- Email: [email protected]
Solvothermal synthesis of TiO2 nanorods to enhance photovoltaic performance of dye-sensitized solar cells
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 132 (2016) 310–320 www.elsevier.com/locate/solener
Solvothermal synthesis of TiO2 nanorods to enhance photovoltaic performance of dye-sensitized solar cells Sasipriya Kathirvel a, Chaochin Su a,⇑, Yung-Jen Shiao a, Ya-Fen Lin b, Bo-Ren Chen b, Wen-Ren Li b,⇑ a
Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 10608, Taiwan b Department of Chemistry, National Central University, Chung-Li 32001, Taiwan Received 6 January 2016; received in revised form 8 March 2016; accepted 12 March 2016
Communicated by: Associate Editor Igor Tyukhov
Abstract In this study, rod shaped TiO2 nanocrystals with excellent crystallinity were obtained by a simple solvothermal approach utilizing isopropyl alcohol without the use of any surfactants or capping agents. The solvothermal process parameters such as the concentration of titanium(IV) isopropoxide and volume of acetic acid were varied to reach optimal synthesis conditions to prepare well-defined and monodispersed TiO2 nanorods. The growth of rod shaped TiO2 nanocrystals has also been investigated by varying the solvothermal reaction durations and temperatures. TiO2 nanorods with various film thicknesses were used as photoanodes to fabricate dye-sensitized solar cells (DSSCs). A power conversion efficiency of 9.21% has been achieved for the DSSC fabricated using a TiO2 nanorod photoanode film with 14 lm thickness. The high photovoltaic performance was attributed to the effective electron transfer of photogenerated electrons in TiO2 nanorod films with enhanced dye adsorption which was favored for efficient light harvesting ability. Ó 2016 Elsevier Ltd. All rights reserved.
Keywords: Rod shaped TiO2 nanocrystals; Solvothermal; Photoanode; Dye-sensitized solar cells
1. Introduction Dye-sensitized solar cells (DSSCs) as a new generation photovoltaic device have attracted prominent interests due to their cost-effective and high-efficiency (Nazeeruddin et al., 2011; O’Regan and Gra¨tzel, 1991). DSSCs consist of a monolayer of dye adsorbed photoanode, an electrolyte, and a counter electrode. Among these, the semiconductor nanoparticle layer covered with dye ⇑ Corresponding authors. Tel.: +886 2 2771 2171x2435; fax: +886 2 2731 7174 (C. Su). Tel.: +886 3 422 7151x65907; fax: +886 3 4277972 (W.-R. Li). E-mail addresses: [email protected] (C. Su), [email protected] (W.-R. Li).
http://dx.doi.org/10.1016/j.solener.2016.03.025 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
molecules is the key element, since it acts as a light harvesting and electron transporting medium (Park and Han, 2014). As a wide band gap semiconductor, TiO2 is the most commonly utilized photoanode material so far compared to ZnO, Nb2O5, SnO2, etc., due to its superior photovoltaic performance (Jose et al., 2009; Mir and Salavati-Niasari, 2012). The TiO2 nanoparticle film with small particle size has pronounced a large surface area for anchoring sufficient dyes (Shiu et al., 2012). However, it has been reported that the nanoparticle film possesses random electron transport and short electron diffusion length due to trapping and detrapping of electrons at the network of numerous grain boundaries (Peter, 2007; Zhang and Cao, 2011). As alternative, one-dimensional TiO2 morphologies like nanotubes, nanorods, or nanowire arrays are considered as
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
good candidates which can provide direct electrical pathways for photogenerated electrons as well as decreased inter-crystalline contacts and thus significantly suppress the charge recombination characteristics (Chen et al., 2012). For instance, the experimental study proved that the electron diffusion length is of the order of 100 lm in TiO2 nanotube arrays based devices (Jennings et al., 2008). However, the overall performances of the above one-dimensional TiO2 nanoarrays based DSSCs are limited due to the low internal surface area, leading to insufficient dye adsorption with low light-harvesting efficiency (Wu et al., 2013; Zhang and Cao, 2011). Therefore, the combination of high surface area and fast electron transport with excellent light harvesting efficiency are essential for an ideal photoanode material. One dimensional anatase rod shaped TiO2 nanocrystals have attracted wide interest since they provide large surface area and efficient electron transport properties to obtain good photovoltaic performance. Many attempts have made in the synthesis of highly crystalline anatase TiO2 nanorods as photoelectrodes for DSSCs (De Marco et al., 2011; Liao et al., 2009). Adachi et al. have reported the TiO2 nanorods with lengths and diameters of 100–300 nm and 20–30 nm, respectively, using surfactant assisted hydrothermal process. The TiO2 anatase nanorods of 16 lm thick photoanode films had reached a power conversion efficiency of 7.06% (Jiu et al., 2006). Lee et al. employed a two-step sol–gel method to prepare shape controlled anatase TiO2 nanocrystals with uniform morphology and size. Different morphologies of TiO2 such as spherical (20 nm diameter), rod (20 nm width and 100 nm length), and wire (20 nm width and 200 nm length) shaped nanocrystals were synthesized. The efficiencies of nanorod (4.7%) and nanowire (4.2%) shaped TiO2 based DSSCs were lower than that of the spherical shaped TiO2 based device (5.3%) (Lee et al., 2010). Although, the lower surface area of longer length nanorods decrease the cell efficiency, these nanorods still show the advantage of charge recombination suppression. Similarly, Baek et al. have reported the large aspect ratio of ellipsoidal like TiO2 with length in the range of 200–350 nm prepared using peroxotitanate solution. The resulted device was reported to have reduced dye loading due to relatively low surface area (43 m2/g) (Baek et al., 2009). On the other hand, the longer length anatase TiO2 nanorods are also functioned as light scattering materials. For instance, TiO2 nanospindles with an average length of 120 nm and diameter of 25 nm were obtained with the addition of diethylenetriamine as a shape controller. The TiO2 nanospindles applied as a scattering layer in DSSC has achieved an efficiency of 6.97%. However, the photoelectrodes based on TiO2 nanospindle as a single active layer has obtained a relatively lower efficiency of 4.75% than commercial P-25 (5.50%) due to its lower dye loading amount (Jiang et al., 2011). All the above works demonstrated the preparation of large aspect ratio of anatase rod like TiO2 nanocrystals with tailored morphology and size either by
311
hydrothermal or sol–gel synthesis with the addition of shape controllers or surfactants. Apart from these methods, the solvothermal synthesis of TiO2 nanocrystals is one of the effective approaches to obtain monodispersed particles with well controlled morphology, size, and crystallinity. Han et al. have reported an aminolysis route to prepare monodisperse anatase titania nanorods with tunable length (12–30 nm) and diameter (2 nm) using 1-octadecene solvent with oleic acid and oleylamine as surfactants (Zhang et al., 2005). Besides, Cozzoli group has developed an anisotropically shaped brookite TiO2 nanocrystals via a nonaqueous surfactant assisted aminolysis route (Buonsanti et al., 2008). Jun et al. also reported the shape evolution of bullet and diamond shaped, rods, and branched rods shaped TiO2 nanocrystals using lauric acid and trioctylphosphine oxide under nonaqueous conditions (Jun et al., 2003). The aforementioned studies are based on the solvothermal synthesis of TiO2 nanocrystals with the presence of surfactants containing organics, which are selectively bind on the surface of crystalline facets leads to different shape evolution with controlled morphology. Therefore the preparation of TiO2 nanocrystals by solvothermal route without the presence of any surfactants or capping agents is more attractive. Marco et al. synthesized an anatase TiO2 nanorods (5 nm 30 nm) using a one-step solvothermal process by a modified benzyl alcohol route. The DSSC based on the smaller nanorods leads to relatively high efficiency of 7.9% by maximizing the dye adsorption (De Marco et al., 2010; Melcarne et al., 2010). Eventhough considerable efforts have been reported on the synthesis of TiO2 nanocrystals, it is still challenging to obtain fine control morphology of TiO2. There are several methods to prepare TiO2 with different morphology, which consists of hydrothermal, surfactant-assisted hydrothermal, solvothermal sol–gel method, electrochemical anodization, and sonochemical, etc. (Chen and Mao, 2007; Jun et al., 2003; Mor et al., 2006; Pavasupree et al., 2006; Zhou et al., 2011). The synthesis of TiO2 nanorods with high surface area via simple preparation methods is more desirable to enhance the cell performance. Based on the above considerations, in the present work, rod shaped TiO2 nanocrystals were synthesized by a simple solvothermal method using isopropyl alcohol (IPA) and acetic acid (AcOH) with the titanium(IV) isopropoxide (TTIP) precursor. Highly crystalline anatase TiO2 nanorods with diameter (10–15 nm) and length (30–50 nm) were obtained. The concentrations of TTIP and volume of AcOH were varied in order to study the effect of different concentrations on the formation of TiO2 nanocrystals. The influence of solvothermal reaction durations and temperatures on the morphology and crystallinity of TiO2 nanocrystals were also investigated. The TiO2 nanorod based photoanode has demonstrated an excellent photoconversion efficiency in DSSC. Further, the effect of different TiO2 film thicknesses on the photoelectric performances of DSSCs was investigated.
312
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
2. Experimental section 2.1. Materials Titanium isopropoxide (TTIP, 98%, ACROS), acetic acid (AcOH, 99.8%, Scharlau), and isopropyl alcohol (IPA, 99.9%, Echo) were used for the synthesis of TiO2. For the TiO2 paste preparation, a-Terpineol (95%, KANTO chemical), ethyl cellulose (SHOWA) and ethanol (99.5%, Echo) were used. Fluorine doped tin oxide (FTO) conducting glass (TEC7 Hartford glass, transmission =80%, sheet resistivity: 8 ohm/h, USA) was used as substrate. Cis-di(thiocyanato)-N,N0 -bis (2,20 -bipyridyl-4-car boxylic acid-40 -tetrabutylammonium carboxylate) ruthenium(II) (N719) was purchased from Solaronix. Acetonitrile (99.9%, J.T. Baker) and tert-Butyl alcohol (99.5%, TEDIA) were the solvents used to dissolve N719 dye. The electrolyte composed of 0.1 M lithium iodide (98%, Acros), 0.05 M iodine, 0.5 M 4-tert-butylpyridine (96%, Aldrich), and 0.5 M 1,2-dimethyl-3-propylimidazolium iodide (99.5%, UR) was dissolved in acetonitrile. All the chemicals were used as received without further purification. 2.2. Synthesis of TiO2 nanorods Fig. 1 shows the steps for the preparation of TiO2 nanorods. In a typical procedure, titanium(IV) isopropoxide (TTIP, 14.21 g) was added into 50 mL of isopropyl alcohol (IPA) and stirred for 10 min. 6 mL of concentrated acetic acid (AcOH, 17 M) was added slowly to the above 1 M solution with continuous stirring. After 2 h, the resulting transparent solution was transferred into a 200 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained in an oven at 150 °C for 8 h (TNR-3). After reaction, the obtained white suspension was washed by centrifugation with ethanol for three times. Subsequently, the sample was dried in air at 110 °C for 8 h.
