Freeze-drying of TiO2 nanorod arrays improves charge transport properties and performance of solar cell

TiO₂ nanorod arrays (NRAs) prepared by hydrothermal treatment, anodic oxidation and sol-gel synthesis have received considerable attention in solar cells, photoelectrochemical water splitting and gas sensors. Conventional air drying of the TiO₂ NRAs could cause aggregation of neighboring nanostructures and distortion of their morphological features, which deleteriously influence their charge transport properties and surface area. In addition, chemically adsorbed halides and alkyl chains might change the surface properties and  influence the interfacial charge transfer process and hence the overall performance of the device. It is important to preserve the vertical alignment of the nanostructures as well as to avoid chemically adsorbed impurities to achieve fast electron transportation, conformal heterojunctions with guest materials and enhanced light scattering. Researchers at School of Advanced Materials and Nanotechnology and Key Lab of Wide Band-Gap Semiconductor Materials and Devices, Xidian University, People’s Republic of China have employed a freeze drying method to dry the TiO₂ nanorod arrays obtained by a two-step hydrothermal process. The freeze-drying post-treatment renders a clean TiO₂ surface and preserves the vertically-aligned nanostructures.

The TiO₂ NRAs were synthesized on FTO (SnO₂:F) substrates by a two-step hydrothermal method. In the first step, a mixture of 1.5 g TiCl₄, 15 ml DI water and 15 ml HCl (36.5 wt%) was subjected to hydrothermal treatment at 150 °C for 6 h. The resultant powder obtained from the first step was treated with a similar solution without TiCl₄ at 150 °C for 3 h in the second step. The air-dried TiO₂ NRAs were directly collected by air-gun blowing, while the freeze-dried samples were obtained after freeze-drying them for 5 h.

The morphological features of air- and freeze-fried TiO₂ NRAs are shown in Fig. 1(a) and Fig. 1(b), respectively. In spite of the method of drying, there is not much difference in the surface morphologies of the TiO₂ NRAs. Nevertheless, the cross-sectional morphology of the air-dried TiO₂ NRAs indicates collapse of the nanorods and destruction of morphology, probably induced by surface tension effect. In contrast, the vertically-aligned nanorod arrays are well preserved for the freeze-dried TiO₂ NRAs since this methodology enables removal of water by sublimation and desorption under vacuum, which avoids solid/liquid interfaces and eliminates the surface tension effect. During freeze-drying, localized energy generated through intermolecular heat transfer enables breaking of the Ti-Cl bonds, desorption and collision of chlorine atoms, resulting in the formation of molecular chlorine. Thus, the freeze-drying post-treatment leads to a “clean” TiO₂ surface with a well-defined morphology of NRAs. The length of the air- and freeze-dried TiO₂ NRAs are ~7.1 μm and ~7.8 μm, respectively. In spite of a slight difference in their colour shade, no apparent difference is observed in their crystallinity.

Fig. 1 Surface and cross-sectional (top insets) SEM images of TiO₂ NRAs obtained by: (a) air-drying; and (b) freeze drying methods (bottom insets: optical images)

The air- and freeze-dried TiO₂ NRAs show a similar UV-visible absorption spectra before loading the dyes. However, after loading the dyes, the freeze-dried TiO₂ NRAs exhibits a decrease in absorption when compared to that of the air-dried ones (Fig. 2(a)). The amount of dye loaded in air-dried TiO₂ NRAs is 69.6 nmol/cm² while for the freeze-dried TiO₂ NRAs it is decreased to 44.3 nmol/cm², following a decrease in its surface area by ~ 36%. However, the perseverance of the ordered nanostructures enables the freeze-dried TiO₂ NRAs to exhibit an enhanced visible-NIR light-scattering performance when compared to that of the air-dried ones (upper inset of Fig. 2(a)). The band gaps of freeze- and air-dried TiO₂ NRAs are 2.91 eV and 2.95 eV, respectively (bottom inset of Fig.2(a)). The flatband potential (E_fb) of the freeze-dried TiO₂ NRA shows a negative shift (~ 0.05 V) when compared to that of the air-dried one. The donor density (N_d) of the freeze-dried TiO₂ NRA is slightly increased when compared to that of the air-dried TiO₂ NRA` from 0.53×10¹⁷/cm³ to 0.6×10¹⁷/cm³. The schematic energy level diagram (Fig.2(b)) indicates that the negative shift in CB and E_F as well as disappearance of the deep acceptor level of freeze-dried TiO₂ NRAs are due to the removal of the adsorbed species that could facilitate charge separation and transport.

Fig. 2 (a) UV-Vis-NIR absorption of the TiO₂ NRAs formed on FTO substrates, (top inset: diffused reflectance spectra; bottom inset: Kubelka-Munk function vs. energy); and (b)  schematic energy level diagrams

The sharp decay in photocurrent with in a second observed for the freeze-dried TiO₂ NRAs when compared to the prolonged duration of decay of photocurrent over 100 s (Fig. 3(a)) observed for the air-dried one suggests the occurrence of an efficient charge extraction across the freeze-dried TiO₂/electrolyte interface. The model dye sensitized solar cells device (top inset of Fig. 3(b)) fabricated using air- and freeze-dried TiO₂ NRAs and the corresponding J-V curves are shown in Fig. 3(b). The PV parameters of the device fabricated using air-dried TiO₂ NRAs are as follows: open-circuit voltage (V_oc) = 0.70 V, short-circuit current density (J_sc) = 6.68 mA/cm², fill factor (FF) = 66%, and power conversion efficiency (PCE) = 3.08%. For the device fabricated using freeze-dried TiO₂ NRAs, the PV parameters are improved as follows: V_oc= 0.68 V, J_sc=8.11 mA/cm², FF= 65%, and PCE=3.60%. The improvement in the PCE of freeze-dried TiO₂ NRAs by ~20% is mainly due to an increase in its J_s.

Fig. 3 (a) Photocurrent decay curves (inset: parallel capacitance vs. applied potential plots); (b) J-V curves (top inset: model of DSSCs using TiO₂ NRAs)

Freeze-drying post-treatment of TiO₂ NRAs preserves the vertically-aligned nanoarchitecture and provides a clean surface, which helps to improve the electronic properties and the performance of the solar cell.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: P. Zhong et al., Freeze-drying as a novel approach to improve charge transport in titanium dioxide nanorod arrays, ChemElectroChem 10.1002/celc.201700572