Preparation And Characterization Of The Conducting Polymer/Inorganic Nano-Crystal Composite And Devices

Infrared materials and devices have important and wide applications in the fields such as optical communication, thermal image, biology image and solar cells. The research on conductive polymer/inorganic quantum dot infrared composite materials and devices has just been started and the device performance is still very low. The study of the energy transfer and the carrier mobility in the composite can provide the fundamental reasons on how to improve the luminescence efficiency and photovoltaic performance.In this work, we synthesized MEH-PPV/PbS, InP quantum dot near-infrared composite, and fabricated a Al/MEH-PPV(InP)/ITO visible-near infrared photo-diode. Using quasi-in-situ transmission spectroscopy and photoluminescence, we successfully observed the quantum-dot size dependence of the Dexter energy transfer rate from MEH-PPV to PbS quantum dots and verified the effect of the optically quasi-forbidden transitions on the Dexter energy transfer. The carrier mobility of a Al/MEH-PPV(InP)/ITO photodiode was studied by using a time-of-flight measurement system, which was built by ourselves, The main contents are as follows:1) A new procedure of synthesizing PbS quantum dots directly in conductive polymer MEH-PPV was developed, and the size and its distribution can be adjusted by controlling the reaction time. Small PbS quantum dots with diameters ranging from 2.5 nm-3 nm are directly synthesized in MEH-PPV at 70℃. According to the quasi-in-situ transmission spectroscopy and photoluminescence (PL) analysis of a series of size-variable PbS quantum dot composite, we found that, as the size of PbS quantum dots increases, the PL of MEH-PPV decreases first and then rises and finally decreases. Combining the theoretical calculation of the energy level structure of PbS quantum dots, we found that, when the diameter of the PbS quantum dots is 2.65 nm, the overlap between the quantum-dot exciton ground state or the first optically forbidden transition and the emission spectrum of MEH-PPV is minimum, resulting a minimum energy transfer rate. The finding is helpful for understanding the working properties of conductive polymer and quantum dot composite.2) In order to study the growth dynamics of PbS nano-crystals, we designed a in-situ transmission spectroscopy system to monitor the growth of PbS nano-crystals. During the synthesis in water, PbS nano-wires are formed if adding surfactant SDS, otherwise, PbS nano-dots are formed. We found that, when the other reaction conditions are the same, as the reaction proceeds, the absorption peak wavelength of PbS red-shifts faster than that of PbS Nano-wires. As the absorption peak wavelength of the nano-wires is determined by the limit of the lateral sizes of the wires, the finding of this phenomenon directly verified that the growth rate along the longitudinal direction of the wires is faster than that of the transverse direction during the growth of the PbS nano-wires.3) Although the synthesis technique of nan-crystals such as CdS, PbS, PbSe etc. is very mature, these materials content poisonous elements, so it is necessary to synthesize environment-friendly materials such as InP. Using organic high-temperature synthesis technique, we successfully grew InP quantum dots and the luminescence efficiency of the InP quantum dots was improved by capping ZnS. After hybridizing MEH-PPV and InP, we fabricated Al/MEH-PPV(InP)/ITO photo-diode. The IV characteristics of the device show that the conductivity of.the composite material is much higher than that of pure MEH-PPV.4) We designed and built a temperature-voltage variable time-of-flight (TOF) measurement system. Using this system, we measured the TOF of Al/MEH-PPV(InP)/ITO photodiodes. We found that the mobility of the composite is Gaussian-disorder type. Compared with the case of pure MEH-PPV, the position disorder is almost unchanged, while the energy disorder is obviously broadened.