Later on, several set of experiments were conducted by varying the concentration of TTIP to 0.5 M (TNR-1), 0.8 M (TNR-2), 1.2 M (TNR-4), and 1.5 M (TNR-5) in 50 mL of IPA and volume of AcOH to 5 mL (TNR-6), 7 mL (TNR-7), and 9 mL (TNR-8) by maintaining the other parameters constant. Further the TiO2 nanocrystals were also prepared at 150 °C for different reaction durations such as 2 h (TNR-3-1), 16 h (TNR-3-2), and 24 h (TNR-3-3) and at different reaction temperatures such as 175 °C (TNR-3-4) and 200 °C (TNR-3-5) for 8 h using TTIP: 1 M, AcOH: 6 mL, and IPA: 50 mL (TNR-3). 2.3. Preparation of TiO2 photoelectrodes The dried TiO2 powder (0.8 g) was fine ground and mixed with 10 mL of anhydrous ethanol and stirred for 24 h. The obtained solution was sonicated for 5 min with pulse time of 2 s on and 3 s off. Subsequently, 3.24 g of a-terpineol was added into the solution and then sonication was continued for 5 min. Then the solution containing 0.24 g of 10 cP ethyl cellulose in 3.76 g of ethanol was added into the above mixture and the sonication was repeated for 5 min. The resultant solution was placed in a rotary evaporator at 50 °C for 20 min to remove the excess ethanol. Finally, the obtained viscous paste was rolled using three roller miller to avoid agglomeration of pastes. FTO glass was cleaned in detergent solution, deionized (DI) water, acetone, and methanol using an ultrasonic bath each for 30 min and dried in hot air oven. The TiO2 paste was coated on FTO glass by screen-printing method and dried at 110 °C for 15 min. The coating was repeated for four times to obtain a thickness of 12–15 lm. The active area was 0.16 cm2. The film was annealed at 110 °C for 30 min, 125 °C for 15 min, 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min with a heating rate of 5 °C/min. The TiO2 film was immersed in 0.3 mM of N719 dye solution at 30 °C for 36 h. 2.4. Solar cells assembly Platinum (Pt) counter electrodes were prepared by depositing 20 nm thin Pt layer on FTO substrate by ion sputtering (Hitachi E-1045) method. Two holes were drilled on the Pt electrodes and were cleaned with ethanol using ultrasonic bath for 30 min and dried. The dyesensitized TiO2 films were sealed with Pt counter electrodes using 60 lm (Dupont 1702) Surlyn under hot condition. The ionic electrolyte (0.1 M lithium iodide, 0.05 M iodine, 0.5 M 4-tert-butylpyridine, and 0.5 M 1,2-dimethyl-3propylimidazolium iodide in acetonitrile) was injected into the two holes and sealed with 30 lm surlyn and microscopic cover glass. 2.5. Characterizations
Fig. 1. Flow chart of the preparation of TiO2 nanorods by solvothermal process.
The morphology and structure of TiO2 samples were examined by X-ray diffraction (XRD, PANalytical, X’pert
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
˚ )), PRO, with nickel filtered CuKa radiation (k = 1.5406 A field emission scanning electron microscopy (FESEM, JSM-6400, JEOL), and transmission electron microscopy (TEM, H-7600, Hitachi). The thickness of the TiO2 films was measured by surface profiler (EZ Step, Force). The photovoltaic performances of the device were measured using an AM 1.5 solar simulator. A 300 W Xenon lamp (Oriel, #91160) was used as light source and its light intensity was adjusted using a NREL-calibrated monocrystalline silicon solar cell (PVM134 reference cell, PV Measurement Inc.) to approximate one sun radiation AM 1.5 G. The current–voltage characteristics were measured by applying external potential bias to the cell and the photocurrent was recorded with a Keithley model 2400 source meter. The incident photon-to-current conversion efficiency (IPCE) spectra were measured as a function of wavelength from 400 to 800 nm. The equipment system consists of a 150 W xenon lamp (Oriel, #66902), a monochromator (Oriel Cornerstone TM 130), a lock-in amplifier (SR830 DSP), and an optical Chopper (SR540, Stanford Research Corporation). In addition, the amount of dye solution adsorbed on TiO2 film was analyzed by UV–Vis spectrophotometer (JASCO, V-630). The dye solution was desorbed using 4 mL of 0.1 M NaOH solution and the absorbance of desorbed solution was measured using UV–Vis analyzes.
313
3. Results and discussion 3.1. Preparation and characterization of TiO2 nanorods 3.1.1. Effect of TTIP and AcOH concentrations on the formation of rod shaped TiO2 nanocrystals TiO2 nanocrystals were prepared at the different concentrations of TTIP while maintaining the other parameters constant. The morphologies of TiO2 samples obtained at different TTIP concentrations are shown in the SEM and TEM images (Figs. 2 and 3), respectively. Initially, a well dispersed rod like morphology of TiO2 nanocrystals with an average length of 40–50 nm and width of 5–10 nm were obtained by using 1 M TTIP in 50 mL IPA and 6 mL of AcOH (Fig. 2c). Closely packed with random arrangement of rod like TiO2 nanocrystals were observed from FESEM image (Fig. 3c). Various concentration of TTIP were also investigated at 0.5, 0.8, 1.2, and 1.5 M. From Figs. 2b and 3b, it was observed that the length of the nanorods were slightly decreased (35–40 nm) while reducing the monomer concentration from 1 to 0.8 M. Further reducing the concentration to 0.5 M, the length of the nanorod decreases to around 25–30 nm as indicated in Figs. 2a and 3a. At lower concentration, the TiO2 nanorods were more aggregated which was due to the interaction of excess organic acid capping layers (Carlucci et al., 2014).
Fig. 2. TEM images of TiO2 nanocrystals obtained at different TTIP concentrations (a) TNR-1 (TTIP – 0.5 M), (b) TNR-2 (TTIP – 0.8 M), (c) TNR-3 (TTIP – 1 M), (d) TNR-4 (TTIP – 1.2 M), and (e) TNR-5 (TTIP – 1.5 M) at 150 °C for 8 h.
314
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
Fig. 3. FESEM images of TiO2 nanocrystals obtained at different TTIP concentrations (a) TNR-1 (TTIP – 0.5 M), (b) TNR-2 (TTIP – 0.8 M), (c) TNR-3 (TTIP – 1 M), (d) TNR-4 (TTIP – 1.2 M), and (e) TNR-5 (TTIP – 1.5 M) at 150 °C for 8 h.
At higher concentration (1.2 M), the formation of nanorods was incomplete and the well defined structure was deteriorated (Figs. 2d and 3d). No specific shape and size of TiO2 nanostructures were observed while further increasing the TTIP concentration to 1.5 M (Figs. 2e and 3e). These observations indicate that the excess amount of monomer concentration could hinder the reaction rate due to the insufficient amount of AcOH under solvothermal conditions leading to the formation of distorted and undefined TiO2 structure. At the lowest TTIP concentration, the small nanorods lead to the aggregation which could be due to the fast nucleation rate and crystal growth process. At the highest TTIP concentration, the low reaction rate suppresses the TiO2 nanocrystal growth (Wang et al., 2014). Fig. 4 illustrates the TEM and FESEM images of TiO2 nanorods prepared with different amount of AcOH. In the case of 6 mL AcOH, the formation of TiO2 nanorods was well defined as discussed above (Figs. 2c and 3c). Accordingly, the amount of AcOH was studied at 5, 7, and 9 mL. No specific morphology of TiO2 particles was observed while reducing the amount of AcOH (TNR-3) to 5 mL (TNR-6) (Fig. 4a). FESEM image (Fig. 4d) also shows no specific shape of crystal structures. These results indicate that the crystal growth was incomplete due to the insufficient reacting agents. This result follows the same trend with the highest TTIP concentration. In the case of 7 mL of AcOH (TNR-7), the rod like particles was formed with slightly reduced length of 30–40 nm (Fig. 4b and e). While further increasing the amount of AcOH to 9 mL (TNR-8), the rod shaped particles were aggregated and the length of the rod decreases to 25–30 nm. From the
TEM image (Fig. 4c), it can be observed that the anisotropic growth of rod like particles was diminishing at higher volume of AcOH. Hence the excess amount of acetic acid tends to induce the aggregation of the rod shaped particles. The XRD patterns of the TiO2 samples prepared at different TTIP concentrations and various amount of AcOH are shown in Fig. 5a and b, respectively. All these samples exhibited the anatase phase TiO2 structures. The intensity of (1 0 1) peak was extremely high and the (0 0 4) peak width was narrow as compared to the lower diffraction intensity of (2 0 0), (1 0 5), (2 0 4) peaks in TNR-3 sample i.e., 1 M of TTIP (Fig. 5a). This implies the formation of rod like nanocrystals with preferred crystal growth orientation along the c-axis of anatase lattice (Cozzoli et al., 2003). The intensities of TNR-1, TNR-2, and TNR-4 sample were relatively lower than the TNR-3 sample, which indicates the low crystallization of the particles with reduced anisotropic growth of particles. At higher TTIP concentration (TNR-5), the diffraction peak was very weak, indicating the incomplete crystallization of particles which was consistent with the morphological analysis. Thus, to obtain well crystalline homogeneous TiO2 nanorods, the use of optimal TTIP concentration of 1 M is essential. The domain size calculated from (1 0 1) diffractions of TNR-1, TNR-2, TNR-3, TNR-4, and TNR-5 were 29.55, 11.49, 10.34, 51.74, and 34.45 nm, respectively, indicating the different crystallization behavior based on the dissimilar characteristics line broadening of XRD patterns. In Fig. 5b, the intensity of TNR-6 sample was the lowest which indicates that the crystallinity of TiO2 was decreasing at lower amount of AcOH as consistent with FESEM and TEM results. As reported elsewhere (Zhao et al., 2011), the reactivity
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
315
Fig. 4. (a–c) TEM and (d–f) FESEM images of TiO2 nanocrystals obtained at different volume of AcOH at 150 °C for 8 h.
