Synthesis And Characterization Of Black Silicon By Silver-Assisted Chemical Etching For Solar Cell
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Date
2021-03
Authors
Noor, Nur Afidah Md.
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Publisher
Universiti Sains Malaysia
Abstract
Crystalline silicon (c-Si) suffers from poor light absorption due to its indirect band gap and high reflection from its surface (about 35% in the visible region). This problem can be solved by texturing the surface of c-Si wafer to reduce its broadband reflection. Black silicon (b-Si) or nanotextured c-Si, has a huge potential for applications in solar cell due to its superior broadband light absorption within 300-1100 nm wavelength region. In this work, two-step metal-assisted chemical etching (MACE) is used to synthesize b-Si by combining low-annealing temperature of silver (Ag) film and short duration of etching of the c-Si wafer. For the b-Si fabrication, p-type (100) c-Si wafers are deposited with 15 nm Ag film using radio frequency (RF) sputtering process. Subsequently, the Ag film is annealed at low temperatures (200-230ᵒC) for 40 min in nitrogen (N2) ambient, producing Ag nanoparticles (NPs). Then, the c-Si wafers with the Ag NPs are etched in a solution containing hydrofluoric acid:hydrogen peroxide:deionized water (HF:H2O2:DI H2O) for a short duration (35-180 s). Effects of etching time, etchant volume ratio and annealing temperature towards surface morphological and optical properties by using atomic force microscope (AFM), field emission scanning electron microscope (FESEM) and UV-Vis-NIR (within 300-1100 nm wavelength region) of b-Si are then investigated. From the investigation, the b-Si with the lowest broadband reflection is produced by annealing at 230ᵒC for 40 min and etched for 70 s using HF:H2O2:DI H2O (1:5:10 by volume). The lowest reflection of the b-Si wafer is 3% at wavelength of 600 nm, with weighted average reflection (WAR) of 8%. The b-Si demonstrates average width and height of about 50-100 nm and 300-400 nm respectively for the nanotextures.
For solar cell fabrication, the b-Si is diffused with phosphoric acid (H3PO4) and 2-butanol using temperatures between 850 to 950oC for 20 min to form front n+ emitter. The emitter is characterized using Hall effect measurement. Ag and aluminium (Al) are thermally evaporated on the front and rear surfaces respectively to form electrical contacts. From current-voltage measurement, the b-Si solar cells demonstrate short-circuit current density (Jsc) of 20 mA/cm2, open-circuit voltage (Voc) of 423 mV and fill factor (FF) of 25.4%. This corresponds to conversion efficiency (η) of 5.2%. On the other hand, the planar c-Si reference solar cells exhibit Jsc of 7 mA/cm2, Voc of 390 mV and FF of 21.7%. The low conversion efficiency of the solar cells is contributed by high shading loss due to the front metal fingers and poor quality of p-n junction formation.
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Physics