Effects of optical interference and energy transfer on exciton diffusion length measurements in organic semiconductors

Abstract

Exciton diffusion is of great importance to the future design of high efficiency organic photovoltaics. Exciton diffusion studies require accurate experimental techniques. This paper addresses two important complications that can arise in exciton diffusion length measurements made by analyzing luminescence from thin films on quenching substrates: namely, the effects of optical interference and of energy transfer to the quencher. When there is modest contrast in the refractive indices of the quencher and organic material, as is the case for titania or C60 and most organic materials, interference effects can overwhelm the measurement, thereby making it impossible to accurately determine the diffusion length of excitons in the organic material. We show that this problem can be fully eliminated by using thin (<5nm) quencher films. The second complication that can occur is energy transfer to the quenching layer. We model the effect this has when fullerenes are used as quenchers. If energy transfer was ignored, one would falsely measure exciton diffusion lengths that are much greater than, and in some cases more than double, the actual diffusion length. Using titania as a quencher we eliminate the possibility of energy transfer, and by using thin titania films we eliminate the effects of interference and accurately measure a diffusion length of 6±1nm for the commonly used polymer poly[2-methoxy,5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene.