Mercury: A Back Junction Back Contact Front Floating Emitter Cell with Novel Design for High Efficiency and Simplified Processing

The back junction back contact cell, and more specifically the interdigitated back contact (IBC) cell is among the most appropriate cell designs to achieve highly efficient solar cells. An important aspect to improve manufacturability (e.g. reduce cost) of the cell and module is to increase the rear side back surface field (BSF) region width, as this currently constitutes the smallest feature size in the diffusion pattern of the IBC cell. We propose a novel design of an IBC cell that enhances the effective lateral transport of minority carriers (holes) and therefore allowing wide BSF regions. The novel design feature is to implement an appropriate conductive and well passivated p++-doped layer, referred to as a front floating emitter (FFE), on the front surface of the IBC cell. This conductive FFE enables equally-sized interdigitated doping patterns of positive and negative polarities on the rear, with similar cell pitch and efficiency compared to traditional IBC cells. It also enables larger interconnection pads for easier module interconnection with marginal loss of cell performance. Additional advantages are expected from relaxed alignment tolerances for patterning as well as interconnection. We report on the proof-of-principle of this new cell concept, which we name “Mercury”, brought forward by 2D simulations and experimental results on small and 6 inch cells, based on an industrial process flow with stable results. So far, these cells yield efficiencies up to 19.4% and short-circuit densities well above 41 mA/cm2. Additionally, the Mercury cells show less efficiency loss at low illumination intensity than a standard p-type H-pattern cell.