Doping Bottleneck in Hematite: Multipole Clustering by Small Polarons

Abstract

Highly effective doping in transition metal oxides is critical to fundamentally overcome low carrier conductivity due to small polaron formation and reach their ideal efficiency for energy conversion applications. However, the optimal doping concentration in polaronic oxides such as hematite has been extremely low, for example, less than a percent, which hinders the benefits of doping for practical applications. In this work, we investigate the underlying mechanism of low optimal doping concentration with group IV (Ti, Zr, and Hf) and XIV (Si, Ge, Sn, and Pb) dopants from first-principles calculations. We find that novel dopant-polaron clustering occurs even at very low dopant concentrations and resembles electric multipoles. These multipoles can be very stable at room temperature and are difficult to fully ionize compared to separate dopants, and thus they are detrimental to carrier concentration improvement. This allows us to uncover mysteries of the doping bottleneck in hematite and provide guidance for optimizing doping and carrier conductivity in polaronic oxides toward highly efficient energy conversion applications.

The impacts of dopants on the small polaron mobility and conductivity in hematite – the role of disorder

“…Since EP transports through thermally activated hopping, the low mobility is caused by overcoming the energy barrier over 0.1 eV, significantly larger than the thermal energy at room temperature k B T (0.0259 eV). The carrier concentration may be increased by atomic doping as investigated in detail earlier; 8 however, the way polaron mobility is affected by doping remains illusive, despite previous studies. [9][10][11][12] Therefore, understanding how atomic doping changes the carrier mobility and in turn the electrical conductivity is essential and highly desired to boost the performance of hematitebased devices.…”

Exploring the effect of partial B-site Al3+–Mg2+ dual substitution on optoelectronic, surface, and photocatalytic properties of BaTaO2N

“…5 A significant increase in the concentration of (co)doped ions decreases the efficiency. By applying a computational method, Smart et al 6 recently found a doping clustering, which traps free-electron polarons and severely lowers the carrier concentration with respect to the doping concentration, to be responsible for the doping bottleneck in a-Fe 2 O 3 , and proposed a codoping with dopants having low binding energies for clustering, such as Sn-Ti, as a solution. The formation of oxygen vacancies in SrTiO 3 , which act as electron-hole recombination centers, was suppressed by Rh-La 7 and La-Al 8 codoping, resulting in apparent quantum yields of 1.1% at 420 nm and 78.43% under 365 nm, respectively.…”

Ideal dopant to increase charge separation efficiency in hematite photoanodes: germanium

“…23,24 Recently, several theoretical and experimental studies have highlighted the benecial effect of germanium (Ge) as an additive to improve the photocurrent of hematite photoanodes. [25][26][27][28][29][30][31] Aer analyzing the experimental studies, we observed a clear dependence of the photoelectrochemical performance of the hematite photoanodes on the synthesis route used to introduce Ge. 31 This dependence is associated with the low chemical compatibility between a-Fe 2 O 3 and GeO 2 .…”

“…Thus, from thermodynamic arguments, we expect the segregation and formation of Ge clusters. [30][31][32] The low reactivity between a-Fe 2 O 3 and GeO 2 made us to believe that to maximize the benets of Ge, it is necessary to use synthetic routes that enable the control of Ge incorporation, generating a metastable solid solution.…”