Nanoscale Photon Management for Solar Energy Harvesting

Transcription

1 Nanoscale Photon Management for Solar Energy Harvesting Speaking: Mark Stanford University Doing the Work: Soo-Jin Kim, Juhyung Kang, Jung Hyun Park Isabell Thomann, Blaise Pinaud, Zhebo Chen. Funding: DOE EFRC, KAUST, Samsung GRO Thank you: Bruce Clemens group, Tom Jaramillo group, Stacey Bent Group.

2 Metals Play a Dual Role in Many Solar Cells Metals are used as electrical contacts and optical backreflectors J.H. Zhao, Martin Green, et al., Appl. Phys. Lett. 73, 1991 (1998) V. Ferry, R. Schropp, H. Atwater, A. Polman, et al., Appl. Phys. Lett. 95, Keisuke Nakayama H.A. Atwater et al., Appl. Phys. Lett. 93, (2008) Organic Solar Cells, Courtesy McGehee group, Stanford

3 Conventional Metallic Contacts Present a Challenge First order estimate of the absorption in a thin film (t < λ) on top of a mirror Electric field distribution above a Perfect Electric Mirror (PEM) 2 Air Spacer A PEM E 1 0 λ Electrically conductive and optically transparent spacer layers can increase absorption Transparent oxides can perform this function, but can add significantly to the solar cell cost.

4 Possible Alternative Strategy: A Magnetic Mirror Definition of conventional Electrical and unconventional Magnetic mirrors In images D. Sievenpiper, E. Yablonovitch, et al., Microwave Theory and Techniques, IEEE Transactions on.47, (1999). J. McVay, N. Engheta, et al., Microwave and Wireless Components Letters, IEEE. 14, (2004). A S Schwanecke, Zheludev et al., J. Opt. A: Pure Appl. Opt. 9 (2007) Magnetic mirrors in the visible! In equations E z (x) = e ikx + re ( +ikx ) Magnetic mirror: r = 1 Electric mirror: r = -1

5 Illustrating the Benefits of a Magnetic Mirror First order estimate of the absorption in a thin film (t<<λ) on top of a mirror Perfect Electric Mirror (PEM) Perfect Magnetic Mirror (PMM) E Air A PEM Air A E PMM MM offers better overlap of standing wave with a thin layer of absorbing medium MM offers an improved broadband performance

6 The world of High Impedance Surfaces and Metasurface Reflectors Subwavelength-engineered reflectors come in many varieties D. Sievenpiper, E. Yablonovitch et al,. IEEE TRANSACTIONS, VOL. 47, (1999) A S Schwanecke Zheludev et al., J. Opt. A: Pure Appl. Opt. 9 (2007) J. Mc Vay, N. Engheta, IEEE Microwave and Wireless Components Letters14, 130 (2004) A. Pors, S. Bozhevolnyi, et al., Scientific Reports, 3, 2155 (2013)

7 Implementation of a Poor Man s Metamaterial Mirror Example: Transforming a regular Electrical mirror into a Magnetic mirror η m Reflection change comes about by modifying the surface impedance: Z S Complex reflection coefficient Governed by Characteristic impedance medium m: Surface impedance of the mirror: Regular mirror: ϕ = π (Z s 0) and Magnetic mirror: ϕ = 0 (Z s >> η m )

8 Analyzing the Electric Fields Above Planar and Grooved Reflectors Subwavelength groove array affect reflection phase for one polarization Regular Ag Mirror Grooved Ag Mirror Grooved Ag Mirror E E E λ = 600 nm Ag Mirror

9 A Magnetic Mirror (MM) is Realized with Properly-size Grooves Subwavelength groove array serves as a MM for one polarization Simulation of electric fields Simulation of magnetic fields E H Note 1: Highest electric field magnitude occurs at the surface = top of the teeth Note 2: It appears as if the light reflects from the bottom of the groove

10 Another Viewpoint: Light Reflects from the Bottom of the Grooves! Light funnels into grooves due to coupling of plane wave to groove modes Plot of the power flow, where: S ext S scat y d = λ Target /4 E The groove dimensions (depth, width) should control the phase pickup!

