Solution-phase synthesis and characterization of fe2ges4 material and photovoltaic devices
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Recently, climate change and the energy crisis are warning human race that it is necessary to develop new renewable energy. Solar energy is considered to be one of the cleanest and most abundant energy sources to replace fossil fuels. Solar cells are extensively studied devices that convert sunlight into electricity. However, recent solar cells have been hampered by high cost, use of toxic materials and low conversion efficiency. To further improve the solar cells, a cheap, earth-abundant, and non-toxic material is required.
The ternary compound Fe2GeS4 (FGS) has attracted considerable attention because it not only has high absorption coefficient and band gap suitable for photovoltaic applications but also has better sustainability. In addition, the elements contained in FGS are relatively abundant in earth’s crust and are less toxic. In my dissertation work, FGS crystals were prepared using two methods, solution-phase synthesis of FGS precursor, followed by a thermal treatment and one-pot synthesis.
In the solution-phase synthesis of FGS precursor, the effects of reaction parameters such as precursors, solvents, reaction times on the chemical and physical properties of the resulting products have been comprehensively investigated. The results show that a stable FGS precursor can be prepared in the mixture of oleylamine (OLA) and 1-Octadecene (ODE), using Fe (III) 2,4-pentanedionate, Ge (Gly)2(H2O)2, elemental sulfur as Fe, Ge and S source, while the reaction time is 15 minutes. Then, the optimization of the annealing conditions for FGS powders has also been studied. Finally, the highly crystalline FGS powders were obtained by a two-step process, solution-phase synthesis of FGS precursors and thermal treatment at 550℃ for 2 hours under sulfur/argon atmosphere. FGS thin films were fabricated by dip-coating the prepared FGS inks on the substrate and performing the same thermal treatment at 550℃ for 2 hours under sulfur/argon atmosphere. The FGS solution-based solar cell using FGS thin film as the light absorbing layer showed a significant open-circuit voltage (VOC) of 361 mV. When the optimized FGS thin film was incorporated into a well-established dye-sensitized solar cell, the fabricated FGS-catalyzed dye-sensitized solar cell achieved a higher open-circuit voltage (VOC), short-circuit current density (JSC) and conversion efficiency (η).
In the one-pot synthesis of FGS NCs, the effects of trioctylphosphine oxide (TOPO) and reaction time on the product have been determined. The results show that the plate-like FGS crystals can be prepared within 5 hours, using FeCl2, Ge(Gly)2(H2O)2, TOPO, and 1-dodecanethiol (1-DDT) as reactants, while OLA acts as the solvent. A high-quality FGS thin film was prepared by dip-coating and thermal treatment at 400 ℃ for 2 hours under sulfur/argon atmosphere. Similar to the solar cells fabricated based on FGS precursor, the photovoltaic devices based on the FGS NCs also exhibited a significant open-circuit voltage.