Study of ultrafast photocarrier dynamics in polycrystalline CdTe films under low illumination

Abstract

The photocarrier dynamics in polycrystalline CdTe films is closely related to device performance. However, the test results obtained at different illumination intensity may be different, and even significantly deviate from the ones in practical application environments. Here, the photocarrier dynamics in polycrystalline CdTe films deposited by thermal evaporation was studied through time-resolved transient absorption (TA) under different pump intensity. It was found that the behavior of photocarriers in CdTe films is strongly dependent on the pump intensity. Under low pump intensity close to operating environment of solar cells, the lifetime of free carriers becomes extremely short compared to high intensity, which is interpreted as a rapid trapping of free carriers by defects. For low pump intensity, numerical fitting by a physical model with one defect level shows that the free carriers in band-edge can be rapidly trapped by defects within tens of picoseconds and then slowly recover to the ground state within several nanoseconds. These results provide a meaningful reference to understand the photocarrier dynamics of CdTe polycrystalline films with a low excess carrier concentration close to that under AM1.5G illumination. • Photocarrier dynamics of CdTe films was characterized under low pump intensity by an ultra-sensitive TA test system. • A physical model was employed to describe the photocarrier dynamics of CdTe films under low illumination. • Experimentally revealed that the photocarrier lifetime of CdTe films under low illumination is tens of picoseconds.