Atomic/molecular layer deposition of p-type conducting copper–sulfur–organic coordination polymer thin films for thermoelectric applications

Here we report a three-precursor atomic/molecular layer deposition (ALD/MLD) process for the fabrication of copper-based coordination polymer thin films for thermoelectric applications. This process mimics the wet chemical synthesis of poly[metal-ethenetetrathiolate] (poly[M-ETT]) polymers based on a trans-metalation reaction. In our ALD/MLD process the Cu-for-Li trans-metalation is realized upon the pulsing of the three precursors, 1,3,4,6-tetrathiapentalene-2,5-dione (TPD), lithium hexamethyldisilazide (Li-HMDS) and copper(ii) acetylacetonate (Cu(acac)₂), in a cyclic manner. The process yields p-type electrically conducting poly[Cu-ETT] thin films with an appreciably high growth-per-cycle (GPC) of ∼11 Å per cycle at a deposition temperature of 220 °C. The targeted chemical composition was confirmed with XPS measurements, which verified the Cu : S ratio at 0.246 (i.e. very close to the ideal 0.25 value). From electrical transport measurements, the room-temperature resistivity and Seebeck coefficient values were determined to be 0.17 Ω m^-1 and 88 µV K^-1, respectively.