ISO 23038:2018

Space systems — Space solar cells — Electron and proton irradiation test methods

ISO 23038:2018 specifies the requirements for electron and proton irradiation test methods of space solar cells. It addresses only test methods for performing electron and proton irradiation of space solar cells and not the method for data analysis.

Systèmes spatiaux — Cellules solaires spatiales — Méthodes d'essai d'irradiation d'électrons et de protons

General Information

• Status: Published
• Publication Date: 14-Feb-2018
• ICS: 49.140 - Space systems and operations
• Technical Committee: ISO/TC 20/SC 14 - Space systems and operations
• Drafting Committee: ISO/TC 20/SC 14 - Space systems and operations
• Current Stage: 9060 - Close of review
• Completion Date: 02-Sep-2028

Relations

• Revises: ISO 23038:2006 - Space systems — Space solar cells — Electron and proton irradiation test methods
• Effective Date: 04-Nov-2015

Overview

ISO 23038:2018 – Space Systems - Space Solar Cells - Electron and Proton Irradiation Test Methods is an internationally recognized standard developed by the International Organization for Standardization (ISO). This document outlines standardized procedures for testing the effects of electron and proton radiation on space solar cells, which are critical components in spacecraft power systems. ISO 23038:2018 defines the requirements for irradiation test methods, ensuring consistency, reliability, and repeatability in evaluating solar cell performance under simulated space radiation environments. Notably, it addresses only the test procedures for irradiation and does not prescribe data analysis methodologies.

Key Topics

• Space Radiation Environment:
  The standard describes the typical radiation environment encountered in space, mainly consisting of energetic electrons and protons, and the impact of phenomena such as solar flares and the Van Allen radiation belts.

• Radiation Effects on Solar Cells:
  Solar cells are vulnerable to degradation due to displacement and ionization damage caused by particle irradiation. ISO 23038:2018 specifies test approaches to simulate these effects under controlled laboratory conditions.

• Test Methodology:

  • Electron irradiation: Procedures for exposing solar cells to electron beams, considering factors like vacuum conditions, temperature control, beam coverage, uniformity, and flux measurement.
  • Proton irradiation: Special considerations for proton exposure due to greater scattering in air, with recommendations for vacuum conditions and material selection for scattering foils.
  • Beam Dosimetry: Techniques for measuring irradiation fluence, including Faraday cup usage and secondary dosimetry methods.
  • Test Sample Handling: Guidance on sample preparation, coverage area, exposure methodologies, and handling to ensure accurate and representative results.

• Test Report Requirements:
  The standard provides a checklist for documentation, including sample details, irradiation and annealing conditions, measurement methods, and uncertainty levels to ensure traceability and reproducibility.

Applications

ISO 23038:2018 supports organizations involved in the design, production, and qualification of space solar cells, including:

• Satellite manufacturers needing to validate solar cell durability and performance before assembly.
• Space agencies and research groups conducting irradiation studies to predict long-term degradation of solar power systems in different mission profiles.
• Component suppliers seeking to demonstrate compliance with international qualification requirements by applying standardized irradiation testing methods.
• Testing laboratories performing irradiation services and generating accredited test reports in line with recognized methodologies.

By following the procedures established in ISO 23038:2018, stakeholders can ensure solar cell reliability under real-world space conditions, minimize mission risk, and facilitate technology selection for specific orbits and mission durations.

Related Standards

For comprehensive qualification of space solar cells and components, the following international standards and resources provide complementary guidance:

• ISO/IEC standards on space system design and radiation environments
• JPL Solar Cell Radiation Handbooks (Silicon and GaAs)
• SRIM, CASINO, MCNPX, and GEANT4 – simulation tools referenced for irradiation transport calculations
• Other ISO/TC 20/SC 14 standards covering space system performance and environmental effects.

