Design of Solar Cells p⁺/n Emitter by Spin-On Technique

Abstract

In this paper spin-on dopant diffusion has been investigated as a technique for fabrication of p + /n monocrystalline silicon solar cell emitters. A homogeneous spreading onto the front wafer surface has been achieved by using 2 ml of boron-dopant solution and three-step spin-profile. Study of the wafers stacking arrangement has revealed that the highest doping level and the best emitter sheet resistance uniformity were obtained using the back-to-back wafers arrangement. The N2/O2 gas ratio variation during the diffusion process has shown that a higher percentage of nitrogen yields a slightly lower emitter sheet resistance. Study on temperature dependence of as-processed emitter resistivity revealed that 910• C results in targeted sheet resistance of around 48 Ω/sq. Using these preliminary experimental results, a batch of 6 silicon wafers was processed. After BSG and BRL chemical removal, the batch average sheet resistance of the emitter was 49.50 Ω/sq. The uniformity of a wafer and of the batch was below 7% and 13%, respectively. The ECV and SIMS depth profiling have shown the electrically active and the total boron surface concentration of 1.5 × 10 20 atoms/cm 3 and 2.5 × 10 20 atoms/cm 3 , respectively. The junction depth was around 0.3 µm. Finally, by increasing the oxygen flow rate we reached an average sheet resistance of 51 Ω/sq. and a junction depth of 0.35 µm.