The maximum voltage of CdTe solar cells is limited by low majority carrier concentration and doping difficulty. Copper that enters from the back contact can form both donors and acceptors in CdTe. It is empirically known that the free carrier concentration is several orders lower than the total Cu concentration. Simplified thermodynamic models of defect compensation after Cu introduction can be found in literature. We present a firstprinciples- based analysis of kinetics of defect formation upon Cu introduction, and show that Cu-i is mobile at room temperature. Calculations of properties of Cu-i-V-Cd and Cu-i-Cu-Cd complexes show that the neutral Cu-i-Cu-Cd complex is mobile at elevated temperatures, while formation of the V-Cd-Cu-i complex is unlikely because it transforms into the Cu-Cd defect. (C) 2012 Elsevier B.V. All rights reserved.