High thermoelectric power factor from multilayer solution-processed organic films

Abstract

We investigate the suitability of the “sequential doping” method of organic semiconductors for thermoelectric applications. The method consists of depositing a dopant (F4TCNQ) containing solution on a previously cast semiconductor (P3HT) thin film to achieve high conductivity, while preserving the morphology. For very thin films (∼25 nm), we achieve a high power factor around 8 μW/mK−2 with a conductivity over 500 S/m. For the increasing film thickness, conductivity and power factor show a decreasing trend, which we attribute to the inability to dope the deeper parts of the film. Since thick films are required to extract significant power from thermoelectric generators, we developed a simple additive technique that allows the deposition of an arbitrary number of layers without significant loss in conductivity or power factor that, for 5 subsequent layers, remain at ∼300 S/m and ∼5 μW/mK−2, respectively, whereas the power output increases almost one order of magnitude as compared to a single layer. The efficient doping in multilayers is further confirmed by an increased intensity of (bi)polaronic features in the UV-Vis spectra.

Conjugated Polymer Blends for Organic Thermoelectrics

“…[56,64] First, the p-type model system P3HT (gray square) will not likely reach relevant ZT values, as it, at ZT ≈ 0.01, already appears to sit close to the maximally obtainable doping concentration. [61] Second, comparing pure PTB7 (gray triangle) with the P3HT:PTB7 blend at maximum in S (gray with green edge) shows that the addition of the (P3HT) trap indeed seems to have increased ZT; a similar move starting from the grey circle would have landed near ZT = 1. Third, although the pure P3HT (gray square) has a higher PF than the blend with PTB7 at maximum in S (gray with green edge), moving along the corresponding orange line for the latter system (improving doping efficiency) would lead to higher ZT, possibly beyond ZT = 0.1.…”

“…[13,17,58] Sequential doping is done by spin-coating a dopant-containing solution over the pristine active layer. [61,62] Revisiting the concept of DOS engineering, Zuo et al used the archetypical P3HT as material A (Figure 4) and mixed it with selected conjugated polymers PTB7 and TQ1 (materials B), having different HOMO energy levels. [56] Sequential (p-type)…”

“…Recent reports indicated that morphology also has a strong influence on the thermoelectric properties of single OTE materials. [13,17,61,62,73,74] For instance, Patel et al showed that the longrange order of vapor-doped PBTTT strongly enhanced the power factor, reaching up to 120 µWm -1 K -2 . [13] Hamidi-Sakr et al showed that the morphology impacts anisotropic thermoelectric properties and charge transport in a series of doped single polythiophene derivatives with different alkyl side chains.…”

Doping High‐Mobility Donor–Acceptor Copolymer Semiconductors with an Organic Salt for High‐Performance Thermoelectric Materials

“…For example, Kemerink et al have shown sequential “surface doping” by spin–coating organic semiconductor (P3HT) solution and dopant (F 4 TCNQ) solution in sequence. A conductivity of 4 S cm −1 of and PF around 7 µW K −2 m −1 were achieved for very thin films (25 nm) 21. Chabinyc et al reported vapor‐doping PBTTT with F 4 TCNQ, and obtained a σ of 670 S cm −1 and a S of 42 µV K −1 , which lead to a large PF of 120 µW K −2 m −1 22.…”

“…A conductivity of 4 S cm −1 of and PF around 7 µW K −2 m −1 were achieved for very thin films (25 nm). [21] Chabinyc et al reported vapor-doping PBTTT with F 4 TCNQ, and obtained a σ of 670 S cm −1 and a S of 42 µV K −1 , which lead to a large PF of 120 µW K −2 m −1 . [22] Inspired by the fact that the performance of n-type OTEs is much lagging behind the p-type counterparts, in recent years researchers have paid intensive attention to investigating OTEs based on n-type semiconductors.…”