Fabrication of low-cost perovskite-based solar cells use recycled lead and lead compounds from depleted sealed-lead acid batteries
Malevu, Thembinkosi Donald
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Perovskite solar cells have been found as promising candidates to offset carbon emissions while providing an alternative way to meet increasing demand in energy consumption. In this thesis, we have successfully fabricated perovskite based solar cell using cheaper materials and environmentally friendly methods. The investigation of these perovskite solar cells covers the operation starting from Synthesis and characterization of high-quality Pbh nanopowders from depleted sealed acid batteries. On the other hand, synthesis and characterization of the nanocrystalline Ti02 and CH3 NH3 I compounds are also carried in fabricating the perovskite solar cells. By utilizing the proposed method, we possibly find alternative ways of improving the power efficiency of the existing solar cells fabricated in ordinary atmospheric conditions. A high quality lead iodide (Pb12) nanoparticles were synthesized from both anode and cathode of a discarded sealed lead-acid accumulator as starting materials. The structure, morphology, chemical composition and optical properties of washed Pbh were investigated using X-ray diffraction, field emission scanning electron microscope, photoluminescence and energy-dispersive X-ray spectrometer. The XRD measurements indicated the presence of pure hexagonal Pbh nanoparticles. Application of the Scherrer equation indicates crystal sizes between 13.70 and 14.32 nm. SEM indicated the presence of spherical particle agglomerations between 1.50 and 3.50 μm in diameter. The measured band gap using two methods (cathode and anode) was consistent at 2.75 eV. EDS results suggest the absence of impurities in the synthesized nanoparticles. The overall results suggest that discarded sealed lead-acid accumulators can source pure hexagonal phase lead iodide nanoparticles with potential applications in perovskite solar cells. By investigating the effects of annealing time and post-melting temperature on the structure and optical properties of synthesized product it was found that Pbl2 nanoparticles consist of the rare 6R polytype which has never been evaluated for a potential applications in photovoltaic. Subjecting this 6R polytype Pbl2 nanoparticles to temperatures higher than the melting point a phase transformation was observed through XRD peaks and the various changes on stoichiometries gave interesting future directions in attempts to enhance solar cell performance. Furthermore, high-quality TiO2 nanocrystals were successfully synthesized using hydrothermal method. The as-prepared samples were subjected to a subsequent annealing at temperatures (AT) ranging from 200 to 600 °C to investigate the effect of AT on the structure, morphology and optical properties of TiO2 nanocrystals. XRD as well as Raman studies suggested the presence of anatase and rutile phases with no traces of other TiO2 phases such as brookite or srilankite as confirmed by Raman Spectroscopy. SEM showed that the preferred facet for TiO2 nanaocrystals at high temperature is (101) and (001) facet and selected area diffraction (SAED) patterns confirmed high crystal quality of the synthesized TiO2 and are monocrystalline. PL data showed three main emission peaks appear at about 407, 416 and 493 nm which are attributed to photoexcited electronhole pairs, band-edge free excitons and bound excitons and oxygen vacancy defects, respectively. UV Vis data showed a decrease in the band-gap from 3.08 to 2.73 eV over the 400-600 °C temperature range. In diode device: To evaluate 6R polytype Pbh nanoparticles contribution in photovoltaic applications, two sets of devices were fabricated i.e diodes and solar cells. A comparison between 6R polytype and common 12R polytype were made. Current-Voltage measurements for both samples show good rectifying behavior of the resulting heterogeneous Schottky diodes. The ideality factors and barrier heights were found to be 4.07/4.09 and 0.500/0.496 eV for the 6R/12R polytypes, respectively. The 6R polytype devices appeared to show improved 1-V characteristics in comparison to the 12R polytype thus suggesting an avenue to enhance the performance of MAPbX3 pevoskite devices. In solar cells: A perovskite solar cells based on TiO2 nanocrystalline was prepared and its photovoltaic performance as a function of annealing temperature was investigated. The optical properties and morphology of the perovskite films were correlated with changes in device performance. It was shown that device efficiency is strongly dependent on annealing temperature. Increasing annealing temperature of perovskite films from 60 °C to 100 °C led to agglomeration of perovskite island in perovskite films, thus increasing the efficiency of the cells from 1.59 % to 2.63 %. At the optimal temperature of 100 °C, ITO/Ti02 /CH3 NH3PbI3/PDOT:PSS/Ag solar cell was found to have a power efficiency of 2.62 %.