Design and characterization of multijunction photovoltaic devices for optical wireless power transmission

Optical wireless optical power transmission (OWPT) is a promising technology for a variety of data communication and sensing applications. It has superb properties compared to conventional wired power transmission methods, such as faster communication rate, longer detection range, better heat management capability especially for compact systems, better durability in various operating conditions, enhanced safety, lighter weight of the module, and improved overall robustness of these devices. A typical OWPT module consists of a light emitting source, such as light-emitting diodes (LEDs), vertical-cavity surface-emitting lasers (VCSELs), or edge-emitting lasers (EELs) which convert the input electrical power at the source into optical power. The output optical power from the light source is transmitted through an intermediary medium (such as air, optical fiber, waveguide) to a detector at the receiving-end of the module. The detector is usually based on an array of series-connected photovoltaic (PV) cells, in which it converts the absorbed optical power back into the electrical power as the system output. In this paper, we will demonstrate the smallest-to-date micro-PV array that can be used as the detector in the OWPT modules. Each array consists of multijunction PV cells with active areas between 30-150μm and are fabricated on semi-insulating GaAs substrate and series-connected to each other to form an array in order to efficiently absorb a monochromatic light at the wavelength of ~850nm. The PV array can be used to convert the incoming optical power into electrical power with the desired output voltage and current levels. High-temperature operating lifetime tests demonstrate the reliability performance of these devices with different aperture sizes at elevated operating temperatures. Also, formation of photo-carriers in the substrate has been shown to form a leakage current path underneath the PV array, in which it could significantly impact the performance of these devices. These results demonstrate the feasibility of fabricating micro-PV arrays for use in optical wireless power transmission systems that can have a wide variety of applications in low-power sensing and energy storage application.