Radical-driven silicon surface passivation for organic-inorganic hybrid photovoltaics
Date
2018
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Surface passivation has become increasingly crucial for thin silicon photovoltaic devices. Quinhydrone/ methanol (QHY/ME) has been utilized in the past as a temporary passivant for silicon surfaces with an outstanding lifetime. The work described in this thesis investigates the passivation mechanism of quinone molecules on silicon. The passivation behaviors of free radicals in quinone molecules, and other radical sources like photoinitiators, are discovered for the first time. ☐ This work confirms that radical intermediates are the reactive species in quinhydrone/ methanol passivation on silicon surfaces. The two constituent parts, p-benzoquinone(BQ) and hydroquinone(HQ), have been studied separately. BQ abstracts the hydrogen atom from methanol to become semi-quinone radicals (QH*). Both QH* and the resulting methanol radical are responsible for the large, instantaneous increase in minority carrier lifetime in BQ/ME, obtaining the lowest surface recombination velocity of 1.6cm/s. HQ releases a hydrogen atom to become QH*. This radical-driven passivation mechanism is also valid on other radical sources like photoinitiators and weak bonds like C-Cl. ☐ The chemical passivation mechanism was further investigated by X-ray photoelectron spectroscopy (XPS), which confirmed the bonding of aromatic groups to the surface. Density functional theory (DFT) results support the possibility of QH* bonding from a thermodynamic perspective. The methanol radical has the most favorable adsorption energy for bonding, followed by QH* and then BQ molecule. ☐ The electronic structure of BQ/ME passivated Si is determined by a combination of the surface band bending and electron affinity/dipole. Both the photoemission and Scanning Kelvin Probe Microscopy (SKPM) techniques indicate a downward band bending of H-Si and BQ treated samples. DFT calculations show that a negative dipole is formed upon bonding of BQ radicals on the surface, decreasing the surface electron affinity and work function. Both the negative dipole and downward band bending contribute to the formation of electron accumulation on n-Si by BQ bonding resulting in the observed surface passivation. ☐ Hybrid organic/silicon devices combining PEDOT:PSS on Si with BQ/ME as a surface passivant were fabricated. The introduction of the BQ passivating layer does not provide a barrier to charge transfer. A device efficiency of 9.6% was achieved. Quantum efficiency data shows a good light absorption near the front of the cell indicating a well-passivated front surface. ☐ At last, another alternative passivation method -- SiOC passivation was studied, where the SiOC films were deposited with plasma-free ultra-low-temperature ALD. The surface passivation effect and stability of the SiOC films were compared with the quinone passivation.
Description
Keywords
Pure sciences, Applied sciences, Chemical passivation, Photovoltaic, Quinhydrone/ methanol, Surface passivation