Measure solar-cell performance

ALSAT-1N carried four experimental thin-film cadmium-telluride solar cells on a small external board. The experiment measured their electrical behavior in orbit. It should not be described as the spacecraft’s complete power-generating array or as a deployed commercial power station. Flying a sample answers a more specific question about a technology under flight conditions.

An illuminated cell has a relationship between voltage and current. At open circuit, current is zero; at short circuit, voltage is approximately zero. Neither endpoint delivers the maximum useful power, because electrical power is voltage multiplied by current. A measurement sweep helps identify the operating point where that product is greatest.

Fill factor compares maximum electrical power with the rectangle formed by open-circuit voltage and short-circuit current: FF = Pmax/(Voc × Isc). It is not conversion efficiency, which compares useful electrical output with incident light power. Two cells can have similar endpoint values but different curve shapes and different useful power.

In a fictional example, Voc is 1 volt, Isc is 0.03 ampere and maximum power is 0.018 watt. The rectangle is 0.03 watt and the fill factor is 0.018/0.03 = 0.60, or 60 percent. If maximum power falls to 0.012 watt with the same endpoints, fill factor becomes 40 percent. These values teach the definition; they are not measured Alsat results. Temperature and illumination must also be considered when comparing actual curves.

Fill factor compares two powers

Underwood and colleagues’ 2023 study found significant fill-factor degradation across the flight record. They associated the decline with falling shunt resistance and proposed diffusion from the gold back contacts as its cause. Their recommendation was to improve the contacting approach. This distinguishes an observed electrical trend from the team’s explanation for its mechanism.

The cells’ continued ability to produce measurements did not mean their performance remained unchanged. A useful technology demonstration can therefore be both a successful experiment and a source of evidence that the design needs improvement. ASAL separately reported continued spacecraft operation in September 2023; that dated status does not certify current operation or qualify the experimental cells for every future mission.