Controlled Synthesis Of Chalcogenide Semiconductor Nanocrystals And Their Optical, Electronic Applications

Due to the unique electronic, optical and magnetic properties, chalcogenidesemiconductor nanocrystalline materials have great potential applications in many fields,such as solar cells, sensors, light detectors and light emitting diodes and field effecttransistors. Currently, the solvothermal method had been the main synthesis method for thefabrication of semiconductor nanocrystals (especially for Group II-VI). This method hadmany advantages, such as simple experimental procedure; enrich solvents and ligandsselection, controllable reaction process, high yield and controllable size and mophologystructure of nanocrystals. Recently, semiconductor nanocrystals have become fascinatingresearch field for there application in electronical and optical devices. For example, based onspin-coating method, the hybrid solar cells and photodetector were prepared by usingsemiconductor nanocrystals as the acceptor while polymer as donor. All inorganicnanocrystalline solar cells, which were realized using semiconductor nanocrystals withdifferent band gap, showed advantages including low-cost, convenient preparation and lowwaste of materials, which made them an important research topic for highly efficientnew-generation solar cells. To date, scientists have achieved numerous impressing researchresults, in which energy conversion efficiency of small-area solar cells has exceed10%,which is very close to that of amorphous silicon solar cells.In this thesis, we firstly review the progress of fabrication and applications of thesemiconductor nanocrystals. Then we summary the controlled synthesis of nanocrystallineswith different morphology by solvothermal methods and investigated their applications inelectronical, optical and magnetical devices. In this thesis, we focused on the preparation andapplications of two kinds of semiconductor nanocrystals with great potential applications inelectronic and optical devices. One is CdSe semiconductor nanocrystals with highlycontrollable size and mophology structure, the band-gap of which was mainly controled bysize and mophology. The levels of conduction band and valence band made them suitable asacceptor phase in the organic-inorganic hybrid solar cells. The other is CdTe semiconductornanocrystals with relatively lower band-gap (~1.5eV) and larger Bohr extinction coefficient(104/cm), which showed great potential for application in highly efficient new-generation solar cells because they are very suitable as light absorption layer. In addition, wesuccessfully prepared a series of semiconductor nanocrystals Ag2+δTe and Ag2+δTexSe1-xthrough elemental exchange reactions using Se and Te nanoline as template and investigatedtheir application in electronic and magnetic properties. To date, we found some of ourresearch results were among the top level in this field after careful investigation:1. We proposed a new in situ-formed ligands mechanism that high quality CdSenanocrystals with controllable mophology (spherical dots, irregular dots, nanorods,tetrapods), controllable size and length/diameter ratio were successfully prepared usinglow-cost cadmium carboxylate as the cadmium precursor and TOPSe as the seleniumprecursor respectively at medium temperature (240oC) without the involvement of additionalsurfactant. Furthermore, we investigated the influence of reaction temperature, TOPSemonomer concentration and chain length of cadmium carboxylate precursor on themorphology of nanocrystals. We found that tetrapods CdSe NCs with selectivity of85%canbe obtained under optimized reaction conditions. The result can compare with CdSe NCs(tetrapod shape) prepared using expensive phosphoric ligand or other ligands to control thesynthesis procedure. Relative experiment results showed that this type of NCs was a verypromising candidate as acceptor in the organic-inorganic hybrid solar cells because of thehigh charge mobility and space expansibility of these NCs with tetrapod shape. The bulkheterojunction (BHJ) solar cell devices were prepared using CdSe NCs andpoly(3-hexylthiophene)(P3HT) as mixture active layer. The device performance of deviceswas studied with consideration of the ratio of CdSe NCs/P3HT, concentration and thermalannealing condition. The energy conversion efficiency of devices thermally annealed at120oC was improved to some extent and achieved1%after optimization. The research of solarcell devices using CdSe NCs and other highly efficient red light-emitting polymer donormaterials is still going on. The formation mechanism and physical properties of CdSe NCswere discussed and analyzed in details by consideration of experiment data and recentprogress in literature.2. In the case of CdTe NCs, CdTe NCs with high-quality and controllable size is moreeasy to prepare by controlling reaction condition as the cohesive energy difference betweenthe nucleation of zinc blende-only with wurtzite-only is lager than that of CdSe NCs. Considering the investigation of literature, CdTe NCs with anisotropy (nanorods, tetrapodsand multi-pod) can be obtained more easily than typical CdSe NCs. We chose suitablecadmium precursor, concentration of precursor, reaction temperature, reaction time aspreviously mentioned solvothermal method, then successfully prepared a series CdTe NCswith controllable morphology and size. The morphology, structure, absorption andphotoluminescence properties were studied and the results showed that the growing processwas highly controllable. The addition of appropriate amount of fatty acids can prevent thegrowth of CdTe NCs along the long axis. The influence of reaction temperature on themorphology as following: at low reaction temperature, the length of CdTe NCs became largerwhile the length of NCs became smaller with increase of diameter at higher reactiontemperature. The high selectivity to CdTe tetrapods can be obtained under optimized reactionconditions. The growth mechanism of CdTe NCs was discussed in details in this chapter.3. CdTe NCs are very suitable as active layer in the organic-inorganic hybrid solar cellsbecause of their large molar extinction coefficient and the reasonable low band-gap (~1.5eV)which match well with the sun absorption spectra. We prepared all-inorganic solar celldevices using the previously prepared CdTe NCs by spin-coating method and we got somecreative results. The device structure were mainly as following: ITO/CdTe/Al (schottky diodesolar cells), ITO/CdTe/ZnO/Al(all-inorganic NCs solar cells). The device performance ofschottky diode solar cells is impressing: the best device with short-circuit current (JSC) of17.56mA/cm2, open circuit voltage (VOC) of0.52V, fill-factor (FF) of56.39%and energyconversion efficiency (PCE) of5.15%were obtained, which is higher than the resultsreported in the literature with similar devices structure.4. Ag2+δTe and Ag2+δTexSe1-xCdSe NCs were prepared by chemical liquor depositmethod and elemental exchange reactions using Se and Te and SexTe1-xalloy nanocrystallineas template. The influence of reaction conditions on the morphology of NCs was investigated.The results showed that the morphology of product was quite different from that of templatematerials due to the difference of various lattice parameters. Dots, nanorods and wires werethe main morphology in our NCs product. NCs film devices containing Ag2+δTe andAg2+δTexSe1-xCdSe NCs were prepared and the magneto-resistance (MR) effectcharacterization was conducted. The study found that the Ag2+δTe and Ag2+δTexSe1-xNCs were promising candidates as magneto-resistive reluctance materials, the magneto-resistance(MR) change of Ag2.2Se0.2Te0.8NCs thin films showed a maximum rate to68%. The resultsshowed prospective great potential for the practical application in information storage andphase changed memory devices.