Lateral Nanoconcentrator Nanowire Multijunction Photovoltaic Cells

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Apr 20, 2009 - junction are carefully chosen (based on InGaP/InGaAs/Ge system) and a metamorphic .... Insets are the optical electric field intensity in the x-direction |Ex|2 plotted for 625nm, .... The field profile at longer wavelengths confirms as well ..... 14e and 14f) by successive drying at 110oC for 24 hours and baking at ...
GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

Lateral Nanoconcentrator Nanowire Multijunction Photovoltaic Cells Investigators Professors: H.-S. Philip Wong (Department of Electrical Engineering) Peter Peumans (Department of Electrical Engineering) Mark Brongersma (Department of Materials Science) Yoshio Nishi (Department of Electrical Engineering) Graduate researchers: Ms. Ying Chen, Mr. Jason Parker, Ms. Trudie Wang Post-doctoral researchers: Dr. Aaron Hryciw, Dr. Xinyu Bao Senior Research Associate: Dr. Jim McVittie Abstract In the past year we focused on developing the metal nanostructure simulation, nanowire growth, and device fabrication. We explored a new geometry with Hertzianlike disc shaped antennas to replace the bowtie antennas we had initially investigated to achieve better performance. Light absorption enhancement of the nanodimers in terms of size and inter space was simulated. A testbed structure based on Si nanowire was developed to quantify the absorption enhancement in single NWs due to the presence of optical feed-gap antenna structures. We also studied a top down etching process for Si NW fabrication, which can be also used for other materials. The Ge core-shell nanowire device was fabricated and tested. The fabrication process is compatible to the integration with metal nanostructures and can be applied to other nanowire devices. An MOCVD tool is under construction which will be used to explore the high efficient III-V nanowire solar cells. GaP nanowire growth has been carried out using other MOCVD facilities to explore the conditions. Single crystalline anatase TiO2 nanowires were fabricated successfully using block copolymer self-assembled templates and dye-sensitized solar cell (DSSC) based on TiO2 nanowires were fabricated and tested. Introduction The objective of this research project is to develop a novel type of multijunction photovoltaic cell that uses lateral arrays of semiconductor nanowires (NWs) of various bandgaps as the elements that convert optical energy into electrical energy. In contrast to conventional stacking multijunction cells, the NWs of varying bandgap will not be connected in series in our approach. Instead, a specially-designed nanostructured metal film is used to split the incident broadband solar spectrum and localize spectral energy in different lateral spatial locations (spectral splitting and concentration) coinciding with the location of the NWs of the optimized bandgap. The same nanostructured metal film also allows for current extraction from each nanowire separately such that photocurrent matching is not required. This allows us to use a wide range of bandgaps (depending on the performance of the lateral metal spectral splitter and concentrator) without requiring current matching. It also allows broader choices on materials which can approach the ideal performance limits due to the less spectral mismatch losses. This removes the most Page 1 of 18

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GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

important efficiency bottleneck of multijunction cells such that efficiencies >45% may be achieved over a wide range of spectral conditions. The NWs can be grown by the vaporliquid-solid (VLS) method and the sol-gel approach, both of which have the potential for low cost manufacturing since epitaxial growth conditions are easily met at moderate temperatures over the short length scale of a NW. To demonstrate the lateral multijunction principle, we will use NWs of materials that span the solar spectrum, including Si, Ge, III-V materials and other abundant, non-toxic, low-cost elements, e.g., TiO2. Background Multijunction cells are currently the photovoltaic technology with the highest demonstrated power conversion efficiency exceeding 40% on monolithic chips. The world record of efficiency for the multijunction solar cells has been increased from 40.8% in 2008 [1] to 41.1% at the beginning of 2009 [2], in which materials for each junction are carefully chosen (based on InGaP/InGaAs/Ge system) and a metamorphic growth technique was developed to improve the spectral and current matching between subcells. While successful, this approach has several inherent limitations. Efficiencies exceeding 50% are theoretically possible, but practically unachievable because it becomes difficult to match the photocurrents of the subcells and the optimal spectral absorption. Solar cells using lateral spectrum splitting with optics have archived a highest efficiency record of 42.9% up to now [3] as schematically shown in Fig. 1a. Efficiency over 50% is expected using this kind of spectral splitting solar cell modules with optics although the monolithic integration is still difficult. The usage of static optics in the design makes the module thin and compact but lacks sun light tracking ability such that the efficient operation is only achieved for a few hours over which the sunlight remains focused on the solar cells rather than over the course of a day.

