**5. Degradation due to space environment**

Space is a hostile environment for the electronic components in general and solar cells in particular. The sun radiates energy in almost the whole electromagnetic spectrum, from radio waves to gamma rays, and abundant charged particles impinging on a surface cause damages which cumulate over the mission lifetime (fluence Φ). The behaviour of solar cells in a radiation environment can be described in terms of the changes in the engineering output parameters of the devices. The radiation usually of interest in the study of degradation of materials and devices consists of energetic or fast massive particles (i.e. electrons, protons, neutrons or ions). The major types of radiation damage phenomena in

exposed so far is sufficient for the design of a solar array for space application at system

0° 20° 40° 50° 60° 70° 80° 90° 100° 110° 120°

<sup>0</sup> <sup>10</sup> <sup>20</sup> <sup>30</sup> <sup>40</sup> <sup>50</sup> <sup>60</sup> <sup>70</sup> <sup>80</sup> <sup>90</sup> <sup>100</sup> <sup>0</sup>

Sat Altitude-Earth radium ratio (H=h/R)

80 deg 70 deg 60 deg 30 deg 0 deg

<sup>105</sup> **Albedo visibility factor as function of altitude, for different values of Beta angle**

Fig. 7. View Factors F12 as function of H=h/R, parameter:

130° 140° 180° 160°

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

View factor F12

Fig. 8. Albedo Visibility factor F as function of h for different β value

Space is a hostile environment for the electronic components in general and solar cells in particular. The sun radiates energy in almost the whole electromagnetic spectrum, from radio waves to gamma rays, and abundant charged particles impinging on a surface cause damages which cumulate over the mission lifetime (fluence Φ). The behaviour of solar cells in a radiation environment can be described in terms of the changes in the engineering output parameters of the devices. The radiation usually of interest in the study of degradation of materials and devices consists of energetic or fast massive particles (i.e. electrons, protons, neutrons or ions). The major types of radiation damage phenomena in

10-4 10-3 10-2 10-1 <sup>100</sup> <sup>102</sup>

Visibility Factor F

**5. Degradation due to space environment** 

103

Altitude [km]

104

level.

solids which are of interest to the solar array designer are ionisation and atomic displacement.

Ionisation occurs when orbital electrons are removed from an atom or molecule in gases, liquids, or solids. The measure of the intensity of ionising radiation is the roentgen. The measure of the absorbed dose in any material of interest is usually defined in terms of absorbed energy per unit mass. The accepted unit of absorbed dose is the rad (100 erg/g or 0.01 J/kg). For electrons, the absorbed dose may be computed from the incident fluence *Φ* (in cm-2) as: Dose (rad) = 1.6x10-8 *dE*/*dx Φ*, where *dE*/*dx* (in MeV cm2 g-1) is the electron stopping power in the material of interest. In this manner, the effects of an exposure to fluxes of trapped electrons of various energies in space can be reduced to an absorbed dose. By the concept of absorbed dose, various radiation exposures can be reduced to absorbed dose units which reflect the degree of ionisation damage in the material of interest. This concept can be applied to electron, gamma, and X-ray radiation of all energies. Several ionisation related effects may degrade the solar cell assemblies. The reduction of transmittance in solar cell cover glasses is an important effect of ionising radiation.

The basis for solar cells damage is the displacement of semiconductor atoms from their lattice sites by fast particles in the crystalline absorber. The displaced atoms and their associated vacancies after various processes form stable defects producing changes in the equilibrium of carrier concentrations and in the minority carrier lifetime. Such displacements require a certain minimum energy similar to that of other atomic movements. Seitz and Koehler [1956] estimated the displacement energy is roughly four times the sublimation energy. Electron threshold energies up to 145 keV have been reported. Particles below this threshold energy cannot produce displacement damage, therefore the space environment energy spectra are cut off below this value. The basic solar cell equations (1) may be used to describe the changes which occur during irradiation. This method would require data regarding the changes in the light generated current, series resistance, shunt resistance, but most investigations have not reported enough data to determine the variations in the above parameters. The usual practice is then to reduce the experimental data in terms of changes in the cell short circuit current (*I*sc), open circuit voltage (*V*oc), and maximum power (*P*max). The variation of common solar cell output parameters during irradiation can be described as shown for *I*sc in the following case:

$$I\_{\rm sc} = I\_{\rm sc0} \text{ - C } \log \left( 1 + \spadesuit \;/\; \oslash\_{\rm x} \right) \tag{13}$$

