**2. Solar cells for space applications**

Since the beginning of the astronautic era, photovoltaic devices have been considered for the generation of electrical power on board spacecrafts because of their high power output per unit mass, associated with the fundamental advantage of not having moving parts, present, instead, in all the most used electrical power generators for both terrestrial and aeronautical applications (turbines, motors, alternators, etc.). Therefore the PV array is static, does not produce vibrations or noise, and does not need an active cooling. The Russians were the first, in 1958, to launch a satellite powered with silicon solar cells.

Architectural Design Criteria for Spacecraft Solar Arrays 163

taken into account when the power conditioning architecture is chosen, and the relevant

**Gaget2 3G28**

Fig. 3. Capacitance identified for the 15 cells string, Gaget2 and 3G28 (AZUR SPACE

The mathematical model of a photovoltaic cell has to take into account the following factors

0 10 20 30 40 50 **String voltage (V)**

The solar cell model, derived from the Mottet-Sombrin's one, is basically a current generator driven by the value of the voltage applied at its terminal according to the equivalent circuit reported below. Generally speaking a solar cell is a particular p-n junction where the diffusion process (diode D1) co-exists with the generation and recombination effect of the charge carrier (diode D2) induced by the presence of crystalline defects. This model was tested using data relevant to the AZUR SPACE 28% solar cell, as reported in the datasheet

exp 1 exp 1 <sup>2</sup>

*q V q V <sup>V</sup> iii <sup>i</sup>*

*D D D*

iO

VO

*k T kT R*

RP

VD

RS

*p*

RO

(1a)

provided by the Manufacturer, and available on company web-site.

iD iR

D1 D2

*oLD R*

devices designed.

products)

**3. Solar cell equivalent circuit** 

1. Intensity of the incident light. 2. Operative absolute temperature. 3. Degradation by cosmic radiation.

Fig. 4. Equivalent Circuit of solar cell The relevant Kirchhoff equations are:

iL

capable to influence the solar cell behaviour.

**Capacitance (nF)**

Solar cells for space applications have to be highly efficient, capable to stand thousands of thermal cycles in orbit where the temperature, according to the mission profile may vary from -150 °C to more than 120 °C. They have to show a limited degradation during time due to cosmic radiations and Ultraviolet, and they have to resist to the mechanical solicitations mainly linear accelerations and vibrations during launch and orbital manoeuvres, because of these constraints the cells for space are smaller than those for terrestrial applications.

In order to have the highest conversion efficiency, solar cells for space application are developed from mono-crystalline materials. In the past silicon was the most used and the reachable bulk efficiency was not higher than 14%. The advent of GaAs based solar cells in the last decade of the 20th century took the efficiency up to 19%, and nowadays triple junction solar cells show more than 30%.

Figure 1 shows a very simplified structure of triple junction cell.

Fig. 1. Triple junction solar cell structure

While figure 2 reports the quantum efficiency for each junction, it can be clearly seen that the increased efficiency is due to wider wavelength coverage of the absorbed radiation.

Fig. 2. Equivalent quantum efficiency as function of wavelength

Triple junction GaAs solar cells are populating more and more solar generators worldwide, while manufacturers are actively working on four to six junction cells as a way forward always increasing conversion efficiency. Consequently, there is a need to improve the understanding of the electrical dynamic behaviour of multi-junction based solar array considering that the proper design of solar array regulators requires, among others, a good mastering of the solar section/regulator interface. In order to better understand EMC aspects connected to the chosen regulation philosophy, which will be discussed further, it is worth to have a quick look at the equivalent capacitance present at the output of a triple junction cell. The figure 3 reports the capacitance measured across strings composed of 15 cells. The cells used are produced by AZUR SPACE Solar Power GmbH. It can be observed that at high voltages the capacitance is considerably increased. Such behaviour has to be

Solar cells for space applications have to be highly efficient, capable to stand thousands of thermal cycles in orbit where the temperature, according to the mission profile may vary from -150 °C to more than 120 °C. They have to show a limited degradation during time due to cosmic radiations and Ultraviolet, and they have to resist to the mechanical solicitations mainly linear accelerations and vibrations during launch and orbital manoeuvres, because of these constraints the cells for space are smaller than those for terrestrial applications. In order to have the highest conversion efficiency, solar cells for space application are developed from mono-crystalline materials. In the past silicon was the most used and the reachable bulk efficiency was not higher than 14%. The advent of GaAs based solar cells in the last decade of the 20th century took the efficiency up to 19%, and nowadays triple

While figure 2 reports the quantum efficiency for each junction, it can be clearly seen that the increased efficiency is due to wider wavelength coverage of the absorbed radiation.

<sup>p</sup> <sup>n</sup> <sup>n</sup> <sup>p</sup>

p

GaInP GaAs

n Ge

Triple junction GaAs solar cells are populating more and more solar generators worldwide, while manufacturers are actively working on four to six junction cells as a way forward always increasing conversion efficiency. Consequently, there is a need to improve the understanding of the electrical dynamic behaviour of multi-junction based solar array considering that the proper design of solar array regulators requires, among others, a good mastering of the solar section/regulator interface. In order to better understand EMC aspects connected to the chosen regulation philosophy, which will be discussed further, it is worth to have a quick look at the equivalent capacitance present at the output of a triple junction cell. The figure 3 reports the capacitance measured across strings composed of 15 cells. The cells used are produced by AZUR SPACE Solar Power GmbH. It can be observed that at high voltages the capacitance is considerably increased. Such behaviour has to be

junction solar cells show more than 30%.

Fig. 1. Triple junction solar cell structure

Figure 1 shows a very simplified structure of triple junction cell.

Tunnel junctions

Front metal

Metal

Fig. 2. Equivalent quantum efficiency as function of wavelength

taken into account when the power conditioning architecture is chosen, and the relevant devices designed.

Fig. 3. Capacitance identified for the 15 cells string, Gaget2 and 3G28 (AZUR SPACE products)
