N-Type Silicon Wafer Screening for IBC Solar Cells by Different Thermal Treatments

Abstract

A small area (4 cm) baseline process for interdigitated back contact (IBC) silicon solar cells has been developed at imec. This process resulted in cell efficiencies up to 23.3 % on 4-inch n-type float-zone (FZ) substrates. Later, 125x125 mm n-type Czochralski (Cz) wafers were integrated successfully in this baseline with efficiencies up to 22.8 %. Transition of small area cell process to 156x156 mm n-type Cz wafers was implemented but best cell efficiencies topped at 20.4 %. The reason for this limited efficiency was believed to be oxygen precipitation in the bulk of the Cz wafer during the series of consecutive thermal processing steps. In this work we can confirm that the applied 156x156 mm wafers, were indeed more sensitive to oxygen precipitation than the applied 125x125 mm wafers. By this, we demonstrate that oxygen concentration and precipitation in the wafer material should be taken into account for the processing of n-type solar cells. Next, we explain the wafer evaluation method that has been setup in the search for n-type Cz wafers of sufficient quality to fulfil the requirements for our IBC baseline process and demonstrate a processing path which can limit oxygen precipitation. With this alternative low temperature processing path, bulk lifetimes are kept high resulting in cell efficiencies around 23%.