold (Au) with a 100 nm thickness as a counter electrode. The evaporation was performed with a metal mask, yielding an active area of 0.05 cm2.
2.3 Characterization of TiO2 films and solar cells
The surface morphology and grain growth of the prepared TiO2 films were investigated by scanning electron microscopy (FE-SEM) (Hitachi, SU8200) and transmission electron microscopy (TEM) (Hitachi, HF-3300) with focused ion beam (FIB) system (Hitachi, NB5000). The crystalline phase of the TiO2 films was analyzed by X-ray diffraction (XRD) (CuKα, 40kV, Panalytical MPD for thin film) with grazing angle of 2° and scan rate of 10°/min. The diffuse reflectance of TiO2 films was measured by UV-vis spectrophotometry (UV-VIS-NIR, CARY5000).
The photovoltaic properties of the solar cells were characterized using a source meter (Keithley, 2400) unit and a solar simulator (Newport, 94022A) to simulate 1.5 AM solar irradiation. External quantum efficiency (EQE) measurements were conducted using an incident photon-to-current efficiency (IPCE) measurement system (McScience, K3100).
3. Results and discussion
SEM was used for the investigation of the surface morphology and nanostructure of TiO2 films prepared from the soaking method. SEM images of the TiO2 films are presented in Fig. 1. Figures 1 (a)-(d) are surface images of the TiO2 films obtained by the soaking method with reaction times of 30, 60, 90, and 120 min, respectively. In the cross-sectional images of TiO2 films, the thickness of TiO2 films was increased with longer reaction times. TiO2 films with 30 and 60 min of soaking show only flat layers, however, 90 and 120 min of soaking resulted in TiO2 films with a double-layered structure, namely, a particulate layer on a flat layer (particulate-flat layer). In the case of the TiO2 film soaked for 120 min, the thickness of the particulate-flat film was approximately 1.38 μm, and the thickness of the individual flat and particulate layers were approximately 500 and 700 nm, respectively.
Fig. 1 Cross-sectional and surface SEM images of TiO2 films deposited by the soaking method with different reaction time. Yellow and green regions in cross-sectional SEM images represent the flat and particulate layer, respectively.
The nanostructure of double-layered TiO2 films deposited by the soaking method is flat at short reaction times such as 30 min and 60 min. At longer reaction times (over 60 min), the films had many particulates on the flat layers. The particulate-flat structure of double-layered TiO2 films seems to be formed by the continuous growth of TiO2 particulates, followed by the formation of flat TiO2 layers. When the reaction time is short, only small grains of TiO2 are grown by the reaction of TiO2 precursors thus, a flat and dense TiO2 layer is formed on the substrate. However, as the reaction time of soaking increases, a dense TiO2 flat layer covers the substrate completely, and some of the TiO2 grains abruptly start to grow into larger TiO2 particulates on the already formed flat TiO2 layer. Nanostructured TiO2 films prepared by the soaking method with sufficient reaction times showed a distinctive double-layered structure with particulate and flat shapes. Unlike the paste-coating method with multiple deposition cycles, double-layered TiO2 films prepared by simple one-step soaking methods can play multiple roles of blocking layer and light scattering layer due to the dense bottom TiO2 layer and many TiO2 particulates on the bottom layer. In addition, TiO2 films with this particulate-flat structure might have larger surface areas than those with a flat structure, which would be advantageous for the improvement of power conversion efficiency.
Figure 2 shows the X-ray diffraction (XRD) pattern of the TiO2 film prepared by the soaking method for 120 min on FTO substrates. As shown in Fig. 2, the peaks of the TiO2 film were located at 2θ = 27.53°, 36.17°, 41.37°, 56.65°, 62.90°, 68.86°, and 69.87°, which correspond to the (110), (011), (111), (121), (002), (130), and (031) crystallographic planes of the TiO2 structure, respectively. These diffraction peaks closely match the values of the standard data (JCPDS No.98-005-3997), which confirms the formation of a rutile TiO2 phase without any impurities [19]. Based on this result, our soaking method is suitable for the preparation of rutile nanostructured TiO2 films.
Fig. 2 XRD graph of TiO2 film deposited on FTO substrate by soaking method with reaction time of 120 min.
