  The dual-phase liquid argon time projection chamber is presently one of the
leading technologies to search for dark matter particles with masses below 10
GeV/c$^2$. This was demonstrated by the DarkSide-50 experiment with
approximately 50 kg of low-radioactivity liquid argon as target material. The
next generation experiment DarkSide-20k, currently under construction, will use
1,000 times more argon and is expected to start operation in 2027. Based on the
DarkSide-50 experience, here we assess the DarkSide-20k sensitivity to models
predicting light dark matter particles, including Weakly Interacting Massive
Particles (WIMPs) and sub-GeV/c$^2$ particles interacting with electrons in
argon atoms. With one year of data, a sensitivity improvement to dark matter
interaction cross-sections by at least one order of magnitude with respect to
DarkSide-50 is expected for all these models. A sensitivity to WIMP--nucleon
interaction cross-sections below $1\times10^{-42}$ cm$^2$ is achievable for
WIMP masses above 800 MeV/c$^2$. With 10 years exposure, the neutrino fog can
be reached for WIMP masses around 5 GeV/c$^2$.
