Radiation-induced defects are responsible for solar cell degradation. The effects of radiation and annealing on the defects of a GaAs/Ge solar cell are modeled and analyzed in this paper. The electrical performance and spectral response of solar cells irradiated with 150 keV proton are examined. Then, thermal annealing was carried out at 120°C. We found that the proportion of defect recovery after annealing decreases with increasing irradiation fluence. The minority carrier lifetime increases with decreasing defect concentration, which means that the electrical performance of the solar cell is improved. We calculated the defect concentration and minority carrier lifetime with numerical simulation and modeled an improved annealing kinetic equation with experimental results.
Space radiation produced damage in solar cells, which will limit the capability and lifetime of satellites. In order to mitigate the space radiation damage, the mechanisms of radiation-induced degradation of the solar cell have been heavily studied [
The minority carrier lifetime for solar cells is extremely sensitive to defects and increases with the decrease of defect concentration [
In this paper, single-junction GaAs/Ge solar cells were irradiated with 150 keV proton, and the irradiated solar cells were annealed at 120°C. Then, we used an improved defect annealing kinetic equation to calculate the defect concentration. And based on the defect concentration, the simulation of short-circuit current during the annealing process was in good agreement with the experimental results.
The samples used in this work are GaAs/Ge single-junction solar cells manufactured by metal organic chemical vapor deposition (MOCVD). The cell size is
Schematic diagrams of GaAs/Ge cells used in the study.
The fluence setting in irradiation experiment.
Group | Fluence (cm-2) |
---|---|
1 | |
2 | |
3 | |
4 | |
5 | 0 |
The
The
The electric performance and EQE for GaAs solar cells irradiated with 150 keV proton: (a) degradation of
The steady-state operating characteristics of solar cells can be described by the minority carrier diffusion equations [
The meaning of all parameters is shown in Table
Parameters in the minority carrier diffusion equations (Equation (
Parameters | Description |
---|---|
Hole diffusion coefficient | |
Electron diffusion coefficient | |
Hole concentration in the n-type material | |
Electron concentration in the p-type material | |
Minority carrier hole lifetimes | |
Minority carrier electron lifetimes | |
Depth from the front surface of the solar cell | |
Optical generation rate of electron-hole pairs |
The current density of emitter, base, and space charge regions can be deduced from the above minority carrier diffusion equations [
The meaning and value of parameters in the above formula are shown in Table
Parameters in the current density equation (Equation (
Parameters | Description | Value |
---|---|---|
Quantity of electric charge | ||
Emitter thickness | 0.1 | |
Base thickness | 2.97 | |
Space charge zone thickness | 0.03 | |
Emission zone minority carrier diffusion coefficient | 6.63 cm2/s | |
Base zone minority carrier diffusion coefficient | 1.93 cm2/s | |
Front surface recombination rate | ||
Back surface recombination rate | ||
Minority carrier lifetime | 38 ns | |
Emission zone minority carrier diffusion length, |
4.17 | |
Base zone minority carrier diffusion length, |
7.72 | |
Simplified front surface recombination rate, |
2.16 | |
Simplified back surface recombination rate, |
4.66 |
The AM0 solar spectrum irradiance and corresponding photon flux.
The photon absorption coefficient of GaAs and front surface reflectance of the solar cell before irradiation.
The short-circuit current density
According to Equations (
We calculated the defect concentration and minority carrier lifetime with 150 keV proton fluence (seen in Figure
The defect concentration and minority carrier lifetime vs. 150 keV proton fluence.
To verify the accuracy and parameter settings of Equation (
The normalized
Studies show that after thermal annealing, the irradiation damage of solar cells will be partially recovered [
In situ measurement for
The annealing kinetics of radiation defects can be expressed as [
Figure
The defect concentration and minority carrier lifetime before and after annealing and the annealing rate and the proportion of irrecoverable defects.
Fluence (1/cm2) | ||||||
---|---|---|---|---|---|---|
Before annealing | After annealing | Before annealing | After annealing | |||
4.49 | 17.45 | 6.30 | ||||
1.42 | 3.02 | 35.94 | ||||
1.11 | 2.50 | 33.22 | ||||
1.04 | 2.07 | 40.17 | ||||
0.06 | 0.12 | 45.58 |
The degradation of GaAs solar cells in electrical properties can be enhanced by increasing the proton fluence. However, it can be improved in the process of thermal annealing. The short-circuit current after irradiation and annealing can be studied by the minority diffusion equation. Based on the experiment results and theoretical analysis, an improved annealing kinetic equation is carried out.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This work was supported by the Shanghai Institute of Space Power-Sources, and the solar cells used in the experiment were also provided by it. The work was funded by the Fundamental Research Funds for the Central Universities, No. NS2019052.