LOW-TEMPERATURE DRIFT FIELD PERFORMANCE IN THE BASE REGION OF CRYSTALLINE SILICON SOLAR CELLS BY HYDROGEN ENHANCED THERMAL DONOR FORMATION

摘要

An innovative method for the low-temperature drift-field formation (graded doping) in p- and n-type type Czochralski-grown (Cz) Si substrates for solar cells is developed. The method is based on the hydrogen enhanced thermal donor (TD) formation process (electrical activation of oxygen-related complexes) in local regions saturated by atomic hydrogen. Two process routes are discussed: (1) a one-step-process, where the gradient doping appears during the hydrogen exposure at 350-450°C; (2) a two step process, where the gradient doping requires annealing at 400-450 °C after a plasma treatment at 250 °C. It is shown that the controlled hydrogen enhanced TDs formation leads to the creation of doping gradients spanning over several tens of microns as required for drift field solar cells. The kinetics of the formation of the doping gradients caused by the gradients of TDs is investigated. It is shown that the difference in doping concentration between the surface region and the bulk of the p-type Si substrates is due to acceptor compensation by TDs. Our study gives a method for a simple low-temperature technology for the creation a drift field in Cz Si. This method can be used as the first step for a low-temperature (< 400 °C) solar cell processes or as the last step for the high-temperature ones.