- Email: [email protected]
Microwave-assisted synthesis of titanium dioxide nanocrystalline for efficient dye-sensitized and perovskite solar cells
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 120 (2015) 345–356 www.elsevier.com/locate/solener
Microwave-assisted synthesis of titanium dioxide nanocrystalline for efficient dye-sensitized and perovskite solar cells Po-Shen Shen a, Chuan-Ming Tseng b, Ta-Chuan Kuo a, Ching-Kuei Shih a, Ming-Hsien Li a, Peter Chen a,c,d,⇑ a
Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan b Institute of Physics, Academia Sinica, Taipei 115, Taiwan c Advanced Optoelectronic Technology Center (AOTC), National Cheng Kung University, Tainan 701, Taiwan d Research Center for Energy Technology and Strategy (RCETS), National Cheng Kung University, Tainan 701, Taiwan Received 16 April 2015; received in revised form 7 July 2015; accepted 14 July 2015
Communicated by: Associate Editor Frank Nu¨esch
Abstract A rapid microwave-assisted hydrothermal synthetic method is reported for the fabrication of TiO2 nanoparticles. Their photovoltaic activities are performed in dye-sensitized and perovskite-based solar cells. Power conversion efficiencies using microwave-assisted synthesized TiO2 are relatively similar compared with those employed TiO2 nanoparticles made of conventional hydrothermal process. The reaction time that is typically 12 h (literature value) was greatly reduced to only 25 min using microwave-heating process for TiO2 nanoparticles formation, which provides an energy-saving and cost-effective method for making the building blocks of photoactive TiO2 nanocrystallines. The material properties of the microwave-synthesized TiO2 nanoparticles are characterized in details by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, fourier transform infrared spectroscopy and photoluminescence emission spectra. An optimal dye-sensitized solar cell with impressive power conversion efficiency of 8.2% was achieved using microwave-synthesized nanoparticles in combination with commercial paste (CCIC HPW-400) as scattering layer. A mesoscopic perovskite-based solar cell employing microwave-assisted synthesized TiO2 nanoparticles obtained power conversion efficiency over 10%. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Titanium dioxide; Dye-sensitized solar cells; Microwave-assisted synthesis; Perovskite solar cells
1. Introduction After nearly two decades’ research and development, dye-sensitized solar cells (DSCs) have been greatly attracting academic and commercial interest as promising inexpensive alternatives to conventional semiconductor-based ⇑ Corresponding author at: Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan. E-mail address: [email protected] (P. Chen).
http://dx.doi.org/10.1016/j.solener.2015.07.036 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
photovoltaic devices (Gra¨tzel, 2001; O’Regan and Gra¨tzel, 1991). As a new and emerging renewable energy technology, the best conversion efficiency of 12% has been reported (Yella et al., 2011; Yum et al., 2014). Dye-sensitized solar cells contain three main device components: an electron-conducting mesoporous network of wide band-gap oxide semiconductor on which light-absorbing dyes are adsorbed, electrolyte composed of hole conducting medium and platinized catalytic counter electrode. The mesoporous network serves as an electron collector and
346
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
an extremely high surface area scaffold hosting light harvester. Nanocrystalline TiO2, a material that is abundant and chemically stable, is widely used for working electrode in photovoltaic devices, photocatalyst coatings, photoelectrochemical system, lithium (or sodium) batteries, chromatic display and so on (Byrne et al., 2014; Fujishima and Honda, 1972; Laskova et al., 2014; Periyat et al., 2010; Wu et al., 2014). For most of the highly efficient DSCs devices, their IPCE responses are nearly 90% at the dye-absorbing region, which implies the charge collection efficiency is close to unity (Gra¨tzel, 2009; Yella et al., 2011). So far, these best-performing devices employed nanocrystalline TiO2 made of hydrothermal autoclaving procedure (Ito et al., 2008; Wang et al., 2003). However, the conventional hydrothermal method for the fabrication of high quality nanocrystalline TiO2 is time-consuming and requires elaborate processes (Ito et al., 2008). Several compromised solutions such as using commercial available powder as starting materials (Chang et al., 2013; Ito et al., 2007), or nonaqueous synthesis routes (Stefik et al., 2013) have been applied to DSCs electrodes for delivering high efficiency. Various nanostructure of nanotube (Jen et al., 2013; Tetreault and Gratzel, 2012), nanofiber (Sabba et al., 2014), nanorods (Yang et al., 2013), nano-composite (Wu et al., 2013a,b), or hierarchical architectures (Lan et al., 2013; Sauvage et al., 2010; Tetreault and Gratzel, 2012; Wu et al., 2013a,b) have been applied to fabricate photoanodes for the purpose of high efficiency device. However, elaborate process in controlling the nanoscale structure is not very straightforward for mass production. Therefore, in order to develop a low energy budget and convenient procedure for the preparation of nanocrystlline TiO2, we propose a method to replace the step of autoclaving in conventional hydrothermal procedure by microwave heating with a focused microwave-assisted reactor (Model Discovery, CEM Corporation). The microwavesynthesized TiO2 is analyzed thoroughly for the material properties and its photovoltaic characteristics. Komarneni et al. first used microwave-assisted method to synthesize TiO2 particles (Komarneni et al., 1999). Since then, to ameliorate the preparation of TiO2 using microwave-assisted method, studies using microwave-assisted synthesis have been conducted for the preparation of TiO2 crystal (Ding et al., 2007; Hart et al., 2004; Li et al., 2009; Periyat et al., 2010; Ribbens et al., 2008; Wilson et al., 2006; Wu and Tai, 2013; Zhang et al., 2009; Zumeta et al., 2009). The microwave-assisted method is an efficient alternative as it allows swift heating to the set temperature and extremely rapid crystallization, leading to the simplification of the preparation procedure (Baghbanzadeh et al., 2011; Baldassari et al., 2005; Bilecka and Niederberger, 2010; Niederberger, 2013). Another virtue of the microwave process is their easy adaptation to the conventional heating processes for the industrial usage. Moreover, its unique advantages of volumetric heating and energy
saving in comparison with the conventional hydrothermal method are impressive (Baghbanzadeh et al., 2011). Recently the microwave synthesis has been applied to fabricate TiO2 or titanate with great success for dye solar cells or battery (Chen et al., 2013; Huang et al., 2011; Manseki et al., 2013; Parmar et al., 2011; Shen et al., 2014; Wang et al., 2011). Lately, mesoscopic or thin film TiO2 has shown its great importance for perovskite solar cells. PSCs using TiO2 as mesoporous layer has achieved power conversion efficiency over 16% while the thin film perovskite solar cell using compact TiO2 received 19% (Jeon et al., 2014; Zhou et al., 2014). In this article, we emphasize on the microwave method that significantly reduced thermal history and energy budget for synthesizing TiO2 nanocrystallites in comparison with the conventional hydrothermal process. Moreover, the microwave-synthesized material is capable of offering comparable photovoltaic performances, both for DSCs and PSCs, to the one prepared by conventional hydrothermal or commercial available materials. We have successfully used microwave-heating method to fabricate TiO2 nanoparticles as building blocks for highly transparent mesoscopic thin film which is beneficial for optical penetration. Besides, compared with the reaction time required for crystallization in hydrothermal autoclaving, our focused microwave-assisted heating greatly reduced the process from 12 h to 25 min. As a result, DSCs using these crystals delivered a remarkable performance of power conversion efficiency of 8.2%, which ranks high among the reported microwave-synthesized TiO2 for DSCs. Meanwhile, we also applied this microwave-synthesized nanoparticles to fabricate organometal halide perovskite solar cells, delivering device efficiency close to 11%. This result presents the first application of microwave-synthesized nanoparticles in organic–inorganic hybrid perovskitebased solar cells. 2. Experimental The experimental procedure is illustrated in Scheme 1 with all the detail processes described in the following sections. 2.1. Materials Commercially available titanium(IV) isopropoxide (TIP, 98%), ethanol (J.T.Baker, 99.5%), acetic acid (J.T.Baker, 100%), P25 TiO2 powders (Degussa), nitric acid (Sigma–Aldrich, 65%) and PbI2 (Sigma–Aldrich, 99.999%) were used as received. Two kinds of pure powders of ethyl celluloses (#46070, Fluka; #46080, Fluka) were dissolved in an ethanol solution with 5 wt% content of each ethyl cellulose (the total ethyl cellulose is 10 wt% in ethanol). The N719 dye powder (Solaronix) was used as received. Commercial TiO2 paste for screen-printing was obtained from JGC CO., Ltd.
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
347
Scheme 1. The experimental procedure for the fabrication of DSCs and PSCs using microwave-synthesized nanoparticles.
2.2. Preparation of TiO2 screen-printing pastes and device fabrication 2.2.1. The synthesis of TiO2 nanoparticles colloid and paste formation The preparation of TiO2 precursor solution was modified from the processes based on the work of Ito (Ito et al., 2008). An amount of 6 g (0.1 mol) of acetic acid was added into 29.3 g (0.1 mol) of titanium isopropoxide under stirring in a three-neck flask at room temperature. After vigorous stirring for 15 min, deionized water of 145 mL was poured into this three-neck flask as quickly as possible and keep stirring (700 rpm). Meanwhile the white precipitate was formed. The resulting precipitate took one hour to achieve complete hydrolysis reaction. After adding 2 ml of 65% nitric acid, the mixture was heated to 80 °C within 40 min and peptized for 60 min. Then, in batches, we loaded 15 mL of the precursors into a 35 mL reaction vessel for microwave heating (Discover SP, CEM Corporation). The set temperature, pressure limit, and heating power were 210 °C, 300 psi, and 200 W, respectively. After heating at 210 °C for 25 min, the resultant solution was then centrifuged, and followed by several times washing with ethanol for solvent exchange. For the preparation of the screen-printing TiO2 paste, the viscous slurry was prepared by the following procedures (Ito et al., 2007). 2 g of microwave-synthesized nanoparticles was well dispersed in ethanol with acetic acid (0.33 mL) as dispersion agent. Terpineol (6.6 g) and ethyl cellulose (10 g of 10% solution in ethanol) were added into the solution one after another. Since the resultant product existed aggregation for the microwave-synthesized crystals, the treatment of titanium ultrasonic horn was performed to break the agglomeration. After the removal of ethanol with a rotary-evaporator, the final paste was obtained when the vapor pressure reached around 23 mbar. The P25 paste was formulated according to the procedures described in the literature (Ito et al., 2007).
