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Title: Silicon nanowires based inorganic/organic hybrid solar cells
Authors: He, Lining.
Keywords: DRNTU::Engineering::Nanotechnology
DRNTU::Engineering::Electrical and electronic engineering::Nanoelectronics
DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
Issue Date: 2013
Abstract: As the importance of renewable energy source becomes more apparent, tremendous efforts have been devoted to developing photovoltaics (PV) since solar energy is plentiful and widespread. However, the high production cost of the conventional high-efficiency Silicon (Si) solar cells poses a barrier for solar energy to compete with fossil fuels. This has led to the search of novel solar cells that are low-cost and highly efficient. Recently, there has been significant interest in the study of inorganic/organic hybrid solar cells incorporating Si nanostructures such as Si nanowires (SiNWs) and organic semiconductors, to leverage on the advantages of the former such as high carrier mobility and excellent light trapping capability, and the latter that include low material cost, low temperature and solution based process. The objective of this work is to design, fabricate and characterize SiNWs based inorganic/organic hybrid solar cells. Firstly, various Si surface texturing structures such as SiNWs and SiNWs-on-pyramid have been investigated to form hybrid cells based on bulk crystalline Si, and the choice of organic materials includes conjugated polymer and small molecule. Next, the effects of Si/organic interface treatment on the cell performance have been studied. Finally, hybrid solar cells have been successfully demonstrated on epitaxial Si thin films to bypass the use of costly bulk Si wafer. A simple approach has been demonstrated to fabricate high-efficiency hybrid solar cells based on n-type SiNWs and a p-type conductive polymer, poly(3,4-ethylene-dioxythiophene): polystyrenesulfonate (PEDOT:PSS). PEDOT is directly spin coated on vertical SiNWs arrays fabricated by metal-catalyzed electroless etching (MCEE) to form the core-sheath heterojunction. Compared to planar Si/PEDOT cell, the power conversion efficiency (PCE) of the SiNWs/PEDOT cell increases greatly from 6.2% to 9.0%. Cells with different SiNWs lengths are also studied and it is found that enhanced aggregations of longer SiNWs lead to poor surface coverage by PEDOT which has long polymer chain, and hence degrade the performance of the hybrid cells. In order to further improve the light trapping of the device, the Si/PEDOT hybrid solar cells have been fabricated using synergistic surface texturing of SiNWs-on-pyramids binary structure. The fabrication process involves the formation of the SiNWs/pyramids binary structure by a two-step chemical etching process, followed by spin coating of a PEDOT layer. The cells are found to have a maximum short-circuit current density (Jsc) of 31.9 mA/cm2 and PCE of 9.9%, which are higher than similar cells fabricated using planar Si, pyramid-textured Si and SiNWs. A new hybrid cell has been fabricated by incorporating a small organic molecule, 2,2’,7,7’-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9’-spirobifluorene (Spiro-OMeTAD), on SiNWs arrays to form a new core-sheath heterojunction. The small molecular size of Spiro-OMeTAD enables it to infiltrate well into the small gaps among the SiNWs arrays, leading to larger heterojunction area and better passivation on the SiNWs surface. A maximum PCE of 10.3% has been obtained from the SiNWs/Spiro cell with a 0.35-µm long SiNWs. The PCE is the highest reported to-date in the field of SiNWs/organic hybrid solar cells. The characteristics of the hybrid cells are investigated as a function of SiNWs length from 0.15 to 5 μm. It is found that enhanced aggregations in longer SiNWs still limit the SiNWs/Spiro cell performance due to increased series resistance and higher carrier recombination in the shorter wavelength region. The effects of Si surface native oxide on the performance of hybrid cells have been studied. The cells were fabricated based on PEDOT and planar Si wafers with (100) and (111) orientations. Compared to cell with hydrogen-terminated Si surface, the cell with oxygen-terminated Si surface reveals a 530-fold increase in PCE from 0.02% to 10.6%. The formation of SiOx-Si bonds poses a net positive surface dipole which leads to a favorable band alignment for charge separation. However, for thicker oxide the cell performance is degraded due to higher series resistance. The PCE of 10.6% demonstrates the highest PCE reported to-date in the field of planar Si/organic hybrid solar cells. The Si/PEDOT hybrid cells have been further fabricated on a 2.2 μm epitaxial Si absorber thin-film. The Jsc and PCE of SiNWs/PEDOT cell increases from 12.5 to 13.6 mA/cm2 and 5.4% to 5.6%, respectively, as compared to planar epitaxial Si/PEDOT cell. A maximum external quantum efficiency (EQE) of 56.6% is obtained for the SiNWs/PEDOT cell. Our results have demonstrated a promising route to realizing low-cost and efficient Si or SiNWs/organic hybrid solar cells using low-cost Si thin films instead of costly bulk Si wafers.
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