Vacuum-Deposited Cu₂BaGe_1-xSn_xSe₄ Films and Solar Cells

Recently, Cu₂-<i>II</i>-<i>IV</i>-<i>X</i>₄ (<i>II</i> = Sr, Ba; <i>IV</i> = Ge, Sn; <i>X </i>= S, Se) compounds have been introduced as an important family of multinary chalcogenide semiconductors.¹Unlike for Cu₂BaSnS₄ (CBTS), which is one of the first members of the Cu₂-<i>II</i>-<i>IV</i>-<i>X</i>₄ family to be studied in detail, there are only a handful of studies for the related isostructural Cu₂BaGeSe₄ (CBGSe) system.^1-3One of the interesting aspects of CBGSe is that its band gap can be tuned from 1.91 eV to 1.57 eV by partial substitution of Ge with Sn (Cu₂BaGe_1-<i>x</i>Sn<i>_x</i>Se₄; CBGTSe), which is in an appropriate range for optoelectronic and single- and multi-junction PV applications.² Here, we present our recent studies on CBGSe and CBGTSe films prepared using vacuum-based techniques.^4,5 The deposition process consists of three steps: (1) sequential deposition of elemental layers, (2) high temperature pre-annealing, and (3) selenization. A variety of characterization methods were performed on the CBGSe films and compared with CBTS counterparts to acquire better understanding of these materials. Hall effect was used to determine the majority carrier types, concentrations, and mobilities for the semiconductor films. The band edge positions (<i>i.e.,</i> electron affinity and ionization energy) have been determined by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The exciton and defect properties of the films were analyzed with temperature-dependent photoluminescence (PL) measurements. Charge carrier kinetics, transport, and recombination properties of the films were examined with optical-pump terahertz-probe spectroscopy (OPTP), and open-cell time-resolved microwave conductivity (oc-TRMC). Additionally, using the CBGSe and CBGTSe films with chemical-bath-deposited CdS as a buffer layer, we demonstrated the first functioning solar cells for these materials. These initial prototype devices showed a maximum power conversion efficiency of ~1.5 % and ~3.1 %, respectively. The various results provide insights on possible approaches for improving the properties of films of these materials and analogous films/devices based on related Cu₂-<i>II</i>-<i>IV</i>-<i>X</i>₄ chalcogenides.<br/>References:<br/>1. Zhu, T.<i>, et al.</i>, <i>Chem. Mater. </i>(2017) <b>29</b> (18), 7868.<br/>2. Wessler, G. C.<i>, et al.</i>, <i>Chem. Mater. </i>(2018) <b>30</b> (18), 6566.<br/>3. Tampier, M., and Johrendt, D., <i>Z. Anorg. Allg. Chem. </i>(2001) <b>627</b> (3), 312.<br/>4. Kim, Y.<i>, et al.</i>, <i>J. Mater. Chem. A </i>(2021), Advance Article (published on-line): https://doi.org/10.1039/D1TA05666B.<br/>5. Kim, Y., and Mitzi, D. B., <i>ACS Appl. Energy Mater. </i>(2021), Advance Article (published on-line): https://doi.org/10.1021/acsaem.1c02259.