Implementing ISO 23038:2018 alongside related standards ensures robust and internationally recognized approaches to space solar cell qualification, strengthening mission assurance and supporting global collaboration in the aerospace industry.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23038
Second edition
2018-02
Space systems — Space solar cells —
Electron and proton irradiation test
methods
Systèmes spatiaux — Cellules solaires spatiales — Méthodes d'essai
d'irradiation d'électrons et de protons
Reference number
©
ISO 2018
© ISO 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva, Switzerland
Tel. [phone]
Fax [phone]
[e-mail]
www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 2
5 Space radiation environments . 2
5.1 Space radiation . 2
5.2 Shielding effects . 3
6 General radiation effects in solar cells . 3
6.1 Solar-cell radiation damage . 3
6.2 Radiation effects on solar cell cover materials . 3
7 Radiation test methods . 4
7.1 General . 4
7.2 Electron irradiation . 5
7.2.1 Vacuum . 5
7.2.2 Temperature . 5
7.2.3 Coverage area . 5
7.2.4 Irradiation beam uniformity . . 6
7.2.5 Flux levels . 6
7.2.6 Dosimetry. 6
7.2.7 Other practical test considerations . 6
7.3 Proton irradiation . 7
7.3.1 General. 7
7.3.2 Vacuum . 7
7.3.3 Coverage area . 7
7.4 Post irradiation annealing phenomena . 7
8 Test report guidelines . 7
Bibliography . 9
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,
Subcommittee SC 14, Space systems and operations.
This second edition cancels and replaces the first edition (ISO 23038:2006), which has been technically
revised. The main changes compared to the previous edition are as follows:
— radiation environment models were updated from AE8/AP8 to AE9/AP9;
— threshold energies for atomic displacement for silicon and GaAs were deleted;
— a statement was added that, whatever the method, the duration or intensity level of the electron and
proton irradiation test is considered a “destructive test”.
iv © ISO 2018 – All rights reserved

INTERNATIONAL STANDARD ISO 23038:2018(E)
Space systems — Space solar cells — Electron and proton
irradiation test methods
1 Scope
This document specifies the requirements for electron and proton irradiation test methods of space
solar cells. It addresses only test methods for performing electron and proton irradiation of space solar
cells and not the method for data analysis.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
NOTE Physical constants are given to four significant figures only and reflect current knowledge.
3.1
differential energy spectrum
spread of energies of some specific group
Note 1 to entry: In this document, this refers to the number of particles possessing an energy value that lies in
the infinitesimal range E, E + dE divided by the size of the range (dE). Integration of the differential particle
spectrum over all particle energies yields the total number of particles. This quantity is given in units of particles
per unit area per unit energy.
3.2
electron
−
e
−31 −19
elementary particle of rest mass m = 9,109 kg × 10 kg, having a negative charge of 1,602 C × 10 C
3.3
flux
number of particles passing through a given area in a specified time
Note 1 to entry: Flux may also be specified in terms of the number of particles per unit time passing through a
unit area from source directions occupying a unit solid angle. Typical units are particles per cm per second per
steradian (sr) (1 sr is the solid angle subtended at the centre of a unit sphere by a unit area of the surface of the
sphere).
3.4
fluence
total number of particles in any given time period given in units of particles per unit area
Note 1 to entry: Fluence is also known as time-integrated flux.
3.5
integral energy spectrum
total number of particles in a specified group that possess energies greater than, or equal to, a specified
value, given in units of particles per unit area
3.6
irradiation
exposure of a substance to energetic particles that penetrate the material and have the potential to
transfer energy to the material
3.7
omnidirectional flux
number of particles of a particular type which have an isotropic distribution over 4π steradians and
that would traverse a test sphere of 1 cm cross-sectional area in 1 s
Note 1 to entry: Expressed in units of particles per cm per second.
3.8
proton
+
p
−27
positively charged particle of mass number one, having a mass of 1,672 kg × 10 kg and a charge equal
in magnitude but of opposite sign to the electron
Note 1 to entry: A proton is the nucleus of a hydrogen atom.
4 Symbols and abbreviated terms
eV electronvolt
NIEL nonionizing energy loss
NOTE 1 eV, is a unit of energy commonly used for ions, electrons, elementary particles, etc.
−19
(1 eV ≈ 1,602 J × 10 J.)
NOTE 2 The rate at which the incident particle transfers energy to the crystal lattice through nonionizing
2 −1
events is referred to as the nonionizing energy loss (NIEL), typical unit is MeV ⋅ cm ⋅ g .
5 Space radiation environments
5.1 Space radiation
Primarily, electrons and protons with a wide range of energies characterize the space radiation
environment (see References [1] and [2]). Gamma rays can be used as a substitute for electron irradiation
with the proper transformation. Some reasonable electron and proton fluence limits usually attained
in typical earth orbit conditions are given below. For 1 MeV electrons and 10 MeV protons, these typical
15 13 2
but not inclusive fluence limits are 10 and 10 particles per cm , respectively. Alpha particles and
other charged particles are usually of negligible quantity as far as solar cell damage is concerned. The
particles come from the solar wind and are trapped by Earth’s magnetic field to form radiation belts
with widely varying intensities. Solar wind is usually associated with particles of low energy (typically
below 100 keV), while particles of concern for solar cells are generally of higher energies. The inner
portion of the belts consists mainly of protons and of an inner electron belt, while the outer portion
consists primarily of electrons. Outside of these radiation belts, there is a likelihood of sudden bursts
of protons and electrons originating from coronal mass ejections from the Sun, referred to generally as
solar flares. Thus, the differential spectrum of electrons and protons for any given mission is dependent
on the specific mission orbit. Due to the large variability of the involved phenomena, the prediction of
the particle spectrum for a given mission is affected by a significant uncertainty. Widely accepted tools
for its calculation include the AP9 (protons) and AE9 (electrons) codes for the trapped particles, while
the solar proton events are modelled with other tools such as the JPL 91 code. Note that there is also
2 © ISO 2018 – All rights reserved

