Design of PV Cells and LEDs Robust to Grid Shadowing Losses in Emission.

Abstract

In photovoltaics, it is generally assumed that the emission and absorption efficiency is linearly affected by the grid coverage fraction. Typically, the top grid is therefore optimized to allow maximal light exposure with minimal electrical resistance, while the optical properties of the grid are not treated to the same extent. In this work, we provide a numerical study that shows that as a result of the optical properties of the grid, the light extraction efficiency and resulting emission changes nonlinearly with grid coverage, contrary to the standard approximation. If the grid is optically lossy while light is mostly trapped in the diode, the loss in emission is more than linear and therefore larger than expected based on the standard grid shadowing assumption. However, with an optically reflective grid and a good light extraction scheme, the structure obtains a robustness against losses from grid, leading to a meaningful increase in the light extraction efficiency. This is shown using a simple 300 nm GaAs light-emitting diode (LED) structure the emissive properties of which generalize to a thin-film PV cell. Specifically, it is found that depending on the design of the grid and backside mirror, at 10% grid coverage the light extraction efficiency need only be reduced less than 4% absolute. Conversely, in particularly detrimental cases, at 10% grid coverage the light extraction efficiency is reduced by over 35% absolute.