One-step synthesis of titanium oxide with trilayer structure for dye-sensitized solar cells

Titanium oxide films with trilayer structure grown on fluorine doped tin oxide substrate were prepared from one-step hydrothermal process. The trilayer structure consists of microflowers, nanorod array and compact nanoparticulates, which is expected to posses the merits of good light harvesting, a high electron transport rate, while avoiding the issues of electron shunting. The photovoltaic performance was comprehensively studied and a 60% enhancement in short circuit photocurrent density was found from microflowers contribution as a light scattering layer. This unique trilayer structure exhibits great potential application in future dyesensitized solar cells. Since impressive light-to-electricity conversion efficiency of dye-sensitized solar cells (DSSCs) based on photosensitization of TiO2 nanocrystalline film was reported in 1991, DSSCs have attracted increasing attention as one of the most promising candidates for low cost solar energy conversion devices. Recently, experimental and theoretical research indicated that the geometric shape of the semiconductor plays an important role in dictating its physical properties and the performance of associated devices. Thus, considerable effort has been devoted to developing various architectures in TiO2, in particular one dimensional (1D) structure as it has the potential to offer direct electrical pathways for photogenerated electrons, and hence increase the electron transport rate and improve the conversion efficiency. Several approaches have been adopted to achieve 1D arrays grown directly on fluorine doped tin oxide (FTO) substrate, e.g., hydrothermal/solvothermal and electrochemical synthesis processes. However, one issue of using 1D array film for DSSC application is the electron shunting (direct reaction with the electrolyte) at the FTO/electrolyte interface. Another issue is the low light absorption, which is often caused by its small specific surface area and poor light trapping, resulting in significant light loss and unsatisfactory device performance. Therefore, it is necessary to tailor the film structure to improve the performance. An ideal structure of a TiO2 film for DSSC application should consist of three layers: a compact layer, a nanorod array layer and a light scattering layer (Fig. 1a). The compact layer is composed of nanoparticles which efficiently prevent the electron shunting at the FTO/electrolyte interface. The TiO2 nanorod array forms the second layer for electron generation and transport. The light scattering layer with micron sized TiO2 particles enhances the light harvesting efficiency. In this paper, we successfully prepared TiO2 films with a trilayer structure (3LS) grown directly on FTO substrate using one-step hydrothermal process. The assembled DSSC device demonstrated improved photovoltaic performance. To prepare the 3LS TiO2 film via a hydrothermal process, a cleaned FTO substrate (TEC 15, Libbey Owens Ford) was placed into a sealed Teflon reactor (45mL, Parr Instrument) containing 0.045−0.15 mL (1.5, 3 and 5 mM) titanium butoxide (Sigma-Aldrich), 10 mL hydrochloric acid (32 wt%) and 20 mL deionized water. The synthesis was conducted at 160 °C for 12 h in an electric oven. After cooling, the film was thoroughly rinsed with deionized water and then dried. For comparison, TiO2 powder with flower-like shape was also prepared using 0.45 mL (15 mM) titanium butoxide in the above described process without the FTO substrate. The morphology and crystal structure of TiO2 films and powder were examined using scanning electron microscopy (SEM, FEI Quanta 200) and X-ray diffraction (XRD, PANanalytical Xpert Pro), respectively. DSSCs were assembled using the same procedure reported before. Briefly, the asprepared 3LS TiO2 film was sintered at 500 oC for 30 min followed by a TiCl4 (40 mM aqueous solution) treatment. After washing with deionized water, the film was sintered at 450 oC for 30 min, and then immersed in absolute ethanol containing 0.25 mM N719 (Dyesol, Australia) for 16 h. The DSSCs were finally packed by sealing the dye coated TiO2 electrode with a thermally platinized FTO counter electrode through a thin thermal plastic film (Surlyn, 25 μm, Solaronix). An electrolyte composed of 0.1 M N-methyl benzimidiazoium, 0.6 M 1propyl-3-methylimidiazolium iodide, 0.05 M I2, 0.1 M guanidinium thiocyanate and 0.2 M NaI in 3-methoxypropionitrile was introduced into the cell through a predrilled hole in the counter electrode. The hole was subsequently sealed with a microscope slip using Surlyn film. To study the light scattering effect from the microflower on the DSSC performance, three different cells were assembled with (1) TiO2 nanoparticles only (film thickness of 6.5 μm), (2) TiO2 nanoparticles covered with a microflower layer; and (3) TiO2 microflowers only (film thickness of 3 μm). The detailed assembling process with these nanoparticles can be found in Ref. 20. The microflower based cell was made via the so-called doctor-blading technique with a solution containing 0.1 g microflower powder, 0.4 ml ethanol and 0.02 ml α-terpinol. The photocurrent-voltage (J-V) characteristics of the DSSCs were evaluated