【作者】 夏勇；

【导师】 张建兵；

【作者基本信息】 华中科技大学 ， 微电子学与固体电子学， 2020， 博士

【摘要】 硫化铅（PbS）胶体量子点因具有低成本溶液工艺制备、高消光系数、良好的稳定性和多激子产生（MEG）效应等优点,被视为极具发展前景的光伏材料。基于此,小尺寸（带隙为1.3～1.4 eV）PbS量子点太阳能电池经认证后的效率高达12.3%。此外,由于强烈的量子尺寸效应,PbS量子点带隙可以在0.5～1.9 eV内大范围可调,既可以满足单结太阳能电池的需求,还可以制备量子点红外光伏器件,并与其他电池构成互补,实现对太阳光宽光谱的有效利用。基于此,大尺寸（带隙小于1.1 eV）PbS量子点太阳能电池经硅片过滤后的效率可达1.34%。PbS量子点光伏器件性能提升的核心和关键是表面缺陷态的控制,其主要与量子点合成和后续表面处理密切相关。在此之前,绝大部分研究者把精力投放在后续表面处理上,但是与缺陷密切相关的合成却鲜有人问津。针对此问题,本论文围绕合成之初量子点表面的缺陷控制而展开课题研究,分别从量子点晶面控制和表面原位卤素离子钝化入手,以此改善量子点的光电性能,并致力于发展高效率的PbS量子点太阳能电池。本论文的具体研究成果如下:（1）提出了从源头上抑制小尺寸量子点{100}面缺陷的解决方案。当前研究者主要聚焦于合成或后处理过程中引入配体来对{100}面缺陷进行修复,而并未予以彻底清除。为此,本论文提出一种创新性的解决方案来从源头上抑制表面缺陷态,即通过控制量子点生长过程中的动力学和热力学平衡来实现晶面的有效调控。研究表明,在动力学占主导的生长条件下,量子点各向异性生长,获得的～3 nm PbS量子点的几何结构近似为八面体,表面几乎仅含{111}晶面;而在热力学占主导的生长条件下,量子点各向同性生长,获得的量子点几何结构为截角八面体,表面具有{111}和{100}晶面。由于{111}面配体钝化完全,因此表面几乎仅含{111}晶面的量子点具有更小的斯托克斯位移、更高的荧光量子产率（PLQY=50%）和更长的荧光衰减寿命（1.41μs）。（2）为了获得良好的表面钝化,同时解决阳离子交换尺寸分布不佳的问题,本论文提出一种极具创新性的阳离子交换策略,即Zn S纳米棒阳离子交换合成大尺寸PbS量子点。其基本思想是依靠由棒到点转变过程中因部分溶解而释放的S来维持一定的过饱和度,促进量子点的生长并维持较好的尺寸分布。研究表明,量子点表面具有原位Cl^–钝化,展现出了极小的斯托克斯位移和更高的荧光量子产率（PLQY=36.6%）;另外,空气存储6个月后激子吸收峰未发生蓝移现象,量子点稳定性优异。此外,量子点具有高度的单分散性,激子吸收峰的半高半宽最小值仅为15.3 meV,尺寸分布偏差小于2.9%。（3）为了研究晶面控制抑制表面缺陷对量子点光伏特性的影响,本论文将上述两种小尺寸量子点制备成光伏器件,并通过材料性能表征和器件物理分析来研究器件性能差异的主要原因。研究表明,表面仅{111}晶面的量子点薄膜具有更少的表面俘获态和带隙间的缺陷态,从而有利于抑制光生载流子的缺陷辅助复合;此外,其具有更小量子点间的间距和更浅的带尾态分布,从而利于提升载流子迁移率,降低光伏器件开路电压(V_OC)的损失。基于此,仅{111}暴露晶面的量子点光伏器件效率高达11.5%,相比{111}和{100}暴露晶面的量子点光伏器件效率（9.2%）提升了25%。此外,前者具有很高的稳定性,在未封装的情况下,空气中存放42天后效率还能够保持最高值的90%,持续光照4小时后效率还能够维持初始值的95%。（4）为了研究从棒到点阳离子交换合成的量子点光伏特性,本论文将上述大尺寸量子点制备成红外太阳能电池,并与传统方案合成的大尺寸量子点光伏器件进行对比。随后,结合材料性能表征和器件物理分析来研究产生器件性能差异的主要原因。研究表明,量子点由于具有原位卤素离子钝化和极为优异的尺寸分布,则有利于成膜后缺陷态的控制和载流子迁移率的提升。基于此,带隙为～0.95 eV的大尺寸量子点其器件性能国际领先,在AM1.5下,效率高达10%,800 nm长通滤光片下的效率为4.2%,1100 nm长通滤光片下的效率为1.1%。此外,本论文还制备了带隙更小的量子点红外太阳能电池,均获得了良好的能量转换效率。更多还原

