High Aspect Ratio Plasmonic Nanocones for Enhanced Light

Sep 9, 2014 - Design of Ag nanograting for broadband absorption enhancement in amorphous silicon thin film solar cells. Ping Liu , Shi-e Yang , Yanxia...
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High Aspect Ratio Plasmonic Nanocones for Enhanced Light Absorption in Ultrathin Amorphous Silicon Films V. Gusak,† B. Kasemo,† and C. Hag̈ glund*,‡,§ †

Department of Applied Physics, Chalmers University of Technology, 41296 Göteborg, Sweden Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States

‡

S Supporting Information *

ABSTRACT: Strategies for enabling high light absorption in ultrathin solar cell layers may contribute importantly to more viable photovoltaics. To this end, we investigate the effect of the enhanced near-field, associated with nanoparticle plasmon resonances, on the light absorption in ultrathin (20 nm) hydrogenated amorphous silicon (a-Si:H) films. In order to maintain the dipolar plasmon resonance above the a-Si:H optical gap, we employ high aspect ratio Ag nanocones coated with the a-Si:H by chemical vapor deposition. Experiments were performed for Ag/a-Si:H nanocomposites on glass and on a spacer-reflector resonant cavity, used to boost and tailor the optical response. Finite-element calculations were employed to model and extract the absorption rates of the different components of the samples, and to help explain the origin of the spectral features. The highest integrated absorption in the aSi:H film, corresponding to an ideal photocurrent of 12.5 mA/cm2, was observed for a nanocone/a-Si:H system on a 40 nm thick TiO2 spacer placed on an Al reflector. Due to the rather high (217 nm) structures employed, the a-Si:H absorptance was, however, not very sensitive to the spacer thickness. Numerical comparison with systems where the core Ag cones were substituted by dielectric cones, demonstrated that the effect of the plasmon resonance added significantly to the benefits of the increased amount of absorber material and enhanced light interaction imposed by other geometrical effects.

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INTRODUCTION Focusing light on the nanoscale in order to achieve high absorption in tiny quantities of material can have applications in many fields. One such area is photovoltaics, which would benefit significantly if the light absorbing layers could be made thinner without sacrificing the absorption. Such thinning would enable more efficient carrier collection, voltage generation and especially fill factors.1 Even with perfect carrier collection, a reduction of the absorber layer thickness and concentration of the absorption is advantageous for the efficiency, in analogy with the efficiency typically gained with increased light intensities; a higher photovoltage and fill factor are attainable when the carrier density is enhanced.2 Hence, it is of high interest to reduce the absorber layer thickness in order to improve on the best demonstrated efficiency of 10.1%,3 which is much below the Shockley-Queisser limit of approximately 27% for the a-Si:H bandgap of 1.7 eV. In this work, we focus on enhancing absorption of light in hydrogenated amorphous silicon (a-Si:H) thin films. For this material, the undesirable Staebler-Wronski effect4 has been claimed to diminish as the thickness decreases,5 which provides additional motivation for making the corresponding solar cells thinner. One way to concentrate light in a solar cell absorber is to use localized surface plasmon resonances in metal nanostructures.6 Such resonances are the result of collective excitations of free © 2014 American Chemical Society

electrons when confined on the nanoscale. A plasmonic nanostructure at resonance typically scatters light (a far-field effect) and also generates an intense electric field close to its surface (a near-field effect, dominating at distances much below the wavelength of light). Enhanced scattering prolongs the path of light through the solar cell material, which increases the probability for its absorption. Likewise, the enhanced near-field increases the absorption rate in an adjacent solar cell material, since this rate is proportional to the field intensity.7 Depending on the particle material, size, and surrounding media, the local near-field intensity can be orders of magnitude higher than the incident field intensity. Both far-field and near-field plasmonic effects have been explored in attempts to enhance light absorption in solar cell materials. For a-Si:H solar cells, however, most efforts have been devoted to the enhanced scattering (the far-field) mechanism,8−12 and mostly through the employment of nanostructured Ag back reflectors.8,10,11 Nevertheless, theoretical works predict a very high potential for beneficial thickness reduction if optimal exploitation of the near-field effect can be realized, especially in combination with a reflective support.7,13,14 Since a-Si:H has a high absorption coefficient, it is capable of strong damping and absorption of Received: May 19, 2014 Revised: July 31, 2014 Published: September 9, 2014 22840