Fig. 5. XRD patterns of TiO2 nanocrystals obtained at (a) different TTIP concentrations and (b) various amount of AcOH at 150 °C for 8 h.
of TTIP precursor can be tuned by AcOH owing to the chelating effect, which influences the nucleation and growth of TiO2 nanocrystals. At the lowest volume of AcOH, the incomplete reaction leads to the formation of undefined TiO2 structures with poor crystallinity similar to the case of the highest TTIP concentration. Increasing the AcOH content results in the decrease of particle size with more aggregation due to the interaction of capping layers (Xu et al., 2013). The crystalline size of TNR-6, TNR-7, and TNR-8 were 10.34, 9.40, and 10.34 nm, respectively. No significant difference in the crystalline size was observed at the changes of different amount of AcOH. 3.1.2. Effect of solvothermal reaction durations and temperatures on the formation of rod shaped TiO2 nanocrystals Fig. 6 shows TEM images of TiO2 samples prepared at the different solvothermal reaction durations and
temperatures using 1 M of TTIP, 6 mL of AcOH, and 50 mL of IPA. All the other parameters were maintained constant while changing these conditions individually. No specific morphology of TiO2 was observed after 2 h (TNR-3-1) of reaction time as shown in Fig. 6a. The formation of TiO2 nanocrystals was incomplete as the TiO2 precursors were not fully reacted with AcOH and alcohols at the short reaction time. When increasing the time to 8 h (TNR-3), well-defined rod shaped TiO2 nanocrystals were observed as discussed in the above section (Fig. 2c). The formation of nanocrystals become well defined while increasing the reaction time from 2 to 8 h. The length of the TiO2 rods (TNR-3-2) was slightly increased to 45– 50 nm at 16 h reaction time as shown in Fig. 6b. At this condition, the edges of the rod tend to become sharp due to the anisotropic growth. As the reaction time was further extended to 24 h (TNR-3-3), the length of the rod shaped particles (55–60 nm) becomes large and the edges of the
316
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
Fig. 6. TEM images of TiO2 nanocrystals obtained using TTIP: 1 M, IPA: 50 mL, and AcOH: 6 mL at 150 °C under different reaction durations (a) TNR3-1 (150 °C, 2 h), (b) TNR-3-2 (150 °C, 16 h), and (c) TNR-3-3 (150 °C, 24 h and for 8 h at different reaction temperatures (d) TNR-3-4 (175 °C, 8 h), and (e) TNR-3-5 (200 °C, 8 h).
rods were become sharp (Fig. 6c) (Rui et al., 2012). The longer reaction time is beneficial for the complete growth of the TiO2 nanocrystals. The TiO2 samples were prepared at the reaction temperatures of 150, 175, and 200 °C. While increasing the solvothermal reaction temperature from 150 (TNR-3) to 175 °C (TNR-3-4), the length (35–40 nm) of the nanorod was slightly decreased and the width (10 nm) was slightly increased as shown in Fig. 6d. The sharpness of the edges of the rods was diminished. While further increasing to 200 °C (TNR-3-5), the length of the rod was further decreased to 30 nm and the edges of the rod was smooth (Fig. 6e), which indicated the better crystalline structure. The XRD patterns of TiO2 samples obtained at different reaction durations and temperatures are displayed in Fig. 7. No diffraction peak was appeared for the TiO2 sample prepared at 2 h reaction duration, which revealed the amorphous nature of the sample (Fig. 7a). This observation suggests that the short reaction time leads to a poorly crystallized and aggregated TiO2 products due to insufficient hydrolysis reaction (Liao et al., 2012). The XRD peaks were appeared while increasing the reaction time to 8 h which confirmed the formation of anatase phase TiO2 structures. The sharpening of (0 0 4) peak intensity indicated the growth of TiO2 crystal along [0 0 1] direction which was consistent with the TEM image. Further all the diffraction peaks become strong as the reaction time was prolonged to 16 and 24 h (Fig. 7a) which confirmed the complete formation of anatase phase TiO2 structure.
The crystalline size of TNR-3-2 (34.47 nm) and TNR-3-3 (41.36 nm) showed the increasing trend at the longer reaction durations. The XRD patterns of the TiO2 samples obtained at different reaction temperatures are shown in Fig. 7b. Every diffraction peaks were assigned to anatase phase TiO2 for all the samples. In particular the diffraction peak of (1 0 1) was intensified while increasing the reaction temperature from 150 to 175 and 200 °C. This indicated the enhancement of crystallization of the TiO2 samples at higher autoclave reaction temperatures. The crystalline size of TNR-3-4 and TNR-3-5 were 29.55 and 10.34 nm, respectively. The TEM analyzes were consistent with the XRD results. Hence, it was observed that with increasing the reaction durations and temperatures, the XRD peak intensities of anatase phase TiO2 become stronger and the width of the peaks become narrower. This observation indicated the formation of large size crystallites with enhanced crystallinity. These changes confirmed that the reaction temperature and time play an important role in determining the size (length of rod like particles) and crystallinity of TiO2 (Liao et al., 2012; Yu et al., 2013). 3.2. Photovoltaic performances of DSSCs 3.2.1. Different TiO2 photoanodes Fig. 8a shows the I–V curves of the DSSCs fabricated using three TiO2 photoanode films (TNR-3, TNR-3-3, and TNR-3-5). The resultant photovoltaic parameters are
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
317
Fig. 7. XRD patterns of TiO2 nanocrystals obtained at using TTIP: 1 M, IPA: 50 mL, and AcOH: 6 mL at 150 °C under different (a) reaction durations and (b) reaction temperatures.
Fig. 8. (a) I–V characteristics and (b) IPCE spectra of the DSSCs with the photoanode TNR-3 (150 °C, 8 h), TNR-3-3 (150 °C, 24 h), and TNR-3-5 (200 ° C, 8 h).
summarized in Table 1. The DSSCs fabricated from TNR3-3 and TNR-3-5 photoanodes showed higher Jsc of 18.59 and 18.39 mA/cm2, respectively, than that of the device using TNR-3 photoanode. This was due to the enhanced dye loading ability (0.30 lmol/cm2) in the TiO2 nanorods which have better crystallinity. However, both Voc and FF values of the TNR-3 device were higher than the other two cells. The TiO2 nanorods with uniform length and diameter and the negative shift of the Fermi level of TiO2 could result in the lower electron recombination and the enhancement of the Voc value. The better power conversion efficiency of 9.21% for the TNR-3 based DSSC was due to the combination of enhanced Voc and FF value. The IPCE spectra of the DSSCs fabricated using the three different TiO2 photoelectrodes are shown in Fig. 8b. IPCE spectra shows 66%, 62%, and 64% of IPCE value at the wavelength of around 540 nm for TNR-3, TNR-3-3, and TNR-3-5, respectively. The IPCE spectra overlap at the red spectral
range for all the three different photoanodes which perform similar light harvesting efficiencies. 3.2.2. TiO2 nanorods with different film thicknesses The current–voltage (I–V) characteristics of DSSCs based on TiO2 nanorod (TNR-3) photoanode with various film thicknesses are shown in Fig. 9a. The corresponding photovoltaic parameters such as short-circuit photocurrent density (Jsc), open circuit voltage (Voc), fill factor (FF), and conversion efficiency (g) are summarized in Table 2. The different thicknesses of TiO2 films play an important role on the photovoltaic performances of the DSSCs, leading to the variation in the dye adsorption, Jsc, Voc, FF, and g of devices. From Table 2, it can be observed that when the film thickness increases from 5 to 14 lm, the efficiency of DSSCs increases from 6.40% to 9.21% which is 44% of increment. Further increase in the thickness from 14 to 19 and 24 lm leads to a decrease in efficiency of 8.61%
Table 1 Photovoltaic performance of DSSCs based on different TiO2 photoanodes. Photoelectrodes
Thickness (lm)
Dye ads. (lmol/cm2)
Jsc (mA/cm2)
Voc (V)
FF
g (%)
TNR-3 TNR-3-3 TNR-3-5
14 14 14
0.29 0.30 0.30
17.75 18.59 18.39
0.75 0.73 0.73
0.70 0.66 0.67
9.21 8.98 9.02
318
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
Fig. 9. (a) I–V characteristics and (b) IPCE spectra of the DSSCs based on TiO2 nanorod (TNR-3) photoanodes with various film thicknesses.
Table 2 Photovoltaic performance parameters and amount of adsorbed dye for DSSCs based on TiO2 nanorod (TNR-3) photoanode with various film thicknesses. TNR-3 cells
Thickness (lm)
Dye ads. (lmol/cm2)
Jsc (mA/cm2)
Voc (V)
FF
g (%)
1 2 3 4 5
5 9 14 19 24
0.07 0.17 0.29 0.35 0.43
10.97 16.24 17.75 16.83 16.81
0.80 0.76 0.75 0.72 0.71
0.73 0.70 0.70 0.70 0.69
6.40 8.59 9.21 8.61 8.27
and 8.27%, respectively. The maximum efficiency of 9.21% was obtained with a film thickness of 14 lm in which Jsc was at a maximum value of 17.75 mA/cm2. The dye adsorption, short-circuit current, open-circuit voltage, fill factor, and efficiency as a function of film thickness are plotted in Fig. 10. The increase of dye adsorption can be observed for TNR-3 cells with increasing film thickness. The thicker photoanodes possessed a large surface area leading to the enhanced adsorption of dye molecules which in turn captures more light to generate photoexcited electrons. As larger amount of the sensitizers adsorbed on TiO2 surface to have more electron injection from excited state of dyes to the conduction band of TiO2, the Jsc of the cell usually increases accordingly (Wang et al., 2004).
The short-circuit current density of the cell increased with increasing film thickness from 5 to 14 lm to its maximum value and then decreased from 14 to 24 lm. The reduced Jsc value at higher film thickness could be due to the longer diffusion length of electron transport to the electrode which enhancing the probability of recombination (Tsai et al., 2013). The efficiency of the cell was mainly corresponding to the Jsc value even though Voc and FF vary with thickness. The open circuit voltage (Voc) decreased linearly with increasing the film thickness which may be attribute to the increased charge recombination sites and restricted electron transport in thicker films. The lower Voc could also be cause by the increase of surface trapping sites at the TiO2 film. The FF decreased from 0.73 to 0.70 while
Fig. 10. Dependence of dye adsorption, short-circuit current density, open-circuit voltage, fill factor, and efficiency on film thickness.
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
increasing the film thickness from 5 to 19 lm. At particular increase of thickness, FF gets saturated and then further decreased to 0.69 at higher thickness (24 lm). The decrease in the FF value at 24 lm thick film could be due to increased series resistance. Hence the best cell efficiency was obtained for the optimal TiO2 film thickness of 14 lm. Fig. 9b shows the IPCE spectra as a function of wavelength for DSSCs based on different thickness of TNR-3 films. The IPCE data were in good agreement with the efficiencies of the devices. The device using the 14 lm thick TiO2 film (TNR-3 cell-3) has the highest quantum efficiency of about 67% at 530 nm, which was approximately 48% and 36% higher than that of the 5 lm (TNR3 cell-1) and 24 lm (TNR-3 cell-5) films, respectively. 4. Conclusions In this work, rod shaped TiO2 nanocrystals were prepared by a simple solvothermal method. The effect of different reaction parameters such as TTIP concentrations and AcOH volume on the formation of TiO2 nanorods was studied. At the lowest TTIP concentration or at the highest AcOH concentration, the size of the rod shaped particles reduced and the aggregation of particles was observed. Similarly, at the highest concentration of TTIP and the lowest amount of AcOH, no specific structure was obtained due to the incomplete reactions. A well dispersed rod shaped TiO2 (TNR-3) with a diameter and length of 10–15 and 30–50 nm was obtained using 1 M of TTIP in 50 mL of IPA and 6 mL of AcOH. The formation of anatase phase TNR-3 with high crystallinity was revealed by XRD analysis. DSSCs were then fabricated using TNR-3 of various film thicknesses (5, 9, 14, 19, and 24 lm). The highest efficiency of 9.21% was achieved for 14 lm thick TiO2 film with enhanced light harvesting capacity which was confirmed by the IPCE data. Further increase of film thickness would cause a decrease of Jsc due to its longer electron diffusion length and thus leading to the reduction in efficiency. This study demonstrates that solvothermal reaction parameters play an important role on the formation of TiO2 nanorods and their effects on the photovoltaic properties of DSSCs. Acknowledgements We thank the Ministry of Science and Technology, Taiwan, R.O.C., for financial support (grants MOST-1042113-M-027-007-MY3 and MOST-103-2113-M-008-008MY3). References Baek, I.C., Vithal, M., Chang, J.A., Yum, J.-H., Nazeeruddin, M.K., Gra¨tzel, M., Chung, Y.-C., Seok, S.I., 2009. Facile preparation of large aspect ratio ellipsoidal anatase TiO2 nanoparticles and their application to dye-sensitized solar cell. Electrochem. Commun. 11, 909–912. http://dx.doi.org/10.1016/j.elecom.2009.02.026.
