Nanostrukturphysik (Nanostructure Physics)

Transcription

1 Nanostrukturphysik (Nanostructure Physics) Prof. Yong Lei & Dr. Yang Xu Fachgebiet 3D-Nanostrukturierung, Institut für Physik Contact: Office: Unterpoerlitzer Straße 38 (Heisenbergbau) (tel: 3748) Vorlesung: Wednesday 7:00 8:30, Faradaybau 3001 Übung: Friday (G), 11:00 12:30, C 110 (a) (b 1 ) (b 2 ) UTAM-prepared free-standing one-dimensional surface nanostructures on Si substrates: Ni nanowire arrays (a) and carbon nanotube arrays (b).

2 Light management with nanostructures for photovoltaics Traditional approaches Semiconductor nanostructures Metal nanostructures

3 Traditional solar cell light absorption approaches Optical Lost Optical lost because the light is reflected from the front surface, or because it is not absorbed in the solar cell. Normally, bare silicon has a high surface reflection of over 30%.

4 1. Anti-Reflection coating Advantages: the thickness of the anti-reflection layer is usually designed to a quarter of wavelength, making the phase difference of the incident light and reflected light half a wavelength, thus reflection is suppressed through destructive interference.

5 SiO 2 anti-reflection coating SiO 2 monolayer on glass 200 nm 500 nm 700 nm Appl. Phys. B 2010, 100, 547.

6 Si 3 N 4 anti-reflection coating Thickness can be measured by ellipsometry or simply look at the color. Four polycrystalline wafers covered with films of silicon nitride. The difference in color is solely due to the thickness of the film.

7 Disadvantages: It works best only for an individual wavelength. If the incident light is oblique, the effect will be weakened due to change of the light travel path. Fabrication process involves vacuum deposition equipment thus is usually not low-cost.

8 2. Surface texturization A single crystalline substrate can be textured by etching along the faces of the crystal planes to formed pyramid. Inversed pyramid

9 Reflection and transmission of light for a textured silicon solar cell. A textured surface will not only reduce reflection as previously described but will also couple light obliquely into the silicon, thus giving a longer optical path length than the physical device thickness. For example, a solar cell with no light trapping features may have an optical path length of one device thickness, while a solar cell with good light trapping may have an optical path length of 50, indicating that light bounces back and forth within the cell many times.

10 Rear Reflectors In actual devices, the front surface is also textured using schemes such as the random pyramids mentioned earlier. Light trapping using a randomized reflector on the rear of the cell. Light less than the critical angle escapes the cell but light greater than the critical angle is totally internally reflected inside the cell.

11 Light management with nanostructures for photovoltaics Semiconductor nanostructures Metal nanostructures Arrays of sub-wavelength nanostructures can realize a smooth or stepwise transition of effective refractive index from air to semiconductor materials. The effective refractive index of a nanostructured film is defined as n eff = n*fr, where n is the refractive index of material and FR is the material filling ratio.

12 Optical Absorption Enhancement with nanostructure: theoretical investigations Figure 1 Schematic drawing of the periodic silicon nanowire structure. The parameters are the length L, the period a, and the diameter d. In the figure, θ and φ are the zenith and azimuthal angles, respectively. where is the frequency of the incident wave, and R, T, and A are the frequency dependent reflectance, transmittance, and absorptance of the wire structure. Lu Hu; Gang Chen; Nano Lett. 2007, 7,

13 Nanowires (NWs) Si NWs fabricated by deep reactive ion etching (DRIE) using a monolayer film of silica beads as a mask. Pitch and diameter of Si NWs can be controlled by silica beads. Length is determined by etch time. 2μm: transmittance is lower than 10% from 400 to 1000 nm. 5μm: close to zero. Nano Lett. 2011, 11, 1851

14 500 nm InAs NWs grown in a high vacuum chemical beam epitaxy (CBE) unit by using electron beam lithography (EBL) defined gold dots as seeds. The colors of NWs are different with different diameter and length. Calculated electric field intensity distribution: vertical resonance Optical absorption of 850 nm is stronger than 450 nm. Nano Lett. 2012, 10, 2649.

15 Experiments for realizing the idea nanostructure array -Vapor-Liquid-Solid growth of Si wires for enhancing light absorption NATURE MATERIALS,VOL 9,MARCH 2010

16 NATURE MATERIALS,VOL 9,MARCH 2010

17 Nanopillars (NPLs) More advantages than NWs: smaller surface area and less surface recombination. Single-diameter NPLs (Ge): similar to NWs, showing a strongdiameter dependent absorption. Finite difference time domain (FDTD) simulations with Ge: optimal NPL structures can be identified with the best absorption ~80%. Nano Energy 2013, 2, 951.

