Preparation And Characterization Of Selenide Semiconductor Materials For Photoelectric And Thermoelectric Applications

With the acceleration of global industrialization process and improvement of living standards of the population, energy crisis and environmental issues have become two main challenges in today's world. They are severely restricting the social long-term stable development. To find renewable energy sources has been a kind of inevitable trend for sustainable development. As the only source of all renewable energies, solar energy has received much attention due to its acceptable energy density on the Earth and has occupied a central position among the renewable energies. Considering its diffuse and intermittent features, the biggest challenge for solar energy utilization is to efficiently capture, convert and store this energy. This article made the summary of the currently used or considered technologies for harvesting and converting the solar energy. It mainly includes photothermal conversion, photochemical conversion, photovoltaic conversion and thermoelectric conversion whose principles have been introduced and present situation have been analyzed in detail. According to the current research status of all kinds of solar energy conversion materials and technology, it's urgent to develop new kinds of solar energy conversion materials in order to broaden the solar energy application field in high efficiency, environmental protection and low cost way. Our research priorities mainly focus on the preparation and synthesis of metal selenide semiconductor materials. In this article, preparation methods of metal selenide ceramics and nanometer materials were studied in detail. A variety of characterization methods were used to study their phases, compositions, morphologies, structures, and performances. The crystal growth processes have been studied by parameters controlling, such as reaction temperature, reactants ratio, reaction time etc., in order to give full play to their excellent photoelectric and thermoelectric performance.The main conclusions of this article are as follows:(1) Using the polycrystalline germanium, antimony, selenium and copper as starting materials, the Cu2GeSe3-Sb2Se3 ceramics with an interpenetrating heterojunction network have been prepared by melt-quenching method. The influence of different reactants ratios on the phase composition, microstructure and photoelectric properties has been studied. It has been found that the reactants ratios don't change the phase types of the ceramics in which there are only Cu2GeSe3 and Sb2Se3. With the increase of Cu(Ge)/Sb ratio, the rod-like Sb2Se3 has progressively drowned in the dense Cu2GeSe3 crystals. The Cu2GeSe3/Sb2Se3 ratio has no significant influence on the macroscopic photoelectric characteristics of the ceramics. All ceramics have similar conductivity, I-V curves, and only n type semiconductor photocurrent enhancement behaviors. The most intense photocurrent, up to 50 ?A/cm2 at the bias of -0.6 V, was observed under a chopped illumination of 400 W·m2 which is much higher than the two semiconducting phases separately. The influence of partly substitution of Cu by CuI on the phases, structures and properties of the Cu2GeSe3-Sb2Se3 based ceramics has been investigated. It leads directly to the formation of n-type SbSel with higher conductivity and the breakage of the conductive Cu2GeSe3 channels. The obtained ceramics show simultaneous n-type and p-type semiconducting behaviors, demonstrating the existence of p-n junctions and the conductive channels including both p type and n type semiconductors. The conducting mechanism of the Cu2GeSe3-Sb2Se3 ceramics are proposed based on the research contents, which is of course of great significance for the application of this ceramics in the solar cell devices.(2) Using the polycrystalline antimony, selenium, copper and Cul as starting materials, the Cu3SbSe4-Sb2Se3 ceramics have been prepared by melt-quenching method. The influence of different reactants ratios on the phase composition, microstructure and photoelectric properties has been studied. The influence of reactants ratio on the Cu3SbSe4-Sb2Se3 ceramics is similar with the Cu2GeSe3-Sb2Se3 system. With the introduction of I, the ceramics indicate simultaneous n-type and p-type semiconducting behaviors. The photo-electro-chemical (PEC) measurements under a chopped light source (400 W·m-2) showed a photocurrent of 50 uA/cm2 at the bias of -0.6 V. The p-n junction interface was directly observed by means of TEM and EDS. This study indicates that Cu3SbSe4 can substitute the Cu2GeSe3 to form heterojunctions with Sb2Se3, leading to efficient charge separation and transport. It was shown that even small quantities of iodine can increase the conductivity of Sb2Se3 by more than 105 times without generating new phases. This is quite important for the improvement of the application of Sb2Se3 with excellent optical absorption and thermoelectric properties while limited by its high resistance. The influence of I on the Cu3SbSe4-Sb2Se3 ceramics is similar with the Cu2GeSe3-Sb2Se3 system. Even though there's no generation of SbSel phase, it leads to the breakage of the conductive Cu3SbSe4 channels. With the improvement of conductivity of Sb2Se3 by I introduction, the photo-generated charge carriers could go through not noly Cu3SbSe4 but also the modified Sb2Se3, showing simultaneous n-type and p-type semiconducting behaviors.(3) The copper selenide nanoparticles and high-quality single-crystalline two-dimensional hexagonal nanoplatelets have been synthesized by a hot-injection approach. The possible growth mechanism of the nanoplates is based on the "oriented attachment" nanoparticles. During the synthesis process, Al3+ ions in the solution have a significant impact on the growth rate of the nanoplates. It is demonstrated that CuSe band gap can be widely tuned simply by modifying the synthesized time. This gives the possibility to adjust this important parameter of the CuSe semiconductor according to the different applications.(4) We have demonstrated a rapid-injection route to synthesize the Cu3Sb1-xSnxSe4 nanoparticles which showed a monodisperse and quasi-spherical morphology. The mean nanoparticle size could be adjusted from 15 nm to 110 nm by controlling the reaction time. Those nanoparticles with a smaller size and controlled shape have been used as the starting powders for preparing bulk thermoelectric materials, which resulted in increased grain boundary scattering to reduce the lattice thermal conductivity. The maximum ZT value reached 0.50 at 575 K for the Cu3Sb0.98Sn0.02Se4 bulk materials which were prepared by hot-pressing of the precursor nanoparticles.