For the indirect bandgap, a similar but more pronounced trend is observed ranging from 2.86 eV to 2.37 eV. Both the direct and indirect band gap depend linearly on the amount of bipyridine doping (Figure ). However, with increasing bipyridine content a more obvious Urbach tail is visible in the Tauc plots, indicating the formation of localized or mid-band gap states from CTF-02 to CTF-02-Bpy (Figure ), in line with the increasing n-doping in the series. Electron paramagnetic resonance (EPR) experiments in a water-triethanolamine mixture confirm the assumption of localized states. Upon illumination, the photo excited CTFs are reductively quenched by reaction with TEOA to yield the reduced CTF radical anion, highlighted by a sharp signal in the EPR spectrum at 337 mT (g ≈ 1.995 -1.999) characteristic for unpaired electrons on aromatic rings with most likely odd numbers of carbon atoms. The EPR signal intensity increases in the same order as the Urbach tail, showing the strongest signal intensity for CTF-02-Bpy (Figure ). As expected, bipyridine not only brings additional electrons into the system, but also acts as a donor moiety to the triazine acceptor function, resulting in the observed increase in signal intensity and thus available unpaired electrons. Although this should reduce electron hole recombination, it does not automatically lead to a higher number of available charge carriers, as transient photocurrent measurements depict a different result (Figure ).