Electronic grade and flexible semiconductor film employing oriented attachment of colloidal ligand-free PbS and PbSe nanocrystals at room temperature

Abstract

Electronic grade semiconductor films have been obtained via the sintering of solution processed PbS and PbSe nanocrystals at room temperature. Prior attempts to achieve similar films required the sintering of nanocrystals at higher temperatures (>350 °C), which inhibits the processing of such films on a flexible polymer substrate, and it is also expensive. We reduced the sintering temperature by employing two important strategies: (i) use of ligand-free nanocrystals and (ii) oriented attachment of nanocrystals. Colloidal ligand-free PbS and PbSe nanocrystals were synthesized at 70 °C with high yield (∼70%). However, these nanocrystals start to agglomerate with time in formamide, and upon the removal of the solvation energy, nanocrystals undergo oriented attachment, forming larger elongated crystals. PbS and PbSe nanocrystal films made on both glass and flexible substrates at room temperature exhibit Ohmic behavior with optimum DC conductivities of 0.03 S m−1 and 0.08 S m−1, respectively. Mild annealing of the films at 150 °C increases the conductivity values to 1.1 S m−1 and 137 S m−1 for PbS and PbSe nanocrystal films, respectively. AC impedance was measured to distinguish the contributions from grain and grain boundaries to the charge transport mechanism. Charge transport properties remain similar after the repeated bending of the film on a flexible polymer substrate. Reasonably high thermoelectric Seebeck coefficients of 600 μV K−1 and 335 μV K−1 for PbS and PbSe nanocrystal pellets, respectively, were obtained at room temperature.