The central energy source elated from the extraterrestrial space, solar energy capacities to surpass the almanac world’s energy request by a large border. Given the long forecast era of the sun, solar energy is also considered the ultimate renewable source that can be harvested on the planet, earth. In 1991, a new photoelectric converter which named dye-sensitized solar cell (DSSC) was successfully prepared under appropriate process conditions by O’Regan and Gratzel (Oregan and Grätzel, 1991). Thereafter, DSSC has gained wide attention recent years as potential low-cost solar cells, owing to their simple manufacturing process, environmentally friendly material and relatively high power conversion efficiency (PCE) (Chen et al., 2017; Ponken et al., 2019; Ünlü and Özacar, 2020). Until now, a η of nearly 14% has been presented. Normally, photoanode material, the key component of the DSSC, plays a decisive role for preparing highly efficient DSSCs (Lee et al., 2014; Park and Dhayal, 2014; Jang et al., 2015; Wang et al., 2015). For example, the thin TiO₂ photoelectrode can absorb dye sensitizer, transfer photo generated electrons to the conductive substrate fluorine-doped tin oxide (FTO) and promote the diffusion of electrolyte into the adsorbed dye (Steffy et al., 2017; Maurya et al., 2019). Among the semiconductor metal-oxides, nanostructures TiO₂ materials have been widely utilized as an efficient mediator to transfer electrons and promote the separation of photoinduced electron-hole pairs in the working electrode of the DSSCs (Jadhav et al., 2014; Chandrakala et al., 2016). In the past few years, a lot of studies had been concentrated on design and preparation of excellent TiO₂ photoanode materials. It is well-known that two requirements are desirable for satisfactory photoanode materials, one is the large specific surface area for more dye loading and the other is the uniform structure for more optical scattering and faster electronic transmissions (Hore et al., 2006; Zhang et al., 2019). So it is highly desirable if one could simply prepare unique TiO₂ nanostructures with a large specific surface area which will promote their widely used in DSSCs.

Herein, we successfully synthesized TiO₂ HSN via one-step hydrothermal method by using TiO₂ nanobelts as growing framework, tetrabutyl titanate (TBOT) as titanium source, and the glacial acetic acid (HAC) solutions as regulating agent. This novel micro-sized material and TiO₂ P25 were used to fabricated the bi-layer TiO₂ photoanode film based DSSC, in which the photoanode consisting of the nano-sized TiO₂ P25 film as under-layer and the micro-sized TiO₂ HSN film as upper-layer. This is a larger breakthrough in preparing a double layered TiO₂ photoanode DSSC.

TiO₂ HSN was prepared using the hydrothermal synthesis method, the main factors influencing HSN growth process were investigated in our previous work (Jiang et al., 2014). The morphology of the prepared HSN is shown in Figure 1. As we can see, high homogeneous bunchy structures composed of plentiful micro-spheres, and the regular array of 3D TiO₂ micro-spheres are consist of a large number of TiO₂ thin nanosheets. Furthermore, we can see clearly from Figures 1A,B, the diameter of self-assembly micro-spheres are nearly 1.5 μm, meanwhile, a nanobelt with a dozen microns length acts as the growing framework Figure 1C which throughout the structure (Figures 1B,C). More interestingly, a large number of TiO₂ nanoparticles adhere to the 2D TiO₂ nanosheets, and the measurement shows that the diameter of every nanoparticle is about 10–30 nm, as shown in Figure 1D. We all know that the existence of tiny nanoparticles will greatly increase the specific surface area of new structural materials.

For detailed analysis, the edge of the micro-spheres image is also shown in Figure 1. We can see the nanosheets and nanoparticles clearly in Figure 1D, the local regions landmarks are corresponds to the Figures 1E,F. As stated earlier, the internal section exhibits better crystallized state, and there are clear lattice fringes on the nanosheets and nanoparticles. As show in Figure 1E, the lattice spacing of ∼0.235 nm and ∼0.35 nm are calculated at the edge of nanosheet which coincide with the (001) and (101) crystal planes belong to anatase TiO₂. While Figure 1F shows the lattice distance to the nanoparticals facets is ca 0.19 nm, which coincides well with (110) crystal planes of anatase TiO₂. It has been proved by many researches that anatase TiO₂ especially high exposure (001) crystal planes has great advantages in the field of DSSCs (Khalil et al., 2019; Lara et al., 2019; Wang et al., 2019).

