Figure 3: Absorption and electroabsorption spectra of pentacene/C₆₀ CT states.

(a) Schematic of the photovoltaic device structure used for external quantum efficiency (EQE) and electroabsorption (EA) measurements. (b) Polarized EQE spectra measured for the device. The s- and p-polarized EQE data overlap at energies >1.5 eV, where isotropic bulk layer exciton transitions dominate, but the polarizations differ markedly in the low-energy region (E<1.4 eV) which is dominated by anisotropic CT-state absorption. The simulated s- and p-polarized EQE spectra (solid blue and red lines, respectively) in the excitonic region are based on the measured complex refractive indices of Pn and C₆₀ (Supplementary Fig. 1). The inset shows the CT absorption coefficient extracted from the EQE spectra. (c) Polarized EA spectra in the CT region (E<1.4 eV), where the p-polarized response again dominates due to the alignment of CT states normal to the pentacene/C₆₀ interface.