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.

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Within the different organic photovoltaic devices the conjugated polymer/fullerene bulk heterojunction approach is one of the foci of today's research interest. These devices are highly dependent on the solid state nanoscale morphology of the two components (donor/acceptor) in the photoactive layer. The need for finely phase separated polymer–fullerene blends is expressed by the limited exciton diffusion length present in organic semiconductors. Typical distances that these photo-excitations can travel within a pristine material are around 10–20 nm. In an efficient bulk heterojunction the scale of phase separation is therefore closely related to the respective exciton diffusion lengths of the two materials involved. Once the excitons reach the donor/acceptor interface, the photoinduced charge transfer results in the charge separation. After the charges have been separated they require percolated pathways to the respective charge extracting electrodes in order to supply an external direct current. Thus also an effective charge transport relies on the development of a suitable nanomorphology i.e. bicontinuous interpenetrating phase structures within these blend films. The present feature article combines and summarizes the experimental findings on this nanomorphology–efficiency relationship.

Morphology of polymer/fullerene bulk heterojunction solar cells

Processible conjugated polymers: from organic semiconductors to organic metals and superconductors

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

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.

Z. Ni

Papers

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

Polymers and an Unusual Molecular Crystal with Nonlinear Optical Properties

Fibres of poly (methoxy-2-ethyl-hexyloxy) phenylenevinylene prepared from the soluble, fully conjugated polymer