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1. Trap-dominated nitrogen dioxide and ammonia responses of air-stable p-channel conjugated polymers from detailed bias stress analysis

   https://doi.org/10.1039/D0TC05458E  

   Mukhopadhyaya, Tushita ; Katz, Howard E. ( March 2021 , Journal of Materials Chemistry C)

   null (Ed.)

   The improvement of conjugated polymer-based gas sensors involves fine tuning the backbone electronic structure and solid-state microstructure to combine high stability and sensitivity. We had previously developed a series of diketopyrrolopyrrole (DPP)-based polymer semiconductors by introducing a variety of fluorene linkers to study the trends and mechanisms governing gas sensitivities and electronic stability in air and under gate and drain bias stress. The proportional on-current change of organic field-effect transistors (OFETs) using a dithienyl DPP–fluorene polymer reached ∼600% for a sequential exposure from 0.5–20 ppm of NO 2 for 5 minutes and also a high response-to-drift ratio under dynamic bias stress. In the present work we specify the roles of static bias stress and traps in the sensing process for the first time. Apart from electronic structure, defects at the molecular and microstructural levels govern the ability to form and sustain traps and subsequent backbone dopability. A polymer with a twisted backbone was observed to be capable of creating an energetically broad trap distribution while a polymer with a high degree of solid-state order shows a tendency to form an energetically narrow trap distribution and a fast passivation of traps on exposure to air. The stability and energetic distribution of traps on subjecting the polymers to bias stress was related to electronic structure and solid-state packing; and the ability of NO 2 and NH 3 to fill/create traps further was evaluated. At a bias stress condition of V G = V D = −80 V, the polymers retain their NO 2 sensitivity both post NO 2 -aided recovery and air-aided recovery. In order to verify the ability of NH 3 to create traps, traps were erased from the OFET sensors by charging with the aid of a positive gate voltage leading to an increase in the NH 3 response when compared to air controls. This work demonstrates that the charge-trap filling and generation response mechanism is predominant and can even be leveraged for higher responses to vapors. Backbone dopability appears to be a minor contributor to responses in this category of polymeric semiconductors with engineered defects. Finally, bias stress generally does not preclude this category of OFET vapor sensors from recovering their original sensitivities. 

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2. Organic Doping at Ultralow Concentrations

   https://doi.org/10.1002/adom.202100089  

   Radha Krishnan, Raj Kishen ; Liu, Shiyi ; Dahal, Drona ; Paudel, Pushpa Raj ; Lüssem, Björn ( April 2021 , Advanced Optical Materials)

   Organic doping is widely used for defining the majority charge carriers of organic thin films, tuning the Fermi level, and improving and stabilizing the performance of organic light‐emitting diodes and organic solar cells. However, in contrast to inorganic semiconductors, the doping concentrations commonly used are quite high (in the wt% range). Such high concentrations not only limit the scope of doping in organic field‐effect transistors (OFETs), but also limit the doping process itself resulting in a low doping efficiency. Here, the mechanism of doping at ultralow doping concentrations is studied. Doped C₆₀metal‐oxide‐semiconductor (MOS) junctions are used to study doping at the 100 ppm level. With the help of a small‐signal drift‐diffusion model, it is possible to disentangle effects of traps at the gate dielectric/organic semiconductor interface from effects of doping and to determine the doping efficiency and activation energy of the doping process. Doped C₆₀OFETs with an ultralow operation voltage of 800 mV and an excellent on/off ratio of up to 10⁷are realized. The devices have low subthreshold swing in the range of 80 mV dec⁻¹and a large transconductance of up to 8 mS mm⁻¹.

    

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3. Doped N‐Type Organic Field‐Effect Transistors Based on Faux‐Hawk Fullerene

   https://doi.org/10.1002/aelm.201900109  

   Liu, Shiyi ; DeWeerd, Nicholas J. ; Reeves, Brian J. ; San, Long K. ; Dahal, Drona ; Radha Krishnan, Raj Kishen ; Strauss, Steven H. ; Boltalina, Olga V. ; Lüssem, Björn ( May 2019 , Advanced Electronic Materials)

