We have investigated the role of the Si excess on the photoluminescence properties of Er-doped Si nanoclusters (Si nc). We demonstrate that the Si excess has two competing roles: when agglomerated to form Si nc it enhances the Er excitation efficiency but it also introduces new non-radiative decay channels. When Er is excited through an energy transfer from Si nc, the beneficial effect on the enhanced excitation efficiency prevails and the Er emission increases with increasing Si content. The luminescence quenching processes limiting quantum efficiency in Er-doped Si nc light emitting devices are investigated and identified. It is found that carrier injection, while needed to excite Er ions through electron–hole recombination, at the same time produces an efficient non-radiative Auger de-excitation with trapped carriers. The maximum excited Er fraction in this system is only of the order of percent. In order to …