  The control over material properties attainable through molecular doping is
essential to many technological applications of organic semiconductors, such as
OLED or thermoelectrics. These excitonic semiconductors typically reach the
degenerate limit only at impurity concentrations of 5-10\%, a phenomenon that
has been put in relation to the strong Coulomb binding between charge carriers
and ionized dopants, and whose comprehension remained elusive so far. This
study proposes a general mechanism for the release of carriers at finite doping
in terms of collective screening phenomena. A multiscale model for the
dielectric properties of doped organic semiconductor is set up by combining
first principles and microelectrostatic calculations. Our results predict a
large nonlinear enhancement of the dielectric constant (ten-fold at 8\% load)
as the system approaches a dielectric instability (catastrophe) upon increasing
doping. This can be attributed to the presence of highly polarizable
host-dopant complexes, plus a nontrivial leading contribution from dipolar
interactions in the disordered and heterogeneous system. The enhanced screening
in the material drastically reduces the (free) energy barriers for
electron-hole separation, rationalizing the possibility for thermal charge
release. The proposed mechanism is consistent with conductivity data and sets
the basis for achieving higher conductivities at lower doping loads.
