Hole transport in nonstoichiometric and doped wüstite

We propose ways to enhance the conductivity of wüstite (i.e., naturally p-type FeO), a visible-light-absorbing, inexpensive, abundant, and nontoxic potential alternative material for solar energy conversion devices. Unfortunately, the conversion efficiency of FeO is inhibited by its low hole conductivity. Increasing the iron vacancy concentration or adding p-type dopants improves FeO conductivity by increasing the number of holes; however, it is not known which strategy introduces larger energy traps that would hinder hole conductivity. Here we employ the small polaron model along with ab initio calculations on electrostatically embedded clusters to analyze the local trapping effects of iron vacancies and several substitutional p-type dopants that are soluble in FeO, including copper, nitrogen, lithium, and sodium, and also hydrogen as an interstitial dopant for comparison. We find that vacancies create stronger traps than dopants and that copper and nitrogen dopants form deeper traps than lithium, sodium, or hydrogen. Furthermore, hydrogen repels the hole and substantially decreases the trap formed by an iron vacancy. Because of the shallower traps formed compared to vacancies, lithium-, sodium-, or hydrogen-doped, nanostructured or alloyed FeO may be worth evaluating as a p-type semiconductor for solar energy conversion applications.