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湿式化学成長した超極薄AlドープSiOxにより形成されたAl誘起酸化物反転層を有する新規Si系太陽電池

Author(s)中島, 寛記CitationIssue Date2024-03

TypeThesis or Dissertation

Text version

ETDURLhttp://hdl.handle.net/10119/19072

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Supervisor: 大平 圭介, 先端科学技術研究科, 博士

Japan Advanced Institute of Science and Technology

ABSTRACT
Carrier-selective passivating contacts would realize excellent surface passivation for crystalline Si (c-Si) solar cells witha high conductivity without direct metal electrode contacts, causing a deterioration of the interface properties. Hence, itwould be a breakthrough technology to access a practical limit of solar cell performance.
This thesis addresses using a simple process to realize low-cost and high-efficiency c-Si solar cells with asimple device structure of carrier-selective passivating contacts whose emitter is induced by external electrostatic charges.
The main major contribution of this work is developing a novel surface passivation method using a simple wet
chemical process, which does not require vacuum, high temperature, or hydrogen treatment processes, which are essential
processes in industrial c-Si solar cells. The process is just immersing Si wafers into a hot aluminum nitrate (Al(NO 3)3)
aqueous solution to form an ultrathin Al-doped SiOx layer possessing hole selectivity. On a planar n-type c-Si surface, themaximum effective surface recombination velocity (Seff, max) reached as low as 16 cm s−1. The analyzed saturation current
density (J0) of 65 fA cm−2 and contact resistivity (ρc) of 20 mΩ cm2, the hole selectivity (S10) value could be estimated tobe 13.3 which is competitive to other selective contacts created using vacuum and high-temperature process. Since theprocess eliminates the complexity of solar cell fabrication processes with an excellent carrier-selective passivating contact
property, this study suggests that a wet chemical process using an aqueous solution of Al(NO3)3 can be an industrial solar
cell fabrication process. In addition, the mechanism of the hole-selective passivating contact property was revealed. The
field-effect passivation is mainly attributed to the surface passivation.
The second contribution of this work is a demonstration of the device operation of a novel device structurewith an inversion layer induced by the negative fixed charges existing in ultrathin Al-doped SiOx. This result implies thatthe charge-induced inversion layer acts as a p+ emitter formed by a simple wet chemical process. Hence, using a surface
charge-induced emitter, this type of solar cell realizes high-throughput, low-cost p+ emitter formation by a simple wet
chemical process. This novel solar cell was named an Al-induced charged oxide inversion layer (Al-COIL) solar cell. The
conversion efficiency (η) of >10% was achieved for the device fabricated on a pyramidal textured Si substrate. In
particular, an excellent FF of 0.794 was realized using a fine Ag grid electrode on the emitter side. The reduction due toa high emitter sheet resistance could be mitigated by improving carrier collection through a fine Ag grid electrode covering
a relatively larger emitter surface area. However, still low Jsc and Voc were confirmed. PC1D simulation implies that thereductions might originate from carrier recombination at the emitter surface due to the large number of interface states
and the increased minority carrier (electron) density by the diminished fixed charge density (Qf) less than −1×1011 cm−2.
The third contribution of this work is a discovery that surface passivation, and the sheet resistance of ultrathin
Al-doped SiOx/Si improve by the light illumination, which is the variable for addressing the origin of the hole-selective
passivating contact property. The significant increment of these properties implies that the excited electron trapping in
the Al-induced acceptor states existing in the Al-doped SiOx layer forms negatively charged AlO4− structures and enhancesfield effect passivation induced by the fixed charges.
Lastly, achieving an excellent surface passivation quality on a pyramidal textured c-Si surface was crucial. Thelower Seff, max was confirmed as low as 13.3 cm s−1, corresponding to iVoc of 682 mV and J0 of 59 fA cm−2, respectively.
The light-trapping enhancement and slight space charge region compression compensate for the relatively enormous
amount of interface state density due to the surface area increment on the textured Si surface, reducing surface
recombination velocity and increasing solar cell performance. Hence, this study provides insight into the novel surface
passivation mechanism on a practical surface structure for the c-Si solar cell. It requires a suitable surface structure design
with a nano-scale size for increasing a hole-selective passivating contact by effectively exploiting electron trapping andspace charge region compression.
Keywords: c-Si solar cells, carrier-selective passivating contacts, electrostatic charges, ultrathin Al-doped SiOx, wetchemical oxidation