EN19.09.11 : Powering Internet of Things with Indoor Lighting

5:00 PM–7:00 PM Apr 4, 2018

PCC North, 300 Level, Exhibit Hall C-E

Stuart Bowden1 André Augusto1

1, Arizona State University, Tempe, Arizona, United States

Wireless communication is essential for the vision of internet of things. A true wireless world calls for wireless power solutions both indoors and out. In this work, we present a detailed study of solar cell performance under indoor lighting conditions. In the past, crystalline solar cells were disregarded due to low performance under low illumination combined with relatively high cost. New solar cell designs now allow crystalline silicon solar cells to reach efficiencies close to 27%, and their cost has fallen 70% in the last 5 years. The conditions and behavior of solar cells under indoor lighting are very different from standard AM 1.5 conditions of full sunlight. At AM 1.5 the spectral range is broad and the power density is 1000 W/m2. Most indoor light is in the visible range of 400-750 nm, and the intensities are in the range of 1-10 W/m2 (100 to 1000 lux), i.e. 100-1000 times lower power than full sunlight. We show that the poor performance of crystalline silicon solar cells under indoor lightening is due to shunt resistance. Shunt resistivities (Rshunt) of kΩ.cm2 have negligible effect on the solar cell performance under outdoor light conditions but it is catastrophic under low light. For Rshunt of few kΩ.cm2, the efficiency of a solar cell drops from 22% to less than 5% under indoor lighting conditions. Even for the best commercial cells with Rshunt values of 10-100 kΩ.cm2 the efficiency drops below 10% under low light. However, efficiency under low light conditions is significantly improved for Rshunt greater than 1 MΩ.cm2. With Rshunt greater than 1 MΩ.cm2, a silicon solar cell with an AM1.5 efficiency of 22% shows an efficiency of 17% for illumination of 1W/m2. Such high Rshunt are typical on silicon heterojunction solar cells. These solar cells are shown to outperform cells such as amorphous silicon that are often used for indoor light conversion. Typical solar cell illuminated IV measurement systems do not accurately report the Rshunt values over 10 kΩ.cm2. In this work, we used a more sensitive source measure unit. These results show the potential of silicon heterojunction solar cells to power indoor devices. Moreover, the narrow and short wavelength range means that indoor silicon cells no longer need light trapping or thick substrates so that only 1-10 µm of material is needed. Thinner substrates lead to flexible solar cell devices. Finally, using crystalline silicon based solar cells provides an opportunity to integrate the logic circuits and electronics in the same substrate.