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

Abstract

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.

Enhanced n‐Type Thermoelectric Performance of Conjugated Polymers Based on an Indandione‐Terminated Quinoidal Unit through Comonomer Optimization

“…The same trend in the evolution of HOMO and LUMO energy levels was observed in DFT calculation (Figure S12). Since low‐lying LUMO energy levels are beneficial for electron transfer between the host polymer and the n‐type dopant, [ 1g,3a,5d ] we believe that the modification of the comonomer with electron‐withdrawing groups can affect the doping efficiency, and thus tune the thermoelectric performance of the polymers.…”

“…[ 17 ] This feature is appealing for thermoelectric applications because low‐crystallinity polymer thin films are more easily doped and the multiple packing orientations can provide various transition regions to accommodate dopants or dopant cations, thus enabling good miscibility between the polymer matrix and dopant. [ 3e,4c,18 ] The three polymers possessed similar lamellar packing distances because of their identical alkyl chains. The lamellar packing distances calculated from the out‐of‐plane (100) diffraction peaks are ca .…”

“…After incorporating N‐DMBI, the XRD patterns of the three polymers kept nearly unchanged, indicating that the dopants were mainly dispersed in the amorphous region and did not disturb the polymer packing. [ 3d‐e ]…”

“…Since the thermoelectric apparatus require both p‐type and n‐type thermoelectric elements, the construction of high‐performance n‐type thermoelectric materials is therefore of utmost importance. [ 3 ]…”

Organic thermoelectric generators: working principles, materials, and fabrication techniques

“…However, n-type thermoelectric polymers are lagging behind their p-type counterparts in terms of thermoelectric performance and air stability, therefore there is a growing research interest on this materials. [84][85][86][87] Low electrical conductivity, and zT values are mainly due to their challenging doping mechanism owing to their high electron affinities (À3 to À4 eV). Additionally, their tendency to ambient degradation (oxygen and water) and, in some cases, insolubility challenges require certain attention.…”