Synthesis And Optoelectronic Application Of Short-Wave Infrared PbS Quantum Dot Ink

Short-wave infrared spectrum(～1-3 μm)has always been at the forefront of research attention due to its large variety of applications,such as night vision,biomedical imaging,and optical communication,etc.In addition,the SWIR photons in solar spectrum account for more than 20%of the available power in the whole solar radiation.Therefore,employing short-wave infrared photovoltaic materials to use this region of energy to build a tandem solar cell with silicon or perovskites is an effective strategy to increase the power conversion efficiency(PCE)of current solar cells.According to the theoretical calculation,using this kind of tandem solar cells can produce up to 6%and 12%additional PCE points on silicon or perovskite,respectively.PbS colloidal quantum dot(CQD)as a photo-responsive semiconductor that allows solution process,are ideal materials for the preparation of optoelectronic devices due to their strong quantum confinement effect and multi-exciton effect.Even more,the bandgap of PbS CQDs can easily cover most of the SWIR region by tuning their sizes,making them one of the most promising candidates to fabricate SWIR photodetectors,solar cells,and light-emitting diodes to replace expensive III-V SWIR semiconductors.At present,most PbS CQDs are synthesized through hot-injection method with their surface capped with insulating surface ligands.In the process of preparing optoelectronic devices,it is necessary to remove the insulating organic ligands on the surface of CQDs through ligand exchange,which makes the preparation process cumbersome and introduces new defect states.Recently,our group has developed a ’one-step synthesis’method of PbS CQD semi-conductive inks,which can be directly deposited to prepare devices without ligand exchange.This method is simple,low-cost and scalable,which is very promising for future industrial applications.However,the reaction mechanism of this method is unclear,making the size tunability difficult.It is still hard to realize the direct synthesis of PbS CQD ink with absorbance covering the SWIR region.Aiming at these problems,this project systematically studies the size control mechanism of directly synthesized PbS CQD ink,and extends its band gap to the SWIR band.With the further improvement of surface passivation and colloidal stability,highperformance SWIR solar cells and photodetectors were fabricated based on these inks.The main research work is as follows:(1)Direct synthesis of SWIR PbS CQD semiconductor ink and their photovoltaic performance.Through the study of the reaction mechanism,it is found that PbI2 cannot directly react with the sulfur source,but first dissociates into lead-iodine highcoordination compounds such as PbI3-and PbI42-in polar solvents,and then participates in the reaction.By controlling the concentration of lead iodide,the reaction equilibrium of its transformation into lead-iodine high-coordination compounds can be adjusted to regulate the nucleation number and eventually the size of quantum dots.Further by introducing halogen ions,the surface passivation can be improved.At the same time,the colloidal stability of quantum dots is improved.Finally,the preparation of SWIR solar cells is explored based on the directly synthesized PbS CQD semiconductor inks.(2)Research on the hole transport layer(HTL)of SWIR PbS quantum dot photoelectric conversion devices.Based on the previous work,we further studied the effect of different types of hole transport materials on the performance of SWIR PbS QD optoelectronic devices.The study found that the performance of the device can be effectively improved by constructing a composite HTL of quantum dot modification layer and organic polymer.Finally,the infrared PCE has achieved a maximum value of 1.44%(IR-PCE,tested with a 1100 nm long-pass filter added between the device and the light source)based on the directly synthesized SWIR PbS CQD inks,which is the highest value reported in the same wavelength band so far.This means that an additional 1.44%absolute photoelectric conversion efficiency can be produced by constructing a fourterminal laminated photovoltaic device with crystalline silicon.Furthermore,we applied the material to SWIR photodetectors,and the specific detection rate in the 1310 nm band reached 4.08×1011 Jones,with good on-off ratio and response/recovery speed.The results of this study show that the directly synthesized SWIR PbS QD semiconductor inks can exhibit excellent photoelectric conversion performance,combined with its advantages of simple preparation process,low cost and batch production,it is expected to promote the applications of high-performance,low-cost SWIR photoelectric devices.