Photoluminescence in Organometal Halide Perovskites: Free Carrier Versus Exciton

The photoexcitation dynamics in CH3NH3 PbI3 thin films has been studied by nanosecond time-resolved photoluminescence (PL) and transient photoinduced absorption. Under excitation intensity from similar to 0.1 to similar to 100 mu J/cm(2), free carrier is the dominant photoexcitation species. However, in the intermediate excitation intensity from similar to 1 to similar to 10 mu J/cm(2), a small fraction of photoexcitations are excitons, which, however, are responsible for PL within this excitation intensity range. The branching ratio between excitons and free carriers at 2 mu J/cm(2) was estimated to be 17%. Furthermore, the spectral narrowing PL was found at the low temperature after the formation of excitons, indicating that laser action could be due to excitonic emission. Our findings provide fundamental insights into the optical properties of organometal halide perovskites with direct implications for optoelectronics.