  Oxygen plasma is a widely used treatment to change the surface properties of
organic layers. This treatment is particularly interesting to enable the
deposition from solution of
poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) on top of
the active layer of organic solar cells or photodetectors. However, oxygen is
known to be detrimental to organic devices, as the active layer is very
sensitive to oxygen and photo-oxidation. In this study, we aim to determine the
impact of oxygen plasma surface treatment on the performance of organic
photodetectors (OPD). We show a significant reduction of the sensitivity as
well as a change in the shape of the external quantum efficiency (EQE) of the
device. Using hole density and conductivity measurements, we demonstrate the
p-doping of the active layer induced by oxygen plasma. Admittance spectroscopy
shows the formation of trap states approximately 350 meV above the highest
occupied molecular orbital of the active organic semiconductor layer. Numerical
simulations are carried out to understand the impact of p-doping and traps on
the electrical characteristics and performance of the OPDs.