11 The Groove Dimensions Control the Reflection Phase Pickup Simulation of light reflection (λ = 600 nm) from a grooved metamaterial mirror Active layer Ag -d O Distance Reflection phase can continuously be tuned by varying the groove depth d

12 Exploring Wavelength Dependence of E-field near the Mirror Surface Illustrating potentially favorable broadband optical response Comparison of optical fields in front of perfect electrical mirrors and metamaterials mirror Active layer Meta Mirror Meta Mirror

13 Application of Metamaterial Mirrors to Bulk Heterojunction Solar Cells Basic cell operation 1. Photon absorption Transparent contact Donor Acceptor Metal contact 2. Exciton diffusion 3. Charge transfer Photon 4. Charge separation 5. Carrier collection Many different organic solar cell designs exist Low carrier mobility limits thickness of the solar cell to < 100 nm

14 Fabrication of subλ groove arrays in a Ag Back Contact SEM images of grooves made by Focus Ion Beam (FIB) Milling Top view Cross sectional view 500 nm Note: FIBed grooves feature a sloped sidewall

15 Application of Metamaterial Mirror to a Basic Organic Solar Cell Full-field simulations demonstrate broadband absorption enhancement Patterned metamaterials mirror provide a broadband enhancement upto a factor 1.5 Let s analyze the field distribution inside the device

16 Full Field Simulations of an Optimized Solar Cell Evolution of the field profile λ = nm Electric field distribution (unit cell average) Note: Small near-field contribution Note: Resonance and field overlap are important

17 Optical properties of subλ groove arrays in Ag Optical reflection images of sample with the organic layer on the grooves. Polarization illumination E E Groove orientation Response is clearly polarization-dependent Enhanced absorption occurs in the active organic layer and metal for correct polarization

18 Demonstration of Enhanced Absorption with Metamaterial Mirror Experimental demonstration metamaterial mirror effect by photocurrent measurements SEM image Optical image Photocurrent map:

19 Metamaterial Mirror Provides Broadband Absorption Enhancement Measurements of enhancement at various wavelengths Enhancement was found to be broadband Enhancement was found to be peaked around 650 nm Enhancement was found to be a high as 1.2 x (as compared to 1.5 in simulations)

20 Nanostructuring to Enhance Solar Energy Conversion to Fuels Use nanostructuring to achieve and control electronic and optical confinement Enhance absorption Use atomic-scale engineering to realize new photocatalysts materials Enhance catalytic activity Improve water splitting across multiple length-scales (Photonic, Electronic, and Atomic)

21 The Road Towards Inexpensive and Efficient Solar Energy Conversion In search of the ideal photocatalyst material... Solar-to-hydrogen conversion efficiency >10% Earth-abundant, non-toxic, low-cost Chemically stable Strong light absorption Favorable electronic bandgap Favorable band alignment Excellent charge transport Scalable synthesis How do I find the ideal photocatalyst...? Example: Rapid screening techniques Screening binary oxides: Jacobsen/Norskov Groups There is no material yet that simultaneously fullfils all of our requirements Now what? Nanophotonics may hold the key

22 Let s Consider a Prototypical Water Splitting Catalyst Desired properties of iron oxide Inexpensive, Earth-abundant, Non-toxic, Corrosion stable Bandgap (~ 2eV) suitable for visible-light driven hydrogen production h + d I Challenges with iron oxide: Poor light absorption near the band edge Poor carrier transport I.Thomann et. al. Nano Lett. 2011, 11, 3440 absorption length [microns] Photonic and Electronic length scales Carrier diffusion length ~ 20 nm wavelength in nm

23 Photon Management to the Rescue! Approach: Exploit and engineer optical resonances in nanostructured photocatalysts Example: Driving optical Mie resonances in Fe 2 O 3 nanobeams : Fe 2 O 3 Benefits of our approach Optical resonances can increase the useful absorption per unit volume Optical resonances allow for spatial engineering of optical fields in devices (e.g. near interfaces) Approach is scalable to larger areas Thinner films exhibiting poor carrier transport can be used (opens the door to using cheaper and more catalytically active materials that have not been considered viable before) Enhanced light absorption faciliates higher electrochemical potentials and open circuit voltages

24 High-Index Nanostructures Naturally Exhibit Optical Resonances Example: Light scattering from a silicon nanowire I scatter (a.u.) I scatter Linyou Cao et al., Nano Lett., , 10, 2010.

25 Tuning the Optical Properties of High Index Nanostructures Size 30 nm 180 nm Polarization Shape Coupling 130 nm 50nm L. Cao, et al., Nano Lett., , 10, L. Cao et al., Nano Lett., 10, (2010).

26 Optical Properties of Dielectric/Semiconductor Structures Example: Optical properties of high index nanowires Free space photons can couple to Mie or leaky mode resonances Intuitive resonance condition: mλ eff = 2πr For top-illumination resonances split in TM and TE modes Nomenclature TM ml H 2 m: # wavelengths l : # radial maxima

27 Simplest Geometry to Test Concepts: Fe 2 O 3 Nanobeam Arrays Goal: Joint optimization of local (Mie) and waveguide coupling resonances Geometry of the nanostructured photoelectrode Example: Illuminating a beam at λ = 600 nm 200 nm One can enhance light absorption in the top nm layer near the Fe 2 O 3 /H 2 O interface by going from a planar photoelectrode to a nanostructured photoelectrode