18 Multi-diameter NPL (MNPL): smallest diameter tip for minimal reflectance and largest diameter base for maximal effective absorption coefficient. Ge MNPL: AAO template-assisted vapor-liquid-solid (VLS) growth, 60 nm tip and 130 nm base. Drastic improvement: % absorption between nm. Nano Lett. 2010, 10, 3823.

19 Nanocones (NCNs) Cu(In,Ga)Se 2 NCNs (CIGS NCNs) from CIGS thin films via direct sputtering of a CIGS target in conjunction with an Ar + milling process. Angular dependant reflectance mapping: a reflectance less than 0.1% for incident wavelengths from 400 to 1000 nm. Nano Lett. 2011, 11, 4443.

20 Thin Film NWs NCNs Hydrogenated amorphous Si (a-si:h): CVD growth on ITO using silica nanoparticles as an etch mask in the RIE process. NCN arrays show the best refractive index transition from air to a- Si:H and absorption above 93% between nm. Nano Lett. 2009, 9, 279.

21 Nanodomes (NDMs) Electrodes Active material Back reflector 500 nm NCN substrate with 100 nm base diameter, 450 nm spacing, and 150 nm height Significantly enhance the optical absorption comparing to flat control samples. Adv. Energy Mater. 2012, 2, 1254.

22 High aspect ratio tapered nanostructures Nanoneedles (NNs), nanospikes (NSPs), and nanosyringes. Strong light scattering. A gradual change of the effective refractive index from the top to the bottom. Efficient photon capture.

23 Black Ge Black Ge NNs based on crystalline/amorphous core/shell arrays. Quasi-vertical orientation, high density. Drastic reduction of reflectance of NNs comparing to thin film (TF) and NWs. A reflectance of <1% for all wavelengths with a length >1 μm. Nano Lett. 2010, 10, 520.

24 3D Al NSP arrays on thin Al foil via direct current anodization of Al substrates under high voltage in conjunction with wet chemical etching. 100 nm amorphous Si thin film conformally deposited on the Al NSPs. Strong light absorption over a broad range of wavelengths ( nm) and incident angles (0-60 ). ACS Nano 2011, 5, 9291.

25 3D nanoholes (NHLs) and nanowells (NWLs) Previous nanostructures: positive structures with respect to the substrates, meaning that the structures protrude out from the substrates into free space. Light trapping in the positive structure: photon multiple scattering within the nanostructures, increasing effective optical path length of a photon and absorption probability. NHLs and NWLs: negative structures with cylindrical cavities, providing geometric confinement for incoming photons naturally.

26 (a) Schematic illustrations of nanohole and nanorod arrays. Light is incident from above. (b) Calculated absorptance at λ = 670 nm as a function of c-si filling fraction for the nanohole and the nanorod array structures occupying a half space. Nano Lett. 2010, 10, The maximum efficiency of 31.4% for a homogeneous film is achieved for nanohole arrays with the thickness of only around 7 μm and for nanorod arrays of over 9 μm. A 300 μm thick homogeneous film with the AR coating specified earlier has an efficiency of 40.5%. The AR-coated nanohole array of 50 μm thickness gives an identical Efficiency.

27 Amorphous Si NWLs on Al substrate Nano Lett. 2012, 12, 3682.

28 Light management with nanostructures for photovoltaics Semiconductor nanostructures Metal nanostructures

29 Surface plasmons Au colloids in water (M. Faraday ~1856) Wavelength selective scattering from silver nanoparticles observed by darkfield microscopy.

30 Different fabrication process for realizing large scale nano-particles. Chem. Rev. 2011, 111,

31 Plasmonic effect Hot ejection: transfer of charge between metal/oxide ( hot electron-hole pairs) Localized surface plasmon resonance: plasmonically induced heating and an induced electromagnetic field Sensitize the semiconductor to below-band gap light Smaller amounts of semiconductor to achieve complete light absorption More light scattering and lower light reflection Near-edge light absorption

32 Images of various plasmonic colour filters taken in microscope transmission mode under a white light illumination. Figure 4. CVs spectra of ZnO [Fe (CN) 6] 3 /4 (a), and ZnO/ZnS [Fe (CN) 6 ]3 /4 (b) at different scan rate (0.02 Vs V s 1 ). The (c and d) curves

33 Plasmonic light-trapping geometries for thin-film solar cells a. Light trapping by scattering from metal nanoparticles at the surface of the solar cell. Light is preferentially scattered and trapped into the semiconductor thin film by multiple and highangle scattering, causing an increase in the effective optical path length in the cell. b. Light trapping by the excitation of localized surface plasmons in metal nanoparticles embedded in the semiconductor. The excited particles near-field causes the creation of electron hole pairs in the semiconductor. c. Light trapping by the excitation of surface plasmon polaritons at the metal/semiconductor interface. A corrugated metal back surface couples light to surface plasmon polariton or photonic modes that propagate in the plane of the semiconductor layer.

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41 Promising large-area metal nanopatterns for plasmonic solar cells.

42 Thank you!