Leveraging Sequential Doping of Semiconducting Polymers to Enable Functionally Graded Materials for Organic Thermoelectrics

Abstract

With the ability to modulate electronic properties through molecular doping coupled with ease in processability, semiconducting polymers are at the forefront in enabling organic thermoelectric devices for thermal energy management. In contrast to uniform thermoelectric material properties, an alternative route focuses on functionally graded materials (FGMs) where one spatially controls and optimizes transport properties across the length of a thermoelectric material. While primarily studied in the context of inorganic materials, the concept of FGMs for organic thermoelectrics has not been explored. Herein, we introduce how molecular doping of semiconducting polymers enables spatial compositional control of thin-film FGMs. Specifically, we use sequential vapor doping of poly­[2,5-bis­(3-tetradecyl­thiophen-2-yl)­thieno­[3,2-b]­thiophene] (PBTTT) with the small molecule acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano­quinodimethane (F4TCNQ) to fabricate the simplest form of FGMsdouble-segmented thin films. The two thin-film segments are of equal length (7.5 mm) but each set to different doping levels. Our study focuses on understanding the thermoelectric properties (Seebeck coefficient, α, and electronic conductivity, σ) and structural properties (through X-ray scattering, UV–vis–NIR spectroscopy, and Raman spectroscopy) within and across the two segments. We observe the presence of a small diffuse interfacial region of 0.5–1 mm between the two segments where the doping level and transport properties vary continuously. Despite the diffuse interface, the measured α across the two segments is simply the average of α within each segment. Importantly, this experimental result is consistent with reported mathematical models describing the spatial average of α in graded thermoelectric materials. Our results demonstrate the facile fabrication and characterization of functionally graded organic thermoelectric materials, providing guidelines for further development on more complex FGMs.

Electron transport in a sequentially doped naphthalene diimide polymer

“…3,4 Sequential doping avoids possible complications that arise if the doped material is poorly soluble in the casting solvent and in some cases may allow for the preservation of some of the structural order present in the pristine semiconductor film. Although widely applied to the p-doping of P3HT [5][6][7][8] and PBTTT, 9,10 there are only few reports of the sequential n-doping of solution-processed polymers. 11 The air-stability of oxidants, hole-transport materials, and their doped combinations often allow for easy handling in air.…”

Correlating conductivity and Seebeck coefficient to doping within crystalline and amorphous domains in poly(3‐(methoxyethoxyethoxy)thiophene)

“…We calculate side‐chain distance d 100 = 2π/q 100 as a function of FnTCNQ doping time, the result of which is shown in Figure 4(A). Upon introducing FnTCNQ we observe an increase in d 100 for all three dopants due to the intercalation of dopant into the side‐chain spacing, a widely observed phenomenon in polythiophene‐based polymers 17,22,43,49,50 . Specifically, d 100 increases initially from 1.79 nm for neat P3MEET to 1.94 nm, 2.01 nm and 2.06 nm for F1TCNQ, F2TCNQ and F4TCNQ‐doped thin films, respectively.…”

“…Figure 1(B) shows a schematic of our home‐built apparatus for controlled vapor doping of polymer thin films in an argon atmosphere glovebox. More details are provided in the methods section and our previous publication 43 …”

“…Seebeck coefficient was measured with two 1 mm 2 gold pads, which are 3 mm apart. A detailed schematic is provided elsewhere 43 . Four probe conductivity measurements were performed using a custom‐designed probe station in an Argon glovebox.…”

Interfacial Molecular Doping at Donor and Acceptor Interface in Bilayer Organic Solar Cells

“…To avoid the aforementioned drawbacks, different research groups recently exposed undoped conjugated polymer films to dopants to achieve molecular doping, including gasphase or liquid-phase approaches, which we refer to as sequential interfacial doping. [2,[11][12][13] The technique for interfacial doping in solution is to spin coat the dopant directly from the solution onto the precast polymer film.…”