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Final Report Summary - BEST (Bridging length and timescales of Electronic processes in organic SemiconducTor devices)

Unveiling the relationship between structure, properties and functionality stands as a formidable challenge in complex organic materials of interest for application as active elements in the next generation of cheap and sustainable electronic devices. While the theoretical modelling is expected to be a major drive toward the rational design of improved materials and devices, theoretical and computational tools bridging the length and timescales pertaining to electronic processes in organic solids are not yet widely available. The project BEST addressed this crucial knowledge gap, specifically focusing on the development of an integrated multiscale modelling platform for the accurate description of the electronic processes taking place in organic semiconductors, and its application applications to selected problems identified as crucial bottlenecks for the advancement of organic electronics.
On the methodological side, this project proposed original solutions to bridge isolated research efforts, regarding the morphology of complex (disordered and heterogeneous) molecular systems and their electronic structure and dynamics, into a synergistic multiscale modelling platform. This has been then applied to address timely research questions in the context of organic photovoltaics, singlet fission and doped organic semiconductors, contributing to the advancement of these research lines as summarized below.
Photovoltaic panels based on organic materials are promising candidates to satisfy the growing demand for low-cost renewable energy. Our investigations contributed in shedding light on the elementary electronic processes taking places upon sunlight absorption in the active layer, which consists of a blend of an electron acceptor (typically fullerene derivatives) and an electron donor (usually polymers) organic semiconductors. We showed that the electronic states in soluble fullerene derivatives (e.g. PCBMs) are characterized by localized charge carriers at room temperature, as a result of the energetic disorder arising from the dipolar side groups, quite irrespective on the sample crystallinity. However, all fullerene derivatives sustain high-energy delocalized states that can play a role for solar cell application, as we showed for a prototypical polymer/fullerene interface. In fact, at these hetero-interfaces bound localized charge transfer states are found to coexist with a large majority of thermally accessible delocalized space-separated charges. The latter can be also reached by direct photoexcitation, thanks to their strong hybridization with singlet (Frenkel) polymer excitons. Our findings reconciled the recent experimental reports of ultrafast exciton separation (“hot” process) with the evidence that high quantum yields do not require excess electronic or vibrational energy (“cold” process). Furthermore, we showed that delocalization, by increasing the effective electron−hole distances, tends to reduce energy losses through charge recombination, a process that strongly limits the efficiency of organic solar cells.
Singlet fission is an interesting spin-allowed process in which a singlet excitation is converted into two triplets. This process can be used to increase the efficiency of organic solar cells beyond the Shockley-Queisser limit, by allowing the creation of two electron-hole pairs per absorbed photon. In collaborations with an international team of researchers, both experimentalists (Cambridge University, Humboldt University Berlin) and theoreticians (Tohoku University, Japan), we performed an extensive study of the electronic and excited state properties of pentacene-perfluoropentacene co-crystals. This system represents an ideal case study for the fundamental investigations on singlet fission, whose mechanism is not well understood with ongoing debate in the literature on the role of charge-transfer excitations as possible mediators of the process. Our modelling proved successful in two respects: first, theoretical calculations allowed the comprehension of the complex electronic structure of these co-crystals, allowing the interpretation of anomalous features characterizing ultraviolet photoelectron spectra. In second instance, time-resolved spectroscopy measurements complemented by quantum dynamics simulations of singlet fission, based on the accurate determination of charge-transfer excitation energies, allowed making decisive step forward towards the full comprehension of singlet fission in molecular crystals, as will be disclosed in a forthcoming publication.
The lack of a clear understanding of the mechanism for molecular doping in organic semiconductors is one of the main factors currently hampering the application of this promising technology. Within this project we proposed and applied a general model for the study of molecular doping in organic semiconductor that can be fully parametrized from first principles calculations and that can be applied irrespectively to molecular and polymer systems. Mostly focusing on the paradigmatic case of the pentacene crystal doped with the strong electron acceptor molecule F4TCNQ, our calculations allowed reconciling contrasting experimental evidences and invited to completely revise the well-established textbook picture of doping in the case of organic semiconductors. Our calculations indeed showed that excitonic (electron-hole interaction) and polaronic (structural relaxation) effects, both missed in standard pictures, are crucial for the ionization of dopant molecules. The new approach to molecular doping provided a new robust framework for the systematic study of structure-property relationships in doped semiconductors.
This project also contributed to development, in collaboration with Dr. Xavier Blase (CNRS Grenoble), of a cutting-edge computational method merging in a multiscale fashion many-body perturbation theory (GW formalism and Bethe-Salpeter equation) with classical polarizable models of atomistic resolution. This new tool allows for an accurate description of charged and optical excitations in large and complex molecular systems of interest for applications, fully accounting for the effect of the molecular environment. This has been efficiently implemented in the computer code FIESTA, paving the way to several possible applications in materials science, chemistry, biochemistry and biophysics.

This project exposed the Researcher to new scientific challenges and international collaborations, strengthening his profile of established researcher and bringing him to complete professional maturity and independence. This is best proven by the permanent researcher position he was able to obtain at the Institut Neel of CNRS in Grenoble (France).

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