Effects of dipole orientation and birefringence on the optical emission from thin films

Abstract Polymers are frequently used as the emissive material in optical devices such as the light emitting diode. The long chain molecular structure of typical polymer materials together with the spin coating technique by which thin layers are fabricated results in significant optical birefringence. We outline a classical model for spontaneous emission from within such birefringent materials in planar multi-layer structures. We compare the results obtained using this model with experimental data on the radiation pattern produced by a thin film of the conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy) 1,4 phenylenevinylene] (MEH-PPV). As a control we also measured the radiation pattern from the dye Lumogen, dispersed as a guest in an optically isotropic thin film of polycarbonate. We find that birefringence and the orientation of the dipole moment of the polymer have to be taken into account to understand the departure from the expected Lambertian radiation pattern that we observe from MEH-PPV.