Unveiling the Interplay among End Group, Molecular Packing, Doping Level, and Charge Transport in N-Doped Small-Molecule Organic Semiconductors

Doped small molecules with high electrical conductivity are desired because they typically show a larger Seebeck coefficient and lower thermal conductivity than their polymer counterparts. However, compared with conjugated polymers, only a few small molecules can show high electrical conductivities. In this study, three n-type small-molecule organic semiconductors with different end functional groups are synthesized to explore the reasons for the low electrical conductivity issue in n-doped small-molecule semiconductors. Charge carrier mobility and doping level are usually considered as two major parameters for achieving high electrical conductivity. TDPP-ThIC with high electron mobility of 0.77 cm² V⁻¹ s⁻¹ and high electron affinity, which can be easily n-doped; however, it only displays an electrical conductivity ≈10⁻³ S cm⁻¹. To explore the reasons, the single crystal structure of TDPP-ThIC and the grazing incidence wide-angle X-ray scattering of its n-doped films are carefully analyzed. TDPP-ThIC with a 1D column packing is disclosed and easily distorted by the enthetic n-dopants, which damages the charge transport pathways, and thereby results in low electrical conductivity. The results suggests that only high intrinsic charge carrier mobility and high doping level cannot guarantee high electrical conductivity, and keeping good charge transport pathways after doping is also critical.