Abstract
This review outlines the design strategies which aim to develop high performing n-type materials in the fields of organic thin film transistors (OTFT), organic electrochemical transistors (OECT) and organic thermoelectrics (OTE). Figures of merit for each application and the limitations in obtaining these are set out, and the challenges with achieving consistent and comparable measurements are addressed. We present a thorough discussion of the limitations of n-type materials, particularly their ambient operational instability, and suggest synthetic methods to overcome these. This instability originates from the oxidation of the negative polaron of the organic semiconductor (OSC) by water and oxygen, the potentials of which commonly fall within the electrochemical window of n-type OSCs, and consequently require a LUMO level deeper than ∼−4 eV for a material with ambient stability. Recent high performing n-type materials are detailed for each application and their design principles are discussed to explain how synthetic modifications can enhance performance. This can be achieved through a number of strategies, including utilising an electron deficient acceptor–acceptor backbone repeat unit motif, introducing electron-withdrawing groups or heteroatoms, rigidification and planarisation of the polymer backbone and through increasing the conjugation length. By studying the fundamental synthetic design principles which have been employed to date, this review highlights a path to the development of promising polymers for n-type OSC applications in the future.
Highlights
When the heteroatom has a larger electronegativity than carbon, this has the effect of reducing the electron density in the conjugated pi electron system, increasing the electron affinity of the polymer
organic thermoelectrics (OTE) generators convert thermal energy into power by applying a thermal gradient across organic semiconductor (OSC). This gradient causes charge carriers to diffuse away from the heated side of the OTE material, thereby generating a potential difference across the material, known as a thermovoltage and given by the Seebeck coefficient (S), which is the ratio of voltage difference to temperature difference across the material
This review has summarised the various design strategies employed to maximise performance of electron transporting OSC materials for use in organic thin film transistors (OTFT), organic electrochemical transistors (OECT) and OTE device applications as well as figures of merit used to measure performance
Summary
Journal of Materials Chemistry C more complex circuits and increased operational stability.[7,8,9] OTE generators require OSCs for both p-type and n-type operation with well-matched electrical and thermal conductivities. Materials that efficiently transport charge and are deemed ‘‘high mobility’’ generally contain adequate regions of high order, with short connecting sections of amorphous material.[27] An example of a high-mobility n-type polymer is the branched NDI derivate P(NDI2OD-T2), where large regions of crystallinity have been observed.[28] This polymer takes advantage of a face-on packing texture,[29] with the aromatic polymer backbones stacked directly on top of each other, parallel to the substrate This facilitates the hopping mechanism of electrons between chains, due to a stronger orbital overlap and interchain interaction. More efficient packing leads to improved charge carrier transport, higher mobilities[29] and short contact distances.[30,31,32,33] Factors which alter the packing of a material are strong dipole–dipole interactions,[34,35] degree of backbone planarity and steric locking of polymer backbones.[36,37]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
