Figure 3: Exciton diffusion in PC₇₁BM.

(a) Chemical synthesis scheme of the electron donor-functionalized diketopyrrolopyrrole derivative DPP-NMe₂, with synthesis described in the Methods. (b) Absorption spectrum of the DPP-NMe₂ molecule that is used in the quenching measurements. The PL spectrum of PC₇₁BM is also shown, indicating that there is some spectral overlap between the fullerene emission and DPP-NMe₂ absorption. The shaded vertical bands indicate two spectral regions of fullerene emission, 640–690 nm and 690–740 nm—these regions were used to monitor spectral diffusion of the fullerene exciton and thus determine the hopping time. (c) PL quenching of PC₇₁BM emission when varying the concentration of the DPP-NMe₂ quencher from 0 to 5 wt%. The solid lines are fits to the experimental data using the model as defined in the text. The deduced diffusion coefficient as defined in the text is D=1.6 × 10⁻⁴ cm² s⁻¹ and radius R_AD=1.5 nm. (d) PL dynamics of PC₇₁BM when looking at two spectral windows, 640–690 nm (open circles) and 690–740 nm (closed squares), as defined in b. A PL decay is observed on the blue side with a best fit (solid line) decay time constant of 7 and 157 ps. In the spectral window 690–740 nm, a rise-time is fitted (solid line) of 7 ps. The instrument response function is shown as a dotted line (3 ps full-width half-maximum).