Entropy-Ruled Method on Density of States Proportion for Charge-Energy Transport in Organic Molecular Crystals: A Computational Study

What is it about?

In this article, we introduce and implement a new term "DOS Proportion" to study the effects of thermodynamics and bias-driven degeneracy weightage on charge-energy transport in molecular crystalline solids. Importantly, we develop the degenerate and large energy flux-approximated entropy-ruled mobility equation to explore the charge transport, which provides band-like mobility from intermediate range (far from hopping, but proximity to band transport). Here, the DOS proportion acts as a key descriptor not only for mobility, conductivity, and also for current density, diode principle too.

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Why is it important?

The variation of normal electronic density of states (DOS) due to doping, applied field, effect of thermal polaron scattering, typical disorder (weak or strong) etc. can be accounted for by DOS proportion, which is the energy (here, chemical potential) scaled entropy of a given Fermionic system (molecule or material). The bias-driven (here, electric field) degeneracy weightage and the effect of thermodynamics on CT in the molecular crystals are addressed through a term named "Density of States (DOS) Proportion". Here, we use 'DOS proportion' principle instead of 'DOS' to quantify the exact charge transport parameters (mobility, conductivity, current density and diode principle) via appropriate QM/MM calculations/measurement in the dialkyl substituted dithienothiophene (DSDTT)-based molecular crystal systems. Using the entropy-ruled method under degenerate and large carrier energy flux approximation, the calculated hole mobility values are around 0.85 and 0.19 cm2/V s for DSDTT-1 and DSDTT-2 molecular crystals, respectively, reaching a proximity value with an experimentally obtained average mobility.

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