Mitsuru Imaizumi Space Solar Cells -- II -- SLATS Spacecraft Power System, Kyusyu Inst. Tech. Oct. 26, 2018 Space Solar Cells -- II -- SLATS Mitsuru Imaizumi

Contents Operation principle and fundamentals Radiation damage and effects Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell

Contents Operation principle and fundamentals Radiation damage and effects Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell

High efficiency Si solar cell InGaP/GaAs/Ge triple-junction solar cell Space Solar Cells Buried bypass diodes Bypass diode Inter-connector Cell size: 2×2 cm2 Size: 40mm×60mm Size: 37mm×76mm High efficiency Si solar cell InGaP/GaAs/Ge triple-junction solar cell

Energy band in semiconductor Operation Principle Intrinsic N-type P-type Energy band in semiconductor

Operation Principle

Current in Solar Cell V Iph Id I + _

Output Characteristics Current-Voltage (I-V) characteristics Photo- generation current (a) Under dark (b) Under light Current-Voltage (I-V) characteristics

Output Characteristics

Output Performance Parameters

I-V Characteristics of a 3J solar cell Output Performance I-V Characteristics of a 3J solar cell

Quantum Efficiency of a high efficiency Si solar cell Spectral Response Quantum Efficiency of a high efficiency Si solar cell

Equivalent Circuit of Solar Cell Id Vd Rs = 0 Iph Rsh = ∞ V n = 1 Ish

Ideal Current Output of Solar Cell

Ideal Current Output of Solar Cell

Ideal Current Output of Solar Cell

Ideal Current/Voltage Output of Solar Cell

Ideal Current/Voltage Output of Solar Cell Sun Light Sun Light

Spectral Response Output current estimation ×

Contents Operation principle and fundamentals Radiation damage and effects Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell

Radiation Damage in Solar Cell N-region Electron Hole Light Defect High-energy particles P-region Loss Incident of high-energy particles (electrons/protons) ↓ Elastic/non-elastic collision with atoms Formation of vacancy-interstitial (Flenkel) pairs (Some defect reactions) Generation of minority-carrier recombination center(s) and majority-carrier trap(s)

Operation Principle

Radiation Degradation Minority-carrier recombination

Radiation Degradation Majority-carrier reduction Before irradiation After irradiation Majority-carrier reduction

Radiation Damage in Solar Cell Introduction of minority carrier recombination centers Change in minority carrier diffusion length (L) Introduction of majority carrier traps Change in majority carrier concentration (p)

Equivalent Circuit of Solar Cell Rs Iph Rsh V n

Effect of generation current decrease

Effect of shunt resistance decrease

Effect of series resistance increase

Output Performance Degradation Cell size: 2×2 cm2 Irradiation Decrease in output power Degradation trend (Pmax)

Degradation trend of high-efficiency Si solar cell (a) Absolute values (b) Remaining factors Degradation trend of high-efficiency Si solar cell

Contents Operation principle and fundamentals Radiation damage and effects Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell

Degradation of Single-junction Solar Cell B doped p-Si (100) base 10 Wcm (2×1015cm-3) p+-Si back surface field Ti/Pd/Ag contact Al back surface reflector Ti/Pd/Ag contacts AR coating (TiO2/Al2O3) P doped n+-Si emitter x j = 0.15 mm 50/100 mm Structure of sample solar cell

Degradation of Single-junction Solar Cell (a) 10MeV protons (b) 1MeV electrons Degradation trend of high-efficiency Si solar cell

Degradation of Single-junction Solar Cell Low fluence region: Gradual decrease ↓ Transition region: Anomalous increase in Isc High fluence region: Drastic decrease/ Sudden death

Anomalous Degradation Analysis Short circuit current (Isc) is expressed by First stage: Reduction of L leads to a decrease in Isc. Second stage: Reduction of p leads to an increase in W and consequently an increase in Isc. Third stage: Reduction of p leads to an increase in resistivity and consequently abrupt decrease in Isc.

