  A highly conductive metallic gas that is quantum mechanically confined at a
solid-state interface is an ideal platform to explore nontrivial electronic
states that are otherwise inaccessible in bulk materials. Although
two-dimensional electron gas (2DEG) has been realized in conventional
semiconductor interfaces, examples of two-dimensional hole gas (2DHG), which is
the counter analogue of 2DEG, are still limited. Here, we report the
observation of a 2DHG in solution-processed organic semiconductors in
conjunction with an electric double-layer using ionic liquids. A molecularly
flat single crystal of high mobility organic semiconductors serves as a
defect-free interface that facilitates two-dimensional confinement of
high-density holes. Remarkably low sheet resistance of 6 k$\Omega$ and high
hole gas density of 10$^{14}$ cm$^{-2}$ result in a metal-insulator transition
at ambient pressure. The measured degenerated holes in the organic
semiconductors provide a broad opportunity to tailor low-dimensional electronic
states using molecularly engineered heterointerfaces.
