The role of CdCl2 treatments and annealing in the formation of sintered CdTe nanocrystal solar cells

Solution processing of CdTe nanocrystals is a promising approach for the fabrication of low cost photovoltaic devices at low process temperatures. However despite the inherent advantages of this approach, the nanoscale physical mechanisms underlying the fabrication process are not well understood. Here we demonstrate that chemical treatment of CdTe nanoparticles leads to the production of a deep valence band edge. However, the removal of ligands also introduces sizable quantities of excess oxygen into the films in the form of TeO2, as proved by XPS. Thermal annealing of the CdTe films in air removes this excess oxygen and the TeO2 signal. Annealing further increases the energy of the valence band edge, decreases the energy of the conduction band edge, and is responsible for the p-type conductivity observed in NC CdTe films resulting in enhanced photovoltaic performance. Importantly, the presence of oxygen during the annealing step is crucial because it leads to an increase in hole concentrations needed for high performance devices.