Li, S.; Lloyd, M.A.; Hempel, H.; Hages, C.; Márquez, J.A.; Unold, T.; Eichberger, R.; McCandless, B.; Baxter, J.: Relating Carrier Dynamics and Photovoltaic Device Performance of Single-Crystalline Cu2ZnSnSe4. Physical Review Applied 11 (2019), p. 034005/1-13
10.1103/PhysRevApplied.11.034005
Open Accesn Version

Abstract:
Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination mechanisms to structural properties 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), but TRPL may not be suitable or straightforward for all absorbers. Alternative non-contact methods can enable measurement of ultrafast carrier dynamics for a wider range of materials. Here we demonstrate the complementary use of time-resolved terahertz spectroscopy (TRTS) and near-infrared transient reflectance (NIR-TR) spectroscopy along with TRPL to elucidate photoexcited carrier dynamics in a high-quality copper-poor, zinc-rich kesterite Cu2ZnSnSe4 (CZTSe) single crystal. The single-crystalline nature of the sample eliminates complications arising from grain boundaries, secondary phases, and interfaces associated with thin film growth. A single-crystal-based PV device exhibited an efficiency of 6.2% and Voc of 400 mV, consistent with the quasi-Fermi level splitting determined using absolute photoluminescence. NIR-TR showed picosecond-scale cooling and relaxation of carriers into a distribution of band tail states while TRTS revealed a characteristic time scale of 200 – 260 ps for recombination. Hall effect and TRTS measurements revealed electron and hole mobilities in the range of 50 – 100 cm2/Vs and a majority carrier density of 9x1016 cm-3. These dynamics result in a characteristic minority carrier diffusion length of less than 200 nm, leading to incomplete carrier collection, as confirmed by a strongly decreasing external quantum efficiency at long wavelengths on a device prepared on the crystal. Our approach combining ultrafast spectroscopy and device measurements can lead to more detailed understanding of performance-limiting photophysical processes and can subsequently accelerate the development of more efficient PVs.