experiments that it is equivalent to the onset of photogenera- tion. By comparing this data to optical absorption, the exciton binding energy is determined for regioregular P3HT to about 0.7 eV. Moreover, from a comparison to field dependent pho- toluminescence measurements, we can provide a lower limit of 0.42 eV for the transition from a singlet exciton to a bound polaron pair and a polaron pair binding energy of about 0.3 eV. As the method of field induced photoluminescence quenching was used commonly to determine singlet exciton binding en- ergies, but actually yields the polaron pair binding energy, pre- vious experimental results should be reconsidered. All in all, we provide a comprehensive insight into the energy levels and binding energies of excited states in the frequently used com- pound P3HT. We emphasize that the presented experimental approach is applicable to conjugated polymers in general, and will be used to expand our findings in view of a comprehen- sive understanding of this class of materials. to be reconsidered. In Ref.$^{28}$, two peaks assigned to local- ized polarons, P$_{1}$ and P$_{2}$, are reported to absorb at 0.45 eV and 1.3 eV in regioregular and regiorandom P3HT. Two other peaks assigned to delocalized polarons, DP$_{1}$ and DP$_{2}$, are only observed in regioregular P3HT at 60 meV resp. 1.8 eV. ¨ Osterbacka et al. explain their findings on basis of charged HOMO and LUMO levels moving closer together into the HOMO–LUMO band gap due to the polaronic relaxation. The two polaron signals are then explained by optical transitions from the HOMO to the charged HOMO, corresponding to P$_{1}$, and from there to the charged LUMO, corresponding to P$_{2}$. In this interpretation, the HOMO–LUMO gap should corre- spond to 2 × P$_{1}$ + P$_{2}$=2.1 eV resp. to 2 × DP$_{1}$ + DP$_{2}$=1.92 eV. Both these energy differences are much smaller than the trans- port gap—which we determined to be 2.6 eV with two inde- pendent techniques—let alone the HOMO–LUMO gap. How- ever, on basis of our data we currently cannot present a concise alternative explanation. 5 Discussions with K. Kanai are acknowledged. The current work is supported by the Bundesministerium f¨ur Bildung und Forschung in the framework of the GREKOS project (contract no. 03SF0356B) and by the DFG (GK 1221). J.G.’s work is funded by the solarNtype project (contract no. 035533-2) within the 6th framework programme of the EU. V.D.’s work at the ZAE Bayern is financed by the Bavarian Ministry of Economic Affairs, Infrastructure, Transport and Technology. 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