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Dan Hampton and Tim Gfroerer , Davidson College, Davidson, NC

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Presentation on theme: "Dan Hampton and Tim Gfroerer , Davidson College, Davidson, NC"— Presentation transcript:

1 Photocapacitance measurements on GaP alloys for high efficiency solar cells
Dan Hampton and Tim Gfroerer , Davidson College, Davidson, NC Mark Wanlass, National Renewable Energy Lab, Golden, CO Acknowledgements Davidson Research Initiative American Chemical Society Petroleum Research Fund Jeff Carapella for growing and processing test structures

2 Solar Cell Operation

3 Conduction Band Conduction Band Valence Band Valence Band Valence Band
E=hc/λ Energy + Valence Band Valence Band Valence Band Valence Band

4 Conduction Band E=hc/λ Energy + Valence Band Valence Band

5 Conduction Band Conduction Band Conduction Band Conduction Band
Energy Electron-Hole Pair + + Valence Band Valence Band

6 Conduction Band Conduction Band Conduction Band Conduction Band
Band Edge Energy + + Valence Band Valence Band

7 Conduction Band Conduction Band Conduction Band Conduction Band
Heat Conduction Band Conduction Band Conduction Band Conduction Band Band Edge Energy Ephoton – Band Gap Energy = Energy Lost + + Valence Band Valence Band

8

9 Defects

10 Conduction Band Conduction Band Valence Band Valence Band Valence Band
Defect Levels Increasing Energy Trap Depth + + + + + + Valence Band Valence Band Valence Band Valence Band Hole

11 Conduction Band Conduction Band Valence Band Valence Band Valence Band
+ Defect Levels Defect Levels Defect Levels + Increasing Energy Trap Depth Capture + + + + + Valence Band Valence Band Valence Band Hole

12 Conduction Band Conduction Band Valence Band Valence Band Valence Band
Defect Levels Defect Levels Defect Levels + Increasing Energy Escape Trap Depth + + + + + + Valence Band Valence Band Valence Band Hole

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14 1 mm Picture of Device

15 N+ P - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Photocapacitance - - - - + + - - - - + - - - + - - - + N+ - P - - + - - + - - - - + - - + - - - - - + - + Depletion with bias

16 Depletion without bias
Photocapacitance - - - - + + - - - - + - - - + - - - + N+ - P - - + - - + - - - - + + - - - - - - - + - + Depletion without bias

17 Depletion without bias
Depletion Layer - - - - + - + - - - + - - - - + - + - - N+ - P - - + - - - - + - + - + - + - - - - - - + Depletion without bias

18 Depletion without bias
Depletion Layer T = 77 K - - - - + - + - - - + - - - - + - + - - N+ - P - - + - - + - - - - + + - + - - - - - - + Depletion without bias

19 Depletion without bias
Depletion Layer T = 77 K - - - - + + - - - - + - - - + - - - + N+ - P - - + - - + - - - - + - + - - - - - - + - + Depletion without bias

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21

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23 Conclusions Optical threshold escape energies are higher than thermal threshold energies → Configuration Dependent Traps GaAsP has a very large trap density, comparable to the density of dopants. This suggests zinc is contributing to the formation of the traps.

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