C8.3 - Photovoltaic Energy Harvesting under Low Lighting Conditions

The design rules for standard outdoor solar cells are not applicable for cells which are used under low lighting conditions and spectra deviating from AM 1.5. This paper will discuss the major influences on cell efficiencies and their impact on the design of photovoltaic cells for energy self- sufficient sensor systems. This is done for different material classes like crystalline silicon, amorphous silicon and III-V materials. Keywords-energy harvesting; indoor photovoltaics; low- lighting conditions; photovoltaic cells I. INTRODUCTION Nowadays, solar cells are optimized with respect to the standard test conditions (STC) with the AM 1.5 sun spectrum and an intensity of 1000 W/m 2 . In fact, these conditions are rarely given in outdoor applications (1), (2) and especially not for indoor or low lighting applications. A commercially available crystalline silicon solar cell with an efficiency of 18%abs at STC may only provide efficiencies of less than 5%abs when it is operated at typical indoor intensities (3), (4). Similar relations hold for other technologies. The lack of dedicated research on optimizing photovoltaic devices has hindered potential improvement so far. This paper describes the main influences on cell efficiencies that have to be taken into account for a future photovoltaic cell optimization.