319
Buonsanti, R., Grillo, V., Carlino, E., Giannini, C., Kipp, T., Cingolani, R., Cozzoli, P.D., 2008. Nonhydrolytic synthesis of high-quality anisotropically shaped brookite TiO2 nanocrystals. J. Am. Chem. Soc. 130, 11223–11233. http://dx.doi.org/10.1021/ja803559b. Carlucci, C., Xu, H., Scremin, B.F., Giannini, C., Altamura, D., Carlino, E., Videtta, V., Conciauro, F., Gigli, G., Ciccarella, G., 2014. Selective synthesis of TiO2 nanocrystals with morphology control with the microwave-solvothermal method. CrystEngComm 16, 1817–1824. http://dx.doi.org/10.1039/C3CE41477A. Chen, H.-Y., Kuang, D.-B., Su, C.-Y., 2012. Hierarchically micro/nanostructured photoanode materials for dye-sensitized solar cells. J. Mater. Chem. 22, 15475–15489. http://dx.doi.org/10.1039/C2JM32402D. Chen, X., Mao, S.S., 2007. Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chem. Rev. 107, 2891– 2959. http://dx.doi.org/10.1021/cr0500535. Cozzoli, P.D., Kornowski, A., Weller, H., 2003. Low-temperature synthesis of soluble and processable organic-capped anatase TiO2 nanorods. J. Am. Chem. Soc. 125, 14539–14548. http://dx.doi.org/ 10.1021/ja036505h. De Marco, L., Manca, M., Buonsanti, R., Giannuzzi, R., Malara, F., Pareo, P., Martiradonna, L., Giancaspro, N.M., Cozzoli, P.D., Gigli, G., 2011. High-quality photoelectrodes based on shape-tailored TiO2 nanocrystals for dye-sensitized solar cells. J. Mater. Chem. 21, 13371– 13379. http://dx.doi.org/10.1039/C1JM11887K. De Marco, L., Manca, M., Giannuzzi, R., Malara, F., Melcarne, G., Ciccarella, G., Zama, I., Cingolani, R., Gigli, G., 2010. Novel preparation method of TiO2-nanorod-based photoelectrodes for dyesensitized solar cells with improved light-harvesting efficiency. J. Phys. Chem. C 114, 4228–4236. http://dx.doi.org/10.1021/jp910346d. Jennings, J.R., Ghicov, A., Peter, L.M., Schmuki, P., Walker, A.B., 2008. Dye-sensitized solar cells based on oriented TiO2 nanotube arrays: transport, trapping, and transfer of electrons. J. Am. Chem. Soc. 130, 13364–13372. http://dx.doi.org/10.1021/ja804852z. Jiang, J., Gu, F., Ren, X., Wang, Y., Shao, W., Li, C., Huang, G., 2011. Efficient light scattering from one-pot solvothermally derived TiO2 nanospindles. Ind. Eng. Chem. Res. 50, 9003–9008. http://dx.doi.org/ 10.1021/ie200803q. Jiu, J., Isoda, S., Wang, F., Adachi, M., 2006. Dye-sensitized solar cells based on a single-crystalline TiO2 nanorod film. J. Phys. Chem. B 110, 2087–2092. http://dx.doi.org/10.1021/jp055824n. Jose, R., Thavasi, V., Ramakrishna, S., 2009. Metal oxides for dyesensitized solar cells. J. Am. Ceram. Soc. 92, 289–301. http://dx.doi. org/10.1111/j.1551-2916.2008.02870.x. Jun, Y.-W., Casula, M.F., Sim, J.-H., Kim, S.Y., Cheon, J., Alivisatos, A. P., 2003. Surfactant-assisted elimination of a high energy facet as a means of controlling the shapes of TiO2 nanocrystals. J. Am. Chem. Soc. 125, 15981–15985. http://dx.doi.org/10.1021/ja0369515. Lee, S., Cho, I.-S., Lee, J.H., Kim, D.H., Kim, D.W., Kim, J.Y., Shin, H., Lee, J.-K., Jung, H.S., Park, N.-G., Kim, K., Ko, M.J., Hong, K.S., 2010. Two-step sol–gel method-based TiO2 nanoparticles with uniform morphology and size for efficient photo-energy conversion devices. Chem. Mater. 22, 1958–1965. http://dx.doi.org/10.1021/cm902842k. Liao, J.-Y., He, J.-W., Xu, H., Kuang, D.-B., Su, C.-Y., 2012. Effect of TiO2 morphology on photovoltaic performance of dye-sensitized solar cells: nanoparticles, nanofibers, hierarchical spheres and ellipsoid spheres. J. Mater. Chem. 22, 7910–7918. http://dx.doi.org/10.1039/ C2JM16148F. Liao, J., Shi, L., Yuan, S., Zhao, Y., Fang, J., 2009. Solvothermal synthesis of TiO2 nanocrystal colloids from peroxotitanate complex solution and their photocatalytic activities. J. Phys. Chem. C 113, 18778–18783. http://dx.doi.org/10.1021/jp905720g. Melcarne, G., De Marco, L., Carlino, E., Martina, F., Manca, M., Cingolani, R., Gigli, G., Ciccarella, G., 2010. Surfactant-free synthesis of pure anatase TiO2 nanorods suitable for dye-sensitized solar cells. J. Mater. Chem. 20, 7248–7254. http://dx.doi.org/10.1039/C0JM01167C. Mir, N., Salavati-Niasari, M., 2012. Photovoltaic properties of corresponding dye sensitized solar cells: effect of active sites of growth
320
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
controller on TiO2 nanostructures. Sol. Energy 86, 3397–3404. http:// dx.doi.org/10.1016/j.solener.2012.08.016. Mor, G.K., Varghese, O.K., Paulose, M., Shankar, K., Grimes, C.A., 2006. A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties, and solar energy applications. Sol. Energy Mater. Sol. C 90, 2011–2075. http://dx.doi.org/10.1016/ j.solmat.2006.04.007. Nazeeruddin, M.K., Baranoff, E., Gra¨tzel, M., 2011. Dye-sensitized solar cells: a brief overview. Sol. Energy 85, 1172–1178. http://dx.doi.org/ 10.1016/j.solener.2011.01.018. O’Regan, B., Gra¨tzel, M., 1991. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737–740. http://dx. doi.org/10.1038/353737a0. Park, S.K., Han, Y.S., 2014. Efficient dye-sensitized solar cells with surface-modified photoelectrodes. Sol. Energy 110, 260–267. http://dx. doi.org/10.1016/j.solener.2014.09.019. Pavasupree, S., Ngamsinlapasathian, S., Suzuki, Y., Yoshikawa, S., 2006. Hydrothermal synthesis of nanorods/nanoparticles TiO2 for photocatalytic activity and dye-sensitized solar cell applications. MRS Online Proc. Libr. 951. http://dx.doi.org/10.1557/PROC-0951-E09-04. Peter, L.M., 2007. Dye-sensitized nanocrystalline solar cells. Phys. Chem. Chem. Phys. 9, 2630–2642. http://dx.doi.org/10.1039/B617073K. Rui, Y., Li, Y., Wang, H., Zhang, Q., 2012. Photoanode based on chainshaped anatase TiO2 nanorods for high-efficiency dye-sensitized solar cells. Chem. Asian J. 7, 2313–2320. http://dx.doi.org/10.1002/ asia.201200590. Shiu, J.-W., Lan, C.-M., Chang, Y.-C., Wu, H.-P., Huang, W.-K., Diau, E.W.-G., 2012. Size-controlled anatase titania single crystals with octahedron-like morphology for dye-sensitized solar cells. ACS Nano 6, 10862–10873. http://dx.doi.org/10.1021/nn3042418. Tsai, J., Hsu, W., Wu, T., Meen, T., Chong, W., 2013. Effect of compressed TiO2 nanoparticle thin film thickness on the performance of dye-sensitized solar cells. Nanoscale Res. Lett. 8 (459), 1–6,. Wang, S., Ding, Y., Xu, S., Zhang, Y., Li, G., Hu, L., Dai, S., 2014. TiO2 nanospheres: a facile size-tunable synthesis and effective light-harvest-
ing layer for dye-sensitized solar cells. Chem. Eur. J. 20, 4916–4920. http://dx.doi.org/10.1002/chem.201304963. Wang, Z.-S., Kawauchi, H., Kashima, T., Arakawa, H., 2004. Significant influence of TiO2 photoelectrode morphology on the energy conversion efficiency of N719 dye-sensitized solar cell. Coord. Chem. Rev. 248, 1381–1389. http://dx.doi.org/10.1016/j.ccr.2004.03.006. Wu, W.-Q., Rao, H.-S., Xu, Y.-F., Wang, Y.-F., Su, C.-Y., Kuang, D.-B., 2013. Hierarchical oriented anatase TiO2 nanostructure arrays on flexible substrate for efficient dye-sensitized solar cells. Sci. Rep. 3, 1–7. http://dx.doi.org/10.1038/srep01892. Xu, H., Picca, R.A., De Marco, L., Carlucci, C., Scrascia, A., Papadia, P., Scremin, B.F., Carlino, E., Giannini, C., Malitesta, C., Mazzeo, M., Gigli, G., Ciccarella, G., 2013. Nonhydrolytic route to boron-doped TiO2 nanocrystals. Eur. J. Inorg. Chem. 364–374. http://dx.doi.org/ 10.1002/ejic.201200842. Yu, X., Wang, H., Liu, Y., Zhou, X., Li, B., Xin, L., Zhou, Y., Shen, H., 2013. One-step ammonia hydrothermal synthesis of single crystal anatase TiO2 nanowires for highly efficient dye-sensitized solar cells. J. Mater. Chem. A 1, 2110–2117. http://dx.doi.org/10.1039/ C2TA00494A. Zhang, Q., Cao, G., 2011. Nanostructured photoelectrodes for dyesensitized solar cells. Nano Today 6, 91–109. http://dx.doi.org/ 10.1016/j.nantod.2010.12.007. Zhang, Z., Zhong, X., Liu, S., Li, D., Han, M., 2005. Aminolysis route to monodisperse titania nanorods with tunable aspect ratio. Angew. Chem. Int. Ed. 44, 3466–3470. http://dx.doi.org/10.1002/ anie.200500410. Zhao, X., Jin, W., Cai, J., Ye, J., Li, Z., Ma, Y., Xie, J., Qi, L., 2011. Shape- and size-controlled synthesis of uniform anatase TiO2 nanocuboids enclosed by active {1 0 0} and {0 0 1} facets. Adv. Funct. Mater. 21, 3554–3563. http://dx.doi.org/10.1002/adfm.201100629. Zhou, J., Zhao, G., Song, B., Han, G., 2011. Solvent-controlled synthesis of three-dimensional TiO2 nanostructuresvia a one-step solvothermal route. CrystEngComm 13, 2294–2302. http://dx.doi.org/10.1039/ C0CE00793E.
