Lateral transport in silicon solar cells
Jan Haschke, Gabriel Christmann, Christoph Messmer, Martin Bivour, Mathieu Boccard, Christophe Ballif
Abstract
We investigate lateral charge carrier transport in crystalline silicon solar cells. Under typical operation illumination of high-efficiency solar cells, a significant population of electrons and holes exist in the silicon wafer, leading to a non-negligible sheet conductance for both carrier types. To investigate the contribution of these sheet conductances to lateral transport in solar cells, we develop a model that calculates the effective series resistance of two sheet resistances coupled via a contact resistance. In solar cells, the upper sheet resistance describes the highly conductive region like a diffusion or a transparent conductive oxide, whereas the lower sheet resistance describes the silicon absorber. We find that the coupling contact resistance needs to be low to benefit from the lateral current flow in the silicon absorber. We show experimentally for silicon heterojunction solar cells that the silicon absorber supports lateral minority charge carrier transport for well-passivated devices. Another finding is that there is no principle advantage for coupling of the two sheet resistances for rear-junction or front-junction solar cells, as the pn-junction (for front-junction solar cells) does not prevent coupling. We suggest that for n-type silicon heterojunction solar cells, the observed advantage of the rear-junction over the front-junction architecture is due to practically lower contact resistance and higher mobility of electrons vs holes. We also confirm experimentally the importance of a low contact resistivity between the highly conductive region and the silicon absorber for effective coupling and present an innovative technique to extract contact resistance from comparing Suns-VOC and current–voltage measurements.