Theoretical efficiency limits of a 2 terminal dual junction step cell

In this paper we present theoretical analysis for upper efficiency limit of a novel 2 terminal dual junction stepcell. Results show that step cell design relaxes bandgap requirements for efficient tandem cell. While conventional tandem cell with optimized bandgap combination (1.64 / 0.96 eV) has the highest efficiency (45.78 %), the step-cell design provides significant efficiency improvement for cells with non-optimized bandgap values. Efficiency upper limit for Si based step-cell with top cell bandgap equal to 1.41 eV (~ GaAs), efficiency upper limit increases from ~21% in conventional tandem cell to 38.7% for optimized step-cell design. Step-cell design provides opportunity for wider selection of materials used in tandem solar cell applications.