Effect of Series Resistances on Conversion Efficiency of GaAs/Si Tandem Solar Cells With Areal Current-Matching Technique

The electrical losses due to the series resistances-the bonding resistance and sheet resistance-in a two-terminal (2T) GaAs/Si tandem solar cell that implements the areal current-matching (ACM) technique was investigated. A 2T GaAs/Si ACM tandem solar cell has a smaller top cell area and a larger bottom cell area, to achieve current matching between the top and bottom cells. The effect of the series resistances on the performance of such an ACM tandem cell was evaluated through device simulations, in which the subcell area ratio (bottom cell area / top cell area) and series resistances were varied under 1-sun and 3-sun conditions. The simulation results revealed that the optimal subcell area ratio at which the tandem cell exhibits the highest efficiency is affected mainly by the bonding resistance, due to a unique characteristic of the fill factor. A larger bonding resistance results in a smaller optimal subcell area ratio, i.e., a larger top cell area, which requires the use of a larger amount of expensive III-V materials. This trend is more prominent under 3-sun conditions. The indoor experiments conducted on an in-house 2T GaAs/Si ACM tandem cell verified the simulated results and showed that a lower external quantum efficiency (EQE) also resulted in a smaller optimal subcell area. Therefore, low bonding resistance and high EQE are of primary importance for realizing high-efficiency low-cost 2T ACM tandem solar cells.