To investigate the effect of the nanostructure of double-layered TiO2 films on photovoltaic properties, we have fabricated solar cell devices of FTO/TiO2/Sb2S3/P3HT/Au and measured I-V characteristics using a solar simulator (Table 1), Fig. 3.Because our double-layered TiO2 films have a dense TiO2 layer on the bottom side, we did not deposit a compact TiO2 blocking layer on the FTO substrate. The performance of solar cell devices was closely related to the reaction time of the soaking method. The power conversion efficiency was improved from 1.78 to 3.67% as the reaction time increased. Among the TiO2 films prepared by the soaking method, the solar cell using a TiO2 film with 120 min of soaking showed the best performance, with short-circuit photocurrent density (JSC), open-circuit voltage (VOC), fill factor (FF), and efficiency (η) values of 12.94 mA / cm2, 498 mV, 56.98%, and 3.67%, respectively, under AM 1.5G full sunlight (99.8 mW / cm2). Whereas VOC had similar values regardless of soaking time, JSC was considerably dependent on the reaction time. This difference of JSC would be closely related with the existence of particulate structure on double-layered TiO2 films. In the case of 90 and 120 min, there are many particulates on top of the TiO2 photoelectrode, which might be advantageous for the sufficient adsorption of Sb2S3 sensitizer and light scattering effect of TiO2 photoelectrode.
Table 1. Device characteristics of ISSC fabricated using nanostructured TiO2 films by soaking method under AM 1.5G.
Fig. 3 Photocurrent-voltage (J-V) curve of ISSC fabricated using nanostructured TiO2 films by soaking method under AM 1.5G.
In order to investigate the structure of photoelectrode and solar cell devices in detail, FE-TEM analysis of solar cell devices was performed. Figure 4 shows cross-sectional TEM images of solar cell devices with different soaking time with EDX mapping. In the case of 30 min soaking time, a very thin TiO2 layer (pink color) is deposited on the FTO substrate and Sb2S3 (green color) covers the top of the TiO2 layer. As the soaking time increases to 60 min, thickness of the TiO2 layer increases to about 300 nm. However, for both 30 and 60 min soaking times, there was no particulate on flat TiO2 films. Contrary to 30 and 60 min soaking time, TiO2 films with 90 and 120 min soaking times show double-layered structure of TiO2. Sb2S3 sensitizers are uniformly distributed on not only the surface but also the particulates of TiO2 photoelectrodes. Because Sb2S3 sensitizers are deposited using an ALD process, Sb2S3 sensitizers are uniformly distributed on the double-structured TiO2 films. In the case of particulate-flat TiO2 films with 90 and 120 min soaking time, a large amount of Sb2S3 sensitizers seems to be located in the surface region of TiO2 photoelectrodes compared with 30 and 60 min.
Fig. 4 TEM cross-sectional images of TiO2 films deposited by soaking method with different reaction time with EDX mapping. (pink: Ti, green: Sb)
The particulate structure of double-layered TiO2 films also could contribute to the light scattering effect of photoelectrodes. In order to confirm the enhanced light-scattering of nanostructured TiO2 films prepared by the soaking method, diffuse reflectance spectra were measured (Fig. 5).The reflectance over the entire wavelength range increased with increased TiO2 reaction time for TiO2 prepared by the soaking method. Further observation indicates that the TiO2 film with 120 min soaking time exhibited the highest diffuse reflectance, approximately 30% in the range from 400 to 800 nm wavelength. The diffuse reflectance of TiO2 films with 90 and 120 min of soaking is higher than that of TiO2 films with 30 and 60 min of soaking, indicating that the particulate structure of the nanostructured TiO2 has a higher light-scattering ability than that of flat TiO2 films. In the case of conventional photoelectrodes, 10 ~20 nm-sized TiO2 nanoparticles are used for transparent photoelectrodes, which has a weak light scattering effect. Therefore, large nanoparticles (100 ~400 nm) have been incorporated as light scattering centers to increase the optical length in TiO2 films, and enhanced light harvesting has been demonstrated both experimentally and theoretically [8–10]. Therefore, it is reasonable to infer that TiO2 films with large nanoparticles (100 nm) and those with the particulate-flat structure would have similar light scattering effect [20,21]. The nanostructure of double-layered TiO2 films prepared by the soaking method was easily controlled by the reaction time, and it is expected that the double-layered TiO2 films can contribute to the solar cell performance through the light scattering effect. TiO2 particulates of double-layered TiO2 films are beneficial for enhancing the reflectance of light, and the optical path length of light in the photoelectrode can be effectively improved [22,23].
