We report the electrochemical, photoluminescence, and electroluminescence properties of four fluorinated cationic iridium complexes bearing pyridyltriazole ancillary ligands. All the complexes display unstructured emission in the true blue region at 298 K with photoluminescent l em ranging from 452 to 487 nm in acetonitrile solution, in powder and in PMMA doped thin films. The nature of the emission is a mixed metal-to-ligand/ligand-to-ligand charge transfer state. Photoluminescence (PL) quantum efficiencies both in solution and in the solid state were low while excited state decay kinetics were found to be multiexponential. Each complex undergoes quasi-reversible oxidation and irreversible reduction with large HOMO-LUMO gaps. A detailed computational investigation corroborates the spectroscopic assignments. Additionally, light-emitting electrochemical cells (LEECs) were fabricated for each of the four complexes. The electroluminescence (EL) spectra of all complexes were red-shifted relative to the PL spectra. The LEEC containing 2a is the bluest emitter (l max ¼ 487 nm) of the family of complexes.

The performance of fullerene-based organic photovoltaic devices (OPVs) with low donor concentrations is not limited by the tradeoff between short-circuit current density (J sc ) and open-circuit voltage (V oc ), unlike bulk heterojunction OPVs. While the high V oc in this novel type of OPVs has been studied, here we investigate the mechanisms that govern J sc , which are not well understood. Three mechanisms, diffusion limited exciton relaxation, geminate recombination during exciton dissociation, and nongeminate recombination during charge transport, are studied analytically by combining various experimental techniques and transfer matrix simulation. We find that exciton dissociation at donor/acceptor interfaces is the dominant factor to produce high J sc , and at low P3HT concentrations exciton relaxation limits photocurrent generation. With more P3HT inclusion, the creation of interfaces promotes exciton dissocation but also reduces fullerene crystallinity, weakening the driving force for charge separation, and introduces nongeminate recombination sites. Quantitative analyses show that the magnitude of measured J sc and the donor concentration dependence are well accounted for by these three competing mechanisms.

Substrates can significantly affect the electronic properties of organic semiconductors. In this paper, we report the effects of contact-induced doping, arising from charge transfer between a high work function hole extraction layer (HEL) and the organic active layer, on organic photovoltaic device performance. Employing a high work function HEL is found to increase doping in the active layer and decrease photocurrent. Combined experimental and modeling investigations reveal that higher doping increases polaron-exciton quenching and carrier recombination within the field-free region. Consequently, there exists an optimal HEL work function that enables a large built-in field while keeping the active layer doping low. This value is found to be ~0.4 eV larger than the pinning level of the active layer material. These understandings establish a criterion for optimal design of the HEL when adapting a new active layer system and can shed light on optimizing performance in other organic electronic devices.

Surface chemistry of gold nanoparticles produced by pulsed laser ablation in liquids method is investigated by X-ray Photoelectron Spectroscopy (XPS). The presence of surface oxide expected on these systems is investigated using synchrotron radiation in conditions close to their original state in solvent, but free from substrate or solvent effects which could affect the interpretation of spectroscopic observations. For that purpose, we performed the experiment on a controlled free-standing nanoparticle beam produced by the combination of an atomizer and an aerodynamic lens system. These results are compared with those obtained by the standard situation of deposited nanoparticles on silicon substrate. An accurate analysis based on Bayesian statistics concludes that the existence of oxide in the free-standing conditions cannot be solely confirmed by the recorded core-level 4f spectra. If present, our data indicate an upper limit of 2.15 ± 0.68 % of oxide. However, a higher credence to the hypothesis of its existence is brought by the structureless valence profile of the free-standing beam. Moreover, the cross-comparison with the deposited nanoparticles case clearly evidences an important misleading substrate effect. Experiment with free-standing nanoparticles is then demonstrated to be the right way to further investigate oxidation states on Au-NP.