Ultrafast Charge and Triplet State Formation in Diketopyrrolopyrrole Low Band Gap Polymer/Fullerene Blends: Influence of Nanoscale Morphology of Organic Photovoltaic Materials on Charge Recombination to the Triplet State

Figure 5

fs-TA data analysis of the blend films. (a) Representative target model used for the analysis. Note the multiple branching pathways. The transition from A to E is singlet fission. See below for key. (b) Representative concentration profiles of the species-associated difference spectra (SADS) obtained for the FINE sample (cosolvent is ODCB) using a laser fluence of 12 μJ/cm². Decay times (in ps) belonging to the various species are 0.29 (FC), 2.86 (S1), 40 (CT), 410 (CT), and 4.5 ns (T); see below for key. (c) Representative normalized SADS obtained for the FINE sample (cosolvent is ODCB) using a laser power of 28 μJ/cm². Decay times (in ps) belonging to the various species are 0.39 (S1), 5.5 (CT), 60 (CT), and 2.5 ns (T). FC species is not shown. (d) Relative triplet and charge transfer state formation efficiency as functions of laser fluence for the COARSE and the FINE blend films, with 2nd-order polynomial fit. (e) Change of triplet and charge transfer excited state lifetimes as function of laser fluence for the COARSE and the FINE blend films, with linear fit. Key: Franck-Condon (FC) state = black, hot S₁ (S₁) = pink, hot charge transfer state (CT) = red, charge transfer state (CT) = blue, triplet (T) = green. It has to be noted that the hot CT state is not a vibrational hot CT state but a collection of higher lying CT states within the CT density of states.

(a) Target model

(b) Kinetics of SADS

(c) SADS

(d) Fluence effects on yields

(e) Fluence effects on decay times