Overview of light degradation research on crystalline silicon solar cells

Abstract

Recent research on light degradation of crystalline Si materials and solar cells is reviewed. The first paper on the issue was published in 1973 when efficiency of solar cells using 1 Ω cm, B‐doped CZ wafers degraded under illumination and recovered by annealing at a low temperature of around 200°C. In the 1990's, several studies have been performed to investigate the mechanism of the light degradation and also to provide practical solutions to suppress the degradation. Numerous experiments have been carried out regarding the effects of impurities including B, Ga, P, O and processing parameters such as oxidation temperature. To suppress the degradation, reducing the concentration of B and O or substituting boron by gallium as dopant was found to be effective. These findings are in agreement with a model attributing the lifetime degradation to oxygen and boron. Copyright © 2000 John Wiley & Sons, Ltd.

Crystalline silicon cells and modules in present photovoltaics

“…Crystalline silicon photovoltaic cells are usually fabricated from boron doped P-type Si. The boron dopant has a relatively high segregation coefficient of 0.8 [14] that results in low resistivity variations along the entire length of the boule [15], [16] that increases yield of crystalline silicon material with defined parameters.…”

“…cells fabricated from starting P-type material, both monocrystalline and multicrystalline and allow to reach cell efficiency close to 20%. Unfortunately, boron doped material prepared by Czochralski method or block casting contain the metastable boron-oxygen complexes that lead to the carrier lifetime degradation (after illumination it will rapidly decay from the hundreds of microseconds to the tens of microseconds [15]) and, in consequence, it limits the cell efficiency by 20%. Pophorous doped N-type silicon wafers retain lifetimes on the order of milliseconds under the same stresses [22], [23] and therefore it can be used as a starting material for high efficient solar cells.…”

Effective bulk doping concentration of diffused and undiffused silicon wafers obtained from combined photoconductance and photoluminescence measurements

“…Other calibration methods, for instance the self-consistent method [10], can be used only if the effective lifetimes before and after illumination are identical. This condition is not met in some practical cases, such as the presence of oxygen complexes in p-type CZ substrates [37,38] or iron contamination in boron-doped multicrystalline wafers [39]. This new method is immune from any variation of the effective lifetime, as both Δn pl (t) and Δn pc (t) are measured simultaneously and will be equally influenced by variations in effective lifetime.…”

High efficiency photovoltaics: on the way to becoming a major electricity source

“…However, Cz-Si contains a high concentration of oxygen, which is not active by itself but can lower the lifetimes after forming oxygen complexes in the material. 31 Compared with sc-Si, mc-Si has crystal defects such as grain boundaries and dislocations; moreover, high concentration metals that are introduced in faster solidification will aggravate recombination and lower the lifetimes. 32 In large volume production, the growth environment has more contamination sources, and the impurities and defects are more active after high temperature processing.…”