Current-induced degradation in polythiophene

The three-year collaboration between Simmons College and the Cornell Center for Materials Research (CCMR) is focused on undergraduate student/faculty research in organic light emitting diodes (OLEDs). The physics of OLED devices is characterized by three major processes: charge injection, charge transfer, and light emission as a result of the electron-hole recombination. In the first year of the program our research has been related to the first two stages. OLEDs based on small molecule as well as polymeric layers have been investigated, The devices were prepared using mostly aluminum (also nickel and iron) as electrodes and PC:TPD or polythiophene as the organic layer. Electrodes of about 20 nm were formed by vacuum evaporation, and organic layers of approximately 100-200 nm were spin-coated. The current-voltage characteristics, measured under forward and reverse bias up to 10 volts, demonstrate typical semiconductor S-shape behavior, and show variations dependent on aging, thickness of the polymer layer, and type and combination of electrodes. The results presented here specifically track the degradation of devices using polythiophene sandwiched between aluminum electrodes. The I-V curves and successive current response as a function of time and under constant voltage drive are presented along with complementary mass spectra and UV-visible and infrared absorption spectra. These measurements along with preliminary computer modeling of HOMO and LUMO energies for a series of thiophene oligomers suggest a correlation between internal changes in the polymer and variations in the electrical characteristics of the devices

Degradation of Ru(bpy) <inf>3</inf>²⁺-based OLEDs

Analysis of the possible mechanisms of degradation of Ru(bpy) 32+-based OLEDs has led to the idea of quencher formation in the metalloorganic area close to the cathode. It has been suggested that the quencher results from an electrochemical process where one of the bipyridine (bpy) groups is replaced with two water molecules or from reduction of Ru(bpy) 32+ to Ru(bpy) 30. We have tested these and other degradation ideas for Ru(bpy) 32+-based OLEDs, both prepared and tested with considerable exposure to the ambient environment and using materials and procedures that emphasize cost of preparation rather than overall efficiency. In order to understand the mechanisms involved in these particular devices, we have correlated changes in the devices' electrical and optical properties with MALDI-TOF mass spectra and UV-vis absorption and fluorescence spectra. © 2005 Materials Research Society

A supramolecularly-caged ionic iridium (III) complex yielding bright and very stable solid-state light-emitting electrochemical cells

A new iridium(III) complex showing intramolecular interligand pi-stacking has been synthesized and used to improve the stability of single-component, solid-state light-emitting electrochemical cell (LEC) devices. The pi-stacking results in the formation of a very stable supramolecularly caged complex. LECs using this complex show extraordinary stabilities (estimated lifetime of 600 h) and luminance values (average luminance of 230 cd m-2) indicating the path toward stable ionic complexes for use in LECs reaching stabilities required for practical applications

Intramolecular pi-Stacking in a Phenylpyrazole-Based Iridium Complex and Its Use in Light-Emitting Electrochemical Cells

A novel iridium(III) complex, [Ir(dmppz)2pbpy][PF6] (Hdmppz = 3,5-dimethyl-1-phenylpyrazole and pbpy = 6-phenyl-(2,2′-bipyridine)), is reported. The complex shows an intramolecular face-to-face π-stacking between the phenyl ring of the dmppz ligand and the pendant phenyl of the pbpy ligand. This interaction provides a supramolecular cage formation that holds also in the excited states. Light-emitting electrochemical cells (LECs) using the novel complex show extraordinary lifetimes of ∼2000 h. The high stability is favored by the presence of pendant methyl groups on the dmppz ligands that hinder the entrance of water molecules rendering the degradation of the complex more difficult