Recently, two-dimensional (2D) layered materials have emerged as possible anode materials for lithium ion batteries.¹ Among them, the Group IV monochalcogenides are proposed as anode materials in the forms of nano- crystals² and sheets.³ Herein, by using the density functional theory (DFT), we show that the GeSe monolayer is a promising candidate for an anode material for Na and K ion batteries. The GeSe monolayer is thermodynamically stable and can be experimentally exfoliated from its bulk counterpart.⁴ Our calculation shows that the GeSe monolayer has fast charge and discharge rates and low voltages suitable for alkali metal ion batteries. The theoretical capacities of Na and K ions are calculated to be 707 and 353 mAh g^-1, respectively, which are larger than that of typical 2D anode materials. We also studied the dye sensitized solar cells (DSSCs) by simulating the adsorption of organic⁵ and organo-metallic⁶ sensitizers on a (TiO₂)₁₆ cluster. The electronic coupling between dye and semiconductor and the position of the TiO₂ conduction band are strongly influenced by the dye@TiO₂ interface and the adsorption configuration. We studied five organic double-donor-acceptor sensitizers bridged with different alkyl chain spacers to examine the effects of their alkyl chain lengths on the overall efficiency. The adsorption strengths of the dyes on TiO₂ were correlated with overall efficiencies. We have also rationalized the effect of cyclometalation of a ruthenium sensitizer, an organo-metallic complex on the electron injection into the semiconductor. The band gap of the semiconductor decreased due to the sharply occupied molecular orbitals of the dye. Also, the LUMOs of the dyes strongly overlap with the TiO₂ conduction bands to favor the electron injection. The wider broadening of the LUMO of cyclometalated ruthenium dye signifies the stronger electron overlap and better electron injection into semiconductor over its corresponding non-cyclometalated ruthenium dye.

References

1. H. R. Jiang, W. S., M. Liu, L. Wei, M. C. Wu and T. S. Zhao, Boron phosphide monolayer as a potential anode material for alkali metal-based batteries. J. Mater. Chem. A 2017, 5 (2), 672-679.
2. Im, H. S. L., Y. R.; Cho, Y. J.; Park, J.; Cha, E. H.; Kang, H. S., Germanium and Tin Selenide Nanocrystals for High-Capacity Lithium Ion Batteries: Comparative Phase Conversion of Germanium and Tin. J. Phys. Chem. C 2014, 118 (38), 21884-21888.
3. Ramasamy, P. K., D.; Lim, D.-H.; Ra, H.-S.; Lee, J.-S., Solution synthesis of GeS and GeSe nanosheets for high-sensitivity photodetectors. J. Mater. Chem. C 2016, 4 (3), 479-485.
4. Karmakar, S. C., C.; Datta, A., Two-Dimensional Group IV Monochalcogenides: Anode Materials for Li-Ion Batteries. J. Phys. Chem. C 2016, 120 (27), 14522-14530.
5. Lee, Y. H. C., R. K.; Jang, B. Y.; Jang, J.; Thogiti, S.; Kim, J. H., Alkyl chain length dependence of the charge-transfer, recombination and electron diffusion length on the photovoltaic performance in double donor-acceptor-based organic dyes for dye sensitized solar cells. Dyes Pigm. 2016, 133, 161-172.
6. Chitumalla, R. K. J., J., Effects of Cyclometalation on the Panchromatic Ruthenium Sensitizer for DSSC Applications. Bull. Korean Chem. Soc. 2017, 38 (10), 1209-1213.