Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic speci®cations for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Ultraviolet photoelectron spectroscopy (UPS) has been used to observe changes in the /r-band structure in poly(3-hexylthiophene) induced by doping using NOPF6. The charge-induced movement of the Fermi energy and a finite density of electronic states at the Fermi energy in the most highly doped (saturated) material are seen unambiguously with UPS for the first time in a conjugated polymer. The results imply the existence of a new, recently predicted theoretically, but hitherto unobserved, polaronlattice state of this conjugated polymer at the saturation-doping level.

We report the results of a theoretical study of the interaction between aluminum and poly(p-phenylenevinylene) and derivatives of poly(p-phenylenevinylene). This allows us to investigate the initial stages of formation of the metal/conjugated polymer interface. Applying the semiempirical Austin model 1 method, we find that aluminum reacts preferentially by forming covalent bonds with the vinylene linkages in both poly(p-phenylenevinylene) and poly(2,5-dimethoxy-p-phenylenevinylene). When carbonyl groups replace the methoxy groups, i.e., to form poly(2,5-dialdehyde-p-phenylenevinylene), new reactive sites are induced, leading to structures involving aluminum–oxygen bonds, with stabilities comparable to those involving reaction with the vinylene groups. In all of the three systems investigated, the interaction with aluminum induces major modifications of the polymer chains with interruptions of the π system caused by formation of sp3 sites. Charge distribution analysis indicates that electron charge transfer takes place from the aluminum atoms to the polymer chains.

A detailed understanding of the electronic structure of π-conjugated materials can be reached by means of two widely available semiempirical quantum-chemical methods: Austin model 1 (AM1) and intermediate neglect of differential overlap (INDO). This is illustrated by calculating the ultraviolet photoelectron spectra (UPS) of π-conjugated oligomers and polymers and comparing the theoretical data to experimental spectra. The approach is applied here to a series of compounds with varying molecular topology and chemical constitution: oligomers of p-phenylenevinylene and various derivatives, fluorinated derivatives of polyisothianaphthene, and 4,4′-bis(m-tolyphenylamino)biphenyl (TPD). The AM1-and INDOcalculated UPS spectra are also compared to data obtained with the valence effective Hamiltonian method, which is known to provide reliable results for the simulation of UPS spectra of these types of molecules. An easily applicable procedure is proposed to obtain the best fit to the experimental spectra from the AM1 and INDO molecular orbital energies. Both techniques accurately reproduce the lower energy part of the spectrum, which contains the most important part of the π electronic structure; INDO is also found to perform well for the inner part of the UPS spectrum, which mainly corresponds to the σ electronic states.