Mitsuru Imaizumi HTV-5 Spacecraft Power System, Kyusyu Inst. Tech. Dec. 11, 2015

2 1.Operation principle and fundamentals 2.Radiation damage and effects 3.Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell Contents

3 1.Operation principle and fundamentals 2.Radiation damage and effects 3.Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell Contents

High efficiency Si solar cell Cell size: 2 × 2 cm 2 InGaP/GaAs/Ge triple-junction solar cell Size: 40mm×60mm Size: 37mm×76mm Bypass diodeInter-connector Space Solar Cells Buried bypass diodes 4

Operation Principle Intrinsic N-type P-type Energy band in semiconductor 5

Operation Principle 6

Current in Solar Cell V I ph IdId I + _ 7

Current-Voltage (I-V) characteristics Output Characteristics (a) Under dark(b) Under light Photo- generation current 8

Output Characteristics 9

Output Performance Parameters 10

I-V Characteristics of a 3J solar cell Output Performance 11

Spectral Response Quantum Efficiency of a high efficiency Si solar cell 12

Equivalent Circuit of Solar Cell V RsRs R sh I ph VdVd IdId I sh = 0 = ∞ n = 1 I 13

Ideal Current Output of Solar Cell 14

Ideal Current Output of Solar Cell 15

Ideal Current Output of Solar Cell 16

Ideal Current/Voltage Output of Solar Cell 17

18 Sun Light Ideal Current/Voltage Output of Solar Cell

Output current estimation × Spectral Response 19

20 1.Operation principle and fundamentals 2.Radiation damage and effects 3.Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell Contents

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) N-region Electron Hole Light Defect High-energy particles P-regionLoss Radiation Damage in Solar Cell 21

Operation Principle 22

Radiation Degradation 23 Minority-carrier recombination

Before irradiation Majority-carrier reduction 24 After irradiation Radiation Degradation

Equivalent Circuit of Solar Cell V RsRs R sh I ph n I 25

少数キャリア寿命低下の影響 放射線による性能劣化 26

Effect of shunt resistance decrease 27

Effect of series resistance increase 28

Decrease in output power Cell size: 2 × 2 cm 2 Output Performance Degradation Degradation trend (Pmax) Irradiation 29

Degradation trend of high-efficiency Si solar cell Degradation Trend (a) Absolute values (b) Remaining factors 30

Introduction of minority carrier recombination centers Change in minority carrier diffusion length (L) Introduction of majority carrier traps Change in majority carrier concentration (p) Decrease in carrier concentration (p) Increase in resistivity (  ) and depletion region width (W) Radiation Damage in Solar Cell 31

32 1.Operation principle and fundamentals 2.Radiation damage and effects 3.Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell Contents

B doped p-Si (100) base 10  cm (2 × cm -3 ) p + -Si back surface field Ti/Pd/Ag contact Al back surface reflector Ti/Pd/Ag contacts AR coating (TiO 2 /Al 2 O 3 ) P doped n + -Si emitter x j = 0.15  m 50/100  m Degradation of Single-junction Solar Cell Structure of sample solar cell 33

Degradation trend of high-efficiency Si solar cell (a) 10MeV protons (b) 1MeV electrons Degradation of Single-junction Solar Cell 34

Low fluence region: Gradual decrease ↓ Transition region: Anomalous increase in Isc ↓ High fluence region: Drastic decrease/ Sudden death Degradation of Single-junction Solar Cell 35

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 36

Anomalous Degradation Analysis 37

Experimental Results K L = 2×10 -7 R C = 50 cm -1 Anomalous Degradation Analysis 38

K L = 2×10 -7 R C = 50 cm -1 Anomalous Degradation Analysis 39

40 1.Operation principle and fundamentals 2.Radiation damage and effects 3.Radiation degradation characteristics 4-1. Single-junction solar cell 4-2. Multi-junction solar cell Contents

Structure of Space Solar Cell InGaP/GaAs/Ge triple-junction solar cell 41

Structure of 3J solar cells TRIM simulation of 3MeV proton irradiation onto 3J solar cell InGaP top cell GaAs middle cell Ge bottom cell (substrate) Substrate 140  m Epi layers ~10  m P-electrode N-electrode ARC Degradation of Multi-junction Solar Cell 42

Degradation of Multi-junction Solar Cell Degradation trend curves Energy dependence of remaining factors 43

Degradation of Sub-cells in 3J Solar Cell Isc degradation Voc degradation 44

Radiation-hardening of 3J Solar Cells Spectral response of 3J cell Estimation of Isc of sub-cells in irradiated 3J solar cells 45