Where *Φ*x represents the radiation fluence at which *I*sc starts to change to a linear function of the logarithm of the fluence. The constant *C* represents the decrease in *I*sc per decade in radiation fluence in the logarithmic region. In a similar way, for the Voc it can be written;

$$V\_{\rm oc} = V\_{\rm oc0} \text{--C}^{\prime} \log \left( 1 + \text{@} \, / \, \text{@} \chi \right). \tag{14}$$

And for the maximum power;

$$P\_{\text{max}} = P\_{\text{max}0} \cdot C' \log \left( 1 + \Phi / \phi\_{\text{x}} \right) . \tag{15}$$

In the space environment a wide range of electron and proton energies is present; therefore some method for describing the effects of various types of radiation is needed in order to get a radiation environment which can be reproduced in laboratory. It is possible to determine an equivalent damage due to irradiation based upon the changes in solar cell parameters which are in some way related to the minority carrier diffusion length.

Architectural Design Criteria for Spacecraft Solar Arrays 171

fluence is determined for a given space environment, the parameter degradation can be evaluated in the laboratory by irradiating the solar cell with the calculated fluence level of unidirectional normally incident flux. The equivalent fluence is normally expressed in terms of 1 MeV electrons or 10 MeV protons. The three basic input elements necessary to perform

1. degradation data for solar cells under normal incidence 1 MeV electron irradiation; 2. effective relative damage coefficients for omni-directional space electrons and protons

The equivalent 10 MeV proton fluence can be converted to equivalent 1 MeV electron

In cases when the cell degradation is entirely dominated by proton damage, the cell degradation could be estimated more accurately by calculating the equivalent 10 MeV proton fluence and using 10 MeV proton cell damage data, than by using the equivalent 1

To use cover-glass darkening data, a procedure is necessary to evaluate the absorbed dose produced by the various radiation components of the space environment. The procedure is similar to that used for equivalent fluence, with the exception that the absorbed dose varies

The starting point for the solar array sizing is the correct identification of the power demand

Such power demand may change during the satellite lifetime either because of different operational modes foreseen during the mission or, more simply, because of degradation of

Taking into consideration what just said, an analysis of power demand is performed, including peak power, of all the loads installed either in the platform or as payload for each identified phase of the mission. Because of presence of sun eclipses, and possible depointings along the orbit, an analysis of the energy demand is also performed, this because in case of insufficient illumination the on board battery will supply the electrical power, and the solar array has to be sized in order to provide also the necessary power for its recharge. The power budget is based on peak power demands of the loads, while the

Several electronic units work in cold or hot redundancy; this has to be taken into account

Once the power demand is defined including the margins above, it is advisable to add 20% extra margin at system level and defined at the beginning of the project. Such margin is particularly useful during the satellite development in order to manage eventual power excesses of some units beyond the margins defined at unit level. In this way eventual Request For Deviation (RFD) issued by the subcontractors can be successfully processed

the electrical performances of the electrical loads (in majority electronic units).

It is good practice consider power margins both at unit and electrical system level. The consumption of each unit is calculated considering the following criteria: 20% margin with respect to expected power demand if the unit design is new. 10% margin if the unit design has a heritage from a previous similar one.

of various energies for solar cells with various cover-glass thicknesses;

3. Space radiation environment data for the orbit of interest.

degradation calculations are:

fluence as follows: *Φ*1MeV e = 3000 *Φi*10MeV p .

MeV electron fluence and electron data.

**6. The power and energy budget** 

throughout the whole mission of the spacecraft.

energy budget is based on average consumptions.

5% margin if the unit is recurrent.

when summing the power demands.

with depth in the cover material.

The *I*sc variation in each environment is described by the equation for *I*sc. In this case, two constants, *C* and *Φ*x, are required to describe the changes in *I*sc. It has been shown that the constant *C*, under solar illumination, does not greatly vary for different radiation environments. For electron irradiations in the 1 MeV and greater range, *C* is about 4.5 to 5.5 mA cm-2/decade. In case of proton and neutron, *C* approaches 6 to 7 mA cm-2 /decade.

For solar cells with the same initial *I*sc, the constant *Φ*x is a measure of the damage effectiveness of different radiation environments. The constant *Φ*x for a particular radiation can be determined graphically on a semi-log plot at the intersection of the starting *I*sc and the extrapolation of the linear degradation region.