complementary information in Reference [3]. The definitions of the various particle radiation spectra
can be obtained using freely available resources such as the Space Environment Information System
(www .spenvis .oma .be).
5.2 Shielding effects
Space solar cells are typically flown with some material covering the cell surface, usually a piece of
glass (coverglass), and are mounted on some support structure. These front and rear covering materials
act to shield the solar cell from some of the incident irradiation. Because of this, the solar cell in space
is actually irradiated by a modified particle spectrum, usually referred to as a slowed down spectrum.
An example showing such a slowed down spectrum calculation can be found in References [4] and [5].
Shielding materials may themselves be sensitive to radiation (see 6.2).
The response of the cell to particle radiation is typically tested on unshielded cells. This enables the
radiation analysis conducted on the bare solar cell to be made applicable to all combinations of shielding
that might be used at solar array level. In practical terms, it also avoids potential complication of the
analysis due to broadening by the shielding materials of a nominally monoenergetic particle beam,
which can be significant.
6 General radiation effects in solar cells
6.1 Solar-cell radiation damage
Solar cells, like all semiconductor devices, are subject to electrical degradation when exposed to particle
irradiation. In terms of radiation damage to solar cells used in space, the primary particles of interest
are electrons and protons. When these energetic particles are incident upon the solar cell material,
they collide with the atoms of the crystal lattice of the solar cell. In these atomic collisions, energy is
transferred from the incident particle to the target atom. This energy can be transferred in several
ways. The majority of the energy is transferred through ionization of the target atom, where electrons
of the target atom absorb the transferred energy and are promoted to higher energy levels. Another
energy transfer mechanism is through nonionizing events, which results in the displacement of the
target atom. If enough energy is transferred in a nonionizing event, then the displaced target atom may,
in turn, displace other atoms, creating a cascade of displaced atoms. The displacement damage induced
by the nonionizing interactions is the primary cause of most solar cell degradation.
When an atom is displaced in a lattice, the electron energy band structure of the material is disturbed,
and localized energy levels can be created near the site of the defect. These defect energy levels can
act to trap electrical charge carriers, thus restricting their ability to move through the material, which
is characterized by a reduction in the minority carrier diffusion length. Since solar cell operation
depends on the motion of photogenerated charge carriers through the material, these defect sites tend
to degrade the solar cell performance.
The amount of displacement damage caused by an incident particle is a function of the type of incident
particle (i.e. electron or proton), the particle energy, and the composition of the crystal lattice. The
rate at which the incident particle transfers energy to the crystal lattice through nonionizing events
is referred to as the nonionizing energy loss (NIEL). Electrons become more damaging as the incident
electron energy increases. The opposite is true for protons, where the lower energy protons are the
most damaging. Also, protons are significantly more damaging in comparison to electrons, primarily
due to the increased proton differential scattering cross section for atomic displacements. There is a
lower limit to displacement damage corresponding to the threshold energy for atomic displacements.
This threshold energy is dependent on the semiconductor material that constitutes the solar cell.
6.2 Radiation effects on solar cell cover materials
Although not specifically a solar cell radiation effect, it is appropriate in this document to note the
effects of irradiation on solar cell coverglass materials and the adhesives which are typically used
to attach them. Certain solar cell coverglass material has been shown to darken under ultraviolet or
[6]
particle irradiation thereby absorbing some of the incident light . This increased light absorption can
reduce the solar cell output in one of two ways: (1) reduction of the amount of light that reaches the cell,
and (2) increase in operating temperature of array that reduces the cell electrical conversion efficiency.
Whereas crystalline solar cells are typically degraded by atomic displacement damage, coverglasses
[1]
are typically more sensitive to ionization effects . The “absorbed doses” associated with the radiation
environment of a particular mission can be calculated and then simulated by laboratory testing.
Testing cells with attached coverglass or different geometries require special care (see References [1]
and [2]).
7 Radiation test methods
7.1 General
As described in Clause 5, the space radiation environment consists of an omnidirectional spectrum of
particle energies, and as described in this clause, solar cell radiation damage is energy dependent. The
...

