Fabrication And Performance Of PbS Quantum Dots/TiO₂ Heterojunction Devices

In recent years, colloidal quantum dots（CQDS） have attracted more and more attention because of their characteristics of solution preparation, adjustable band gap and multiple exciton generation effect. Especially, IV-VI group quantum dots such as PbS have been paid much attention because their light absorption range can be extended to near infrared region, making them more potential as light absorption layer in solar cell. The performance of PbS CQD-based solar cells is closely related to dynamic behaviors and processes of internal carriers, including carrier generation, transfer, transport and extraction, which are strongly affected by the surface properties and energy level structures of each functional layer in device. Moreover, the active layer thickness of conventional planar heterojunction QD photovoltaic devices is between 200 to 300 nm, which limits the full utilization of sunlight in solar cells. In view of the above problems, the effects of different sizes and ligands on the properties of heterojunction cells were analyzed by using PbS QDs as active layer; With rutile Ti O2 nanorods as electron transport layer, the performance of PbS QDs/TiO₂ nanorods heterojunction devices was analyzed. The main results are as follows: 1. Preparation and properties of PbS quantum dots/TiO₂ nanoparticles heterojunction devices: Colloidal PbS quantum dots with sizes of 2.6 4.5 nm were synthesized via hot injection method using bis（trimethylsilyl） sulphide and PbO as sulfur source and lead source, respectively. The conduction band minimum and valence band maximum of colloidal PbS quantum dots with different sizes and surface ligands（oleic acid or tetrabutylammonium iodide, TBAI） were determined by electrochemical cyclic voltammetry and absorption measurements. Furthermore, the effect of quantum dot sizes on the performance of PbS quantum dots/TiO₂ heterojunction solar cells prepared under air atmosphere was also studied. Our results show that, as the quantum dot size increases from 2.6 nm to 4.5 nm, the conduction band minimum of pristine PbS quantum dots with oleic acid ligands decreases from-3.67 eV to-4.0 eV, while their valence band maximum increases from-5.19 eV to-4.97 eV. However, for PbS quantum dots passivated by TBAI ligands, their conduction band minimum and valence band maximum change from-4.15 and-5.61 eV to-4.51 and-5.46 eV, respectively. Devices made of the Pb S quantum dots with a size of 3.9 nm show the highest power conversion efficiency of 2.34%, perhaps owing to their bandgap, crystalline structure, and favorable energy-level alignment at the PbS quantum dots/TiO₂ interface. Compared with the organic molecular ligands（3-Mercaptopropionic acid）, halogen atoms as ligands shows a higher air stability owing to the efficient passivation for PbS QDs;The devices fabricated with atomic ligand treatements showed better stability over air exposure than those with molecular ligands.2. Preparation and properties of PbS quantum dots/TiO₂ nanorods heterojunction devices: TiO₂ nanorods with different lengths were prepared by hydrothermal method. The optical properties, crystal structure and morphology of TiO₂ nanorods were analyzed by ultraviolet-visible- near infrared spectroscopy, X-ray diffraction and scanning electron microscopy. The effects of hydrothermal concentration, reaction time and temperature on the growth of TiO₂ nanorods were analyzed. Meanwhile, the effects of nanorods with different length on the performance of PbS QDs/TiO₂ nanorods heterojunction devices were investigated. Results show that the heterojunction cells prepared on nanorods with around 0.6 μm length exhibited the highest performance, perhaps because their active layers composing of three-dimensional heterojunction region and PbS QD capping layer with appropriate thicknesses enabled the most effective solar energy harvesting and the least carrier recombination simultaneously.The research results will help further deepen the understanding of the influence mechanism of the surface properties of PbS QDs on the energy level structure and device performance. Moreover, it will lay the foundation for further performance optimization of CQD-based solar cells by constructing three-dimensional heterojunction structure.