Performance Optimization And Characterization Of Thiophenimide-Based N-Type Organic Thermoelectric Materials

Organic semiconductors have been widely investigated in organic solar cells,organic field-effect transistors and organic Light-Emitting Diode.Recently,as the development of this kind of materials,they are becoming appealing as organic thermoelectric materials.Both p-type and n-type organic semiconductors are required to construct an effective organic thermoelectric generator.However,the performance of n-type organic thermoelectric materials is lagged far behind of p-type due to their poor doping efficiency and ambient stability.Therefore,it is important to realize high efficiency n-type doping and thermoelectric performance through optimizing doping method,device fabrication process and film morphology.Unpaired electrons of organic radicals can offer high electrical conductivity without doping,but they typically suffer from low stability.In the first program,we report two organic diradicaloids based on quinoidal oligothiophene derivative?QOT?,that is,BTICN and QTICN,with high stability and conductivity by employing imidebridged fused molecular frameworks.The attachment of a strong electronwithdrawing imide group to the tetracyano-capped QOT backbones enables extremely deeply aligned LUMO levels?from-4.58 to-4.69 e V?,cross-conjugated diradical characters,and remarkable ambient stabilities of the diradicaloids with half-lives > 60 days,which are among the highest for QOT diradicals and also the widely explored polyaromatic hydrocarbon?PAH?-based diradicals.Specifically,QTICN based on a tetrathiophene imide exhibits a cross-conjugation assisted self-doping in the film state as revealed by XPS and Raman studies.This property in combination with its ordered packing yields a high electrical conductivity of 0.34 S cm^-1 for the QTICN films with substantial ambient stability,which is also among the highest values in organic radical-based undoped conductive materials reported to date.When used as an n-type thermoelectric material,QTICN shows a promising power factor of 1.52 ?W m^-1 K^-2.Our results not only provide new insights into the electron conduction mechanism of the self-doped QOT diradicaloids but also demonstrate the great potential of fused quinoidal oligothiophene imides in developing stable diradicals and highperformance doping-free n-type conductive materials.Solution processed n-doping of organic semiconductors is realized by utilizing air-stable dopants.For these dopants,the endothermic process for bond cleavage directly effect the following thermodynamics and kinetics of doping reaction.Although Pd-based catalyst has been widely used in organic synthesis to promote bond cleavage,it was never applied in n-doping process.In the second program,we utilized Pd-based catalyst to investigate its influence on n-doping reaction.We synthesized a new n-type organic semiconductors TEG-BTI2-DFT with modified side chain.After a series optimization work,it was proved that Pd-based catalyst promote the doping reaction,thus dramatically improved the carrier concentration.Finally,a conductivity over 80 S cm^-1 was achieved.