Durability evaluation of InGaP/GaAs/Ge triple-junction solar cells in HIHT environments for Mercury exploration mission

The Japan Aerospace Exploration Agency has been developing the Mercury Magnetospheric Orbiter (MMO), which is Japanese part of the BepiColombo mission. During its mission around Mercury, the spacecraft will be exposed to high solar irradiance of up to 11 suns, with an estimated maximum solar panel temperature of 230°C. In such an environment, solar cells are required to operate under high intensity and high temperature (HIHT) conditions. Therefore, it is necessary to evaluate the durability of solar cells to meet the power requirements throughout the mission life. We conducted a continuous operation test under HIHT conditions to examine the validity of the solar array configuration, using the interior planetary thermal vacuum chamber. Our HIHT tests clarified the following facts: (i) Transparency of the coverglass and the performance of the solar cells do not degrade and (ii) transparency of the DC93-500 adhesive in the top cell response region degrades mainly due to ultraviolet exposure at high temperatures. We decided to use AR0213 coverglass (from JDSU) with a thickness of 300 μm, which have a longer cut-on wavelength in ultraviolet region. With this configuration, the predicted decrease in Pmax due to the HIHT environment is 17.3% and that due to radiation effects is 11.0% Our new design will offer the available power at EOL of 394.2 W, which is 46.7 W greater than the required power.