Thermoelectric properties of p-type lead telluride doped with silver or potassium

Single crystals of p-type PbTe were prepared by the Bridgman method, where either Ag2Te or K was doped as the source material of silver or potassium, respectively. Carrier concentration and Hall mobility were measured from 77 to 300 K. The hole concentration was successfully controlled in the range from 2.0 x 10(24) to 9.0 x 10(24) m(-3) by doping 100 to 700 mol ppm Ag2Te, while the conduction type changed from p- to n-type in the region more than 1000 mol ppm Ag2Te. In the case of K-doping, the carrier concentration was controlled in the region of 8.0 x 10(24) to 4.0 x 10(25) m(-3) with 1000 to 25000 mol ppm it. Figure of merits (Z) of p-PbTe at 300 K were 8.0 x 10(-4) K-1 at a hole concentration of 1.34 x 10(25) m(-3) (2400 mol ppm K) and 1.38 x 10(-3) K-1 at 2.34 x 10(24) m(-3) (200 mol ppm Ag2Te). Diffusion lengths of dopant elements of Ag, K and Sn for p-type and I for n-type PbTe were estimated using the literature data. The result indicates that long distance diffusion occurs in a short period in the case of Ag and K. Therefore, the stable functionally graded materials (FGM) of PbTe might be prepared by gradient I doping or Sn alloying, while those by Ag or K doping will be soon changed into non-FGM due to the rapid diffusion resulting in the decreased efficiency of thermoelectric energy conversion.