Fig. 9. Variation of solar cell short circuit current with fluence for various radiations

It is the practice to define an arbitrary constant referred to as the critical fluence *Φ*c. One method of defining this value is that fluence which degrades a solar cell parameter 25% below its BOL state. But such a parameter is valid only when comparing cells with similar initial parameters. To eliminate this problem, critical fluence may be alternatively defined as that fluence which will degrade a cell parameter to a certain value. By use of the critical fluence or the diffusion length damage coefficient, it is possible to construct a model in which the various components of a combined radiation environment can be described in terms of a damage equivalent fluence of a selected mono-energetic particle. 1 MeV Electrons are a common and significant component of space radiation and can be produced conveniently in a test environment. For this reason, 1 MeV electron fluence has been used as a basis of the damage equivalent fluences which describe solar cell degradation.

The degradation due to radiation effects on solar cell cover-glass material in space is difficult to assess. The different radiation components of the environment act both individually and synergistically on the elements of the shielding material and also cause changes in the interaction of shielding elements. However, the most significant radiation effects in cover materials involve changes in the transmission of light in the visible and near infrared region.

The methods for estimating solar cell degradation in space are based on the techniques described by *Brown et al.* [1963] and *Tada* [1973ab]. In summary, the omni-directional space radiation is converted to a damage equivalent unidirectional fluence at a normalised energy and in terms of a specific radiation particle. This equivalent fluence will produce the same damage as that produced by omni directional space radiation considered if the relative damage coefficient (RDC) is properly defined to allow the conversion. When the equivalent

The *I*sc variation in each environment is described by the equation for *I*sc. In this case, two constants, *C* and *Φ*x, are required to describe the changes in *I*sc. It has been shown that the constant *C*, under solar illumination, does not greatly vary for different radiation environments. For electron irradiations in the 1 MeV and greater range, *C* is about 4.5 to 5.5 mA cm-2/decade. In case of proton and neutron, *C* approaches 6 to 7 mA cm-2 /decade. For solar cells with the same initial *I*sc, the constant *Φ*x is a measure of the damage effectiveness of different radiation environments. The constant *Φ*x for a particular radiation can be determined graphically on a semi-log plot at the intersection of the starting *I*sc and the

Fig. 9. Variation of solar cell short circuit current with fluence for various radiations

a basis of the damage equivalent fluences which describe solar cell degradation.

It is the practice to define an arbitrary constant referred to as the critical fluence *Φ*c. One method of defining this value is that fluence which degrades a solar cell parameter 25% below its BOL state. But such a parameter is valid only when comparing cells with similar initial parameters. To eliminate this problem, critical fluence may be alternatively defined as that fluence which will degrade a cell parameter to a certain value. By use of the critical fluence or the diffusion length damage coefficient, it is possible to construct a model in which the various components of a combined radiation environment can be described in terms of a damage equivalent fluence of a selected mono-energetic particle. 1 MeV Electrons are a common and significant component of space radiation and can be produced conveniently in a test environment. For this reason, 1 MeV electron fluence has been used as

The degradation due to radiation effects on solar cell cover-glass material in space is difficult to assess. The different radiation components of the environment act both individually and synergistically on the elements of the shielding material and also cause changes in the interaction of shielding elements. However, the most significant radiation effects in cover materials involve changes in the transmission of light in the visible and near

The methods for estimating solar cell degradation in space are based on the techniques described by *Brown et al.* [1963] and *Tada* [1973ab]. In summary, the omni-directional space radiation is converted to a damage equivalent unidirectional fluence at a normalised energy and in terms of a specific radiation particle. This equivalent fluence will produce the same damage as that produced by omni directional space radiation considered if the relative damage coefficient (RDC) is properly defined to allow the conversion. When the equivalent

extrapolation of the linear degradation region.

infrared region.

fluence is determined for a given space environment, the parameter degradation can be evaluated in the laboratory by irradiating the solar cell with the calculated fluence level of unidirectional normally incident flux. The equivalent fluence is normally expressed in terms of 1 MeV electrons or 10 MeV protons. The three basic input elements necessary to perform degradation calculations are:


The equivalent 10 MeV proton fluence can be converted to equivalent 1 MeV electron fluence as follows: *Φ*1MeV e = 3000 *Φi*10MeV p .

In cases when the cell degradation is entirely dominated by proton damage, the cell degradation could be estimated more accurately by calculating the equivalent 10 MeV proton fluence and using 10 MeV proton cell damage data, than by using the equivalent 1 MeV electron fluence and electron data.

To use cover-glass darkening data, a procedure is necessary to evaluate the absorbed dose produced by the various radiation components of the space environment. The procedure is similar to that used for equivalent fluence, with the exception that the absorbed dose varies with depth in the cover material.
