Carrier Lifetime and Diffusion Measurement using Free-carrier Absorption Imaging

Abstract

At the moment, when energy and environmental issues are of concerned in our society, photovoltaic technology has received tremendous development and demand. Because carrier lifetime and diffusion coefficient are the important indicators to determine the recombination level, which influences the efficiency of solar cells to a large extent, they are regarded as key in choosing solar cell materials. A technique for effective lifetime measurement, modulated free-carrier absorption (FCA), can extract lifetime and diffusion coefficient simultaneously, which is supported by a general mathematical model that predicts the experimental signal accounting for the 3-dimensional (3D) charge-carrier transport and recombination within the semiconductor. A single mode 1064 nm laser modulated by an EO modulator is used as the pump and a 2050 nm modulated LED is used as probe in this experiment as the pump/probe parts. An IR camera detects the frequency-domain diffusion image from the tested silicon sample at the tested frequency range between 1 kHz to 200 kHz and the lifetime can be extracted by frequency-domain free-carrier concentration equation, which is a Lorentzian model. By simulating the diffusion data from the camera with the 3D free-carrier absorption model, we can extract lifetime and diffusion coefficient simultaneously. The fitted lifetime from frequency-domain free-carrier absorption equation is 33.5 ± 1.3 μs, and the fitted lifetime from this 3D FCA model is 32.8 ± 1.5 μs, which match to within the error bars. The fitted diffusion coefficient from this 3D FCA model is 15.6 ± 0.7 cm2/s, which agrees with the theoretical value of 16 cm2/s for silicon. Good quantitative agreement is found among the model, experimental data, and theory.