We have studied photoexcitation dynamics in thin films of poly(3-butyl-thiophene) [P3BT] using ps transient and cw photomodulation techniques, streak camera imaging, electroabsorption and optically detected magnetic resonance spectroscopies. We have determined that intrachain excitons are the primary photoexcitations with a characteristic transient photomodulation spectrum consisting of two photoinduced absorption bands in the near IR spectral range and a broad stimulated emission band in the visible spectral region. The photogenerated excitons are relatively short-lived (150 ps) and can subsequently decay into polaron pairs and triplets, which are longer-lived excitations with lifetimes of order ms. We do not find stimulated emission in very thin P3BT films and this is attributed to high defect concentration close to the film surface. (C) 1998 Published by Elsevier Science B.V.

The optical properties of ordered and disordered α-sexithiophene (α-6T) thin films are investigated by a variety of spectroscopies. The dominant low-lying even and odd parity excitons are identified by absorption and electroabsorption spectroscopies. Intermolecular interactions in ordered α-6T films results in Davydov splitting of the absorption band of the lowest odd parity exciton. Charged excitations in α-6T films were studied by photoinduced absorption and optically detected magnetic resonance. Both polarons and bipolarons are observed in disordered samples. As a consequence of higher carrier mobilities, only bipolarons were observed in ordered samples. Spin pairing of excitations broadens the observed ODMR spectra.

Using photoinduced absorption and its version of optically detected magnetic resonance, we observe a vibronically split triplet-triplet absorption band in sexithiophene films. The observed vibration frequency is then used to determine the triplet manifold electronic susceptibility and the ground triplet manifold level 1(3)B(u).

We present a comparative study of photoluminescence (PL) and photoinduced absorption (PA) excitation spectroscopy, resonant Raman (RR) spectroscopy and electroabsorption (EA) of C-60 and C-70 from 1.5 eV to 4.5 eV. In C-60 solid-state films, a salient feature at 2.3 eV was observed, which manifests itself as a peak in the PL excitation and RR spectra, as an onset in the excitation spectra of charge carriers and triplet excitons, and as the first prominent derivative feature in the EA spectrum. In contrast, these features are absent in the case of isolated C-60 molecules. We attribute the 2.3 eV feature to a charge transfer state in solid C-60. A similar charge transfer state was also observed in solid C-70 at 2.15 eV.

We have studied photogenerated charged excitations in sexithiophene and a variety of π-conjugated polymers by photoinduced absorption (PA) and PA-detected magnetic resonance (PADMR). We found distinct spin signatures for polarons and bipolarons, which enabled us to separate their contributions to the PA spectrum. In all cases, we find that polarons have two subgap optical transitions, whereas bipolarons have one. Having unambiguously identified the energy levels of polarons and bipolarons, we show that bipolarons are stable photo excitations, with negative effective correlation energy. We have also identified polaron parr species in the PADMR of C60-doped DOO-PPV films by the existence of an anisotropic spin-exchange interaction.

The X-band spin 1/2 photoluminescence-detected magnetic resonance (PLDMR) of C-60 and C-60 isolated in a toluene/polystyrene matrix and various films is reviewed. The fluorescence (F)-enhancing triplet resonances of the isolated molecules are attributed to ground-state repopulation by a nonradiative triplet state delocalized over the molecule. The phosphorescence-detected resonance of C-70 glasses is due to the direct radiative decay of the long-lived triplets. Its strong temperature dependence results apparently from a dynamic Jahn-Teller effect.The PLDMR of C-60 and C-70 films is similar, and includes a narrow F-enhancing spin 1/2 resonance (Delta H(1/2)similar to 8G) at g similar to 2.0025 and narrow and broad triplet patterns at full- and half-fields. The spill 1/2 resonance is discussed in relation to polaron recombination or a radical-triplet pair mechanism. The narrow triplet pattern attributed to a triplet delocalized over the molecule, is slightly narrower than in C-60, consistent with a slightly larger exciton. The broad pattern is similar to that observed in pi-conjugated polymers and is believed to result from a triplet localized oil a pentagon or hexagon adjacent to a neighboring molecule.

Photoexcitations in poly[2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and composites of MEH-PPV and C-60 (MEH-PPV/C-60) were studied by photoinduced absorption and absorption-detected magnetic-resonance spectroscopies. We report direct evidence that the prominent triplet photoexcitations in pristine MEH-PPV are effectively quenched in MEH-PPV/C-60. In contrast, the dominant photoexcitations in MEH-PPV/C-60 are spin-1/2 excitations: polarons on the polymer chains; and polarons on the C-60 (C-60(-)).

We have studied photoexcitations in mixed compounds of C60 with derivatives of poly (p-phenylenevinylene) and poly(phenyleneacetylene) using a variety of CW and picosecond (ps) transient spectroscopies. The CW techniques used include photoinduced absorption (PA), photoluminescence (PL) and optically detected PA (ADMR), whereas the ps transient techniques used were time-resolved PA, PL and stimulated emission (SE). Compared with the pristine polymers, all mixed C60 compounds have shown enhancement of PA bands associated with charged photoexcitations. These PA bands have shown a single spin-half ADMR resonance, identified as due to spins on the polymer chains; no spin-half ADMR signal from photogenerated C60− has been observed. In the ps time domain we have observed a pronounced reduction of the transient PL and SE in the mixed compounds. Due to the similarity found between the ps transient and CW PA bands, we have identified the ps photoexcitations in the mixed compounds as polarons (P+−P− ) on the polymer chains, which are indirectly photogenerated as a result of exciton dissociation in C60− related defect centers. The ultrafast charge transfer, claimed to exist between the polymer chains and C60 molecules, has not been observed in the ps time domain.