Anomalous Degradation Analysis

Anomalous Degradation Analysis Experimental Results KL = 2×10-7 RC = 50 cm-1 Anomalous Degradation Analysis

Anomalous Degradation Analysis KL = 2×10-7 RC = 50 cm-1

Contents Operation principle and fundamentals Radiation damage and effects Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell

Structure of Space Solar Cell InGaP/GaAs/Ge triple-junction solar cell

Degradation of Multi-junction Solar Cell Structure of 3J solar cells InGaP top cell GaAs middle cell Ge bottom cell (substrate) Substrate 140mm Epi layers ~10mm P-electrode N-electrode ARC Structure of 3J solar cells TRIM simulation of 3MeV proton irradiation onto 3J solar cell

Degradation of Multi-junction Solar Cell Degradation trend curves Energy dependence of remaining factors

Degradation of Sub-cells in 3J Solar Cell The last is comparison of radiation tolerance of the three sub-cells. This is the first for such trial using the same irradiation facility and the same measurement apparatus as far as we know. Left hand side is degradation of Isc, and right hand side is that of Voc as a function of 10 MeV proton fluence. For both, InGaP top cell exhibits highest radiation tolrance, while GaAs middle cell shows the lowest. Ge bottom cell has interesting features. Isc tolerance is as low as GaAs middle cell, but Voc tolerance is as high as InGaP top cell. Isc degradation Voc degradation

Degradation of Sub-cells in 3J Solar Cell InGaP top-cell GaAs middle-cell InGaAs bottom-cell Graded buffer Epi layers ~20mm The last is comparison of radiation tolerance of the three sub-cells. This is the first for such trial using the same irradiation facility and the same measurement apparatus as far as we know. Left hand side is degradation of Isc, and right hand side is that of Voc as a function of 10 MeV proton fluence. For both, InGaP top cell exhibits highest radiation tolrance, while GaAs middle cell shows the lowest. Ge bottom cell has interesting features. Isc tolerance is as low as GaAs middle cell, but Voc tolerance is as high as InGaP top cell. Thin Film IMM-3J cell

Degradation of Sub-cells in 3J Solar Cell The last is comparison of radiation tolerance of the three sub-cells. This is the first for such trial using the same irradiation facility and the same measurement apparatus as far as we know. Left hand side is degradation of Isc, and right hand side is that of Voc as a function of 10 MeV proton fluence. For both, InGaP top cell exhibits highest radiation tolrance, while GaAs middle cell shows the lowest. Ge bottom cell has interesting features. Isc tolerance is as low as GaAs middle cell, but Voc tolerance is as high as InGaP top cell. Electrons Protons Isc degradation

Degradation of Sub-cells in 3J Solar Cell The last is comparison of radiation tolerance of the three sub-cells. This is the first for such trial using the same irradiation facility and the same measurement apparatus as far as we know. Left hand side is degradation of Isc, and right hand side is that of Voc as a function of 10 MeV proton fluence. For both, InGaP top cell exhibits highest radiation tolrance, while GaAs middle cell shows the lowest. Ge bottom cell has interesting features. Isc tolerance is as low as GaAs middle cell, but Voc tolerance is as high as InGaP top cell. Electrons Protons Voc degradation

Degradation of Sub-cells in 3J Solar Cell The last is comparison of radiation tolerance of the three sub-cells. This is the first for such trial using the same irradiation facility and the same measurement apparatus as far as we know. Left hand side is degradation of Isc, and right hand side is that of Voc as a function of 10 MeV proton fluence. For both, InGaP top cell exhibits highest radiation tolrance, while GaAs middle cell shows the lowest. Ge bottom cell has interesting features. Isc tolerance is as low as GaAs middle cell, but Voc tolerance is as high as InGaP top cell. Electrons Protons Pmax degradation