3 1.0 0.8 0.6 0.4 0.2 0.0 Q Braun-Onsager (F) 10 6 2 4 6 10 7 2 4 6 10 8 2 4 6 10 9 F [V/m] 8x10 -2 6 4 2 0 Q(F) experiment  P3HT Braun-Onsager model  E$_{B}$ = 0.33 eV  E$_{B}$ = 0.37 eV  E$_{B}$ = 0.42 eV  E$_{B}$ = 0.50 eV  E$_{B}$ = 0.60 eV 2.0 1.5 1.0 0.5 0.0 Photoluminescence (PL) [arb.u.] 3.0 2.5 2.0 1.5 1.0 0.5 Energy [eV] 0.6 0.4 0.2 0.0 EQE [%], Absorption [arb.u.] 1000  700 500 Wavelength [nm] P3HT  Absorption  PL  EQE FIG. 4. (Color Online) The photoluminescence quenching yield Q ( F ) = ( PL ( 0 ) − PL ( F )) / PL ( 0 ) of a P3HT diode with dielectric layers. At high fields, it becomes apparent that not all luminescing singlet excitons can be quenched by the fields attainable with the ex- perimental setup. FIG. 3. (Color Online) Absorption (solid line), photoluminescence (dotted line) and external quantum efficiency (EQE) of P3HT thin films. The thin lines show how the onsets of the respective spectra were determined. The onset of the spectrally resolved photocurrent, the EQE, starts just above the absorption onset of 1.86 eV. The major photocurrent contribution, however, is seen at the energy of 2.58 eV, which corresponds to the transport gap extracted in Fig. 2. III. EXPERIMENTAL RESULTS photon-conversion yield, i.e., the fraction of generated and ex- tracted charges per incident photon in dependence on the pho- ton energy. The low energy region of such an EQE spectrum of the P3HT sample is presented in Fig. 3, measured under short circuit conditions. It shows a clear—although weak— onset of the photocurrent already at the absorption edge of 1.85 eV, with a quantum efficiency of about 0 . 1%. This im- plies that a fraction of the singlet excitons is already dissoci- ated without excess energy. Synthesis residues such as Ni— typically below 100 parts per million relative to P3HT—or dissociation at the electrodes can be responsible for this low energy photocurrent, and would explain its low magnitude. Apart from this relatively weak signal, which is thus probably due to extrinsic effects, the photocurrent strongly increases at photon energies above approx. 2.6 eV, and thus equals the in- trinsic transport gap as determined by our photoemission ex- periments. The combined UPS and IPES spectra of P3HT are pre- sented in Fig. 2. The band edges can be derived from the onset values of the HOMO and LUMO signals, as indicated by the lines used for evaluation. 15 The resulting values, deter- mined and given relative to the Fermi level E$_{F}$ , are -0.75 eV for the HOMO and 1.85 eV for the LUMO. They can be as- sociated with the respective relaxed ionic states, which corre- spond to the rearrangement of the electronic system due to the additional positive (negative) charge generated by (inverse) photoemission. 16,17 In consequence, these values provide the position of the energy levels relevant for charge transport. The corresponding transport gap is the energetic separation be- tween the respective transport levels and can be determined to 2.6 eV. Note that this is significantly different from other data in literature, 18 where the band energies are determined not from the onset values but the maxima of the signal peaks. In Fig. 3 the absorption and photoluminescence spectra are shown for a 110 nm thick P3HT film on ITO. The absorp- tion onset is at 1.85 eV and the maximum of the first singlet exciton transition $^{1}$B$_{u}$ is at about 2.1 eV. By applying a Gaus- sian fit to the highest energy photoluminescence peak at about 1.85 eV, we can determine the line width to$_{s}$ = 68 meV. This distribution also describes the absorption edge very well, and is assigned to the variance of the gaussian density of states distribution. The Stokes shift between the 0–0 absorption and emission band is about 250 meV. These measurements are complemented by EQE spectra. This experimental technique shows the spectrally resolved The electric field dependence of the P3HT luminescence peak of 1.65 eV is shown in Fig. 4. The spectral shape of the PL is field independent, only the magnitude changes (not shown). The field-induced photoluminescence is a suit- able technique to determine the P3HT singlet exciton bind- ing energy. We point out that the magnitude of the binding energy cannot be extracted directly. Therefore, we related our field-dependent PL quenching measurements to the es- tablished Onsager–Braun 19,20 (OB) theory for calculating the separation yield of the bound state, which considers the finite lifetime of the excited state. Although this empirical theory 21 was originally derived for ion pair dissociation, is has been found suitable for calculating the exciton binding energy. 4,8 Within this framework, the dissociation probability P ( F ) is given by 22 P ( F ) = k$_{d}$ ( F ) k$_{d}$ ( F ) + k$_{f}$ = $_{k$_{d}$}$( F ) $_{k$_{d}$}$( F ) + ( µ$_{t}$$_{f}$ ) − $_{1}$. (1)