Research

The current project pursues the theoretical-experimental design and characterization of novel molecular materials and their implementation in light-emitting (OLEDs and LECs) devices, which permit a considerable energy saving, and photovoltaic devices able to generate energy. The requirements for potential materials and the device physics are shared by both light-emitting and photovoltaic devices and, thus, these optoelectronic applications can be studied in parallel.

The type of electroluminescence and photovoltaic devices studied in this project are briefly described as following:

OLEDs

In the last decades organic light-emitting diodes (OLEDs) have been extensively investigated as the potential next generation technology for flat-panel display and lighting. The interest in this technology has been triggered due to the reports of new breakthroughs in device efficiencies, lifetimes, and achievable colors, including white. However, these high performance levels are only obtained using multilayered devices. The multilayer architecture is obtained by sequentially evaporating the active species under high-vacuum conditions. Additionally, these devices use air-sensitive metals or charge-injection layers, which require rigorous encapsulation to prevent degradation. A successful entry of OLEDs into the general lighting market requires, apart from high performance levels, a significant cost reduction of the devices. In this respect, it is of particular importance to be able to generate electroluminescence from devices using air-stable charge-injection interfaces. Some examples exist; however these devices rely on the presence of ionic charges to generate a dipole across the metal–light-emitting layer interface, and their reported lifetimes are low. Metal oxides hold, in principle, the promise of good charge injection, as they combine properties such as high transparency, good electrical conductivities, tuneable morphology, and the possibility of deposition on large areas with low-cost techniques. Recently, reports concerning the use of metal oxides as charge-injection layers have been published.

LECs

Solid-state light-emitting electrochemical cells (LECs) are single-component electroluminescent devices consisting of a charged luminescent material. The main characteristic of these devices is the insensitivity to the workfunction of the electrodes employed. Therefore, in contrast to organic light-emitting diodes (OLEDs, http://www.oled-info.com/devices.html), air-stable electrodes, such as gold, silver, or aluminum can be used, which is an initial requirement for obtaining unencapsulated devices. Prototype examples of such devices were based on conjugated polymers to which inorganic salts were added. More recently, single-component, solid-state light-emitting devices based on ionic transition-metal complexes (iTMCs) have been reported. In iTMC-based LECs, the ionic complexes perform all the necessary roles for the generation of light: a) the lowering of the injection barrier by the displacement of ions, b) the transport of electrons and holes by consecutive reduction and oxidation, respectively, of the iTMC, and c) the generation of the photons by phosphorescence

OSCs

Organic solar cells belong to the class of photovoltaic cells known as excitonic solar cells, which are characterized by strongly bound electron–hole pairs (excitons) that are formed after excitation with light. Strongly bound excitons exist in these materials as a consequence of the low dielectric constants in the organic components, which are insufficient to affect direct electron–hole dissociation, as is found in their high dielectric inorganic counterparts. In excitonic solar cells, exciton dissociation occurs almost exclusively at the interface between two materials of differing electron affinities (and/or ionization potentials): the electron donor (or simply donor) and the electron acceptor (or simply acceptor). To generate an effective photocurrent in these organic solar cells, an appropriate donor–acceptor pair and device architecture must be selected.