(775a) Measurement and Modeling of Carrier Dynamics in Photovoltaic Cztse

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination mechanisms to structure and processing of photovoltaic (PV) absorber materials is critical to the design of efficient solar cells. Carrier dynamics in PV absorbers have conventionally been characterized by time-resolved photoluminescence (TRPL). However, not all materials are strongly emissive or otherwise suitable for TRPL. Alternative non-contact probes could enable measurement of ultrafast carrier dynamics for a wider range of materials. Here we demonstrate the use of time-resolved terahertz spectroscopy (TRTS) and surface-sensitive transient reflectance (TR) spectroscopy coupled with numerical modeling of the transport equations to elucidate photoexcited carrier dynamics in photovoltaic absorbers. We show that critical parameters including surface recombination velocity, mobility, and Shockley-Read-Hall lifetime can be obtained. We used a high-quality Cu₂ZnSnSe₄ single crystal with device efficiency of 6.2% as a model case. The single-crystal sample eliminates complications arising from grain boundaries and high densities of point defects and secondary phases associated with thin film growth. Surface recombination velocity could be reduced by at least two orders of magnitude, to 3x10³ cm/s as determined by TR, with appropriate chemical and mechanical polishing. Mobility values of 80 cm²/Vs from TRTS and modeling were validated with Hall effect measurements. Shockley-Read-Hall lifetimes beyond 10 nanoseconds indicate high material quality and promise for high photovoltages. The mobility-lifetime product would result in a diffusion length of ~700 nm, which is similar to the thickness necessary for light absorption and high photocurrents.