Non-Lattice Matched III-V Heterostructures for Ultrahigh Efficiency PV

Transcription

1 Non-Lattice Matched III-V Heterostructures for Ultrahigh Efficiency PV Harry Atwater 1, James Zahler 2, Melissa Griggs 1, Anna F. I. Morral 2, Sean Olson 2, Katsuaki Tanabe 1 1. California Institute of Technology, Pasadena, CA Aonex Corporation, Pasadena, CA A Path to Ultrahigh Efficiency PV Layer Splitting and Heterogeneous Integration Microconcentrator Arrays

2 300x 4J III-V Cell Gemstone Cell FP 3J III-V Cell Area Cost-Efficiency Phase Diagram for Photovoltaic Technology Gratzel In/Organic Films c-si Costs are modules per peak W; installed is $5-10/W; $0.35-$1.5/kW-hr

3 Limits to Efficiency: Spectral Absorption E c Energy Band-to-Band Absorption Hot Carrier Excitation E v Subgap Photon Energy S. M. Sze, Physics of Semiconductor Devices, (Wiley) 1981.

4 III-V Compound Solar Materials Palette A Four Junction Cell: Energy Gap (ev) 6 AlN 0.2 N 5 P As GaN Sb AlP AlAs InN GaP GaAs AlSb Si 1 InP GaSb 1.0 Ge InSb InAs Lattice constant (A) Wavelength (µm) InP Si Substrate GaInP hν = 1.9eV GaAs hν = 1.42eV InGaAsP hν = 1.05eV InGaAs hν = 0.72eV W m -2 µm Wavelength (µm)

5 Detailed Balance Calculations: All photons of energy greater band gap are absorbed to form electron hole pairs. All recombination of carriers occurs radiatively (absorber is a perfect defect-free material). Radiation is non-thermal with a chemical potential equal to the separation of the electron and hole quasi-fermi levels, i.e. the cell operating potential The number of photons absorbed by the cell equals: the number of photons reemitted via radiative recombination plus the number of electron-hole pairs extracted at the cell chemical potential by the contacts. In Layered Multijunction Cells, Solar Irradiance Photo-current: Each above-bandgap photon for subcell under consideration but below the band gap of the subcell above is absorbed to generate an electron-hole pair. Radiative Emission from the subcell above. Radiative recombination in subcell above the subcell under consideration consists entirely of photons in excess of band gap energy of the subcell under consideration. Thus energy that is reemitted into subcell under consideration is perfectly This is a modification of the original Shockley and Quiesser formulation of the detailed-balance model which has been adopted by recent analyses of proposed photon-conversion next-generation solar cells by Green et al..the basis for using this radiation model is described by Würfel as an extension of Plank s law for non-thermal radiation from a two-level photon gas at a chemical potential,

6 Detailed Balance Calculations: Optimal Bandgap Sequence Two- and Three- junction multijunction cells in both parallel and series connected geometries at 300 K 100 sun AM1.5 spectrum Two-junction Tandem Parallel Series Maximum Maximum Ga 0.5 In 0.5 P / GaAs / Ge Bandgap (ev) Cell Cell Efficiency Three-junction Tandem Bandgap (ev) Cell Cell Cell 3 (Ge) Efficiency

7 2.5 Effect of Series vs. Parallel Subcell Connection E1 (ev) 2J w/ parallelconnected subcells at 300 K,100 suns (a) E1 (ev) 2J w/ seriesconnected subcells at 300 K,100 suns (Maximum efficiency for single junction cell on thick substrate is marked by the dashed line) (b) E2 (ev)

8 2.50 Optimal Bandgap Sequence in Triple Junction Cell J w/ parallelconnected subcells, Ge bottom cell, at 300 K,100 suns; E1 (ev) (a) E1 (ev) 3J w/ seriesconnected subcells, Ge bottom cell at 300 K,100 suns (Maximum efficiency for single junction cell on thick substrate is marked by the 1.00 dashed line) 1.00 (b) E2 (ev)

9 Efficiency vs. Bottom Subcell Bandgap Variation in 4 Junction Cell Ge or GaAs bonded to MOCVD InP GaInP 2 hν = 1.9eV GaAs hν = 1.42eV InGaAsP E 3 InGaAs E 4 InP substrate E 3 (ev) E 4 (ev) Iso-effiency plot for the variation of the bottom two subcell bandgaps E 3 and E 4 in a four-junction solar cell operated under 100 sun AM1.5 illumination at 300 K.

10 Efficiency vs. Bandgap Variation in 4 Junction Cell 0.7 n + GaAs Contact Cap 0.6 η = (2.00 ev, 1.49 ev, 1.12 ev, 0.72 ev) n AlGaInP n AlGaInP p AlGaInP Window Emitter Base 0.5 n InGaAsP p InGaAsP p GaAs n Si Tunnel Junction Emitter Base Transferred Layer Bonded Interface Emitter Efficiency E 1 p Si Base 0.2 E 2 p + Si n + Ge p Ge Backside Field Bonded Interface Emitter Base Contact 0.1 E 3 E E (ev) Variation of efficiency of optimal 100 sun AM1.5 series-connected four-junction solar cell with changes of each subcell bandgap. Each subcell is varied independently maintaining the other subcells at their optimum bandgap of 2.00, 1.49, 1.12, and 0.72 ev respectively.

