Synthesis Of PbS Quantum Dots And Their Novel Heterojunction Photovoltaic Devices

PbS is a direct band gap semiconductor material which has the bulk band gap of 0.41 e V and a high absorption coefficient.The exciton Bohr radius of PbS is 18 nm,which makes it easily to get a tunable energy band gap that covers the optimal band gap range for single and multi-junction solar cells by controlling the synthetic conditions.Furthermore,in the past one-decade,a great attention has been paid to PbS quantum dots?QDs?for photovoltaic applications due to the enhanced multiple exciton generation and the power conversion efficiency has increased rapidly from initial less than 1% to more than 10%.However,there are many challenges existing in the further commercial production.For example,compared to the 1st and 2st generation solar cells,the power conversion efficiency is still lower and the performance stability is relatively poor in the PbS QDs solar cells.Moreover,the low reactivity of sulfur source limits the synthesis of PbS quantum dots and increases the synthesis costs,and the assembly process makes it difficult for large-scale industrial production.Therefore,developing green,low-cost,large-scale PbS QDs synthesis and device assembly processes,exploring mechanism and improving power conversion efficiency have great significance.For the above difficulty,this dissertation started with a new n-type CdS alternative electron transport layer material to construct the PbS QDs heterojunction solar cells.For PbS QDs synthesis,a cation exchange reaction was employed to obtain highly monodisperse and well-size-controlled PbS QDs.Using these cation-exchanged QDs,multi-junction heterojunction cells were formed.The main results are as follows:1.The monodisperse and stable PbS QDs were obtained by thermal injection using TMS as a sulfur source.The presynthesized OA-coated CdS QDs to dissolve in octadecene?ODE?were injected into the PbCl₂/OLA or PbNO₃/OLA mixture to form the highly monodisperse PbS QDs,with the first excitonic absorption peak from 800 nm to 1202 nm and the solid-state cation diffusion mechanism in this exchange reaction was explored.The cation exchange reaction solves the limitation of low activity for sulfur powder and the high price and poor stability for TMS in traditional chemical synthesis,provides a new idea in the synthesis of monodisperse,well-controlled PbS QDs,and in situ inorganic ligands passivation in QDs surface.2.The controlled growth of CdS thin films was achieved on the FTO substrate through the regulation of reaction time,solution concentration and so on.Then,a spin-coating process was developed for PbS QDs films on CdS substrate to construct a novel CdS/PbS quantum dot?depleted heterojunction?DH photovoltaic cells.The MoO₃ layer was inserted to achieve good ohmic contact between the PbS layer and the metal electrode and improve hole transport.By optimizing the CdS thickness,the highest efficiency?5.22%?was obtained in CdS/PbS heterojunction solar cells.This CBD method,which has the advantages of lower synthesis temperature,larger process scale and more selectivity for the substrate,provides a prospect for large-scale industrial production.Furthermore,CdS thickness-dependent performance is of great reference value for the construction of such photovoltaic devices3.We first constructed a 3-D DH PbS QDs solar cells employing CdS NRs as electron acceptor.The PbS QDs were synthesized through the solution process method,and the CdS nanorods?NRs?were synthesized through a facile two-step template free hydrothermal approach,which allows PbS QDs to infiltrate into the CdS nanorod arrays by spin-coating to form a new 3-D DH structure.In a typical experiment,the gap in the nanorod arrays was adjusted by regulating the thickness of the seed layer,the permeability was improved by increasing the spin-coating speed.Also,the morphology,transmission spectra and the power conversion efficiency of the different layers of PbS QDs fillms were monitored to obtain the optimal dense 3D heterostructures.The highest power conversion efficiency of 4.78% has been achieved,approximately five times higher than that of a planar design of similar thickness.4.The bandgap in the QDs can be modified by the TBAI and EDT ligands.A multijunction solar cell of the FTO/TiO₂/PbS-TBAI/PbS-EDT/Au architecture was formed using the cation exchanged QDs in this work.After systematically optimization in the construction of multi-junction heterojunction and passivation of halogen ions,a high power conversion efficiency of 7.89% was obtained in this improved architecture,the Jsc and Voc are measured to be 30.96 m A/cm2 and 0.49 V with the FF of 51.9%.The carrier transport mechanism caused by the high EDT-PbS conduction band position in this multi-junction structure was studied.PbS-TBAI exchanged QDs film was served as the mainly photoactive region whereas PbS-EDT exchanged one was served as the electron-blocking layer.Furthermore,in-situ chloride passivated QDs solar cells showed good stability in air and a longer carrier diffusion length.The heterojunction device can be also used as self-powered photodetector with fast response speed,the rise time??r?and fall time??f?can be determined to be 4.4?s and 37.2?s under a pulsed light?f=10 k Hz?.