Dye-sensitized solar cells (DSSCs) with economy, environmental friendliness, mechanical stability, simple preparation process, and high efficiency of 14%, are considered as a favorable choice compared to the silicon device^[1−3]. As an important component of DSSC, photoanode is not only a support and adsorption carrier for dye molecules, but also an electron transport carrier. Titanium dioxide (TiO₂) is the most representative for its rich optical, dielectric, catalytic, and antimicrobial properties, which leads to a variety of uses in solar cells, photocatalytic, and fuel cells^[4−7]. Among three polymorphs of rutile, anatase, and brookite, rutile TiO₂ has more stable physicochemical properties, higher refractive index and light-scattering capability compared to anatase^[8−10]. Also, the morphology of TiO₂ shows significant effect on its physical and chemical properties, such as the one-dimensional (1-D) structure, whose charge transport is preferred compare to the mesoporous structure due to the reduction in grain boundaries and lattice imperfections, thereby slowing down the electron hole recombination rate and accelerating electron transfer, and helping to improve the performance of various optoelectronic applications^[11−13]. Sadhu et al. reported the fabrication of a three-dimensional (3-D) rutile-phase TiO₂ microsphere arrays on fluorine-doped conducting oxide (FTO), and showed superior photon-harvesting performance owing to the increase in light-scattering in DSSC^[14]. Ri et al. spotlighted growth of a sea urchin-like rutile TiO₂ hierarchical microsphere on Ti foil, which showed much better light-scattering capability in visible region than the bare Ti foil^[15]. Sun et al. recorded a bilayer DSSC with an efficiency of 7.2% based on a bilayer TiO₂ film consisting of a 1-D nanowire arrays as underlayer and a 3-D dendritic microsphere as light-scattering overlayer^[16]. These results indicate that designing DSSC photoanodes with different morphologies is of great significance for improving the performance of DSSC.

Moreover, the substrate of traditional DSSC is FTO, and its features of rigid and heavy can not meet all application scenarios^[17]. To solve this issue, some flexible substrates (e.g., ITO/PET, ITO/PEN and metals) are used to replace FTO glass. Nevertheless, the ITO/PET and ITO/PEN substrates are high-cost and intolerant of temperatures higher than 150 °C, which is much lower than the temperature required for manufacturing photoanodes on FTO glass (450−500 °C), thus resulting in the relatively poor performance of plastic-based DSSCs. Therefore, high-temperature resistant metal foils (such as Ti and stainless steel) are used to fabricate flexible DSSCs (FDSSCs) with encouraging performance compared with the plastic-based ones^{[18, 19]}.

In this contribution, the anatase phase TiO₂ with four morphologies of nanoparticles, nanowires, nanorods and nanotubes (abbreviated as NP, NW, NR and NT) were prepared by using a in-situ hydrothermal synthesis and served as photoanode in FDSSCs. The influences of the morphology of TiO₂ on the adsorption capacity of dyes and charge transfer performances of FDSSCs were extensively investigated. The power conversion efficiencies of the FDSSCs based on NP, NW, NR, and NT TiO₂ photoanodes and poly (3,4-ethylenedioxythiophene) （PEDOT）counter electrode reached 6.96%, 7.36%, 7.65%, and 7.83%, respectively under rear illumination of 100 mW·cm⁻² and the optimized condition.

A reserved thin TiO₂ blocking layer was prepared by immersing the Ti substrate in 0.15 M of TiCl₄ isopropanol solution for 12 h, followed by sintering at 450 °C for 30 min in air.

Next, four TiO₂ photoanodes with different morphologies were prepared.

(1) TiO₂ NPs paste were prepared as described previously^[20−22].

