Multipole Clustering is Responsible for the Doping Bottleneck in Hematite

Optimal doping of transition metal oxides is crucial for their development in photoelectrochemical (PEC) applications to mitigate the adverse effects of polaron formation which limits carrier conductivity. However, the optimal doping concentration in hematite has been extremely low, for example less than a percent, which hinders the benefits of doping for practical applications. In this work, we investigated the underlying mechanism of low optimal doping concentration from first-principles calculations with group IV (Ti, Zr, Hf) and XIV (Si, Ge, Sn, Pb) doping. We find that novel dopant-polaron clustering can form even at low dopant concentrations and which resemble electric multipoles. These multipoles are very stable and difficult to be fully ionized compared to separate dopants, and thus detrimental to carrier concentration and conductivity. We also exclude intrinsic vacancy as a cause for dopant clustering. This allows us to resolve mysteries of the doping bottleneck in hematite and provide guidance for optimizing doping and carrier conductivity in polaronic systems towards highly efficient PEC application.