ScienceDirect Solar Energy 132 (2016) 310–320 www.elsevier.com/locate/solener
Solvothermal synthesis of TiO2 nanorods to enhance photovoltaic performance of dye-sensitized solar cells Sasipriya Kathirvel a, Chaochin Su a,⇑, Yung-Jen Shiao a, Ya-Fen Lin b, Bo-Ren Chen b, Wen-Ren Li b,⇑ a
Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 10608, Taiwan b Department of Chemistry, National Central University, Chung-Li 32001, Taiwan Received 6 January 2016; received in revised form 8 March 2016; accepted 12 March 2016
Communicated by: Associate Editor Igor Tyukhov
Abstract In this study, rod shaped TiO2 nanocrystals with excellent crystallinity were obtained by a simple solvothermal approach utilizing isopropyl alcohol without the use of any surfactants or capping agents. The solvothermal process parameters such as the concentration of titanium(IV) isopropoxide and volume of acetic acid were varied to reach optimal synthesis conditions to prepare well-defined and monodispersed TiO2 nanorods. The growth of rod shaped TiO2 nanocrystals has also been investigated by varying the solvothermal reaction durations and temperatures. TiO2 nanorods with various film thicknesses were used as photoanodes to fabricate dye-sensitized solar cells (DSSCs). A power conversion efficiency of 9.21% has been achieved for the DSSC fabricated using a TiO2 nanorod photoanode film with 14 lm thickness. The high photovoltaic performance was attributed to the effective electron transfer of photogenerated electrons in TiO2 nanorod films with enhanced dye adsorption which was favored for efficient light harvesting ability. Ó 2016 Elsevier Ltd. All rights reserved.
Keywords: Rod shaped TiO2 nanocrystals; Solvothermal; Photoanode; Dye-sensitized solar cells
1. Introduction Dye-sensitized solar cells (DSSCs) as a new generation photovoltaic device have attracted prominent interests due to their cost-effective and high-efficiency (Nazeeruddin et al., 2011; O’Regan and Gra¨tzel, 1991). DSSCs consist of a monolayer of dye adsorbed photoanode, an electrolyte, and a counter electrode. Among these, the semiconductor nanoparticle layer covered with dye ⇑ Corresponding authors. Tel.: +886 2 2771 2171x2435; fax: +886 2 2731 7174 (C. Su). Tel.: +886 3 422 7151x65907; fax: +886 3 4277972 (W.-R. Li). E-mail addresses: [email protected] (C. Su), [email protected] (W.-R. Li).
http://dx.doi.org/10.1016/j.solener.2016.03.025 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
molecules is the key element, since it acts as a light harvesting and electron transporting medium (Park and Han, 2014). As a wide band gap semiconductor, TiO2 is the most commonly utilized photoanode material so far compared to ZnO, Nb2O5, SnO2, etc., due to its superior photovoltaic performance (Jose et al., 2009; Mir and Salavati-Niasari, 2012). The TiO2 nanoparticle film with small particle size has pronounced a large surface area for anchoring sufficient dyes (Shiu et al., 2012). However, it has been reported that the nanoparticle film possesses random electron transport and short electron diffusion length due to trapping and detrapping of electrons at the network of numerous grain boundaries (Peter, 2007; Zhang and Cao, 2011). As alternative, one-dimensional TiO2 morphologies like nanotubes, nanorods, or nanowire arrays are considered as
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
good candidates which can provide direct electrical pathways for photogenerated electrons as well as decreased inter-crystalline contacts and thus significantly suppress the charge recombination characteristics (Chen et al., 2012). For instance, the experimental study proved that the electron diffusion length is of the order of 100 lm in TiO2 nanotube arrays based devices (Jennings et al., 2008). However, the overall performances of the above one-dimensional TiO2 nanoarrays based DSSCs are limited due to the low internal surface area, leading to insufficient dye adsorption with low light-harvesting efficiency (Wu et al., 2013; Zhang and Cao, 2011). Therefore, the combination of high surface area and fast electron transport with excellent light harvesting efficiency are essential for an ideal photoanode material. One dimensional anatase rod shaped TiO2 nanocrystals have attracted wide interest since they provide large surface area and efficient electron transport properties to obtain good photovoltaic performance. Many attempts have made in the synthesis of highly crystalline anatase TiO2 nanorods as photoelectrodes for DSSCs (De Marco et al., 2011; Liao et al., 2009). Adachi et al. have reported the TiO2 nanorods with lengths and diameters of 100–300 nm and 20–30 nm, respectively, using surfactant assisted hydrothermal process. The TiO2 anatase nanorods of 16 lm thick photoanode films had reached a power conversion efficiency of 7.06% (Jiu et al., 2006). Lee et al. employed a two-step sol–gel method to prepare shape controlled anatase TiO2 nanocrystals with uniform morphology and size. Different morphologies of TiO2 such as spherical (20 nm diameter), rod (20 nm width and 100 nm length), and wire (20 nm width and 200 nm length) shaped nanocrystals were synthesized. The efficiencies of nanorod (4.7%) and nanowire (4.2%) shaped TiO2 based DSSCs were lower than that of the spherical shaped TiO2 based device (5.3%) (Lee et al., 2010). Although, the lower surface area of longer length nanorods decrease the cell efficiency, these nanorods still show the advantage of charge recombination suppression. Similarly, Baek et al. have reported the large aspect ratio of ellipsoidal like TiO2 with length in the range of 200–350 nm prepared using peroxotitanate solution. The resulted device was reported to have reduced dye loading due to relatively low surface area (43 m2/g) (Baek et al., 2009). On the other hand, the longer length anatase TiO2 nanorods are also functioned as light scattering materials. For instance, TiO2 nanospindles with an average length of 120 nm and diameter of 25 nm were obtained with the addition of diethylenetriamine as a shape controller. The TiO2 nanospindles applied as a scattering layer in DSSC has achieved an efficiency of 6.97%. However, the photoelectrodes based on TiO2 nanospindle as a single active layer has obtained a relatively lower efficiency of 4.75% than commercial P-25 (5.50%) due to its lower dye loading amount (Jiang et al., 2011). All the above works demonstrated the preparation of large aspect ratio of anatase rod like TiO2 nanocrystals with tailored morphology and size either by
311
hydrothermal or sol–gel synthesis with the addition of shape controllers or surfactants. Apart from these methods, the solvothermal synthesis of TiO2 nanocrystals is one of the effective approaches to obtain monodispersed particles with well controlled morphology, size, and crystallinity. Han et al. have reported an aminolysis route to prepare monodisperse anatase titania nanorods with tunable length (12–30 nm) and diameter (2 nm) using 1-octadecene solvent with oleic acid and oleylamine as surfactants (Zhang et al., 2005). Besides, Cozzoli group has developed an anisotropically shaped brookite TiO2 nanocrystals via a nonaqueous surfactant assisted aminolysis route (Buonsanti et al., 2008). Jun et al. also reported the shape evolution of bullet and diamond shaped, rods, and branched rods shaped TiO2 nanocrystals using lauric acid and trioctylphosphine oxide under nonaqueous conditions (Jun et al., 2003). The aforementioned studies are based on the solvothermal synthesis of TiO2 nanocrystals with the presence of surfactants containing organics, which are selectively bind on the surface of crystalline facets leads to different shape evolution with controlled morphology. Therefore the preparation of TiO2 nanocrystals by solvothermal route without the presence of any surfactants or capping agents is more attractive. Marco et al. synthesized an anatase TiO2 nanorods (5 nm 30 nm) using a one-step solvothermal process by a modified benzyl alcohol route. The DSSC based on the smaller nanorods leads to relatively high efficiency of 7.9% by maximizing the dye adsorption (De Marco et al., 2010; Melcarne et al., 2010). Eventhough considerable efforts have been reported on the synthesis of TiO2 nanocrystals, it is still challenging to obtain fine control morphology of TiO2. There are several methods to prepare TiO2 with different morphology, which consists of hydrothermal, surfactant-assisted hydrothermal, solvothermal sol–gel method, electrochemical anodization, and sonochemical, etc. (Chen and Mao, 2007; Jun et al., 2003; Mor et al., 2006; Pavasupree et al., 2006; Zhou et al., 2011). The synthesis of TiO2 nanorods with high surface area via simple preparation methods is more desirable to enhance the cell performance. Based on the above considerations, in the present work, rod shaped TiO2 nanocrystals were synthesized by a simple solvothermal method using isopropyl alcohol (IPA) and acetic acid (AcOH) with the titanium(IV) isopropoxide (TTIP) precursor. Highly crystalline anatase TiO2 nanorods with diameter (10–15 nm) and length (30–50 nm) were obtained. The concentrations of TTIP and volume of AcOH were varied in order to study the effect of different concentrations on the formation of TiO2 nanocrystals. The influence of solvothermal reaction durations and temperatures on the morphology and crystallinity of TiO2 nanocrystals were also investigated. The TiO2 nanorod based photoanode has demonstrated an excellent photoconversion efficiency in DSSC. Further, the effect of different TiO2 film thicknesses on the photoelectric performances of DSSCs was investigated.
312
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
2. Experimental section 2.1. Materials Titanium isopropoxide (TTIP, 98%, ACROS), acetic acid (AcOH, 99.8%, Scharlau), and isopropyl alcohol (IPA, 99.9%, Echo) were used for the synthesis of TiO2. For the TiO2 paste preparation, a-Terpineol (95%, KANTO chemical), ethyl cellulose (SHOWA) and ethanol (99.5%, Echo) were used. Fluorine doped tin oxide (FTO) conducting glass (TEC7 Hartford glass, transmission =80%, sheet resistivity: 8 ohm/h, USA) was used as substrate. Cis-di(thiocyanato)-N,N0 -bis (2,20 -bipyridyl-4-car boxylic acid-40 -tetrabutylammonium carboxylate) ruthenium(II) (N719) was purchased from Solaronix. Acetonitrile (99.9%, J.T. Baker) and tert-Butyl alcohol (99.5%, TEDIA) were the solvents used to dissolve N719 dye. The electrolyte composed of 0.1 M lithium iodide (98%, Acros), 0.05 M iodine, 0.5 M 4-tert-butylpyridine (96%, Aldrich), and 0.5 M 1,2-dimethyl-3-propylimidazolium iodide (99.5%, UR) was dissolved in acetonitrile. All the chemicals were used as received without further purification. 2.2. Synthesis of TiO2 nanorods Fig. 1 shows the steps for the preparation of TiO2 nanorods. In a typical procedure, titanium(IV) isopropoxide (TTIP, 14.21 g) was added into 50 mL of isopropyl alcohol (IPA) and stirred for 10 min. 6 mL of concentrated acetic acid (AcOH, 17 M) was added slowly to the above 1 M solution with continuous stirring. After 2 h, the resulting transparent solution was transferred into a 200 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained in an oven at 150 °C for 8 h (TNR-3). After reaction, the obtained white suspension was washed by centrifugation with ethanol for three times. Subsequently, the sample was dried in air at 110 °C for 8 h.
