posted on 2021-06-16, 15:08authored byJian Liu, Bas Van der Zee, Diego R. Villava, Gang Ye, Simon Kahmann, Max Kamperman, Jingjin Dong, Li Qiu, Giuseppe Portale, Maria Antonietta Loi, Jan C. Hummelen, Ryan C. Chiechi, Derya Baran, L. Jan Anton Koster
Molecular doping
makes possible tunable electronic properties of
organic semiconductors, yet a lack of control of the doping process
narrows its scope for advancing organic electronics. Here, we demonstrate
that the molecular doping process can be improved by introducing a
neutral radical molecule, namely nitroxyl radical (2,2,6,6-teramethylpiperidin-i-yl)
oxyl (TEMPO). Fullerene derivatives are used as the host and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazoles (DMBI-H) as the n-type dopant. TEMPO
can abstract a hydrogen atom from DMBI-H and transform the latter
into a much stronger reducing agent DMBI•, which
efficiently dopes the fullerene derivative to yield an electrical
conductivity of 4.4 S cm–1. However, without TEMPO,
the fullerene derivative is only weakly doped likely by a hydride
transfer following by an inefficient electron transfer. This work
unambiguously identifies the doping pathway in fullerene derivative/DMBI-H
systems in the presence of TEMPO as the transfer of a hydrogen atom
accompanied by electron transfer. In the absence of TEMPO, the doping
process inevitably leads to the formation of less symmetrical hydrogenated
fullerene derivative anions or radicals, which adversely affect the
molecular packing. By adding TEMPO we can exclude the formation of
such species and, thus, improve charge transport. In addition, a lower
temperature is sufficient to meet an efficient doping process in the
presence of TEMPO. Thereby, we provide an extra control of the doping
process, enabling enhanced thermoelectric performance at a low processing
temperature.