Evidence for photoinduced electron transfer from the excited state of a conducting polymer onto buckminsterfullerene, C(60), is reported. After photo-excitation of the conjugated polymer with light of energy greater than the pi-pi* gap, an electron transfer to the C(60) molecule is initiated. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite as compared to the separate components, consistent with photo-excited charge transfer. A photoinduced electron spin resonance signal exhibits signatures of both the conducting polymer cation and the C(60) anion. Because the photoluminescence in the conducting polymer is quenched by interaction with C(60), the data imply that charge transfer from the excited state occurs on a picosecond time scale. The charge-separated state in composite films is metastable at low temperatures.

https://science.sciencemag.org/content/sci/258/5087/1474.full.pdf

Photoinduced electron transfer from a conducting polymer to buckminsterfullerene.

Light‐emitting diodes made with poly(2‐methoxy‐5(2′‐ethyl)hexoxy‐phenylenevinylene) (MEH‐ PPV) using indium‐tin‐oxide (ITO) as anode and Ca as cathode have been examined as they age during operation in a dry inert atmosphere. Two primary modes of degradation are identified. First, oxidation of the polymer leads to the formation of aromatic aldehyde, i.e., carbonyl which quenches the fluorescence. The concomitant chain scission results in reduced carrier mobility. ITO is identified as a likely source of oxygen. The second process involves the formation of localized electrical shorts which do not necessarily cause immediate complete failure because they can be isolated by self‐induced melting of the surrounding cathode metal. We have not identified the origin of the shorts, but once they are initiated, thermal runaway appears to accelerate their development. The ultimate failure of many MEH‐PPV devices occurs when the regions of damaged cathode start to coalesce.

Degradation and failure of MEH‐PPV light‐emitting diodes

It has been experimentally discovered that a low magnetic field (less than 500 mT) can substantially change the electroluminescence, photoluminescence, photocurrent, and electrical-injection current in nonmagnetic organic semiconducting materials, leading to magnetic-field effects (M FEs). Recently, there has been significant driving force in understanding the fundamental mechanisms of magnetic responses from nonmagnetic organic materials because of two potential impacts. First, MFEs can be powerful experimental tools in revealing and elucidating useful and non-useful excited processes occurring in organic electronic, optical, and optoelectronic devices. Second, MFEs can lead to the development of new multifunctional organic devices with integrated electronic, optical, and magnetic properties for energy conversion, optical communication, and sensing technologies. This progress report discusses magnetically sensitive excited states and charge-transport processes involved in MFEs. The discussions focus on both fundamental theories and tuning mechanisms of MFEs in nonmagnetic organic semiconducting materials.

Magnetic‐Field Effects in Organic Semiconducting Materials and Devices

Conjugated conducting polymers constitute a subject of research lying at the interface between solid-state physics and molecular science. Electronic absorption and vibrational spectroscopies of doped conjugated polymers, whose ground states are nondegenerate, are reviewed from a viewpoint of molecular spectroscopy. Raman spectroscopy is a powerful tool for studying the structures of polarons and bipolarons, which are associated with electrical, magnetic, and optical properties of the polymers. Electronic absorption and Raman spectra of doped polymers have been analyzed on the basis of those of model compounds. These analyses have led us to the conclusion that polarons are the major species generated by doping in most nondegenerate conjugated polymers such as polythiophene, poly(p-phenylene), and poly(p-phenylenevinylene), in contrast with the previous view that bipolarons are the major species. The theoretical and experimental bases of these two contradictory views are discussed.

Electronic absorption and vibrational spectroscopies of conjugated conducting polymers

Visible light LEDs are produced having a layer of conjugated polymer which is cast directly from solution or formed as a gel-processed admixture with a carrier polymer The LEDs can be formed so as to emit polarized light

/pdf/visible-light-emitting-diodes-fabricated-from-soluble-tnbsrdvcsc.pdf

Visible light emitting diodes fabricated from soluble semiconducting polymers

We have studied the vibrational and electronic structure and the elementary charged excitations of poly(1,4-phenylene-vinylene) and a series of its (2,5-alkoxy) derivatives including poly(2,5-methoxy-PV), poly(2,5-hexoxy-PV), poly(2,5-octoxy-PV), and poly[2-methoxy,5-(2'ethyl-hexoxy)-PV] by absorption, photoinduced (PI) absorption, and doping-induced (DI) absorption. For the pristine materials, we observe that the band gap decreases by as much as \ensuremath{\sim}0.3 eV with the introduction of alkoxy side chains. We assign the observed ir-active phonons to specific structural vibrations. Upon either PI or DI carrier injection, we observe a series of infrared-active vibrational (IRAV) modes and two subgap electronic absorptions that we associate with the formation of bipolarons. Assignment of the IRAV modes indicates that the charged bipolarons couple strongly to lattice modes of the polymer backbone, and the measured IRAV mode frequencies indicate that the pinning strength of bipolarons in a given system decreases as the length of the side chain increases. In addition, from the energy of the two subgap electronic absorptions, we determine that the confinement parameter \ensuremath{\gamma} increases, the Coulomb interaction energy ${\mathit{U}}_{\mathit{b}}$ remains unchanged, and the Coulomb binding energy ${\mathit{E}}_{\mathit{b}}$ decreases with side-chain length.

Substitution effects on bipolarons in alkoxy derivatives of poly(1,4-phenylene-vinylene).

Linear and nonlinear optical studies of poly(p-phenylene vinylene) derivatives and polydiacetylene-4BCMU

Preparation de polymeres [CH 2 -C(OC 6 H 13 ) 2 ] n de degre de polymerisation variant entre 1200 et 3000 par polymerisation en masse de l'acetal de cetene correspondant amorcee par le triflate de cadmium. Le polymere est soluble dans les solvants organiques usuels precipite sous forme cristalline dans le melange THF/methanol, et est stable jusqu'a 240°. Son hydrolyse donne le polycetene dont la solubilite et la conductivite electrique sont mesurees

S. Askari

Papers

Substitution effects on bipolarons in alkoxy derivatives of poly(1,4-phenylene-vinylene).

Linear and nonlinear optical studies of poly(p-phenylene vinylene) derivatives and polydiacetylene-4BCMU

High molecular weight poly(ketene dihexyl acetal): anion effect in a cationic polymerization