(355d) Composition-Dependent Ultrafast Carrier Dynamics in Cu2ZnSnSe4 Single Crystals
Ultrafast spectroscopy can enable the measurement of charge carrier dynamics on femtosecond to nanosecond time scales. Linking photoexcited carrier lifetimes, recombination mechanisms, and mobilities to the structure and processing of photovoltaic (PV) absorber materials is critical to the design of efficient solar cells. Non-toxic, earth abundant, and inexpensive Cu2ZnSn(S,Se)4 (CZTSSe) is a promising candidate for PV applications. However, the record efficiency of 12.6% for thin film CZTSSe solar cells is less than half of the theoretical limit. In this study, we used high-quality, quasi-equilibrium CZTSe single crystals to eliminate complications arising from grain boundaries and secondary phases associated with thin film growth. We utilized complementary time-resolved terahertz spectroscopy (TRTS) and near infrared transient reflectance spectroscopy (NIR-TR) to measure photoexcited carrier dynamics for a range of single-crystal CZTSe cation compositions. According to TRTS and NIR-TR, near-stoichiometric compositions had carrier lifetimes of less than 10 ps, but copper-poor, zinc-rich compositions showed the longest carrier lifetimes of ~260 ps. This trend in lifetime is consistent with trends reported for thin film device efficiency, where the best cells also are copper-poor, zinc-rich. For a typical copper-poor, zinc-rich crystal, TRTS showed that the sum of carrier mobilities was 125 cm2/Vs, while Hall effect measurement indicated hole mobility of 89 cm2/Vs. From these data, we calculated a minority diffusion length on the order of 0.1 µm, which would result in incomplete carrier collection. This result is consistent with PV device measurements, where the external quantum efficiency decreases toward the red because lower energy photons have larger penetration depths. The power conversion efficiency of the device is 6.2%. Efficiency is limited not only by incomplete charge collection, but also by a large Voc deficit. This Voc deficit is likely due to potential fluctuations resulting from high densities of CuZn and ZnCu antisite defects. Since the covalent radius of Ag is approximately 15% larger than that of Cu and Zn, Ag was used as a substitute for Cu to reduce the density of antisite defects that can trap photoexcited carriers. With Ag/(Ag+Cu)=0.29, lifetime increases to 4 ns, with associated increase in diffusion length up to ~0.6 µm. Incorporating Ag into CZTSSe devices may lead to improved efficiencies.