Electrically Driven Cascaded Photon Emission in a Single Molecule

Controlling electrically stimulated quantum light sources (QLSs) is key for developing integrated and low-scale quantum devices. The underlying mechanisms leading to electrically driven quantum emission, however, are complex, as a large number of electronic states of the system can be involved and, thus, impact the emission dynamics. Here, we use a scanning tunneling microscope to electrically excite a model QLS, namely, a single ZnPc molecule, and disentangle the interplay of charge transfer and excited state formation. The luminescence spectra reveal two lines, associated to the emission of the neutral (exciton) and positively charged (trion) ZnPc, both exhibiting single-photon source behavior. In addition, we find a correlation between the charged and neutral emission, specifically, the signature of a photon cascade in which the radiative decay of the molecular trion is followed by the formation and decay of the exciton. By adjusting the charging vs discharging rate, we show that we can control these emission statistics. This generic strategy is further established by a comprehensive rate equation model comprising a variety of states that mediate excited state formation in the electrically driven single and cascaded photon emission process, revealing the complex internal dynamics of the molecular junction.