2.2.2. Device assembling For dye-sensitized solar cells (DSCs) fabrication: The FTO glass substrates were cleaned and then immersed into a 40 mM TiCl4 aqueous solution at 70 °C for 30 min and subsequently washed with water and ethanol. Then the microwave-synthesized nanoparticles paste was screen-printed onto an FTO substrate serving as working electrode. After sintering at 500 °C, the photoanodes were again treated with 40 mM TiCl4 solution as mentioned above and sintered at 500 °C for 30 min. When the temperature of the substrates were cooling down to 80 °C, the working electrodes were sensitized by immersing into dye solutions of 0.5 mM N719 dye (Solaronix) in acetonitrile and tertiary butanol (in the volume ratio of 1:1) at room temperature for 24 h. The dye-covered TiO2 films and the platinized counter electrodes were assembled into a sandwich-type cell and sealed with a hot-melt sealing foil (SX1170-25, 25 lm thickness, Solaronix) used as spacers between the electrodes. A hole was drilled beforehand in the counter electrode, allowing the internal space between the two electrodes to be filled with electrolyte solution, which is composed of 0.60 M butylmethylimidazolium iodide, 0.03 M I2, 0.10 M guanidinium thiocyanate, and 0.50 M 4-tert-butylpyridine in a mixture of acetonitrile and valeronitrile (v/v 85:15), using a vacuum backfilling system. After electrolyte filling, the hole was sealed with a thin glass sheet. For perovskite-based solar cells (PSCs) fabrication: The FTO glass substrate was coated with dense TiO2 (cp-TiO2) film via spray pyrolytic deposition of 0.068 M titanium diisopropoxide bis(acetylacetonate) solution in ethanol. Mesoporous TiO2 layer of 200 nm was spin coated using microwave synthesized TiO2 paste diluted in ethanol (1:5 weight ratio) and sintered at 500 °C for 30 min. The perovskite precursor solution was prepared by mixing CH3NH3I (1.2 M) and PbI2 (1.2 M) in the mixture of c-butyrolactone (GBL) and dimethyl sulphoxide (DMSO). The prepared MAPbI3 solution was coated onto mp-TiO2/cp-TiO2/FTO substrate by continuous two-step
348
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
spin-coating process at 1000 rpm and 5000 rpm for 10 s and 30 s, respectively. During the second step, a small amount of toluene was dropped on the center of the substrate. The perovskite-covered substrate was dried at 110 °C for 10 min. The hole-transporting material was subsequently spin coated onto the device (5000 rpm, 30 s). The solution was prepared by dissolving 72.3 mg Spiro-MeOTAD, 28.8 lL 4-tert-butylpyridine (tBP), and 17.5 lL of a stock solution of 17 mg/mL lithium bis(tri fluoromethylsulphonyl)imide (Li-TFSI) in acetonitrile in 1 ml chlorobenzene. Finally, device fabrication was completed with thermal evaporation of 60 nm gold as cathode. 2.2.3. Material characterizations Characterization of the TiO2 thin film crystal structure was measured by glancing incident X-ray diffraction (Rigaku, XRD-D/max2500) with Cu Ka radiation of wavelength k = 0.15418 nm, high-resolution transmission electron microscope (HRTEM) at 40 keV (JEOL, JEM-2100F), and isotherm of N2 adsorption at 77 k (See SI), to get TiO2 crystalline structures and particles size, respectively. Analysis of TiO2 surface was performed by X-ray photoelectron spectroscopy (XPS, K-alpha, 236 Thermo-Fisher, USA), and the fourier transformed infrared (FTIR) spectroscopy (Bruker, VERTEX 70). Raman spectroscopy (Bruker, SENTERRA) is applied to measure the vibration modes for TiO2. The optical properties are examined by photoluminescence (PL) measurement, UV– Vis reflectance spectra (Hitachi, U-4100). The morphology and structure of the TiO2 electrodes were imaged by field-emission scanning electron microscope (ZEISS GeminiSEM supra 500). 2.2.4. Photovoltaic characterizations An AM 1.5 solar simulator equipped with a 300 W xenon lamp (Model No. 91160, Oriel) was employed as light source and the power of simulator was calibrated to 100 mW/cm2 by using a reference Si diode (Hamamatsu S1133) equipped with an IR-cutoff filter (KG-5, Schott) to reduce the mismatch between the simulated light and standard AM 1.5G. The incident photon to electron conversion efficiency (IPCE) measurement was performed under monochromatic light illumination at short-circuit condition. The incident light was generated by a 300 W xenon lamp (Oriel) equipped with a monochromator (Oriel Cornerstone 260 1/4 m). The active area of the dye-coated TiO2 film was 0.16 cm2. The photocurrent and photovoltage transient measurements were conducted using an array composed of white (10 w) and red (3 w) light emitting diodes (LEDs, Cree XLamp). The white light diodes generate a continuous bias illumination on the devices. A short pulse (500 ms) generated by the red LEDs can be superimposed on the device to generate a perturbation on the cell with a transient current or voltage output. By fitting the photovoltage decays with various bias light intensities under open-circuit condition, the
electron recombination lifetime was obtained. The capacitance was derived from the equation: DQ/DV, where DQ is the total charge per unit area integrated by the photocurrent decays under short-circuit condition induced by the pump pulse and DV is the voltage difference caused by the pump pulse with the same intensity as applied in current decay (O’Regan et al., 2005). 3. Results and discussion 3.1. X-ray diffraction (XRD) The XRD patterns of TiO2 films for microwave-synthesized TiO2 (denoted as MW), commercial paste JGC 18NRT (denoted as JGC), and commercial P25 paste (denoted as P25), and as-synthesized TiO2 crystallites for MW are displayed in Fig. 1. The XRD result of the as-synthesized TiO2 implies that the precursor was successfully transformed into anatase structures after microwave heating process. Except for the P25 containing a portion of rutile crystallites which has peak corresponding to the plane (1 1 0), all the peaks of MW and JGC are referred to the anatase structures (JCPDS Card No. 21-1272) (Adachi et al., 2004) corresponding to the crystal planes of (1 0 1), (0 0 4) and etc. By comparing the FWHM value of MW with other TiO2 crystals shown in Fig. 1, the broaden peaks in MW nanoparticles obviously suggest smaller average grain size with less crystalline quality compared with the others. We have tried different temperatures to synthesize TiO2 nanoparticles ranging from 160 °C to 220 °C. As temperature increases, the XRD patterns (In Fig. S1) show better crystallinity and larger crystallite size. However, the differences in crystallite size between these temperatures are marginal. Since the machine for microwave synthesis has constraint on the reaction temperature and process pressure, we chose 210 °C as the reaction
Fig. 1. X-ray diffraction patterns of thin films made of commercial paste JGC (denoted JGC) and P25, microwave-synthesized nanoparticles (denoted MW) and microwave as-synthesized nanocrystallites.
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
temperature that does not approach the limit of the synthesis tool. It has been found that the crystalline growth of TiO2 in microwave synthesis is mainly dominated by thermodynamic control rather than kinetics (Wilson et al., 2006; Wu and Tai, 2013). The hydrothermal time has minor influence on the particle size within 180 min once the reaction is complete. 3.2. Raman spectroscopy Fig. 2 presents the Raman spectra of the TiO2 films for sample JGC, MW, and P25, respectively. As illustrated in Fig. 2, the anatase phases of TiO2 showed major Raman bands at 143, 195, 395, 515, 639 cm 1 which are attributed to the Raman-active modes of anatase phases with Eg, Eg, B1g, A1g + B1g, Eg, respectively (Gupta et al., 2010). Although the commercial P25 contains a portion of rutile crystals, the content is so small that the Raman characteristic bands of rutile phase are negligible of their appearance. By comparing the Eg peak of MW and JGC at 146 and 143.5 cm 1 respectively, the one of MW is slightly blue shifted and as is found in the literature (Gupta et al., 2010). This blue shift may be due to the grain size effect which indicates that the grain size of MW TiO2 crystals is smaller than the one of JGC TiO2 crystals. Meanwhile, the fact that the intensities of the first Eg peak implies that the JGC TiO2 crystals have higher degree of crystallinity, which agrees with the XRD results. 3.3. Scanning and Transmission Electron Microscopy (SEM and TEM) Fig. 3 shows the SEM and the TEM images for the thin film morphologies of commercial TiO2 JGC and microwave prepared TiO2 nanoparticles coated onto FTO substrate. As revealed in Fig. 3a–d, the morphology of TiO2
Fig. 2. Raman spectra of TiO2 films made of commercial paste JGC, microwave-synthesized nanoparticles (denoted as MW), and commercial P25 nanoparticles.
349
surface for MW (Fig. 3c and d) is comparatively smooth compared with the surface of JGC 18NRT (Fig. 3a and b) with adequate dispersion. From the SAED patterns displayed in the inset of Fig. 3e, the results of diffraction rings exactly agree with the XRD patterns of the corresponding planes (1 0 1), (0 0 4), and so on. Both the XRD and TEM lattice spacing confirmed the anatase phase for the microwave-synthesized TiO2 nanoparticles. Meanwhile, the particle size of about 12–15 nm for microwave-synthesized nanoparticles was observed from the high-resolution transmission electron microscopy as shown in the inset in Fig. 3e. The N2-sorption isotherm for microwave-synthesized nanoparticles with its corresponding pore size distribution (inset) is demonstrated in Fig. S2. A type H1 hysteresis loop observed at relative high pressures (P/P0 = 0.8–0.9) indicates high degree of pore size uniformity with size distribution peak centered around 8 nm. This guarantees a well dispersion of highly transparent porous electrode for dye uptake and pore-filling (for perovskite) with nicely distributed particle size. From transmittance spectra presented in Fig. S3, thin film of microwave-synthesized nanoparticles shows comparable transparency with respect to that of JGC 18NRT. 3.4. X-ray photoelectron spectroscopy (XPS) Quantitative XPS analysis was performed on the thin films derived from microwave-synthesized nanoparticles and commercial paste JGC 18NRT. The high-resolution XPS spectra were presented in Figs. 4 and 5. As seen in Fig. 4, the high-resolution spectra of Ti 2p signals showed two major peaks in the spectrum for both TiO2 films of JGC 18NRT and microwave-synthesized nanoparticles. After peak fitting, Ti 2p1/2 with a binding energy (BE) of 464.5 eV and Ti 2p3/2 with a binding energy of 458.8 eV are assigned to TiO2 (Aronsson et al., 1996; Viornery et al., 2002). From the O 1s peak spectra in Fig. 5, the peak with a binding energy about 530 eV is assigned to O2 for TiO2. Another peak with a binding energy of 531.6 eV is assigned to hydroxyl species. As seen in Fig. 4, there are relatively more defeat states presented as Ti2O3 on the MW TiO2 surface than on the JGC TiO2 surface. This result could be attributed to comparatively inferior crystallinity for MW TiO2 compared to JGC TiO2 as demonstrated in XRD and Raman specstroscopy. In addition, as shown in Fig. 5, it is apparently that higher ratio of [OH]/[O2 ] exists for microwave-synthesized nanoparticles than that for JGC 18NRT, indicating more trapping states presenting in the MW TiO2 (Erdem et al., 2001). Those trapping states existing in the MW TiO2 would to some degree induce a loss in photocurrent although MW TiO2 has greater number of hydroxyl groups which are favorable for dye anchoring. Accordingly, the dye-loading test was performed to examine the total number of adsorbed dye molecules per unit volume (The results are discussed in Section 3.6 Current–voltage characteristics). The Photoluminescence (PL) emission spectra of TiO2 films
350
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Fig. 3. SEM top-view images of TiO2 film for commercial paste JGC (a), (b) and microwave-synthesized nanoparticles (c), (d) of magnification of 100,000 and 350,000, respectively. (e) TEM images of microwave-synthesized TiO2 nanoparticles. (The inset on the top left is the high-resolution image and one on the bottom-left is the SAED pattern.) The scale bar is 50 nm for the low magnification image and 5 nm for the high-resolution image (top-left inset).
for MW, JGC 18NRT, and P25 are shown in Fig. S4. For MW and JGC 18NRT, they display similar emission band around 460 nm. This strong emission in the region of 400– 550 nm might be assigned to the energy states of surface traps (oxygen vacancies) composed of different emission bands at 2.99, 2.84, 2.65, and 2.37 eV (Yoon et al., 2005). Since rutile TiO2 has different band structure from anatase TiO2 with slightly narrower band gap, the PL emission band for P25 is relatively broad and red shifted due to its mixture phases of anatase and rutile. The PL spectra suggested that the microwave-synthesized TiO2 particles exhibit defect states of anatase phase similar to that of conventional hydrothermal synthesized TiO2.
3.5. Fourier transform infrared spectroscopy (FTIR) Fig. 6 demonstrates the FTIR spectra of TiO2 films for MW, JGC 18NRT, and P25, respectively, in the frequency range of 4000–1000 cm 1. As seen in Fig. 6, a broader band centered at 3400 cm 1 is attributed to the hydroxyl groups stretching vibration on the TiO2 surface and the band at 1630 cm 1 is assigned as the HO-H bonding vibrations (Erdem et al., 2001). By comparing the area under the curve for the broadening bands at 3400 cm 1, it is noticeable that MW sample has the largest area under hydroxyl band; that is, more hydroxyl groups stretched upon the surface of MW sample. This result further confirms that the
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Fig. 4. Ti 2p XPS spectra for the TiO2 thin films made of (a) JGC commercial paste and (b) microwave-synthesized nanoparticles.
351
Fig. 5. O 1s XPS spectra for the TiO2 films made of (a) JGC commercial paste and (b) microwave-synthesized nanoparticles.
MW TiO2 surface was capable of possessing higher ratio of [OH]/[O2 ] than JGC TiO2 surface, as illustrated in Fig. 5 for XPS O1s results. 3.6. Current–voltage characteristics The microwave-synthesized TiO2 nanoparticles and commercially available P25 or JGC 18NRT made by conventional hydrothermal synthesized process were used as photoanodes to assemble the dye-sensitized solar cells with same film thickness and the photovoltaic parameters (open-circuit voltage VOC, short-circuit current density JSC, fill factor FF and conversion efficiency g) were listed in Table S1 and plotted in Fig. S5. In comparison with the results of P25 or JGC 18NRT with conversion efficiency of 4.9% or 6.9%, device efficiency of 6.2% for microwave-synthesized nanoparticles exhibits a result that is relatively higher or comparable to the commercially available products. In light of examining the difference on the number of adsorbed dye molecules between photoanodes of MW and JGC 18NRT, dye-loading test was conducted. MW photoanode was found to obtain higher dye-adsorbed
Fig. 6. FTIR spectra of TiO2 films made of commercial paste JGC, microwave-synthesized nanoparticles (denoted as MW), and commercial P25 nanoparticles.
concentration per unit volume and the total number of adsorbed dye molecules is 4.86 10 5 and 4.93 10 5 mole per cubic cm for JGC 18NRT and MW, respectively.
