What Limits the Voltage of GaAs Photovoltaic Devices under Low Indoor Light?

The external radiative efficiency of a photovoltaic device is a useful metric to compare different photovoltaic technologies and material systems. However, under low‐intensity indoor lighting, a particular open‐circuit voltage dependence arises that cannot be described using the assumption of a constant external radiative efficiency, as often assumed at higher illumination intensities. As a result, the external radiative efficiency at 1 sun conditions no longer becomes a useful comparison metric for devices at indoor lighting levels. By use of absolute electroluminescence measurements of gallium arsenide photovoltaic (PV) cells, the current density dependence of the external radiative efficiency is explored and it is shown that the functional form of this dependence dictates the open‐circuit voltage losses in the low‐intensity limit. This dependence is quantified with a mathematical treatment that takes advantage of a widely accepted recombination current model in III–V photovoltaic devices, and this model is used to study the dependence of PV device open‐circuit voltage at low intensity with varying non‐radiative recombination rates. This work uses absolute electroluminescence measurements of GaAs photovoltaic cells to explore the current density dependence of the external radiative efficiency in these devices. Measurement results and modeling work show that the functional form of this dependence dictates the open‐circuit voltage losses in the low‐intensity limit appropriate for indoor photovoltaics.