Later on, several set of experiments were conducted by varying the concentration of TTIP to 0.5 M (TNR-1), 0.8 M (TNR-2), 1.2 M (TNR-4), and 1.5 M (TNR-5) in 50 mL of IPA and volume of AcOH to 5 mL (TNR-6), 7 mL (TNR-7), and 9 mL (TNR-8) by maintaining the other parameters constant. Further the TiO2 nanocrystals were also prepared at 150 °C for different reaction durations such as 2 h (TNR-3-1), 16 h (TNR-3-2), and 24 h (TNR-3-3) and at different reaction temperatures such as 175 °C (TNR-3-4) and 200 °C (TNR-3-5) for 8 h using TTIP: 1 M, AcOH: 6 mL, and IPA: 50 mL (TNR-3). 2.3. Preparation of TiO2 photoelectrodes The dried TiO2 powder (0.8 g) was fine ground and mixed with 10 mL of anhydrous ethanol and stirred for 24 h. The obtained solution was sonicated for 5 min with pulse time of 2 s on and 3 s off. Subsequently, 3.24 g of a-terpineol was added into the solution and then sonication was continued for 5 min. Then the solution containing 0.24 g of 10 cP ethyl cellulose in 3.76 g of ethanol was added into the above mixture and the sonication was repeated for 5 min. The resultant solution was placed in a rotary evaporator at 50 °C for 20 min to remove the excess ethanol. Finally, the obtained viscous paste was rolled using three roller miller to avoid agglomeration of pastes. FTO glass was cleaned in detergent solution, deionized (DI) water, acetone, and methanol using an ultrasonic bath each for 30 min and dried in hot air oven. The TiO2 paste was coated on FTO glass by screen-printing method and dried at 110 °C for 15 min. The coating was repeated for four times to obtain a thickness of 12–15 lm. The active area was 0.16 cm2. The film was annealed at 110 °C for 30 min, 125 °C for 15 min, 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min with a heating rate of 5 °C/min. The TiO2 film was immersed in 0.3 mM of N719 dye solution at 30 °C for 36 h. 2.4. Solar cells assembly Platinum (Pt) counter electrodes were prepared by depositing 20 nm thin Pt layer on FTO substrate by ion sputtering (Hitachi E-1045) method. Two holes were drilled on the Pt electrodes and were cleaned with ethanol using ultrasonic bath for 30 min and dried. The dyesensitized TiO2 films were sealed with Pt counter electrodes using 60 lm (Dupont 1702) Surlyn under hot condition. The ionic electrolyte (0.1 M lithium iodide, 0.05 M iodine, 0.5 M 4-tert-butylpyridine, and 0.5 M 1,2-dimethyl-3propylimidazolium iodide in acetonitrile) was injected into the two holes and sealed with 30 lm surlyn and microscopic cover glass. 2.5. Characterizations
Fig. 1. Flow chart of the preparation of TiO2 nanorods by solvothermal process.
The morphology and structure of TiO2 samples were examined by X-ray diffraction (XRD, PANalytical, X’pert
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
˚ )), PRO, with nickel filtered CuKa radiation (k = 1.5406 A field emission scanning electron microscopy (FESEM, JSM-6400, JEOL), and transmission electron microscopy (TEM, H-7600, Hitachi). The thickness of the TiO2 films was measured by surface profiler (EZ Step, Force). The photovoltaic performances of the device were measured using an AM 1.5 solar simulator. A 300 W Xenon lamp (Oriel, #91160) was used as light source and its light intensity was adjusted using a NREL-calibrated monocrystalline silicon solar cell (PVM134 reference cell, PV Measurement Inc.) to approximate one sun radiation AM 1.5 G. The current–voltage characteristics were measured by applying external potential bias to the cell and the photocurrent was recorded with a Keithley model 2400 source meter. The incident photon-to-current conversion efficiency (IPCE) spectra were measured as a function of wavelength from 400 to 800 nm. The equipment system consists of a 150 W xenon lamp (Oriel, #66902), a monochromator (Oriel Cornerstone TM 130), a lock-in amplifier (SR830 DSP), and an optical Chopper (SR540, Stanford Research Corporation). In addition, the amount of dye solution adsorbed on TiO2 film was analyzed by UV–Vis spectrophotometer (JASCO, V-630). The dye solution was desorbed using 4 mL of 0.1 M NaOH solution and the absorbance of desorbed solution was measured using UV–Vis analyzes.
313
3. Results and discussion 3.1. Preparation and characterization of TiO2 nanorods 3.1.1. Effect of TTIP and AcOH concentrations on the formation of rod shaped TiO2 nanocrystals TiO2 nanocrystals were prepared at the different concentrations of TTIP while maintaining the other parameters constant. The morphologies of TiO2 samples obtained at different TTIP concentrations are shown in the SEM and TEM images (Figs. 2 and 3), respectively. Initially, a well dispersed rod like morphology of TiO2 nanocrystals with an average length of 40–50 nm and width of 5–10 nm were obtained by using 1 M TTIP in 50 mL IPA and 6 mL of AcOH (Fig. 2c). Closely packed with random arrangement of rod like TiO2 nanocrystals were observed from FESEM image (Fig. 3c). Various concentration of TTIP were also investigated at 0.5, 0.8, 1.2, and 1.5 M. From Figs. 2b and 3b, it was observed that the length of the nanorods were slightly decreased (35–40 nm) while reducing the monomer concentration from 1 to 0.8 M. Further reducing the concentration to 0.5 M, the length of the nanorod decreases to around 25–30 nm as indicated in Figs. 2a and 3a. At lower concentration, the TiO2 nanorods were more aggregated which was due to the interaction of excess organic acid capping layers (Carlucci et al., 2014).
Fig. 2. TEM images of TiO2 nanocrystals obtained at different TTIP concentrations (a) TNR-1 (TTIP – 0.5 M), (b) TNR-2 (TTIP – 0.8 M), (c) TNR-3 (TTIP – 1 M), (d) TNR-4 (TTIP – 1.2 M), and (e) TNR-5 (TTIP – 1.5 M) at 150 °C for 8 h.
314
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
Fig. 3. FESEM images of TiO2 nanocrystals obtained at different TTIP concentrations (a) TNR-1 (TTIP – 0.5 M), (b) TNR-2 (TTIP – 0.8 M), (c) TNR-3 (TTIP – 1 M), (d) TNR-4 (TTIP – 1.2 M), and (e) TNR-5 (TTIP – 1.5 M) at 150 °C for 8 h.
At higher concentration (1.2 M), the formation of nanorods was incomplete and the well defined structure was deteriorated (Figs. 2d and 3d). No specific shape and size of TiO2 nanostructures were observed while further increasing the TTIP concentration to 1.5 M (Figs. 2e and 3e). These observations indicate that the excess amount of monomer concentration could hinder the reaction rate due to the insufficient amount of AcOH under solvothermal conditions leading to the formation of distorted and undefined TiO2 structure. At the lowest TTIP concentration, the small nanorods lead to the aggregation which could be due to the fast nucleation rate and crystal growth process. At the highest TTIP concentration, the low reaction rate suppresses the TiO2 nanocrystal growth (Wang et al., 2014). Fig. 4 illustrates the TEM and FESEM images of TiO2 nanorods prepared with different amount of AcOH. In the case of 6 mL AcOH, the formation of TiO2 nanorods was well defined as discussed above (Figs. 2c and 3c). Accordingly, the amount of AcOH was studied at 5, 7, and 9 mL. No specific morphology of TiO2 particles was observed while reducing the amount of AcOH (TNR-3) to 5 mL (TNR-6) (Fig. 4a). FESEM image (Fig. 4d) also shows no specific shape of crystal structures. These results indicate that the crystal growth was incomplete due to the insufficient reacting agents. This result follows the same trend with the highest TTIP concentration. In the case of 7 mL of AcOH (TNR-7), the rod like particles was formed with slightly reduced length of 30–40 nm (Fig. 4b and e). While further increasing the amount of AcOH to 9 mL (TNR-8), the rod shaped particles were aggregated and the length of the rod decreases to 25–30 nm. From the
TEM image (Fig. 4c), it can be observed that the anisotropic growth of rod like particles was diminishing at higher volume of AcOH. Hence the excess amount of acetic acid tends to induce the aggregation of the rod shaped particles. The XRD patterns of the TiO2 samples prepared at different TTIP concentrations and various amount of AcOH are shown in Fig. 5a and b, respectively. All these samples exhibited the anatase phase TiO2 structures. The intensity of (1 0 1) peak was extremely high and the (0 0 4) peak width was narrow as compared to the lower diffraction intensity of (2 0 0), (1 0 5), (2 0 4) peaks in TNR-3 sample i.e., 1 M of TTIP (Fig. 5a). This implies the formation of rod like nanocrystals with preferred crystal growth orientation along the c-axis of anatase lattice (Cozzoli et al., 2003). The intensities of TNR-1, TNR-2, and TNR-4 sample were relatively lower than the TNR-3 sample, which indicates the low crystallization of the particles with reduced anisotropic growth of particles. At higher TTIP concentration (TNR-5), the diffraction peak was very weak, indicating the incomplete crystallization of particles which was consistent with the morphological analysis. Thus, to obtain well crystalline homogeneous TiO2 nanorods, the use of optimal TTIP concentration of 1 M is essential. The domain size calculated from (1 0 1) diffractions of TNR-1, TNR-2, TNR-3, TNR-4, and TNR-5 were 29.55, 11.49, 10.34, 51.74, and 34.45 nm, respectively, indicating the different crystallization behavior based on the dissimilar characteristics line broadening of XRD patterns. In Fig. 5b, the intensity of TNR-6 sample was the lowest which indicates that the crystallinity of TiO2 was decreasing at lower amount of AcOH as consistent with FESEM and TEM results. As reported elsewhere (Zhao et al., 2011), the reactivity
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
315
Fig. 4. (a–c) TEM and (d–f) FESEM images of TiO2 nanocrystals obtained at different volume of AcOH at 150 °C for 8 h.
