Stability of Charge Transfer States in F₄TCNQ-Doped P3HT

Abstract

Printable electronic devices from organic semiconductors are strongly desired but limited by their low conductivity and stability relative to those of their inorganic counterparts. p-Doping of poly­(3-hexyl)­thiophene (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane increases conductivity through integer charge transfer (ICT) to form mobile carriers in P3HT. An alternate undesired reaction pathway is formation of a partial charge transfer complex (CPX), which results in a localized, traplike state for the hole on P3HT. This effort addresses the stability of the free carrier states, once formed. Herein, we demonstrate that, while the ICT state may be kinetically preferred, the CPX state is thermodynamically more stable. Conversion of the ICT state to the CPX state is monitored here over time using a combination of infrared and photoelectron spectroscopies and supported by a complete loss of film conductivity with an increased CPX state concentration. Both the fraction and the rate of conversion to the CPX state are influenced by polymer molecular weight, dopant concentration, and storage conditions, with ambient storage conditions accelerating the conversion. This work suggests that a renewed focus on dopant–matrix reaction chemistry should be considered in the context of both kinetic and thermodynamic considerations.

Doping-related broadening of the density of states governs integer-charge transfer in P3HT

“…In principle, suppressing FCT is desired to maximize the doping efficiency, 11 while, in turn, the presence of a moderate fraction of FCT has recently been found beneficial for maintaining the performance of thermoelectric devices over time. 5,6 Notably, although FCT does not entail immediate ionization, significant increases in conductivity have been reported for this scenario as well. [9][10][11] The necessary precondition for FCT is the overlap between the highest occupied molecular orbital (HOMO) of the OSC and the lowest unoccupied molecular orbital (LUMO) of the p-dopant, which results in intermolecular rehybridization.…”

“…We thereby exploit that some vibrational modes of TCNQ derivatives undergo a softening which can be related to the degree of charge transfer δ . [5][6][7]10,11,15,16,[24][25][26][27][28] Especially, the C≡N stretch modes from the FxTCNQ 5 This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.…”

“…[1][2][3] Conjugated polymers are promising in this field as they exhibit low thermal conductivities while molecular doping can be used to achieve high electrical conductivities, and both properties are key for thermoelectric applications because they promote the thermoelectric figure of merit (ZT ). 2,[4][5][6][7][8] In a simplified view of the fundamental processes at work in the molecular p-doping of OSCs, the dopant molecule, typically a strong electron acceptor, receives an electron from the surrounding OSC host and thus populates it with holes as potentially mobile charge carriers. Energetically, this requires the electron affinity of the dopant (EA) to exceed the ionization energy of the OSC host material (IE).…”

“…Knowing the limits of charge transfer in one and the other scenario is of practical importance as it can promote the use of weaker dopants whose decreased reactivity can lead to higher environmental stability of OSC devices. 14 In that regard, the OSC poly(3-hexylthiophene) (P3HT) p-doped with tetrafluorotetracyanoquinodimethane (F4TCNQ) has been widely investigated as it was found that both ICT and FCT can occur under some circumstances, [5][6][7][15][16][17] and that regio-random (rra) P3HT is more prone to FCT than its regio-regular (rr) variety 16 which has been ascribed to a higher IE. 18 However, (semi)crystalline rr-P3HT and amorphous rra-P3HT are structurally and energetically only comparable to a limited extent, in particular if we assume that disorder may play a crucial role in the doping process.…”

Conductive Polymer Work Function Changes due to Residual Water: Impact of Temperature‐Dependent Dielectric Constant

“…[5][6][7] Molecular doping of polythiophenes, in which a small amount of dopant is added to the host polymer, has proven to be a successful approach to increase the density of mobile charge carriers and thus conductivity above 1 S cm −1 . [8][9][10][11][12][13][14] A key challenge to the successful application of molecularly doped polymers lies in their relatively poor thermal stability, which ultimately leads to dopant diffusion and desorption at elevated temperatures. [15][16][17] A conductive polymer from the thiophene family that presently dominates in device applications is p-doped poly(ethylenedioxythiophene) (PEDOT).…”