352
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Since smaller particle size for MW leads to a higher surface area for dye uptake, the result that more dye molecules adsorbed onto MW photoanode seems reasonable. However, the benefit of more adsorbed dye concentration for devices of MW does not contribute to higher photocurrent density than that of JGC 18NRT. As mentioned above in XPS O1s analysis, more trapping states exist below the conduction band edge of MW TiO2 than that of JGC TiO2. This photocurrent loss due to those trapped charges could account for the lower photocurrent density of device made of MW TiO2. The results of lower voltage in the MW-DSC device also suggest more recombination process occurred at the interfacial defects. Moreover, to further enhance the PCE, we used double-layer structure (9 + 5 lm) by adding commercial paste (CCIC HPW-400) as scattering layer to obtain an optimized result of 8.2%, with VOC of 780 mV, JSC of 14.9 mA/cm2 and FF of 70%. This device performance of 8.2% ranks high among the relevant reports that apply microwave-synthesized nanomaterial for dye-sensitized solar cells. The photovoltaic parameters for total amount of 11 fabricated devices (including the champion cell) are demonstrated in Fig. S6 for the reproducibility. Meanwhile, devices parameters for CH3NH3PbI3 perovskite-based solar cells employing microwave-synthesized nanoparticles as mesoscopic layer are demonstrated in comparison with that of commercial JGC TiO2. The J–V curves and the incident photon to electron conversion efficiency (IPCE) spectra of perovskite-based devices (PSCs) as well as that of the best dye solar cells (MW-DSCs) are presented in Fig. 7 with their photovoltaic parameters summarized in Table 1. Obviously, the device MW-PSCs showed both higher VOC and JSC than MW-DSCs due to the use of the wide range and high extinction perovskite absorber, leading to VOC of 943 mV and JSC of 18.1 mA/cm2 with device efficiency close to 11%. Similar to the results of dye solar cells shown in Table S1, perovskite-based device of commercial JGC 18NRT (JGC-PSCs) showed slightly higher VOC of 1030 mV and JSC of 19.0 mA/cm2 than the MW-PSCs. The PCE of 10.7% for the application of microwave-synthesized nanoparticles in CH3NH3PbI3 perovskite solar cells is fairly remarkable compared with the reported nanostructured TiO2 for PSCs (Jiang et al., 2014; Kim et al., 2013; Qiu et al., 2013; Sabba et al., 2014). Efficient perovskite solar cells have been reported using hydrothermally synthesized nanostructured titania materials with various morphology such as nanorods or nanowires. For example, CH3NH3PbI3 perovskite cells incorporating with well aligned rutile TiO2 nanorods of length 0.6 lm as photoanodes reached conversion efficiency of 9.8% and device efficiency declined with increasing nanorod length. In addition, rutile nanowire-based perovskite solar cells exhibited 11.7% with nanowire length 1 lm. However, these nanostructure-based devices generally produce relatively low voltage output (below 800 mV), especially while the structure length further increases. Compared with the aforementioned nanostructures
Fig. 7. (a) Current–voltage curves and (b) the incident photon-to-electron conversion efficiencies (IPCE) for dye-sensitized solar cells (DSCs) and perovskite solar cells (PSCs) using microwave-synthesized TiO2.
Table 1 Photovoltaic parameters for dye-sensitized solar cell (DSCs) and perovskite-based solar cells (PSCs) using microwave-synthesized TiO2 (MW) and commercial TiO2 (JGC). Sample name
VOC (mV)
JSC (mA/cm2)
FF
PCE (%)
MW-DSCs MW-PSCs JGC-PSCs
780 943 1030
14.9 18.1 19.0
0.70 0.63 0.60
8.2 10.7 11.8
developed for PSCs, the microwave-assisted synthesis route provides a facile and energy efficient method for making the building blocks. Yet, the photovoltaic performances obtained are close to those PSCs composed of hydrothermally synthesized TiO2. MW-PSCs with thickness 200 nm of MW nanoparticles is capable of delivering comparably high VOC and JSC in comparison with those reported results. As for the IPCE, the enhancement on the red part for the perovskite is clear as perovskite has narrower band gap with broader absorption region (from 350 nm to 800 nm) than molecular dye. It is obvious that the significant difference of IPCE values between wavelength of
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
650 nm and 800 nm are mainly accountable for the enhancement of photocurrent for MW-PSCs than MW-DSCs. Compared with all the published results, we believe our conversion efficiency of 8.2% for MW-DSCs and 10.7% for MW-PSCs are remarkable in consideration of the use of microwave-synthesized TiO2 for solar cells. 3.7. Transient analysis The transient photocurrent/photovoltage decay measurements were performed for DSCs fabricated using each material as photoanodes. The corresponding electron recombination lifetime vs VOC, capacitance vs VOC, and DOS vs VOC were plotted in Fig. 8. As shown in Fig. 8a, the recombination lifetime of electrons from TiO2 conduction band to electrolyte as a function of VOC was similar for both JGC 18NRT and microwave-synthesized nanoparticles. The results indicated that it makes marginal differences for electrons recombination from TiO2
Fig. 8. Transient measurements for (a) electron lifetime and (b) capacitance, DOS, which are determined by photocurrent and photovoltage decay, as funtion of open-circuit voltage for the DSCs made of microwave-synthesized nanoparticles and JGC commercial paste with same 6-lm thick film.
353
conduction band to electrolyte between these two samples with JGC device showing slightly longer recombination lifetime. In Fig. 8b, the DOS mapping vs VOC curves were illustrated. The DOS distribution revealed small differences in curvature with perceptible shift to higher energy level for JGC devices. This suggests that the JGC TiO2 has negative shift of conduction band edge potential and would be favorable for higher open-circuit voltage. Though the crystal quality presented in XRD and XPS suggested inferior crystal quality for the MW compared with JGC crystal, there is no detrimental effect in terms of photovoltaic performance for DSCs. To the best of our knowledge, there were about twenty publications on the study of microwave-synthesized TiO2 for the DSCs application. Recent publications using microwave-synthesized TiO2 for efficient DSCs applications together with their DSCs efficiencies and material morphologies are summarized in Table 2. Currently, three research groups reported DSCs over 8% using microwave-synthesized TiO2 and the highest efficiency obtained for microwave-synthesized TiO2 DSCs is 8.32% with film thickness of 23 lm (Manseki et al., 2013). The high efficiency is originated from the anisotropic V-shaped single nanocrystal which could facilitate the charge transport. However, the cost for the single crystal formation would require a longer reaction time of 60 min. Another method worth noting is the microwave-solvothermal process that prevented the water content in the reaction which could avoid the subsequent tedious solvent exchange procedure (Dar et al., 2014; Shen et al., 2014). We have achieved 7.8% efficiency DSCs with the microwave-solvothermal synthesized TiO2 in a previous publication (Shen et al., 2014). The PCE is slightly lower than the current microwave-hydrothermal route. However, the solvothermal recipe has the advantage to avoid the tremendous effort for solvent exchange in the subsequent paste formation procedures. In comparison with those methods in Table 2, our low energy budget recipe takes 25 min for microwave hydrothermal process and the resultant TiO2 possesses considerably high specific surface area of 168 m2/g, which is beneficial for photocurrent generation. Our approach delivered high photovoltaic response with low energy budget which is advantageous for future scale up mass production. Besides, the power conversion efficiency of device reached respectful 8.2%, which is relatively high compared with those listed in Table 2 under the film thickness condition of 14 lm thick (9 lm of nanocrystalline layer +5 lm scattering layer). Furthermore, its application in perovskite-based solar cells obtained device efficiency over 10%. The microwave-assisted synthetic route of TiO2 in this report could be a promising alternative for high quality TiO2 fabrication and for industrial mass production. We believe with further efforts to control the synthesis procedures, improved material quality are expected to fulfill requirements for photovoltaic and other applications.
354
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Table 2 Recent reports using microwave-synthesized TiO2 for efficient dye-sensitized solar cells applications with their photovoltaic efficiencies and material descriptions. Reaction time (min)
SBET (m2/ g)
Fill factor
DSCs best efficiency (%)
Film thickness (lm)
PSCs efficiency (%)
This report
25 Nanoparticle
168
0.70
8.2
10.7
Huang et al. (2011) Parmar et al. (2011) Chen et al. (2013)
120 Nanoparticle 10 Nanorice
217 79
N.A 0.694
7.1 8.05
Nanocrystalline 9 + scatter layer 5 Nanocrystalline 7.4 Nanocrystalline 16
80 Nanosphere
132
0.74
6.92
Shen et al. (2014)
30 Nanoparticle
152
0.71
7.8
Manseki et al. (2013) Dar et al. (2014)
60 Single Nanocrystal Nanorod 10–60 Nanoparticle
50
0.718
8.32
N.A
0.71
6.54
Yang et al. (2011)
N.A Nanorod
N.A
0.63
3.7
Nanocrystalline 8 + scatter layer 4 Nanocrystalline 6 + scatter layer 4 Single Nanocrystal 23 Nanocrystalline 6 + scatter layer 2 Nanorods 2.5
N.A N.A N.A N.A N.A N.A N.A
4. Conclusion
References
In summary, we have successfully applied microwave-assisted synthesis to fabricate TiO2 nanoparticles with remarkable photovoltaic response. Material characteristics such as crystal phases, particle sizes, and surface properties are scrutinized in detail. The material properties showed similar quality comparable with commercial product made from conventional hydrothermal method. The photoanodes derived from microwave-synthesized nanoparticles delivered an optimal PCE of 8.2% for dye-sensitized solar cell. Furthermore, we demonstrate the first application of microwave-assisted synthesized TiO2 nanoparticles in CH3NH3PbI3 pervoskite solar cells with device efficiency as high as 10.7%. It is expected that our promising results of microwave heating process, which are facile, cost-effective and energy saving, could be a potential alternative for the fabrication of photovoltaic functional nanocrystals comparable to the conventional hydrothermal method.
Adachi, M., Murata, Y., Takao, J., Jiu, J.T., Sakamoto, M., Wang, F.M., 2004. Highly efficient dye-sensitized solar cells with a titania thin-film electrode composed of a network structure of single-crystal-like TiO2 Nanowires Made by the “Oriented Attachment” Mechanism. J. Am. Chem. Soc. 126, 14943–14949. Aronsson, B.-O., Krozer, A., Lausmaa, J., Kasemo, B., 1996. Commercially pure titanium and Ti6Al4V: XPS comparison between different commercial Ti dental implants and foils prepared by various oxidation procedures. Surf. Sci. Spectra. 4, 42–89. Baghbanzadeh, M., Carbone, L., Cozzoli, P.D., Kappe, C.O., 2011. Microwave-assisted synthesis of colloidal inorganic nanocrystals. Angew. Chem. Int. Ed. 50, 11312–11359. Baldassari, S., Komarneni, S., Mariani, E., Villa, C., 2005. Rapid microwave-hydrothermal synthesis of anatase form of titanium dioxide. J. Am. Ceram. Soc. 88, 3238–3240. Bilecka, I., Niederberger, M., 2010. Microwave chemistry for inorganic nanomaterials synthesis. Nanoscale 2, 1358–1374. Byrne, O., Ahmad, I., Surolia, P.K., Gun’ko, Y.K., Thampi, K.R., 2014. The optimisation of dye sensitised solar cell working electrodes for graphene and SWCNTs containing quasi-solid state electrolytes. Sol. Energy 110, 239–246. Chang, H., Yang, Y.-J., Li, H.-C., Hsu, C.-C., Cheng, I.C., Chen, J.-Z., 2013. Preparation of nanoporous TiO2 films for DSSC application by a rapid atmospheric pressure plasma jet sintering process. J. Power Sources 234, 16–22. Chen, P., Peng, J.-D., Liao, C.-H., Shen, P.-S., Kuo, P.-L., 2013. Rapid microwave synthesis of porous TiO2 spheres and their applications in dye-sensitized solar cells. J. Nanopart. Res. 15, 1465. Dar, M.I., Chandiran, A.K., Gratzel, M., Nazeeruddin, M.K., Shivashankar, S.A., 2014. Controlled synthesis of TiO2 nanoparticles and nanospheres using a microwave assisted approach for their application in dye-sensitized solar cells. J. Mater. Chem. A 2, 1662– 1667. Ding, K., Miao, Z., Liu, Z., Zhang, Z., Han, B., An, G., Miao, S., Xie, Y., 2007. Facile synthesis of high quality TiO2 nanocrystals in ionic liquid via a microwave-assisted process. J. Am. Chem. Soc. 129, 6362–6363. Erdem, B., Hunsicker, R.A., Simmons, G.W., Sudol, E.D., Dimonie, V.L., El-Aasser, M.S., 2001. XPS and FTIR surface characterization of TiO2. Langmuir 17, 2664–2669. Fujishima, A., Honda, K., 1972. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38. Gra¨tzel, M., 2001. Photoelectrochemical cells. Nature 414, 338–344.