【Abstract】 PbS colloidal quantum dots（CQDs）have many advantages,such as low-cost solution processability,high extinction coefficient,excellent air stability and multi exciton generation（MEG）effect,which have been regarded as a promising photovoltaic material.The performance of PbS CQD（band gap of 1.3～1.4 eV）solar cells has been improved rapidly,achieving a certified power conversion efficiency（PCE）of 12.3%.In addition,due to the strong quantum size effect,the band gap of PbS CQD can be adjusted in a wide range of0.5～1.9 eV.PbS CQDs can not only meet the needs of single junction solar cells,but also can be used to fabricat infrared CQDs photovoltaic devices,which are complementary with other devices for achieving effective use of the broad spectrum of sunlight.Based on this,the PCE of large sized（band gap less than 1.1 eV）PbS CQDs solar cells can reach 1.34%with silicon wafer filtration of the solar irradiation.However,the defect states control of PbS CQDs is the core and key to improve the performance of photovoltaic devices.The defect states are closely related to the synthesis of CQDs and subsequent surface passivation.Heretofore,researchers have invested more research in the latter,but few attention has been paid to the former.For this reason,this thesis focuses on the defect states control in the synthesis of PbS CQDs,and it is expected to further improve the performance of PbS CQD solar cells.The specific research results of this article are as follows:（1）The{100}facets of PbS CQDs,which are self-passivated,are important origins of trap states.However,previous investigations focused on synthesis,ligand exchange or passivation approaches and ignored the control of{100}facets for a given dot size.Herein,we suppress trap states from the source via facet control.The{100}facets of～3 nm PbS CQDs are reduced by tuning the balance between the growth kinetics and thermodynamics in the synthesis.The PbS CQDs synthesized at a relatively low temperature with a high oversaturation follow a kinetics-dominated growth,producing nearly octahedral nanoparticles terminated with almost only{111}facets.The PbS CQDs synthesized at a relatively high temperature follow a thermodynamic growth,thus a spherical shape is preferred,producing truncated octahedral nanoparticles with more{100}facets.Therefore,the PbS CQDs with nearly only{111}facets have smaller Stokes shift,higher fluorescence quantum yield（PLQY=50%）and longer fluorescence decay life（1.41μs）.（2）In order to obtain excellent surface passivation and highly monodisperse PbS CQDs.We developed a new synthesis for PbS CQDs based on cation-exchange,i.e.,Zn S nanorods（NRs）were converted to PbS CQDs.Sulfur precursors are released via the dissolution of the Zn S NRs during thecation exchange,which promotes size focusing of PbS CQDs.The results show that the surface of CQDs is in situ Cl^–passivated,showing a very small Stokes shift and a higher fluorescence quantum yield（PLQY=36.6%）.In addition,the exciton absorption peak does not blue shift after six months of air storage,and the CQDs have excellent stability.Furthermore,the minimum value of half-width at half-maximum（HWHM）of the frst exciton peak is only 15.3 meV,and the size distribution deviation is less than 2.9%.（3）In order to study the effect of facet control and suppression of defect states on the photovoltaic properties of CQDs.Photovoltaic devices are fabricated based on the above small sized CQDs,and the main reasons for the differences in device performance are studied by material characterization and device physical analysis.The results show that PbS CQDs with{111}facets has less trap states,which is helpful to restrain the defect recombination of photogenerated carriers;In addition,it has a closer inter-dot distance and shallower band tail states,which is conducive to improving carrier mobility and reducing the open circuit voltage(V_OC)loss of photovoltaic devices.Due to the effective suppression of trap states via facet control,the PbS CQDs with{111}facets lead to more efficient solar cells than the CQDs with{111}and{100}facets,achieving a PCE of 11.5%,and the former also show excellent stability of air-storage（42 days）and illumination（4 h）.（4）In order to study the photovoltaic properties of CQDs synthesized by rod-to-dot cation exchange,the above-mentioned large sized CQDs were used to fabricate infrared solar cells which are compared with the devices based PbS CQDs synthesized by conventional methods.The main reasons for the differences of device performance are studied by material characterization and device physical analysis.The results show that CQDs with in situ halogen passivation and excellent size distribution are beneficial to the control of trap states and the enhancement of carrier mobility.Based on this,the device performance of large sized CQDs with a band gap of～0.95 eV is internationally leading.Under AM1.5,the efficiency is as high as 10%,under 800 nm long-pass filter,the efficiency is 4.2%,and under 1100 nm long-pass filter,the efficiency is 1.1%.In addition,the CQD infrared solar cells with smaller band gaps have been fabricated,which also have excellent device performance.更多还原