Fig. 5. XRD patterns of TiO2 nanocrystals obtained at (a) different TTIP concentrations and (b) various amount of AcOH at 150 °C for 8 h.
of TTIP precursor can be tuned by AcOH owing to the chelating effect, which influences the nucleation and growth of TiO2 nanocrystals. At the lowest volume of AcOH, the incomplete reaction leads to the formation of undefined TiO2 structures with poor crystallinity similar to the case of the highest TTIP concentration. Increasing the AcOH content results in the decrease of particle size with more aggregation due to the interaction of capping layers (Xu et al., 2013). The crystalline size of TNR-6, TNR-7, and TNR-8 were 10.34, 9.40, and 10.34 nm, respectively. No significant difference in the crystalline size was observed at the changes of different amount of AcOH. 3.1.2. Effect of solvothermal reaction durations and temperatures on the formation of rod shaped TiO2 nanocrystals Fig. 6 shows TEM images of TiO2 samples prepared at the different solvothermal reaction durations and
temperatures using 1 M of TTIP, 6 mL of AcOH, and 50 mL of IPA. All the other parameters were maintained constant while changing these conditions individually. No specific morphology of TiO2 was observed after 2 h (TNR-3-1) of reaction time as shown in Fig. 6a. The formation of TiO2 nanocrystals was incomplete as the TiO2 precursors were not fully reacted with AcOH and alcohols at the short reaction time. When increasing the time to 8 h (TNR-3), well-defined rod shaped TiO2 nanocrystals were observed as discussed in the above section (Fig. 2c). The formation of nanocrystals become well defined while increasing the reaction time from 2 to 8 h. The length of the TiO2 rods (TNR-3-2) was slightly increased to 45– 50 nm at 16 h reaction time as shown in Fig. 6b. At this condition, the edges of the rod tend to become sharp due to the anisotropic growth. As the reaction time was further extended to 24 h (TNR-3-3), the length of the rod shaped particles (55–60 nm) becomes large and the edges of the
316
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
Fig. 6. TEM images of TiO2 nanocrystals obtained using TTIP: 1 M, IPA: 50 mL, and AcOH: 6 mL at 150 °C under different reaction durations (a) TNR3-1 (150 °C, 2 h), (b) TNR-3-2 (150 °C, 16 h), and (c) TNR-3-3 (150 °C, 24 h and for 8 h at different reaction temperatures (d) TNR-3-4 (175 °C, 8 h), and (e) TNR-3-5 (200 °C, 8 h).
rods were become sharp (Fig. 6c) (Rui et al., 2012). The longer reaction time is beneficial for the complete growth of the TiO2 nanocrystals. The TiO2 samples were prepared at the reaction temperatures of 150, 175, and 200 °C. While increasing the solvothermal reaction temperature from 150 (TNR-3) to 175 °C (TNR-3-4), the length (35–40 nm) of the nanorod was slightly decreased and the width (10 nm) was slightly increased as shown in Fig. 6d. The sharpness of the edges of the rods was diminished. While further increasing to 200 °C (TNR-3-5), the length of the rod was further decreased to 30 nm and the edges of the rod was smooth (Fig. 6e), which indicated the better crystalline structure. The XRD patterns of TiO2 samples obtained at different reaction durations and temperatures are displayed in Fig. 7. No diffraction peak was appeared for the TiO2 sample prepared at 2 h reaction duration, which revealed the amorphous nature of the sample (Fig. 7a). This observation suggests that the short reaction time leads to a poorly crystallized and aggregated TiO2 products due to insufficient hydrolysis reaction (Liao et al., 2012). The XRD peaks were appeared while increasing the reaction time to 8 h which confirmed the formation of anatase phase TiO2 structures. The sharpening of (0 0 4) peak intensity indicated the growth of TiO2 crystal along [0 0 1] direction which was consistent with the TEM image. Further all the diffraction peaks become strong as the reaction time was prolonged to 16 and 24 h (Fig. 7a) which confirmed the complete formation of anatase phase TiO2 structure.
The crystalline size of TNR-3-2 (34.47 nm) and TNR-3-3 (41.36 nm) showed the increasing trend at the longer reaction durations. The XRD patterns of the TiO2 samples obtained at different reaction temperatures are shown in Fig. 7b. Every diffraction peaks were assigned to anatase phase TiO2 for all the samples. In particular the diffraction peak of (1 0 1) was intensified while increasing the reaction temperature from 150 to 175 and 200 °C. This indicated the enhancement of crystallization of the TiO2 samples at higher autoclave reaction temperatures. The crystalline size of TNR-3-4 and TNR-3-5 were 29.55 and 10.34 nm, respectively. The TEM analyzes were consistent with the XRD results. Hence, it was observed that with increasing the reaction durations and temperatures, the XRD peak intensities of anatase phase TiO2 become stronger and the width of the peaks become narrower. This observation indicated the formation of large size crystallites with enhanced crystallinity. These changes confirmed that the reaction temperature and time play an important role in determining the size (length of rod like particles) and crystallinity of TiO2 (Liao et al., 2012; Yu et al., 2013). 3.2. Photovoltaic performances of DSSCs 3.2.1. Different TiO2 photoanodes Fig. 8a shows the I–V curves of the DSSCs fabricated using three TiO2 photoanode films (TNR-3, TNR-3-3, and TNR-3-5). The resultant photovoltaic parameters are
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
317
Fig. 7. XRD patterns of TiO2 nanocrystals obtained at using TTIP: 1 M, IPA: 50 mL, and AcOH: 6 mL at 150 °C under different (a) reaction durations and (b) reaction temperatures.
Fig. 8. (a) I–V characteristics and (b) IPCE spectra of the DSSCs with the photoanode TNR-3 (150 °C, 8 h), TNR-3-3 (150 °C, 24 h), and TNR-3-5 (200 ° C, 8 h).
summarized in Table 1. The DSSCs fabricated from TNR3-3 and TNR-3-5 photoanodes showed higher Jsc of 18.59 and 18.39 mA/cm2, respectively, than that of the device using TNR-3 photoanode. This was due to the enhanced dye loading ability (0.30 lmol/cm2) in the TiO2 nanorods which have better crystallinity. However, both Voc and FF values of the TNR-3 device were higher than the other two cells. The TiO2 nanorods with uniform length and diameter and the negative shift of the Fermi level of TiO2 could result in the lower electron recombination and the enhancement of the Voc value. The better power conversion efficiency of 9.21% for the TNR-3 based DSSC was due to the combination of enhanced Voc and FF value. The IPCE spectra of the DSSCs fabricated using the three different TiO2 photoelectrodes are shown in Fig. 8b. IPCE spectra shows 66%, 62%, and 64% of IPCE value at the wavelength of around 540 nm for TNR-3, TNR-3-3, and TNR-3-5, respectively. The IPCE spectra overlap at the red spectral
range for all the three different photoanodes which perform similar light harvesting efficiencies. 3.2.2. TiO2 nanorods with different film thicknesses The current–voltage (I–V) characteristics of DSSCs based on TiO2 nanorod (TNR-3) photoanode with various film thicknesses are shown in Fig. 9a. The corresponding photovoltaic parameters such as short-circuit photocurrent density (Jsc), open circuit voltage (Voc), fill factor (FF), and conversion efficiency (g) are summarized in Table 2. The different thicknesses of TiO2 films play an important role on the photovoltaic performances of the DSSCs, leading to the variation in the dye adsorption, Jsc, Voc, FF, and g of devices. From Table 2, it can be observed that when the film thickness increases from 5 to 14 lm, the efficiency of DSSCs increases from 6.40% to 9.21% which is 44% of increment. Further increase in the thickness from 14 to 19 and 24 lm leads to a decrease in efficiency of 8.61%
Table 1 Photovoltaic performance of DSSCs based on different TiO2 photoanodes. Photoelectrodes
Thickness (lm)
Dye ads. (lmol/cm2)
Jsc (mA/cm2)
Voc (V)
FF
g (%)
TNR-3 TNR-3-3 TNR-3-5
14 14 14
0.29 0.30 0.30
17.75 18.59 18.39
0.75 0.73 0.73
0.70 0.66 0.67
9.21 8.98 9.02
318
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
Fig. 9. (a) I–V characteristics and (b) IPCE spectra of the DSSCs based on TiO2 nanorod (TNR-3) photoanodes with various film thicknesses.
Table 2 Photovoltaic performance parameters and amount of adsorbed dye for DSSCs based on TiO2 nanorod (TNR-3) photoanode with various film thicknesses. TNR-3 cells
Thickness (lm)
Dye ads. (lmol/cm2)
Jsc (mA/cm2)
Voc (V)
FF
g (%)
1 2 3 4 5
5 9 14 19 24
0.07 0.17 0.29 0.35 0.43
10.97 16.24 17.75 16.83 16.81
0.80 0.76 0.75 0.72 0.71
0.73 0.70 0.70 0.70 0.69
6.40 8.59 9.21 8.61 8.27
and 8.27%, respectively. The maximum efficiency of 9.21% was obtained with a film thickness of 14 lm in which Jsc was at a maximum value of 17.75 mA/cm2. The dye adsorption, short-circuit current, open-circuit voltage, fill factor, and efficiency as a function of film thickness are plotted in Fig. 10. The increase of dye adsorption can be observed for TNR-3 cells with increasing film thickness. The thicker photoanodes possessed a large surface area leading to the enhanced adsorption of dye molecules which in turn captures more light to generate photoexcited electrons. As larger amount of the sensitizers adsorbed on TiO2 surface to have more electron injection from excited state of dyes to the conduction band of TiO2, the Jsc of the cell usually increases accordingly (Wang et al., 2004).
The short-circuit current density of the cell increased with increasing film thickness from 5 to 14 lm to its maximum value and then decreased from 14 to 24 lm. The reduced Jsc value at higher film thickness could be due to the longer diffusion length of electron transport to the electrode which enhancing the probability of recombination (Tsai et al., 2013). The efficiency of the cell was mainly corresponding to the Jsc value even though Voc and FF vary with thickness. The open circuit voltage (Voc) decreased linearly with increasing the film thickness which may be attribute to the increased charge recombination sites and restricted electron transport in thicker films. The lower Voc could also be cause by the increase of surface trapping sites at the TiO2 film. The FF decreased from 0.73 to 0.70 while
Fig. 10. Dependence of dye adsorption, short-circuit current density, open-circuit voltage, fill factor, and efficiency on film thickness.
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
increasing the film thickness from 5 to 19 lm. At particular increase of thickness, FF gets saturated and then further decreased to 0.69 at higher thickness (24 lm). The decrease in the FF value at 24 lm thick film could be due to increased series resistance. Hence the best cell efficiency was obtained for the optimal TiO2 film thickness of 14 lm. Fig. 9b shows the IPCE spectra as a function of wavelength for DSSCs based on different thickness of TNR-3 films. The IPCE data were in good agreement with the efficiencies of the devices. The device using the 14 lm thick TiO2 film (TNR-3 cell-3) has the highest quantum efficiency of about 67% at 530 nm, which was approximately 48% and 36% higher than that of the 5 lm (TNR3 cell-1) and 24 lm (TNR-3 cell-5) films, respectively. 4. Conclusions In this work, rod shaped TiO2 nanocrystals were prepared by a simple solvothermal method. The effect of different reaction parameters such as TTIP concentrations and AcOH volume on the formation of TiO2 nanorods was studied. At the lowest TTIP concentration or at the highest AcOH concentration, the size of the rod shaped particles reduced and the aggregation of particles was observed. Similarly, at the highest concentration of TTIP and the lowest amount of AcOH, no specific structure was obtained due to the incomplete reactions. A well dispersed rod shaped TiO2 (TNR-3) with a diameter and length of 10–15 and 30–50 nm was obtained using 1 M of TTIP in 50 mL of IPA and 6 mL of AcOH. The formation of anatase phase TNR-3 with high crystallinity was revealed by XRD analysis. DSSCs were then fabricated using TNR-3 of various film thicknesses (5, 9, 14, 19, and 24 lm). The highest efficiency of 9.21% was achieved for 14 lm thick TiO2 film with enhanced light harvesting capacity which was confirmed by the IPCE data. Further increase of film thickness would cause a decrease of Jsc due to its longer electron diffusion length and thus leading to the reduction in efficiency. This study demonstrates that solvothermal reaction parameters play an important role on the formation of TiO2 nanorods and their effects on the photovoltaic properties of DSSCs. Acknowledgements We thank the Ministry of Science and Technology, Taiwan, R.O.C., for financial support (grants MOST-1042113-M-027-007-MY3 and MOST-103-2113-M-008-008MY3). References Baek, I.C., Vithal, M., Chang, J.A., Yum, J.-H., Nazeeruddin, M.K., Gra¨tzel, M., Chung, Y.-C., Seok, S.I., 2009. Facile preparation of large aspect ratio ellipsoidal anatase TiO2 nanoparticles and their application to dye-sensitized solar cell. Electrochem. Commun. 11, 909–912. http://dx.doi.org/10.1016/j.elecom.2009.02.026.