(2) TiO₂ NRs were prepared as following^[23−25]. 5 mL Tetrabutyltitanate was rapidly added to 75 mL absolute ethyl alcohol under stirring for 0.5 h and followed by addition of 0.33 mL concentrated sulfuric acid and 0.3 mL deionized water to the solution, and then transferred into 100 mL Teflon-liner and autoclaved at 180 °C for 4 h to form a milky white slurry. The slurry was dissolved into 50 mL, 10 M NaOH solution. After ultrasonication and stirring for 0.5 h until a homogeneous solution was achieved and then the obtained solution was transferred into Teflon-liner and autoclaved at 150 °C for 24 h. The precipitate was collected by filter and washed with distilled water and 0.3 M HCl for 3 times, followed by washing with distilled water some times and annealing at 450 °C for 1 h.

(3) TiO₂ NTs were prepared in a two-steps hydrothermal method^{[26, 27]}. 3 g P25 powder was added into 10 M NaOH solution and then transferred into 100 mL Teflon-liner and autoclaved at 160 °C for 4 h and 110 °C for 20 h, respectively. After that, the obtained produce was washed with distilled water and HCl aqueous solution (pH = 2), and then with distilled water for several times until the pH is 7. The products were calcined at 400 °C in air for 1 h. Thus the TiO₂ NTs were obtained.

The TiO₂ NRs and NTs paste were prepared including of 10 mL absolute ethyl alcohol, 5 mL N-methyl-2-pyrrolidone, 0.1 g polyvinylidene fluoride, 1 g TiO₂ NRs and NTs kept stirring for 12 h at 50 °C. Soon afterwards, the TiO₂ NPs, NRs, and NTs films were coated onto the preprepared blocking layer by using doctor blade method, and then sintered at 450 °C for 30 min in air.

(4) TiO₂ NWs photoanode were prepared as the reported^[28−30]. 10 mL dioxane and 1 mL HCl with concentrated of 35% were mixed with stirring, and then added 1 mL titanium butoxide and TiCl₄ to above solution. The obtained solution was transferred into a 25 mL Teflon-liner. The Ti substrate with a thin TiO₂ blocking layer was placed into above solution at an angle against the wall, and the hydrothermal synthesis was conducted at 180 °C for 6 h. The TiO₂ NWs photoanode was obtained.

At last, the dye was loaded by immersing the bilayer TiO₂ photoanodes in 0.3 mM of N719 ethanol solution for 12 h. Thus the flexible dye-sensitized TiO₂ photoanodes with thickness of 4−5 μm was obtained.

The PEDOT counter electrode (CE) was electropolymerization from the base polymerization solution consisted of 0.1 M EDOT, 0.1 M LiClO₄ and PEG-600 dissolved in anhydrous ethanol. The FDSSCs assembled by clipping the dye-sensitized TiO₂ photoanode and Pt CE, and the liquid electrolyte consisted of 0.05 M of iodine, 0.1 M of lituium iodide, 0.6 M of tetrabutylammonium iodide and 0.5 M of 4-tert-butyl-pyridine in acetonitrile was injected in the aperture between the two electrodes.

The morphology, microstructure and the elements of the samples were investigated by the transmission electron microscopy (TEM, JEM-2100F, Japan) equipped with energy dispersive spectrometer (EDS) and the field emission scanning electron microscopy (SEM, JSM-7001F, Japan). The crystalline structures of the composites were investigated by glancing incident X-ray diffractometer (X‘Pert Pro, PANalytical B.V., Netherlands). The photovoltaic tests of FDSSCs with an exposed area of 0.2 cm² were carried out by measuring photocurrent−photovoltage (J−V) characteristic curves under rear irradiation of 100 mW·cm⁻² from the solar simulator (CEL-S500, Beijing China Education Au-light Co., Ltd) in ambient atmosphere.