From the above TEM and diffuse reflectance analysis, it was found that particulate structure of double-layered TiO2 films is beneficial for the sufficient adsorption of Sb2S3 sensitizer and the enhancement of light harvesting of photoelectrodes by light scattering effect. These two factors are closely related with photocurrent generation of solar cell devices. For a more detailed investigation of the effect of the nanostructure of TiO2 films on the photocurrent characteristic of solar cells, we have measured EQE data for the solar cell devices. Figure 6 shows the EQE spectra of solar cells using nanostructured TiO2 films prepared by the soaking method. A solar cell using TiO2 films with 120 min soaking time exhibited the highest EQE of all other solar cells. Overall, EQE increased as soaking time increases and the absolute EQE of 120 min soaking time is higher than the other TiO2 films over the entire wavelength region, which is in good agreement with the higher JSC of 120 min soaking time in Fig. 3. By the way, contrary to EQE at wavelengths over 450 nm, a solar cell using TiO2 film with 120 min soaking time exhibits a similar EQE with 60 and 90 min soaking time in the short wavelength region below 450 nm. The similar EQE at shorter wavelength might be attributed to the similar presence of the Sb2S3 inorganic sensitizer (absorption coefficient = 1.8 × 105 cm−1 at 450 nm). However, a solar cell using the TiO2 film with 30 min soaking time shows incomparably lower EQE even at wavelengths below 450 nm, which might be attributed to the smaller amount of Sb2S3 sensitizers on a TiO2 photoelectrode. The higher IPCE at long wavelength is most likely due to the superior light-scattering effect of double-layered TiO2 films. At wavelengths over 450 nm, solar cells using TiO2 films with 90 and 120 min soaking time show higher EQE than 60 min soaking time, which might be closely related with the particulate structure of double-layered TiO2 films. Light-scattering is effective in the long wavelength region because most light is transmitted through a TiO2 electrode and scattered in this wavelength region [22–24]. This result indicates that the nanostructure of double-layered TiO2 photoelectrode is essential for improving the solar cell performance through photocurrent improvement.
Fig. 6 IPCE of ISSC based on nanostructured ISSC fabricated using nanostructured TiO2 films by soaking method with various reaction time.
4. Conclusion
We have fabricated double-layered TiO2 films using a simple one-step soaking method. Thickness and morphology of double-layered TiO2 films were closely related with the reaction time of soaking method. The nanostructured TiO2 films with sufficient soaking time (over 90 min) show unique double-layered structures, which are composed of dense TiO2 bottom layer and TiO2 particulates on the bottom layer. The dense TiO2 bottom layer could substitute for the existing blocking layer and TiO2 particulates could play an important role of inorganic sensitizer adsorption sites and light scattering effects. To determine the effect of the nanostructure of double-layered TiO2 films on the photovoltaic characteristics, ALD Sb2S3–based solar cell devices using double-layered TiO2 as photoelectrodes were fabricated. The TiO2 particulate was found to be advantageous for securing sufficient adsorption of Sb2S3 sensitizer and enhancement of light harvesting through light scattering effects, which could contribute to the increase of short circuit current of solar cell devices. A solar cell device using a double-layered TiO2 film with 120 min soaking time showed the best result with JSC, VOC, FF, and η values of 12.94 mA / cm2, 498 mV, 57.0%, and 3.67%, respectively. Double-layered TiO2 photoelectrodes prepared by a simple one-step soaking method were found to be useful for the fabrication of high performance ISSC.
Acknowledgments
This work was supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (14-EN-03).
References and links
1. Q. Zhan, W. Li, and S. Liu, “Controlled fabrication of nanosized TiO2 hollow sphere particles via acid catalytic hydrolysis/hydrothermal,” Powder Technol. 212(1), 145–150 (2011). [CrossRef]
2. H. Hu, H. Shen, C. Cui, D. Liang, P. Li, S. Xu, and W. Tang, “Preparation and photoelectrochemical properties of TiO2 hollow spheres embedded TiO2/CdS photoanodes for quantum-dot-sensitized solar cells,” J. Alloy. Comp. 560, 1–5 (2013). [CrossRef]
3. L. Feng, J. Jia, Y. Fang, X. Zhou, and Y. Lin, “TiO2 flowers and spheres for ionic liquid electrolytes based dye-sensitized solar cells,” Electrochim. Acta 87, 629–636 (2013). [CrossRef]
