Electronic properties of highly-doped and compensated solar-grade silicon wafers and solar cells

Abstract

Compensation effects are intensively studied on two highly doped ingots grown from solar-grade silicon feedstocks purified using metallurgical routes, through a comparison of the electrical properties at iso-carrier densities. Working at given carrier densities enables a clearer extraction of the compensation effects, at the wafer and solar cell levels. At the wafer level, the majority carrier mobility and the carrier lifetime are investigated. Regarding the mobilities, it was found that current models may underestimate the amount of incomplete ionization of boron leading to underestimated mobilities. In addition, the majority carrier mobility was found to be strongly affected at high compensation level. Regarding the carrier lifetimes, our results show that after a phosphorus diffusion step, dopants alone — and especially boron — can limit the lifetime in highly doped solar-grade silicon. At the cell level, I-V characteristics under standard illumination were studied. In particular, the observed reductions in short-circuit current on solar cells having a very high compensation level could be explained in terms of a compensation-induced reduction in the minority carrier mobility. We also report high conversion efficiencies of up to 15.9% on solar cells showing a boron content greater than two ppmw (2.6 × 1017 cm−3), which is generally considered unsuitable for solar cell manufacturing.