Acknowledgement The authors are grateful to the research grant from the National Science Council (MOST 103-2221-E-006-029-MY3). PC thanks the financial support from the Top-Notch Project under the Headquarter of University Advancement at National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan, ROC. The funding from the Research Center for Energy Technology and Strategy (RCETS) and Advanced Optoelectronic Technology Center (AOTC), National Cheng Kung University is acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.solener.2015.07.036.
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356 Gra¨tzel, M., 2009. Recent advances in sensitized mesoscopic solar cells. Acc. Chem. Res. 42, 1788–1798. Gupta, S.K., Desai, R., Jha, P.K., Sahoo, S., Kirin, D., 2010. Titanium dioxide synthesized using titanium chloride: size effect study using raman spectroscopy and photoluminescence. J. Raman Spectrosc. 41, 350–355. Hart, J.N., Cervini, R., Cheng, Y.B., Simon, G.P., Spiccia, L., 2004. Formation of anatase TiO2 by microwave processing. Sol. Energy Mater. Sol. Cells 84, 135–143. Huang, C.-H., Yang, Y.-T., Doong, R.-A., 2011. Microwave-assisted hydrothermal synthesis of mesoporous anatase TiO2 via sol–gel process for dye-sensitized solar cells. Micropor. Mesopor. Mater. 142, 473–480. Ito, S., Chen, P., Comte, P., Nazeeruddin, M.K., Liska, P., Pechy, P., Gra¨tzel, M., 2007. Fabrication of screen-printing pastes from TiO2 powders for dye-sensitized solar cells. Prog. Photovolt: Res. Appl. 15, 603–612. Ito, S., Murakami, T.N., Comte, P., Liska, P., Gra¨tzel, C., Nazeeruddin, M.K., Gra¨tzel, M., 2008. Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%. Thin Solid Films 516, 4613–4619. Jen, H.-P., Lin, M.-H., Li, L.-L., Wu, H.-P., Huang, W.-K., Cheng, P.-J., Diau, E.W.-G., 2013. High-performance large-scale flexible dyesensitized solar cells based on anodic TiO2 nanotube arrays. ACS Appl. Mater. Interf. 5, 10098–10104. Jeon, N.J., Noh, J.H., Kim, Y.C., Yang, W.S., Ryu, S., Seok, S.I., 2014. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nat. Mater. 13, 897–903. Jiang, Q., Sheng, X., Li, Y., Feng, X., Xu, T., 2014. Rutile TiO2 nanowirebased perovskite solar cells. Chem. Comm. 50, 14720–14723. Kim, H.-S., Lee, J.-W., Yantara, N., Boix, P.P., Kulkarni, S.A., Mhaisalkar, S., Gra¨tzel, M., Park, N.-G., 2013. High efficiency solidstate sensitized solar cell-based on submicrometer Rutile TiO2 nanorod and CH3NH3PbI3 perovskite sensitizer. Nano Lett. 13, 2412–2417. Komarneni, S., Rajha, R.K., Katsuki, H., 1999. Microwave-hydrothermal processing of titanium dioxide. Mater. Chem. Phys. 61, 50–54. Lan, C.-M., Liu, S.-E., Shiu, J.-W., Hu, J.-Y., Lin, M.-H., Diau, E.W.-G., 2013. Formation of size-tunable dandelion-like hierarchical rutile titania nanospheres for dye-sensitized solar cells. RSC Adv. 3, 559– 565. Laskova, B., Zukalova, M., Zukal, A., Bousa, M., Kavan, L., 2014. Capacitive contribution to Li-storage in TiO2 (B) and TiO2 (Anatase). J. Power Sources 246, 103–109. Li, Y., Li, H., Li, T., Li, G., Cao, R., 2009. Facile synthesis of mesoporous titanium dioxide nanocomposites with controllable phase compositions by microwave- assisted esterification. Micropor. Mesopor. Mater. 117, 444–449. Manseki, K., Kondo, Y., Ban, T., Sugiura, T., Yoshida, T., 2013. Sizecontrolled synthesis of anisotropic TiO2 single nanocrystals using microwave irradiation and their application for dye-sensitized solar cells. Dalton Trans. 42, 3295–3299. Niederberger, M., 2013. Microwaves in Nanoparticle Synthesis: Fundamentals and Applications. Wiley-VCH, Weinheim. O’Regan, B., Gra¨tzel, M., 1991. High-efficiency solar cell based on dyesensitized colloidal TiO2 films. Nature 335, 737–740. O’Regan, B.C., Scully, S., Mayer, A.C., Palomares, E., Durrant, J., 2005. The effect of Al2O3 barrier layers in TiO2/Dye/CuSCN photovoltaic cells explored by recombination and DOS characterization using transient photovoltage measurements. J. Phys. Chem. B 109, 4616– 4623. Parmar, K.P.S., Ramasamy, E., Lee, J., Lee, J.S., 2011. Rapid (10 min) synthesis of single-crystalline, nanorice TiO2 mesoprticles with a high photovoltaic efficiency of above 8%. Chem. Comm. 47, 8572–8574. Periyat, P., Leyland, N., McCormack, D.E., Colreavy, J., Corr, D., Pillai, S.C., 2010. Rapid microwave synthesis of mesoporous TiO2 for nanochromic displays. J. Mater. Chem. 20, 3650–3655. Qiu, J., Qiu, Y., Yan, K., Zhong, M., Mu, C., Yan, H., Yang, S., 2013. All-solid-state hybrid solar cells based on a new organometal halide
355
perovskite sensitizer and one-dimensional TiO2 nanowire arrays. Nanoscale 5, 3245–3248. Ribbens, S., Meynen, V., Tendeloo, G.V., Ke, X., Mertens, M., Maes, B.U.W., Cool, P., Vansant, E.F., 2008. Development of photocatalytic efficient Ti-based nanotubes and nanoribbons by conventional and microwave assisted synthesis strategies. Micropor. Mesopor. Mater. 114, 401–409. Sabba, D., Kumar, H.M., Yantara, N., Pham, T.T.T., Park, N.-G., Gratzel, M., Mhaisalkar, S.G., Mathews, N., Boix, P.P., 2014. High efficiency electrospun TiO2 nanofiber based hybrid organic–inorganic perovskite solar cell. Nanoscale 6, 1675–1679. Sauvage, F., Di Fonzo, F., Li Bassi, A., Casari, C.S., Russo, V., Divitini, G., Ducati, C., Bottani, C.E., Comte, P., Graetzel, M., 2010. Hierarchical TiO2 photoanode for dye-sensitized solar cells. Nano Lett. 10, 2562–2567. Shen, P.-S., Tai, Y.-C., Chen, P., Wu, Y.-C., 2014. Clean and timeeffective synthesis of anatase TiO2 nanocrystalline by microwaveassisted solvothermal method for dye-sensitized solar cells. J. Power Sources 247, 444–451. Stefik, M., Heiligtag, F.J., Niederberger, M., Gra¨tzel, M., 2013. Improved nonaqueous synthesis of TiO2 for dye-sensitized solar cells. ACS Nano. 7, 8981–8989. Tetreault, N., Gratzel, M., 2012. Novel nanostructures for next generation dye-sensitized solar cells. Energy Environ. Sci. 5, 8506–8516. Viornery, C., Chevolot, Y., Le´onard, D., Aronsson, B.-O., Pe´chy, P., Mathieu, H.J., Descouts, P., Gra¨tzel, M., 2002. Surface modification of titaniium with phosphonic acid to improve bone bonding: characterization by XPS and ToF-SIMS. Langmuir 18, 2582–2589. Wang, H.-E., Zheng, L.-X., Liu, C.-P., Liu, Y.-K., Luan, C.-Y., Cheng, H., Li, Y.Y., Martinu, L., Zapien, J.A., Bello, I., 2011. Rapid microwave synthesis of porous TiO2 spheres and their applications in dye-sensitized solar cells. J. Phys. Chem. C 115, 10419–10425. Wang, P., Zakeeruddin, S.M., Comte, P., Charvet, R., Humphry-Baker, R., Gratzel, M., 2003. Enhance the performance of dye-sensitized solar cells by co-grafting amphiphilic sensitizer and hexadecylmalonic acid on TiO2 nanocrystals. J. Phys. Chem. B 107, 14336–14341. Wilson, G.J., Matijasevich, A.S., Mitchell, D.R.G., Schulz, J.C., Will, G.D., 2006. Modification of TiO2 for enhanced surface properties: finite ostwald ripening by a microwave hydrothermal process. Langmuir 22, 2016–2027. Wu, H.-P., Lan, C.-M., Hu, J.-Y., Huang, W.-K., Shiu, J.-W., Lan, Z.-J., Tsai, C.-M., Su, C.-H., Diau, E.W.-G., 2013a. Hybrid titania photoanodes with a nanostructured multi-layer configuration for highly efficient dye-sensitized solar cells. J. Phys. Chem. Lett. 4, 1570– 1577. Wu, L., Buchholz, D., Bresser, D., Gomes Chagas, L., Passerini, S., 2014. Anatase TiO2 nanoparticles for high power sodium-ion anodes. J. Power Sources 251, 379–385. Wu, W.-Q., Xu, Y.-F., Rao, H.-S., Su, C.-Y., Kuang, D.-B., 2013b. A double layered TiO2 photoanode consisting of hierarchical flowers and nanoparticles for high-efficiency dye-sensitized solar cells. Nanoscale 5, 4362–4369. Wu, Y.-C., Tai, Y.-C., 2013. Effects of alcohol solvents on anatase TiO2 nanocrystals prepared by microwave-assisted solvothermal method. J. Nanopart. Res. 15, 1–11. Yang, M., Ding, B., Lee, S., Lee, J.-K., 2011. Carrier transport in dyesensitized solar cells using single crystalline TiO2 nanorods grown by a microwave-assisted hydrothermal reaction. J. Phys. Chem. C 115, 14534–14541. Yang, M., Neupane, S., Wang, X., He, J., Li, W., Pala, N., 2013. Multiple step growth of single crystalline rutile nanorods with the assistance of self-assembled monolayer for dye sensitized solar cells. ACS Appl. Mater. Interf. 5, 9809–9815. Yella, A., Lee, H.-W., Tsao, H.N., Yi, C., Chandiran, A.K., Nazeeruddin, M., Diau, E.W.-G., Yeh, C.-Y., Zakeeruddin, S.M., Gra¨tzel, M., 2011. Porphyrin-sensitized solar cells with cobalt (II/III)–based redox electrolyte exceed 12 percent efficiency. Science 334, 629–634.
356
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Yoon, M., Seo, M., Jeong, C., Jang, J.H., Jeon, K.S., 2005. Synthesis of liposome-templated titania nanodisks: optical properties and photocatalytic activities. Chem. Mater. 17, 6069–6079. Yum, J.-H., Lee, J.-W., Kim, Y., Humphry-Baker, R., Park, N.-G., Gra¨tzel, M., 2014. Panchromatic light harvesting by dye- and quantum dot-sensitized solar cells. Sol. Energy 109, 183–188. Zhang, P., Yin, S., Sato, T., 2009. Synthesis of high-activity TiO2 photocatalyst via environmentally friendly and novel microwave assisted hydrothermal process. Appl. Catal. B: Environ. 89, 118–122.
Zhou, H., Chen, Q., Li, G., Luo, S., Song, T.-B., Duan, H.-S., Hong, Z., You, J., Liu, Y., Yang, Y., 2014. Interface engineering of highly efficient perovskite solar cells. Science 345, 542–546. Zumeta, I., Ayllo´n, J.A., Gonzalez, B., Dome`nech, X., Vigil, E., 2009. TiO2 films obtained by microwave-activated chemical-bath deposition used to improve TiO2-conducting glass contact. Sol. Energy Mater. Sol. Cells 93, 1728–1732.
