Abstract
POLYPHON challenges how modifications in the lattice dynamics of organic semiconductors (OSC) reflect in a change of their electronic properties. POLYPHON exploits a robust synergic approach between precise structure definition, lattice dynamics and electronic transport, always combining experiments and theoretical modelling. Such a knowledge is of significant value because of the lack of predictive understanding of the inter-relationship between solid-state packing and performance of the device. Thus, despite OSC are attractive candidates as active layers in several new technologies, their use is currently hampered. In fact, different crystal structures (i.e. polymorphs) are available for the same molecular compound. Different polymorphs also have different vibrational patterns, which return different electron-phonon coupling (EPC), shaping the transport behaviour. Thus, polymorphism has a deep, but yet unpredictable, impact on OSC electronic properties. POLYPHON tackles its ambitious goals by addressing classes of molecules (further detailed in part B) like variably substituted, also asymmetric, BTBT, DNTT, TIPS and TMTES; and charge transfer compounds like (BTBT)(F -TCNQ/TCNNQ). x Each class will be first investigated by XRD in bulk as pre-screening, in mutual assessment with Raman, to fasten the subsequent work on thin films. The latter, necessary for most of real-world OSC applications, will be studied by identifying the actual substrate-selected polymorphs, including mixed phases, for the proper evaluation of the EPC, and via transport experiments in FET, Hall effect and van der Paw geometries, also with alternated current up to 1 GHz. Characterizations at different temperatures will enable to explore the polymorph landscape and phase diagram. Inhomogeneous mixed phases will receive special attention, the boundaries between different polymorphs will be investigated, mainly computationally, and included in the transport simulations, with continuous mutual feedback with the experiments. Transport simulations will follow a semiclassical approach developed by the PI (github.com/PatrizioGraziosi/ELECTRA) where the EPC can be fully considered in the anisotropic evaluation of the atomistic scattering rates, and where extrinsic effects, like mixed phases, can be included. POLYPHON’s comprehensive approach will allow to: i) achieve fundamental and transferrable knowledge on how the polymorphism impacts on the electronic properties with the possibility to be predictive on how a change in the lattice dynamics will reflect in a change in the electronic properties; ii) disentangle molecular structure, solid-state packing, nature and quantity of polymorphs, nature of the boundary between polymorphs, in respect of the electronic properties; iii) recognize how the substrate nature selects metastable polymorphs; iv) release to the community open-source simulation tools for the data analysis and the modelling of polymorphic OSC.
Results achieved
POLYPHON has achieved its main scientific and methodological objectives by establishing an integrated experimental/computational framework to connect molecular structure, polymorphism, lattice dynamics, electron-phonon coupling and charge transport in organic semiconductors. The consortium consolidated the selection of target materials, including substituted BTBT and DNTT derivatives, TIPS-pentacene and other high-mobility molecular semiconductors, and applied a combined strategy based on X-ray diffraction, Raman/IR spectroscopy and atomistic transport modelling. Experimental work identified polymorphic and orientational fingerprints in bulk materials and thin films. BTBT-based systems were studied as a function of deposition conditions, substrate environment and thermal treatment, showing that processing and polymer interlayers can strongly affect crystallinity and mobility, even when the polymorph is not changed. TIPS-pentacene thin films were investigated by grazing-incidence X-ray diffraction and polarized Raman spectroscopy, showing preservation of the stable polymorph together with strong out-of-plane and in-plane texture. Variable-temperature measurements on selected systems enabled monitoring of phase transitions, metastable forms and recrystallization processes through low-frequency vibrational modes. The computational work delivered validated protocols for phonons, Raman spectra, electron-phonon coupling and charge-transport simulations. These protocols were first benchmarked on oligoacenes and then extended to representative polymorphic organic semiconductors, including BTBT/DNTT derivatives, TIPS-pentacene, rubrene, diF-TES-ADT and related systems. The project developed theoretical models to account not only for intrinsic electron-phonon-coupling-limited transport, but also for extrinsic effects such as phase mixing, effective barriers, thermal broadening and traps. Open-source routines supporting electron-phonon coupling calculations and charge-transport analysis have been released through the PI GitHub repository, including EPHOS and ELECTRA. The project has also produced significant dissemination outputs: fourteen conference contributions, eight scientific publications already published, and further manuscripts under preparation on TIPS-pentacene, BTBT/DNTT systems, rubrene, polymorph coexistence and related model-validation studies. Access to large-scale computational and experimental infrastructures was obtained, including more than 2 million GPU hours, more than 3 million CPU hours, and Elettra proposals on THz nano-imaging, nano-IR mapping and UV resonant Raman spectroscopy. Overall, POLYPHON has generated transferable knowledge on how polymorphism, lattice dynamics and microstructural effects shape charge transport in organic semiconductor materials. Beyond the specific case studies, the project has established a robust and transferable experimental/computational framework linking solid-state packing, vibrational fingerprints, electron–phonon coupling, atomistic scattering processes and charge mobility. The combined use of spectroscopy, diffraction and modelling proved effective for identifying structure–property relationships in complex systems, homologous molecular series, thin films and metastable phases. These results provide a solid basis for future predictive studies aimed at rationalising transport behaviour across different classes of organic semiconductors and supporting the design of materials with improved performance. Further impact is expected through the planned publications, conference presentations, future collaborative activities, and the continued development and use of the open-source modelling tools validated during the project. At the same time, POLYPHON has strengthened a national network integrating spectroscopy, diffraction, electronic-structure theory and charge-transport modelling.Project details
Unibo Team Leader: Elisabetta Venuti
Unibo involved Department/s:
Dipartimento di Chimica Industriale "Toso Montanari"
Coordinator:
CNR - Consiglio Nazionale delle Ricerche(Italy)
Total Unibo Contribution: Euro (EUR) 67.473,00
Project Duration in months: 24
Start Date:
28/09/2023
End Date:
28/02/2026
