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Solar Cells, Sluggish Capacitance, and a Puzzling Observation Tim Gfroerer Davidson College, Davidson, NC with Mark Wanlass National Renewable Energy Lab, CO ~ Supported by Bechtel Bettis, Inc. and the American Chemical Society – Petroleum Research Fund ~
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Experiments by... Kiril Simov (Davidson ’05) Patten Priestley (Davidson ’03) and Malu Fairley (Spelman ’03)
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Outline Semiconductors, defects, and solar cells Diode capacitance and the DLTS experiment Our measurements and an unusual result A new model for minority carrier trapping/escape during DLTS
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Semiconductors Periodic Potential Physlet
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InGaAs Bandgap vs. Alloy Composition Bandgap vs. Lattice Physlet
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Semiconductor Defects Lattice-Mismatch Applet Defect Level Physlet (from the forthcoming Physlet Quantum Physics: An Interactive Introduction to Quantum Theory by Mario Belloni et al., due out this Fall
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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. An internal electric field sweeps the electrons and holes away, creating electricity.
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Defect-Related Trapping and Recombination Conduction Band Valence Band ENERGY Defect Level - + PHONONS But electrons can recombine with holes by hopping through defect levels and releasing phonons (heat). This loss mechanism reduces the efficiency of a solar cell.
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Defect-Related Transition Probabilities P ~ 10 -3 P ~ (0.5) 10 ~ 10 -3 P ~ 10 -5 P ~ 10 -1 P ~ (0.5) 16 ~ 10 -5 P ~ (0.5) 4 ~ 10 -1 The probability P of transitions involving phonon emission depends on the number of phonons required, which is determined by the position of the defect level in the gap. - +++ --
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p/n Junction Formation N P+ + + + + + + + + + + - - - - - ++ + + + + + + + + + + Depletion Layer + - + + - - + + + - - - + + + - - - +
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Bias-Dependent Depletion + + - - N P+ + + + + + + + + + + - - - ++ + + + + + + + + + + Depletion Layer + - + - - + + + - - + - - With Bias
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Diode Capacitance No bias Reverse bias d1d1 V built-in V built-in +V applied d2d2 C = Q/ V ~ 0 A/d Reverse bias increases the separation between the layers where free charge is added or taken away. ENERGY
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Defect characterization via DLTS + + - - N P+ + + + + + + + + + + - - - ++ + + + + + + + + + + Depletion Layer With Bias + - + - - + + + - - + Temporary Reduced Bias Depletion Layer With Bias - - + + - - Temporary Reduced Bias + +
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Typical DLTS Measurements
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DLTS Experimental Setup Computer with LabVIEW Temp Controller Pulse Generator Cryostat with sample Digital Scope (Tektronix) (1) (2) (3) (4) (5) Oxford 77K Agilent Capacitance meter (Boonton)
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Device Structure and Band Diagram {
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Transient Capacitance: Escape
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Filling Pulse Dependence: Capture
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Proposed Model
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Testing the Model
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Variable-Bandgap Lattice-Mismatched Stuctures Undoped InAs y P 1-y, 30 nm Undoped In x Ga 1-x As, 1.5 μm Undoped InAs y P 1-y buffer, 1 μm Undoped InAs y P 1-y step-grade region: 0.3 μm/step (~ -0.2% LMM/step), n steps Undoped InP substrate
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Radiative Recombination Conduction Band Valence Band PHOTON ENERGY - + light in = heat + light out radiative efficiency = light out / light in heat light in light out
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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.
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Radiative Efficiency Measurements heat light
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Four Conclusions 0.29eV hole trap is observed in n-type InGaAs under reverse bias Temperature-dependent capture and escape rates are symmetrical Rates level off at cold temperatures due to tunneling Device modeling points to defect states near the p+/n junction Two References T.H. Gfroerer et al., APL 80, 4570 (2003). T.H. Gfroerer et al., IPRM (2005).
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