3D hierarchical rutile TiO2 and metal-free organic sensitizer producing dye-sensitized solar cells 8.6% conversion efficiency

Abstract

Three-dimensional (3D) hierarchical nanoscale architectures comprised of building blocks, with specifically engineered morphologies, are expected to play important roles in the fabrication of 'next generation' microelectronic and optoelectronic devices due to their high surface-to-volume ratio as well as opto-electronic properties. Herein, a series of well-defined 3D hierarchical rutile TiO2 architectures (HRT) were successfully prepared using a facile hydrothermal method without any surfactant or template, simply by changing the concentration of hydrochloric acid used in the synthesis. The production of these materials provides, to the best of our knowledge, the first identified example of a ledgewise growth mechanism in a rutile TiO2 structure. Also for the first time, a Dye-sensitized Solar Cell (DSC) combining a HRT is reported in conjunction with a high-extinction-coefficient metal-free organic sensitizer (D149), achieving a conversion efficiency of 5.5%, which is superior to ones employing P25 (4.5%), comparable to state-of-the-art commercial transparent titania anatase paste (5.8%). Further to this, an overall conversion efficiency 8.6% was achieved when HRT was used as the light scattering layer, a considerable improvement over the commercial transparent/reflector titania anatase paste (7.6%), a significantly smaller gap in performance than has been seen previously.

Figure 1. FESEM images of the as-prepared 3D hierarchical rutile TiO₂ architectures (HRT) [denoted as HRT-1 (a), HRT-2 (b), HRT-3 (c), HRT-4 (d), HRT-5 (e), HRT-6 (f), HRT-7 (g), HRT-8 (h)], synthesized from a reaction solution containing 0.5 mL aqueous tetrabutyl titanate (TT) solution with 25 mL X M hydrochloric acid (HCl) (X = 1–8).

Figure 2. (a, c, e, g, i, k, m, o) High-magnification SEM images of HRT-1 to HRT-8, respectively; (b, d, f, h, j) TEM images of individual nanorods; (l, n, p) TEM images of nanorod bundles.

The insets of (b, d, f, h, j, l, n, p) are the corresponding FFTs (b, d, f, h, j) and electron diffraction patterns (l, n, p).

Figure 3. Ledgewise growth of rutile nanorod (HRT-1).

(a) Macro growth direction of a rutile nanorode; preferential growth to [001] direction, (b) Thickening of a nanorode by ledgewise growth of surface, (c) atomic structure of a rutile nanorod.

Figure 4

(a) J-V characteristics of P25-, HRT-1- and Dyesol-T-based DSCs. Cells were illuminated at an intensity of 100 mW cm⁻² with a spectrum approximately AM 1.5 G and an active area of 0.16 cm²; inset: incident photon to current conversion efficiency (IPCE) curves of P25-, HRT-1- and Dyesol-T-based DSCs. (b) diffuse reflectance spectra of P25, HRT-1 and Dyesol-T films. Impedance spectra of DSCs containing P25, HRT-1 and Dyesol-T photoanodes measured at V_oc under illumination of 100 mW cm⁻²: (c) Nyquist plots, with the inset showing the equivalent circuit, and (d) Bode phase plots.

Figure 5. J-V curves of optimized DSCs based on a Dyesol-S film as the light scattering layer over a Dyesol-T under layer [Dyesol-T (12 μm)/Dyesol-S (4 μm)] and a HRT-1 film as the light scattering layer over a Dyesol-T under layer [Dyesol-T (12 μm)/HRT-1 (4 μm)] under AM 1.5 G one sun intensity.

Inset is a titled SEM cross-sectional image of Dyesol-T (12 μm)/HRT-1 (4 μm) on the fluorine-doped tin oxide (FTO) and its schematic illustration.