Y. Ding

Photoluminescence, electroluminescence, and optically detected magnetic resonance study of 2,5-dialkoxy derivatives of poly(p-phenyleneacetylene) (PPA) and PPA-based light-emitting diodes

Abstract The photoluminescence (PL) (i.e., optically)-detected magnetic resonance (ODMR) of the 2,5-dialkoxy derivatives of poly(p-phenyleneacetylene) (PPA) films and blends, and the electroluminescence (EL), conductivity (σ)-, and EL-detected magnetic resonance of PPA-based LEDs is described. The weak vibronic structure of the intense PL peaking at ∼1.95 eV and the broad tail extending beyond 800 nm are consistent with a relatively high degree of disorder. The ODMR spectra are similar to those of poly(3-alkylthiophenes) (P3ATs) and poly(p-phenylenevinylenes (PPVs), and attributable to magnetic resonance enhancement of singlet exciton generation by either polaron-polaron or triplet-triplet fusion. The broad, nearly symmetric half-field triplet ODMR is also consistent with considerably greater disorder than in P3ATs or PPVs. The EL spectrum of PPA-based diodes also reveals considerable disorder. They yeild intense EL-quenching but σ-enhancing resonances at g ≈ 2.002, which are discussed in terms of polaron-to-bipolaron decay, and half-field EL- and σ-detected resonances, assigned to triplet-triplet exciton fusion.

Electrically Symmetric Poly(Phenylene Acetylene) Diodes

Abstract A diode has been fabricated with poly(phenylene acetylene) [PPA] as the electroluminescent polymer. The device exhibited an unusual symmetric (positive and negative bias) I-V characteristic and electroluminescent output. These experimental results are discussed in terms of tunneling of electrons and holes via localized states.

Conductivity, optical absorption, photoluminescence, and X-band optically detected magnetic resonance of novel poly(2,5-dibutoxyparaphenyleneacetylene) (PDBOPA)

Abstract The optical properties of novel PDBOPA (average molecular weight ≈ 48 000) are described and discussed. Exposure to I2 vapor at ≈80 °C increases the conductivity to ≈5 × 10−3 S/cm, but the I2 rapidly evolves from the film at room temperature. The absorption onsets at 2.53 eV and peaks at 2.81 eV. The 2.71 eV-excited photoluminescence (PL) peaks at ≈2.0 eV in films and solutions. In all cases, it is very intense and largely structureless. This contrasts with the clear vibronic structure of the PL of poly(3-alkylthiophene) (P3AT) and poly(2,5-dialkoxyparaphenylenevinylene) (PDOPV) films and blends. As in P3AT and PDOPV, three PL-enhancing optically detected magnetic resonance (ODMR) features are observed: (i) a strong narrow (≈15 G wide) resonance at g = 2.0025, attributed to intrachain polaron recombination; (ii) a ≈1.0 kG wide triplet powder pattern around g ∼ 2, and (iii) the Δms = 2 transitions of these excitons at g≊4.07 . The results are discussed in relation to conformational defects resulting from the near cylindrical symmetry of the backbone acetylene units.

UV-Excited Optically Detected Magnetic Resonance (UV-ODMR) Study of π-Conjugated Polymers

Abstract The photoluminescence (PL) and X-band ODMR of poly(p-phenylenevinylene) (PPV), poly(p-phenyleneacetylene) (PPA), and polythiophene (PT)derivative films excited at 254 44 λex≤400 nm is compared to those excited at 488 nm. In PPV and PT, but not in PPA, UV excitation yields a strong narrow PL-quenching ODMR in addition to the narrow PL-enhancing polaron resonance. In PPV and PT, the PL yield and the triplet exciton ODMR weaken at short λex. The nature of the narrow resonance is discussed in relation to recombination of polarons, which may yield luminescent 11Bu singlet excitons but in any case eliminates them as nonradiative singlet and triplet exciton quenching centers. The UV-excited PL-quenching resonance is then discussed in relation to several scenarios, including geminate polaron recombination and spin-dependent decay of like-charged polarons into bipolarons. The behavior of PPA derivatives is suspected to result from structural defects which trap the charged excitations into polarons.

Electroabsorption and Charged-Excitation Spectroscopy of II-Conjugated Polymers

Abstract Electroabsorption (EA) and photoinduced absorption excitation (PAE) spectra for charged excitations have been measured for luminescent [poly(phenylene acetylene), PPA] and nonluminescent (polydiethynyl silane, PDES) π-conjugated polymers at 80K, over the spectral range from 1.5 to 4 eV. Based on these measurements, a discussion of the locations of the singlet 1BU and Ag states is presented, as well as an analysis of the first- and second-derivative nature of the EA lineshapes. It is argued that luminescence (or lack of) in these polymers depends on the location of an Ag state in close proximity to the 1Bu state.

Picosecond to millisecond photoexcitations in poly(phenylene acetylene)

Abstract Using ps transient and cw photoinduced absorption and photoluminescence, and their respective versions of optically detected magnetic resonance, we elucidate the origin of photoexcitations in thin films of poly(phenylene acetylene) (PPA). We clearly identified two kinds of photoexcited singlet excitons: the first type is radiative and unrelaxed whereas the second type is nonradiative, self trapped and thus long-lived. Other photoexcitations such as triplets and polarons play only a minor role in PPA.

X-Band Optically Detected Magnetic Resonance (ODMR) Study of C60-Doped π-Conjugated Polymers

Abstract The X-band ODMR of 0.1–10 mol.% C60-doped 2,5-dihexoxy poly (p-phenylenevinylene) (DHOPPV) and 2,5-dibutoxy poly(p-phenylene-acetylene) (DBOPPA) is described. As invariably observed in undoped π-conjugated polymers, it includes a narrow PL-enhancing polaron resonance at g ≊ 2.002, and full- and half-field triplet exciton powder patterns. The polaron resonance is surprisingly weak in undoped DHOPPV, but its intensity increases twentyfold at 0.1 mol.% C60, and then decreases one hundredfold at 10 mol.%. In undoped DBOPPA the narrow resonance is intense and initially decreases with C60 content. C60 doping generally drastically weakens the full- and half-field triplet exciton resonances. The results are discussed in relation to the role of positive (p+) and negative (p−) polarons and bipolarons as nonradiative singlet and triplet quenching centers; the polaron resonance is then attributed to their removal by spin-dependent p+−p− recombination, which may also generate radiative singlet excitons.