Bulk degradation of P-, Sb- and As-doped Cz Si wafers and TOPCon cells due to light and elevated temperature

Abstract

We assessed light- and elevated-temperature induced degradation (LeTID) in n -type Czochralski (Cz) Si wafers, and industrial tunnel oxide passivating contact (TOPCon) Si solar cells processed with and without the laser-enhanced contact optimization (LECO). A recovery pre-treatment at 20 °C under 2 sun improved wafer and TOPCon cell performance, after which the samples were subjected to LeTID at 120–130 °C under 1 sun. P- and Sb-doped Cz Si wafers showed similar recovery and LeTID behavior, whereas As-doped wafers exhibited higher recovery and degradation, likely due to the differences in grown-in intrinsic defect concentrations. Liquid surface passivation confirmed that changes in the lifetimes during recovery pre-treatment and LeTID are due to the changes in defects in the bulk. Lifetime analysis suggests defects responsible for recovery are either shallow-level defects, or deep-level defects with higher capture cross-section for electrons than holes. The defect responsible for LeTID in n -type Cz Si appears similar in properties to that known in p -type Cz Si. In TOPCon cells that were fired with a higher cooling rate of ∼100 °C/s, LeTID reduced photoconversion efficiency by up to 0.7% absolute. These cells, if processed with LECO, showed smaller changes in device performance parameters during recovery and LeTID. When these TOPCon cells were fired with a slower cooling rate of ∼30 °C/s, performance shifts were minimal. For another fully metallized cell with LECO obtained from an industrial production line, performance shifts were minimal and remained within typical warranty tolerances after LeTID, indicating incorporation of mitigation strategies in current manufacturing.