Testing solar cells in the stratosphere dates back to the mid 1960s, with the beginning of NASA’s programme for calibration of reference solar cells onboard high altitude balloons. Such procedure quickly became a standard and is still in use both for calibration and for testing of new photovoltaic technologies in near-space conditions. The stratosphere - albeit different from LEO as for particle radiation, vacuum level and temperature extremes - has very similar insolation conditions to AM0 and provides an excellent and easily accessible environment for early testing of new spacebound solar cells, so to advance technology and climb up the TRL ladder. It also provides, of course, the proper enviroment where testing of advanced photovoltaics for the stratosphere-dwelling, forthcoming large High Altitude Platform Stations (HAPS - also known as High Altitude Pseudo Satellites) shall take place.

The Space Systems group at UniPi, mostly concerned with small satellites and with the associated power generation systems, recently got an opportunity to fly a solar cell test package in the frame of the H2020 HEMERA programme. To this end, we designed and built a very compact system for in-flight measurement of the I-V curves of solar cells of various kind, including perovskites, CIGS, terrestrial silicon cells and state-of-the-art triple junction GaAs. Our ECAPS experiment (“Experimental Characterization of Advanced Photovoltaics in the Stratosphere”) will fly on the SOLAR mission, scheduled for launch in August 2022 onboard a large zero-pressure balloon from Timmins, Canada, under CNES management. The balloon gondola is equipped with an attitude control system that guarantees correct exposure towards the Sun throughout the flight; ECAPS will sit side-by-side with the much larger institutional/commercial CASOLBA facility of CNES, providing an invaluable opportunity for cross-check of the solar cell measurement system performance.

We present ECAPS design and expected performance, as well as the dedicated solar cell testing system, derived form the ECAPS experience, that we fly on our proprietary miniaturized stratospheric platform based on COTS sounding balloons. Equipped with attitude stabilizers and an optional Sun-pointing mechanism, such novel platform provides flight opportunities to 30-35 km altitude, with a permanence time of 1 to 12 hours at altitude, minimal pre-flight integration effort and very low cost.