11 Four-junction 40% efficiency solar cell Proposed four-junction solar cell : 40% efficiency Lattice mismatched band gap selection avoids use of InGaAsN Bonding processes enable materials integration: InP to Si epitaxial template for InGaAs / InGaAsP structure Ge or GaAs to InP epitaxial template for GaAs / GaInP 2 potential for >4-junction band gap optimized solar cell Ge or GaAs bonded to MOCVD InP Si Substrate GaInP 2 hν = 1.9eV GaAs hν = 1.42eV InGaAsP hν = 1.05eV InGaAs hν = 0.72eV InP bonded to Si substrate *Sharps, P. et al. 26 th IEEE PVSC (1997).

12 Lattice-Mismatched Materials Integration via Hydrophobic Wafer Bonding/ Layer Transfer 1. H-implant of donor substrate to desired peak range 3. Room temperature bond initiation ~1MPa H + Donor Substrate Donor Substrate Handle Substrate Clean, hydrophobic surface 2. Hydrophobic surface passivation 4. Wafer bonding and layer splitting: Pressure to accommodate thermomechanical stress Temperature to blister and form covalent bonds O O 3 O 3 CO 2 O CO 2 Pressure, Temperature Micro-crack formation and layer transfer 5. Wafer bonded heterostructure ~1 µm film

13 Structure and Formation of Hydrophobic Bond Room temperature >400 C >600 C = Ge = Si = H Conceptual approach: room temperature weak van der Waals forces >400 C desorption and diffusion of H Reduction of hydration >600 C formation of covalent bonds Ohmic, metal free contact Source: Weldon et al., J. Vac. Sci. Tech. B 1996, 14(4)

14 JZ8 Exfoliation: TEM 1x10 17 cm -2, un-annealed 1x10 17 cm -2, 250ºC [100] 300 nm 300 nm 25 nm [100] 25 nm [100] Animate magnified view

15 Slide 14 JZ8 TEM images of defect microstructure as a function of temperature. Important points: - Dense defect network near peak range - Defects consist of platelet structures and extended defects - Upon annealing cracks begin to form in the peak damage region and travel predominantly along the (100) plane James Zahler, 4/8/2004

16 JZ9 Experimental procedure: MIT-FTIR MIR prism geometry MIT realized: d/λ = Ge-H modes = cm -1 Implant depth (range ± straggle) = nm Estimated enhancement: Parallel, I/I x = Parallel, I/I y = Perpendicular, I/I z = Spectra processed with an un-implanted reference z Detector p-polarized y x s-polarized θ = 45 o

17 Slide 15 JZ9 Discuss expected enhancement due to MIT measurement. James Zahler, 4/8/2004

18 JZ11 FTIR: low temperature features Ge-H 2 * Bending mode 765 cm -1 Stretch mode 1774 cm -1 Stretch mode 1989 cm -1 Nielsen et al. Phys. Rev. B 54 (8) Un-annealed 57 o C 129 o C 170 o C 1763 cm Un-annealed 57 o C 129 o C 170 o C 2050 cm -1 Absorbance Absorbance cm cm cm cm cm cm cm Wave Number (cm -1 ) Wave Number (cm -1 )

19 Slide 16 JZ11 Comment on presence of discrete defects observable at low temperatures, in particular H2*. James Zahler, 4/8/2004

20 FTIR: 5x10 16 cm -2 un-annealed spectra 1.2 P-polarization S-polarization cm Wave Absorbance 1923 cm cm cm cm cm cm cm -1 Absorbance 1870 cm cm cm cm Number (cm -1 ) Wave Number (cm -1 ) 2050 cm-1 platelet? Anneal Temperature ( o C)

21 FTIR: 5x10 16 cm ºC anneal 1.2 P-polarization S-polarization cm 1923 cm cm cm -1 Absorbance 1816 cm cm cm cm cm cm cm ve Number (cm -1 ) Wave Number (cm -1 ) Anneal Temperature ( o C) Wa Absorbance -1

22 FTIR: 5x10 16 cm ºC anneal 1.2 P-polarization S-polarization 2050 cm cm cm cm cm -1 Absorbance Wave Number (cm -1 ) Wave Number (cm -1 ) Absorbance 1845 cm cm cm cm -1 Cartoons? Anneal Temperature ( o C)

23 FTIR: 5x10 16 cm ºC anneal Absorbance cm -1 P-polarization 2026 cm Wave Number (cm -1 ) Absorbance cm cm cm cm Wave Number (cm -1 ) S-polarization 2031 interacting (100) monohydride 2050 consumed platelets in the formation of monohydride Anneal Temperature ( o C)