ScienceDirect Solar Energy 120 (2015) 345–356 www.elsevier.com/locate/solener
Microwave-assisted synthesis of titanium dioxide nanocrystalline for efficient dye-sensitized and perovskite solar cells Po-Shen Shen a, Chuan-Ming Tseng b, Ta-Chuan Kuo a, Ching-Kuei Shih a, Ming-Hsien Li a, Peter Chen a,c,d,⇑ a
Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan b Institute of Physics, Academia Sinica, Taipei 115, Taiwan c Advanced Optoelectronic Technology Center (AOTC), National Cheng Kung University, Tainan 701, Taiwan d Research Center for Energy Technology and Strategy (RCETS), National Cheng Kung University, Tainan 701, Taiwan Received 16 April 2015; received in revised form 7 July 2015; accepted 14 July 2015
Communicated by: Associate Editor Frank Nu¨esch
Abstract A rapid microwave-assisted hydrothermal synthetic method is reported for the fabrication of TiO2 nanoparticles. Their photovoltaic activities are performed in dye-sensitized and perovskite-based solar cells. Power conversion efficiencies using microwave-assisted synthesized TiO2 are relatively similar compared with those employed TiO2 nanoparticles made of conventional hydrothermal process. The reaction time that is typically 12 h (literature value) was greatly reduced to only 25 min using microwave-heating process for TiO2 nanoparticles formation, which provides an energy-saving and cost-effective method for making the building blocks of photoactive TiO2 nanocrystallines. The material properties of the microwave-synthesized TiO2 nanoparticles are characterized in details by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, fourier transform infrared spectroscopy and photoluminescence emission spectra. An optimal dye-sensitized solar cell with impressive power conversion efficiency of 8.2% was achieved using microwave-synthesized nanoparticles in combination with commercial paste (CCIC HPW-400) as scattering layer. A mesoscopic perovskite-based solar cell employing microwave-assisted synthesized TiO2 nanoparticles obtained power conversion efficiency over 10%. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Titanium dioxide; Dye-sensitized solar cells; Microwave-assisted synthesis; Perovskite solar cells
1. Introduction After nearly two decades’ research and development, dye-sensitized solar cells (DSCs) have been greatly attracting academic and commercial interest as promising inexpensive alternatives to conventional semiconductor-based ⇑ Corresponding author at: Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan. E-mail address: [email protected] (P. Chen).
http://dx.doi.org/10.1016/j.solener.2015.07.036 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
photovoltaic devices (Gra¨tzel, 2001; O’Regan and Gra¨tzel, 1991). As a new and emerging renewable energy technology, the best conversion efficiency of 12% has been reported (Yella et al., 2011; Yum et al., 2014). Dye-sensitized solar cells contain three main device components: an electron-conducting mesoporous network of wide band-gap oxide semiconductor on which light-absorbing dyes are adsorbed, electrolyte composed of hole conducting medium and platinized catalytic counter electrode. The mesoporous network serves as an electron collector and
346
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
an extremely high surface area scaffold hosting light harvester. Nanocrystalline TiO2, a material that is abundant and chemically stable, is widely used for working electrode in photovoltaic devices, photocatalyst coatings, photoelectrochemical system, lithium (or sodium) batteries, chromatic display and so on (Byrne et al., 2014; Fujishima and Honda, 1972; Laskova et al., 2014; Periyat et al., 2010; Wu et al., 2014). For most of the highly efficient DSCs devices, their IPCE responses are nearly 90% at the dye-absorbing region, which implies the charge collection efficiency is close to unity (Gra¨tzel, 2009; Yella et al., 2011). So far, these best-performing devices employed nanocrystalline TiO2 made of hydrothermal autoclaving procedure (Ito et al., 2008; Wang et al., 2003). However, the conventional hydrothermal method for the fabrication of high quality nanocrystalline TiO2 is time-consuming and requires elaborate processes (Ito et al., 2008). Several compromised solutions such as using commercial available powder as starting materials (Chang et al., 2013; Ito et al., 2007), or nonaqueous synthesis routes (Stefik et al., 2013) have been applied to DSCs electrodes for delivering high efficiency. Various nanostructure of nanotube (Jen et al., 2013; Tetreault and Gratzel, 2012), nanofiber (Sabba et al., 2014), nanorods (Yang et al., 2013), nano-composite (Wu et al., 2013a,b), or hierarchical architectures (Lan et al., 2013; Sauvage et al., 2010; Tetreault and Gratzel, 2012; Wu et al., 2013a,b) have been applied to fabricate photoanodes for the purpose of high efficiency device. However, elaborate process in controlling the nanoscale structure is not very straightforward for mass production. Therefore, in order to develop a low energy budget and convenient procedure for the preparation of nanocrystlline TiO2, we propose a method to replace the step of autoclaving in conventional hydrothermal procedure by microwave heating with a focused microwave-assisted reactor (Model Discovery, CEM Corporation). The microwavesynthesized TiO2 is analyzed thoroughly for the material properties and its photovoltaic characteristics. Komarneni et al. first used microwave-assisted method to synthesize TiO2 particles (Komarneni et al., 1999). Since then, to ameliorate the preparation of TiO2 using microwave-assisted method, studies using microwave-assisted synthesis have been conducted for the preparation of TiO2 crystal (Ding et al., 2007; Hart et al., 2004; Li et al., 2009; Periyat et al., 2010; Ribbens et al., 2008; Wilson et al., 2006; Wu and Tai, 2013; Zhang et al., 2009; Zumeta et al., 2009). The microwave-assisted method is an efficient alternative as it allows swift heating to the set temperature and extremely rapid crystallization, leading to the simplification of the preparation procedure (Baghbanzadeh et al., 2011; Baldassari et al., 2005; Bilecka and Niederberger, 2010; Niederberger, 2013). Another virtue of the microwave process is their easy adaptation to the conventional heating processes for the industrial usage. Moreover, its unique advantages of volumetric heating and energy
saving in comparison with the conventional hydrothermal method are impressive (Baghbanzadeh et al., 2011). Recently the microwave synthesis has been applied to fabricate TiO2 or titanate with great success for dye solar cells or battery (Chen et al., 2013; Huang et al., 2011; Manseki et al., 2013; Parmar et al., 2011; Shen et al., 2014; Wang et al., 2011). Lately, mesoscopic or thin film TiO2 has shown its great importance for perovskite solar cells. PSCs using TiO2 as mesoporous layer has achieved power conversion efficiency over 16% while the thin film perovskite solar cell using compact TiO2 received 19% (Jeon et al., 2014; Zhou et al., 2014). In this article, we emphasize on the microwave method that significantly reduced thermal history and energy budget for synthesizing TiO2 nanocrystallites in comparison with the conventional hydrothermal process. Moreover, the microwave-synthesized material is capable of offering comparable photovoltaic performances, both for DSCs and PSCs, to the one prepared by conventional hydrothermal or commercial available materials. We have successfully used microwave-heating method to fabricate TiO2 nanoparticles as building blocks for highly transparent mesoscopic thin film which is beneficial for optical penetration. Besides, compared with the reaction time required for crystallization in hydrothermal autoclaving, our focused microwave-assisted heating greatly reduced the process from 12 h to 25 min. As a result, DSCs using these crystals delivered a remarkable performance of power conversion efficiency of 8.2%, which ranks high among the reported microwave-synthesized TiO2 for DSCs. Meanwhile, we also applied this microwave-synthesized nanoparticles to fabricate organometal halide perovskite solar cells, delivering device efficiency close to 11%. This result presents the first application of microwave-synthesized nanoparticles in organic–inorganic hybrid perovskitebased solar cells. 2. Experimental The experimental procedure is illustrated in Scheme 1 with all the detail processes described in the following sections. 2.1. Materials Commercially available titanium(IV) isopropoxide (TIP, 98%), ethanol (J.T.Baker, 99.5%), acetic acid (J.T.Baker, 100%), P25 TiO2 powders (Degussa), nitric acid (Sigma–Aldrich, 65%) and PbI2 (Sigma–Aldrich, 99.999%) were used as received. Two kinds of pure powders of ethyl celluloses (#46070, Fluka; #46080, Fluka) were dissolved in an ethanol solution with 5 wt% content of each ethyl cellulose (the total ethyl cellulose is 10 wt% in ethanol). The N719 dye powder (Solaronix) was used as received. Commercial TiO2 paste for screen-printing was obtained from JGC CO., Ltd.
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
347
Scheme 1. The experimental procedure for the fabrication of DSCs and PSCs using microwave-synthesized nanoparticles.
2.2. Preparation of TiO2 screen-printing pastes and device fabrication 2.2.1. The synthesis of TiO2 nanoparticles colloid and paste formation The preparation of TiO2 precursor solution was modified from the processes based on the work of Ito (Ito et al., 2008). An amount of 6 g (0.1 mol) of acetic acid was added into 29.3 g (0.1 mol) of titanium isopropoxide under stirring in a three-neck flask at room temperature. After vigorous stirring for 15 min, deionized water of 145 mL was poured into this three-neck flask as quickly as possible and keep stirring (700 rpm). Meanwhile the white precipitate was formed. The resulting precipitate took one hour to achieve complete hydrolysis reaction. After adding 2 ml of 65% nitric acid, the mixture was heated to 80 °C within 40 min and peptized for 60 min. Then, in batches, we loaded 15 mL of the precursors into a 35 mL reaction vessel for microwave heating (Discover SP, CEM Corporation). The set temperature, pressure limit, and heating power were 210 °C, 300 psi, and 200 W, respectively. After heating at 210 °C for 25 min, the resultant solution was then centrifuged, and followed by several times washing with ethanol for solvent exchange. For the preparation of the screen-printing TiO2 paste, the viscous slurry was prepared by the following procedures (Ito et al., 2007). 2 g of microwave-synthesized nanoparticles was well dispersed in ethanol with acetic acid (0.33 mL) as dispersion agent. Terpineol (6.6 g) and ethyl cellulose (10 g of 10% solution in ethanol) were added into the solution one after another. Since the resultant product existed aggregation for the microwave-synthesized crystals, the treatment of titanium ultrasonic horn was performed to break the agglomeration. After the removal of ethanol with a rotary-evaporator, the final paste was obtained when the vapor pressure reached around 23 mbar. The P25 paste was formulated according to the procedures described in the literature (Ito et al., 2007).
