State-of-the-Art Reviews and Analyses of Emerging Research Findings and Achievements of Thermoelectric Materials over the Past Years

This review is focused on state-of-the-art thermoelectric materials (or thermoelements), from which the thermoelements with the highest figures of merit (z) along with the those having the greatest research interest and findings were surveyed and analyzed. These were in addition to the statistical analyses made in this review for categorizing z achievement ranges for all types of thermoelements. Almost 56% of positive thermoelements and 39.6% of negative thermoelements were discovered from 1950 to 2017, and a total of 62.2% of thermoelement research findings were reported in 2010–2017. Furthermore, nearly 47.65% of the discovered thermoelements preserved z in the range of 1–4.99 × 10 K, and only about 2.52% possessed less than 9.9 × 10 K. Chalcogenide was the major type of thermoelement studied to date, with overall representation of 37.2%. Nearly 68.9% of chalcogenide thermoelements were capable of reach 1–4.99 × 10 K, while 53% of metal oxide thermoelements ranged within 0.1–0.499 × 10 K. Nanostructure thermoelements achieved the highest z of 47 × 10 K and 17 × 10 K at 300 K, for BiTe quantum wires and BiTe quantum wells, respectively. Correspondingly, hybrid and conducting polymer thermoelements also reached z as high as 16 × 10 K at 300 K for positive thermoelement: nano-Ag/regioregular poly(3-octylthiophene-2,5-diyl) and negative thermoelement: graphdiyne.