Enhancing thermoelectric performance of the CH3NH3PbI3 polycrystalline thin films by using the excited state on photoexcitation

Abstract

Abstract Recently, organic-inorganic hybrid perovskite materials (CH3NH3PbI3) have made great progress in the solar cells field, however its thermoelectric properties are not good, one of the reason is that the carrier concentration is too low, resulting in low electrical conductivity. Another reason is that the phase structure changes from the tetragonal phase to the cubic phase when the temperature over 57 °C, which distorted the octahedron structure of PbI6. The structural distortion restricted the carrier's transport, leading to decreasing of the carrier's mobility. And the Seebeck coefficient was also rapidly dropped with the temperature increasing. In this article, we used photoexcitation-generated excited states could effectively restrain the decrease of electrical conductivity and Seebeck coefficient of CH3NH3PbI3 polycrystalline thin film, because the photoexcitation could greatly enhance the density of excited state, which increased charge carrier concentration and electrical conductivity. Moreover, thanks to the photoexcitation, the surface charge accumulation was increased significantly, enhancing the surface polarization at metal/Perovskite interface, simultaneously, enhancing the polarization effect of CH3NH3PbI3. The enhanced surface polarization functions as an additional driving force to diffuse the charge carriers from hot side to cold side, thus the carrier mobility was increased effectively, restraining the Seebeck coefficient's decrease caused by the crystal phase transition. Our experimental results showed that photoexcitation increased the electrical conductivity from 7.8 × 10−6 S/m in the dark to 4 × 10−5 S/m in illumination condition, the mobility increased from the 0.141 cm2/v.s in the dark to 13.122 cm2/v.s in illumination condition. Simultaneously, the Seebeck coefficient was also increased from 26 μV/K to −17 mV/K with increasing light intensity from 0 to 100%. The Power Factor (PF) value of 11.51 nW/mK2 in the illumination condition was further bigger than the PF of 5.27 × 10−6 nW/mK2 in the dark condition. As a result, our experiment further demonstrated that photoexcitation can be used as an effective way to simultaneously develop high Seebeck coefficient and electrical conductivity through excited states in an organic-inorganic semiconducting material with multilayer electrode/perovskite/electrode thin-film device.