Screen printing allows for direct conversion of thermoelectric nanocrystals into flexible energy harvesters and coolers. However, obtaining flexible thermoelectric materials with high figure of merit ZT through printing is an exacting challenge due to the difficulties to synthesize high-performance thermoelectric inks and the poor density and electrical conductivity of the printed films. Here, we demonstrate high-performance flexible films and devices by screen printing bismuth telluride based nanocrystal inks synthesized using a microwave-stimulated wet-chemical method. Thermoelectric films of several tens of microns thickness were screen printed onto a flexible polyimide substrate followed by cold compaction and sintering. The n-type and p-type films demonstrate peak ZTs of 0.43 and 0.8 along with superior flexibility, which is among the highest reported ZT values in flexible thermoelectric materials. The printed nanostructured thermoelectric films present ultralow thermal conductivities due to the nanoscale grains and pores, which significantly scatter the phonon transport and reduce phonon mean free path.
A flexible thermoelectric device fabricated using the printed films produces a high power density of 4.1 mW/cm2 with 60 ºC temperature difference between the hot side and cold side. The highly scalable and low cost process to fabricate flexible thermoelectric materials and devices demonstrated here opens up many opportunities to transform thermoelectric energy harvesting and cooling applications.