4. D. W. Liu, I. C. Cheng, J. Z. Chen, H. W. Chen, K. C. Ho, and C. C. Chiang, “Enhanced optical absorption of dye-sensitized solar cells with microcavity-embedded TiO2 photoanodes,” Opt. Express 20(S2Suppl 2), A168–A176 (2012). [CrossRef] [PubMed]
5. S. H. Lee, H. Jin, D. Y. Kim, K. Song, S. H. Oh, S. Kim, E. F. Schubert, and J. K. Kim, “Enhanced power conversion efficiency of quantum dot sensitized solar cells with near single-crystalline TiO₂ nanohelixes used as photoanodes,” Opt. Express 22(S3Suppl 3), A867–A879 (2014). [CrossRef] [PubMed]
6. I. G. Yu, Y. J. Kim, H. J. Kim, C. Lee, and W. I. Lee, “Size-dependent light-scattering of nanoporous TiO2 spheres in dye-sensitized solar cells,” J. Mater. Chem. 21(2), 532–538 (2010). [CrossRef]
7. Y. J. Kim, M. H. Lee, H. J. Kim, G. Kim, Y. S. Choi, N. G. Park, K. Kim, and W. I. Lee, “Formation of highly efficient dye-sensitized solar cells by hierachical pore generation with nanoporous TiO2 spheres,” Adv. Mater. 21(36), 1–6 (2009). [CrossRef]
8. L. Zhao, J. Li, Y. Shi, S. Wang, J. Hu, B. Dong, H. Lu, and P. Wang, “Double light scattering layer film based on TiO2 hollow spheres and TiO2 nanosheets: improved efficiency in dye-sensitized solar cells,” J. Alloy. Comp. 575, 168–173 (2013). [CrossRef]
9. B. Tan and Y. Wu, “Dye-sensitized solar cells based on anatase TiO2 nanoparticle/nanowire composites,” J. Phys. Chem. B 110(32), 15932–15938 (2006). [CrossRef] [PubMed]
10. A. Usami, “Theoretical study of application of multiple scattering of light to a dye-sensitized nanocrystalline photoelectrochemical cell,” Chem. Phys. Lett. 277(1-3), 105–108 (1997). [CrossRef]
11. P. P. Boix, Y. H. Lee, F. Fabregat-Santiago, S. H. Im, I. Mora-Sero, J. Bisquert, and S. I. Seok, “From flat to nanostructured photovoltaics: balance between thickness of the absorber and charge screening in sensitized solar cells,” ACS Nano 6(1), 873–880 (2012). [CrossRef] [PubMed]
12. M. Grätzel, U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissörtel, J. Salbeck, and H. Spreitzer, “Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies,” Nature 395(6702), 583–585 (1998). [CrossRef]
13. M. Wang, J. Liu, N.-L. Cevey-Ha, S.-J. Moon, P. Liska, R. Humphry-Baker, J.-E. Moser, C. Grätzel, P. Wang, and S. M. Zakeeruddin, “High efficiency solid-state sensitized heterojunction photovoltaic device,” Nano Today 5(3), 169–174 (2010). [CrossRef]
14. H. Wedemeyer, J. Michels, R. Chmielowski, S. Bourdais, T. Muto, M. Sugiura, G. Dennler, and J. Bachmann, “Nanocrystalline solar cells with an antimony sulfide solid absorber by atomic layer deposition,” Energy Environ. Sci. 6(1), 67–71 (2012). [CrossRef]
15. Y. Itzhaik, O. Niitsoo, M. Page, and G. Hodes, “Sb2S3-Sensitized nanoporous TiO2 solar cell,” J. Phys. Chem. C 113(11), 4254–4256 (2009). [CrossRef]
16. S. H. Im, H. J. Kim, J. H. Rhee, C. S. Lim, and S. I. Seok, “Performance improvement of Sb2S3-sensitized solar cell by introducing hole buffer layer in cobalt complex electrolyte,” Energy Environ. Sci. 4(8), 2799–2802 (2011). [CrossRef]
17. J. Krüger, R. Plass, L. Cevey, M. Piccirelli, M. Gratzel, and U. Bach, “High efficiency solid-state photovoltaic device due to inhibition of interface charge recombination,” Appl. Phys. Lett. 79(13), 2085–2087 (2001). [CrossRef]
18. A. Burke, S. Ito, H. Snaith, U. Bach, J. Kwiatkowski, and M. Grätzel, “The function of a TiO2 compact layer in dye-sensitized solar cells incorporating “planar” organic dyes,” Nano Lett. 8(4), 977–981 (2008). [CrossRef] [PubMed]
19. JCPDS 98–005–3997.
20. J. Yu, J. Fan, and K. Lv, “Anatase TiO2 nanosheets with exposed (001) facets: improved photoelectric conversion efficiency in dye-sensitized solar cells,” Nanoscale 2(10), 2144–2149 (2010). [CrossRef] [PubMed]
21. J. Yu, J. Fan, and L. Zhao, “Dye-sensitized solar cells based on hollow anatase TiO2 spheres prepared by self-transformation method,” Electrochim. Acta 55(3), 597–602 (2010). [CrossRef]
22. Y. Zhang, J. Zhang, P. Wang, G. Yang, Q. Sun, J. Zheng, and Y. Zhu, “Anatase TiO2 hollow spheres embedded TiO2 nanocrystalline photoanode for dye-sensitized solar cells,” Mater. Chem. Phys. 123(2-3), 595–600 (2010). [CrossRef]
23. Z. S. Wang, H. Kawauchi, T. Kashima, and H. Arakawa, “Significant influence of TiO2 photoelectrode morphology on the energy conversion efficiency of N719 dye-sensitized solar cell,” Coord. Chem. Rev. 248(13-14), 1381–1389 (2004). [CrossRef]
24. H. J. Koo, Y. J. Kim, Y. H. Lee, W. I. Lee, K. Kim, and N. G. Park, “Nano-embossed hollow spherical TiO2 as bifunctional material for high-efficiency dye-sensitized solar cells,” Adv. Mater. 20(1), 195–199 (2008). [CrossRef]
