1. Defects in solar cell materials: the good, the bad, and the ugly Tim Gfroerer Davidson College, Davidson, NC with Yong Zhang University of NC @ Charlotte and Mark Wanlass National Renewable Energy Lab, Golden, CO ~ Supported by the Charlotte Research Institute and the American Chemical Society – Petroleum Research Fund ~

2. Some of the experiments and analysis by... Ryan Crum and Mark Crowley (’11) Mac Read and Caroline Vaughan (’10)

3. Outline Semiconductors, solar cells, and defects Recombination, radiative efficiency, and dependence on defect level distributions Photoluminescence imaging and modeling Confocal photoluminescence microscopy and the role of diffusion

4. Semiconductors Periodic Potential Physlet* * Physlet Quantum Physics: An Interactive Introduction by Mario Belloni et al. (2006).

5. Solar Cell Operation Conduction Band Valence Band PHOTON ENERGY ELECTRON E-Field E- HOLE E-Field E- ++ + + - - - - - CURRENT ABSORPTION When a photon is absorbed, an electron is excited into the conduction band, leaving a hole behind in the valence band. Some heat is lost, reducing efficiency. Then an internal electric field sweeps the electrons and holes away, creating electricity. HEAT

6. Good Defects: Impurities for p/n Junction Formation N P+ + + + + + + + + + + - - - - - ++ + + + + + + + + + + + - + + - - + + + - - - + + + - - - +

7. Semiconductor Defects Dislocation Applet Defect Level Physlet ~ from Physlet Quantum Physics: An Interactive Introduction by Mario Belloni et al. (2006).

8. Bad Defects: Defect-Related Trapping and Recombination Conduction Band Valence Band ENERGY Defect Level - + HEAT Electrons can recombine with holes by hopping through defect levels and releasing more heat. This loss mechanism also reduces the efficiency of a solar cell. HEAT

9. Radiative Recombination and Efficiency Conduction Band Valence Band PHOTON ENERGY - + Radiative Efficiency = (light out) / (light in) = (radiative rate) / (total recombination rate) heat light in light out Radiative Rate ~ n x p

10. Photoluminescence Imaging Laser Camera Lowpass filter Sample Experiment Excitation-Dependent Images

11. Top View of Diffusion to Dislocations

12. Simulated Images ExperimentSimulation

13. Simulation Details 2 nd Simulation1 st Simulation Generation, recombination, and diffusion with augmented defect-related recombination in dislocation pixel: Recombination Assumptions: 1.Defect levels clustered near the middle of the gap – no thermal excitation out of traps 2.(# of electrons) = (# of holes) = n Theoretical Efficiency: Recombination Improvements: 1.Defect level distribution can be tailored to achieve the best fit 2.Theory accounts for thermal excitation out of traps 3.(# of e - s in conduction band) = n can differ from (# of holes in valence band) = p 4.(# of trapped e - s) = dn can differ from (# of trapped holes) = dp Theoretical Efficiency:

14. Better Simulated Images ExperimentSimulation

15. The Defect-Related Density of States Valence Band Conduction Band ENERGY The distribution of defect levels within the bandgap can be represented by a density of states (DOS) function as shown above.

16. Defect-Related Density of States Used for Better Simulation

17. Confocal Photoluminescence Microscopy Laser Spectrometer Notch Filter Mirror Sample Lens Translation Stage Lens Experiment Contrast Map 20 microns Photoluminescence Contrast Aperture Lens

18. Confocal Maps Before

19. Confocal Maps BeforeAfter: Ugly Defects!

20. Confocal Maps of an Ugly Defect High Magnification Low Magnification

21. Radial Contrast Profile

23. Effective Diffusion Length

24. Electrons? Holes?

25. D Dislocation Electron Hole Excitation & Detection L D Mid-Excitation P P P P P P P P P P + P Pt. Defect - + P P P L D Low-Excitation P P P P P P P P P P + P P P + + + + + + + + - - - - - - - - - - Top View of Confocal Measurement with Diffusion to a Dislocation

26. D Dislocation Electron Hole Excitation & Detection L D P P P P P P P P P P + P Pt. Defect - + P P P + + + + + + + - - - - - - - - Top View of Confocal Measurement with Diffusion to a Dislocation Mid-Excitation L D P P P P P P P P P P + P P P + + + + + + - - - - - - - - High-Excitation - - - - + + + +

27. Side View of a Solar Cell Under High Illumination DISLOCATIONS PHOTONS + - + p/n Junction E-Field ELECTRICITY! -

28. Side View of a Solar Cell Under Low Illumination DISLOCATIONS PHOTONS + - + LOST! - p/n Junction E-Field OK

29. Conclusions Defects reduce solar cell efficiency by providing new recombination pathways (loss) Photoluminescence is a powerful tool for examining the properties of defects Depletion of electrons and holes near dislocations depends strongly on illumination The physics of confocal microscopy differs dramatically from the physics of imaging Ultimately, understanding diffusion near defects will facilitate better solar cell design