Limiting efficiency of indoor silicon photovoltaic devices.

Energy harvesting from ambient light can be used to power wireless sensors and other standalone electronic devices. The intensity of light used for illumination is 300-3000x lower than sunlight and the spectrum of artificial light is normally narrowly concentrated in the visible range. As a result, the optimal design of photovoltaic devices for energy harvesting from ambient light differs from conventional solar cells. We calculate the maximum efficiency for Si photovoltaic devices operating under conditions expected indoors as a function of the cell thickness, taking into account the relevant properties of Si. The optimum thickness for devices operating under 250 lux illumination produced by white LED's is 1.8 µm and the efficiency is 29.0%, whereas for direct sunlight, the optimum thickness is much larger at 109 µm, while the maximum efficiency is almost the same (29.7%). The relative efficiency increases logarithmically with light intensity at 8.5% per decade.