1. PREDICTION OF SPACE DEGRADATION OF MULTIJUNCTION SOLAR CELLS
GESEC R&D Inc.
Semiconductor Materials Engineering
PREDICTION OF SPACE DEGRADATION OF MULTIJUNCTION SOLAR CELLS
J. C. BOURGOIN, S. MAKHAM & G.C. SUN
GESEC R&D, 68 av. de la forêt, Avon, FRANCE .

2.  = 71 /Enl (eV.cm-1) , ( in cm-2)
A – Semi-empirical method
For GaAs:
 = 71 /Enl (eV.cm-1) , ( in cm-2)

3. Experimental verification
Using GaAs cell NASA data (proton energies from 0,2 to 9,5keV)*:
Standard: the degradation curve of Pm corresponding to 1 MeV electron irradiation.
(*) B.E. Anspaugh Solar Cell Radiation Handbook, JPL Pub, (1996).

4. B – Non empirical method
1-Classical calculation of the current-voltage characteristics of each subcell knowing the minority carrier lifetimes τ of the base and emitter for a given fluence φ:
1/= 1/0 + k
k: Introduction rate  
n & p : Capture cross sections of minority carriers 
2- Construction of the degradation of the MJ cell from that of each subcell i:
Isc(φ) = minimum (Isci (φ))
Voc (φ) = i (Voci(φ))

5. Experimental verification
Case of a GaInP/GaAs cell (Emcore):

6. Necessary knowledge, for both base and emitter for each subcell
k, introduction rate (or concentration kφ).
σb, σe, minority capture carrier cross section.
τ0e, τ0b, initial lifetimes.
thicknesses .
Recombination velocities at interfaces.
alloy composition x.
Techniques to determine these parameters:
k  DLTS, I-V in dark, electroluminescence.
x  Low temperature electroluminescence.
τ0e,b  I-V under illumination.

7. Prediction of degradation (2J Emcore)

8. Determination of k versus energy