N‐Type Organic Thermoelectrics: Improved Power Factor by Tailoring Host–Dopant Miscibility

Abstract

In this contribution, for the first time, the polarity of fullerene derivatives is tailored to enhance the miscibility between the host and dopant molecules. A fullerene derivative with a hydrophilic triethylene glycol type side chain (PTEG-1) is used as the host and (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine n-DMBI) as the dopant. Thereby, the doping efficiency can be greatly improved to around 18% (<1% for a nonpolar reference sample) with optimized electrical conductivity of 2.05 S cm , which represents the best result for solution-processed fullerene derivatives. An in-depth microstructural study indicates that the PTEG-1 molecules readily form layered structures parallel to the substrate after solution processing. The fullerene cage plane is alternated by the triethylene glycol side chain plane; the n-DMBI dopants are mainly incorporated in the side chain plane without disturbing the π-π packing of PTEG-1. This new microstructure, which is rarely observed for codeposited thin films from solution, formed by PTEG-1 and n-DMBI molecules explains the increased miscibility of the host/dopant system at a nanoscale level and the high electrical conductivity. Finally, a power factor of 16.7 µW m K is achieved at 40% dopant concentration. This work introduces a new strategy for improving the conductivity of solution-processed n-type organic thermoelectrics.

N‐Type Organic Thermoelectrics of Donor–Acceptor Copolymers: Improved Power Factor by Molecular Tailoring of the Density of States

“…For the practical TE applications, both efficient p‐type and n‐type TE materials are required; however, the development of the latter lags much behind that of the former . For this reason, growing research efforts are being directed towards developing better n‐type TE materials . So far, advanced conjugated‐backbone designs targeting favorable energetics, high mobility, planar structure, and good host/dopant miscibility have been reported .…”

High Seebeck Coefficient and Power Factor in n‐Type Organic Thermoelectrics

“…[14] Liu et al demonstrated a modified fullerene derivative, PTEG-1, that shows an increased miscibility at the nanoscale level and thereby achieved a PF of 16.7 µW m −1 K −2 with σ ≈ 205 S m −1 at 40 mol% of N-DMBI. [15] For p-type OTE, most reported experimental data seem to follow an empirical quasi-universal power law relationship between the Seebeck coefficient S and conductivity as S ∝ σ − 1/4 . [2,3] In view of the limited number of reported data, it is still unclear whether n-type OTE also follow this power law, and different power law slopes have been reported.…”

“…Gray and yellow dots indicate p-and n-type OSC, respectively; light yellow data are taken from refs. [11,12,15,28] and dark yellow from this work; for the latter, dots w/ and w/o red perimeter are for inverse-sequential doping and bulk doping, respectively; All p-type data come from our group. [23] Lines are model calculations with parameters: intersite distance a NN = 1.8 nm; Gaussian disorder σ DOS = 0.08 eV; temperature T = 300 K; inverse localization length α = 5 × 10 8 nm −1 .…”

“…[2,3] In view of the limited number of reported data, it is still unclear whether n-type OTE also follow this power law, and different power law slopes have been reported. [15,16] Here, we introduce a novel, inverse-sequential doping procedure that mitigates the need for solvent orthogonality in conventional sequential doping to investigate the potential of The n-type thermoelectric properties of [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) are investigated for different solution-based doping methods. A novel inverse-sequential doping method where the semiconductor (PCBM) is deposited on a previously cast dopant 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-diphenylaniline film to achieve a very high power factor PF ≈ 35 µW m −1 K −2 with a conductivity σ ≈ 40 S m −1 is introduced.…”

“…[2,3] In view of the limited number of reported data, it is still unclear whether n-type OTE also follow this power law, and different power law slopes have been reported. [15,16] Here, we introduce a novel, inverse-sequential doping procedure that mitigates the need for solvent orthogonality in conventional sequential doping to investigate the potential of deposited on a previously cast dopant 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-diphenylaniline film to achieve a very high power factor PF ≈ 35 µW m −1 K −2 with a conductivity σ ≈ 40 S m −1 is introduced. It is also shown that n-type organic semiconductors obey the −1/4 power law relation between Seebeck coefficient S and σ that are previously found for p-type materials.…”