Counts [arb.u.] -4 -2 0 2 4 Binding Energy [eV] UPS IPES E$_{g,s}$=2.6 eV E$_{HOMO}$ + = -0.75 eV E$_{LUMO}$ - = 1.85 eV P3HT LUMO HOMO E g no charge polaron levels (+ or -) polaron pair, singlet exciton (+ and -) E g,s LUMO - HOMO + E g,PP PP 1 PP 0 band gap transport gap optical gap S 1 S 0 E g,abs E F (i) (ii) Δ E $_{S}$→$_{PP}$  = E g,PP  - E g,abs Δ E $_{PP}$→$_{P}$  = E g,s - E g,PP (i) (ii) Δ E $_{S}$→$_{P}$  = E g,s  - E g,abs } FIG. 2. (Color Online) UPS (black) and IPES (grey) spectra of a 70nm thick P3HT film on ITO/glass. Measured data are plotted as points, the straight lines are 5 point averages. The transport levels were derived from the band onsets as indicated by the lines. The transport gap is 2.6 eV. The molecular structure of P3HT is shown in the inset. FIG. 1. (Color Online) Energy levels in a polymer in the ground state (a), during charge transport (b), and after photoexcitation (c). The details are described in the text. glass/ITO/(optional PMMA)/P3HT/(optional PMMA)/Al. All UV photoelectron and inverse photoelectron spec- troscopy measurements (UPS and IPES, respectively) were performed in a UHV system, described in detail elsewhere. 14 The base pressure was below 2 · 10 − 10 mbar and the samples were at room temperature. The He-I line of a gas discharge lamp was used for UPS. The IPES detector (a Geiger–M¨uller detector with SrF$_{2}$ window filled with Ar and I$_{2}$) was used in the isochromatic mode with a fixed photon energy of about 9.5 eV. The electron beam was defocussed over the sample to minimize radiation damage, and data acquisition was stopped immediately when changes in the IPES spectra occurred be- tween two scans. spective excitonic levels S 0 and S 1 is the optical gap E$_{g}$$_{,}$$_{abs}$ . During exciton dissociation, a strongly bound polaron pair (PP) with the levels PP 0 and PP 1 and the energy separation E$_{g}$$_{,}$$_{PP}$ can be formed. While the ground state band gap E$_{g}$ and the uncharged HOMO–LUMO levels can generally not be determined directly—particularly the common electrochemi- cal methods being only sensitive to singly charged levels—a combination of techniques can shed light on the energetics in- volved in the generation and separation of charge. The optical gap can be measured by optical absorption. The position of the polaronic transport levels (relative to the Fermi level E$_{F}$ ) and the transport gap can be accessed by photoelectron spec- troscopy. In addition, photoluminescence quenching and ex- ternal quantum efficiency measurements can provide the en- ergies necessary to transfer the excitonic into bound or free polaron states, respectively. The field-induced photoluminescence was performed on samples held in a helium cold finger cryostat under vacuum. The excitation source was a mechanically modulated solid state laser at a wavelength 532 nm with a power of 30 mW. The photoluminescence was measured using a Cornerstone monochromator with a silicon diode and a liquid nitrogen cooled InSb-detector, and recorded by a lock-in amplifier. The electric field was applied by a Keithley 237 source measure unit. After each voltage step field and light were switched off for 4 minutes, which we found was the minimum time lag to avoid charging. External quantum efficiency spectra were recorded by a homemade, lock-in based setup equipped with a calibrated reference silicon photodiode. A 300 W Xenon lamp was ap- plied as light source. Thin films were processed by spin-coating a solution of 20 mg/ml of regioregular P3HT in chlorobenzene on oxygen- plasma treated indium tin oxide (ITO) coated glass substrates. The resulting layer thickness was 70 nm and 110 nm, as de- termined by a surface profiler (Veeco). All samples were thermally treated at 120 $^{◦}$C for 10 min. For diodes, 80 nm thick Al cathodes were thermally evaporated. The P3HT was purchased from Rieke Metals. According to the manufac- turer, it has a regioregularity of between 90 and 93%, and typically contains less than 0.08% Ni, 0.5% Zn and 0.7% Br as residues from the synthesis. For field-induced photo- luminescence measurements, we additionally deposited two dielectric layers of 200 nm thickness by spincoating a so- lution of polymethylmethacrylat (PMMA), in order to avoid charge carrier injection. Thus, the device configuration was II. EXPERIMENTAL METHODS 2