1. Inverted High-efficiency Triple-junction Solar Cells Based on GaAs Substrates EECS 235 Paper Review Presentation Xiaojun Zhang

2. Papers Reviewed 1.J.F. Geisz, S. Kurtz, et al, High-efficiency GaInP/GaAs/InGaAs triple-junction solar cells grown inverted with a metamorphic bottom junction, Appl. Phys. Lett. 91, 023502 (2007) 2.J.F. Geisz, D.J. Friedman, et al, 40.8% efficient inverted triple-junction solar cell with two independently metamorphic junctions, Appl. Phys. Lett. 93, 123505 (2008)

3. Triple-junction Solar Cells Theoretical Bandgaps: E g1 = 1.86 eV; E g2 = 1.34 eV; E g3 = 0.93 eV

4. Schematic of an inverted triple- junction solar cell Growth Direction

5. Why inverted? The top cell produces more power than the lower ones. Importance: Cell 1 > Cell 2> Cell 3. The top cell produces more power than the lower ones. Importance: Cell 1 > Cell 2> Cell 3. By inverting the growth sequence, the top layers, which are more important, can be grown first and thus have a better quality. By inverting the growth sequence, the top layers, which are more important, can be grown first and thus have a better quality.

6. In order to achieve high efficiency Two important factors to be considered Lattice Match Lattice Match Bandgap Match (Theoretical Bandgaps: Bandgap Match (Theoretical Bandgaps: 1.86 eV; 1.34 eV; 0.93 eV)

7. In order to achieve high efficiency Considering bandgap match ( ): Considering bandgap match (1.86 eV; 1.34 eV; 0.93 eV ): GaInP/GaInAs/Ge : 1.86 eV, 1.39 eV, 0.67 eV, Ge, not good!  Ga 0.5 In 0.5 P/GaAs/In 0.3 Ga 0.7 As: 1.8 eV; 1.4 eV, 1.0 eV (paper 1), Acceptable! Ga 0.51 In 0.49 P/In 0.04 Ga 0.96 As/In 0.37 Ga 0.63 As: 1.83 eV; 1.34 eV; 0.89 eV (paper 2), Best!

8. Experimentals Grown by atmospheric-pressure organometallic vapor phase epitaxy (OMVPE) on a (001) GaAs substrate. Gold was electroplated. The sample was then mounted to a handle with epoxy. The GaAs substrate was removed using selective chemical etching. Antireflective coating and gold grids on the front layer were deposited.

9. Results Paper 1Paper 2 Test results under the direct terrestrial spectrum as a function of concentration

10. Thank you! Any questions?