4-Junction GaAs Based Thin Film Photonic Power Converter with Back Surface Reflector for Medical Applications
Energy supply of medical implants is a challenging task that poses requirements regarding electrical performance, safety, mechanical dimensions, biological compatibility and ideally compatibility with diagnostic methods such as magnetic resonance imaging. In this paper we report about photovoltaic cells that are optimized to power medical implants optically. We demonstrate 4-junction thin film photonic power converters (PPCs) with integrated back reflector designed to operate under monochromatic near-infrared light with a wavelength of 855 nm. The influence of the incident laser wavelength and spectral sensitivity are investigated under 809 nm and 855 nm laser light. Measurements under 855 nm light at a temperature of 25 Â°C suggest almost perfect current match. However, already at an operating temperature of 40 Â°C, as expected for operation of the implant inside the body, current mismatch becomes apparent. The impact of spectral sensitivity and current mismatch on the shape of the I-V curve is discussed based on measurements at a detuned wavelength of 809 nm. Furthermore, the effect of a gold back surface reflector on cell performance is examined and a lower sensitivity on current mismatch conditions due to the mirror is found. Despite the dependence of the device performance on temperature and wavelength, the maximum power point voltage remains well above 3.3 V under all relevant operating conditions, which makes these PPCs well suited to drive smart implant electronics without the need for voltage upconversion.