INTERNATIONAL ISO
STANDARD 23038
Second edition
2018-02
Space systems — Space solar cells —
Electron and proton irradiation test
methods
Systèmes spatiaux — Cellules solaires spatiales — Méthodes d'essai
d'irradiation d'électrons et de protons
Reference number
©
ISO 2018
© ISO 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva, Switzerland
Tel. [phone]
Fax [phone]
[e-mail]
www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 2
5 Space radiation environments . 2
5.1 Space radiation . 2
5.2 Shielding effects . 3
6 General radiation effects in solar cells . 3
6.1 Solar-cell radiation damage . 3
6.2 Radiation effects on solar cell cover materials . 3
7 Radiation test methods . 4
7.1 General . 4
7.2 Electron irradiation . 5
7.2.1 Vacuum . 5
7.2.2 Temperature . 5
7.2.3 Coverage area . 5
7.2.4 Irradiation beam uniformity . . 6
7.2.5 Flux levels . 6
7.2.6 Dosimetry. 6
7.2.7 Other practical test considerations . 6
7.3 Proton irradiation . 7
7.3.1 General. 7
7.3.2 Vacuum . 7
7.3.3 Coverage area . 7
7.4 Post irradiation annealing phenomena . 7
8 Test report guidelines . 7
Bibliography . 9
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,
Subcommittee SC 14, Space systems and operations.
This second edition cancels and replaces the first edition (ISO 23038:2006), which has been technically
revised. The main changes compared to the previous edition are as follows:
— radiation environment models were updated from AE8/AP8 to AE9/AP9;
— threshold energies for atomic displacement for silicon and GaAs were deleted;
— a statement was added that, whatever the method, the duration or intensity level of the electron and
proton irradiation test is considered a “destructive test”.
iv © ISO 2018 – All rights reserved

