Photoinduced changes in the complex index of refraction resulting from photoinduced charge transfer in conjugated polymer/fullerene blends

Abstract

We report steady-state photoinduced absorption (PIA) and photoinduced reflectance (PIR) in films of MEH-PPV and BCHA- PPV blended with fullerene-based acceptors. Absorption from the metastable charge-transferred state is probed by PIA; the modulated absorption spectrum causes changes in the real part of the index of refraction, (Delta) n, which can be measured directly by PIR. The charge transfer gives rise to pronounced features in (Delta) n, including vibrational structure in the mid- and near-IR. Our measurements over a wide spectral range allow quantitative comparison of (Delta) n obtained from PIR with that obtained from Kramers- Kronig transformation of the PIA data. We find good agreement throughout the IR, indicating that our method for measuring (Delta) n is useful as an analytical tool for optical characterization and for prediction of optical spectral ranges for nonlinear optical response.