The adsorption property of TiO₂ HSN and TiO₂ P25 were further determined. Figure 2A and Figure 2B show N₂ adsorption isotherms and pore size distribution curves of TiO₂ P25 and TiO₂ HSN. The related parameters are summarized in Table 1. By the full analysis of the curve in Figure. 2A, TiO₂ HSN exhibits a loose and mesoporous structure, which displays a typical type IV isotherms on the basis of IUPAC classification, and the S_BET of the TiO₂ HSN and TiO₂ P25 are 144.432 m²/g and 50.025 m²/g, respectively. Obviously, the novel product shows relatively higher specific surface areas, which is beneficial to the adsorption of dyes for DSSC. Furthermore, the average pore volume and average pore diameters of the TiO₂ HSN are 1.048 cm³/g and 61.463 nm , for P25 are only 0.6693cm³/ g and 18.805 nm. Prior studies have suggested that the advantageous large pore size distributions is beneficial to facilitate the fast diffusion of ions in electrolyte (Zhao et al., 2018).

As we know, photoanode films made from pure nano-sized TiO₂ particles usually exhibit low light harvesting efficiency. Therefore, the photoanode films with the scattering layer are very essential which can effectively increase the propagation path of sunlight (Chava et al., 2017; Cui et al., 2017; Xu et al., 2019; Xie et al., 2020). N₂ adsorption experiments show that the novel TiO₂ HSN with huge specific surface area and the uniform nanosheets structure is an excellent scattering layer for DSSC. In our study, in order to verify the effect of HSN scattering layer, three different photoanode films (P25 13 μm, P25 13 μm /P25 7 μm, P25 13 μm /HSN 7 μm) were prepared. The cross-sectional images of the pure P25 electrode (P25), P25 absorbent layer/ P25 scattering layer (P25/P25) and P25/HSN scattering layer (P25/HSN) are shown in Figure.3. It can be seen from Figures 3A,B, the individual TiO₂ P25 films are 13 μm and 20 μm thick (can be considered to 13 μm absorbing layer and 7 μm scattering layer) respectively. Meanwhile, the bi-functional structure layer of P25/HSN which consist of 13 μm P25 absorbing layer and 7 μm HSN scattering layer is shown in Figure 3C. Even better, for ease of understanding, the schematic diagram of the P25/HSN photoanode DSSC device is vividly depicted, as shown in Figure 3D.

In order to validate the light scattering effect, the reflectance spectra of P25 (13 μm ) photoanode film, TiO2 P25 (20 μm) photoanode film and P25/HSN photoanode film were investigated. As shown in Figure 4A, the P25/HSN photoanode with HSN scattering layer exhibits the strongest reflectivity which is ascribed to the potential scattering properties of the unique HSN structure. Several studies have described that the scattering materials with a size similar to the wavelength of visible light could cause great light dispersion effect on the basis of Mie theory (Konevskikh et al., 2018). Accordingly, the upper-layer TiO₂ HSN can reflect most of sunlight to the under-layer TiO₂ P25, which can improve the utilization of sunlight. Further, after dye adsorption process, the UV-vis absorption spectras of the three photoanode films were tested, as shown in Figure 4B, the P25/HSN photoanode film shows the best light absorption property on the full spectrum. It is verified that the addition of TiO₂ HSN will greatly improve the efficient utilization of sunlight.

The current-voltage (I-V) characteristics of the three cells were tested, the results are shown in Figure 4C, and the main performance parameters are summarized in Table 2. The cell P25 (13 μm) exhibits a short-circuit current (Jsc) of 13.64 mA/cm2 and PCE of 6.72%, whereas cell P25 (20 μm) only attains a 11.44 mA/cm2 and PCE of 3.67%, and there is 45.39% decrease in PCE compared to cell P25 (13 μm). Conspicuously, cell with HSN as the scattering layer (cell P25/HSN) demonstrates the highest Jsc ( 17.86 mA/cm²) among the three cells, while the voltage and fill factor (FF) remain similar with P25 (13 μm) cells. As a result, the PCE increases to the hightest of 8.08%, corresponding to 20.24% increment compared to cell P25 (13 μm). However, the cell with P25 (20 μm) photoanode shows the worst power conversion efficiency which attributes to the high recombination of electron-hole pairs of TiO₂ nanoparticals. So further enhancing the thickness of the TiO₂ P25 film can not increase the PCE, even if the adsorption quantity of dye increases. By a detailed analysis, the P25/HSN cell shows the best power conversion efficiency.

In order to compare the effects of different photoanode films (P25/HSN and P25 [13 μm, P25 (20 nm)] on photocurrent of DSSCs, the internal quantum efficiency were measured via IPCE measurement. The result reflects that the morphology, size, porosity and S_BET of the photoanode materials all influence the IPCE data (Sauvage et al., 2010; Kavan et al., 2011; Xue et al., 2012). As shown in Figure 4D, P25/HSN photoanode cell exhibits the highest IPCE at 530 nm. In addition, because of the large specific surface area of HSN which obviously boost the dye adsorption, the P25/HSN-DSSC shows a better performance on IPCE from 400 nm to 600 nm wavelength. More significantly, due to the participation of HSN, P25/HSN cell also shows excellent values in the wavelength range of 600–700 nm. Based on the analysis of the basic characteristics (has low absorption ability in the region of 600–700 nm of N719), the excellent scattering performance of TiO₂ HSN leads to the high optical energy utilization ratio in the wavelength range of 600–700 nm. Stated thus, the enhanced photocurrent of P25/HSN-DSSC is mainly attributed to the coordinated actions of excellent dye adsorption and strong scattering performance of TiO₂ HSN. In a follow-up experiment, we studied for only P25 (13 μm)-DSSC and bi-layer P25/HSN-DSSC.

The electron transport and recombination properties at the photoanode films-dye-electrolyte interface were also tested by using electrochemical impedance spectroscopy (EIS)(Wang et al., 2005). As we can see, two well-defined semicircles are shown in Figure 4E, since the impedance of other components unchanged, we only focus on the curves in multi-channel which represents the electron transfer/charge recombination at the photoanode film-dye-electrolyte interface. At last, the values of charge transfer resistance (Rct) have been calculated, for P25 (13 μm) cell is 35 Ω, and for P25/HSN cell is only 21 Ω. Therefore, we can say that the existence of TiO₂ 1D namobelts and 2D nanosheets lead to the more excellent redox ability of I^3-/I^- pairs at photoanode-dye-electrolyte interface, and thus the lower Rct of P25/HSN-DSSC.

In order to study the dye adsorption properties of different photoanode films, dye adsorption-desorption experiment of photoanode films were studied. Figure 4F shows the optical absorption spectra of all photoanode films, and the adsorption quantities of dye by P25 and P25/HSN photoanodes are presented in Table 2. It indicates that when using the P25/HSN photoanode, dye adsorption capacity is enhanced , thus the absorption of sunlight is increased. To further prove the superiority of TiO₂ HSN as a scattering layer, the dye adsorption amount of the photoanode film with a 20 μm thick P25 layer was also measured by the same method. As shown in Table 2, the loading amount of dyes in P25 (20 μm) photoanode is obviously improved when increasing the thickness of the P25 film, nevertheless, the J_SC, V_OC, FF all decrease compared to the P25 (13 μm), which attributes to the thicker P25 film brings more recombination sites. Therefore, it also shows that the dye loading does not the most important factor affecting the efficiency of DSSCs. The good scattering ability and excellent electronic transmission performance of the HSN are of great significance in improving the performance of DSSCs.

In summary, a novel bunchy TiO₂ hierarchical nanostructure was successfully synthesized via facile hydrothermal approach. The TiO₂ hierarchical nanostructure is assembled by anatase nanosheet based microspheres and nanobelt framework. The new TiO₂ structure shows excellent dye adsorption capability and powerful scattering ability. The TiO₂ HSN act as a scattering layer in photoanode film for DSSCs, which can greatly enhance the performance of DSSCs. The PCE of the P25/HSN-DSSC has reached 8.08%, showing a 20.24% increment over that derived from DSSC with single-layer nanocrystalline TiO₂ P25. The experimental results can be explained by the several favorable potential advantages of TiO₂ HSN scattering layer which can offer large specific surface area, excellent light scattering capability and direct electron transport pathways. It is evidential from the findings of this work that the novel TiO₂ HSN could be promising photoanode systems for DSSC applications.