Sheet Resistance Optimization in (Al)GaInP Solar Cells for Concentrator Quadruple-Junction Solar Cells

The reduction of the series resistance in multi-junction solar cells is of high importance for attaining peak efficiencies in concentrator photovoltaics. This study showcases strategies to reduce the sheet resistance of the uppermost subcell of a direct wafer bonded four-junction devices, since it contributes significantly to the series resistance. Therefore, electron mobilities in n-type AlGaInP, lattice matched to GaAs, are investigated across bandgap energies between 1.9 eV and 2.1 eV and for various doping concentrations. The sheet resistances for AlGaInP rear heterojunction cells are determined for the integration in a two terminal four-junction solar cell. The rear heterojunction cell architecture effectively addresses the sheet resistance optimization because it features a thick n-type doped absorption layer. The sheet resistance of our current world record quadruple-junction solar cell with 47.6% efficiency under the 665-fold concentrated AM1.5d spectrum is 550 Ω sq-1. Here, we show how optimizing the n-absorption layer in the 1.90 eV GaInP top cell can reduce the sheet resistance to 250 Ω sq-1 without deteriorating the short-circuit current density and current matching. This reduction corresponds to a 5 mΩ cm2 improvement in specific series resistance, which would elevate the efficiency of this device to 48.2%.