24 FTIR: 5x10 16 cm ºC anneal Absorbance cm cm cm -1 P-polarization S-polarization Wave Number (cm -1 ) Absorbance cm cm cm Wave Number (cm -1 ) 2008 cm-1 possibly (111) platelets or interacting defects Anneal Temperature ( o C)

25 FTIR: 5x10 16 cm ºC anneal 1.2 P-polarization S-polarization 500 Absorbance cm cm Wave Number (cm -1 ) Absorbance cm cm cm cm Wave Number (cm -1 ) Anneal Temperature ( o C)

26 FTIR: 5x10 16 cm ºC anneal Absorbance P-polarization 1955 cm cm cm Wave Number (cm -1 ) Absorbance cm cm Wave Number (cm -1 ) S-polarization Ge-H physical mechanism understood: *Platelet nucleation Separation of interacting Ge(100) monohydride surfaces Coalescing of H 2 in defects Pressure induced crack opening (100) free surfaces Anneal Temperature ( o C)

27 Layer Transfer of Ge on Si (IR transmission image) Conditions: Low power fast O2 +N2 plasma, 900mbar pressure Long pre-anneal, graphite sheet on top

28 Split Ge interface and surface morphology x-axis (µ m) z-axis (Å) y-axis (µm) 4 0 z-axis (Å) y-axis (µm) kev H+ 1x1017 cm-2; 250oC, 10 min x-axis (µm) nm 80 kev H+ 1x1017 cm-2; >300oC rough surface nm rms highly defective near surface region

29 2 Wafer Transfer of InP on Si IR transmission image Conditions: O 2 plasma, 900mbar pressure

30 Wafer bonding results: InP/Si Optical micrograph Si InP 25 µm Chemical Etching of Damaged Layer In HCl:H 3 PO 4 :H 2 O 2 ; 1:2:4 AFM: rms roughness = 0.9 nm

31 Hydrophobic Bonding Ohmic Contact Current [A cm -2 ] p + -Ge/p + -Si p + -Ge/n + -Si Slope = 1 / R Al contacts Ge wafer Si substrate Voltage [V] Degenerately doped p + -Ge/p + -Si and p annealed to 400 C Contact resistances of <0.1 Ω cm in p + Ge/ p + Si tunnel junction in p + Ge/ p + Si Ge/n + -Si wafer bonded structures No twist angle dependence for degenerately doped substrates

32 MOCVD: three-junction solar cell on Ge/Si Sample Pre-MOCVD Ge roughness (Å) Post-MOCVD Ge roughness (Å) Bulk Ge <5 147 Ge/Si Ge/Si rough III-V interfaces Ge/Si-1 Si Substrate 6 µm GaAs Cap (1.4µm) Active InGaP (0.7µm) Active GaAs (3.0µm) GaAs Buffer (1.5µm) Transferred Ge (~0.7µm) Bulk Ge Ge Substrate 5 µm

33 MOCVD results: GaAs cap photoluminescence PL Intensity [a.u.] 1.2x x x x x x Bulk Ge Ge/Si-2 Ge/Si-1 τ Ge Bulk = 0.23 ns τ Sample 2 = 0.20 ns Wavelength [nm] Strong GaAs band edge emission at ~880 nm PL Intensity ~ (surface roughness) -1 GaAs Cap GaInP GaAs Base Region λ = 458nm

34 Epitaxial growth on InP/Si templates 2x10 5 cladding layers transferred substrate InGaAsP InGaAs InGaAsP InP Si PL Intensity (a.u.) 1x Wavelength (nm) Bulk InP (25 mw) InP/Si x 43.5 (75 mw)

35 Thin Film Microcell with Microconcentrator LED-like Technology for Photovoltaics: Composite Fresnel/SIL Optics for Concentration (cm 2 ) PDMS (RTV Silicone) Microlense Array, Suns, ( few cm 2 ) High Efficiency 4 J MicroCells (2-5 mm x 2-5 mm)

36 Fabrication of Fresnel Lenses From an existing glass fresnel lens make a mold of the lens pour PDMS into petri dish degas and cure lay glass lens onto PDMS until it sticks cover the lens completely with an excess of PDMS degas and cure peel the layers apart real and imaginary images

37 Thin Film Microcell Microconcentrator UHE PV Arrays 3J or 4J Tandem MicroCells (2-5 mm x 2-5 mm) PDMS Microlense Array, Suns PC Board Like-Substrate; Cells Surface Mounted

38 Summary Multijunction Absorbers Essential for Ultrahigh Efficiency Lattice-mismatched III-V Semiconductors Enable Very Flexible Absorber Design Wafer Bonding and Layer Transfer: New Design Freedoms for High Efficiency PV Detailed Balance Calculations can be used as Materials Selection/Interconnection Guide Microconcentrators: LED-like Technology for Low-Cost Multijunction III-V Terrestrial PV