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the energy in the plasmon resonance, especially if a low-loss metal, such as Ag, is chosen for the plasmonic structure.7 Analytical and numerical calculations by Hägglund et al.13 suggest a maximum efficiency of 18% for an optically optimized a-Si:H solar cell with Ag nanoparticles, using an absorber layer of only 20 nm volume equivalent thickness. This requires prolate Ag nanoparticles uniformly coated by a 10 nm thick aSi:H layer, on an optimized dielectric spacer/Al reflector structure.13 The reflector suppresses transmittance, while the absorber layer, when separated from the reflector by an optimal distance, allows for the suppression of the reflectance through impedance matching. An absorber/spacer/reflector structure was recently used to demonstrate near complete absorption of visible light in a gold nanoparticle array of only 1.6 nm volume equivalent thickness.15 Experimentally the absorption enhancement via the plasmonic near-field has been less studied for a-Si:H solar cells. Santbergen et al.16 investigated Ag particles directly embedded into a 300 nm thick a-Si:H solar cell at different locations, and observed a slight photocurrent enhancement with nanoparticles compared to a reference cell. Moulin et al.17 observed an enhanced photocurrent with embedded Ag nanoparticles in a 40 nm thick a-Si:H absorber layer. The enhancement mechanism in the near-infrared was in this case attributed to the decay of plasmons into electron−hole pairs and subsequent injection of (hot) electrons into the a-Si:H film. Another possibility would be plasmon near-field induced transitions from defect states in the band gap of the a-Si:H, as was claimed for a similar solar cell configuration by Lükermann et al.18 A previous work by us19 demonstrated enhancement of photoconductivity of ultrathin (10−70 nm) aSi:H films in the presence of Ag nanodiscs, which we attributed to the enhanced near-field through comparison with numerical calculations. We found the highest plasmon induced absorptance for about 10 nm thick a-Si:H films.19 Cai et al.20 recently suggested heterostructured Ag nanoparticles, which could increase the photocurrent of a 20 nm thick a-Si:H solar to the value achievable for the standard 300 nm thick cell. The enhancement is a result of combination of the far-field, the near-field, and the geometrical effects. A particular challenge with the use of a-Si:H films stems from its high dielectric constant in combination with its relatively high optical gap of around 1.7 eV. The plasmon resonance significantly red-shifts when the metal nanoparticles are coated with a-Si:H,19 often beyond the optical gap threshold wavelength.17,18 While a resonance in this region may still provide some additional charge carrier generation via the photoemission mechanism17 or excitation from defect states,18 it is not ideal for maximizing the overall efficiency for extremely thin absorber layers. In this work, we therefore focus on the light absorption in high aspect ratio (height per width) plasmonic Ag nanocones. Because the plasmon resonance wavelength decreases in a close to linear fashion as the aspect ratio of a nanostructure increases,21 the resonance may in this way be retained in the useful spectral range for a-Si:H. The cones are coated by an approximately 9 nm thick a-Si:H layer, corresponding to a volume equivalent thickness of 20 nm, which is close to optimized conditions found theoretically.13 The coated cone structure is placed on a dielectric (TiO2) spacer, separating it from an optically thick (Al) reflector (Figure 1). Variation of the spacer thickness allows for tailoring and optimization of the optical response. For better separation of various effects, we

Figure 1. Schematics of Ag/a-Si:H nanocomposite (A) on glass and (B) on the spacer-reflector structure. SEM images of the large cones, (C) bare and (D) coated with 20 nm of a-Si:H, on a Si wafer coated with 60 nm of TiO2.

compare this system to the corresponding absorber structure on a glass substrate, as well as to flat reference samples without metal structures. Further, we numerically investigate the effect of exchanging the nanocone material from Ag to SiO2. Light absorption was measured and calculated numerically. The latter allowed us to analyze the origin of the absorption and to distinguish absorption in different parts of the structure. The absorption in the Al reflector and the Ag nanocones was typically found to sensitively depend on the thickness of the spacer. However, for sufficiently high cones (217 nm in this case), the absorption of the a-Si:H layer did not vary much with the spacer thickness. The highest overall light absorption in the a-Si:H layer was observed for a TiO2 spacer thickness of 40 nm, for which an ideal photocurrent value of 12.5 mA/cm2 was calculated. This is about 75% of the photocurrent obtained in record a-Si:H solar cells3 but in a much more compact volume of a-Si:H.

