4 summarized in Tab. I. A central result is the finding that the transport gap E$_{g}$$_{,}$$_{s}$ of 2.6 eV directly corresponds to the en- ergy needed for achieving an efficient photocurrent. We point out that the two independent methods involved in determin- ing this single particle gap—photoemission spectroscopy and external quantum efficiency—rely on very different princi- ples. The former is very surface sensitive and is performed on thin films, whereas the latter relies on charge generation and transport in the bulk of a device. The nevertheless same resulting energy for the transport gap is consistent with for- mer work. 25 The difference between E$_{g}$$_{,}$$_{s}$ and the absorption onset E$_{g}$$_{,}$$_{abs}$ = 1 . 85 eV can be attributed to the exciton binding energy $_{D}$E$_{S}$$_{→}$$_{P}$ ≃ 0 . 7 eV. Sakurai et al. 12 investigated poly(3- octylthiophene) and found the $^{1}$A$_{g}$ polaron pair state 0.5 eV above the $^{1}$B$_{u}$ exciton by electroabsorption and two-photon absorption. Using electroabsorption, Liess et al. 11 determined 0.6 eV for the transition from the $^{1}$B$_{u}$ exciton state to the $^{m}$A$_{g}$ state of another poly(3-alkyl thiophene), pointing out that this might be lower limit for the continuum band threshold. Van der Horst et al. calculated 0.61 eV for the latter transition. 13 We note that our experimental approach, in contrast to Sakurai et al. , Liess et al. and van der Horst et al. , probes the transition from singlet excitons to free polarons, thus giving the exciton binding energy relevant for applications such as photovoltaics. The apparently lower exciton binding energy found by PL( F ) measurements, 0 . 42 eV instead of 0 . 7 eV, can be under- stood as follows. In order to quench the PL, it is sufficient to separate the constituents of the exciton partly, i.e., generating a—still bound—polaron pair. Therefore, not the full energy to generate a free polaron from the exciton has to be invested. In other words, the PL quenching experiment determines the energy of the transition from singlet exciton to polaron pair, $_{D}$E$_{S}$$_{→}$$_{PP}$ ≥ 0 . 42 eV. Note that from the levelling off of the field dependent PL shown in Fig. 4 it is clear that only a fraction of the singlet exciton radiative decay is suppressed. This effect is probably related to the semicrystalline regions in P3HT, 26 leading to self-quenching of PL even at zero field. Also, the exciton binding energy, and specifically the exciton to polaron pair transition, is probably distributed in energy. The exper- imentally determined value of 0.42 eV thus presents a lower limit for the exciton-to-polaron-pair transition energy. It cor- responds well with the density functional theory calculations of van der Horst 13 for polythiophene, where 0.45 eV were determined for this transition, and the photophysical experi- ments of Sakurai et al. 12 on poly(3-alkyl thiophene) (0.55 eV). The remaining energy difference between polaron pair and free polaron, which is necessary to create a photocurrent, can be expressed as $_{D}$E$_{S}$$_{→}$$_{P}$ −$_{D}$ E$_{S}$$_{→}$$_{PP}$ . Experimentally, we find that ≤ 0 . 3 eV are needed for this transition. This energy dif- ference corresponds to a Coulomb binding energy. Assum- ing a dielectric constant $_{e}$= 3 . 4, an electron–hole distance of about 1.4 nm can be derived, which is similar to the d-spacing of 1.6 nm found in X-ray diffraction. 27 The latter can be at- tributed to twice the hexyl side chain length of P3HT, repre- senting the stacking of different molecular units. We point out that our results suggest that the interpretation of ¨ Osterbacka et al. 28 concerning polaron transitions observed in P3HT by photoinduced absorption spectroscopy might need The experimentally determined energy levels of regioreg- ular P3HT, schematically shown and explained in Fig. 1, are D being the diffusion coefficient. For µ$_{t}$ = 5 · 10 − 16 m$^{2}$/V, L$_{D}$ becomes 3 . 5 nm. Using the singlet exciton lifetime as determined recently for regioregular P3HT, 24 $_{t}$= 0 . 4 ns, we determine a mobility of the strongly bound charges of about 1 . 3 · 10 − 6 m $^{2}$/ Vs from the above mentioned µ$_{t}$ product used to describe the data in Fig. 4. IV. DISCUSSION Using the OB model with µ$_{t}$ = 5 · 10 − 16 m$^{2}$/V,$_{e}$ = 3 . 4 and a maximum quenching of 9%, we could achieve a reasonable fit to our experiment, yielding E$_{b}$ = 0 . 42 eV as shown in Fig. 4. The discrepancy between the shoulder at low fields in the ex- perimental data and the fit can be explained by the reasonable assumption that the exciton binding energies are distributed over a finite energy range. We attribute the different magni- tudes of experimental and modelled the field dependent pho- toluminescence quenching to higher energy transistions from singlet exciton to polaron pair, which are beyond our measure- ment range. Using the Einstein relation, the µ$_{t}$ product can also be ex- pressed in terms of a diffusion length L$_{D}$ , where $_{g}$= qµ /$_{ee}$$_{0}$ is the Langevin recombination factor, 23 r$_{S}$ is the initial exciton radius, E$_{b}$ = e $^{2}$/ ( 4$_{pee}$$_{0}$ r$_{S}$ ) is the exciton binding energy, kT the thermal energy, J$_{1}$ the Bessel function of order one, and b = e $^{3}$F / ( 8$_{pee}$$_{0}$ ( kT ) $^{2}$) . F is the electric field, e is the elementary charge, and ee 0 the effective dielec- tric constant of the organic semiconductor blend. F is the electric field,$_{t}$$_{f}$ = k − 1 f the exciton lifetime, µ the sum of electron and hole mobilities. The field dependent dissocia- tion rate k$_{d}$ ( F ) = µ$_{k}$$_{d}$ ( F ) is k$_{d}$ ( F ) = 3$_{g}$ 4$_{p}$ r 3 S exp$^{(}$ − E$_{b}$ kT $^{)}$J$_{1}$ $^{(}$2 √ − 2 b ) √ − 2 b (2) L$_{D}$ = √ D$_{t}$ =$^{√}$ kT e µ$_{t}$ , (3) TABLE I. Experimentally determined energy levels of regioregular P3HT. The corresponding energies are shown and explained in Fig. 1. Energy [eV] Method E$_{g}$$_{,}$$_{s}$ 2 . 6 PES/IPES LUMO − relative to E$_{F}$ 1 . 85 PES/IPES HOMO $^{+}$relative to E$_{F}$ − 0 . 75 PES/IPES E$_{g}$$_{,}$$_{abs}$ 1 . 85 absorption E$_{g}$$_{,}$$_{s}$ 2 . 58 EQE E$_{g}$$_{,}$$_{abs}$ 1 . 88 EQE $_{D}$E$_{S}$$_{→}$$_{PP}$ ≥ 0 . 42 field dependent PL $_{D}$E$_{S}$$_{→}$$_{P}$ ≈ 0 . 7 PES/IPES vs. absorption, EQE