III-V-Based Photovoltaics for the »Internet of Things« and Indoor Spaces

In our increasingly digitalized and interconnected world, the “Internet of Things (IoT)” is becoming more and more important. New applications are rapidly arising here for consumers, commerce, industry, infrastructure and the energy system. In most of these applications, there is a call for small and wireless devices with integrated sensors, which can simply be positioned anywhere. Two decisive factors in the development of such devices are the implementation of bi-directional communications and the power supply. Autonomous solutions for the latter task are interesting to enable truly self-sufficient operation without an external supply of energy. This means that neither cables nor batteries and their maintenance are needed. Alternatively, a wireless optical power supply that converts light from an external source can be implemented (Power-by-Light).

We are addressing these topics at Fraunhofer ISE with III-Vbased photovoltaic cells. Photovoltaic cells based on gallium indium phosphide have material properties which suit them well for conversion of indoor light. Their absorption behavior ideally suits the emission spectra of artificial light sources (white LED sources and fluorescent lamps), and the solar cells feature excellent low-light behavior due to the high material quality. In initial measurements with a 1 cm2 cell, we achieved an efficiency value of 15.5 % under low light conditions (1.7 W/m2 corresponds to about 210 lux).

For applications in the »Internet of Things (IoT)«, we are also working on technology which allows a power supply and data communication to be integrated into a single component. This makes compact designs feasible and saves costs. For simultaneous optical wireless information and power-transfer (SWIPT), Fraunhofer ISE developed a gallium arsenide-based photovoltaic cell. With the support of the University of Edinburgh we have recorded a peak data rate of 0.5 % Gb/s (3dB bandwidth: 24.5 MHz). Under monochromatic illumination this cell was characterized by an efficiency value of 42 % (847 nm, 0.46 W/cm2). A comparable cell, which was prepared by thin-film technology with a back-surface mirror, featured a peak monochromatic efficiency value of 67 % (860 nm, 10 W/cm2).

In addition to highly efficient conversion of the incident light (photovoltaic cell) and fast data reception (photodiode) we are currently working on the option of using the same cell also as a light-emitting data transmitter (LED). Efficient light extraction is achieved by the high radiative efficiency of the III-V materials.