#### Transcript Mines Colloquium 2005

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 ~ Experiments by . . . Kiril Simov (Davidson ’05) Patten Priestley (Davidson ’03) and Malu Fairley (Spelman ’03) 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 Semiconductors a free atoms V(r) r atomic crystal Energy levels Spacing decreasing n=3 n=2 n=1 - a Periodic Potential Physlet InGaAs Bandgap vs. Alloy Composition 1.6 1.4 GaAs InP Bandgap (eV) 1.2 Substrate 1.0 Bandgap vs. Lattice Physlet Severe Mismatch 0.8 0.6 0.4 0.2 5.6 InAs 5.7 5.8 5.9 6.0 Lattice parameter (Angstroms) 6.1 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 Solar Cell Operation Conduction Band - E-Field - ELECTRON ABSORPTION CURRENT PHOTON - Valence Band HOLE + E-Field + + + 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. Defect-Related Trapping and Recombination Conduction Band ENERGY - Defect Level PHONONS PHONONS + Valence Band 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. Defect-Related Transition Probabilities - P ~ (0.5)10 ~ 10-3 P ~ 10-1 P ~ (0.5)16 ~ 10-5 P ~ 10-5 P ~ 10-3 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. p/n Junction Formation + + + + + + + + + + + + + P+ + + + + + + + + + + + + + + + + - Depletion Layer - - N - Bias-Dependent Depletion + + + + + + + + + + + + P+ + + + + + + + + + + + - + - + + + Depletion Layer - + - N+ - With Bias - + Diode Capacitance d1 No bias Vbuilt-in d2 Reverse bias C = DQ/DV ~ e0A/d Vbuilt-in+Vapplied Reverse bias increases the separation between the layers where free charge is added or taken away. Defect characterization via DLTS + + + + + + + + + + + + + P+ + + + + + + + + + + + + - + + + - + - - + - N+ - Depletion Temporary Layer With Bias Reduced Bias + Typical DLTS Measurements 0 e T = 200K T = 180K T = 160K T = 140K -1 Capacitance Change (a.u.) e -2 e Pulse toward zero bias free carriers -3 e Return to steady-state reverse bias -4 e -5 e trapped carriers -6 e 0.0 0.1 0.2 0.3 Time (ms) 0.4 0.5 DLTS Experimental Setup Computer with LabVIEW (5) Digital Scope (Tektronix) Capacitance meter (Boonton) (4) Cryostat with sample (1) (2) 77K (3) Oxford Agilent Temp Controller Pulse Generator Device Structure and Band Diagram 0.05m (Zn) In0.53Ga 0.47As 19 -3 NA = 1x10 cm 0.05m (Zn) InP 18 -3 N A = 2x10 cm 0.05m (Zn) In0.53Ga 0.47As 19 -3 NA = 1x10 cm 0.5m (S) In 0.53Ga0.47As 16 -3 ND = 3x10 cm 0.1m (S) InP 19 -3 ND = 1x10 cm W Energy Depletion region Quasi EF,p ++++ ----- Quasi E F,n Position p+/n Junction m (S) InP 18 -3 ND = 3x10 cm Conduction band Valence band Transient Capacitance: Escape -2 T = 130K T = 140K T = 145K T = 150K T = 160K 10 e -4 Steady-State Bias = -1.1V Pulse = +0.1V -6 e esc = 110 s -1 e Average Ea = 0.29 eV 12 e Escape Rate (s ) Capacitance Change (a.u.) e 8 e 6 e 4 e -8 e Pulse = +0.1V relative to SS and and and SS Bias = -0.1V SS Bias = -1.1V SS Bias = -2.1V 2 e 0.0 0.1 0.2 0.3 0.4 0.5 70 80 90 -1 Time (ms) 1 / kT (eV ) 100 Filling Pulse Dependence: Capture -2 e -1 e e -3 -3 e DC0 - DCtraps (a.u.) -2 Capacitance Change (a.u.) T = 77K Pulse Length: 10 s 30 s 100 s 200 s e T = 77K -4 e -5 e cap = 113 +/- 2 s -4 e -5 e -6 e -6 e -7 e Steady-State bias = -0.3V Pulse: +0.2V (relative to SS) -7 e -8 -200 0 200 400 Time (s) 600 800 e 0 200 400 Pulse Length (s) 600 Proposed Model W Energy Depletion region Quasi EF,p ++++ Conduction band ----- Traps d Quasi E F,n Position p+/n Junction Valence band Testing the Model 3 e Bias = -0.1V Bias = -1.1V Bias = -2.1V Bias = -3.1V -2 2 e Pulse = +0.1V T = 77K -4 e Thickness (nm) Capacitance change DC0 (a.u.) e -6 e 1 e 0 e -8 e Dd DW D C0 (a.u.) -1 e -10 e 0.0 0.2 Time (ms) 0.4 0.6 0 1 2 Reverse Bias (V) 3 Variable-Bandgap Lattice-Mismatched Stuctures Undoped InAsyP1-y, 30 nm Undoped InxGa1-xAs, 1.5 μm Undoped InAsyP1-y buffer, 1 μm Undoped InAsyP1-y step-grade region: 0.3 μm/step (~ -0.2% LMM/step), n steps Undoped InP substrate Radiative Recombination Conduction Band - light in heat PHOTON + Valence Band light out light in = heat + light out radiative efficiency = light out / light in 10 16 10 12 -3 -1 Density of States (cm eV ) Defect-Related Density of States 10 8 10 4 10 0 0.0 0.1 Valence Band 0.2 0.3 0.4 Energy (eV) 0.5 0.6 Conduction Band The distribution of defect levels within the bandgap can be represented by a density of states (DOS) function as shown above. Radiative Efficiency Measurements 100 Eg = 0.80 eV Log(DOS) 100 light 80 40 20 heat 0 10 19 10 21 EV Energy EV Energy EC 60 Log(DOS) Radiative Efficiency (%) 60 Log(DOS) Radiative Efficiency (%) 80 40 EV Energy EC 20 EC Eg = 0.68 eV 0 10 23 10 25 -3 -1 e-h Pair Generation and Recombination (cm s ) 18 10 20 10 22 10 24 10 -3 -1 e-h Pair Generation and Recombination (cm s ) 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).