Figure 3: Experimental evaluation of vibrational modulation effect.

(a) The vibrational part of the PPP response, measured at negative delay time (−100 ps). The signal amplitude is normalized to the spectral density of the infrared (IR) push source. The spectrum is obtained using two PPP spectra, each covering a different but overlapping wavenumber range; these are scaled to match the amplitude of the 1,430 cm⁻¹ mode that is present in both spectra. For comparison, the absorption spectrum of the pentacene/C₆₀ layer is presented in red. (b) The influence of different vibrations on device photocurrent, estimated by normalizing the amplitude of the PPP signal to the absorbed infrared intensity. The change of photocurrent absolute value corresponds to a flat 0.5 μJ cm⁻² per cm⁻¹ spectral density of exciting infrared light, fully absorbed by the vibrations. The error bars (red for fullerene modes, blue for pentacene vibrations) are s.d. for measurements on different devices; the number of measurements was 10 for 1,300–1,450 cm⁻¹ modes and 4 for all other modes.