Composition change-driven texturing and doping in solution-processed SnSe thermoelectric thin films

Seung Hwae Heo,Hosun Shin,Fredrick Kim,Garam Choi,Jaeyoung Jang,Hyewon Jeong,Seungki Jo,Won Bo Lee,Hyo Seok Kim,Jae Yong Song,No-Jin Park,Jun-Yun Kang,Jaemin Jung,Jae Sung Son,Hyeong Woo Ban

doi:10.1038/s41467-019-08883-x

Abstract

The discovery of SnSe single crystals with record high thermoelectric efficiency along the b-axis has led to the search for ways to synthesize polycrystalline SnSe with similar efficiencies. However, due to weak texturing and difficulties in doping, such high thermoelectric efficiencies have not been realized in polycrystals or thin films. Here, we show that highly textured and hole doped SnSe thin films with thermoelectric power factors at the single crystal level can be prepared by solution process. Purification step in the synthetic process produced a SnSe-based chalcogenidometallate precursor, which decomposes to form the SnSe2 phase. We show that the strong textures of the thin films in the b–c plane originate from the transition of two dimensional SnSe2 to SnSe. This composition change-driven transition offers wide control over composition and doping of the thin films. Our optimum SnSe thin films exhibit a thermoelectric power factor of 4.27 μW cm−1 K−2.

Highlights

The discovery of SnSe single crystals with record high thermoelectric efficiency along the baxis has led to the search for ways to synthesize polycrystalline SnSe with similar efficiencies
In summary, we showed that highly textured SnSe thin films exhibiting TE power factors at the single crystal level (Supplementary Fig. 11) can be prepared by solution process using SnSe-based ChaM precursor
The high structural quality and texturing of the thin films were achieved through a purification step in the synthetic process to produce the molecular Sn2Se64− ChaMs

Summary

Result

At 400 °C, these peaks are totally disappeared and new peaks at 70, 127 and 150 cm−1 and at 107 cm−1 appeared in the Raman spectrum (Supplementary Fig. 5), which are exactly matched to the B3g and to the Ag vibration modes of the SnSe phase[50] These experimental observations demonstrated that the highly oriented texture in the current SnSe thin film originates from the transition of the pre-formed textured SnSe2. With increasing heat treatment time, the hole concentration progressively decreased from 4.0 × 1018 cm−3 for the 1-min-heated sample to 2.7 × 1018 cm−3 for the 13-minheated sample These hole concentrations are an order of magnitude higher than that of single crystalline SnSe and were similar to the reported value for spark plasma sintered Sn1−x Se polycrystal that exhibit peak ZT value of 2.1, indicating the hole doping effect of SnSe thin films by compositional engineering[60]. Considering that thin films typically have much lower thermal conductivity than bulk materials owing to interface or surface scattering of phonons, the actual ZT value of the current SnSe thin films is expected to be higher than the calculated

Discussion

Methods