Ultrafast Carrier Dynamics in Methylammonium Lead Iodide (MAPbI ₃ ) Perovskite Thin Films With Coexisting Orthorhombic and Tetragonal Phases

Methylammonium lead iodide (MAPbI ₃ ) is a prototypical soft‐lattice semiconductor whose orthorhombic and tetragonal polymorphs can coexist and interconvert under optical excitation or changes in temperature. The mixed‐phase regime attracts attention because of the carrier transfer between phases, which enables optoelectronic functionalities such as optical gain. Here, we investigate carrier dynamics in mixed‐phase MAPbI ₃ thin films by comparing laser‐induced phase coexistence at 77 K using time‐resolved photoluminescence with thermally‐induced phase coexistence at 148 K using transient absorption (TA) spectroscopy. By using low‐repetition‐rate TA to mitigate cumulative heating and avoid thermally induced phase conversion, we resolve the distinct spectral fingerprints of the orthorhombic and tetragonal phases and identify two carrier‐transfer timescales of 36 and 539 ps, attributed to interfacial and bulk‐mediated transfer, respectively. Despite carrier funneling in the mixed phase and high carrier densities, these conditions do not inherently lead to observable optical gain in the studied films.