Inverted High-efficiency Triple-junction Solar Cells Based on GaAs Substrates

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: Eg1 = 1.86 eV; Eg2 = 1.34 eV; Eg3 = 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. • 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 • Bandgap Match (Theoretical Bandgaps: 1.86 eV; 1.34 eV; 0.93 eV)

7. In order to achieve high efficiency • 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!  • Ga0.5In0.5P/GaAs/In0.3Ga0.7As: 1.8 eV; 1.4 eV, 1.0 eV (paper 1), Acceptable! • Ga0.51In0.49P/In0.04Ga0.96As/In0.37Ga0.63As: 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 1 Paper 2 Test results under the direct terrestrial spectrum as a function of concentration

10. Thank you! Any questions?