Identifying structure-absorption relationships and predicting absorption strength of non-fullerene acceptors for organic photovoltaics

Non-fullerene acceptors (NFAs) are excellent light harvesters, yet the origin of their high optical extinction is not well understood. In this work, we investigate the absorption strength of NFAs by building a database of time-dependent density functional theory (TDDFT) calculations of ∼500 π-conjugated molecules. The calculations are first validated by comparison with experimental measurements in solution and solid state using common fullerene and non-fullerene acceptors. We find that the molar extinction coefficient (εd,max) shows reasonable agreement between calculation in vacuum and experiment for molecules in solution, highlighting the effectiveness of TDDFT for predicting optical properties of organic π-conjugated molecules. We then perform a statistical analysis based on molecular descriptors to identify which features are important in defining the absorption strength. This allows us to identify structural features that are correlated with high absorption strength in NFAs and could be used to guide molecular design: highly absorbing NFAs should possess a planar, linear, and fully conjugated molecular backbone with highly polarisable heteroatoms. We then exploit a random decision forest algorithm to draw predictions for εd,max using a computational framework based on extended tight-binding Hamiltonians, which shows reasonable predicting accuracy with lower computational cost than TDDFT. This work provides a general understanding of the relationship between molecular structure and absorption strength in π-conjugated organic molecules, including NFAs, while introducing predictive machine-learning models of low computational cost

Study of nanostructured ultra-refractory Tantalum-Hafnium-Carbide electrodes with wide electrochemical stability window

Transition metal carbides have gathered increasing attention in energy and electrochemistry applications, mainly due to their high structural and physicochemical properties. Their high refractory properties have made them an ideal candidate coating technology and more recently their electronic similarity to the platinum group has expanded their use to energy and catalysis. Here, we demonstrate that the nanostructuring and stoichiometry control of the highest melting point material to this date (Ta-Hf-C) results in outstanding electrochemical stability. Our results show one of the largest windows of stability of a single component electrode in a broad range pH. These experiments provide a new perspective on the electrochemical, thermoelectric and mechanical behavior of Ta-Hf-C nanocomposites, towards a broad range of applications in energy production, catalysis and analytical chemistry.E.C. acknowledges the partial financial support of the National Science Center (NCN) of Poland under the OPUS grant (UMO-2019/35/B/ST5/00248). K.Z. acknowledges the partial financial support from the National Science Centre of Poland by the SONATA Project No. UMO-2016/23/D/ST3/02121. K.S. acknowledges the partial financial support of NCN under the SONATA-BIS grant (2017/26/E/ST5/00416). B.G. and V.B acknowledge the partial financial support of the foundation for polish science (FNP) under the FIRST TEAM program (POIR.04.04.00-00-5D1B/18). Y.K. acknowledges the partial financial support by Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education (NRF-2017R1A6A1A06015181) of Republic of Korea. J.S.R. and B. D.: acknowledge financial support from the Spanish Ministry of Economy, Industry, and Competitiveness through the “Severo Ochoa” Program for Centers of Excellence in R&D (SEV-2015-0496), and MAT2017-90024-P (TANGENTS)-EI/FEDER.With funding from the Spanishgovernment through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe