Abstract
This dissertation presents investigations of the design and synthesis of optoelectronic materials with earth abundant element as well as novel experimental design methodologies. First, we developed three different processes to synthesize quaternary chalcogenide compound for solar cell application, including CZTS colloidal nanoparticles (NPs) by microwave assisted heating process, multi-step selenization process, and fast ramping annealing processes. In microwave heating process, we utilize oleylamine (OLA) and trioctylphosphine oxide (TOPO) as the reaction solvents. With appropriate ratio of two complementary solvents, this microwave heating method can shorten the reaction time from 200 min to 10 min with high quality of CZTS NPs. Our results proved that the crystalline CZTS NPs with appropriate stoichiometry and reasonable energy band gap (~1.5 eV) could be achieved. Meanwhile, we also proposed a multi-step selenization process for the Cu-Zn/Sn metallic stacked precursor to prepare Cu2ZnSnSe4 (CZTSe) absorber. Then the reaction in fixed Se vapour pressure in a series of increasing temperatures was studied. By precisely controlling the nucleation temperatures from 150 oC to 500 oC during 4-step selenization, the homogeneity and crystal quality of CZTSe can be achieved, and the binary phase can be totally ruled out. Finally, stoichiometry with less impurity CZTSe thin film formed at the optimum annealing conditions 500 oC for 10 min: lower or higher temperature lead to insufficient crystallization or undesirable phase segregation. A device efficiency of 5.8 % for the CZTSe solar cell have been achieved with an open circuit voltage of 370 mV, short circuit current of 31.99 mA/cm2, and a fill factor of 48.3%. In the third part, we synthesized high quality CZTSSe with fast ramping heating process with multi-stacking metallic layers. We demonstrated that precursor deposition numbers and inter-diffusion issue have a significant effect on the quality of thin film and device performance. The device prepared with conventional 3 layers stacked, with excessive Cu-rich secondary phase iv formation at the back contact region, results in poor performance of devices due to the poor interdiffusion of precursors. By using the modified 9 layer stacked precursor and fast ramping heating process the device efficiency can be improved from 4.8 to 7.7% with open circuit voltage enhancement from 0.44V to 0.5V due to a compact, smooth microstructure, and the suppression of Cu-rich bi-layer formation. Finally, we introduced a new method to fabricate SWNTs network films with high transparent, high electrical conductivity, and uniform in large (10 cm*10 cm) scale by ultrasonic spray. Due to van der Waals' force within individual SWNTs, dispersion of SWNTs in solvent is a challenging issue; therefore, in order to facilitate SWNTs dissolution in solvent, we functionalize surface of SNWTs with conductive polymer. As SWNTs dissolved, we centrifuged the solution, making the bundle SWNTs, amorphous carbon, and well-dispersed SWNTs to be separated. Finally, dispersive SWNTs solution is ultrasonically sprayed, permitting accurate quantity of SWNTs to be deposited onto substrate with large area uniformity, forming ultra-high smoother, high transmission, and high conductivity transparent conductive film. Hopefully, the optical and electrical transport properties of the SWNTs will be appropriate candidate for multiple-junction solar cells, thermo-photovoltaics, and other applications benefiting from a p-type transparent conductor application due to high near-infrared transmission.