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Theoretical aspect of radiation damage
Irradiation damage to the solar cells is mostly caused by atomic displacements which break periodic lattice structure of the semiconducting materials and they interfere the movement of minority carriers resulting in decrease of carrier lifetime. These irradiation atomic displacements can also affect properties of other electrical devices such as battery, detectors and communication instruments which are equipped for a space mission. For this reason, the radiation effect has gained a lot of interests in the study of degradation of this kind of materials and devices including solar cells. In space, the origin of irradiation is mostly due to energetic particles like electrons and protons. When these particles hit the surface of materials and enter into, they interact in several ways with these materials since they have mass, energy and some particles are charged. Once a charged particle penetrates a material, it slows down by consuming or transferring its energy with electrons and nuclei in the material. In this process, several types of interactions can occur and these interactions can also vary with the speed and the energy of an incident particle [13].
Basically, two types of interactions exist between charged particles and matter; elastic collisions and inelastic collisions. First, the inelastic collisions occur between the projectile and the cloud of electrons of target. By doing interactions with electrons, the incident particles lose its energy and slow down its velocity of moving. Independent on target materials, once the velocity of moving ion is two times slower than that of electrons at the top of the Fermi level, electrons cannot be excited. This threshold energy can be determined for each material, in keV. Thus, below this incident energy, the collision between the projectile and the target is mainly elastic. By the elastic collision, the projectile directly transfers the energy to the target atom, not losing the energy by ionization of the target. As a consequence, the energy transfer of projectile-target is almost conserved. This process is a main cause of displacement damage and responsible for the degradation of solar cell.
Displacement damage and atomic displacement
Considering only the elastic collision process of radiation of heavy charged particles, we will see how the particle is transferring its energy to the target atom and the equation describing irradiations with electrons, comparing the relativistic velocities. In practice, depending on the energy of incident particle, elastic collisions are distinguished. If the particles have higher energies so that the projectile can penetrate the cloud of electrons surrounding the target atom and transfer the energy directly to the atom, it is called Rutherford collisions. Meanwhile, when the particles have lower energies, they cannot penetrate the electron cloud. As a result, the collisions occur between the projectile and the cloud electrons, known as hard sphere collisions. The displacements induced by the interaction between the incident-charged particle and the target atom are considered as primary displacements. Depending on the initial energy of incident particle, the primary atomic displacement can be either due to Rutherford collisions or hard sphere collisions. When the atoms are detached from his lattice site by collisions with the projectile, these species are called primary knock-ons (PKA) atom and they have enough kinetic energy to produce other displacements known as secondary displacements. In elastic collisions the interaction of two atoms can be described with a screened Coulomb potential energy given in the form of:
Nature of irradiation-induced defects in solar cell materials
The study of defect is one of the most important problem in semiconductor physics. In crystalline or amorphous structure, the existence of defects can affect its electrical or optical properties in complex ways. Today, it is possible to theoretically predict a qualitative energy levels associated with some ideal simple intrinsic defects [22]. However, it is still not yet possible to qualitatively identify defects for the lattice distortion, and relaxation. To verify the theoretical prediction of defects, the experiments must be carried out to produce simple defects because tracking its mechanism after the production is already very complicated. The primarily created intrinsic defects, i.e. vacancies and interstitials are presumably moved out very fast and interact with other defects or impurities. Therefore, to irradiate with electrons is a proper choice to properly identify defects in a material. Then, once the defects are sufficiently identified, the comparison with proton irradiation result can be fulfilled. In this section, we collected and summarized some identified defects and their characteristics from the literature. We will discuss the production of defects and their behaviors in different kind of solar cell materials (GaAs, GaInP, and Ge) depending on the type of irradiation and temperatures. However, we have to keep in mind that the identified defects are limited as single defects, that is, complex of defects like cluster and their outcome property might not be measurable with modern measurement techniques. Furthermore, as we will mainly discuss below, most of defects that we are interested in for our study have been analyzed through either magnetic or electric way. So, we should be aware of that there could be still more veiled or non-identified defects by our irradiation conditions.
