Solar cell efficiencies have grown in recent years, but further improvements must be made in order for this sustainable energy technology to see widespread commercial use. Traditional solar cells use a junction near the top surface of the cell to separate charge carriers to create electric current; however, with new advances in technology and improved material quality, the role of the junction has become less clear. Recently designed high-efficiency solar cells have taken advantage of high charge carrier lifetimes to shrink the base and move the junction toward the back of the cell, away from the source of carrier generation. For example, in 2013, a GaInP solar cell was created using a rear-junction design with a base width of just 40 nanometers, yielding a record single-junction efficiency of 20.8%. The reason for this improvement, however, is not well understood. In this study, we develop a model of this record efficiency cell in a numerical device simulator to discover the mechanisms leading to the rise in efficiency. By matching simulation parameters with experimental and theoretical characteristics, we are able to show that the large electric field at the rear junction may diminish recombination due to defects in the bulk region in the cell. We also demonstrate consistent improvement in cell efficiency as the junction is moved toward the back of the GaInP cell. These results provide us with a deeper understanding of present-day high-efficiency solar cell operation and suggest how future efficiencies can be pushed closer to their theoretical limit.