Molecular Descriptor and Criterion for Efficient N-Doping of Organic Semiconductors

Precursor dopants have been extensively applied for n-doping of organic semiconductors (OSCs) to address the trade-off between dopant reducibility and stability. N-DMBI-H derivates are believed as one of the most successful air-stable n-dopants. Two doping mechanisms, i.e., hydrogen atom transfer (HAT) and hydride transfer (HYT), have been suggested but the dominance of them remains controversial. In this work, we rationalize the thermodynamics and kinetics of HAT and HYT reactions between N-DMBI-H derivates and a variety of OSCs and conjugated polymers based on the density functional theory, and manifest that the HYT via concerted electron and hydrogen atom transfer is the most viable doping mechanism. We find two linear relations between the free energy change of HYT, \Delta G_HYT, and the electron affinity (EA) of OSCs as well as the ionization energy (IE) of dopant radicals, D, and the Brnsted-Evans-Polanyi relation between the \Delta G_HYT and the activation barrier of HYT, \Delta G^≠. By correlating important thermodynamic and kinetic parameters of HYT to facile properties of OSCs and dopants, molecular descriptors of n-doping are uncovered. Furthermore, an EA(OSC) - IE(D) > 1.0 eV criterion is proposed for achieving a high doping efficiency based on the requirement \Delta G_HYT < 0. Finally, new quinoid-structure polymers with EA > 4.0 eV and good backbone planarity are designed as potential n-type OSCs. The molecular descriptors and criterion discovered provide a simple and operable way to speed up the development of n-type OSCs and dopants by high-throughput screening and machine learning technique