Fig. 5.

Applications of the three-state approach. a Chemical structures of PIPCP, PC₆₀BM, PBTTT, and PC₇₀BM. b Experimental and fitted extinction coefficient of the PBTTT/PC₇₀BM blend. Since photo-thermal deflection spectroscopy (PDS) can only be used to determine the spectrum shape of CT absorption and not its absolute value, in the experiment³² the PDS spectrum (black line) was therefore scaled to absolute absorption units by matching the PDS spectrum in the strongly absorbing region with an integrating sphere UV-vis transmission and reflection measurement (blue line); a Gaussian fit (dashed gray line) was used to model the experimental CT absorption shoulder in the experiment³². Here, our three-state fit (dashed yellow line) is also employed to fit the full absorption spectrum of PBTTT/PC₇₀BM. c Experimental (dashed lines) and fitted (solid lines) absorption spectra of pure PIPCP and the PIPCP/PC₆₀BM blend. The experimental absorption spectra of the pure PIPCP polymer and the PIPCP/PC₆₀BM blend were obtained via PDS measurements²⁹. In the inset, the region filled in blue shows an additional absorption feature at the low-energy edge of the blend absorption compared with that of the pure polymer