28 Fabrication of Fe 2 O 3 Nanobeam Arrays Multistep fabrication process for the nanobeam arrays Fe nanobeams are first defined lithographically Nanobeams are then overcoated with a 10-nm-thin Fe film to reach full coverage of Fe Hematite (α-fe 2 O 3 ) phase is realized using an anneal at 600 C in air SEM and optical images of the nanobeam photo-electrodes

29 Step 1: Optimization of a Single of Fe 2 O 3 Nanobeam Nanobeams support a series of optical resonances Lowest order resonances at λ = 600 nm Absorption enhancement in top 10 nm Increasing size

30 Step 2: Optimizing the Nanobeam Array Period for 160 nm Beam Optimization array period P or magnitude reciprocal lattice vector G = 2π/P Optimization is performed using light absorption enhancement maps Absorption Enhancement Map (nanobeam) / (planar) Coupling to Quasi-guided Mode 1 2 Coupling to local mode These maps allow visualization, separation, and optimization of enhancement mechanisms

31 Step 2: Optimizing the Nanobeam Array Period for 160 nm Beam Optimization array period P or magnitude reciprocal lattice vector G = 2π/P Optimization is performed using light absorption enhancement maps Absorption Enhancement Map (nanobeam) / (planar) Absorption spectra for different periods Absorbed photon fraction These maps allow visualization, separation, and optimization of enhancement mechanisms Optimization suggest a period of about 300 nm to produce 1.5 ma/cm 2

32 Experimental Photocurrent Enhancement Spectra Taken from an Optimized Fe 2 O 3 Nanobeam Array Photocurrent measurements Measurement of I ph (Nanobeam) / I ph (Planar) Samples with different nanobeam pitch

33 Experimental Photocurrent Enhancement Spectra Taken from an Optimized Fe 2 O 3 Nanobeam Array Photocurrent measurements Measurement of I ph (Nanobeam) / I ph (Planar) Broadband enhancements of up to 3 times are observed Largest enhancement at the predicted location (coincidence of Mie and waveguide resonance)

34 Transparent Electrodes: A Great Use of Metallic Nanostructures Metal nanostructures enable: electrical conduction, reduced reflection, and light trapping Lee, Peumans et al. Nano Letters 8, 689 (2008) Wu et al. Nano Letters 10, 4242 (2010)

35 Plasmonic Nanowelding of a Ag Nanowire Mesh Regular furnace anneal Ag nanowires synthesized by polyol, chemical process 200 nm 500 nm New approach: plasmonic nanowelding Tungsten halogen lamp I = 30W/cm nm Erik Garnett et al., Nature Materials (2012 )

36 TEM Before and After Plasmonic Nanowelding Before welding After welding 50 nm 50 nm Erik Garnett et al., Nature Materials (2012 ) As synthesized: Ag nanowires feature a pentagonally twinned crystal structure After welding: Twinning defects continue through the junction only for top nanowire Conclusion: Bottom nanowire always recrystalizes onto top nanowire at the junction H. Chen et al., J. Phys. Chem. B 108, (2004).

37 Self-limited Plasmonic Nanowelding Simulations 100 nm E k gap Before melting : 2 nm gap Heat generation focused in bottom wire near junction After melting: 2 nm overlap Heat generation is self-limited! Erik Garnett et al., Nature Materials (2012) and consistent w work by Nordlander, Aizpurua, Garcia de Abajo,

38 Optical Nanowelding: A Plasmonic Resonance Effect Heating is most effective when E-field is polarized to enable surface plasmon excitation Heating occurs near the surface plasmon resonance frequency of an individual Ag nanowire

39 Light Scattering Properties of Ag Nanowires Bright-field optical reflection images of Ag nanowires Notable difference in the back reflection of white light 2µm 2µm E-field // to the length of the nanowire causes a mirror-like reflection E-field nanowire causes a surface plasmon excitation and heating (+ forward scattering)

40 Electrical Conduction Properties of Individual Welded Wires Determining of resistance of two welded Ag nanowires 2 µm We can measure single on resistance is similar to the wire resistance Junction resistance is similar to the single wire resistance Erik Garnett et al., Nature Materials (2012 )

41 Electrical and Optical Properties of Welded Wires Welding nanowire meshes Welding two Ag nanowires R = 580 Ω/ Nanowire meshes can be welded on heat sensitive saran wrap Nanowire meshes can be welded low thermal budget organic solar cells Erik Garnett et al., Nature Materials (2012 )

42 Summary and Conclusions New types of mirrors in devices can be realized with subwavelength groove-arrays 2D Mie resonance can be employed to enhance the rate of water splitting H. Rostami, et al., Carbon, 48, 3659 (2010). Au coated C nanotubes Optical resonances in nanostructures can also be applied in nanosynthesis 50 nm 2 µm