Hot exciton dissociation in polymer solar cells.

The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC(60)BM system. Exciton splitting occurs within the first 50 fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200 fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.

The critical role of interfacial dynamics in the stability of organic photovoltaic devices.

Understanding the stability and degradation mechanisms of organic solar materials is required to achieve long device lifetimes. Here we study photodegradation mechanisms of the (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]):[6,6]-phenyl-C61-butyric acid methyl ester (PCPDTBT:PCBM) low band gap-based photovoltaic blend. We apply quasi steady state Photo-induced Absorption Optical Spectroscopy, time-resolved Electron Spin Resonance Spectroscopy and theoretical modeling to investigate the dynamics of long-lived photoexcited species. The role of the interfacial physics in the efficiency and robustness of the photovoltaic blend is clarified. We demonstrate that the polymer triplet state (T), populated through the interfacial charge transfer (CT) state recombination, coexists with charge carriers. However, in contrast to previous suggestions, it has no role in the degradation process caused by air exposure. Instead, the long-lived emissive interfacial CT state is responsible for the blend degradation in air. It mediates direct electron transfer to contaminants, leading to the formation of reactive and harmful species, such as the superoxide.

Overview of transcriptome changes and phenomic profile of sanitized artichoke vis-à-vis non-sanitized plants.

Plant tissue in vitro culture is increasingly used in agriculture to improve crop production, nutritional quality, and commercial value. In plant virology, the technique is used as sanitation protocol to produce virus-free plants. Sanitized (S) artichokes show increased vigour compared to their non-sanitized (NS) counterparts, because viral infections lead to a decline of growth and development. To investigate mechanisms that control the complex traits related to morphology, growth, and yield in S artichokes compared to NS plants, RNAseq analysis and phenotyping by imaging were used. The role of peroxidases (POD) was also investigated to understand their involvement in sanitized plant development. Results showed that virus infection affected regulation of cell cycle, gene expression and signal transduction modulating cellular response to stimulus/stress. Moreover, primary metabolism and photosynthesis were also influenced, contributing to explain the main morphological differences observed between S and NS artichokes. Sanitized artichokes are also characterized by higher POD activity, probably associated with increased plant growth, rather than strengthening of cell walls. Overall, results show that the differences in development of S artichokes may be derived from the in vitro culture stressor, as well as through pathogen elimination, which, in turn, improve qualitative and quantitative artichoke production.

Effect of polymer morphology on P3HT-based solid-state dye sensitized solar cells: an ultrafast spectroscopic investigation.

Solid-state dye sensitized solar cell devices are fabricated with poly(3-hexylthiophene) (P3HT) as the hole transporting layer. Upon annealing treatment we obtained ≈ 70% increase in the device efficiency compared to un-annealed devices. Our investigation, by means of ultrafast transient absorption spectroscopic characterization, correlates the increased device performances to a more efficient hole-transfer at the dye/polymer interface in the thermally treated P3HT.

X-ray photoemission spectroscopy investigation of the interaction between 4-mercaptopyridine and the anatase TiO2 surface.

In polymer-metal oxide hybrid solar cells, an extremely careful engineering of the interface is required to ensure good device performances. Recently, very promising results have been obtained by functionalizing titanium dioxide (TiO2) by means of 4-mercaptopyridine (4-MPy) molecules, showing the beneficial effect of these molecules on the interface morphology. This study investigates the nature of the interaction of 4-MPy molecules with the TiO2 surface by means of X-ray photoemission spectroscopy. In order to mimic the device processing conditions, our analysis is carried out on molecules adsorbed from solution on a nanocrystalline surface. According to our analysis, 4-MPy molecules (C5H5NS) are likely bound with the oxide through the nitrogen atom. The bonding precedes either via a covalent interaction with Lewis surface sites, or via hydrogen mediation, possibly in the form of hydrogen bonds. Interestingly, in the latter case, we also observe strong changes in the spectroscopic features attributed to the thiol group.

High-Throughput Phenotyping in Plant Stress Response: Methods and Potential Applications to Polyamine Field.

High-throughput phenotyping has opened whole new perspectives for crop improvement and better understanding of quantitative traits in plants. Generation of loss-of-function and gain-of-function plant mutants requires processing and imaging a large number of plants in order to determine unknown gene functions and phenotypic changes generated by genetic modifications or selection of new traits. The use of phenomics for the evaluation of transgenic lines contributed significantly to the identification of plants more tolerant to biotic/abiotic stresses and furthermore, helped in the identification of unknown gene functions. In this chapter we describe the High-throughput phenotyping (HTP) platform working in our facility, drawing the general protocol and showing some examples of data obtainable from the platform. Tomato transgenic plants over-expressing the arginine decarboxylase 2 gene, which is involved in the polyamine biosynthetic pathway, were analyzed through our HTP facility for their tolerance to abiotic stress and significant differences in water content and ability to recover after drought stress where highlighted. This demonstrates the applicability of this methodology to the plant polyamine field.

CH3NH3PbI3 perovskite single crystals: surface photophysics and their interaction with the environment.

Here we identify structural inhomogeneity on a micrometer scale across the surface of a CH3NH3PbI3 perovskite single crystal. At the crystal edge a local distortion of the crystal lattice is responsible for a widening of the optical bandgap and faster photo-carrier recombination. These effects are inherently present at the edge of the crystal, and further enhanced upon water intercalation, as a preliminary step in the hydration of the perovskite material.