Quantitative electrochemical control over optical gain in quantum-dot solids

Realizing solution processed quantum dot (QD) lasers is one of the holy-grails of nanoscience. The reason that QD lasers are not yet commercialized is that the lasing threshold is too high: one needs > 1 exciton per QD, which is hard to achieve due to fast non-radiative Auger recombination. The optical gain threshold can be reduced by electronic doping of the QDs, which lowers the absorption near the band-edge, such that the stimulated emission (SE) can easily outcompete absorption. Here, we show that by electrochemically doping films of CdSe/CdS/ZnS QDs we achieve quantitative control over the gain threshold. We obtain stable and reversible doping with up to two electrons per QD. We quantify the gain threshold and the charge carrier dynamics using ultrafast spectroelectrochemistry and achieve quantitative agreement between experiments and theory. Over a range of wavelengths with appreciable gain coefficients, the gain thresholds reach record-low values of ~10^-5 excitons per QD. These results demonstrate an unprecedented level of control over the gain threshold in doped QD solids, paving the way for the creation of cheap, solution-processable low-threshold QD-lasers.