Solvent-free synthesis of organometallic halides CH3NH3PbI3 and (CH3NH3)3Bi2I9 and their thermoelectric transport properties

Abstract

Organometallic halides are great candidates for optoelectronics. As an important family of semiconductors, understanding their thermoelectric transport properties is also important. This has been a challenging task as many of such compounds are highly intrinsic. In this work, we synthesized two halides, CH3NH3PbI3 and (CH3NH3)3Bi2I9, using a solvent-free method. We found an extraordinarily high Seebeck coefficient of +2600 ± 200 μV/K in (CH3NH3)3Bi2I9. For CH3NH3PbI3, our synthesis method led to a negative Seebeck coefficient of −1350 ± 50 μV/K, in contrast to positive values observed in solvent synthesized samples. We also found the thermal conductivity of CH3NH3PbI3 to be 0.38 W/m K, largely independent of temperature from 300 K to 450 K, despite a tetragonal-to-cubic phase transition. (CH3NH3)3Bi2I9 has an even lower thermal conductivity of 0.21 W/m K (also temperature independent) which is due to its soft phonon dispersion and weak bonds.

Halide Perovskites: Thermal Transport and Prospects for Thermoelectricity

“…Recently, ultralow κ was observed for zero‐dimensional (MA) 3 Bi 2 I 9 perovskite. [ 119 ] Zero‐dimensional hybrids exhibit constrained charge and thermal transport. [ 64 ] These lower dimensional materials can be explored as low‐temperature thermal insulation materials and microsensors expanding the functionality of halide perovskites.…”

“…Compared to 3D perovskites, lower κ is observed in the case of 0D perovskites such as (MA) 3 Bi 2 I 9 and Cs 4 PbCl 6 . [ 93,94,119 ] Discrete octahedra structure with low group velocities, reduced phonon lifetimes, and the localization of vibrational energy leads to the ultralow κ in 0D perovskites. [ 64,93 ]…”

Tackling Challenges in Seebeck Coefficient Measurement of Ultra‐High Resistance Samples with an AC Technique

“…Together with their proximity it indicates sufficient quality of samples prepared this way. CH 3 NH 3 PbI 3 is an important photovoltaic material, which is a highly intrinsic semiconductor 20,23,24. No report can be found on successful measurement of Seebeck coefficient from a thin film.…”

“…It should be clarified that even though this problem seems to be due to “high resistance” of the sample, the resistance itself is not the only issue, the non‐ideal sample behavior that is more likely to be found with high resistances is probably more important. In fact, we were able to measure Seebeck coefficient from certain samples >100 GΩ using DC method with reasonable precision (±15%) 20…”

Supercompliant and Soft (CH3NH3)3Bi<

“…To demonstrate the ultralow thermal conductivity, we measured thermal conductivity of both (CH 3 NH 3 ) 3 Bi 2 I 9 and CH 3 NH 3 PbI 3 polycrystals using the laser flash method to have a direct comparison and details could be found in SI [47]. We found that thermal conductivity of CH 3 NH 3 PbI 3 polycrystals is 0.36±0.01 W/(mK) at 300 K, which falls into the range of previous reported values (0.3 W/(mK) [41], 0.6 W/(mK) [42]).…”