Doping Dependent In‐Plane and Cross‐Plane Thermoelectric Performance of Thin n‐Type Polymer P(NDI2OD‐T2) Films

Abstract

Thermoelectric generators pose a promising approach in renewable energies as they can convert waste heat into electricity. In order to build high efficiency devices, suitable thermoelectric materials, both n-and p-type, are needed. Here, the n-type high-mobility polymer poly[N,N′-bis(2-octyldodecyl) naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) (P(NDI2OD-T2)) is focused upon. Via solution doping with 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-diphenylaniline (N-DPBI), a maximum power factor of (1.84 ± 0.13) µW K −2 m −1 is achieved in an in-plane geometry for 5 wt% dopant concentration. Additionally, UV-vis spectroscopy and grazingincidence wide-angle X-ray scattering are applied to elucidate the mechanisms of the doping process and to explain the discrepancy in thermoelectric performance depending on the charge carriers being either transported in-plane or cross-plane. Morphological changes are found such that the crystallites, built-up by extended polymer chains interacting via lamellar and π-π stacking, re-arrange from face-to edge-on orientation upon doping. At high doping concentrations, dopant molecules disturb the crystallinity of the polymer, hindering charge transport and leading to a decreased power factor at high dopant concentrations. These observations explain why an intermediate doping concentration of N-DPBI leads to an optimized thermoelectric performance of P(NDI2OD-T2) in an in-plane geometry as compared to the cross-plane case.

Dichlorinated Dithienylethene‐Based Copolymers for Air‐Stable n‐Type Conductivity and Thermoelectricity

“…[15][16][17][18] Peter Müller-Buschbaum et al achieved high power factor of (1.84 ± 0.13) µW m −1 K −2 using common n-type polymer of P(NDI2OD-T2) doped with N-DPBI. [19] n-Type solution-processible conjugated polymers FBDPPV present high σ of 14 S cm −1 and PF of 28 µW m −1 K −2 . [15] 1.…”

“…achieved high power factor of (1.84 ± 0.13) µW m −1 K −2 using common n‐type polymer of P(NDI2OD‐T2) doped with N‐DPBI. [ 19 ]…”

A Solution‐Processable Polymer‐Based Thin‐Film Thermoelectric Generator

“…Motivated by the design of inorganic TEGs and an organic device consisting of thousands of junctions manufactured by roll‐to‐roll production, we demonstrate a new design approach in so‐called pseudo‐in‐plane geometry. In this regard, we first report that in this geometry, N‐DPBI‐doped P(NDI2OD‐T2) thin films can reach electrical conductivities of up to (0.33 ± 0.05) S cm −1 , which is not only higher than in in‐plane geometry reported previously [ 2,32,35,41 ] but also competitive with other organic n‐type materials. [ 3,4 ] The corresponding power factor amounts to (2.4 ± 0.27) μW K −2 m −1 .…”

“…[ 2,3,35,36 ] The conductivity increase at lower dopant concentrations is ascribed to the dopant filling trap states, whereas at high dopant concentrations the crystallinity of the polymer was reported to be disturbed most likely by dopant agglomerates. [ 32,35,36 ] As shown in Figure 1b, thermovoltages of the order of 100 μV K −1 are obtained at a mean temperature of 70 °C. The highest Seebeck coefficient is obtained for the undoped sample, which is most likely due to its low electrical conductivity as the inverse trends of conductivity and thermovoltage with respect to doping concentration were observed for different polymers.…”

“…We choose the air‐stable, high‐mobility poly[[ N , N ′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐ alt ‐5,5′‐(2,2′‐bithiophene)] (P(NDI2OD‐T2)) [ 21 ] doped with 4‐(1,3‐dimethyl‐2,3‐dihydro‐1 H ‐benzoimidazol‐2‐yl)‐ N , N ‐diphenylaniline (N‐DPBI) to enhance its power factor [ 2 ] as an n‐type component of our TEG. [ 32 ] As p‐type leg, the well‐studied PEDOT:PSS is implemented and posttreated with ethylene glycol (EG) as this is known to enhance its electrical conductivity. [ 33,34 ] Normalized to a temperature difference of 1 K, one thermocouple, and an active area of 1 cm 2 , the TEG possesses a power output of 2.4 × 10 −13 W cm −2 K −1 , which is competitive with the results of former studies (see Table S1, Supporting Information).…”