Polymer Solar Cells-Interfacial Processes Related to Performance Issues.

Harnessing solar energy with solar cells based on organic materials (in particular polymeric solar cells) is an attractive alternative to silicon-based solar cells due to the advantages of lower weight, flexibility, lower manufacturing costs, easier integration with other products, low environmental impact during manufacturing and operations and short energy payback times. However, even with the latest efficiencies reported up to 17%, the reproducibility of these efficiencies is not up to par, with a significant variation in the efficiencies reported across the literature. Since these devices are based on ultrathin multilayer organic films, interfaces play a major role in their operation and performance. This review gives a concise account of the major interfacial issues that are responsible for influencing the device performance, with emphasis on their physical mechanisms. After an introduction to the basic principles of polymeric solar cells, it briefly discusses charge generation and recombination occurring at the donor-acceptor bulk heterojunction interface. It then discusses interfacial morphology for the active layer and how it affects the performance and stability of these devices. Next, the formation of injection and extraction barriers and their role in the device performance is discussed. Finally, it addresses the most common approaches to change these barriers for improving the solar cell efficiency, including the use of interface dipoles. These issues are interrelated to each other and give a clear and concise understanding of the problem of the underperformance due to interfacial phenomena occurring within the device. This review not only discusses some of the implemented approaches that have been adopted in order to address these problems, but also highlights interfacial issues that are yet to be fully understood in organic solar cells.

Polarized emission from stretched PPV films viewed at the molecular level.

We present a study on the photoluminescence (PL) of thin films of poly-(p-phenylene vinylene) (PPV), non-stretched and uniaxially stretched. The experimental study was carried out using linear polarized light as the excitation beam, oriented either parallel or perpendicular to the stretch axis (S). The results showed that when the excitation light source has polarization perpendicularly oriented to the stretch direction, the emitted PL presents maximum intensity in the orientation S, and a minimum in the direction orthogonal to S. In order to understand this interesting phenomenon, we employ theoretical simulations at the atomistic level. We use classical molecular dynamics to simulate amorphous PPV films, non-stretched and stretched, from which we find a tendency of overall alignment of PV units to S, and of local clustering in herring-bone and π-stacking partial symmetries. Our study of optical activity of these kinds of clusters, performed through a quantum semi-empirical method, allows us to explain this polarization conversion behavior, and indicates the possibility of using underivatized PPV as the active layer for polarized electroluminescent devices.

Probing the functionalization of gold surfaces and protein adsorption by PM-IRRAS.

The control of morphology and coating of metal surfaces is essential for a number of organic electronic devices including photovoltaic cells and sensors. In this study, we monitor the functionalization of gold surfaces with 11-mercaptoundecanoic acid (MUA, HS(CH(2) )(10) CO(2) H) and cysteamine, aiming at passivating the surfaces for application in surface plasmon resonance (SPR) biosensors. Using polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), cyclic voltammetry, atomic force microscopy and quartz crystal microbalance, we observed a time-dependent organization process of the adsorbed MUA monolayer with alkyl chains perpendicular to the gold surface. Such optimized condition for surface passivation was obtained with a systematic search for experimental parameters leading to the lowest electrochemical signal of the functionalized gold electrode. The ability to build supramolecular architectures was also confirmed by detecting with PM-IRRAS the adsorption of streptavidin on the MUA-functionalized gold. As the approaches used for surface functionalization and its verification with PM-IRRAS are generic, one may now envisage monitoring the fabrication of tailored electrodes for a variety of applications.

Dynamic scale theory for characterizing surface morphology of layer-by-layer films of poly(o-methoxyaniline).

The dynamic scale theory and fractal concepts are employed in the characterization of surface morphological properties of layer-by-layer (LBL) films from poly(o-methoxyaniline) (POMA) alternated with poly(vinyl sulfonic acid) (PVS). The fractal dimensions are found to depend on the procedures to fabricate the POMA/PVS multilayers, particularly with regard to the drying procedures. LBL films obtained via drying in ambient air show a more homogeneous surface, compared to films dried under vacuum or a flow of nitrogen, due to a uniform rearrangement of polymer molecules during solvent evaporation.