Program summary for position 2
Hybrid organic/inorganic systems resulting from the coupling of 2DMs with organic chromophores are promising ingredients in a number of light-harvesting technologies, aimed at converting solar power into chemical energy (such as, e.g. in artificial photosynthesis or in water splitting) or directly into electric power (such as in photovoltaics), as well as in organic electronics. In all these technologies, processes such as exciton generation and dissociation, charge transport and recombination, as well as charge transfer across different materials and mobility therein, are key in determining the ultimate energy-conversion efficiency of light harvesting devices or the switching dynamics of electronic devices. The overarching goal of this project is to fathom all these processes by state-of-art numerical simulations and time-resolved spectroscopies, and optimize their combined effects on the conversion efficiency, by nano-structuring the inorganic substrate by e.g. stacking different 2D layers into a heterostructure, or by morphing them into spongy quasi-3D micro- or nano-scaffolds hosting the photoactive medium. To this end, we will deploy a multi-disciplinary, nationally distributed, research group covering the entire investigation chain, from materials growth, to characterization, time-resolved spectroscopy, and computer simulation.
We will create new stacks of 2D vertical heterostructures made of graphene, Transition Metal Dichalcogenides (TMDC), h-BN, phosphorene, and borophene, allowing us to fine tune the electronic properties of an electrodic backbone. The latter will be decorated with organic chromophore thin-films (tetrapyrroles), yielding tailored and tunable optical and electronic properties, by developing controlled deposition protocols in all experimental laboratories. We will characterize their morphology, crystalline and electronic structures, and the level alignment at the interfaces using a combination of state-of-the art resonant-photoemission, X-ray absorption, photoemission spectroscopies, and numerical modelling techniques.
The elementary steps of the exciton and charge dynamics in these novel materials will be addressed by quantum-mechanical simulation techniques based on (time-dependent, TD) density-functional (DFT) and many-body perturbation theories (MBPT), and by time resolved and non-linear spectroscopic techniques. The exciton decay channels in the fs-ps time scale will be studied by pump-probe photoemission and absorption spectroscopies. Resonant photoemission will yield information on the charge transfer at interfaces in the hybrid material in the fs time scale. Information on vibronic effects the organic adlayer will be obtained by non-linear sum-frequency generation spectroscopy. While the determination of the vibrational energies from the data will be straightforward, insight into the excitons dynamics will be unraveled by the analysis of the phase signal with the aid of the theoretical description. Computer modeling will assist materials characterization and allow us to discriminate amongst different microscopic mechanisms responsible for the observed spectra.
Based on the structural and dynamical knowledge thus acquired, we will select suitable combinations of the 2D layers and organic components, which promise to yield better device performances. We will consider also the use of nanoporous 2D scaffolds which can overcome the shortcomings of present photovoltaic devices based on organic materials. In fact, charge transport to the electrode competes with electron-hole recombination in organic films, thus implying that charges can only travel a short path in the active material. On the other hand, relatively thick films are needed for efficient absorption of light and nanostructuration of the electrodes has been considered as a possible solution to overcome this problem. The use of inorganic synthetic sponges that can be obtained from micro- and nano-porous 2D graphene will offer the advantages of an efficient continuous electrode and at the same time of a nanostructured scaffold where the photoexcited charges need to cover just a very short path to reach the electrode still having available a sufficiently thick layer of active material to absorb most of the radiation. Structural stability and sample morphology will be modeled by quantum-mechanical methods based on DFT, crucially featuring dispersion forces. Level alignment at the organic/inorganic interface, upon which charge transfer critically depends, will be determined by the GW method, and electronic dynamical effects modeled by TDDFT and ab initio molecular and quantum dynamics.