with a solar simulator under illumination intensity of 100 mW⋅cm (AM 1.5). The open-circuit photovoltage decay was recorded by an electrochemical work station (Princeton Applied Research) after illumination. XRD (Fig. 1b) result shows that the as-prepared TiO2 film is rutile phase with space group of P42/mnm (JCPDS card No. 21-1276). The diffraction peaks labelled by star are from FTO substrate. Figure 1c shows the cross section of the as-prepared film. Some microflowers are observed on the top layer. Those flowers are composed of several rods with the length of 1−3 μm and diameter of 200−500 nm. Underneath the microflowers is a layer of TiO2 nanorod array, which grows vertically on the substrate. The diameter and length of the rod are about 100 nm and 600 nm, respectively (Fig. 1d). The layer of nanoparticles underneath the TiO2 nanorod array is shown in Fig. 1e. The nanoparticles are closely packed together and form a dense layer. Note that this 3LS film was obtained from such a simple one-step process. Growth of TiO2 nanowire/nanorod array films on FTO substrate by hydrothermal method has been reported before. To the best of our knowledge, this is the first time to achieve a 3LS film directly on FTO substrate without any seed and template. We found that the concentration of titanium butoxide was a critical parameter for the morphology control. As shown in Fig. 2a, with 1.5 mM titanium butoxide, only nanorod array is formed, as reported previously. Increasing the concentration to 3 mM, one layer of microflowers on nanorod array was observed. The distribution of the microflowers is quite uniform (Fig. 2b) and the diameter of nanorods is around 80 nm (Fig. 2c). Further increasing the concentration to 5 mM, the diameter of nanorods increases to around 200 nm (Fig. 2d), and the microflowers aggregate severely on the top layer (Fig. 2e). Figure 2f shows TiO2 powders with flower-like shape obtained from 15 mM titanium butoxide in the precursor without FTO substrate. It can be seen that the microflower size is around 5 μm, similar to those in Fig. 2e. Figure 3a shows the J-V characteristics of a DSSC with 3LS film under illumination intensity of 100 mW∙cm. The short-circuit photocurrent density (Jsc), open-circuit voltage (Voc) and fill factor (FF) of the cell is 1.74 mA∙cm, 0.804 V and 0.68 respectively, leading to an overall power conversion efficiency of 1.19%. Considering the short length (~600 nm) of the nanorod prepared in this work, we believe that higher efficiencies could be achieved with longer nanorods. It is noted that the Voc and FF of the cell are promising, which may be attributed to this unique 3LS structure. The contribution of the TiO2 microflowers as light scattering layer on the performance of the DSSCs is shown in Fig. 3b. It can be seen that the Jsc increases from 6.8 to 10.8 mA∙cm (60% enhancement) when adding one layer of microflower on TiO2 nanoparticles. However, the voltage of the cell with this microflower layer is 10 mV lower than that of the cell without it. This is due to the higher electron density which causes a higher recombination between electrons in the TiO2 film and the oxidized species of the electrolyte. Benefiting from the significant enhancement in Jsc, the efficiency of the cell increases from 2.59% to 3.74%. As mentioned before, we also investigated the J-V performance of the cell with the TiO2 film only consisting of microflowers. The Jsc and Voc of the cell are 0.04 mA∙cm and 0.39 V, respectively (inset of Fig. 3b). Clearly, the contribution of the microflower layer is considered to enhance light scattering effect, but not to generate additional photocurrent. The open-circuit photovoltage decay of the DSSCs with a 3LS and a nanoparticle TiO2 films are shown in Fig. 3c. Apparently, the voltage of the cell with the nanoparticle film quickly drops to zero within 4 s after the light was switched off, suggesting a severe electron recombination at the interface of bare FTO/electrolyte. In contrast, the cell with 3LS TiO2 film possesses a high electron density as the voltage of 320 mV is maintained even after 180 s in dark. This clearly demonstrates a nearly perfect ‘blocking’ effect on electrical shunting from the compact layer in the 3LS film. It is well known that a compact layer in DSSC can improve the Voc, and therefore the FF as well. Therefore, the high Voc (804 mV) and good FF (0.68) observed in this work may be due to the formation of the compact layer in the 3LS film. In summary, trilayer structured TiO2 film directly grown on FTO substrate was fabricated by a simple one-step hydrothermal process. The first layer is TiO2 nanoparticles based compact layer, the middle layer is TiO2 nanorod array with a thickness of about 600 nm, and the top layer is featured by TiO2 microflowers. DSSC assembled with this thin 3LS film demonstrated an energy conversion efficiency of 1.19%. The microflowers can be used as a light scattering layer in DSSC, and nearly 60% enhancement in Jsc was achieved. Therefore, this unique trilayer structure shows a great potential for practical applications. Future work is required to obtain longer nanorods and further improve light extinction coefficient of dye materials for better conversion efficiency.