Multi-segment photovoltaic laser power converters and their electrical losses
The electrical power losses of small area six-segment monolithic interconnected GaAs laser power converter are analyzed based on a variety of measurements and an experimentally validated numerical 2.5-dimensional distributed electrical model. Input parameters for the developed model were extracted from a wide-range of independent measurements performed on the manufactured devices. The complex geometry of the device is implemented in the model with digital images of photolithographic masks. Validity of the built model is confirmed by a good agreement between measured and simulated spatial electroluminescence response and a good agreement between measured and simulated dark and illuminated J-V curves is observed. It is shown that distributed ohmic losses limit the efficiency of the studied device above the optimal (Goptimal=13.2 W/cm2) monochromatic irradiance (0=809 nm). At high irradiance (Ghigh=83.1 W/cm2), the photovoltaic conversion efficiency is decreased by 17%abs., due to combined ohmic losses, where more than 88%rel. of the losses are caused by Joule heating in the lateral conduction layer (which is specific to this kind of multi-segment device). Below the optimal irradiance, minority carrier recombination at the perimeter of the pn-junction is identified as the limiting factor. At Glow=1.8 W/cm2 perimeter recombination causes 5.5%abs. decrease of the conversion efficiency. Additionally, photo-induced conductivity in the semi-insulating GaAs substrate leads to a leakage current between adjacent segments which reduces the efficiency for all studied irradiances equally by 1.2%abs.