B←N-Incorporated Dibenzo-azaacenes as n-Type Thermoelectric Materials

Isomers of n-Type Poly(thiophene-alt-co-thiazole) for Organic Thermoelectrics

n-Type polythiophene represents a promising category of n-type polymer thermoelectric materials known for their straightforward structure and scalable synthesis. However, n-type polythiophene often suffers from a twisted backbone and poor stacking property when introducing high-density electron-withdrawing groups for a lower lowest unoccupied molecular orbital (LUMO) level, which is considered to be beneficial for n-doping efficiency. Herein, we developed two isomers of polythiophene derivatives, PTTz1 and PTTz2, by inserting thiazole units into the polythiophene backbone composed of thieno[3,4-c]pyrrole-4,6-dione (TPD) and thiophene-3,4-dicarbonitrile (2CNT). Although PTTz1 and PTTz2 share a similar polymer skeleton, they differ in thiazole configuration, with the nitrogen atoms of the thiazole units oriented toward TPD and 2CNT, respectively. The insertion of thiazole units significantly planarizes the polythiophene backbone while largely preserving low LUMO levels. Notably, PTTz2 exhibits a more coplanar backbone and closer π-stacking compared to PTTz1, resulting in a greatly enhanced electron mobility. Both PTTz1 and PTTz2 can be easily n-doped due to their deep LUMO levels. PTTz2 demonstrates superior thermoelectric performance, with an electrical conductivity of 50.3 S cm-1 and a power factor of 23.8 μW m-1 K-2, which is approximately double that of PTTz1. This study highlights the impact of the thiazole unit on n-type polythiophene derivatives and provides valuable guidelines for the design of high-performance n-type polymer thermoelectric materials.

Terminal Cyano-Functionalized Fused Bithiophene Imide Dimer-Based n-Type Small Molecular Semiconductors: Synthesis, Structure-Property Correlations, and Thermoelectric Performances

n-Doped small molecular organic thermoelectric materials (OTMs) hold advantages of high Seebeck coefficient and better performance reproducibility over their polymeric analogues; however, high-performance n-type small molecular OTMs are severely lacking. We report here a class of small molecular OTMs based on terminal cyanation of a bithiophene imide-based ladder-type heteroarene BTI2. It was found that the cyanation could effectively lower the lowest unoccupied molecular orbital (LUMO) level from -2.90 eV (BTI2) to -4.14 eV (BTI2-4CN) and thus lead to significantly improved n-doping efficiency. Additionally, terminal cyano-functionalization can maintain the close packing and efficient intermolecular charge transfer between these cyanated molecules, thus yielding high electron mobilities of up to 0.40 cm2 V-1 s-1. Benefiting from its low LUMO-enabled efficient n-doping and high electron mobility, an encouraging n-type electrical conductivity of 0.43 S cm-1 and power factor (PF) of 6.34 μW m-1 K-2 were achieved for tetracyanated BTI2-4CN, significantly outperforming those of its noncynated BTI2 (<10-7 S cm-1, PF undetectable) and dicyanated BTI2-2CN (0.24 S cm-1, 1.78 μW m-1 K-2). These results suggest the great potential of the terminal cyanation strategy of ladder-type heteroarenes for developing high-performance small molecular OTMs.

Enhancing the Performance of N-Type Thermoelectric Devices via Tuning the Crystallinity of Small Molecule Semiconductors

In the development of high-performance organic thermoelectric devices, n-type materials, especially with small molecule semiconductors, lags far behind p-type materials. In this paper, three small molecules are synthesized based on electron-deficient naphthalene bis-isatin building blocks bearing different alkyl chains with the terminal functionalized with 3-ethylrhodanine unit and studied their aggregation and doping mechanism in detail. It is found that crystallinity plays an essential role in tuning the doping behavior of small molecules. Molecules with too strong crystallinity tend to aggregate with each other to form large crystalline domains, which cause significant performance degradation. While molecules with weak crystallinity can tolerate more dopants, most of them exhibit low mobility. By tuning the crystallinity carefully, organic thermoelectric devices based on C12NR can maintain high mobility and realize effective doping simultaneously, and a high power factor of 1.07 µW m^-1 K^-2 at 100 °C is realized. This delicate molecular design by modulating crystallinity provides a new avenue for realizing high-performance organic thermoelectric devices.

Recent Advances and Prospects of Small Molecular Organic Thermoelectric Materials

Thermoelectric (TE) materials possess unique energy conversion capabilities between heat and electrical energy. Small organic semiconductors have aroused widespread attention for the fabrication of TE devices due to their advantages of low toxicity, large area, light weight, and easy fabrication. However, the low TE properties hinder their large-scale commercial application. Herein, the basic knowledge about TE materials, including parameters affecting the TE performance and the remaining challenges of the organic thermoelectric (OTE) materials, are initially summarized in detail. Second, the optimization strategies of power factor, including the selection and design of dopants and structural modification of the dope-host are introduced. Third, some achievements of p- and n-type small molecular OTE materials are highlighted to briefly provide their future developing trend; finally, insights on the future development of OTE materials are also provided in this study.

Recent Developments of n-Type Organic Thermoelectric Materials: Influence of Structure Modification on Molecule Arrangement and Solution Processing

Organic semiconductor (OSCs) thermoelectric materials have been studied widely due to their low thermal conductivity and solution processing characteristics. Currently, the high conductivity (up to 1000 s cm^-1 ) has boosted the performance of p-type organic thermoelectric materials substantially. In contrast, the development of n-type organic thermoelectric materials is still limited by their low mobility, inferior air stability, and poor doping efficiency, which is relevant to the molecule structure and dopant dispersion. Herein, the recent development of n-type organic thermoelectric materials was reviewed with an emphasis on molecule structure modification and solution processing. Methods for optimizing conjugate structure were summarized from the effects of conjugated backbone modification and side chains diversification on molecular stacking. The primary n-type dopants were also summarized briefly. Especially, the role of solution aggregation controlling on film preparation and properties was given special attention. Additionally, the emergence of organic diradicals with low lowest unoccupied molecular orbital energy level and no doping was introduced, which shows great potential in n-type organic thermoelectric materials. All these endeavors have led to the development of n-type OSCs materials. This Review is aimed at illustrating the state-of-the-art progress and providing some guideline for the design of organic thermoelectric materials in the future.