INTERNATIONAL STANDARD ISO 23038:2018(E)
Space systems — Space solar cells — Electron and proton
irradiation test methods
1 Scope
This document specifies the requirements for electron and proton irradiation test methods of space
solar cells. It addresses only test methods for performing electron and proton irradiation of space solar
cells and not the method for data analysis.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
NOTE Physical constants are given to four significant figures only and reflect current knowledge.
3.1
differential energy spectrum
spread of energies of some specific group
Note 1 to entry: In this document, this refers to the number of particles possessing an energy value that lies in
the infinitesimal range E, E + dE divided by the size of the range (dE). Integration of the differential particle
spectrum over all particle energies yields the total number of particles. This quantity is given in units of particles
per unit area per unit energy.
3.2
electron
−
e
−31 −19
elementary particle of rest mass m = 9,109 kg × 10 kg, having a negative charge of 1,602 C × 10 C
3.3
flux
number of particles passing through a given area in a specified time
Note 1 to entry: Flux may also be specified in terms of the number of particles per unit time passing through a
unit area from source directions occupying a unit solid angle. Typical units are particles per cm per second per
steradian (sr) (1 sr is the solid angle subtended at the centre of a unit sphere by a unit area of the surface of the
sphere).
3.4
fluence
total number of particles in any given time period given in units of particles per unit area
Note 1 to entry: Fluence is also known as time-integrated flux.
3.5
integral energy spectrum
total number of particles in a specified group that possess energies greater than, or equal to, a specified
value, given in units of particles per unit area
3.6
irradiation
exposure of a substance to energetic particles that penetrate the material and have the potential to
transfer energy to the material
3.7
omnidirectional flux
number of particles of a particular type which have an isotropic distribution over 4π steradians and
that would traverse a test sphere of 1 cm cross-sectional area in 1 s
Note 1 to entry: Expressed in units of particles per cm per second.
3.8
proton
+
p
−27
positively charged particle of mass number one, having a mass of 1,672 kg × 10 kg and a charge equal
in magnitude but of opposite sign to the electron
Note 1 to entry: A proton is the nucleus of a hydrogen atom.
4 Symbols and abbreviated terms
eV electronvolt
NIEL nonionizing energy loss
NOTE 1 eV, is a unit of energy commonly used for ions, electrons, elementary particles, etc.
−19
(1 eV ≈ 1,602 J × 10 J.)
NOTE 2 The rate at which the incident particle transfers energy to the crystal lattice through nonionizing
2 −1
events is referred to as the nonionizing energy loss (NIEL), typical unit is MeV ⋅ cm ⋅ g .
5 Space radiation environments
5.1 Space radiation
Primarily, electrons and protons with a wide range of energies characterize the space radiation
environment (see References [1] and [2]). Gamma rays can be used as a substitute for electron irradiation
with the proper transformation. Some reasonable electron and proton fluence limits usually attained
in typical earth orbit conditions are given below. For 1 MeV electrons and 10 MeV protons, these typical
15 13 2
but not inclusive fluence limits are 10 and 10 particles per cm , respectively. Alpha particles and
other charged particles are usually of negligible quantity as far as solar cell damage is concerned. The
particles come from the solar wind and are trapped by Earth’s magnetic field to form radiation belts
with widely varying intensities. Solar wind is usually associated with particles of low energy (typically
below 100 keV), while particles of concern for solar cells are generally of higher energies. The inner
portion of the belts consists mainly of protons and of an inner electron belt, while the outer portion
consists primarily of electrons. Outside of these radiation belts, there is a likelihood of sudden bursts
of protons and electrons originating from coronal mass ejections from the Sun, referred to generally as
solar flares. Thus, the differential spectrum of electrons and protons for any given mission is dependent
on the specific mission orbit. Due to the large variability of the involved phenomena, the prediction of
the particle spectrum for a given mission is affected by a significant uncertainty. Widely accepted tools
for its calculation include the AP9 (protons) and AE9 (electrons) codes for the trapped particles, while
the solar proton events are modelled with other tools such as the JPL 91 code. Note that there is also
2 © ISO 2018 – All rights reserved

