Lightly Doped Emitters for High Efficiency Silicon Wafer Solar Cells

Tube diffusion is the de facto industry standard for emitter formation for silicon wafer solar cells. In this paper, we report on optimised dopant profiles to produce a lightly doped emitter (LDE) with a lower surface dopant concentration. Solar cells are fabricated with a LDE and achieve a significant gain in open-circuit voltage (Voc) due to a reduced front surface and emitter recombination. The POCl3 tube diffusion process is optimised by introducing a pre-oxidation and multi-plateau drive-in during the diffusion process. Using the optimised process, the surface dopant concentration of our tube diffused n+ emitter is reduced from 4.5x1020 to 2.5x1020 atoms/cm3 for a ∼70 Ohms/sq emitter. An increase of 4mV in Voc, 0.2mA/cm2 in short-circuit current Jsc and 0.2% absolute increase in efficiency is achieved for solar cells using a LDE compared to solar cells with our standard emitter profile. The average fill factors for the standard and LDE batches are above 79%, suggesting the Ag paste (DuPont Solamet PV17F) used in this work had no issues in contacting the lower surface concentration of the LDE.