1. (a1, b1) XRD patterns of samples after storage in hygrothermal environment for different time with (a2, b2) EDS results of material surface after storage in hygrothermal environment for 600 h

2. Surface XPS spectra of n-type Bi₂Se_0.21Te_2.79 material (a1-b1) before and (a2-b2) after storage in hygrothermal environment for 600 h

3. Surface XPS spectra of p-type Bi_0.4Sb_1.6Te₃ material (a1-c1) before and (a2-c2) after storage in hygrothermal environment for 600 h

4. FESEM images of (a1, a2) n-type Bi₂Se_0.21Te_2.79 and (b1, b2) p-type Bi_0.4Sb_1.6Te₃ material (a1, b1) before and (a2, b2) after storage in hygrothermal environment for 600 h

5. (a) TEM image of the area close to the surface of n-type Bi₂Se_0.21Te_2.79 material exposed to hygrothermal environment for 600 h; (b) elemental surface distribution profiles of the square region in (a); (c) HRTEM image of the square region in (a); (d) IFFT image of (c); (e) HAADF-STEM image of the area close to the surface of p-type Bi_0.4Sb_1.6Te₃ material exposed to hygrothermal environment and (f) its elemental surface distribution profiles of the region

S1. Thermoelectric performance of n-type Bi₂Se_0.21Te_2.79 in hygrothermal environment (85 ℃, 85% RH)

S2. Thermoelectric performance of p-type Bi_0.4Sb_1.6Te₃ in hygrothermal environment (85 ℃, 85% RH)

S3. FESEM images of samples exposed to hygrothermal environment for different time n-type Bi₂Se_0.21Te_2.79: (a1) 200 h; (a2) 400 h; p-type Bi_0.4Sb_1.6Te₃: (b1) 200 h; (b2) 400 h

Table 1. Room temperature thermoelectric performance of n-type Bi₂Se_0.21Te_2.79 and p-type Bi_0.4Sb_1.6Te₃ materials before and after storage in hygrothermal environment (85 ℃, 85% RH) for 600 h