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METHODS Ag nanocones were fabricated using hole-mask colloidal lithography (HCL), as described in detail elsewhere.19,22 A 350 nm thick PMMA film produced by spin-coating was covered with a solution of 0.2 wt % charged polystyrene (PS) spheres with a mean diameter of either (i) 60 or (ii) 110 nm. The charged PS spheres adsorb to the PMMA through electrostatic interaction, while repelling each other over a scale determined by the Debye screening length, which in turn depends on the ion concentration of the solution. Dissolved salt (NaCl, 0.3 mM in case (i) and 0.4 mM in case (ii)) served to achieve sufficiently high particle densities here.22 A 15 nm Au mask was subsequently evaporated onto the samples, after which the PS particles were stripped off with tape. O2 plasma (50W, 250 mTorr for 7 min) was employed to ash the PMMA in the holes left in the mask. Ag films with a thickness of 120 and 220 nm for case (i) and (ii), respectively, were subsequently evaporated. The evaporated Ag deposited in the holes (producing the desirable particles), but also on the walls of the PMMA mask, leading to gradual closing of the mask and, thus, producing structures of conical (instead of cylindrical) shapes.22 The remaining PMMA and excess Ag on top of it was finally lifted off in acetone. 22841

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the cones (9 nm) and the corresponding surface areas, from SEM images. The equivalent volume thickness turned out to be the same (ca. 20 nm) for both cone sizes and very similar to the flat sample (19 nm). This is not a coincidence, but a consequence of the relatively long mean free path of the CVD species compared to the cone heights. The different types of samples studied here thus include essentially the same amount of a-Si:H. The TiO2 spacer thickness was varied from about 0 to 80 nm. The combination of roughness and native oxide on the Al reflector surface was accounted for by an additional 5 nm of Al2O3 in the model. The thicknesses and optical constants of the individual layers were obtained from spectroscopic ellipsometry (see Methods for details), while the Ag cone sizes were determined through cross-sectional SEM analysis (Figure 1C,D). Figure 2A,B shows measured and calculated absorptance for cones on glass. The agreement between the measurements and

The resulting cone patterns were characterized by the following parameters, as estimated from SEM images (taken both at 90° and 0°): (i) diameter 69.8 ± 5.4 nm, height 112.0 ± 2.9 nm, base angle 81.8 ± 2.6°, surface coverage 16%, and (ii) diameter 114.2 ± 6.6 nm, height 216.4 ± 3.6 nm, base angle 82.5 ± 2.1°, surface coverage 22%. Plasma-enhanced chemical vapor deposition (PE-CVD) was used to deposit a-Si:H films from a mixture of SiH4 and Ar gases at a rate of about 10 nm/min. The deposition was performed in parallel on the different types of substrates, at room temperature. The thicknesses and optical constants of the films were obtained from spectroscopic ellipsometry (Woollam M2000 with WVASE). The Al films were modeled with a Drude and a Tauc-Lorentz oscillator. A flat Al2O3 film with optical constants from Palik was used to represent the interfacial oxide. The fitted oxide thickness increased from 4.2 to 5.9 nm after the HCL process. The TiO2 films were represented with a CodyLorentz oscillator and a roughness layer on top (a Bruggeman effective medium approximation (EMA) layer of the underlying TiO2 and air). The TiO2 film thicknesses were 21.3 nm with a 0.0 nm EMA; 40.9 nm with a 2.9 nm EMA; 59.6 nm with a 4.6 nm EMA; and 78.5 nm with a 5.2 nm EMA. The a-Si:H films were also modeled with a Cody-Lorentz oscillator. In this case, a SiO2 layer or a mixed SiO2/air EMA layer did not improve the fit. The a-Si:H thickness turned out to be 19.4 nm. The thickness of the a-Si:H around the Ag cones was lower than on the TiO2, about 9 nm as estimated from SEM images at 90° (see Figure 1C,D). The light absorption was measured at a wavelength interval of 1 nm using a Varian Cary 5000 spectrophotometer equipped with an integrating sphere. Three dimensional numerical calculations were performed using the finite element method (RF module of Comsol Multiphysics), as described in more detail elsewhere.13 The geometry was deduced from SEM images (see above), with a fixed 1.3 nm thickness of silver oxide, Ag2O, determined through fitting the peak position of the small cones on glass without any a-Si:H coating. Absorptance of individual layers was calculated using the ratio of absorptance of the layer and the total absorptance, calculated from simulations, and multiplying this branching ratio by the total measured absorptance. For example, the absorptance of an a-Si:H layer in a particular sample was calc meas obtained from Aa‑Si:H = Acalc a‑Si:H/Atotal × Atotal .

