Defects in solar cell materials: the good, the bad, and the ugly Tim Gfroerer Davidson College, Davidson, NC with Yong Zhang University of Charlotte.

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Presentation transcript:

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

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

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

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

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

Good Defects: Impurities for p/n Junction Formation N P

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

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

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

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

Top View of Diffusion to Dislocations

Simulated Images ExperimentSimulation

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:

Better Simulated Images ExperimentSimulation

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.

Defect-Related Density of States Used for Better Simulation

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

Confocal Maps Before

Confocal Maps BeforeAfter: Ugly Defects!

Confocal Maps of an Ugly Defect High Magnification Low Magnification

Radial Contrast Profile

Effective Diffusion Length

Electrons? Holes?

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

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

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

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

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