Ultrahigh thermoelectric power factor in flexible hybrid inorganic-organic superlattice

Hybrid inorganic–organic superlattice with an electron-transmitting but phonon-blocking structure has emerged as a promising flexible thin film thermoelectric material. However, the substantial challenge in optimizing carrier concentration without disrupting the superlattice structure prevents further improvement of the thermoelectric performance. Here we demonstrate a strategy for carrier optimization in a hybrid inorganic–organic superlattice of TiS2[tetrabutylammonium]x[hexylammonium]y, where the organic layers are composed of a random mixture of tetrabutylammonium and hexylammonium molecules. By vacuum heating the hybrid materials at an intermediate temperature, the hexylammonium molecules with a lower boiling point are selectively de-intercalated, which reduces the electron density due to the requirement of electroneutrality. The tetrabutylammonium molecules with a higher boiling point remain to support and stabilize the superlattice structure. The carrier concentration can thus be effectively reduced, resulting in a remarkably high power factor of 904 µW m−1 K−2 at 300 K for flexible thermoelectrics, approaching the values achieved in conventional inorganic semiconductors.

and TiS 2 /organic superlattice by chemical vapor transport (CVT) method and electrochemcial intercalation method. TiS2 has a CdI2 structure with space group P-3m1. It is a quasi-2-dimensional structure, where the layers are connected by the van der Waals gap. The TiS2 single crystal was obtained by a standard CVT method.
Titanium and sulfur powder mixture was sealed in evacuated silica tube. The silica tube was horizontally placed with the powders put at one end. Then the tube was put into a three-zone furnace, in which a temperature gradient across the tube was established.
The optimized temperature for the hot end and cold end are 732 o C and 632 o C, respectively. Additional sulfur was added as the agent which carries the vapor of Ti and Sulfur form the hot end to the cold end, where the single crystal of TiS2 was obtained.
To fabricate the TiS2/organic layered material, an electrochemical intercalation process was used, which is very similar to the electrochemical reaction of the lithium 3 ion battery electrodes. When the voltage is applied, the TiS2 layers were negatively charged and the positive organic cations were intercalated into the van der Waals gaps driven by the electrostatic force, forming a layer by layer structure. The typical voltage is -1.5 V, and the electrolyte concentration is 0.5 M and the electrochemical reaction time is 15 minutes. However, in this paper, the ratios from 1:1 to 1:7 were tried with the obtained Seebeck coefficient varying from -115 to -142 µV/K (The 1:10 composition is excluded here, as the content of TBA is too few to support the stage-1 structure after evaporation of the HA molecules, which is confirmed in Fig. S7).  was first cut into many small pieces and then stacked and bonded using epoxy resin. It was then put into melted wax and solidified. Both sides of the sample were polished using a sand paper. The wax was removed by heating and then washing in ethyl ether.
(b) Thermal diffusivity of the prepared sample was measured by a standard laser flash analysis equipment (ULVAC, TC-9000).
Supplementary Figure 11. The sample (4mm×4mm×150µm) was firmly attached onto the surface of glass tubes with different radii. The electrical resistance was measured using the van der Pauw method as a function of the bending radii, namely, the radii of the glass tube. 13

Supplementary Note 1. Dependence of the thermoelectric properties on the carrier concentration of the TiS 2 -based inorganic-organic superlattice
For pure n-type TiS2 single crystals, the carrier concentration is ~ 2.8*10 20 cm -3 . [2] During the electrochemical process, the TiS2 layers would be electrochemically reduced and the negative electron charge was balanced by the intercalated organic cations (e.g.
where, n is the carrier concentration and m* is the effective mass of the carrier.
Assuming that the intercalation does not affect the effect mass of the carrier (estimated to be ~ 4.8 me [3] ), the Seebeck coefficient (absolute value) was reduced monotonously from 251 μV/K to 54 μV/K, after intercalation, as shown in Fig. S1(a).
The positive ions (e.g. hexylammonium) could potentially scatter the electrons inside the TiS2 layers due to the Coulomb interaction. The drift mobility μ for acoustic phonon scattering and arbitrary electron degeneracy can be expressed by equation (2)  Ξ is deformation potential, m* is effective mass of carriers, the above parameters are assumed to be unchanged after intercalation herein. The drift mobility μ for acoustic phonon scattering and arbitrary electron degeneracy can be expressed by equation (4) in ref. 4, where the change of sample density is neglected. The μ only depends on the reduced Fermi level η, which can be estimated by equation 3 and 4. The carrier concentration dependent η and μ is shown in Fig. S1(b).
The electrical conductivity of the hybrid superlattices can be then calculated using σ = nµq. Due to the increase of the carrier concentration, the electrical conductivity was increased monotonously from 58*10 4 Sm -1 to 345*10 4 Sm -1 , as shown in Fig. S1. The power factor thus decreases from 37 to 10 mW cm -1 K -2 . However, it remains high when the carrier concentration does not change too much.