Efficient introduction of oxygen into a solid-state organic film is demonstrated by applying a high oxygen pressure of 200 MPa to the film. The oxygen-introduced film shows strong suppression of singlet-triplet annihilation (STA) because triplet excitons are quenched by oxygen introduced inside the film. It is demonstrated that oxygen molecules are uniformly distributed in the film and that oxygen molecules are left in the film to some extent when the film is in a vacuum. The concentration of oxygen inside the film is calculated to be a very high value on the order of 4.4 x 10(20) cm(-3). (C) 2015 Elsevier B.V. All rights reserved.