Performance Enhancement Of PbS Quantum Dots Based Thin-film Optoelectronic Devices

Quantum dot（QD）is a low-dimensional nanomaterial,composed by limited number of atoms.For conventionally bulk semi-conductor materials,their properties in many aspects such as the physical,optoelectronic and chemical properties,will be enhanced evidently when their dimension is decreased into nano-meter scale.In recent years,IV-VI group semi-conductor materials,e.g.PbS,being with the large excitons Bohr radius and narrow band-gap,and multiple exciton generation,and combining the advantages of easy-fabrication and low-cost,are widely investigated and applied in diverse fields,such as in infrared and visible light detection,electroluminescence,and light harvesting.Therefore,in this thesis,we focus on the performance enhancement of the colloidal quantum dot（CQDs）based optoelectronic devices,i.e.photodetectors and solar cells.Starting from the synthesis of PbS nanomaterials,we proposed and verified a series of methods to improve the device performance,and the following creative results were achieved:（1）By using hot-injection method,we synthesized high-quality PbS CQDs.By the exact control over the matching of precursor,the reaction time and the reaction temperature,sphere PbS CQDs with diameter of～4 nm were synthesized,as well as with good mono-dispersion of dimension and good auto-alignment.In this way,it laid a firm foundation for the subsequent research steps.（2）Photodetectors based on PbS QDs were fabricated and enhancement of device performance were achieved by energy-band alignment design and film-thickness optimization.The PbS CQDs photodetectors ITO/Zn O/PbS-TBAI/PbS-EDT/Au were fabricated by means of layer-by-layer spin-coating method.After using a combined ligand-exchanging strategy,the defects of semi-conductor film are remarkably decreased and its film conductivity is largely enhanced.On the other hand,an appropriate energy band alignment was designed and realized,to benefit to the transportation,separation and collection of charge carriers.The influence of film thickness of different CQD layers on the performance of photo detector was also investigated profoundly.We realized a remarkable specific detectivity of 3.93×10¹³Jones and 5.52×10¹³ Jones under under 2.0μW/cm2 910 nm and 1.9μW/cm2 500 nm illuminations,respectively,which reaches the level of commercial Si based photo-diode.And the device show good time response and stable performance in air.（3）We fabricated PbS QD base solar cell,and achieved performance amelioration by means of post-annealing treatment and graphene oxide（GO）interface modification.The PbS QD based solar cell was prepared by means of layer by layer spin-coating method,which achieved the ITO/Zn O/PbS-TBAI/Au multi-layer structure.Under carefully choosed temperature of annealing,we realized the power conversion efficiency of 4.52%,38.6%of improvement compared with control device without annealing treatment.The reason can be attribute to the sintering process of PbS CQD which reinforce the coupling of neighbored quantum dots,and the increased doping density caused the annealing.We implemented a GO modification layer between the PbS QD active layer and the Au electrode,and achieved an evident decrease of interface defect and interface resistance for PbS CQD base solar cell with structure ITO/Zn O/PbS-TBAI/GO/Au.After the post-annealing treatment of GO layer,the PCE of device was improved to 5.35%.The experimental results verify the interface modification as well as the hole transport function of the GO layer after annealing,which contribute greatly to the performance improvement of device.