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

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.