Defect Control And Photovoltaic Properties Of Lead Sulfide Colloidal Quantum Dots

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(1.41?s).(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.