Production of Nano silicon particles from sugarcane bagasse ash for solar cell application

Farirai, Fortunate
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Beneficiation of solid waste like agricultural waste materials as renewable sources of energy for sustainable and renewable energy production is well applauded. Silicon (Si), a major semiconductor material used in the production of solar cells has been obtained from sugarcane bagasse ash, a solid waste product from sugarcane processing industries. Currently, pure silicon is obtained from quartz via a high energy-intensive carbothermal process, translating into a huge operating cost, and consequently, the high cost of the solar cells produced thereof. Sugarcane bagasse ash, a waste product from sugar and bio-ethanol processing industries, has been conventionally used as an additive to cement due to its good pozollanic properties and high silica content. For valorization of wastes, the complete characterization of the materials is necessary for potential applications. Therefore, the first investigation of this study was to carry out the composition analysis of sugarcane bagasse and its ash to ascertain its suitability for the production of nano silicon. The chemical composition, surface chemistry, degree of crystallinity, calorific value, and morphology were checked by XRF, FTIR, XRD, bomb calorimetry, SEM and TEM. The results showed that sugarcane bagasse ash contains silica and other oxides. The sugarcane bagasse bottom ash contained 71.49 wt. % silica. The FTIR results revealed the presence of polymerized silica between the wave number 1011 cm1 and 1080 cm-1. The ash content of raw sugarcane bagasse obtained was 5.2 wt. %. The moisture content, volatile matter and fixed carbon were 6.4, 58.54 and 29.86 wt%, respectively. Optimization by citric acid leaching was conducted prior to extraction. The optimum conditions obtained for acid concentration, temperature and time were 2 M, 90 ºC and 60 minutes, respectively. In this study, sol-gel coupled with adsorption, using activated carbon was used to then extract high purity amorphous silica from the sodium silicate solution produced. in the leaching process. Physicochemical characteristics of the extracted silica samples were checked using XRF, XRD, FTIR, N2 adsorption-desorption method, SEM and TEM. Most metal impurities which included iron, manganese, magnesium, calcium, potassium, titanium and chromium were adsorbed by the activated carbon due to its large specific surface area of 1308 m2/g which enhances the physical adsorption of dissolved or dispersed substances from the solution. Amorphous nano-silica of 98.92% purity with a particle size range of 6-24 nm was obtained. From the N2 adsorption-desorption analysis, the silica powder exhibited the following properties: 240 m2/g for specific surface area, 0.58 cm3/g average pore volume, and 10 nm for average pore diameter. The extracted silica was used as the precursor in the production of nano silicon by a magnesiothermic reduction for 4 h at 700 ℃, followed by acid leaching of unreacted magnesium and silica. The physicochemical properties of the as-produced silicon sample were checked using Raman spectroscopy, Fourier Transform Infrared spectroscopy and N2 physisorption at 77 K. A strong peak around 520 cm-1 were observed from the Raman spectrum, confirming the presence of crystalline silicon. The FTIR analysis reveals a sharp peak between 1010 - 1050 cm-1, indicating the presence of Si-O-Si functional group. N2 physisorption at 77 K reveals that the surface area, pore volume and pore diameter of the as-produced silicon was 30.14 m2/g, 0.19 cm3/g and 25 nm, respectively. The produced silicon nanoparticles were incorporated into the hole transport layer poly(3,4- ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) of an organic solar cell based on a blend of poly(3-hexylthiophene) (P3HT) and [6:6]-phenyl-C61butyric acid (PCBM). Consequently, 4 solar cell devices were fabricated: F1 (reference cell), F2 (with SiO2NPs), F3 (with SiNPs) and F4 (with p-type SiNPs) in the hole transport layer (PEDOT: PSS). The current-voltage (J-V) characteristics of the solar cells were analyzed to establish the performance of the P3HT: PCBM organic solar cells. The power conversion efficiency (PCE) of the fabricated cells achieved 0.83% (F1), 0.001% (F2), 0.14% (F3) and 0.63% (F4). From this study, a proof of concept for the application of SiNPs in solar cells has been established. However, this work opens an area for future work to improve the PCE.
A Thesis Submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa, in fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering