Energetics of excited states in the conjugated polymer poly(3-hexylthiophene)

There is an enormous potential in applying conjugated polymers in novel organic optoelectronic devices such as light-emitting diodes and solar cells. Although prototypes and first products exist, a comprehensive understanding of the fundamental processes and energetics involved during photoexcitation is still lacking and limits further device optimizations. Here we report on a unique analysis of the excited states involved in charge generation by photoexcitation. On the model system poly(3-hexylthiophene), we demonstrate the general applicability of our approach. From photoemission spectroscopy of occupied and unoccupied states, we determine the transport gap to 2.6 eV, which we show to be in agreement with the onset of photoconductivity by spectrally resolved photocurrent measurements. For photogenerated singlet exciton at the absorption edge, 0.7 eV of excess energy are required to overcome the binding energy; the intermediate charge-transfer state is situated only 0.3 eV above the singlet exciton. Our results give direct evidence of energy levels involved in the photogeneration and charge transport within conjugated polymers.

Figure 1

(Color online) (a) Energy levels in a polymer in the ground state, (b) during charge transport, and (c) after photoexcitation. The details are described in the text.

Figure 2

(Color online) UPS (black) and IPES (gray) spectra of a 70-nm-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.

Figure 3

(Color online) Absorption (solid line), photoluminescence (dotted line), and 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.

Figure 4

(Color online) The photoluminescence quenching yield $Q\left(F\right)=\left[\text{PL}\left(0\right)-\text{PL}\left(F\right)\right]/\text{PL}\left(0\right)$ 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 experimental setup.