The Optoelectronic Properties And Applications In Thin Film Solar Cells Of PbS And Pyrite FeS₂Nanocrystals

As one of important sources of green and renewable energy, solar energyis clean and inexhaustible. The development of low-cost and high-efficiencysolar cells will have potential to resolve the current energy issue. Thelarge-scale application of traditional photovoltaic devices based on silicon islimited due to their high cost. Next generation solar cells aiming forhigh-efficiency and low-cost develop fast in recent years. PbS quantum dotsolar cell, as one of next-generation photovoltaic devices, has the potential forcombining lower cost with higher efficiency via solution processing andpossible multiple exciton generation, which in turn evokes more extensiveresearch in this field. Most of the PbS quantum dot solar cells reported in theliterature adopts planar heterojunction structure. Constrained by the diffusionlength of charge carriers in quantum dot thin film, the thickness of lightabsorption layer in planar junction solar cells are usually limited into about200nm, which is not favorable for the full absorption of sunlight and theimprovement of the collection efficiency of photon-generated carrier. In orderto solve the problems, PbS quantum dot solar cells with3D heterojunctionstructures were proposed and fabricated in the work. The performance of solarcells with3D heterojunctions was studied and compared with that of planarheterojunction solar cells. Even though the efficiency of PbS quantum dot solar cells keeps boostingcontinuously, its mass applications will probably be limited for potentialenvironmental issues due to the toxic elements contained in the active layer,.In the second section of the workwe focus on an environmental friendly solarcell material, pyrite iron disulfide (FeS2). On the basis of synthesis of ironpyrite nanocrystals, the optical and electronic properties of thenanocrystalsand solid films with various capping ligands are systematically investigated.The main contents of the dissertation are asfollows:（1）ZnO sol-gel (Sol-Gel), zinc salt ethanol solution (ES) and ZnOnanoparticle solution(NP) were used as precursors to grow seed layers on ITOsubstrates. One-dimensional ZnO nanorod arrays were synthesized bychemical bath deposition method (CBD) on the ITO substrates.Three-dimensional (3D) heterojunction PbS quantum dot solar cells werefabricated with ZnO nanorod arrays grown on sol-gel seed layer. Scanningelectron microscopy, X-ray diffraction and transmission spectroscopy wereemployed to analyze effect of seed layers on the morphologies, structure andoptical properties of ZnO nanorod arrays. The performance of solar cells with3D heterojunctions was compared with that of planar heterojunction solar cells.The results show that ZnO nanorods grown on ES and NP seed layers arewell-aligned and free-standing, respectively, and the orientation of thosegrown on Sol-Gel seed layer is medium. The transmittance of ZnO nanorodarrays grown on ES and Sol-Gel seed layers for2hours is around80%,favoring the fabrication of solar cells. Compared to PbS quantum dotphotovoltaic devices with planar heterojunctions, the short-circuit currentdensity of3D heterojunction quantum dot solar cells based on ZnO nanorodarrays was improved by40%, indicating that3D heterojunction structurefacilitates the dissociation and transportation of carriers.（2）On the basis of synthesis of iron pyrite FeS2nanocrystals, the optical and electronic properties of the nanocrystals and solid films withvarious capping ligands are systematically investigated by UV-Vis-NIRabsorption spectroscopy, cyclic voltammetry and current density-voltagecharacteristic measurements. The electron affinity and ionization potential ofFeS2nanocrystals determined through cyclic voltammetry measurements showstrong ligand dependence. Up to190meV of energy level shift is obtainedamong pyrite nanocrystals capped with ligands employed in this work. Ironpyrite nanocrystal films capped with iodide exhibit the largest band edgeenergy shift and obvious photoconductivity. Our results highlight theimportance of surface chemistry of pyrite nanocrystals on their electronicenergies and provide useful optical and electronic parameters for their furtherusage as the infrared optical absorption materials and/or active layer of solarcells.