Ab initio study of electron and hole transport in pure and doped MnO and MnO:ZnO alloy

Abstract

Rationally engineering photocatalytic devices that power water splitting or CO 2 reduction reactions requires identifying economical materials that efficiently absorb sunlight and have suitable band edge placements. Recent theoretical investigations have predicted that a 1 : 1 alloy of MnO and ZnO meets these criteria. However, poor hole conductivity in undoped MnO:ZnO alloys (with up to 10% ZnO) severely limits this material's utility in electronic devices, and its electron conductivity has not yet been characterized. Here we investigate carrier transport in pure and doped MnO and MnO:ZnO with ab initio quantum chemistry calculations. Electrostatically embedded clusters are used to compute and compare relative electron/hole transfer barriers within the small polaron model. We assess the effects of Al, Ga, In, Sc, Y, Ti, Sb, Gd, F (n-type dopants) and Li (a p-type dopant) to determine which may enhance conductivity in MnO:ZnO. Our findings indicate that Ga, Sc, Ti, F, and Sb dopants create deep traps whereas In forms shallower traps that merit further investigation. Y, Al, Gd, and Li dopants should increase the carrier concentration while maintaining favorable electron and hole transport pathways.The latter are recommended for increasing the conductivity of MnO:ZnO and its effectiveness for solar energy conversion.

Design of Semiconducting TetrahedralMn1−xZnxOAlloys and Their Application to Solar Water Splitting

“…The feature of a dispersive conduction band was also recently found for ternary Mn(II) oxides [31]. Thus, electron transport in MnO-ZnO alloys should be described as large-polaron band conduction and not as small-polaron hopping conduction [13]. For hole carriers, the transport mechanism depends on the structure: In the octahedral coordination environment of RS MnO, the small-polaron formation is exothermic, i.e., the self-trapping energy E ST for the process Mn þII þ h þ → Mn þIII is negative (here, h þ denotes a positively charged hole carrier at the VBM).…”

“…Small-polaron hopping transport is generally characterized by low mobilities, usually assumed to be μ ≪ 0.1 cm 2 =Vs ("Bosman-van Daal limit") [4]. A recent study on the polaron hopping transport in RS structure MnO-ZnO alloys [13] found sizable energy barriers around 0.5 eV, suggesting hole mobilities below 10 −6 cm 2 =Vs at room temperature. In the context of solar absorber materials, an important additional implication is that the small polaron creates a deep defect state inside the band gap that acts as a detrimental recombination center [8].…”

“…There is a body of literature on thin-film growth of Mn-doped ZnO by pulsed laser deposition, and Mn 1−x Zn x O alloys in the Zn-rich composition range have attracted much interest as diluted magnetic oxides, with focus on the magnetic properties of Mn impurities in ZnO [10]. However, there are no experimental reports of alloys in the Mn-rich composition range, where the structural transition between the octahedral rocksalt structure (MnO ground state) and the tetrahedral wurtzite structure (ZnO ground state) is expected [11], and previous theoretical investigations for such alloys were restricted to the RS structure [12,13]. The alloy composition range of interest lies inside the miscibility gap [14], and the properties of such alloys are unknown.…”

First‐Principles Modeling of Electrochemical Water Oxidation on MnO:ZnO(001)

“…One model of solvation involves adding water layers with water molecules explicitly bound to the surface and water molecules hydrogenbonded to each other in multilayered structures. Y, Al, or Gd) [32] might reveal a dopant that provides the optimal relative binding strengths that minimize the overpotential. [28] Similarly, the effects of solvation on the remaining intermediates (*O and *OOH) need to be determined.…”

“…[21,22,31] Evaluating the scaling relationships for the reaction intermediates on MnO:ZnO in the presence of recently proposed n-dopants (e.g. Y, Al, or Gd) [32] might reveal a dopant that provides the optimal relative binding strengths that minimize the overpotential. However, previous work on doping hematite did not find doping to appreciably break the linear scaling relationships.…”