Fig. 2. Voltage losses and CT state properties in the solar cells based on DCDA polymers.

$\mathrm{EQE}$ and electroluminescence (EL) spectra of the solar cells based on a PBDB-T-NDI(C6), b PBDB-T-NDI(C12), and c PBDB-T-NDI(C18). The lower energy parts of the $\mathrm{EQE}$ spectra are fitted by using the method described in the literature^67–70 to determine the values for $E_{\mathrm{CT}}$, and the fit results are used to calculate the emission spectra, which agree well with the measured EL spectra of the solar cells. Details regarding the determination of $E_{\mathrm{CT}}$ are provided in Supplementary Note 5.