Fig. 1: Schematics of solar cells using conventional optical concentrator and spectral splitter (a) and metallic concentrator and splitter with NWs (b). The high optical absorption of NW arrays has been theoretically analyzed [4] and the recent experimental measurements confirmed the high absorption/low reflection of various of NWs such as Si, InP, and GaP [5, 6]. Theoretical studies also show the Page 2 of 18

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GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

efficient charge separation [7] and spatial carrier confinement [8] in the core-shell NW structures. These unique properties of NWs are highly desired for high efficiency photovoltaics but hardly achievable in planar structures. Our approach (Fig. 1b) uses lateral arrays of semiconductor NWs of various bandgaps as the elements that convert optical energy into electrical energy. To achieve spectral splitting needed for efficient multijunction operation, the NWs are located on top of a specifically designed metal nanostructure that acts as (1) an array of electrodes to the nanowires, a (2) lateral spectral splitter, and (3) as a lateral concentrator, which makes the monolithic integration possible. The plasmon nature of the metal concentrator and splitter, and the unique absorption properties of NWs also enable high efficient operation at varying illumination conditions. Results I. Electromagnetic Simulations of the Lateral Concentrator To investigate the better absorption of semiconductor nanowires enhanced by metal nanostructures, we explored an alternative geometry with Hertzian-like disc shaped antennas replacing the bowtie antennas we initially investigated last year as a proof of concept. Although bowtie antennas offered much sharper tips for high field enhancement, the tapered structure resulted in significant power loss in the antenna itself. With the disc antennas, we anticipated less loss because the displacement current entering the metallic nanodimer can spread much faster all over the disc and is less spatially confined compared to the thin bar- or prism- shaped nanoantennas at the same length scale. This reduced displacement current density results in the nanodimer being subject to less material losses since a lower current density is trying to force its way through the structure [9]. This makes the nanodimer geometry ideal for optical antenna operation since the biggest obstacle facing these antennas is the tradeoff between field enhancement and material losses inside the antenna. And despite the absence of sharp points or tips in the geometry, the rounded disc shape still allows the antennas to be brought into close proximity of one another with an extremely small gap size between them at a localized point. This means that high spatially confined field enhancement is still possible. Nanofabrication of rounded shapes is also potentially easier due to the absence of sharp points in the antenna geometry and their relatively lower aspect ratio when compared to dipole antennas which use the bar- or prism-shaped geometries. We analyzed the field enhancing properties and spectral selectivity of plasmonic Au nanodimers in the form of closely spaced 20nm thick elliptical nanoparticles. Figure 2a shows one period of these nanodimers, with the boundary conditions being periodic in both the x- and y-directions. In this geometry, the plane wave propagates into the x-y plane along the z-axis and is polarized along the major axis of the nanodimers in the xdirection. Since field concentration occurs in gaps that lie on the axis parallel to the field direction, four different gap sizes along this direction were created in a single period of the geometry to explore the effects of gap size on the frequency and magnitude of field enhancement at resonance. The relative positions of the gaps were controlled independently by varying the major axis diameters dx1 and dx2 as well as the xdisplacements Δx1 and Δx2 of the ellipses. Varying the relative positions allows us to Page 3 of 18

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Wong, Peumans, Brongersma and Nishi

qualitatively determine the range of field enhancement as well as whether or not there is coupling between nanodimers along the different axes. Note that in all cases, the minor axis diameters of the nanodimers were kept constant and equal at 70nm and the ydisplacements were also kept constant at 0nm and 100nm for rows 1 and 2 respectively. In all geometries, the nanowires are made from Ge with 10nm diameters and located midgap between two nanodimers. Both the nanowires and nanodimers are 20nm thick and deposited on SiO2 substrates in an air environment. The nanowire is kept short since field concentration due to plasmonic resonance is a near-field effect and so we are interested only in enhancement for nanowire volumes near the nanodimers. gy

∅dx2 nm

gx2,1

gx2,2 (104,100)

∅dx1 nm

gx1,1

y

gx1,2 μm

x

Fig. 2: (a) Dimensions shown in 2-dimensions on a single period of the geometry. (b) Geometry shown in 3-dimensions. Table 1: Geometries implemented for modeling. Geometry

gap11,12,21,22 [nm]

∅dy1,y2 [nm]

