Effect of metal films on the photoluminescence and electroluminescence of conjugated polymers

We report the modification of photoluminescence (PL) and electroluminescence (EL) from conjugated polymers due to the proximity of metal films. The presence of a metal film alters the radiative decay rate of an emitter via interference effects, and also opens up an efficient nonradiative decay channel via energy transfer to the metal film. We show that these effects lead to substantial changes in the PL and EL quantum efficiencies and the emission spectra of the polymers studied here [cyano derivatives of poly(p-phenylenevinylene), PPV] as a function of the distance of the emitting dipoles from the metal film. We have measured the PL quantum efficiency directly using an integrating sphere, and found its distance dependence to be in good agreement with earlier theoretical predictions. Using the spectral dependence of the emission, we have been able to investigate the effect of interference on the radiative rate as a function of the wavelength and the distance between I-he emitter and the mirror. We compare our results with simulations of the radiative power of an oscillating dipole in a similar system. From our results we can determine the orientation of the dipoles in the polymer film, and the branching ratio that gives the fraction of absorbed photons leading to singlet excitons. We propose design rules for light-emitting diodes (LED's) and photovoltaic cells that optimize the effects of the metal film. By making optimum use of above effects we have substantially increased the EL quantum efficiencies of PPV/cyano-PPV double-layer LED's.