Thermoelectric Properties of Poly(3-hexylthiophene) (P3HT) Doped with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) by Vapor-Phase Infiltration

Abstract

Doping of thin films of semiconducting polymers provides control of their electrical conductivity and thermopower. The electrical conductivity of semiconducting polymers rises nonlinearly with the carrier concentration, and there is a lack of understanding of the detailed factors that lead to this behavior. We report a study of the morphological effects of doping on the electrical conductivity of poly­(3-hexylthiophene) (P3HT) thin films doped with small molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Resonant soft X-ray scattering shows that the morphology of films of P3HT is not strongly changed by infiltration of F4TCNQ from the vapor phase. We show that the local ordering of P3HT, the texture and form factor of crystallites, and the long-range connectivity of crystalline domains contribute to the electrical conductivity in thin films. The thermopower of films of P3HT doped with F4TCNQ from the vapor phase is not strongly enhanced relative to films doped from solution, but the electrical conductivity is significantly higher, improving the thermoelectric power factor.

X‐Ray Scattering Reveals Ion‐Induced Microstructural Changes During Electrochemical Gating of Poly(3‐Hexylthiophene)

“…As the magnitude of the bias increases, the alkyl stacking distance increases sharply to 18.5 Å (Figure c) along with a decrease in π–π stacking distance to 3.62 Å (Figure d). The structural changes observed suggest that the TFSI − counterions reside in the alkyl chains of the crystallites, consistent with other dopant molecules used for P3HT . As expected, the positions of the scattering peaks of the PIL (P1, P2, and P3) are not influenced by application of a gate bias.…”

The Role of Electrostatic Interaction between Free Charge Carriers and Counterions in Thermoelectric Power Factor of Conducting Polymers: From Crystalline to Polycrystalline Domains

“…We also found that most of the experimental results for P3HT are within the range of ≈10 −1 –10 2 µW m −1 K −2 , located within our calculated orders of magnitude. It is worth noting that very few experimental work can directly measure the free carrier concentration of a polymer sample, and several reports showed that their measured carrier concentrations are about 10 21 cm −3 , [ 24,31 ] higher than our predicted optimal doping level (1 × 10 20 –5 × 10 20 cm −3 ), which obviously are over‐doped. Therefore, direct comparison of the absolute value of power factor in the case of unknown carrier concentration is pointless; however, the trend and range of values are meaningful and crucial.…”

“…For example, the mobility of P3HT:F4TCNQ films increases from around 0.2–0.9 cm 2 V −1 s −1 when the carrier concentration increases from 2 × 10 19 to 5 × 10 20 cm −3 . [ 24 ] Such phenomenon can be attributed to the easy trapping of the charge carriers by counterions at low concentrations due to the strong electrostatic attractive interactions. However, a portion of carriers can fill the traps at higher concentrations, and thereby the remaining carriers undergo the trap‐free transport.…”

Highly Passivated n‐Type Colloidal Quantum Dots for Solution‐Processed Thermoelectric Generators with Large Output Voltage

“…[21,22] To date, CQD thermoelectrics have been reported with advanced performance approaching that of conventional bulk Bi 2 Te 3 -based systems. While p-type films [17,[28][29][30][31] have seen rapid advancement, the thermoelectric performance of n-type counterparts still lags behind. [16,19,20] Film-based CQD structures offer the potential for micropower applications such as self-powered devices in sensor networks and the internet of things.…”