Passivation of Molecular n‐Doping: Exploring the Limits of Air Stability

Abstract

Molecular doping is a key technique for fl exible and low-cost organic complementary semiconductor technologies that requires both effi cient and stable p-and n-type doping. However, in contrast to molecular p-dopants, highly effi cient n-type dopants are commonly sensitive to rapid degradation in air due to their low ionization energies ( IE s) required for electron donation, e.g., IE = 2.4 eV for tetrakis(1,3,4,6,7,8-hexahydro-2H -pyrimido[1,2-a ]pyrimidinato) ditungsten(II) (W 2 (hpp) 4 ). Here, the air stability of various host:W 2 (hpp) 4 combinations is compared by conductivity measurements and photoemission spectroscopy. A partial passivation of the n-doping against degradation is found, with this effect identifi ed to depend on the specifi c energy levels of the host material. Since host-W 2 (hpp) 4 electronic wavefunction hybridization is unlikely due to confi nement of the dopant highest occupied molecular orbital (HOMO) to its molecular center, this fi nding is explained via stabilization of the dopant by single-electron transfer to a host material whose energy levels are suffi ciently low for avoiding further charge transfer to oxygen-water complexes. Our results show the feasibility of temporarily handling n-doped organic thin fi lms in air, e.g., during structuring of organic fi eld effect transistors (OFETs) by lithography.

Air‐Stable n‐Type Thermoelectric Materials Enabled by Organic Diradicaloids

“…The electrical conductivities of 2DQQT‐S and 2DQQT‐Se slightly increased at the beginning and then remained at the initial values for over 260 hours. This is the current highest reported air stability among n‐type OTE materials (Figure ), and demonstrates that diradicaloid NSOCs are excellent candidates for thermoelectric applications.…”

“… Attenuation ratios of electrical conductivity of neutral 2DQQT‐S and 2DQQT‐Se thin films in air (humidity ≈20 %; temperature ≈25 °C) with exposure time in comparison to data reported for bis‐HFI‐NTCDI and ClBDPPV ,…”

Cholesteric Aggregation at the Quinoidal-to-Diradical Border Enabled Stable n-Doped Conductor

“…Recently, Bré das et al utilized an organic metal dopant (W 2 (hpp) 4 ) to dope a series of semiconductors with LUMO energy levels ranging from À2.7 to À4.0 eV and found that the best in-air electrical conductivity of bis-HFI-NTCDI (10 À4 S cm À1 ) decreased to 33% of the initial value within half an hour. 12 Katz et al utilized the air-stable organic salt tetrabutylammonium fluoride (TBAF) to dope the polymer ClBDPPV with a LUMO energy level of À4.3 eV. However, the electrical conductivity of n-doped ClBDPPV (0.62 S cm À1 ) with thickness of around 8 mm for self-encapsulation decreased to 50% within 24 h, which is the most stable n-doped system reported to date.…”

Impact of Morphology on Charge Carrier Transport and Thermoelectric Properties of N‐Type FBDOPV‐Based Polymers

“…LUMO levels below –4.0 eV are favorable to ensure thermodynamic stability of n‐type materials regarding ambient conditions (i.e., stability towards oxidation in air). [ 40,41 ] The shape of the reduction waves of FBDOPV‐based polymers suggests the contribution of two anionic species, confirmed by CV measurements of P(FBDOPV‐F) in chloroform solution (Figure S17a, Supporting Information). The stepwise one‐electron transfers suggest the formation of monoanionic species followed by di‐anionic species, tentatively attributed to the successive reductions of each carbonyl group of the fumaraldehyde moiety composing the FBDOPV core (proposed structures drawn in Figure S17b, Supporting Information).…”