319
Buonsanti, R., Grillo, V., Carlino, E., Giannini, C., Kipp, T., Cingolani, R., Cozzoli, P.D., 2008. Nonhydrolytic synthesis of high-quality anisotropically shaped brookite TiO2 nanocrystals. J. Am. Chem. Soc. 130, 11223–11233. http://dx.doi.org/10.1021/ja803559b. Carlucci, C., Xu, H., Scremin, B.F., Giannini, C., Altamura, D., Carlino, E., Videtta, V., Conciauro, F., Gigli, G., Ciccarella, G., 2014. Selective synthesis of TiO2 nanocrystals with morphology control with the microwave-solvothermal method. CrystEngComm 16, 1817–1824. http://dx.doi.org/10.1039/C3CE41477A. Chen, H.-Y., Kuang, D.-B., Su, C.-Y., 2012. Hierarchically micro/nanostructured photoanode materials for dye-sensitized solar cells. J. Mater. Chem. 22, 15475–15489. http://dx.doi.org/10.1039/C2JM32402D. Chen, X., Mao, S.S., 2007. Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chem. Rev. 107, 2891– 2959. http://dx.doi.org/10.1021/cr0500535. Cozzoli, P.D., Kornowski, A., Weller, H., 2003. Low-temperature synthesis of soluble and processable organic-capped anatase TiO2 nanorods. J. Am. Chem. Soc. 125, 14539–14548. http://dx.doi.org/ 10.1021/ja036505h. De Marco, L., Manca, M., Buonsanti, R., Giannuzzi, R., Malara, F., Pareo, P., Martiradonna, L., Giancaspro, N.M., Cozzoli, P.D., Gigli, G., 2011. High-quality photoelectrodes based on shape-tailored TiO2 nanocrystals for dye-sensitized solar cells. J. Mater. Chem. 21, 13371– 13379. http://dx.doi.org/10.1039/C1JM11887K. De Marco, L., Manca, M., Giannuzzi, R., Malara, F., Melcarne, G., Ciccarella, G., Zama, I., Cingolani, R., Gigli, G., 2010. Novel preparation method of TiO2-nanorod-based photoelectrodes for dyesensitized solar cells with improved light-harvesting efficiency. J. Phys. Chem. C 114, 4228–4236. http://dx.doi.org/10.1021/jp910346d. Jennings, J.R., Ghicov, A., Peter, L.M., Schmuki, P., Walker, A.B., 2008. Dye-sensitized solar cells based on oriented TiO2 nanotube arrays: transport, trapping, and transfer of electrons. J. Am. Chem. Soc. 130, 13364–13372. http://dx.doi.org/10.1021/ja804852z. Jiang, J., Gu, F., Ren, X., Wang, Y., Shao, W., Li, C., Huang, G., 2011. Efficient light scattering from one-pot solvothermally derived TiO2 nanospindles. Ind. Eng. Chem. Res. 50, 9003–9008. http://dx.doi.org/ 10.1021/ie200803q. Jiu, J., Isoda, S., Wang, F., Adachi, M., 2006. Dye-sensitized solar cells based on a single-crystalline TiO2 nanorod film. J. Phys. Chem. B 110, 2087–2092. http://dx.doi.org/10.1021/jp055824n. Jose, R., Thavasi, V., Ramakrishna, S., 2009. Metal oxides for dyesensitized solar cells. J. Am. Ceram. Soc. 92, 289–301. http://dx.doi. org/10.1111/j.1551-2916.2008.02870.x. Jun, Y.-W., Casula, M.F., Sim, J.-H., Kim, S.Y., Cheon, J., Alivisatos, A. P., 2003. Surfactant-assisted elimination of a high energy facet as a means of controlling the shapes of TiO2 nanocrystals. J. Am. Chem. Soc. 125, 15981–15985. http://dx.doi.org/10.1021/ja0369515. Lee, S., Cho, I.-S., Lee, J.H., Kim, D.H., Kim, D.W., Kim, J.Y., Shin, H., Lee, J.-K., Jung, H.S., Park, N.-G., Kim, K., Ko, M.J., Hong, K.S., 2010. Two-step sol–gel method-based TiO2 nanoparticles with uniform morphology and size for efficient photo-energy conversion devices. Chem. Mater. 22, 1958–1965. http://dx.doi.org/10.1021/cm902842k. Liao, J.-Y., He, J.-W., Xu, H., Kuang, D.-B., Su, C.-Y., 2012. Effect of TiO2 morphology on photovoltaic performance of dye-sensitized solar cells: nanoparticles, nanofibers, hierarchical spheres and ellipsoid spheres. J. Mater. Chem. 22, 7910–7918. http://dx.doi.org/10.1039/ C2JM16148F. Liao, J., Shi, L., Yuan, S., Zhao, Y., Fang, J., 2009. Solvothermal synthesis of TiO2 nanocrystal colloids from peroxotitanate complex solution and their photocatalytic activities. J. Phys. Chem. C 113, 18778–18783. http://dx.doi.org/10.1021/jp905720g. Melcarne, G., De Marco, L., Carlino, E., Martina, F., Manca, M., Cingolani, R., Gigli, G., Ciccarella, G., 2010. Surfactant-free synthesis of pure anatase TiO2 nanorods suitable for dye-sensitized solar cells. J. Mater. Chem. 20, 7248–7254. http://dx.doi.org/10.1039/C0JM01167C. Mir, N., Salavati-Niasari, M., 2012. Photovoltaic properties of corresponding dye sensitized solar cells: effect of active sites of growth
320
S. Kathirvel et al. / Solar Energy 132 (2016) 310–320
controller on TiO2 nanostructures. Sol. Energy 86, 3397–3404. http:// dx.doi.org/10.1016/j.solener.2012.08.016. Mor, G.K., Varghese, O.K., Paulose, M., Shankar, K., Grimes, C.A., 2006. A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties, and solar energy applications. Sol. Energy Mater. Sol. C 90, 2011–2075. http://dx.doi.org/10.1016/ j.solmat.2006.04.007. Nazeeruddin, M.K., Baranoff, E., Gra¨tzel, M., 2011. Dye-sensitized solar cells: a brief overview. Sol. Energy 85, 1172–1178. http://dx.doi.org/ 10.1016/j.solener.2011.01.018. O’Regan, B., Gra¨tzel, M., 1991. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737–740. http://dx. doi.org/10.1038/353737a0. Park, S.K., Han, Y.S., 2014. Efficient dye-sensitized solar cells with surface-modified photoelectrodes. Sol. Energy 110, 260–267. http://dx. doi.org/10.1016/j.solener.2014.09.019. Pavasupree, S., Ngamsinlapasathian, S., Suzuki, Y., Yoshikawa, S., 2006. Hydrothermal synthesis of nanorods/nanoparticles TiO2 for photocatalytic activity and dye-sensitized solar cell applications. MRS Online Proc. Libr. 951. http://dx.doi.org/10.1557/PROC-0951-E09-04. Peter, L.M., 2007. Dye-sensitized nanocrystalline solar cells. Phys. Chem. Chem. Phys. 9, 2630–2642. http://dx.doi.org/10.1039/B617073K. Rui, Y., Li, Y., Wang, H., Zhang, Q., 2012. Photoanode based on chainshaped anatase TiO2 nanorods for high-efficiency dye-sensitized solar cells. Chem. Asian J. 7, 2313–2320. http://dx.doi.org/10.1002/ asia.201200590. Shiu, J.-W., Lan, C.-M., Chang, Y.-C., Wu, H.-P., Huang, W.-K., Diau, E.W.-G., 2012. Size-controlled anatase titania single crystals with octahedron-like morphology for dye-sensitized solar cells. ACS Nano 6, 10862–10873. http://dx.doi.org/10.1021/nn3042418. Tsai, J., Hsu, W., Wu, T., Meen, T., Chong, W., 2013. Effect of compressed TiO2 nanoparticle thin film thickness on the performance of dye-sensitized solar cells. Nanoscale Res. Lett. 8 (459), 1–6,
ing layer for dye-sensitized solar cells. Chem. Eur. J. 20, 4916–4920. http://dx.doi.org/10.1002/chem.201304963. Wang, Z.-S., Kawauchi, H., Kashima, T., Arakawa, H., 2004. Significant influence of TiO2 photoelectrode morphology on the energy conversion efficiency of N719 dye-sensitized solar cell. Coord. Chem. Rev. 248, 1381–1389. http://dx.doi.org/10.1016/j.ccr.2004.03.006. Wu, W.-Q., Rao, H.-S., Xu, Y.-F., Wang, Y.-F., Su, C.-Y., Kuang, D.-B., 2013. Hierarchical oriented anatase TiO2 nanostructure arrays on flexible substrate for efficient dye-sensitized solar cells. Sci. Rep. 3, 1–7. http://dx.doi.org/10.1038/srep01892. Xu, H., Picca, R.A., De Marco, L., Carlucci, C., Scrascia, A., Papadia, P., Scremin, B.F., Carlino, E., Giannini, C., Malitesta, C., Mazzeo, M., Gigli, G., Ciccarella, G., 2013. Nonhydrolytic route to boron-doped TiO2 nanocrystals. Eur. J. Inorg. Chem. 364–374. http://dx.doi.org/ 10.1002/ejic.201200842. Yu, X., Wang, H., Liu, Y., Zhou, X., Li, B., Xin, L., Zhou, Y., Shen, H., 2013. One-step ammonia hydrothermal synthesis of single crystal anatase TiO2 nanowires for highly efficient dye-sensitized solar cells. J. Mater. Chem. A 1, 2110–2117. http://dx.doi.org/10.1039/ C2TA00494A. Zhang, Q., Cao, G., 2011. Nanostructured photoelectrodes for dyesensitized solar cells. Nano Today 6, 91–109. http://dx.doi.org/ 10.1016/j.nantod.2010.12.007. Zhang, Z., Zhong, X., Liu, S., Li, D., Han, M., 2005. Aminolysis route to monodisperse titania nanorods with tunable aspect ratio. Angew. Chem. Int. Ed. 44, 3466–3470. http://dx.doi.org/10.1002/ anie.200500410. Zhao, X., Jin, W., Cai, J., Ye, J., Li, Z., Ma, Y., Xie, J., Qi, L., 2011. Shape- and size-controlled synthesis of uniform anatase TiO2 nanocuboids enclosed by active {1 0 0} and {0 0 1} facets. Adv. Funct. Mater. 21, 3554–3563. http://dx.doi.org/10.1002/adfm.201100629. Zhou, J., Zhao, G., Song, B., Han, G., 2011. Solvent-controlled synthesis of three-dimensional TiO2 nanostructuresvia a one-step solvothermal route. CrystEngComm 13, 2294–2302. http://dx.doi.org/10.1039/ C0CE00793E.