Contacting bare silicon solar cells with advanced cell metallisation
Improvements in cell metallization result in busbars with high resistance or of non-continuous or nonuniform geometry. Measurements of bare solar cells at calibration laboratories demand a procedure which is free of any influence of lab-specific contacting schemes. For most large area cells with front side contacts shadowing of the active cell area by the contacting unit during current-voltage measurements cannot be avoided. We present an advanced multiple step measurement procedure as performed at the Fraunhofer ISE calibration laboratory which is suitable for many actual metallisation schemes. We explain the requirements for the measurement setup and discuss the influence of contacting resistances. Insights are transferred to current-voltage measurements of cells with an unequal collection of current per busbar. For those cells, we propose a new measurement technique with adjustable resistors to enable homogeneous operating conditions across the cell. We demonstrate the influence on fill factor measurements with circuit simulations and validate our method experimentally with a specially designed solar cell.