Light-tunable thermoelectric transport and reversible carrier-type switching in methylammonium lead bromide single crystals via defect-mediated modulation

We examine the thermoelectric properties of methylammonium lead bromide (MAPbBr₃) single crystals under controlled illumination. In the absence of light (dark conditions), the Seebeck coefficient is positive, indicating hole-dominated charge transport (P-type). Upon illumination with green and blue lasers, the Seebeck coefficient undergoes a shift to negative values, signaling a transition to electron-dominated transport (N-type). As the intensity of the light increases, the Seebeck coefficient becomes increasingly negative, which correlates with a rise in the electron concentration. The increase in the magnitude of the negative Seebeck coefficient under illumination can be attributed to electrons excited from surface defects. The change in Seebeck coefficient type was analyzed using the Two-Band model, which revealed that the relative position of the Fermi level with respect to the conduction and valence bands determines the carrier type. Defects generated by high-energy laser illumination induce lattice deformation, which in turn drives shifts in the Fermi level position. These findings indicate that illumination significantly modulates the behavior of defect states and influences carrier dynamics in MAPbBr₃ single crystals. This suggests the potential for tuning the thermoelectric properties of MAPbBr₃ via optoelectronic control, providing valuable insights into the use of hybrid perovskites for thermoelectric applications.