Δy1,y2 [nm]

Size

34,26,18,10

35,43

4,4

Having previously verified through Finite Element modeling (FEM) the experimental results that showed the metal bowtie nanoantenna’s ability to focus light below the diffraction limit, the aim now is to optimize the structure such that the amount of energy absorbed by the semiconductor nanowires is maximized when they are located in the space where there is maximum field enhancement at plasmonic resonance. Since this is only realizable within a realistic time frame if both the geometry and material properties of the bowtie and absorber can be rapidly altered, simulated and optimized for spectrally selective enhancement, the Finite Difference Time Domain (FDTD) method was selected to optimize our geometry. The FDTD algorithm in 3D is considerably more demanding than 2D calculations, especially for plasmonic materials in the optical regime. Since the structure we are modeling is periodic, however, a very fast electromagnetic field model based on the rigorous-coupled-wave-analysis (RCWA) is an alternative method we have

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explored to analytically solve and optimize the nanodimer structures. Table 1 shows the geometry parameters that were explored.

625nm

750nm

900nm

Fig. 3: Absorption percentage plotted as a function of wavelength for the various gap sizes for Au nanodimers simulated by FDTD. Normalized for rx1= rx2=35nm, Δx=4nm. Insets are the optical electric field intensity in the x-direction |Ex|2 plotted for 625nm, 750nm and 900nm, corresponding to the dotted lines beside. In Fig. 3, the spectral response of the geometry is shown by plotting percent absorption of the absorbers in each gap as a function of wavelength. The field intensity in the x-direction (|Ex|2) is also plotted as insets at various wavelengths. The intensity is evaluated across the z = 10nm plane midway through the antenna. The structure exhibits a resonant peak for all gap sizes at around 625nm, in agreement with the plasmon resonance of around 600nm for Au spheroids with an approximate length scale of 100nm. The slight redshift can be accounted for by the slightly elongated shape of the nanodimers in the E-field direction, the coupling along this direction due to the periodic array, and the confinement in the z-direction. The latter point has been noted in several other plasmonic resonant setups done by various groups, since an increase in the volume of a plasmonic nanodevice often results in an increase in resonant frequency due to less concentration of the fields around the plasmonic interfaces and hence less constraint against the motion of oscillating electrons [9]. Several trends and characteristics can be noted from the spectral response and intensity in Fig. 3. The most noticeable trend is that the smaller the gap size, the more the field gets concentrated and hence the more absorption is enhanced in the nanowire. This is generally true across the entire spectrum we looked at. However, with all gap sizes there is a dip in absorption that occurs, and for larger gaps this dip occurs at longer Page 5 of 18

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wavelengths. Because of these dips, the nanowires in the larger gaps are able to absorb more than the nanowires in the smaller gaps at certain wavelengths. While the dip for the 10nm gap is not significant enough in this geometry to result in low absorption, the 18nm gap absorption dips significantly at 750nm and consequently the nanowires in the larger gaps are able to absorb more power. This spectral response with two regions of enhanced absorption and a dip in-between can be understood by considering the behavior of these nanodimers at both long and short wavelengths. At long wavelengths, the quasistatic approximation can be made and each nanodimer is treated like an oscillating dipole in a constant E-field which can induce dipoles in adjacent nanodimer antennas. This particle-like behavior corresponds to localized plasmon excitation in each nanodimer and if the nanodimers are in close enough proximity, the dipoles can constructively interact and lead to enhanced absorption in the nanowire. In the short wavelength regime, even though the size of the nanoantenna is within a few tens of nanometers, its size remains comparable with the optical wavelength of visible light. This makes the nanodimer operation quite different from the dipole antenna at shorter wavelengths. The infinite arrays of nanodimers now effectively behave like a thin film, resulting in a maximum absorption at around 625 nm which corresponds to the excitation of propagating surface plasmons. This plasmon excitation is a material resonance characteristic of gold’s frequency dependent permittivity and so as the plot clearly shows, is independent of geometry. The nanoscaled features of the antennas are required only to couple light into the gold “film” in this regime. The dip in absorption occurs where the “film” of nanodimers becomes reflecting [10, 11]. Because the material resonance of gold for discs of length scales around 100nm lies near 600nm, the high field enhancement also most likely meant large losses in the gold itself near these frequencies. Hence it is desirable to use a material like silver to enhance absorption at these wavelengths without resulting in so much loss since silver is not as lossy at these wavelengths where it is not at resonance but still enhances the field. The gold nanodimers could play a similar role at silver’s resonant frequency range around 300nm. Hence using silver in conjunction with gold would allow us to capitalize on the field enhancing properties of the plasmons while potentially avoiding the lossiness of material resonances. The geometry we explored is similar to that shown in Fig 1 but in this case, the two rows of nanodimers are now geometrically identical each with a 10nm and 34nm gap with the bottom row formed from silver nanodimers and the top row formed from gold nanodimers. Because we anticipate little coupling between rows, the spectral field enhancements consequence of material properties are expected to remain independent. As Fig. 4 clearly shows, there is indeed complimentary enhancement of absorption when silver and gold are used in separate rows. The dotted and solid lines represent the 10nm and 34nm gaps respectively, while the red and blue lines represent the spectral responses of the silver and gold rows. At around 300nm, there is field enhancement where the gold nanodimers lie with less loss in the gold itself, while closer to 600nm, the silver discs cause strong field enhancement without the loss associated with gold’s material resonance. At all parts of the spectrum for the smaller gap size, one can see that absorption in generally enhanced by at least an order of magnitude either by one material or the other, even where Ge is a poor absorber at longer wavelengths. This Page 6 of 18