Figure 2. Measured (dashed lines) and calculated (solid lines) absorptance of Ag cones with base diameters of 71 nm (A) and 118 nm (B) on glass, with and without a 20 nm coating of a-Si:H. The calculated absorptance of the a-Si:H in the presence of cones, as well as the measured absorptance for a corresponding flat reference a-Si:H film, are also shown. (C, D) Calculated distribution of the electric field along the z-axis for the large cones; bare, at 500 nm (C), and coated with the a-Si:H film, at 540 nm (D). The corresponding conditions are marked with crosses in panel (B). Both field plots are on the same color scale, where the extremes were truncated for improved color contrast overall.

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RESULTS AND DISCUSSION The structures of the samples fabricated in this work are presented in Figure 1A,B. Two Ag cone sizes were considered, with the smaller one having a base diameter of 71 nm and a height of 112 nm, and the larger one having a base diameter of 118 nm and a height of 217 nm. These cone dimensions include a fixed thickness of silver oxide (Ag2O)23 on the surface of the cones, fitted to 1.3 nm (see Methods). For simplicity, they will be referred to as “small” and “large” cones in the remainder of this paper. We considered two sets of samples; one having uncoated cones, and an equivalent set where the samples were coated with an a-Si:H film of about 20 nm volume equivalent thickness. The latter parameter is the thickness of a planar film having the same volume as the film under consideration. The volume equivalent thickness was obtained by estimating the volume of material from the thickness on the flat areas (between the cones: 19 nm) and on

the calculations is reasonably good in all cases. The small bare cones display a resonance at about 480 nm, and the large ones at about 500 nm. The measurements show broader peaks than the calculations, which can partially be attributed to inhomogeneous broadening in the experiments due to variations in the cone size and shape. A part of this broadening might also be the result of electron scattering at grain boundaries, although surface scattering was accounted for in the calculations. For both cone sizes, the absorption peaks 22842

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observed without coatings are mainly dipolar in character, with a concentration of the field energy close to the substrate interface. This is seen in a plot of the calculated electric field component in the incident field direction, as shown in Figure 2C at 500 nm for the large cones. When the cones are coated with a-Si:H, the plasmon resonance red-shifts, as expected (red curves in Figure 2). The dipole resonance shifts to about 700 nm in both cases, and additional resonances appear at shorter wavelengths. The electric field distribution associated with the resonance at 540 nm for the large cones coated with a-Si:H is shown in Figure 2D. It has a more quadrupolar character, as signified by opposing field components of comparable strength in the upper/middle and lower parts of the cone. These resonances all boost absorption in the a-Si:H due to the resulting electric field enhancement close to the cone surface. In addition to this plasmon near-field effect, the change in the geometry through the presence of cones also affects the a-Si:H absorption. For instance, the pillar-like geometry may hold a larger amount of aSi:H, and its interaction with light will be enhanced when deposited on the side walls of the cones. In our case, the volume equivalent thickness of the a-Si:H (20 nm) is very close to the thickness on a flat sample (19 nm), so the first effect plays only a minor role. The enhanced interaction in pillar-like structures is well-known, and in addition to this, the cones scatter light to some extent, which also prolongs the optical path length through the structure. Therefore, the resulting aSi:H absorptance is significantly higher than the absorptance of a corresponding flat a-Si:H film on glass. The contributions of these various effects depend on the cone geometry, and although the plasmon resonances for both cone types have similar positions and amplitudes without the a-Si:H coating, this becomes clear when the a-Si:H is added (Figure 2A,B). Overall, the absorptance of the a-Si:H film is higher for the case of the larger cones, resulting in an approximately 20% higher value of the corresponding ideal photocurrent (Figure 5). This is discussed in more detail below. We now turn to the system of a Ag cone/a-Si:H nanocomposite layer supported by a spacer-reflector structure as depicted in Figure 1B. The measured and calculated absorptance for the large cones, when bare and when coated with a-Si:H, is shown as a function of the TiO2 spacer thickness and wavelength in Figure 3A−D. Five TiO2 spacer thicknesses (approximately 0, 20, 40, 60, and 80 nm; see Methods for the details) were investigated and used for the interpolation. The measured peak positions and lineshapes are reproduced well by the calculations, although peak broadening is again observed in the experiments. The latter is seen clearer in the conventional line plots for the same systems, as well as for the small cones, in Figure S1 of the Supporting Information (SI). Two separate ridges are visible in the plots for the bare cones, both in the experiments and the calculations (Figure 3A,B). In these systems, there are two layers that absorb light: the Ag nanocones and the Al reflector. Figure S2A in the SI shows the absorptance of these two components (as deduced from a combination of measured and calculated data; see the Methods section) for different spacer thicknesses. The results suggest that the short-wavelength ridge in Figure 3A,B (at 400−700 nm) yields comparable absorption in both the cones and the reflector, although their individual maxima are generally not coinciding. Absorption in the cones at this ridge corresponds to a quadrupolar-like resonance. Noteworthy, the plasmon peak position varies sensitively with the spacer

