BEXUS 23 OSCAR: Solar Cell I-V Monitoring System for Space Environments

Abstract

The OSCAR (‘Optical Sensors based on CARbon materials’) project aims to explore the use of novel generation carbon based (i.e. polymer, small molecule, perovskite, flexible) solar cells for aerospace applications through in-situ testing during a stratospheric balloon flight. Complementary ex-situ testing before and after flight is performed to observe and understand the impact of the harsh environmental conditions. It is the mission of this project to get an indication of the degradation of the carbon based solar cells when exposed to space-like environments. This could possibly unveil their deployability in aerospace applications, for which these devices’ very high power-to-mass ratio makes them excellent candidates. To achieve this mission goal, a custom I-V (current-voltage) performance monitoring system for a population of 64 solar cells was implemented. The design period identified a set of strict requirements such as low mass, low power, adequate measuring speed, optimal resolution, measurement accuracy, error handling, stability, resistance to software resets, data communication, and data logging, which were successfully addressed in the final build. Furthermore, the cells were divided into 3 different categories based on their operational I-V ranges (BCF, DEdiode and IPV+STRETCH). Each of these had a characteristic applied voltage range, which were from 0 to 1V, 0 to 0.85V, and 0 to 7V, with corresponding current ranges of -6 to 14mA, -5 to 5mA, and -200 to 500mA, in the cited order. By using all 11 effectively available ADC bits within these ranges, measuring accuracies could get as low as 0.5mV and 10uA for BCF, 0.4mV and 5uA for DEdiode, and 3.5mV and 350uA for IPV+STRETCH. Timewise, there was the requirement of acquiring at least one I-V sweep per device every 5 minutes, in order to have enough data. Moreover, the time between measuring the first and last point within a single sweep was not to exceed 20s, to guarantee minimal changes in received sunlight within measurement curves. In this work, solutions to issues ranging from connector reliability to OPAMP ringing and RF interference, will be explained. In the end, it will be shown that this measurement system has proven to function continuously for a period of 4h (until battery drainage) while being exposed to low temperatures down to -50°C, low pressures down to 6 mbar, and RF interference. All data show consistent I-V measurements, yielding a data set of 192000 IV characteristics acquired during flight, currently being analysed for further research and upcoming communications.