Investigation of the High Electron Affinity Molecular Dopant F6‐TCNNQ for Hole‐Transport Materials

Abstract

2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ) is investigated as a molecular p-type dopant in two hole-transport materials, 2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene (Spiro-TAD) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The electron affinity of F6-TCNNQ is determined to be 5.60 eV, one of the strongest organic molecular oxidizing agents used to date in organic electronics. p-Doping is found to be effective in Spiro-TAD (ionization energy = 5.46 eV) but not in TCTA (ionization energy = 5.85 eV). Optical absorption measurements demonstrate that charge transfer is the predominant doping mechanism in Spiro-TAD:F6-TCNNQ. The hostdopant interaction also leads to a significant alteration of the host film morphology. Finally, transport measurements done on Spiro-TAD:F6-TCNNQ as a function of dopant concentration and temperature, and using a highly doped contact layer to ensure negligible hole injection barrier, lead to an accurate measurement of the film conductivity and hole-hopping activation energy.

Host dependence of the electron affinity of molecular dopants

“…In the case of p-type doping, a strong electron acceptor molecule oxidizes the host, a process that is usually discussed in the literature by comparing the electron affinity of the dopant (EA D ) and the ionization potential of the host semiconductor (IP S ), with doping becoming effective when the difference EA D À IP S is negative or vanishingly small. 1,[11][12][13][14] The simple argument that the effectiveness of doping would only depend on the energetics of the donating and accepting energy levels finds plausible confirmations in experimental literature. Common molecular hole-transporting materials, such as pentacene (PEN, IP = 4.9 eV for films of standing molecules 15 ) or N,N 0 -di(1-naphthyl)-N,N 0 -diphenyl-(1,1 0 -biphenyl)-4,4 0 -diamine (NPB or NPD, IP = 5.2 eV 12 ), are successfully doped by strong oxidants such as 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ, EA = 5.08-5.24 eV 16,17 ) or 2,2 0 -(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ, EA = 5.37-5.60 eV 13,18 ).…”

“…1,[11][12][13][14] The simple argument that the effectiveness of doping would only depend on the energetics of the donating and accepting energy levels finds plausible confirmations in experimental literature. Common molecular hole-transporting materials, such as pentacene (PEN, IP = 4.9 eV for films of standing molecules 15 ) or N,N 0 -di(1-naphthyl)-N,N 0 -diphenyl-(1,1 0 -biphenyl)-4,4 0 -diamine (NPB or NPD, IP = 5.2 eV 12 ), are successfully doped by strong oxidants such as 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ, EA = 5.08-5.24 eV 16,17 ) or 2,2 0 -(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ, EA = 5.37-5.60 eV 13,18 ). On the polymer side, F4TCNQ proved able to dope poly(3-hexylthiophene) (P3HT, IP = 4.6 eV 6 ), while the oxidation of a diketopyrrolopyrrole-based polymer with IP = 5.49 eV 14 could be attained only by the very strong acceptor hexacyano-trimethylenecyclopropane 19,20 (CN6-CP, EA = 5.87 eV 14 ).…”

Demonstration of the key substrate-dependent charge transfer mechanisms between monolayer MoS2 and molecular dopants

“…We start by testing the direct CT scenario for the three substrates according to the measured energy levels. Considering the reported electron affinity (EA) of F 6 TCNNQ (5.6 eV) 41 The energy difference between the EA of ML-MoS 2 and the EA of F 6 TCNNQ supports transfer of thermally excited electrons. In this case, the electron density (n e ) in the conduction band is an important factor.…”

Modulation of the Work Function by the Atomic Structure of Strong Organic Electron Acceptors on H‐Si(111)

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Silicon (Si) is the most commonly and widely used semiconductor device component in commercial microelectronics due to its high stability, abundance, and technological readiness. Good candidates for the organic electron acceptor component are 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodi-methane (F4-TCNQ) and 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F6-TCNNQ) due to their high electron affinity of 5.24 [7] and 5.60 eV, [8] respectively, as measured in neat molecular thin films.The quantitative tuning of the properties of such interfaces depends critically on the structure of the organic/inorganic interface, which is an aspect that is rarely properly modeled or measured under realistic device conditions. In particular, atomic arrangements and charge redistribution at interfaces are crucial design parameters for high-quality devices.

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