FFE IBC cells: Impact of Busbars on Cell Performance with Circuit Modelling

IBC cells with Front Floating Emitter (FFE) pose different design challenges compared to more conventional IBC cells with FSF (Front Surface Field). The FFE enables hole transport over distances that are large compared to the typical BSF or emitter width. The core of the cell design is commonly a device simulation in which, because of the computer resources involved, typically one simulates an as small as possible, but representative part of the solar cell. In an IBC cell this corresponds to ½ of the BSF and ½ of the emitter. Such a unit cell does not account for important geometric features, such as busbars and pads, edges or interruptions in metallization fingers. We show how to construct an equivalent circuit for our Mercury FFE IBC cells to model features beyond the unit cell efficiently, taking into account the lateral hole transport in the FFE. We show how its predictions can be correlated with voltage maps of the FFE voltage, and used for optimum design of for instance busbars. The highest efficiency of our Mercury FFE IBC cells to date is 20.9% with a short-circuit current of 41.3 mA/cm2, obtained with screen printed contacts.