Characterization of spherical Si by photoluminescence measurement

Abstract

Spherical silicon (Si) with a size of ∼1mm diameter was fabricated by the dropping method for the applications of spherical Si solar cells. In this research work, we characterized spherical Si by means of photoluminescence (PL) measurement at 4 and 18K. The horn-type spherical Si crystals, formed under large undercooled conditions without a seeding technique, showed D-band luminescence originating from dislocations, whereas intrinsic PL bands of Si were not observed. In contrast, for the tear-type spherical Si crystals, formed under shadow undercooling by a seeding technique with Si powder, the boron (B) bound and Si intrinsic phonon-assisted PL bands were clearly observed both at 4 and 18K. Moreover, the intensity ratio of B bound exciton band to Si intrinsic phonon-assisted PL band showed good correlation to the minority carrier lifetime measured with microwave photoconductance decay method. These experimental results suggested that the crystallinity of the tear-type spherical Si is significantly improved by the seeding technique compared with the horn-type ones, which contain a large amount of B-related defects.

A concentrator module of spherical Si solar cell

“…Since the volume of Si material increases by a factor of 1.1 upon solidification, the increased portion accumulated inside the droplets. This process generated large strain, resulting in a polycrystalline structure that contained high-density defects, dislocations and large grain boundaries [12,13]. The Si sphere formed under these conditions had an almost precisely spherical body with a small horn (denoted as horn-type).…”

“…The details concerning the formation and the characterization of Si spheres with the drop method of this work are described elsewhere [11][12][13]. Briefly, scrapped tops of Czochralski ingots of electronic industry were crushed into small bulks to be used as source material.…”

Mechanism of electrical passivation of Si surfaces with quinhydrone

“…Chemical passivation using quinhydronemethanol on HF-terminated surfaces has been used previously for temporary passivation [6]- [12]. The silicon substrates used for optimizing the quinhydrone concentration is FZ n-type, Si <100> orientation, 460 µm thick, 100 ohm-cm resistivity.…”

Investigation on the crystal growth process of spherical Si single crystals by melting

“…One can image that nucleation randomly starts at impurities in the droplets, releasing latent heat that causes the temperature to increase closely below the melting point. Spherical Si formed by this method suffered from poor crystallinity, containing high-density grain boundaries, point defects and dislocations, which act as recombination centers for photo-generated carriers and thus reduce the efficiency of solar cells [6][7][8][9].…”