2.2.2. Device assembling For dye-sensitized solar cells (DSCs) fabrication: The FTO glass substrates were cleaned and then immersed into a 40 mM TiCl4 aqueous solution at 70 °C for 30 min and subsequently washed with water and ethanol. Then the microwave-synthesized nanoparticles paste was screen-printed onto an FTO substrate serving as working electrode. After sintering at 500 °C, the photoanodes were again treated with 40 mM TiCl4 solution as mentioned above and sintered at 500 °C for 30 min. When the temperature of the substrates were cooling down to 80 °C, the working electrodes were sensitized by immersing into dye solutions of 0.5 mM N719 dye (Solaronix) in acetonitrile and tertiary butanol (in the volume ratio of 1:1) at room temperature for 24 h. The dye-covered TiO2 films and the platinized counter electrodes were assembled into a sandwich-type cell and sealed with a hot-melt sealing foil (SX1170-25, 25 lm thickness, Solaronix) used as spacers between the electrodes. A hole was drilled beforehand in the counter electrode, allowing the internal space between the two electrodes to be filled with electrolyte solution, which is composed of 0.60 M butylmethylimidazolium iodide, 0.03 M I2, 0.10 M guanidinium thiocyanate, and 0.50 M 4-tert-butylpyridine in a mixture of acetonitrile and valeronitrile (v/v 85:15), using a vacuum backfilling system. After electrolyte filling, the hole was sealed with a thin glass sheet. For perovskite-based solar cells (PSCs) fabrication: The FTO glass substrate was coated with dense TiO2 (cp-TiO2) film via spray pyrolytic deposition of 0.068 M titanium diisopropoxide bis(acetylacetonate) solution in ethanol. Mesoporous TiO2 layer of 200 nm was spin coated using microwave synthesized TiO2 paste diluted in ethanol (1:5 weight ratio) and sintered at 500 °C for 30 min. The perovskite precursor solution was prepared by mixing CH3NH3I (1.2 M) and PbI2 (1.2 M) in the mixture of c-butyrolactone (GBL) and dimethyl sulphoxide (DMSO). The prepared MAPbI3 solution was coated onto mp-TiO2/cp-TiO2/FTO substrate by continuous two-step
348
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
spin-coating process at 1000 rpm and 5000 rpm for 10 s and 30 s, respectively. During the second step, a small amount of toluene was dropped on the center of the substrate. The perovskite-covered substrate was dried at 110 °C for 10 min. The hole-transporting material was subsequently spin coated onto the device (5000 rpm, 30 s). The solution was prepared by dissolving 72.3 mg Spiro-MeOTAD, 28.8 lL 4-tert-butylpyridine (tBP), and 17.5 lL of a stock solution of 17 mg/mL lithium bis(tri fluoromethylsulphonyl)imide (Li-TFSI) in acetonitrile in 1 ml chlorobenzene. Finally, device fabrication was completed with thermal evaporation of 60 nm gold as cathode. 2.2.3. Material characterizations Characterization of the TiO2 thin film crystal structure was measured by glancing incident X-ray diffraction (Rigaku, XRD-D/max2500) with Cu Ka radiation of wavelength k = 0.15418 nm, high-resolution transmission electron microscope (HRTEM) at 40 keV (JEOL, JEM-2100F), and isotherm of N2 adsorption at 77 k (See SI), to get TiO2 crystalline structures and particles size, respectively. Analysis of TiO2 surface was performed by X-ray photoelectron spectroscopy (XPS, K-alpha, 236 Thermo-Fisher, USA), and the fourier transformed infrared (FTIR) spectroscopy (Bruker, VERTEX 70). Raman spectroscopy (Bruker, SENTERRA) is applied to measure the vibration modes for TiO2. The optical properties are examined by photoluminescence (PL) measurement, UV– Vis reflectance spectra (Hitachi, U-4100). The morphology and structure of the TiO2 electrodes were imaged by field-emission scanning electron microscope (ZEISS GeminiSEM supra 500). 2.2.4. Photovoltaic characterizations An AM 1.5 solar simulator equipped with a 300 W xenon lamp (Model No. 91160, Oriel) was employed as light source and the power of simulator was calibrated to 100 mW/cm2 by using a reference Si diode (Hamamatsu S1133) equipped with an IR-cutoff filter (KG-5, Schott) to reduce the mismatch between the simulated light and standard AM 1.5G. The incident photon to electron conversion efficiency (IPCE) measurement was performed under monochromatic light illumination at short-circuit condition. The incident light was generated by a 300 W xenon lamp (Oriel) equipped with a monochromator (Oriel Cornerstone 260 1/4 m). The active area of the dye-coated TiO2 film was 0.16 cm2. The photocurrent and photovoltage transient measurements were conducted using an array composed of white (10 w) and red (3 w) light emitting diodes (LEDs, Cree XLamp). The white light diodes generate a continuous bias illumination on the devices. A short pulse (500 ms) generated by the red LEDs can be superimposed on the device to generate a perturbation on the cell with a transient current or voltage output. By fitting the photovoltage decays with various bias light intensities under open-circuit condition, the
electron recombination lifetime was obtained. The capacitance was derived from the equation: DQ/DV, where DQ is the total charge per unit area integrated by the photocurrent decays under short-circuit condition induced by the pump pulse and DV is the voltage difference caused by the pump pulse with the same intensity as applied in current decay (O’Regan et al., 2005). 3. Results and discussion 3.1. X-ray diffraction (XRD) The XRD patterns of TiO2 films for microwave-synthesized TiO2 (denoted as MW), commercial paste JGC 18NRT (denoted as JGC), and commercial P25 paste (denoted as P25), and as-synthesized TiO2 crystallites for MW are displayed in Fig. 1. The XRD result of the as-synthesized TiO2 implies that the precursor was successfully transformed into anatase structures after microwave heating process. Except for the P25 containing a portion of rutile crystallites which has peak corresponding to the plane (1 1 0), all the peaks of MW and JGC are referred to the anatase structures (JCPDS Card No. 21-1272) (Adachi et al., 2004) corresponding to the crystal planes of (1 0 1), (0 0 4) and etc. By comparing the FWHM value of MW with other TiO2 crystals shown in Fig. 1, the broaden peaks in MW nanoparticles obviously suggest smaller average grain size with less crystalline quality compared with the others. We have tried different temperatures to synthesize TiO2 nanoparticles ranging from 160 °C to 220 °C. As temperature increases, the XRD patterns (In Fig. S1) show better crystallinity and larger crystallite size. However, the differences in crystallite size between these temperatures are marginal. Since the machine for microwave synthesis has constraint on the reaction temperature and process pressure, we chose 210 °C as the reaction
Fig. 1. X-ray diffraction patterns of thin films made of commercial paste JGC (denoted JGC) and P25, microwave-synthesized nanoparticles (denoted MW) and microwave as-synthesized nanocrystallites.
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
temperature that does not approach the limit of the synthesis tool. It has been found that the crystalline growth of TiO2 in microwave synthesis is mainly dominated by thermodynamic control rather than kinetics (Wilson et al., 2006; Wu and Tai, 2013). The hydrothermal time has minor influence on the particle size within 180 min once the reaction is complete. 3.2. Raman spectroscopy Fig. 2 presents the Raman spectra of the TiO2 films for sample JGC, MW, and P25, respectively. As illustrated in Fig. 2, the anatase phases of TiO2 showed major Raman bands at 143, 195, 395, 515, 639 cm 1 which are attributed to the Raman-active modes of anatase phases with Eg, Eg, B1g, A1g + B1g, Eg, respectively (Gupta et al., 2010). Although the commercial P25 contains a portion of rutile crystals, the content is so small that the Raman characteristic bands of rutile phase are negligible of their appearance. By comparing the Eg peak of MW and JGC at 146 and 143.5 cm 1 respectively, the one of MW is slightly blue shifted and as is found in the literature (Gupta et al., 2010). This blue shift may be due to the grain size effect which indicates that the grain size of MW TiO2 crystals is smaller than the one of JGC TiO2 crystals. Meanwhile, the fact that the intensities of the first Eg peak implies that the JGC TiO2 crystals have higher degree of crystallinity, which agrees with the XRD results. 3.3. Scanning and Transmission Electron Microscopy (SEM and TEM) Fig. 3 shows the SEM and the TEM images for the thin film morphologies of commercial TiO2 JGC and microwave prepared TiO2 nanoparticles coated onto FTO substrate. As revealed in Fig. 3a–d, the morphology of TiO2
Fig. 2. Raman spectra of TiO2 films made of commercial paste JGC, microwave-synthesized nanoparticles (denoted as MW), and commercial P25 nanoparticles.
349
surface for MW (Fig. 3c and d) is comparatively smooth compared with the surface of JGC 18NRT (Fig. 3a and b) with adequate dispersion. From the SAED patterns displayed in the inset of Fig. 3e, the results of diffraction rings exactly agree with the XRD patterns of the corresponding planes (1 0 1), (0 0 4), and so on. Both the XRD and TEM lattice spacing confirmed the anatase phase for the microwave-synthesized TiO2 nanoparticles. Meanwhile, the particle size of about 12–15 nm for microwave-synthesized nanoparticles was observed from the high-resolution transmission electron microscopy as shown in the inset in Fig. 3e. The N2-sorption isotherm for microwave-synthesized nanoparticles with its corresponding pore size distribution (inset) is demonstrated in Fig. S2. A type H1 hysteresis loop observed at relative high pressures (P/P0 = 0.8–0.9) indicates high degree of pore size uniformity with size distribution peak centered around 8 nm. This guarantees a well dispersion of highly transparent porous electrode for dye uptake and pore-filling (for perovskite) with nicely distributed particle size. From transmittance spectra presented in Fig. S3, thin film of microwave-synthesized nanoparticles shows comparable transparency with respect to that of JGC 18NRT. 3.4. X-ray photoelectron spectroscopy (XPS) Quantitative XPS analysis was performed on the thin films derived from microwave-synthesized nanoparticles and commercial paste JGC 18NRT. The high-resolution XPS spectra were presented in Figs. 4 and 5. As seen in Fig. 4, the high-resolution spectra of Ti 2p signals showed two major peaks in the spectrum for both TiO2 films of JGC 18NRT and microwave-synthesized nanoparticles. After peak fitting, Ti 2p1/2 with a binding energy (BE) of 464.5 eV and Ti 2p3/2 with a binding energy of 458.8 eV are assigned to TiO2 (Aronsson et al., 1996; Viornery et al., 2002). From the O 1s peak spectra in Fig. 5, the peak with a binding energy about 530 eV is assigned to O2 for TiO2. Another peak with a binding energy of 531.6 eV is assigned to hydroxyl species. As seen in Fig. 4, there are relatively more defeat states presented as Ti2O3 on the MW TiO2 surface than on the JGC TiO2 surface. This result could be attributed to comparatively inferior crystallinity for MW TiO2 compared to JGC TiO2 as demonstrated in XRD and Raman specstroscopy. In addition, as shown in Fig. 5, it is apparently that higher ratio of [OH]/[O2 ] exists for microwave-synthesized nanoparticles than that for JGC 18NRT, indicating more trapping states presenting in the MW TiO2 (Erdem et al., 2001). Those trapping states existing in the MW TiO2 would to some degree induce a loss in photocurrent although MW TiO2 has greater number of hydroxyl groups which are favorable for dye anchoring. Accordingly, the dye-loading test was performed to examine the total number of adsorbed dye molecules per unit volume (The results are discussed in Section 3.6 Current–voltage characteristics). The Photoluminescence (PL) emission spectra of TiO2 films
350
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Fig. 3. SEM top-view images of TiO2 film for commercial paste JGC (a), (b) and microwave-synthesized nanoparticles (c), (d) of magnification of 100,000 and 350,000, respectively. (e) TEM images of microwave-synthesized TiO2 nanoparticles. (The inset on the top left is the high-resolution image and one on the bottom-left is the SAED pattern.) The scale bar is 50 nm for the low magnification image and 5 nm for the high-resolution image (top-left inset).
for MW, JGC 18NRT, and P25 are shown in Fig. S4. For MW and JGC 18NRT, they display similar emission band around 460 nm. This strong emission in the region of 400– 550 nm might be assigned to the energy states of surface traps (oxygen vacancies) composed of different emission bands at 2.99, 2.84, 2.65, and 2.37 eV (Yoon et al., 2005). Since rutile TiO2 has different band structure from anatase TiO2 with slightly narrower band gap, the PL emission band for P25 is relatively broad and red shifted due to its mixture phases of anatase and rutile. The PL spectra suggested that the microwave-synthesized TiO2 particles exhibit defect states of anatase phase similar to that of conventional hydrothermal synthesized TiO2.
3.5. Fourier transform infrared spectroscopy (FTIR) Fig. 6 demonstrates the FTIR spectra of TiO2 films for MW, JGC 18NRT, and P25, respectively, in the frequency range of 4000–1000 cm 1. As seen in Fig. 6, a broader band centered at 3400 cm 1 is attributed to the hydroxyl groups stretching vibration on the TiO2 surface and the band at 1630 cm 1 is assigned as the HO-H bonding vibrations (Erdem et al., 2001). By comparing the area under the curve for the broadening bands at 3400 cm 1, it is noticeable that MW sample has the largest area under hydroxyl band; that is, more hydroxyl groups stretched upon the surface of MW sample. This result further confirms that the
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Fig. 4. Ti 2p XPS spectra for the TiO2 thin films made of (a) JGC commercial paste and (b) microwave-synthesized nanoparticles.
351
Fig. 5. O 1s XPS spectra for the TiO2 films made of (a) JGC commercial paste and (b) microwave-synthesized nanoparticles.
MW TiO2 surface was capable of possessing higher ratio of [OH]/[O2 ] than JGC TiO2 surface, as illustrated in Fig. 5 for XPS O1s results. 3.6. Current–voltage characteristics The microwave-synthesized TiO2 nanoparticles and commercially available P25 or JGC 18NRT made by conventional hydrothermal synthesized process were used as photoanodes to assemble the dye-sensitized solar cells with same film thickness and the photovoltaic parameters (open-circuit voltage VOC, short-circuit current density JSC, fill factor FF and conversion efficiency g) were listed in Table S1 and plotted in Fig. S5. In comparison with the results of P25 or JGC 18NRT with conversion efficiency of 4.9% or 6.9%, device efficiency of 6.2% for microwave-synthesized nanoparticles exhibits a result that is relatively higher or comparable to the commercially available products. In light of examining the difference on the number of adsorbed dye molecules between photoanodes of MW and JGC 18NRT, dye-loading test was conducted. MW photoanode was found to obtain higher dye-adsorbed
Fig. 6. FTIR spectra of TiO2 films made of commercial paste JGC, microwave-synthesized nanoparticles (denoted as MW), and commercial P25 nanoparticles.
concentration per unit volume and the total number of adsorbed dye molecules is 4.86 10 5 and 4.93 10 5 mole per cubic cm for JGC 18NRT and MW, respectively.