Figure 3. (A) Measured and (B) calculated total absorptance with large cones on a TiO2 spacer/Al reflector, as a function of wavelength and spacer thickness. The measured (C) and calculated (D) total absorptance of the same systems with the addition of a 20 nm thick aSi:H coating. The electric field distribution at 500 nm (E) and 650 nm (F) for the larger nanocones coated with 20 nm a-Si:H film on a 40 nm TiO2 spacer, corresponding to the conditions marked with crosses in panel (D). The k-vector of the incident light is along the x-axis, and the electric field is along the z-axis. Both field plots use the same truncated color scale.

thickness (Figure S2A in the SI). The peak value moves from about 880 nm for the 20 nm thick TiO2 to about 640 nm for the 60 nm thick spacer. The long-wavelength ridge (>750 nm) corresponds to absorption in both the reflector and the Ag cones (Figure S2A) through a dipolar resonance shifted into that range. Absorptance spectra of the cones coated with a-Si:H display complex structures and generally exhibit high absorption over large parts of the visible spectrum (Figures 3C,D, and S1B,D of the SI). In these systems, three layers contribute to the absorption: the nanocones, the Al reflector, and the a-Si:H film. 22843

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Around 500 nm wavelength on the 40 nm spacer, the Ag nanocones exhibit a resonance centered near the top half of the cones (Figure 3E). As the wavelength increases, the resonance shifts toward the substrate so that the field concentrates around the lower interface (Figure 3F). This shift is analogous to the spectral redshift of localized plasmon resonances when subjected to a higher refractive index environment. In the present case, the shift is instead spatial, into a region of higher (effective) index, and it is a consequence of the red-shifted excitation wavelength. The resonance has both dipolar and quadrupolar components. The absorptance of the Ag cones and the Al reflector are shown in Figure S2B. The peak positions of the systems of small bare cones are reproduced reasonably well by the calculations (Figure S1C). However, when coated with an a-Si:H film, a red shift of the calculated peaks relative to the measured ones is observed (Figure S1D). Despite the red-shift, however, the features and trends are well reproduced by the calculations. Figure 4 shows absorptance in a-Si:H films in a spacerreflector configuration in the presence of large Ag nanocones

Figure 5. Ideal photocurrent for large cones, obtained by weighing the a-Si:H absorptance by the AM1.5 solar spectrum and integrating it up to the a-Si:H band gap (1.7 eV ⇔ 730 nm). The photocurrent values are shown for different TiO2 spacer thicknesses for the large cone configurations. The corresponding values on glass, for both large and small cones with a-Si:H, are also included.