Production of defects in n- and p-doped Galium-Arsenide (GaAs)
Study of irradiation induced-defects in Galium-Arsenide (GaAs) compound has been continued since 1970s. There are several review articles which contain a considerable amount of works [23], [24]. For these studies, electron irradiation has been mainly used since it is an easy way to produce vacancies and interstitials in both Ga and As sublattices, and to follow the transformation of these primary defects when they become mobile and interact with each other or with various impurities in the material. Ions have been also used for irradiation. However, the complications have arisen due to the heavy mass of incident particle, when the ions penetrate into the material, it displaces a large number of atoms from the lattice creating a displacement cascade, i.e. to induce clusters of defects along the heavy ions path. As a consequence, it became one of difficulties to identify defects induced by ions, such as protons. In this reason, proton induced defects have not yet been extensively studied for GaAs. There are not so many data in literature. In fact, in spite of a large amount of work for electron irradiated GaAs, the identification of defects in n- and p-type GaAs is still not fully understood because of the nature of III-V compound material. The direct identification of defects can be provided by electron paramagnetic resonance (EPR) which is difficult to apply to this type of material due to its large magnitude of hyperfine and superhyperfine interactions of the paramagnetic defects with the nuclear spin of the different Gallium isotopes [22] compared to other materials such as II-VI and silicon. Fortunately, by using a combination with more sensitive technique called deep-level transient spectroscopy (DLTS), optical detection of magnetic resonance or of electron-nuclear double resonance, some defects in GaAs could be identified. Following to the section, some identified irradiation induced-defects in n-type and p-type GaAs will be presented.
Table of contents :
Acknowledgements
General introduction
1 Fundamentals of solar cells for space applications
1.1.1 Basic solar cell equations
1.1.2 Diffusion current
1.1.3 Generation-recombination current
1.1.4 Temperature dependence of solar cells
1.1.5 Spectral response of PN solar cells
1.2.1 Displacement damage and atomic displacement
1.2.2 Primary and secondary displacements
1.2.3 Ionization
1.3.1 Production of defects in n- and p-doped Galium-Arsenide (GaAs)
1.3.2 Production of defects in n- and p-doped Galium-Indium-Phosphide (GaInP)
1.3.3 Production of defects in n- and p-doped Germanium (Ge)
1.4.1 Effects in carrier lifetime and diffusion length
1.4.2 Effects in properties of solar cells
1.5.1 The concept of equivalent damage (JPL method)
1.5.2 The concept of displacement damage dose (NRL method)
Reference
2 Experimental details and Materials
2.1.1 Irradiation Facilities
2.1.2 Solar Simulator
2.1.3 Cryostat Chamber and measurement units
2.2 Structure of lattice matched GaInP/GaAs/Ge triple junction solar cell
2.3 Photon recycling effect in a component cell
2.4 In-situ characterization of TJ cells and its component cells
2.4.1 Indirect temperature measurement
2.4.2 Beginning Of Life performance of the cells
2.4.3 Electron and proton irradiation campaigns
References
3 Proton irradiation
3.1.1 Analysis of I-V characteristics before and after 1 MeV proton irradiations
3.1.2 Degradation of key parameters in TJ cells
3.2.1 Degradation of ISC and VOC at different temperatures
3.2.2 Electric field dependence of I-V characteristics
3.2.3 Orientation dependence of proton irradiation
3.2.4 Isochronal annealing in component cells
3.3.1 Temperature and fluence dependences of the degradation
3.3.2 Recovery of proton irradiation-induced defects
3.3.3 Recombination of photo generated current by irradiation-induced defects
Reference
4 Electron irradiation
4.1.1 Analysis of I-V characteristics before and after 1 MeV electron irradiations
4.1.2 Degradation of key parameters in TJ cells
4.2.1 Degradation of ISC and VOC at different temperatures
4.2.2 The excess leakage current in dark I-V characteristics
Discussion of the chapter
4.4.1 Uncertainty of the TJ cell degradation induced by electron irradiations
4.4.2 Origin of the excess current
Conclusion of the chapter 4
Reference
5 General discussion
Distribution of BOL and EOL data set: Case of electron and proton irradiated TJ cells
Correlation of radiation induced defects with electrical property of the solar cell
Conclusion of the chapter 5
Reference
