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. The 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 8.0 x 10(24) to 4.0 x 10(25) m(-3) with 1000 to 20000 mol ppm K. The figures-of-merits Z of PbTe at 300 K were 8.0 x 10(-4) K-1 for p-type at a hole concentration of 1.0 x 10(25) m(-3) (2000 mol ppm K) and 1.38 x 10(-3) K-1 at 2.34 x 10(24) m(-3) (200 mol ppm Ag2Te). The diffusion lengths of dopant elements of Ag and K for p-type and I for n-type PbTe were estimated using the diffusion data. The result indicates that long-distance diffusion occurs in a short period in the case of Ag and K, but not so much in the case of I. Therefore, the PbTe carrier concentration functionally graded material (FGM) prepared by doping Ag or K might be soon changed into non-FGM due to the dopant diffusion, resulting in the decreased efficiency of thermoelectric energy conversion.