“global tilt” spectrum24) and by integrating it from 400 to 730 nm, where 730 nm corresponds to the optical gap (1.7 eV) of our a-Si:H films. This calculation does not take into account that the absorption in doped a-Si:H layers, which are classically needed for an efficient charge carrier collection, does not contribute much to the photocurrent. On the other hand, it has been shown that in case of very thin films, only one of the doped layers (n- or p-) can be sufficient,17 and these layers can be made as thin as 5 nm.25 Similar to the a-Si:H absorptance, the ideal photocurrent does not vary much as a function of the spacer thickness. Its maximum value of 12.5 mA/cm2 is achieved for the TiO2 spacer of 40 nm. Considering the record demonstrated photocurrent of 16.75 mA/cm2 for an a-Si:H solar cell,3 the value of 12.5 mA/cm2 is rather high for an a-Si:H film of only 20 nm equivalent thickness. (The record demonstrated value could have been a stabilized current, which is usually lower than the initial current, the decrease being due to the Staebler-Wronski effect.4 For very thin a-Si:H solar cells, however, the influence of this undesirable effect is believed to be smaller.) While the value of the total ideal current does not change much with the spacer thickness, the contribution to this current caused by the Ag nanocones does. The contribution is most pronounced for the case of 60 nm TiO2 where the flat film system displays a minimum. The values of the total current and the cone-induced current are similar in the cases of the 80 nm spacer and for the system on glass. In the absence of scattering, one would expect a periodic variation of the absorption as the optical path length through the spacer cycled through integer wavelengths, with an average close to that without a reflector, such as on glass. As mentioned, the improvement in the a-Si:H light absorption in the presence of Ag nanocones arise not only because of the enhanced near-field, associated with the plasmon resonance, but also because of geometrical effects. The latter include improved interaction with light due to enhanced optical path length through and confinement to the a-Si:H distributed on the cone walls, as well as some degree of scattering. These effects have been studied in several works.26,27 For instance, Kuang et al.26 recently demonstrated a 3.6% efficient a-Si:H solar cell, where the absorber is an only 25 nm thick coating on 400 nm high ZnO nanorods. To account for these effects and put them in relation to the influence of the plasmon induced

Figure 4. Absorptance of a-Si:H films (A) in the reference system of flat film stacks and (B) in the presence of Ag nanocones, in both cases on a spacer/reflector structure. The TiO2 spacer thicknesses are indicated.

(Figure 4B) and on a flat spacer-reflector (Figure 4A) for different spacer thicknesses. In the case of flat films, Figure 4A indicates a strong dependence on the spacer thickness for the aSi:H absorptance. The highest absorptance is achieved without a spacer (or rather with a ca. 5 nm thick Al2O3 native oxide spacer, see further Methods). A TiO2 spacer that is 60 nm thick is least favorable for absorption in the flat a-Si:H film. The situation changes drastically in the case of the large cones (Figure 4B). Here, the absorptance is broadly enhanced in the a-Si:H and does not change much as a function of the spacer thickness. One may qualitatively understand the latter observation as an effect of the significant cone height, which exceeds 200 nm. This is comparable to the wavelength of light, leading to substantial phase shifts for propagation along the cone axis. Both constructive and destructive interference nodes may therefore develop along this axis, with positive and negative effects of the reflector canceling out to some extent, independently of the spacer thickness. As one would expect from this argument, a stronger dependence on spacer thickness is observed for the a-Si:H absorptance with the smaller cones (Figure S3 in the SI). Figure 5 shows the ideal photocurrent, assuming unit quantum yield, from the estimated absorptance of the a-Si:H in the different configurations. These values were obtained by weighing the absorptance with the AM1.5G solar spectrum (the 22844

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with a 40 nm TiO2 spacer, where Ag cones were substituted with SiO2 cones. The a-Si:H absorptance was shown to be substantially higher in the presence of the plasmonic cones compared to the dielectric ones.

effects here, we performed calculations for the system of a 40 nm TiO2/reflector and replaced the Ag cones with SiO2 cones. Figure 6 shows the calculated absorptance in the a-Si:H coating

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ASSOCIATED CONTENT

S Supporting Information *

Calculated and measured absorbance spectra of the large and the small cones, uncoated and coated with the a-Si:H film, for the different TiO2 spacer thicknesses; calculated absorbance in the individual layers of the systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 6. Comparison of absorptance in an a-Si:H film on top of SiO2 and Ag cones of the same geometry.

Present Address §

Ångström Solar Center, Division of Solid State Electronics, Department of Engineering Sciences, Uppsala University, 75121 Uppsala, Sweden.