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shows not only that absorption can be enhanced, but also that absorption can be spectrally selective and localized spatially. The field profile at longer wavelengths confirms as well that each nanodimer can be treated like an oscillating dipole in a constant E-field in this regime and is independent of material properties. 545nm 923nm

325n

Fig. 4: Absorption percentage plotted as a function of wavelength for the various gap sizes simulated with FDTD. Insets are the optical electric field intensity in the x-direction |Ex|2 plotted for 325nm, 545nm and 923nm, corresponding to the dotted. Note that this geometry is identical to the case in Fig. 3 with the only difference being that the bottom row of nanodimers is now Ag while the top row of nanodimers remains as Au. The E-field profile for the RCWA simulation excited by light at λ = 625nm is shown in Fig. 5 agreeing qualitatively with the FDTD results shown in Fig 3b. However, as can be seen, the field profiles are still not smooth enough and the adaptive mesh incorporated into the most recent version of our code must still be fine tuned before accurate field plots and reliable spectral responses can be produced. A convergence test for the number of diffraction orders required will also help us to verify when the method is fine tuned enough to produce results are accurate quantitatively. In addition to tuning the RCWA tool, future developments of our geometry will continue to try and optimize the design of the metal nanostructure to concentrate and laterally split the solar spectrum in the multijunction PV cell. This will include comparing the field enhancement of disc shaped nanoantennas to the bowties we had previously used and to determine the effects of using an array of antennas by simulating single nanodimer pairs of similar dimensions and gap sizes. The effect of material properties Page 7 of 18

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GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

will also be explored further in the antenna, nanowire and the surrounding mediums as the relative permittivities will undoubtedly result in a shift in the resonance frequencies of the nanodimer. As Engheta has shown in his work with plasmonic nanostructures, geometries can be very sensitive to the change in permittivity of the materials at different wavelengths and an increase in permittivity often leads to a spectral red-shift. Designs will also look to further explore the structures that allow for independent resonance and coupled resonance, and weigh up the advantages of each. Ultimately, the final design will aim to be a polarization independent geometry. |Ex|

|Ey|

|Ez|

|Etot|

Fig. 5: Field components plotted at z = 10nm from the top surface of the nanodimer. Note that these profiles refer to the geometry with the boundaries shown in Fig. 2a. II. Ge NW Growth and Device Fabrication We have fabricated Ge nanowire core-shell devices to verify the fabrication process using the process flow schematized in Fig. 6. In order to define the position of the nanowires, we utilize patterning techniques that employ packed latex beads. The latex beads (polystyrene) are spun coat onto a thermally-grown SiO2 surface and assembled in a close-packed geometry. The bead size is reduced with an O2 plasma etch. The pitch between the latex beads is fixed by the original bead size, but the final bead diameter can be tailored using O2 plasma etches. A Cr layer is then deposited. The beads are lifted off, followed by etching of the underlying oxide to create holes in the oxide. Nanowires are grown via the VLS method, the gold is removed with a wet etch, and the p-type Ge and ITO are grown using CVD. Boron was used to dope the shell.

