Green Synthesis And Process Investigation Of Iron Sulfide Series Photoelectric Materials

Iron sulfide series have been extensively studied for their potential applications in solar cells. However, the lack of controllable and simple synthetic process of pure iron sulfide series limited their large-scale production and wide application. Based on solvothermal and hydrothermal method, one-step and facile synthetic route was developed to prepare high-quality iron sulfide series materials. In atmosphere, the synthetic process of sulfur group semiconductor, such as pyrite FeS2, FeSe2, NiSx, and NiSex were investigated. Meantime, the synthetic route of FeS2nanocrystals (NCs) was developed via hydrothermal method, and it was modified by trioctylphosphine oxide (TOPO), which could improve stability. Pyrite FeS2with various nanostructures had been prepared in an ethanolamine (ETA)/water binary solution. Finally, the film and solar cell were fabricated based on the FeS2NCs modified by TOPO. The achieved results are stated as follows:(1) Process optimization of the synthesis of pyrite FeS2via solvothermal methodThe coligand (oleic acid (OA)-oleylamine (OLA)) system was developed to prepare pure pyrite FeS2. The phase structure was proved by Raman measurement. The absorption peaks of FeS2samples were at1185nm corresponding to a band gap of1.05eV, which was close to that of silicon (Eg=1.1eV). Meantime, the FeS2sample with smaller particle size was obtained through the optimization of solvents and additives, besides the reaction time was decreased to15min and the cost was greatly reduced.(2) Controllable synthesis of flower-like and flake-like FeSe2via solvothermal methodIn order to extend the process route of FeS2, the synthesis of flower-like and flake-like FeSe2crystals were investigated. The flower-like FeSe2crystals could be got from200℃to300℃with the morphologies changed from flower-like to stick-like structure. When the iron source was replaced by ferric nitrate, four morphologies of the FeSe2samples, including globular, flocculent, flake-like, and rice-like, were obtained in a wide temperature range (180℃-300℃). Based on the reaction dynamics, the mechanism was considered that OLA reacted with free Fe3+to form relatively stable Fe-OLA complex, and then combined with selenium powder to form the FeSe2nuclei. Moreover, the OLA dominated the nucleation process and reaction temperature dominated the growth process. (3) Controllable synthesis of nickel sulfide (selenide) series via solvothermal methodBased on the process route of the FeS2crystal, the nickel sulfide (selenide) compounds were oriented to synthesize through one-step approach, and the influence of phase structures and morphologies by the reaction temperature and molar ratio were investigated. With increasing of the sulfur content, the products with four kinds of morphologies were obtained in sequence, which were trigonal Ni3S2, rthorhombic Ni7S6, hexagonal NiS and cubic NiS2-Based on the kinetic parameters, the mechanism was found as Ni2+and S powder combined with OLA to form to Ni-S-OLA precursor, and then decomposed to nickel sulfide series. When the molar ratio of Ni/Se was2:1, hexagonal NiSe gradually transformed into rhombohedral Ni3Se2with the extension of the reaction time. The x value of Ni1-xSe could be adjusted from0to0.15with the reaction temperatures from180℃to260℃, and the star-like orthorhombic NiSe2converted to square cubic NiSe2with the increase of temperature at a certain Ni/Se ratio of1:2.(4) Controllable synthesis of pyrite FeS2via hydrothermal methodTo obtian the smaller size of FeS2NCs, the hydrothermal method was developed to prepare FeS2NCs. The obtianed FeS2NCs were of pure phase, high crystallization, stable surface, and small size. The FeS2NCs modified by TOPO showed excellent air stability over six months. Meantime, pyrite FeS2with various novel nanostructures had been prepared on a large scale in an ETA/water binary solution. When only pure water was used as solvents, hollow sphere of marcasite FeS2sample were achieved, and it would transform into rod-like structure been piled up with nanoparticles. When ETA was added into the solvent, cubic, flake-like, and two kinds of tetrakaidecahedron shape of pyrite pure FeS2crystals were obtained by adjsuting the volume ratio of ETA and H2O.(5) Film preparation and solar cell assembly of FeS2NCsThe film was prepared by spin-coating method with the FeS2NCs modified by TOPO. The FeS2NCs dispersed in the colloid of polystyrene and toluene showed uniform and smooth property of the prepared film, while its large thickness and high porosity hindered the absorbance of light and led to the permeation of the film, decreasing photoeclectric response and causing the short circuit. After dispersing the FeS2NCs into chloroform, the obtained film displayed good smoothness and transparency with suitable transmission of optical signals. Finally, the Schottky and nanocrystalline/polymer composite solar cell were assembled, and produced a clear signal of photoelectric response.