The effect of the oxygen dangling on the thermoelectric properties of organic Thienoisoindigo single-molecule junction

Theoretical investigation for thermoelectric characteristics of organic Thienoisoindigo single-molecule is carried out using the first-principles calculations based on the density functional theory. It reveals that modifying the position or removing oxygen atoms significantly alters the thermoelectric properties. Transmission coefficient calculations show that the lowest unoccupied molecular orbital (LUMO) dominates across all molecular configurations. Repositioning oxygen atoms increases the bandgap from 1.14 to 1.53 eV, while the complete removal of oxygen further increases to 1.8 eV. This change leads to the disruption of constructive quantum interference, which is replaced by destructive one. The electrical conductance is similarly affected by changes in oxygen atom positioning, with values shifting from \(-\) $-$ 1.06 to \(-\) $-$ 1.63. Molecules without oxygen atoms exhibit lower conductance compared to those with dangling oxygen, resulting in reduced semiconductor-like behavior and enhanced insulating properties. The Seebeck coefficient remains stable at \(-\) $-$ 2.99 \(\varvec{\mu }\) $\mathit{\mu }$ V/K when oxygen atoms are repositioned. However, the removal of one oxygen atom changes the coefficient to a positive value (290.14 \(\varvec{\mu }\) $\mathit{\mu }$ V/K), causing the molecule to transition from n-type to p-type behavior. The complete absence of oxygen atoms returns the Seebeck coefficient to a negative value ( \(-\) $-$ 256.08 \(\varvec{\mu }\) $\mathit{\mu }$ V/K), switching the molecule back to n-type conduction.

This investigation was achieved by applying the SIESTA software through density functional theory (DFT) computations. To account for exchange and correlation effects, we use a double-zeta polarized (DZP) basis set in conjunction with the generalized gradient approximation (GGA-PBE) to determine the ideal ground-state atomic locations. By combining the Hamiltonian of each system with the quantum transport code GOLLUM, we can calculate the transmission coefficient, projected density of states, electrical conductance, and Seebeck coefficient to examine the thermoelectric characteristics of the molecular junction.