Organic optoelectronic devices have already found their way into consumer electronics. This is remarkable, considering how little we understand about the microscopic processes involving charge carriers and excitons (bound electron-hole pairs), the protagonists in such devices. To understand the many-body physics of organic films (which are composed of ∼ 10²¹/cm ³ molecules), the quasiparticle concept is often introduced. As a conceptual tool, it allows us to describe these many-body systems in terms of low-lying excitations—the quasiparticles—that are weakly interacting with each other, but include, a priori, their interaction with the crystal ground state.

This thesis concerns the physics of one such type of quasiparticle, the microcavity polariton, formed when a photon confined to a cavity interacts strongly with another quasiparticle, the exciton. By combining the properties of both components, microcavity polaritons enable a wealth of fascinating physics. In inorganic semiconductors, for example, phenomena such as ultrafast parametric amplification, parametric oscillation and Bose-Einstein condensation of polaritons have been demonstrated. In organic semiconductors, these phenomena have yet to be realized.

The first two chapters of this thesis provide an introduction to the photophysics of organic semiconductors and microcavity polaritons, while providing a historical context for the latter. The next section reports on the fabrication of the first polycrystalline and crystalline strongly-coupled organic microcavities and describes their linear optical properties. We find that new physics arise due to the crystalline order, which also serves to reduce disorder-induced localization.

The latter chapters are concerned with the dynamics of organic cavity polaritons. In particular, we consider radiative and non-radiatve relaxation mechanisms in anthracene microcavities and study ultrafast nonlinear processes in a strongly-coupled microcavity containing the organic 3,4,7,8 napthalenetetracarboxylic dianhydride. Finally, this thesis concludes with the first demonstration of polariton lasing in an organic semiconductor and an outlook on future research directions in this field. We find that polariton lasing may provide a route towards electrically pumped organic lasing, but that several challenges remain to be overcome.