352
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Since smaller particle size for MW leads to a higher surface area for dye uptake, the result that more dye molecules adsorbed onto MW photoanode seems reasonable. However, the benefit of more adsorbed dye concentration for devices of MW does not contribute to higher photocurrent density than that of JGC 18NRT. As mentioned above in XPS O1s analysis, more trapping states exist below the conduction band edge of MW TiO2 than that of JGC TiO2. This photocurrent loss due to those trapped charges could account for the lower photocurrent density of device made of MW TiO2. The results of lower voltage in the MW-DSC device also suggest more recombination process occurred at the interfacial defects. Moreover, to further enhance the PCE, we used double-layer structure (9 + 5 lm) by adding commercial paste (CCIC HPW-400) as scattering layer to obtain an optimized result of 8.2%, with VOC of 780 mV, JSC of 14.9 mA/cm2 and FF of 70%. This device performance of 8.2% ranks high among the relevant reports that apply microwave-synthesized nanomaterial for dye-sensitized solar cells. The photovoltaic parameters for total amount of 11 fabricated devices (including the champion cell) are demonstrated in Fig. S6 for the reproducibility. Meanwhile, devices parameters for CH3NH3PbI3 perovskite-based solar cells employing microwave-synthesized nanoparticles as mesoscopic layer are demonstrated in comparison with that of commercial JGC TiO2. The J–V curves and the incident photon to electron conversion efficiency (IPCE) spectra of perovskite-based devices (PSCs) as well as that of the best dye solar cells (MW-DSCs) are presented in Fig. 7 with their photovoltaic parameters summarized in Table 1. Obviously, the device MW-PSCs showed both higher VOC and JSC than MW-DSCs due to the use of the wide range and high extinction perovskite absorber, leading to VOC of 943 mV and JSC of 18.1 mA/cm2 with device efficiency close to 11%. Similar to the results of dye solar cells shown in Table S1, perovskite-based device of commercial JGC 18NRT (JGC-PSCs) showed slightly higher VOC of 1030 mV and JSC of 19.0 mA/cm2 than the MW-PSCs. The PCE of 10.7% for the application of microwave-synthesized nanoparticles in CH3NH3PbI3 perovskite solar cells is fairly remarkable compared with the reported nanostructured TiO2 for PSCs (Jiang et al., 2014; Kim et al., 2013; Qiu et al., 2013; Sabba et al., 2014). Efficient perovskite solar cells have been reported using hydrothermally synthesized nanostructured titania materials with various morphology such as nanorods or nanowires. For example, CH3NH3PbI3 perovskite cells incorporating with well aligned rutile TiO2 nanorods of length 0.6 lm as photoanodes reached conversion efficiency of 9.8% and device efficiency declined with increasing nanorod length. In addition, rutile nanowire-based perovskite solar cells exhibited 11.7% with nanowire length 1 lm. However, these nanostructure-based devices generally produce relatively low voltage output (below 800 mV), especially while the structure length further increases. Compared with the aforementioned nanostructures
Fig. 7. (a) Current–voltage curves and (b) the incident photon-to-electron conversion efficiencies (IPCE) for dye-sensitized solar cells (DSCs) and perovskite solar cells (PSCs) using microwave-synthesized TiO2.
Table 1 Photovoltaic parameters for dye-sensitized solar cell (DSCs) and perovskite-based solar cells (PSCs) using microwave-synthesized TiO2 (MW) and commercial TiO2 (JGC). Sample name
VOC (mV)
JSC (mA/cm2)
FF
PCE (%)
MW-DSCs MW-PSCs JGC-PSCs
780 943 1030
14.9 18.1 19.0
0.70 0.63 0.60
8.2 10.7 11.8
developed for PSCs, the microwave-assisted synthesis route provides a facile and energy efficient method for making the building blocks. Yet, the photovoltaic performances obtained are close to those PSCs composed of hydrothermally synthesized TiO2. MW-PSCs with thickness 200 nm of MW nanoparticles is capable of delivering comparably high VOC and JSC in comparison with those reported results. As for the IPCE, the enhancement on the red part for the perovskite is clear as perovskite has narrower band gap with broader absorption region (from 350 nm to 800 nm) than molecular dye. It is obvious that the significant difference of IPCE values between wavelength of
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
650 nm and 800 nm are mainly accountable for the enhancement of photocurrent for MW-PSCs than MW-DSCs. Compared with all the published results, we believe our conversion efficiency of 8.2% for MW-DSCs and 10.7% for MW-PSCs are remarkable in consideration of the use of microwave-synthesized TiO2 for solar cells. 3.7. Transient analysis The transient photocurrent/photovoltage decay measurements were performed for DSCs fabricated using each material as photoanodes. The corresponding electron recombination lifetime vs VOC, capacitance vs VOC, and DOS vs VOC were plotted in Fig. 8. As shown in Fig. 8a, the recombination lifetime of electrons from TiO2 conduction band to electrolyte as a function of VOC was similar for both JGC 18NRT and microwave-synthesized nanoparticles. The results indicated that it makes marginal differences for electrons recombination from TiO2
Fig. 8. Transient measurements for (a) electron lifetime and (b) capacitance, DOS, which are determined by photocurrent and photovoltage decay, as funtion of open-circuit voltage for the DSCs made of microwave-synthesized nanoparticles and JGC commercial paste with same 6-lm thick film.
353
conduction band to electrolyte between these two samples with JGC device showing slightly longer recombination lifetime. In Fig. 8b, the DOS mapping vs VOC curves were illustrated. The DOS distribution revealed small differences in curvature with perceptible shift to higher energy level for JGC devices. This suggests that the JGC TiO2 has negative shift of conduction band edge potential and would be favorable for higher open-circuit voltage. Though the crystal quality presented in XRD and XPS suggested inferior crystal quality for the MW compared with JGC crystal, there is no detrimental effect in terms of photovoltaic performance for DSCs. To the best of our knowledge, there were about twenty publications on the study of microwave-synthesized TiO2 for the DSCs application. Recent publications using microwave-synthesized TiO2 for efficient DSCs applications together with their DSCs efficiencies and material morphologies are summarized in Table 2. Currently, three research groups reported DSCs over 8% using microwave-synthesized TiO2 and the highest efficiency obtained for microwave-synthesized TiO2 DSCs is 8.32% with film thickness of 23 lm (Manseki et al., 2013). The high efficiency is originated from the anisotropic V-shaped single nanocrystal which could facilitate the charge transport. However, the cost for the single crystal formation would require a longer reaction time of 60 min. Another method worth noting is the microwave-solvothermal process that prevented the water content in the reaction which could avoid the subsequent tedious solvent exchange procedure (Dar et al., 2014; Shen et al., 2014). We have achieved 7.8% efficiency DSCs with the microwave-solvothermal synthesized TiO2 in a previous publication (Shen et al., 2014). The PCE is slightly lower than the current microwave-hydrothermal route. However, the solvothermal recipe has the advantage to avoid the tremendous effort for solvent exchange in the subsequent paste formation procedures. In comparison with those methods in Table 2, our low energy budget recipe takes 25 min for microwave hydrothermal process and the resultant TiO2 possesses considerably high specific surface area of 168 m2/g, which is beneficial for photocurrent generation. Our approach delivered high photovoltaic response with low energy budget which is advantageous for future scale up mass production. Besides, the power conversion efficiency of device reached respectful 8.2%, which is relatively high compared with those listed in Table 2 under the film thickness condition of 14 lm thick (9 lm of nanocrystalline layer +5 lm scattering layer). Furthermore, its application in perovskite-based solar cells obtained device efficiency over 10%. The microwave-assisted synthetic route of TiO2 in this report could be a promising alternative for high quality TiO2 fabrication and for industrial mass production. We believe with further efforts to control the synthesis procedures, improved material quality are expected to fulfill requirements for photovoltaic and other applications.
354
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Table 2 Recent reports using microwave-synthesized TiO2 for efficient dye-sensitized solar cells applications with their photovoltaic efficiencies and material descriptions. Reaction time (min)
SBET (m2/ g)
Fill factor
DSCs best efficiency (%)
Film thickness (lm)
PSCs efficiency (%)
This report
25 Nanoparticle
168
0.70
8.2
10.7
Huang et al. (2011) Parmar et al. (2011) Chen et al. (2013)
120 Nanoparticle 10 Nanorice
217 79
N.A 0.694
7.1 8.05
Nanocrystalline 9 + scatter layer 5 Nanocrystalline 7.4 Nanocrystalline 16
80 Nanosphere
132
0.74
6.92
Shen et al. (2014)
30 Nanoparticle
152
0.71
7.8
Manseki et al. (2013) Dar et al. (2014)
60 Single Nanocrystal Nanorod 10–60 Nanoparticle
50
0.718
8.32
N.A
0.71
6.54
Yang et al. (2011)
N.A Nanorod
N.A
0.63
3.7
Nanocrystalline 8 + scatter layer 4 Nanocrystalline 6 + scatter layer 4 Single Nanocrystal 23 Nanocrystalline 6 + scatter layer 2 Nanorods 2.5
N.A N.A N.A N.A N.A N.A N.A
4. Conclusion
References
In summary, we have successfully applied microwave-assisted synthesis to fabricate TiO2 nanoparticles with remarkable photovoltaic response. Material characteristics such as crystal phases, particle sizes, and surface properties are scrutinized in detail. The material properties showed similar quality comparable with commercial product made from conventional hydrothermal method. The photoanodes derived from microwave-synthesized nanoparticles delivered an optimal PCE of 8.2% for dye-sensitized solar cell. Furthermore, we demonstrate the first application of microwave-assisted synthesized TiO2 nanoparticles in CH3NH3PbI3 pervoskite solar cells with device efficiency as high as 10.7%. It is expected that our promising results of microwave heating process, which are facile, cost-effective and energy saving, could be a potential alternative for the fabrication of photovoltaic functional nanocrystals comparable to the conventional hydrothermal method.
Adachi, M., Murata, Y., Takao, J., Jiu, J.T., Sakamoto, M., Wang, F.M., 2004. Highly efficient dye-sensitized solar cells with a titania thin-film electrode composed of a network structure of single-crystal-like TiO2 Nanowires Made by the “Oriented Attachment” Mechanism. J. Am. Chem. Soc. 126, 14943–14949. Aronsson, B.-O., Krozer, A., Lausmaa, J., Kasemo, B., 1996. Commercially pure titanium and Ti6Al4V: XPS comparison between different commercial Ti dental implants and foils prepared by various oxidation procedures. Surf. Sci. Spectra. 4, 42–89. Baghbanzadeh, M., Carbone, L., Cozzoli, P.D., Kappe, C.O., 2011. Microwave-assisted synthesis of colloidal inorganic nanocrystals. Angew. Chem. Int. Ed. 50, 11312–11359. Baldassari, S., Komarneni, S., Mariani, E., Villa, C., 2005. Rapid microwave-hydrothermal synthesis of anatase form of titanium dioxide. J. Am. Ceram. Soc. 88, 3238–3240. Bilecka, I., Niederberger, M., 2010. Microwave chemistry for inorganic nanomaterials synthesis. Nanoscale 2, 1358–1374. Byrne, O., Ahmad, I., Surolia, P.K., Gun’ko, Y.K., Thampi, K.R., 2014. The optimisation of dye sensitised solar cell working electrodes for graphene and SWCNTs containing quasi-solid state electrolytes. Sol. Energy 110, 239–246. Chang, H., Yang, Y.-J., Li, H.-C., Hsu, C.-C., Cheng, I.C., Chen, J.-Z., 2013. Preparation of nanoporous TiO2 films for DSSC application by a rapid atmospheric pressure plasma jet sintering process. J. Power Sources 234, 16–22. Chen, P., Peng, J.-D., Liao, C.-H., Shen, P.-S., Kuo, P.-L., 2013. Rapid microwave synthesis of porous TiO2 spheres and their applications in dye-sensitized solar cells. J. Nanopart. Res. 15, 1465. Dar, M.I., Chandiran, A.K., Gratzel, M., Nazeeruddin, M.K., Shivashankar, S.A., 2014. Controlled synthesis of TiO2 nanoparticles and nanospheres using a microwave assisted approach for their application in dye-sensitized solar cells. J. Mater. Chem. A 2, 1662– 1667. Ding, K., Miao, Z., Liu, Z., Zhang, Z., Han, B., An, G., Miao, S., Xie, Y., 2007. Facile synthesis of high quality TiO2 nanocrystals in ionic liquid via a microwave-assisted process. J. Am. Chem. Soc. 129, 6362–6363. Erdem, B., Hunsicker, R.A., Simmons, G.W., Sudol, E.D., Dimonie, V.L., El-Aasser, M.S., 2001. XPS and FTIR surface characterization of TiO2. Langmuir 17, 2664–2669. Fujishima, A., Honda, K., 1972. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38. Gra¨tzel, M., 2001. Photoelectrochemical cells. Nature 414, 338–344.