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GCEP Progress Report 2009

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Fig. 6: Ge nanowire core-shell device fabrication process flow. The bead packing structure has made significant strides (Fig. 7). Incomplete coverage is still an issue (Fig. 7d), but surface modification techniques are being investigated to mitigate voids in the pattern.

Fig. 7: (a) Previous, un-optimized latex bead packing structure. (b) Optimized latex bead packing structure due to better control of bead deposition and optimized plasma etch of beads. (c) Optimized latex bead packing structure, less magnification. (d) Optimized latex bead packing structure, less magnification, with images of unwanted voids in the pattern. The transfer of the new, optimized pattern works well (Fig. 8b). The gold electrodeposition in the optimized pattern (Fig. 8d) is not functioning as expected. The most likely reason for this is the oxide etch process had very high variation due to machine instability and subsequent maintenance. The process to etch the holes in the oxide probably over-etched by 2x, resulting in an 2x greater aspect ratio of the holes and problems with the Au electrodeposition solution entering the holes. This issue can be Page 9 of 18

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GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

resolved by re-tuning the oxide etch process once the etch machine stabilizes and running a test etch before each batch of wafers to verify the re-tuned recipe.

Fig. 8: (a) Previous pattern after bead lift-off and oxide etch. (b) Optimized pattern after bead lift-off and oxide etch. (c) Previous pattern after Au electrodeposition. (d) Optimized pattern after Au electrodeposition with deposition difficulties.

(a)

(b)

Fig. 9: (a) Ge nanowires grown out of patterned holes in oxide. Image shown without outer shell and top contact. (b) Device schematic and the I-V characterization. The germanium nanowires growing out of holes in oxide may be seen in Fig. 9a. A transparent insulating layer (photoresist Shipley 3612) was coated to support the nanowires and reduce the leak current. Silver nanowires were deposited to form top transparent contact. Once the top layers are deposited, a current-voltage curve is obtained under a standard solar simulator (Fig. 9b). There are contact problems in our cell and also a path for current leakage. We are in the process of designing this out of our solar cell using other dielectric layers to block leakage, improve the contacts, analyzing how effectively we are doping the nanowires, and shrinking the active area of the cell to minimize possible leakage paths. Page 10 of 18

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GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

III. Si NW Testbed and Etching We have developed a Si NW testbed structure to quantify the effect of optical antenna structures on the photocurrent collected from single Si NWs. The process defines horizontal Si NWs with an approximately square cross-section by etching the device layer of a silicon-on-insulator (SOI) slab (right panel of Fig. 10a). Each die contains 32 individually-addressable NWs with nominal lengths of 1, 2, 5, 10, 20, 50, 100, and 200 μm (four NW of each length), thicknesses determined by the SOI device layer thickness, and widths determined by the silicon nitride spacer layer thickness (described below). Fig. 10a shows an atomic force microscopy (AFM) image of one of the 2-μm-long wires in between Pd contacts. Once the bare NWs have been characterized, optimized metallic optical antennas may be patterned near the NWs via electron-beam lithography. A typical absorption enhancement experiment would consider the current extracted from the NW– antenna assembly as a function of excitation wavelength, normalized to that from a bare NW (Fig. 10b). The device shown in Fig. 10a consists of back-to-back Schottky contacts; for optimal current extraction however, ohmic contacts are desirable. Our short-term focus is therefore to improve the contacts.

Fig. 10: (a) AFM image of a Si NW in between back-to-back Schottky contacts (Pd). The NW shown has a ~100 nm×120 nm cross-section. The process flow is shown in the right panel. (b) Schematic of a photocurrent absorption enhancement experiment. The current collected is normalized to that of the bare NW structure. As an alternative approach to NW fabrication, we have investigated a top-down etching process. Instead of using the vapor-liquid-solid (VLS) growth mechanism, the Si NWs are defined by etching a Si slab patterned via nanosphere lithography. A Si wafer is coated with a close-packed array of silica nanospheres via the Langmuir-Blodgett approach [6]. The L-B technique allows an entire wafer to be covered with a monolayer of close-packed spheres, with relatively few defects. Upon reactive ion etching, the silica spheres act as a mask, defining Si nanopillars beneath them (Fig. 11). Although we have used Si in these tests, this technique should in principle be able to produce NWs of any material for which an anisotropic reactive ion etching recipe possessing good selectivity to silica exists

