Phosphorous Diffusion Gettering of Trapping Centers in Upgraded Metallurgical-Grade Solar Silicon

TL;DR

This study demonstrates that phosphorous diffusion gettering significantly reduces trapping centers in upgraded metallurgical-grade silicon, leading to substantial improvements in minority carrier lifetime, which is crucial for solar cell efficiency.

Contribution

It provides experimental evidence of PDG's effectiveness in removing traps and enhancing lifetime in UMG-Si, a less conventional silicon material.

Findings

Trap centers are effectively reduced after PDG at 780°C.

Minority carrier lifetime improves up to 18-fold, reaching 70 microseconds.

A second, shorter-lived trapping mechanism persists post-gettering.

Abstract

Experimental evidence indicating the beneficial impact of a phosphorous diffusion gettering (PDG) in the reduction of trapping centers is shown, as observed by means of inductively coupled photoconductance (PC) decay and lifetime measurements carried out on upgraded metallurgical-grade silicon (UMG-Si) wafers. The presence of trapping species dominating the long time range of the PC decay of UMG material (slow traps), which is effectively removed after a PDG conducted at 780 Celsius, is detected. Notwithstanding, a second trapping mechanism, characterized by a shorter time constant, still governs the response at very low injection levels after the gettering. Furthermore, the beneficial effect of the PDG is studied as a function of processing time, showing minority carrier bulk lifetime improvements up to 18-fold, up to the range of 70 us. Thereby, the way for developing gettering strategies capable of successfully removing trap centers and improving the bulk lifetime of unconventional Si material is paved.