Publication date: 3rd July 2020
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. A method to reduce the optical gain threshold is by degenerative doping of the QDs, which effectively reduces the absorption near the band-edge, such that the stimulated emission (SE) can easily outcompete this process and reach optical gain. Here, we show that electrochemically doping films composed out of CdSe/CdS/ZnS core-shell-shell QDs effectively removes the threshold for optical gain. We are able to achieve stable doping of the QD films and demonstrate the reversibility of the electrochemical charging process. We quantify the gain cross section and map out the charge carrier dynamics using ultrafast spectroelectrochemistry, a combination of spectro-electrochemistry (SEC) and femtosecond transient-absorption spectroscopy (fsTA), 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 10-3 to 10-5 excitons per QD. The results presented here not only shed light on the carrier dynamics in doped QD solids, but also pave the way for the creation of cheap, solution-processable low-threshold QD-lasers.
AJH, JJG, SG and WvdS gratefully acknowledge financial support from the European Research Council Horizon 2020 ERC Grant Agreement No. 678004 (Doping on Demand). GG acknowledges financial support from NWO-TTW (Project No. 13903, Stable and Non-Toxic Nanocrystal Solar Cells). We gratefully acknowledge fruitful discussions with Dr. Freddy Rabouw, Stijn Hinterding and Sander Vonk (Utrecht University) on modelling the electrochemical electron injection into QDs.
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