Acknowledgement The authors are grateful to the research grant from the National Science Council (MOST 103-2221-E-006-029-MY3). PC thanks the financial support from the Top-Notch Project under the Headquarter of University Advancement at National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan, ROC. The funding from the Research Center for Energy Technology and Strategy (RCETS) and Advanced Optoelectronic Technology Center (AOTC), National Cheng Kung University is acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.solener.2015.07.036.
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356 Gra¨tzel, M., 2009. Recent advances in sensitized mesoscopic solar cells. Acc. Chem. Res. 42, 1788–1798. Gupta, S.K., Desai, R., Jha, P.K., Sahoo, S., Kirin, D., 2010. Titanium dioxide synthesized using titanium chloride: size effect study using raman spectroscopy and photoluminescence. J. Raman Spectrosc. 41, 350–355. Hart, J.N., Cervini, R., Cheng, Y.B., Simon, G.P., Spiccia, L., 2004. Formation of anatase TiO2 by microwave processing. Sol. Energy Mater. Sol. Cells 84, 135–143. Huang, C.-H., Yang, Y.-T., Doong, R.-A., 2011. Microwave-assisted hydrothermal synthesis of mesoporous anatase TiO2 via sol–gel process for dye-sensitized solar cells. Micropor. Mesopor. Mater. 142, 473–480. Ito, S., Chen, P., Comte, P., Nazeeruddin, M.K., Liska, P., Pechy, P., Gra¨tzel, M., 2007. Fabrication of screen-printing pastes from TiO2 powders for dye-sensitized solar cells. Prog. Photovolt: Res. Appl. 15, 603–612. Ito, S., Murakami, T.N., Comte, P., Liska, P., Gra¨tzel, C., Nazeeruddin, M.K., Gra¨tzel, M., 2008. Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%. Thin Solid Films 516, 4613–4619. Jen, H.-P., Lin, M.-H., Li, L.-L., Wu, H.-P., Huang, W.-K., Cheng, P.-J., Diau, E.W.-G., 2013. High-performance large-scale flexible dyesensitized solar cells based on anodic TiO2 nanotube arrays. ACS Appl. Mater. Interf. 5, 10098–10104. Jeon, N.J., Noh, J.H., Kim, Y.C., Yang, W.S., Ryu, S., Seok, S.I., 2014. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nat. Mater. 13, 897–903. Jiang, Q., Sheng, X., Li, Y., Feng, X., Xu, T., 2014. Rutile TiO2 nanowirebased perovskite solar cells. Chem. Comm. 50, 14720–14723. Kim, H.-S., Lee, J.-W., Yantara, N., Boix, P.P., Kulkarni, S.A., Mhaisalkar, S., Gra¨tzel, M., Park, N.-G., 2013. High efficiency solidstate sensitized solar cell-based on submicrometer Rutile TiO2 nanorod and CH3NH3PbI3 perovskite sensitizer. Nano Lett. 13, 2412–2417. Komarneni, S., Rajha, R.K., Katsuki, H., 1999. Microwave-hydrothermal processing of titanium dioxide. Mater. Chem. Phys. 61, 50–54. Lan, C.-M., Liu, S.-E., Shiu, J.-W., Hu, J.-Y., Lin, M.-H., Diau, E.W.-G., 2013. Formation of size-tunable dandelion-like hierarchical rutile titania nanospheres for dye-sensitized solar cells. RSC Adv. 3, 559– 565. Laskova, B., Zukalova, M., Zukal, A., Bousa, M., Kavan, L., 2014. Capacitive contribution to Li-storage in TiO2 (B) and TiO2 (Anatase). J. Power Sources 246, 103–109. Li, Y., Li, H., Li, T., Li, G., Cao, R., 2009. Facile synthesis of mesoporous titanium dioxide nanocomposites with controllable phase compositions by microwave- assisted esterification. Micropor. Mesopor. Mater. 117, 444–449. Manseki, K., Kondo, Y., Ban, T., Sugiura, T., Yoshida, T., 2013. Sizecontrolled synthesis of anisotropic TiO2 single nanocrystals using microwave irradiation and their application for dye-sensitized solar cells. Dalton Trans. 42, 3295–3299. Niederberger, M., 2013. Microwaves in Nanoparticle Synthesis: Fundamentals and Applications. Wiley-VCH, Weinheim. O’Regan, B., Gra¨tzel, M., 1991. High-efficiency solar cell based on dyesensitized colloidal TiO2 films. Nature 335, 737–740. O’Regan, B.C., Scully, S., Mayer, A.C., Palomares, E., Durrant, J., 2005. The effect of Al2O3 barrier layers in TiO2/Dye/CuSCN photovoltaic cells explored by recombination and DOS characterization using transient photovoltage measurements. J. Phys. Chem. B 109, 4616– 4623. Parmar, K.P.S., Ramasamy, E., Lee, J., Lee, J.S., 2011. Rapid (10 min) synthesis of single-crystalline, nanorice TiO2 mesoprticles with a high photovoltaic efficiency of above 8%. Chem. Comm. 47, 8572–8574. Periyat, P., Leyland, N., McCormack, D.E., Colreavy, J., Corr, D., Pillai, S.C., 2010. Rapid microwave synthesis of mesoporous TiO2 for nanochromic displays. J. Mater. Chem. 20, 3650–3655. Qiu, J., Qiu, Y., Yan, K., Zhong, M., Mu, C., Yan, H., Yang, S., 2013. All-solid-state hybrid solar cells based on a new organometal halide
355
perovskite sensitizer and one-dimensional TiO2 nanowire arrays. Nanoscale 5, 3245–3248. Ribbens, S., Meynen, V., Tendeloo, G.V., Ke, X., Mertens, M., Maes, B.U.W., Cool, P., Vansant, E.F., 2008. Development of photocatalytic efficient Ti-based nanotubes and nanoribbons by conventional and microwave assisted synthesis strategies. Micropor. Mesopor. Mater. 114, 401–409. Sabba, D., Kumar, H.M., Yantara, N., Pham, T.T.T., Park, N.-G., Gratzel, M., Mhaisalkar, S.G., Mathews, N., Boix, P.P., 2014. High efficiency electrospun TiO2 nanofiber based hybrid organic–inorganic perovskite solar cell. Nanoscale 6, 1675–1679. Sauvage, F., Di Fonzo, F., Li Bassi, A., Casari, C.S., Russo, V., Divitini, G., Ducati, C., Bottani, C.E., Comte, P., Graetzel, M., 2010. Hierarchical TiO2 photoanode for dye-sensitized solar cells. Nano Lett. 10, 2562–2567. Shen, P.-S., Tai, Y.-C., Chen, P., Wu, Y.-C., 2014. Clean and timeeffective synthesis of anatase TiO2 nanocrystalline by microwaveassisted solvothermal method for dye-sensitized solar cells. J. Power Sources 247, 444–451. Stefik, M., Heiligtag, F.J., Niederberger, M., Gra¨tzel, M., 2013. Improved nonaqueous synthesis of TiO2 for dye-sensitized solar cells. ACS Nano. 7, 8981–8989. Tetreault, N., Gratzel, M., 2012. Novel nanostructures for next generation dye-sensitized solar cells. Energy Environ. Sci. 5, 8506–8516. Viornery, C., Chevolot, Y., Le´onard, D., Aronsson, B.-O., Pe´chy, P., Mathieu, H.J., Descouts, P., Gra¨tzel, M., 2002. Surface modification of titaniium with phosphonic acid to improve bone bonding: characterization by XPS and ToF-SIMS. Langmuir 18, 2582–2589. Wang, H.-E., Zheng, L.-X., Liu, C.-P., Liu, Y.-K., Luan, C.-Y., Cheng, H., Li, Y.Y., Martinu, L., Zapien, J.A., Bello, I., 2011. Rapid microwave synthesis of porous TiO2 spheres and their applications in dye-sensitized solar cells. J. Phys. Chem. C 115, 10419–10425. Wang, P., Zakeeruddin, S.M., Comte, P., Charvet, R., Humphry-Baker, R., Gratzel, M., 2003. Enhance the performance of dye-sensitized solar cells by co-grafting amphiphilic sensitizer and hexadecylmalonic acid on TiO2 nanocrystals. J. Phys. Chem. B 107, 14336–14341. Wilson, G.J., Matijasevich, A.S., Mitchell, D.R.G., Schulz, J.C., Will, G.D., 2006. Modification of TiO2 for enhanced surface properties: finite ostwald ripening by a microwave hydrothermal process. Langmuir 22, 2016–2027. Wu, H.-P., Lan, C.-M., Hu, J.-Y., Huang, W.-K., Shiu, J.-W., Lan, Z.-J., Tsai, C.-M., Su, C.-H., Diau, E.W.-G., 2013a. Hybrid titania photoanodes with a nanostructured multi-layer configuration for highly efficient dye-sensitized solar cells. J. Phys. Chem. Lett. 4, 1570– 1577. Wu, L., Buchholz, D., Bresser, D., Gomes Chagas, L., Passerini, S., 2014. Anatase TiO2 nanoparticles for high power sodium-ion anodes. J. Power Sources 251, 379–385. Wu, W.-Q., Xu, Y.-F., Rao, H.-S., Su, C.-Y., Kuang, D.-B., 2013b. A double layered TiO2 photoanode consisting of hierarchical flowers and nanoparticles for high-efficiency dye-sensitized solar cells. Nanoscale 5, 4362–4369. Wu, Y.-C., Tai, Y.-C., 2013. Effects of alcohol solvents on anatase TiO2 nanocrystals prepared by microwave-assisted solvothermal method. J. Nanopart. Res. 15, 1–11. Yang, M., Ding, B., Lee, S., Lee, J.-K., 2011. Carrier transport in dyesensitized solar cells using single crystalline TiO2 nanorods grown by a microwave-assisted hydrothermal reaction. J. Phys. Chem. C 115, 14534–14541. Yang, M., Neupane, S., Wang, X., He, J., Li, W., Pala, N., 2013. Multiple step growth of single crystalline rutile nanorods with the assistance of self-assembled monolayer for dye sensitized solar cells. ACS Appl. Mater. Interf. 5, 9809–9815. Yella, A., Lee, H.-W., Tsao, H.N., Yi, C., Chandiran, A.K., Nazeeruddin, M., Diau, E.W.-G., Yeh, C.-Y., Zakeeruddin, S.M., Gra¨tzel, M., 2011. Porphyrin-sensitized solar cells with cobalt (II/III)–based redox electrolyte exceed 12 percent efficiency. Science 334, 629–634.
356
P.-S. Shen et al. / Solar Energy 120 (2015) 345–356
Yoon, M., Seo, M., Jeong, C., Jang, J.H., Jeon, K.S., 2005. Synthesis of liposome-templated titania nanodisks: optical properties and photocatalytic activities. Chem. Mater. 17, 6069–6079. Yum, J.-H., Lee, J.-W., Kim, Y., Humphry-Baker, R., Park, N.-G., Gra¨tzel, M., 2014. Panchromatic light harvesting by dye- and quantum dot-sensitized solar cells. Sol. Energy 109, 183–188. Zhang, P., Yin, S., Sato, T., 2009. Synthesis of high-activity TiO2 photocatalyst via environmentally friendly and novel microwave assisted hydrothermal process. Appl. Catal. B: Environ. 89, 118–122.
Zhou, H., Chen, Q., Li, G., Luo, S., Song, T.-B., Duan, H.-S., Hong, Z., You, J., Liu, Y., Yang, Y., 2014. Interface engineering of highly efficient perovskite solar cells. Science 345, 542–546. Zumeta, I., Ayllo´n, J.A., Gonzalez, B., Dome`nech, X., Vigil, E., 2009. TiO2 films obtained by microwave-activated chemical-bath deposition used to improve TiO2-conducting glass contact. Sol. Energy Mater. Sol. Cells 93, 1728–1732.