Recent developments in the design and synthesis of donor–acceptor semiconducting copolymers have significantly contributed to improve the ambipolar transport properties of polymer field-effect transistors (FETs). The further development of such FETs towards real applications would greatly benefit from the knowledge of the specific charge distribution along the channel, especially in the ambipolar regime. Here we accomplish this task by adopting charge modulation spectroscopy (CMS) and microscopy (CMM), electro-optical spectroscopy techniques which allow us to probe in situ charge induced features in the transmission spectra of conjugated polymers, to study a model high mobility donor–acceptor co-polymer (DPPT-TT, poly(N-alkyldiketopyrrolo-pyrrole dithienylthieno[3,2-b]thiophene)). Thanks to the sub-micrometer spatial resolution of confocal microscopy, we are able to isolate the specific contribution of …