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GCEP Progress Report 2009

500 nm

Wong, Peumans, Brongersma and Nishi

500 nm

Fig. 11: (a) Cross-sectional SEM image of Si NWs produced via reactive ion etching of a Si wafer coated with a close packed array of silica nanospheres (300 nm diameter). These wires are approximately 1.5 µm long, with diameters tapering from 200 to 335 nm. (b) Plan-view SEM image of NW array shown in (a) 20° tilt. IV. III-V NW Growth and MOCVD Tool Setting Up III-V compounds are currently the best materials for high efficiency solar cells due to the wide band gap tuning. MOCVD is the major tool to grow III-V solar cells with precise control of layer compositions, thicknesses, and doping while providing nearly perfect crystalline quality. We are setting up an MOCVD tool for III-V nanowire growth. The tool will be able to grow compound III-V materials with group III sources (Ga, In, Al) and group V sources (As and P) on up to 4” wafers. Additional sources lines containing Si, Zn, and Mg will be used for n-type and p-type doping. After setting up, this tool can fulfill the requirement of III-V material growth for solar cell study.

Fig. 12: GaP nanowires grew using MOCVD in N2 with (a) Au nanoparticles, (b) thin Au films, and (c) electrodeposited Au in patterned holes. At the same time, we have initiated III-V nanowire growth at the Molecular Foundry (Lawrence Berkeley National Laboratory). The growth of GaP nanowires has been Page 12 of 18

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studied preliminary using Au catalyst in terms of growth temperature and III-V ratio in nitrogen carrier gas. Si substrates and nitrogen gas were used to reduce the cost. Tertiarybutyl-phosphine (TBP) was used instead of phosphine gas for safety reasons. TBP is nontoxic liquid while phosphine is very toxic gas. Tri-methyl-gallium (TMGa) was used as Ga source as commonly used. Since most of the III-V compound materials are grown with arsine and phosphine in hydrogen carrier gas, we need to explore the new growth conditions with TBP. Three kinds of Au catalyst were used: first, the Au colloidal nanoparticles; second, the e-beam evaporated Au film; third, electro deposited Au films inside the holes patterned by latex beads. This is a step-by-step optimization of GaP nanowire growth at designed locations for “hot spots” from metal nanostructure concentrators. The preliminary results are shown in Fig. The nanowires didn’t grow along the preferred vertical direction which needs further optimization. Only a few nanowires grew from the patterned holes (Fig. 12c) because the SiO2 layers in most of the holes are not completely removed in this experiment.

Fig. 13: GaP nanowire growth at different V/III ratio: (a) 40, (b) 20, and (c) 10. V/III ratio was found to have a significant effect on the nanowire growth as shown in Fig. 13. When reducing the TBP supply as keeping TMGa constant, the growth changed from nanowire to cone, and eventually a small tip with a big base particle (Fig. 13c). EDX results show that the big particles contain a large portion of Au and Ga, but P is below the detection limit. This behavior indicates the different growth conditions between the growth with TBP in N2 carrier gas and the growth with phosphine in H2 carrier gas. The growth conditions can be migrated to our MOCVD system when it is ready since we will also use TBP as P source for safety reasons. V. Titanium Dioxide NW Dye-Sensitized Solar-Cells We investigated templated sol-gel growth of TiO2 nanopillar array and the application of such nano-structures in Dye-Sensitized Solar-Cells (DSSCs). Oxygen plasma was used to etch a transfer layer of polymer PMGI (polydimethylglutarimide) or NFC (hydroxystyrene-based cross-linkable polymer) masked by a layer of self-assembled diblock copolymer. We used the method introduced in [12], which was modified to yield larger aspect ratio. We used electron cyclotron resonant etcher in our work, where the microwave power and the platen power were optimized for NFC and PMGI respectively to increase the reproducible pore lengths in both of them to 300nm. The results are shown in Fig. 14a and 14c. Page 13 of 18

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GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

