Singlet exciton binding energy in poly(phenylene vinylene)

Abstract

The exciton binding energy (Eb) and the band gap energy (Eg) of poly(phenylene vinylene) are determined by high-resolution measurements of the photoconductivity excitation profile as a function of light polarization, applied electric field, and temperature. At high applied electric fields, a peak in the photoconductivity is observed when the sample is pumped at a photon energy just below the onset of the band-to-band -* absorption. This peak is interpreted as resulting from field ionization of a weakly bound exciton with E b Ϸ 60 meV. The binding energy is obtained from the energy of the exciton peak relative to the band edge and independently from analysis of the dependence of the exciton dissociation on field and temperature.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 (E b ) 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 -* absorption (1).The lowest energy -* 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 -* 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 -* 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 (E -); i.e., at[1a]If, however, E b 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 E b ; i.e., atFor 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 infrared 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 thres...

Quasiparticle and excitonic gaps of one-dimensional carbon chains

“…Within the Wannier-Mott model, the unscreened exciton binding energy of polyyne is 1 R * ∞ = µ * /2 = 0.3 eV. In fact we find the DMC staticnucleus exciton binding energy to be 0.3(1) eV, which is consistent with the small measured exciton binding energies of a range of π-conjugated polymers 77,78 . In Fig.…”

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

“…also supported by previous experimental results obtained in our group from highly oriented MEH-PPV chains in a polyethylene matrix, [97][98][99][100] and on evidence obtained from photoconductivity studies and ultrafast investigations of the infrared active vibrational (IRAV) modes in poly(p-phenylene vinylene) (PPV) and MEH-PPV. [101][102][103][104][105][106] The thermalized, more localized and Coulomb bound exciton state is only formed later, following relaxation, on a time scale that we estimate to 1 ps based on an early rise of the emission spectrum in the fluorescence up-conversion data. [37,38] We mentioned before the discrepancy between the distance that a polymer exciton can diffuse in 100 fs (0.1-0.2 nm) and the distance that the photoexcitation needs to cover (≈10 nm) in order to reach a fullerene interface in the BHJ to undergo ultrafast (<100 fs) charge separation.…”

Field-dependent exciton dissociation in organic heterojunction solar cells

“…After the exciton has diffused to an interface between donor and acceptor molecules, the local energy offset promotes a charge transfer across the interface. 5,6 The result is a charge transfer (CT) state, a Coulombically bound electron-hole pair located on adjacent donor and acceptor molecules. The subsequent dissociation of the CT-state, which has been assumed to be assisted by the cell's internal electric field, has been considered to be the ratelimiting step in the free charge carrier generation process.…”

“…While experiments determined τ CT to be a few nanoseconds, 12,13,17 the OB model required τ CT to be on the order of microseconds [8][9][10][11]18,19 to milliseconds 20 in order to reproduce the observed internal quantum efficiencies of up to 100%. 21 Moreover, the influence of the device thickness could not be reproduced, 11,22 and two publications on BHJ cells made of poly(3-hexyl-thiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric-acid methylester (PCBM) reported contradictory results for the material system's zero field CT-state dissociation probability p CT,0 of 40%-60% and 90%, respectively. 11,22 The reason for the discrepancies between model and experiment could have been the nongeminate recombination of free charge carriers.…”