Fullerenes have formed an integral part of high performance organic solar cells over the last 20 years, however their inherent limitations in terms of synthetic flexibility, cost and stability have acted as a motivation to develop replacements; the so-called non-fullerene electron acceptors. A rapid evolution of such materials has taken place over the last few years, yielding a number of promising candidates that can exceed the device performance of fullerenes and provide opportunities to improve upon the stability and processability of organic solar cells. In this review we explore the structure-property relationships of a library of non-fullerene acceptors, highlighting the important chemical modifications that have led to progress in the field and provide an outlook for future innovations in electron acceptors for use in organic photovoltaics.
This review summarises high performing n-type polymers for use in organic thin film transistors, organic electrochemical transistors and organic thermoelectric devices with a focus on stability issues arising in these electron transporting materials.
N‐type conjugated polymers as the semiconducting component of organic electrochemical transistors (OECTs) are still undeveloped with respect to their p‐type counterparts. Herein, we report two rigid n‐type conjugated polymers bearing oligo(ethylene glycol) (OEG) side chains, PgNaN and PgNgN, which demonstrated an essentially torsion‐free π‐conjugated backbone. The planarity and electron‐deficient rigid structures enable the resulting polymers to achieve high electron mobility in an OECT device of up to the 10−3 cm2 V−1 s−1 range, with a deep‐lying LUMO energy level lower than −4.0 eV. Prominently, the polymers exhibited a high device performance with a maximum dimensionally normalized transconductance of 0.212 S cm−1 and the product of charge‐carrier mobility μ and volumetric capacitance C* of 0.662±0.113 F cm−1 V−1 s−1, which are among the highest in n‐type conjugated polymers reported to date. Moreover, the polymers are synthesized via a metal‐free aldol‐condensation polymerization, which is beneficial to their application in bioelectronics.
Expanding the toolbox
of the biology and electronics mutual conjunction
is a primary aim of bioelectronics. The organic electrochemical transistor
(OECT) has undeniably become a predominant device for mixed conduction
materials, offering impressive transconduction properties alongside
a relatively simple device architecture. In this review, we focus
on the discussion of recent material developments in the area of mixed
conductors for bioelectronic applications by means of thorough structure–property
investigation and analysis of current challenges. Fundamental operation
principles of the OECT are revisited, and characterization methods
are highlighted. Current bioelectronic applications of organic mixed
ionic–electronic conductors (OMIECs) are underlined. Challenges
in the performance and operational stability of OECT channel materials
as well as potential strategies for mitigating them, are discussed.
This is further expanded to sketch a synopsis of the history of mixed
conduction materials for both p- and n-type channel operation, detailing
the synthetic challenges and milestones which have been overcome to
frequently produce higher performing OECT devices. The cumulative
work of multiple research groups is summarized, and synthetic design
strategies are extracted to present a series of design principles
that can be utilized to drive figure-of-merit performance values even
further for future OMIEC materials.
Conjugated polymers
achieve redox activity in electrochemical devices
by combining redox-active, electronically conducting backbones with
ion-transporting side chains that can be tuned for different electrolytes.
In aqueous electrolytes, redox activity can be accomplished by attaching
hydrophilic side chains to the polymer backbone, which enables ionic
transport and allows volumetric charging of polymer electrodes. While
this approach has been beneficial for achieving fast electrochemical
charging in aqueous solutions, little is known about the relationship
between water uptake by the polymers during electrochemical charging
and the stability and redox potentials of the electrodes, particularly
for electron-transporting conjugated polymers. We find that excessive
water uptake during the electrochemical charging of polymer electrodes
harms the reversibility of electrochemical processes and results in
irreversible swelling of the polymer. We show that small changes of
the side chain composition can significantly increase the reversibility
of the redox behavior of the materials in aqueous electrolytes, improving
the capacity of the polymer by more than one order of magnitude. Finally,
we show that tuning the local environment of the redox-active polymer
by attaching hydrophilic side chains can help to reach high fractions
of the theoretical capacity for single-phase electrodes in aqueous
electrolytes. Our work shows the importance of chemical design strategies
for achieving high electrochemical stability for conjugated polymers
in aqueous electrolytes.
Donor-acceptor (D-A) polymers are promising materials for organic electrochemical transistors (OECTs), as they minimized etrimental faradaic side-reactions during OECT operation, yet their steady-state OECT performance still lags far behind their all-donor counterparts.W er eport three D-A polymers based on the diketopyrrolopyrrole unit that affordO ECT performances similar to those of all-donor polymers,hence representing asignificant improvement to the previously developed D-A copolymers.I na ddition to improved OECT performance,DFT simulations of the polymers and their respective hole polarons also reveal ap ositive correlation between hole polaron delocalization and steadystate OECT performance,p roviding new insights into the design of OECT materials.I mportantly,w ed emonstrate how polaron delocalization can be tuned directly at the molecular level by selection of the building blocks comprising the polymers conjugated backbone,t hus paving the way for the development of even higher performing OECT polymers.
Recent research demonstrates the viability of organic electrochemical transistors (OECTs) as an emergent technology for biosensor applications. Herein, a comprehensive summary is provided, highlighting the significant progress and most notable advances within the field of OECT‐based biosensors. The working principles of an OECT are detailed, with specific attention given to the current library of organic mixed ionic‐electronic conductor (OMIEC) channel materials utilized in OECT biosensors. The application of OECTs for metabolite, ion, neuromorphic, electrophysiological, and virus sensing as well as immunosensing is reported, detailing the breadth and scope of OECT‐based biosensors. Furthermore, an outlook and perspective on synthetic molecular design of future channel materials, specifically designed for OECT biosensors, is provided. The development of optimized channel materials, creative device architectures, and operational nuances will set the stage for OECT‐based biosensors to thrive and accelerate their clinical prevalence in the near future.
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