  Defect centers in wide-band-gap crystals attracted considerable attention due
to the realisations of qubits, sensors, or single photon emitters at room
temperature. The family of these centers is constantly growing, including
well-known examples such as nitrogen-vacancy centers in diamond,
silicon-vacancy in silicon carbide, chromium substitutions in aluminium oxide,
and many others. Unfortunately, such defect centers embedded in highly
insulating crystals have been notoriously difficult to excite electrically.
Herewith, we present a realisation of insulating light-emitting diodes based on
carbon centers in hexagonal boron nitride. The rational design of the vertical
tunnelling devices via van der Waals technology enabled us to control the
charge dynamics related to non-radiative tunelling, defect-to-band
electroluminescence, and intradefect electroluminescence. The fundamental
understanding of the tunnelling events enabled us to achieve high efficiency of
electrical excitation, which exceeded by a few orders of magnitude the
efficiency of optical excitation in the sub-band-gap regime. A combination of a
Stark effect and screening by band electrons provide a control knob for tuning
the energy of emission. With this work, we solve an outstanding problem of
creating electrically driven devices realised with defect centers in
wide-band-gap crystals, which are relevant in the domain of optoelectronics,
telecommunication, computation, or sensing.