Fig. 1 shows the SEM images of the TiO₂ NPs, NWs, NRs, and NTs. Figs. 1(a)-1 and 1(a)-2 present the porous and loose nanostructure of TiO₂ NPs, and the Figs. 1(b)-1 and 1(b)-2 exhibit a mixture of TiO₂ NWs and NPs, which are beneficial for the charge transfer and electrolyte loading. To compare the effect of the TiO₂ morphology on electron transfer, TiO₂ NRs and NTs were prepared and as shown as in Figs. 1(c) and 1(d). From Figs. 1(c)-1 and 1(c)-2, the TiO₂ NRs with different sizes and lengths can be observed clearly. Compared to TiO₂ NRs, TiO₂ NTs with a similar size and length are shown in Figs. 1(d)-1 and 1(d)-2. In a word, the TiO₂ photoanode with nano-structure provides highly effective contact between the CE and the I^–/I₃^– redox couple electrolyte, thus possibly absorbing more liquid electrolyte and improving the penetration of I⁻/I₃⁻ liquid electrolyte within TiO₂ film, and expecting to obtain an excellent performance for FDSSC.

Fig. 2 displays the XRD patterns of the TiO₂ NPs, NWs, NRs, and NTs photoanode. From Fig. 2, the characteristic peaks for the TiO₂ NPs (2θ ＝ 25.3°, 37.8°, 48.0°, 53.8°, 55.1°, and 62.6°), TiO₂ NWs (2θ ＝ 25.5°, 38.1°, 48.3°, 54.2°, 55.3°, and 63.0°), TiO₂ NRs (2θ ＝ 25.7°, 38.4°, 48.7°, 54.3°, 55.5°, and 63.2°), and TiO₂ NTs (2θ ＝ 26.1°, 38.6°, 48.8°, 54.7°, 55.9°, and 63.5°) associated with the anatase phase are observed in all the XRD patterns^{[29, 30]}. These findings demonstrate that the TiO₂ NPs, NWs, NRs, and NTs we synthesized are titania anatase dioxide.

Fig. 3 and Table 1 show the N₂ adsorption isotherm of the the TiO₂ NPs, NWs, NRs, and NTs and the specific surface area are about 91.9122, 116.6744, 126.3625, and 158.8077 m²/g, respectively. The large specific surface area for the TiO₂ NRs and NTs, which may be the key factors to absorb more dyes and enhance the performance for FDSSC.

To investigate the effect of TiO₂ morphological changes on dye 719 adsorption capacity, Fig. 4 demonstrates the Ultraviolet (UV)–visible absorption spectra of dye N719 made from the flexible TiO₂ photoanode with different morphologies, which shows three absorption peaks at 308, 373, and 503 nm for the dye N719. The TiO₂ NTs photoanode has the strongest signals, and the orders of the signal strength for the nano TiO₂ are NTs﹥NRs﹥NWs﹥NPs. This result indicates that the TiO₂ NTs photoanode loads more dyes according to the Lambert–Beer’s law, which means more incident light harvested and the larger photocurrent occurs^[31].

Figs. 5(a) and 5(b) are the HRTEM images of TiO₂ NTs with approximate lattice spacing of 2.44−3.71 nms (Fig. 5(c)) and mooth surface, which indicates that most of the P25 particles have been changed into TiO₂ NTs. The length of the TiO₂ NTs is about 100 nm and the diameter is around 15 nm. Fig. 5(d) shows the SAED image of TiO₂ NTs. It can be seen that the TiO₂ NTs are multicrystal structure, and belongs to the (101), (004), and (200) crystal faces for anatase phase.

Figs. 6(a) and 6(b) show the SEM images of the porous PEDOT CE. The PEDOT film with uniform network structure is in favour of improving I⁻/I₃⁻ electrolyte’ penetration and enhancing the contact area between CE and electrolyte. Fig. 6(c) shows the CVs of the Pt and PEDOT CEs at the scan rate of 50 mV·s⁻¹. The PEDOT CE have a higher current density (I_pc) absolute value and smaller overpotential (V_pc) than that of the Pt CE. Both the max anode peak current density (I_pa) and I_pc are almost unchanged in Fig. 6(d) which indicates a great electrochemical stability for the PEDOT CE^[32]. This phenomenon signifies that the PEDOT CE possesses the Pt-like electrocatalytic ability for I₃⁻ reduction due to the low overpotential loss and highly conductive for the PEDOT CE^[33].

