Inaugural Article: Singlet exciton binding energy in poly(phenylene vinylene)

A central issue in the field of conjugated polymers is the strength of the electron-electron interaction relative to the bandwidth (1): Is the attraction of a geminate electron-hole pair so strong that the photoexcitations are localized and strongly correlated Frenkel excitons? Or rather, are the charge carriers sufficiently well screened that a band picture supplemented by the electron-phonon interaction (polaron formation) and the electron-electron interaction (weakly bound excitons) is justified? Determination of the exciton binding energy (Eb) is critically important to answering these questions and thereby to understanding the electronic structure of semiconducting polymers. Because this issue has not been resolved, the extensive literature on the optical properties of semiconducting (conjugated) polymers contains two conflicting assignments for the lowest energy π-π* absorption (1). The lowest energy π-π* absorption results from the creation of tightly bound neutral singlet excitons with the onset of the interband transition at a significantly higher energy, as for example, in molecular crystals such as anthracene (2). The lowest energy π-π* absorption results from a direct band-to-band transition, as for example, in direct gap semiconductors such as GaAs. These two different assignments imply very different results for the photogeneration of charged excitations. When the lowest energy π-π* absorption results from a direct band-to-band transition, one expects to observe a threshold for photogeneration of charge carriers close in energy to the onset of absorption (Eπ-π); i.e., at 1a If, however, Eb is large, one expects to observe the threshold for photogeneration of charge carriers via the lowest band-to-band transition at energy greater than the onset of optical absorption by Eb; i.e., at 1b For poly(phenylene vinylene) (PPV) and several of its soluble derivatives, the quantum efficiency for photogeneration of charged excitations (polarons) has been measured, in zero external field, using ultrafast photo-induced absorption by infra-red active vibrational modes (3–5). The results demonstrated charge carrier photogeneration with a single threshold that is close in energy to the onset of absorption, in agreement with Eq. 1a (5). Although photoconductivity data have been reported with a second threshold well above the onset of absorption (6, 7), we have detected and characterized a contribution to the transient and steady-state photocurrent that originates from electron photoemission (8). After quenching the photoemission contribution, the true bulk photoconductivity data in PPV and all of the PPV derivatives show a threshold close to the onset of optical absorption, above which the photocurrent is nearly independent of excitation energy (up to 6.2 eV), again in agreement with Eq. 1a (8). These data demonstrate that the oscillator strength in the lowest energy optical absorption band of PPV (and its soluble derivatives) arises from the lowest energy band-to-band transition. Theoretical models have yielded estimates for the Eb in PPV that range from values of order 0.1 eV to 1 eV (6, 7, 9). Moreover, the photoluminescnce is proportional to the light intensity (I) rather than I2, indicative of emission from a bound electron-hole pair. Thus, finding the exciton with spectroscopic accuracy and measuring the Eb remain as important goals for experimental studies of the photophysics of semiconducting polymers. We have measured the Eb in chain-oriented PPV through high-resolution excitation profile spectroscopy of the steady-state photocurrent (Iphoto) at various external fields (F) and temperatures (T), and in samples with different defect concentrations. The spectral signature of the exciton is observed in the excitation profile as a narrow peak that emerges just below the band edge upon increasing the external electric field or the defect density. Because the exciton absorption and emission are polarized parallel to the chain axis, measuring the excitation profile of Iphoto with light polarized parallel and perpendicular to the PPV chain axis enables the identification (and separation) of carrier generation by means of exciton dissociation from carrier generation by means of the π-π* interband transition. From these studies, we have determined band gap energy (Eg) (2.42 eV) and Eb (≈60 meV), and we have clarified the role of the external field and defects in the carrier photogeneration process.