Morphology Control Of PbX Nanocrystals And Their Application In Optoelectronic Devices

In the past few years,colloidal semiconductor nanocrystals(NCs)with unique size-and shape-dependent electrical and optical properties,which intrinsically result from quantum confinement,have emerged as promising future semiconducting building blocks for low-cost and solution-processed optoelectronic devices.Among them,Pb X(X = S,Se,Te)are one of the most promising materials because they possess large exciton Bohr radius,strong multiple exciton generation,high absorption coefficient and wide tunable bandgap ranging from visible to mid-infrared region.These versatile characteristics make them prospective candidates for high-performance optoelectronic devices such as solar cells,field-effect transistors and photodetectors.In particular,the ability to control and manipulate the physical properties of Pb X NCs largely depends on precisely size-and shape-control techniques.However,morphology control of Pb X NCs becomes a challenging issue in consideration of their centrosymmetric crystal structure.So far,few researchers have explored ways to obtain such control in Pb X NCs.In this dissertation,we studied new synthetic methods for Pb X NC anisotropic growth,their physical properties and performance in photovoltaic devices and field-effect transistors.The main content of this research are as following:Chapter 1: The introduction of fundamental properties of Pb X NCs and the history of morphology control in NCs.And the progress of Pb X NC application in optoelectronic devices was briefly outlined.Chapter 2: Monodisperse ultra-small Pb Se nanorods(NRs)with the diameter approaching 2 nm(Eg > 1.2 e V)was first obtained.We introduced diphenylphodphine(DPP)and trans-2-octenoic(t-2-OA)in this new synthetic method.The addition of DPP can largely increase Pb Se NRs yield and greatly lower reaction temperature which can realize narrow diameters for the obtained NRs.We used short-chain t-2-OA together with traditional long-chain oleic acid as capping agents for the lead precursor,resulting in high monomer reactivity,fast nucleus diffusion and high growth rate.Under these conditions,anisotropic growth of Pb Se NCs happens at low reaction temperature and then produced ultra-small NRs.The Pb Se NRs show n-type properties and high electron mobility in field-effect transistors.With a suitable bandgap for photovoltaic application,Pb Se NRs were used for the first time in solar cells and exhibited improved efficiency.Chapter 3: Well-performing field-effect transistors based on high quality Pb Se NRs with an inorganic iodide-based ligand are presented for the first time.We found that the measurement environment plays an important role in device performance due to the formation of charge traps by adsorbates(e.g.water and oxygen).The transistors at room temperature display ambipolar characteristics with electron mobility of 0.1 cm2V-1s-1 and hole mobility of 1.1 × 10-4 cm2V-1s-1 in the ultraclean environment.Temperature-dependent device performance reveals charge transport mechanism and there's a transition around 200 K between nearest-neighbor and variable-range hopping mechanism.Below 200 K,the charge transport properties are dominated by the severe disorder.Chapter 4: High-quality,ultra-narrow Pb Te NRs were synthesized for the first time in a one-pot,hot-injection method using trans-2-decenoic acid(t-2-DA)as the agents for lead precursors and tris(diethylamino)phosphine telluride(TDPTe)together with free TDP as the telluride precursors.High monomer reactivity,rapid nucleation and fast growth rate derived from the new precursors led to the anisotropic growth of Pb Te NCs at relative low reaction temperatures(< 150 oC).In addition,the aspect raito of Pb Te NRs could be largely adjusted from 4 to 15 by tuning the Pb to Te precursor molar ratio and reaction temperatures.Interestingly,the synthesized ultra-narrow Pb Te NRs exhibited extremely strong quantum confinement and presented unique optical properties.We revealed that the diameter and length of Pb Te NRs could significantly affect their optical properties,which potentially offer them new opportunities in the application of optoelectronic and thermoelectric devices and make them desired subjects for multiple exciton generation and other fundamental physics studies.In summary,we systematically studied synthetic techniques for morphology control of Pb X NCs and successfully obtained high-quality Pb Se and Pb Te NRs.Based on these advanced materials,optoelectronic devices achieved well-controllable and highly-efficient device performance.