Effects of high photon gas density and radiative efficiency on upper bounds of energy conversion efficiency in single-crystal solar cells

Abstract

The strong dependence that open-circuit voltage has on photon recycling may be the key to push photovoltaic cells closer to their Shockley-Queisser efficiency limit. Photon recycling can be improved by the use of a highly reflective mirror at the back-surface. Therefore, it is important to maximize the mirror area while maintaining sufficient conductivity for carrier collection. In this work, efficiency limit calculations are used to model rear surface reflector effects on cell performance while varying reflectance and surface area of the rear reflector. The lateral and contact resistance due to the back-surface field layer and point contact geometry are then calculated. Finally, point contact size and coverage are optimized to maximize benefits from rear reflectance, while providing enough contact coverage to minimize resistive power loss. As a result, an optimized one-sun efficiency of 32.7% is calculated for GaAs solar cells with a practical, rather than idealized, back surface reflector structure. The optimum taking into account the competing effects of enhanced photon recycling vs. back contact series resistance occurs at less than 1% point contact coverage for 4 µm diameter contacts which can be achieved using standard photolithography and physical vapor deposition techniques.