*Article* **The Presence of Charge Transfer Defect Complexes in Intermediate Band CuAl1***−p***Fe***p***S2**

**Christopher Dickens, Adam O. J. Kinsella, Matt Watkins and Matthew Booth \***

School of Mathematics and Physics, University of Lincoln, Lincoln LN6 7TS, UK **\*** Correspondence: mbooth@lincoln.ac.uk

**Abstract:** Despite chalcopyrite (CuFeS2) being one of the oldest known copper ores, it exhibits various properties that are still the subject of debate. For example, the relative concentrations of the ionic states of Fe and Cu in CuFeS2 can vary significantly between different studies. The presence of a plasmonlike resonance in the visible absorption spectrum of CuFeS2 nanocrystals has driven a renewed interest in this material over recent years. The successful synthesis of CuAl1−*p*Fe*p*S2 nanocrystals that exhibit a similar optical resonance has recently been demonstrated in the literature. In this study, we use density functional theory to investigate Fe substitution in CuAlS2 and find that the formation energy of neutral [FeCu] <sup>2</sup><sup>+</sup> + [CuAl] <sup>2</sup><sup>−</sup> defect complexes is comparable to [FeAl] <sup>0</sup> antisites when *p* ≥ 0.5. Analysis of electron density and density of states reveals that charge transfer within these defect complexes leads to the formation of local Cu2+/Fe2+ ionic states that have previously been associated with the optical resonance in the visible absorption of CuFeS2. Finally, we comment on the nature of the optical resonance in CuAl1−*p*Fe*p*S2 in light of our results and discuss the potential for tuning the optical properties of similar systems.

**Keywords:** semiconductors; DFT; chalcopyrite; defects; electronic structure; charge transfer; nanocrystals