complementary information in Reference [3]. The definitions of the various particle radiation spectra
can be obtained using freely available resources such as the Space Environment Information System
(www .spenvis .oma .be).
5.2 Shielding effects
Space solar cells are typically flown with some material covering the cell surface, usually a piece of
glass (coverglass), and are mounted on some support structure. These front and rear covering materials
act to shield the solar cell from some of the incident irradiation. Because of this, the solar cell in space
is actually irradiated by a modified particle spectrum, usually referred to as a slowed down spectrum.
An example showing such a slowed down spectrum calculation can be found in References [4] and [5].
Shielding materials may themselves be sensitive to radiation (see 6.2).
The response of the cell to particle radiation is typically tested on unshielded cells. This enables the
radiation analysis conducted on the bare solar cell to be made applicable to all combinations of shielding
that might be used at solar array level. In practical terms, it also avoids potential complication of the
analysis due to broadening by the shielding materials of a nominally monoenergetic particle beam,
which can be significant.
6 General radiation effects in solar cells
6.1 Solar-cell radiation damage
Solar cells, like all semiconductor devices, are subject to electrical degradation when exposed to particle
irradiation. In terms of radiation damage to solar cells used in space, the primary particles of interest
are electrons and protons. When these energetic particles are incident upon the solar cell material,
they collide with the atoms of the crystal lattice of the solar cell. In these atomic collisions, energy is
transferred from the incident particle to the target atom. This energy can be transferred in several
ways. The majority of the energy is transferred through ionization of the target atom, where electrons
of the target atom absorb the transferred energy and are promoted to higher energy levels. Another
energy transfer mechanism is through nonionizing events, which results in the displacement of the
target atom. If enough energy is transferred in a nonionizing event, then the displaced target atom may,
in turn, displace other atoms, creating a cascade of displaced atoms. The displacement damage induced
by the nonionizing interactions is the primary cause of most solar cell degradation.
When an atom is displaced in a lattice, the electron energy band structure of the material is disturbed,
and localized energy levels can be created near the site of the defect. These defect energy levels can
act to trap electrical charge carriers, thus restricting their ability to move through the material, which
is characterized by a reduction in the minority carrier diffusion length. Since solar cell operation
depends on the motion of photogenerated charge carriers through the material, these defect sites tend
to degrade the solar cell performance.
The amount of displacement damage caused by an incident particle is a function of the type of incident
particle (i.e. electron or proton), the particle energy, and the composition of the crystal lattice. The
rate at which the incident particle transfers energy to the crystal lattice through nonionizing events
is referred to as the nonionizing energy loss (NIEL). Electrons become more damaging as the incident
electron energy increases. The opposite is true for protons, where the lower energy protons are the
most damaging. Also, protons are significantly more damaging in comparison to electrons, primarily
due to the increased proton differential scattering cross section for atomic displacements. There is a
lower limit to displacement damage corresponding to the threshold energy for atomic displacements.
This threshold energy is dependent on the semiconductor material that constitutes the solar cell.
6.2 Radiation effects on solar cell cover materials
Although not specifically a solar cell radiation effect, it is appropriate in this document to note the
effects of irradiation on solar cell coverglass materials and the adhesives which are typically used
to attach them. Certain solar cell coverglass material has been shown to darken under ultraviolet or
[6]
particle irradiation thereby absorbing some of the incident light . This increased light absorption can
reduce the solar cell output in one of two ways: (1) reduction of the amount of light that reaches the cell,
and (2) increase in operating temperature of array that reduces the cell electrical conversion efficiency.
Whereas crystalline solar cells are typically degraded by atomic displacement damage, coverglasses
[1]
are typically more sensitive to ionization effects . The “absorbed doses” associated with the radiation
environment of a particular mission can be calculated and then simulated by laboratory testing.
Testing cells with attached coverglass or different geometries require special care (see References [1]
and [2]).
7 Radiation test methods
7.1 General
As described in Clause 5, the space radiation environment consists of an omnidirectional spectrum of
particle energies, and as described in this clause, solar cell radiation damage is energy dependent. The
...