Mixed Conduction in an N‐Type Organic Semiconductor in the Absence of Hydrophilic Side‐Chains

Abstract

Organic electrochemical transistors (OECTs) are the building blocks of biosensors, neuromorphic devices, and complementary circuits. One rule in the materials design for OECTs is the inclusion of a hydrophilic component in the chemical structure to enable ion transport in the film. Here, it is shown that the ladder-type, side-chain free polymer poly(benzimidazobenzophenanthro line) (BBL) performs significantly better in OECTs than the donor-acceptor type copolymer bearing hydrophilic ethylene glycol side chains (P-90). A combination of electrochemical techniques reveals that BBL exhibits a more efficient ion-to-electron coupling and higher OECT mobility than P-90. In situ atomic force microscopy scans evidence that BBL, which swells negligibly in electrolytes, undergoes a drastic and permanent change in morphology upon electrochemical doping. In contrast, P-90 substantially swells when immersed in electrolytes and shows moderate morphology changes induced by dopant ions. Ex situ grazing incidence wide-angle X-ray scattering suggests that the particular packing of BBL crystallites is minimally affected after doping, in contrast to P-90. BBL's ability to show exceptional mixed transport is due to the crystallites' connectivity, which resists water uptake. This side chain-free route for the design of mixed conductors could bring the n-type OECT performance closer to the bar set by their p-type counterparts.

n-Type organic semiconducting polymers: stability limitations, design considerations and applications

“…In contrast, coherence lengths for P-90 after doping reduced by 15%, showing a decrease in order in the film. 126 This was also verified by examining the p-stacking distances, which is already shorter in BBL (3.51 Å, compared to 3.82 Å in P-90) and further contracts upon doping, which is likely beneficial for charge transport. 126 However, as a result of the lack of sidechains, processing of BBL requires aggressive conditions, such as methanesulfonic acid.…”

“…In OTFTs, an electron mobility of 0.1 cm 2 V À1 s À1 has been achieved, which has been attributed to the close p-stacking of the fused backbone (3.51 Å). 51,126 Thin films of BBL have been shown to contain crystalline domains on the order of 50-125 nm and this crystallinity has been suggested to act as a kinetic barrier to prevent oxygen diffusion into the films, thereby enhancing the stability of OTFTs based on BBL. 127 Another A-A example are polymers containing the thiazole imide moiety.…”

“…A recent study which compared materials, BBL and P-90, managed to improve on the previously high g m 0 value of B0.6 S cm À1 with an increase of one third, to realise a g m 0 of 0.815 S cm À1 . 126 It should be noted that the key difference in this study compared to the original work with BBL in OECT devices is the processing method, which was originally spray coating, and in this work utilised a spin-coating deposition method. 7,126 This emphasises the importance of selecting the appropriate processing conditions to maximise the performance of a material.…”

“…126 It should be noted that the key difference in this study compared to the original work with BBL in OECT devices is the processing method, which was originally spray coating, and in this work utilised a spin-coating deposition method. 7,126 This emphasises the importance of selecting the appropriate processing conditions to maximise the performance of a material. This work also produced a [mC*] value of 1.99 F cm À1 V À1 s À1 , which further highlights the impact of a sidechain-free structure.…”

“…This work also produced a [mC*] value of 1.99 F cm À1 V À1 s À1 , which further highlights the impact of a sidechain-free structure. 126 By conducting ex situ GIWAXS Table 8 The electron affinity (EA), number average and weight average molecular weights (M n and M w ), the electron mobility (m e ), volumetric capacitance (C*), the gold-standard figure of merit, [mC*] and thickness normalised transconductance (g m 0 ) measurements, it could be seen that the coherence lengths, which are reflective of the order in the film, increased upon electrochemical doping by 9% for BBL. In contrast, coherence lengths for P-90 after doping reduced by 15%, showing a decrease in order in the film.…”