Photocarrier transport dynamics in lifetime and relaxation regimes of semiconductors

Understanding photogenerated carrier transport dynamics is important for optimizing the performance of various semiconductor optoelectronic devices, such as photocatalysts, solar cells, and radiation detectors. In this paper, the spatiotemporal evolution of photogenerated carriers after excitation is investigated both analytically and numerically, in order to reveal the origin of two contradictory photocarrier motion directions, i.e., separation and ambipolar transport in the semiconductors. An analytical solution of the separation distance between mean positions of photogenerated electrons and holes is derived, which shows that photocarriers will transport ambipolarly in the lifetime regime, where the carrier lifetime tau(0) is larger than the dielectric relaxation time tau(d), and separate spontaneously in the relaxation regime, where tau(0) < tau(d). Numerical simulation verifies the analytical results and reveals rich dynamics of carrier transport near the boundary of two regimes. In the lifetime regime, the separation distance rises asymptotically to a polarization distance, while there is a transitional sub-region near the regime boundary where majority carriers go through a separating-ambipolar transformation dynamics. This phenomenon originates from two different components of the drift current. In the relaxation regime, majority carriers deplete because of a larger recombination rate in the minority carrier pulse region. Combining the analytical and numerical results, detailed photocarrier transport dynamics are obtained in the lifetime and relaxation regimes of semiconductors.