SiO2 and Ag nanocones, respectively. For both the large and the small cone types, the a-Si:H film is seen to absorb substantially more light in the presence of the plasmonic nanocones, thus, illustrating the usefulness of these components, despite their inherent losses. As a final comment, we note that for flat Al-TiO2 systems, the absorption in the Al reflector is significantly modified by the presence of the TiO2 layer, and that it depends on the thickness of the latter (see Figure S4A). The TiO2 film acts as an antireflection coating, coupling light into the flat Al film. The addition of a plasmonic nanostructure on top of this system further modifies the Al absorptance spectrum, whether it is the bare cones (Figure S5A), or the a-Si:H coated cones (Figure S5B). These nanostructures may to some extent be considered in terms of an effective medium, absorbing and modifying the phase of the propagating light. Because of the strong and varying phase shift contributions from the plasmon resonance, the antireflective effect on the Al absorptance depends sensitively on it.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding from the Swedish Energy Agency Project DNR. 0189-1 “Nanoscience and Nanotechnology for Sustainable Energy and Environment” (V.G., B.K.), Formas Project 229-2009-772 (V.G., B.K.), and the Myfab research infrastructure for micro and nanofabrication (V.G., B.K.). C.H. thanks the Marcus and Amalia Wallenberg Foundation for financial support.

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REFERENCES

(1) Deng, X.; Schiff, E. A., Amorphous Silicon-Based Solar Cells. In Handbook of Photovoltaic Science and Engineering, 2nd ed.; Luque, A., Hegedus, S., Eds.; John Wiley & Sons Ltd: West Sussex, U.K., 2003; pp 505−566. (2) Queisser, H. J. Photovoltaic Conversion at Reduced Dimensions. Physica E 2002, 14, 1−10. (3) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 44). Prog. Photovoltaics 2014, 22, 701−710. (4) Shimizu, T. Staebler-Wronski Effect in Hydrogenated Amorphous Silicon and Related Alloy Films. Jpn. J. Appl. Phys., Part 1 2004, 43 (6A), 3257−3268. (5) Shah, A. V.; Schade, H.; Vanecek, M.; Meier, J.; Vallat-Sauvain, E.; Wyrsch, N.; Kroll, U.; Droz, C.; Bailat, J. Thin-Film Silicon Solar Cell Technology. Prog. Photovoltaics Res. Appl. 2004, 12, 113−142. (6) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (7) Hägglund, C.; Apell, P. S. Plasmonic Near-Field Absorbers for Ultrathin Solar Cells. J. Phys. Chem. Lett. 2012, 3, 1275−1285. (8) Ferry, V. E.; Verschuuren, M. A.; Lare, M. C. v.; Schropp, R. E. I.; Atwater, H. A.; Polman, A. Optimized Spatial Correlations for Broadband Light Trapping Nanopatterns in High Efficiency Ultrathin Film a-Si:H Solar Cells. Nano Lett. 2011, 11, 4239−4245. (9) Ho, C.-I.; Yeh, D.-J.; Su, V.-C.; Yang, C.-H.; Yang, P.-C.; Pu, M.Y.; Kuan, C.-H.; Cheng, I.-C.; Lee, S.-C. Plasmonic Multilayer Nanoparticles Enhanced Photocurrent in Thin Film Hydrogenated Amorphous Silicon Solar Cells. J. Appl. Phys. 2012, 112, 023113. (10) Hsu, C.-M.; Battaglia, C.; Pahud, C.; Ruan, Z.; Haug, F.-J.; Fan, S.; Ballif, C.; Cui, Y. High-Efficiency Amorphous Silicon Solar Cell on a Periodic Nanocone Back Reflector. Adv. Energy Mater. 2012, 2, 628− 633.

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CONCLUSIONS We have measured and calculated light absorption in systems of high aspect ratio Ag nanocones, coated with approximately 9 nm thick a-Si:H films, amounting to approximately 20 nm volume equivalent thickness of a-Si:H. These nanocomposites were fabricated on glass, and also on a dielectric TiO2 spacer separating them from an optically thick Al reflector by varying spacer thicknesses. Reference samples of flat films without nanocones, as well as systems without the a-Si:H coating, were also characterized. Calculations by the finite element method allowed for distinguishing the absorptance in the different components of the systems. We show that while the spacer thickness significantly influences the a-Si:H absorptance in the case of flat film systems, its effect in the presence of the relatively tall nanocones is moderate. This can be qualitatively understood, since the cone height (ca. 220 nm in this work) effectively evens out interference effects along its depth through its significant optical path length. The highest value of the ideal photocurrent, estimated from the a-Si:H absorptance, is 12.5 mA/cm2. It is realized for a silver cone/a-Si:H nanocomposite on a 40 nm TiO2 spacer/reflector structure. In order to separate the plasmonic near-field effect from other geometrical effects associated with the presence of the nanocones, we performed calculations on the optimal system 22845

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp504916p | J. Phys. Chem. C 2014, 118, 22840−22846