Fig. 7 shows the photovoltaic properties of various FDSSCs under the rear illumination of 100 mW·cm⁻² and the detailed parameters are summarized in Table 2. The power conversion efficiencies (PCEs) of the FDSSCs assembled with TiO₂ NTs and the Pt and PEDOT CEs in Fig. 7(a) achieves 5.06% and 7.83%, respectively, which is remarkably higher for the latter than that of the former. This is mainly because the much higher transmittance for the PEDOT CE than that of the Pt CE, and the decrease in incident light intensity via the reflection of Pt CE^{[34, 35]}. Fig. 7(b) shows the J−V curves of the FDSSCs based on the PEDOT CE and the TiO₂ NPs, NWs, NRs, and NTs photoanodes under the rear irradiation. The FDSSCs with the TiO₂ NRs and NTs photoanodes show as high as PCEs of 7.65% and 7.83%, higher than that of the TiO₂ NPs (6.96%) and NWs (7.36%) photoanodes. The distinction in performance of the cells possibly is derived from the differences in electronic transmission of nano TiO₂. The TiO₂ NRs and NTs with directional structure are more conducive to electronic transmission than that of the TiO₂ NPs and NWs. Simultaneously, the reduction of dye loading capacity on TiO₂ photoanode also plays a certain role for the TiO₂ NPs and NWs.

Fig. 8 presents the Nyqyist plots of the devices with TiO₂ NPs, NWs, NRs, and NTs photoanodes and PEDCT CE, in which the performance of devices is directly related to the charge transfer resistance (R_ct1 and R_ct2)^[36]. Because of TiO₂ directional structure could enhance the electron collection efficiency in devices, therefore, the cell based on TiO₂ NTs photoanode shows the lowest values R_ct1 of 2.80 Ω·cm² and R_ct2 of 6.61 Ω·cm², and the other three devices with the TiO₂ NPs, NWs, and NRs photoanodes display the R_ct1 of 5.06, 4.17, and 3.59 Ω·cm², and corresponding to the R_ct2 of 10.09, 9.56, and 8.31 Ω·cm². The former shows the smallest R_ct1 and R_ct2 compared to the latter cells. This implies that the TiO₂ NRs and NTs with directional structure for the devices are better for electronic transmission than that of the TiO₂ NPs and NWs, resulting in a higher fill factor (FF) and short-circuit current density. This is in consistent with the results of UV−visible and J−V photovoltaic performance.

Fig. 9 shows the architecture of a FDSSC with TiO₂ NTs photoanode irradiated from the rear. The function of the TiO₂ blacking layer can reduce the backwash of elections from Ti foil substrate to the electrolyte, thereby enhancing the long-term stability of FDSSC. The TiO₂ NTs provide an unobstructed path for electron transfer, which helps improve the efficiency of charge transfer and electron collection. Moreover, the light emitted from the back can penetrate and be absorbed by the N719 dye very well due to the good transparency of the PEDOT CE, producing more photogenerated charge carriers in FDSSC, and resulting greatly enhanced the short-circuit current density and a superior PCE of 7.83%.

An efficient FDSSCs assembled with net-work structure’ PEDOT CE and TiO₂ NPs, NWs, NRs, and NTs photoanodes achieved power conversion efficiency of 6.96%, 7.36%, 7.65%, and 7.83% under rear illumination of 100 mW·cm⁻². The morphology of TiO₂ has a significant influence on the photovoltaic performances of FDSSCs. These four FDSSCs with various TiO₂ photoanodes morphology possess good fill factor and open circuit voltage, especially for the FDSSCs with TiO₂ NRs and NTs photoanodes, which generate higher short circuit current density. This is responsible for their oriented structure of NRs and NTs, which are more beneficial to electron and hole transmission in the semiconductor compared to the TiO₂ NPs and NWs photoanodes. This strategy can be further improved by designing a better light-splitting device with vertical incidence and applied other devices with transparent CEs.

The authors are very grateful to the joint support by NSFC (No. 61704047). This work is also supported by Science and Technology Development Project of Henan Province (Nos. 212102210126 and 202300410057).