The electrical conductivity of thin film donor doped hematite: from insulator to semiconductor by defect modulation

Abstract

Hematite or α-Fe2O3 has emerged as a highly promising photoanode candidate for photoelectrochemical cells. While significant improvements in its performance have recently been achieved, it remains unclear why the maximum photocurrents still remain well below their theoretical predictions. Here, we report, for the first time, a detailed correlation between the electrical conductivity of undoped and 1 atom% Ti doped hematite and the conditions under which it was annealed (20 ≤ T ≤ 800 °C and 10(-4) ≤ pO2 ≤ 1 atm). Hematite thin films grown by pulsed laser deposition onto sapphire single crystals were evaluated by impedance spectroscopy. Hematite's room temperature conductivity can be increased from ∼10(-11) S cm(-1) for undoped films by as much as nine orders of magnitude by doping with the Ti donor. Furthermore, by controlling the non-stoichiometry of Ti-doped hematite, one can tune its conductivity by up to five orders of magnitude. Depending on processing conditions, donor dopants in hematite may be compensated largely by electrons or by ionic defects (Fe vacancies). A defect model was derived to explain this phenomenon. In addition, a temperature independent value for the electron mobility of 0.01 cm(2) V(-1) s(-1) for a donor density of 4.0 × 10(20) cm(-3) (1% Ti) was derived. These results highlight the importance of carefully controlling photoanode processing conditions, even when operating within the material's extrinsic dopant regime, and more generally, provide a model for the electronic properties of semiconducting metal oxide photoanodes.

Continuous-wave optically pumped green perovskite vertical-cavity surface-emitter

“…The perovskite layer was synthesized and deposited, as previously reported [28]. Figure 1(a) shows the x-ray diffraction (XRD) spectra of CH 3 NH 3 PbBr 3 thin films on a single-crystal sapphire which are consistent with the literature of CH 3 NH 3 PbBr 3 [29,30] and sapphire [31]. The scanning electron microscopy (SEM) micrograph (inset) shows void-free and randomly oriented crystalline micrograins deposited on the DBR as the substrate.…”

Oxygen deficient α-Fe2O3 photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses

“…The enhanced activity, ease of inclusion of V O , ability to process at temperatures as low as 350 °C 21 and reports of a synergistic effect of extrinsic dopants with intrinsic V O sites20,22 has led to a surge of interest in the controlled inclusion of V O in hematite over the last 2 years. Given the potential utility of this approach it is important that the fundamental processes associated with V O inclusion are elucidated 23–26…”

“…It has been known for over 25 years that the inclusions of V O /Fe 2+ sites within α-Fe 2 O 3 leads to formation of a donor band ∼80 meV below the conduction band 27. The inclusion of V O 's is suggested to improve activity through a number of mechanisms including; improved charge transport, higher charge separation yields and decreased contact resistances at the semiconductor/transparent conducting oxide interface, with various studies citing differing contributions from each effect 13,21,23–26. However, to date research has concentrated on material development and few direct measurements of the actual mechanism of enhanced activity for α-Fe 2 O 3– x are reported.…”

Strategies to suppress cation vacancies in metal oxide alloys: consequences for solar energy conversion

“…Our goal here is to determine whether judicious alloying can raise the iron vacancy formation energy, which could be beneficial for improving, e.g., charge transport [6,10]. We predict that the iron vacancy formation energy does indeed increase upon alloying FeO with selected metal oxides.…”

“…In a recent experiment, for example, increasing the iron vacancy concentration was shown to suppress the photocurrent in Ti-doped Fe 2 O 3 [10]. Vacancies can cause dramatic changes, both positive and negative, in material performance, e.g., trapping charge carriers or facilitating electron-hole recombination, thereby reducing electronic conductivity, enhancing ion diffusion in batteries and fuel cells, inducing phase transitions in solar thermochemical devices, and influencing chemical reactivity of photocatalysts ( Fig.…”