We present studies of fluorescence, phosphorescence, photoinduced absorption, and their respective optically detected magnetic resonance of C-70 molecules dispersed in polystyrene glasses. Two distinct triplet excitons are identified with principle zero-field splitting parameters (D approximate to 0.0089 and 0.0098 cm(-1), respectively) almost twice those reported in the literature (D approximate to 0.0052 cm(-1)). The first triplet is axially symmetric (E approximate to 0) and phosphorescent. Dramatic changes in the line shape and intensity of this triplet are observed as the temperature increases due to a dynamic Jahn-Teller effect. The second triplet, which is believed to arise from strained C-70 molecules, has no axial symmetry (E approximate to D/3) and is not phosphorescent.

We have studied photogenerated charged excitations in sexithiophene and a variety of pi-conjugated polymers by photoinduced absorption (PA) and PA-detected magnetic resonance. We, found distinct spin signatures for polarons and bipolarons, which enabled us to separate their contributions to the PA spectrum. In all cases, we find that polarons have two subgap optical transitions, whereas bipolarons have one. Having unambiguously identified the energy levels of polarons and bipolarons, we show that bipolarons are stable photoexcitations, with negative effective correlation energy.

We present photoexcitation studies of vacuum deposited neutral films of alpha,omega-substituted sexithiophene (alpha-6T) using photoinduced absorption (PA) and PA-detected magnetic resonance spectroscopy (PADMR). We fmd evidence for photoinduced polarons having spin 1/2, with two absorption bands at 0.80 and 1.54 eV, respectively, and with negative PADMR signal at g approximate to 2. Similar absorption bands are observed in lightly p-doped sexithiophene and are interpreted as due to 6T(.+) radical cations. In addition, a PA band is found at 1.1 eV, which is shorter lived and decreases faster with increasing temperature than the polaron bands. As similar dication bands (6T(2+)) have been found in heavily-doped alpha-6T, we identify this band as due to spinless bipolarons. Evidence of charge conjugation symmetry breaking in alpha-6T is presented. Finally, we have identified triplet excitons with triplet-triplet transition energy at about 1.45 eV.

We have studied photoexcitation dynamics in pristine and C-60-doped PPV films under bias illumination by photoinduced absorption CPA); optically-detected magnetic resonance (ODMR), and electromodulated PA (EPA). Intrachain polaron pair species are the dominant excitations under these conditions. The polaron pairs on the polymer chains are identified in ODMR by the existence of an anisotropic spin-exchange interaction: their characteristic absorption spectrum is blueshifted with respect to that of isolated polarons by similar to 0.4 eV. We also identified the PPV+/C-60(-) complexes by EPA spectroscopy.

We present photoexcitation studies of vacuum-deposited neutral films of sexithiophene (6T) using photoinduced absorption spectroscopy and photoinduced absorption detected magnetic resonance (PADMR) techniques. We find evidence for photoinduced polarons (photoexcited ‘radical ions’) having spin 1/2, with two absorption bands at 0.80 and 1.54 eV, respectively, and with negative PADMR signal at g≅2. Similar absorption bands are observed in lightly p-doped 6T, and are interpreted as due to 6T+ radical cations. In addition, a photoinduced absorption band is found at about 1.1 eV, which is shorter lived and decreases faster with increasing temperature than the polaron bands. We identify this band as due to spinless bipolarons (photoexcited ‘di-ions’). A similar di-cation band has been identified in heavily p-doped 6T. Finally, we have identified triplet excitons (S=1) with triplet-triplet transition energy at about 1.45 eV.

We have studied the absorption spectra of charged excitations in sexithiophene thin films induced by doping and photogeneration. Absorption bands at 0.80 and 1.54 eV are attributed to spin-1/2 polarons, whereas a single absorption band at 1.1 eV is due to spinless bipolarons. Charge conjugation symmetry breaking is evident for both polarons and bipolarons.

We have studied photoexcitations in various fullerene thin films using transient photomodulation and photoluminescence from 100 fs to 50 ms and absorption-detected magnetic resonance (ADMR). We show that singlet Frenkel type excitons are the primary photoexcitations; their recombination kinetics in the picosecond time domain are dispersive as a result of inhomogeneity. The long-lived photoexcitations, however, are shown to be triplet excitons and charged polarons, identified by the correlation found between their associated optical transitions and ADMR signals with spin 1 and 1/2, respectively.

We have characterized the long lived photoexcitations in thin films of poly(3-butylthiophene) [PBT] and co-polymers made of PBT and acceptor substituted PBT, using photo-induced absorption (PA) and optically detected magnetic resonance PA (PADMR) spectroscopies. The PA spectra reveal two main absorption bands; one at 1.4-1.7 eV, which is composed of contributions from both S=1 and S=12 species, and another at 0.5 eV which is due to S=12 charged species. Our results are explained within a picture where the S=1 species are triplet excitons and the S=12 species are photogenerated polarons.

Data obtained from thin films of C-60 using the techniques of absorption, photoluminescence, resonance Raman scattering, electroabsorption, and transient photomodulation are presented to develop an understanding of the energy levels and their dynamics in this fullerene solid.

We show that one-electron band theory fails to describe optical absorption in PPV and the absorption spectrum can be described only within a Coulomb-correlated model. The lowest optical state is an exciton, whose binding energy is estimated theoretically to be 0.90\ifmmode\pm\else\textpm\fi{}0.15 eV. Measurements of the photoconductivity quantum efficiency in poly[2-methoxy,5-(${2}^{\mathcal{'}}$-ethyl-hexyloxy)-1,4 phenylenevinylene] reveal a binding energy in good agreement with the theoretical value.