Measurement of Carrier Dynamics in Photovoltaic CZTSe by Time-Resolved Terahertz Spectroscopy

We demonstrate the use of time-resolved terahertz spectroscopy coupled with numerical modeling of the transport equations to elucidate photoexcited carrier dynamics in a photovoltaic absorber. By measuring a high-quality CU 2 ZnSnSe 4 single crystal that exhibited device efficiency of 8.6%, we show that critical parameters including mobility, surface recombination velocity, and Shockley-Read-Hall lifetime can be obtained. Mobility values of 80 cm 2 /Vs were validated with Hall effect measurements. Surface recombination velocity could be reduced by at least two orders of magnitude, to $\pmb{10^{4}}\mathbf{cm}/\mathbf{s}$ , with appropriate chemical and mechanical polishing. Carrier lifetimes exceeding 10ns indicate promise for devices with high photovoltage. Terahertz spectroscopy provides complementary insight to conventional time-resolved photoluminescence and is particularly valuable for materials that are not strongly emissive.