Abstract
In this study we investigate the electronic transport, the optical properties, and photocurrent in two-dimensional arrays of silicon nanocrystals (Si NCs) embedded in silicon dioxide, grown on quartz and having sizes in the range between less than 2 and 20 nm. Electronic transport is determined by the collective effect of Coulomb blockade gaps in the Si NCs. Absorption spectra show the well-known upshift of the energy bandgap with decreasing NC size. Photocurrent follows the absorption spectra confirming that it is composed of photo-generated carriers within the Si NCs. In films containing Si NCs with sizes less than 2 nm, strong quantum confinement and exciton localization are observed, resulting in light emission and absence of photocurrent. Our results show that Si NCs are useful building blocks of photovoltaic devices for use as better absorbers than bulk Si in the visible and ultraviolet spectral range. However, when strong quantum confinement effects come into play, carrier transport is significantly reduced due to strong exciton localization and Coulomb blockade effects, thus leading to limited photocurrent.
Highlights
Silicon nanocrystals (Si NCs) embedded in dielectric matrices such as silicon dioxide or silicon nitride have unique electrical and optical properties which are determined by quantum size and Coulomb blockade effects [1,2,3]
Photocurrent spectra followed absorption, revealing that photocurrent is due to electron hole generation within the silicon nanocrystals (Si NCs)
Excitons generated by light absorption within the Si NCs were strongly localized, and no photocurrent was measured
Summary
Silicon nanocrystals (Si NCs) embedded in dielectric matrices such as silicon dioxide or silicon nitride have unique electrical and optical properties which are determined by quantum size and Coulomb blockade effects [1,2,3]. A fundamental problem with the existing silicon (Si) photovoltaics is that a significant part of the solar cell spectrum in the ultraviolet region, i.e., at energies much higher than the bandgap of silicon, is absorbed creating hot electrons and holes which relax to their respective band edges, losing their energy as heat through electron-phonon scattering and subsequent phonon emission. This effect poses a limit to the conversion efficiency of the cell. One way to increase the conversion efficiency beyond this limit is to use a tandem cell, i.e., a stack of absorber layers with different bandgaps to cover a larger range of the solar spectrum
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