Figure 4.

Transient absorption spectroscopy measurements of PC₆₀BM (a) and PC₇₀BM (d) films spin-coated on quartz substrates. The samples were photoexcited at 530 nm at an excitation fluence of 30 μJ/cm² per pulse. The lower panels show the absorption spectra (b, e) and corresponding kinetics (c, f) of the singlet and triplet excitons as extracted from the data by using a numerical method as described in the text. The singlet and triplet kinetics were fitted to a mono-exponential decay and growth model with a shared time constant. The obtained time constants are summarised in Table1.