Simultaneous Enhancement of Thermoelectric Power Factor and Phase Stability of Tin-Based Perovskites by Organic Cation Doping

Abstract

Tin-based halide perovskites are promising room-temperature thermoelectric materials due to their ultralow thermal conductivity and propensity for doping. However, poor phase stability and inferior electrical transport properties hinder their practical application. Herein, we found that organic cation formamidinium (FA) doping, which is not possible by high-temperature processing, could simultaneously enhance the stability and electrical conductivity of CsSnI3 films through a simple solution process. The carrier concentration of 3 × 1019 cm–3 was obtained in FA-doped films, which leads to an 8× enhancement in electrical conductivity to 26.5 S cm–1 but maintains a Seebeck coefficient as high as 131 μV K–1, resulting in a power factor of 45.53 μW m–1 K–2. A further theoretical calculation shows that the enhanced carrier concentration originates from the lowered transition energy of tin vacancies in the doped films. More importantly, remarkable phase stability is achieved under an ambient atmosphere, which stems from a sharply decreased free energy of the B-γ phase compared to the Y phase by FA doping. This work suggests that organic cation doping by facile solution processing is an avenue for the simultaneous improvement of the thermoelectric power factor and phase stability of tin-based halide perovskite. It is applicable for flexible and wearable thermoelectric generators to harvest energy from low-grade heat sources.

Origins and Suppression of Sn(II)/Sn(IV) Oxidation in Tin Halide Perovskite Solar Cells

“…[ 124 ] Furthermore, Kyaw and co‐workers found that the transition energy of tin vacancies in Cs 0.8 FA 0.2 SnI 3 was lower than that in CsSnI 3 , and the doping of FA + caused a sharp decrease in the free energy of the B‐ γ phase compared with the Y phase, which eventually significantly improved the phase stability of CsSnI 3 . [ 125 ] As shown in Figure 9 a,b Kanatzidis and co‐workers incorporated both formamidinium (FA) and ethylenediammonium (en) as A‐site cations into the tin‐perovskite component to form a novel hollow 3D perovskite {en}FASnI 3 . [ 126 ] The SnI 2 unit extrusion resulted in a lower VBM of {en}FASnI 3 , which implied that the {en}FASnI 3 has a lower absolute work function.…”

“…In summary, without changing the 3D perovskite structure that have high carrier transport properties, the component engineering can effectively inhibit Sn(II)/Sn(IV) oxidation in tin perovskite based on the following functions: i) enhance the formation energy of tin vacancies; [125,130] ii) reduce the absolute work function of the tin perovskite; [126] iii) reduce the adsorption between tin perovskite and water molecules; [47,50,131] and iv) achieve in situ antioxidant dense passivation layer. [128,129]…”

Influence of A-Site Cation Composition on the Electronic Properties of Halide Tin Perovskites

“…The formation energy of Sn vacancies in MASnI 3 is lower compared to FASnI 3 . In the present case, the addition of MA to CsSnI 3 increases the number of Sn vacancies, resulting in higher σ of MA 0.2 Cs 0.8 SnI 3 . To elucidate the enhanced σ of MA 0.2 Cs 0.8 SnI 3 relative to its monocationic counterparts (Figure S8a), charge carrier concentration (CCC) was measured by the Hall effect (Figure S8b).…”

Strategic Doping in Metal Halide Perovskites for Thermoelectrics

“…A recent work by Kyaw et al suggested that an incorporation of 20 mol% FA + significantly enhanced phase stability under ambient atmosphere, which originated from a sharply decreased free energy of the B-𝛾 phase via FA-doping. [48] Empirical evidence have indicated that the spontaneous hole charge doping process in Sn-MHPs could be directly accelerated and fixed by oxidation in air as shown in Equation ( 4): [41]…”

“…A recent work by Kyaw et al suggested that an incorporation of 20 mol% FA + significantly enhanced phase stability under ambient atmosphere, which originated from a sharply decreased free energy of the B‐ γ phase via FA‐doping. [ 48 ]…”