Fig. 14: SEM picture of the di-block copolymer masked PMGI (a) and NFC (c) after oxygen plasma etching with electron cyclotron resonance etcher. The pore diameters are about 20nm. TiO2 nanopillars grown in the di-block copolymer masked 200nm thick PMGI (b) and 300nm thick NFC (d) templates after 450oC baking. (e) HRTEM shows the single crystal structure. (f) Raman spectrum of the nanopillars matches the anatase TiO2 spectrum. Cathodic Sol-Gel deposition was used to fill the templates etched above with titanyl hydroxide gel [13]. The potential drop from the reference electrode to the working electrode is regulated at 1.147V. The gel was turned into single crystalline anatase titania (Fig. 14e and 14f) by successive drying at 110oC for 24 hours and baking at 450oC for 5 hours. The organic polymer template was removed during the annealing process. To increase the adhesion between the titania nanopillars and the substrate, a dense layer of 100nm thick titania was pre-coated on the substrate before spin coating of the polymer materials. And the Sol-Gel time was controlled so that a top layer can form to further increase the mechanical stability of the nanopillars. The results are shown in Fig. 14b and 14d. Dye-sensitized solar-cell was fabricated with the titania nanopillar array as the electron collector or cathode material. The process flow was demonstrated in Figure 14. The Si substrate coated first with platinum, a underlayer of titania and with titania nanostructure grown on top were immersed in an ethanolic solution of N719 dye (Solaronix) 24 hours at room temperature. FTO glass e-beam evaporated with 3nm of of platinum served as the anode. The cathode and anode were stuck together with 25um hotmelt. The cell was filled with I-/I3- electrolyte (Solaronix) with the assistance of a vacuum.

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Fig. 15: The process flow of the fabrication of a Dye-Sensitized Solar-Cell with titania nanopillar array as the cathode. The performance of titania nano-structure based Dye-Sensitized Solar-Cells has been studied over different cell fabrication conditions, including Sol-Gel deposition time of titania, the 450oC baking time and the dyeing time. Under AM1.5 illumination, the short circuit current ranges from 30 to 70 µA/cm2 and the open circuit voltage ranges from 200 to 450 mV with a maximum efficiency around 1%. The performance of the DyeSensitized Solar-Cells we fabricated is limited by the absorption of the iodolyte and the platinum catalyst coated on the anode due to the metal substrate and the series Schottky diode formed by Pt (wf>5eV) and titania (wf~4eV). The relatively thin titania nanostructure has also limited the dye-loading.

Fig. 16: SEM pictures of titania nano-structure after (a) 0s; (b) 15s; (c) 120s; (d) 240s of Sol-Gel deposition. Page 15 of 18

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GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

When the Sol-Gel deposition time is prolonged, the short circuit current increased first, then decreased and increased again. We hypothesize that the succuessive growth of titania nanopillars and forming of denser and eventually cracking top layer of titania, as illustrated in Fig. 16, led to this change of short circuit currents. The short circuit current first increased due to the increased titania surface area which facilitates more dye loading (Fig. 16b). When the growth of titania pillars reaches the top of the polymer template, the overgrowth of titania forms a top layer which could be porous at first and becomes denser and denser after prolonged deposition (Fig. 16c). The dense overlayer blocks the free flowing of I-/I3- species in the electrolyte and might cause increased recombination of electrons carried by the titania and holes carried by the I-/I3- ions, which results in lower short circuit currents. When the titania top layer is too thick, it cracks during the drying and baking process (Fig. 16d), which again gives the ions in the electrolyte a path to pass through the top layer and leads to increased short circuit current. No obvious dependence of the open circuit voltages on the Sol-Gel deposition time was found. We also found that by increasing the titania baking/annealing time from 7h to 21h, the short circuit currents were increased a little while the open circuit voltages dropped. This result might have been caused by the improved electron transportation in both the titania underlayer and the titania nanopillars. The reduction of sheet resistance of the titania underlayer helped increasing the short circuit currents, but since the resistance of the titania underlayer is in parallel with the underneath FTO layer, the increase is marginal. On the other hand, the lower resistance in the titania nanopillars increased the dark current and led to the dropping of open circuit voltages. Progress The main progress are: (1) studied an alternative geometry with Hertzian-like disc shaped antennas; (2) developed a Si nanowire testbed structure to quantify absorption enhancement in single NWs due to the presence of optical feed-gap antenna structures; (3) investigated an alternative fabrication technique for the Si NW; (4) fabricated and tested the first Ge core-shell nanowire array device; (5) investigated block copolymer templated sol-gel growth of TiO2 nanopillar array and the application of such nano-structures in Dye-Sensitized Solar-Cells (DSSCs); (6) a MOCVD tool for III-V nanowire growth is under construction; (7) carried out GaP nanowire growth with the MOCVD at LBNL. Based on the simulation results, we can design and fabricate the metal nanostructures for light concentration and splitting. The nanowire growth and device fabrication provides an insight view of the potential of nanowires for solar cell applications. The MOCVD setup and the III-V nanowire growth can provide premium materials with high efficiency. The fabrication process is developed towards the integration of metal nanostructures and nanowires for high efficiency nanowire solar cells with lateral nanoconcentrators. Future Plans We will continue to develop and improve the simulation and NW devices as listed below. The integration process and fabrication of metal nanostructure with NWs will be carried out as well.

