Photoinduced absorption and photoinduced reflectance in conducting polymer methanofullerene films: Nonlinear-optical changes in the complex index of refraction

We report photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectra of poly(3-octyl thiophene)/methanofullerene films. As a result of the efficient photoinduced intermolecular charge transfer, the PIA and PIR spectra of the composite films are significantly enhanced in magnitude over those in either of the component materials. From the PIA and PIR spectra, the corresponding changes in the complex refractive index Delta N = Delta n(omega) + i Delta kappa(omega) are obtained. The results indicate that the PIA spectra are dominated by Delta<kappa(omega); the subgap electronic absorptions peak at approximately 0.1 and 1.6 eV. The PIR spectra are dominated mainly by Delta n(omega), which exhibits a derivative spectral shape with zero crossing at 0.1 eV. The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear-optical applications of these materials.