   Faux‐hawk fullerenes are promising candidates for high‐performance organic field‐effect transistors (OFETs). They show dense molecular packing and high thermal stability. Furthermore, in contrast to most other C₆₀derivates, functionalization of the fullerene core by the fluorinated group C₆F₄CF₂does not increase their lowest unoccupied orbital position, which allows the use of air‐stable molecular n‐dopants to optimize their performance. The influence of n‐doping on the performance of OFETs based on the faux‐hawk fullerene 1,9‐C₆₀(cyclo‐CF₂(2‐C₆F₄)) (C₆₀FHF) is studied. An analytic model for n‐doped transistors is presented and used to clarify the origin of the increase in the subthreshold swing usually observed in doped OFETs. It is shown that the increase in subthreshold swing can be minimized by using a bulk dopant layer at the gate dielectric/C₆₀FHF layer instead of a mixed host:dopant layer. Following an optimization of the OFETs, an average electron mobility of 0.34 cm² V⁻¹ s⁻¹, a subthreshold swing below 400 mV dec⁻¹for doped transistors, and a contact resistance of 10 kΩ cm is obtained, which is among the best performance for fullerene based n‐type semiconductors.

    

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4. Influence of Molecular Weight on the Organic Electrochemical Transistor Performance of Ladder‐Type Conjugated Polymers

   https://doi.org/10.1002/adma.202106235  

   Wu, Han‐Yan ; Yang, Chi‐Yuan ; Li, Qifan ; Kolhe, Nagesh B. ; Strakosas, Xenofon ; Stoeckel, Marc‐Antoine ; Wu, Ziang ; Jin, Wenlong ; Savvakis, Marios ; Kroon, Renee ; et al ( December 2021 , Advanced Materials)

   Organic electrochemical transistors (OECTs) hold promise for developing a variety of high‐performance (bio‐)electronic devices/circuits. While OECTs based on p‐type semiconductors have achieved tremendous progress in recent years, n‐type OECTs still suffer from low performance, hampering the development of power‐efficient electronics. Here, it is demonstrated that fine‐tuning the molecular weight of the rigid, ladder‐type n‐type polymer poly(benzimidazobenzophenanthroline) (BBL) by only one order of magnitude (from 4.9 to 51 kDa) enables the development of n‐type OECTs with record‐high geometry‐normalized transconductance (g_m,norm ≈ 11 S cm⁻¹) and electron mobility × volumetric capacitance (µC* ≈ 26 F cm⁻¹ V⁻¹s⁻¹), fast temporal response (0.38 ms), and low threshold voltage (0.15 V). This enhancement in OECT performance is ascribed to a more efficient intermolecular charge transport in high‐molecular‐weight BBL than in the low‐molecular‐weight counterpart. OECT‐based complementary inverters are also demonstrated with record‐high voltage gains of up to 100 V V⁻¹and ultralow power consumption down to 0.32 nW, depending on the supply voltage. These devices are among the best sub‐1 V complementary inverters reported to date. These findings demonstrate the importance of molecular weight in optimizing the OECT performance of rigid organic mixed ionic–electronic conductors and open for a new generation of power‐efficient organic (bio‐)electronic devices.

    

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5. Benzimidazolone‐Dioxazine Pigments‐Based Conjugated Polymers for Organic Field‐Effect Transistor

   https://doi.org/10.1002/marc.202200297  

   Tung, Wei‐Yao ; Pu, Cheng ; Huang, Yi‐Fan ; Xie, Wei ; Cheng, Chung‐Fu ; Lai, Yun‐Yu ; Li, Xiang ; Lin, Heng‐Yi ; Lai, Yueh‐Ting ; Chen, Kun ; et al ( June 2022 , Macromolecular Rapid Communications)

   Molecules based on benzimidazolone‐dioxazine are known as blue/violet pigments and have been commercialized for decades. However, unfavorable solubility limits the application of these structures as building blocks of conjugated polymers despite their low band gaps. Herein, a series of donor–acceptor conjugated polymers containing soluble benzimidazolone‐dioxazine structures as the acceptors and oligothiophene as donors are synthesized and investigated. With increasing numbers of thiophene rings, the steric hindrance diminishes and high molecular weight polymers can be achieved, leading to an improved performance in organic field effect transistor devices. The hole mobility of polymers with three to six thiophene units is in the order of 10⁻¹cm²V⁻¹s⁻¹. Among all the polymers, polymer P3 with three thiophene units between benzimidazolone‐dioxazine structures shows the best hole mobility of 0.4 cm²V⁻¹s⁻¹. Grazing‐incidence wide‐angle X‐ray scattering results reveal that the high mobility of organic field‐effect transistors (OFETs) can be accredited by matched donor–acceptor packing in the solid thin films.

    

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