Nanocrystal (NC) films have been proposed as an alternative to bulk semiconductors for electronic applications such as solar cells and photodetectors. One outstanding challenge in NC electronics is to robustly control the carrier type to create stable p-n homojunction-based devices. We demonstrate that the postsynthetic addition of Cd to InAs nanocrystals switches the resulting InAs:Cd NC films from n-type to p-type when operating in a field effect transistor. This method presents a stable, facile way to control the carrier type of InAs nanocrystals prior to deposition. We present two mechanisms to explain the observed switch in carrier type. In mechanism 1, Cd atoms are incorporated at In sites in the lattice and act as acceptor defects, forming a partially compensated p-type semiconductor. In mechanism 2, Cd atoms passivate donor-type InAs surface states and create acceptor-type surface states. This work represents a critical step toward the creation of p-n homojunction-based NC electronics.