Defeating Loss Mechanisms in 1D TiO₂‐Based Hybrid Solar Cells

Abstract

Titanium dioxide (TiO2) semiconductors are widely used in energy conversion, energy storage, catalysis, and other electronic applications. Hydrothermally grown TiO2 nanowires are an alternative to mesoporous TiO2 nanostructures due to possible direct charge transport, but their full potential waits to be unleashed. Latest findings show a peculiar defect structure, consisting of small single‐crystalline fingers and free internal surfaces, which supposedly promote a number of loss mechanisms. In this paper, the influence of these defects on charge transport is studied on the basis of hybrid solar cells featuring a TiO2/dye/polymer interface. Electrical, optical, and structural characterization identifies a number of loss mechanisms, which are inhibited by the introduction of particular annealing steps at specific processing points during fabrication. An increase in power‐conversion efficiency of 35% is obtained, resulting in 2.71% and surpassing mesoporous films of the same material combination. These results suggest that caution has to be exercised when dealing with defect structures possibly present in metal oxides which appear single‐crystalline by conventional analysis methods.

Controlling the Spatial Direction of Hydrothermally Grown Rutile TiO2 Nanocrystals by the Orientation of Seed Crystals

“…Previously, we found that the fine structure results from crystal defects in the early growth state and propagates throughout the growth [27]. The additional grain boundaries affect not only the chemical stability but also the electronic properties such as charge carrier mobility [28][29][30][31]. Within a nanorod, these nanofingers merge and form a single crystal for sufficiently high annealing temperatures [32].…”

Role of Vacancy Condensation in the Formation of Voids in Rutile TiO2 Nanowires

“…However, devices incorporating these hydrothermally grown TiO2 NW arrays fall short of expectations, which were predicted for highly crystalline 1D materials. [14][15] One reason for the low efficiency of e.g. hybrid solar cells is the high recombination rate of charge carriers due to intrinsic point defects (e.g.…”

“…Wisnet et al used a combination of TiCl4 treatment and annealing of NW arrays to reduce the recombination in hybrid solar cells. 14 Recently, Liu et al proposed a combination of helium ion implantation and subsequent annealing to introduce voids inside rutile TiO2 NWs and thereby they enhanced the photoelectrochemical water splitting. 15 Another approach is the thermal treatment of H2Ti3O7 NWs, which results in voids inside TiO2 NWs.…”

Tuning the Electronic Conductivity in Hydrothermally Grown Rutile TiO2 Nanowires: Effect of Heat Treatment in Different Environments

“…The high-angle annular dark-field scanning transmission electron microscopy ((S)TEM) images in Figure 2 , all taken from the central part of appropriate NWs, show more significant changes inside the NWs due to the annealing. While the as-grown NW is built by a bundle of nanofibers, as indicated from the SEM image, the annealed NWs are a single-crystalline material, which is interspersed with voids [ 31 , 32 ]. Nevertheless, SEM showed that even for the annealed NWs there are still residuals of the former nanofiber bundle at the tip ( Figure 1 b,c).…”