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GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

(1) Optimize the design of the metal nanostructure to concentrate and laterally split the solar spectrum. Fabricate the metal nanostructures according to the design to test the performance using NSOM. (2) Improve the contacts for the Si NW testbed to achieve Ohmic contact. Optimize the fabrication process to achieve flat top surface of the etched Si NWs. Use Si NW testbed to test the metal nanostructures. (3) Fabricate Dye-Sensitized Solar-Cells on FTO glass and use front-side illumination to improve external quantum efficiency and avoid the forming of series Schottky diode. Increase the surface area of titania nano-structure by improving the template etching technique to increase the thickness of the template and by application of a TiCl4 treatment so as to increase the roughness of the surface of the titania nano-structure [14]. (4) Improve Ge core-shell NW device fabrication and test the photovoltaic efficiency. (5) Setup the MOCVD within the next six months and start nanowire growth after that. Continue the III-V nanowire growth using LBNL facility before our MOCVD is ready. Study the InGaP nanowire growth, doping, and n-p core-shell structure growth. (6) Set up single nanowire transport measurement facility and process to characterize the nanowire properties such as doping level and junction quality for the NW solar cell optimization. Publications No publication yet. Contacts Professor H.-S. Philip Wong: Professor Peter Peumans: Professor Mark Brongersma: Professor Yoshio Nishi:

[email protected] [email protected] [email protected] [email protected]

Graduate researchers: Ying Chen: [email protected] Jason Parker: [email protected] Trudie Wang: [email protected] Post-doctoral researchers: Aaron Hryciw: [email protected] Xinyu Bao: [email protected] Research Associates: Jim McVittie: [email protected]

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April 20, 2009

GCEP Progress Report 2009

Wong, Peumans, Brongersma and Nishi

References [1] Geisz, J. F., et al. 40.8% efficient inverted triple-junction solar cell with two independently metamorphic junctions. Applied Physics Letters 2008, 93, 123505. [2] World Record: 41.1% efficiency reached for multi-junction solar cells at Fraunhofer ISE. http://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-ise [3] Barnett, A., et al., Milestones Toward 50% Efficient Solar Cell Modules. In 22nd European Photovoltaic Solar Energy Conference, Milan, Italy, 2007. [4] Hu, L., et al. Analysis of Optical Absorption in Silicon Nanowire Arrays for Photovoltaic Applications. Nano Letters 2007, 7, 3249-3252. [5] Muskens, O. L., et al. Design of light scattering in nanowire materials for photovoltaic applications. Nano Letters 2008, 8, 2638-2642. [6] Zhu, J., et al. Optical Absorption Enhancement in Amorphous Silicon Nanowire and Nanocone Arrays. Nano Letters 2009, 9, 279-282. [7] Zhang, Y., et al. Quantum Coaxial Cables for Solar Energy Harvesting. Nano Letters 2007, 7, 12641269. [8] Nduwimana, A., et al. Spatial Carrier Confinement in Core-Shell and Multishell Nanowire Heterostructures. Nano Letters 2008, 8, 3341-3344. [9] Alu, A., et al. Hertzian plasmonic nanodimer as an efficient optical nanoantenna. Physical Review B 2008, 78, 6. [10] Ferry, V. E., et al. Plasmonic Nanostructure Design for Efficient Light Coupling into Solar Cells. Nano Letters 2008, 8, 4391-4397. [11] Zhao, J., et al. Methods for Describing the Electromagnetic Properties of Silver and Gold Nanoparticles. Accounts of Chemical Research 2008, 41, 1710-1720. [12] Park, O.-H., et al. High Aspect-Ratio Cylindrical Nanopore Arrays and Their Use for Templating Titania Nanoposts. Advanced Materials 2008, 20, 738-742. [13] Karuppuchamy, S., et al. Cathodic electrodeposition of oxide semiconductor thin films and their application to dye-sensitized solar cells. Solid State Ionics 2002, 151, 19-27. [14] Mor, G. K., et al. Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells. Nano Letters 2006, 6, 215-218.

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