Plasmonic Hole Arrays for Combined Photon and Electron Management

Transcription

1 Plasmonic Hole Arrays for Combined Photon and Electron Management Andreas C. Liapis, 1, a) Matthew Y. Sfeir, 1 1, b) and Charles T. Black Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States. Material architectures that balance optical transparency and electrical conductivity are highly sought after for thin-film device applications. However, these are competing properties, since the electronic structure that gives rise to conductivity typically also leads to optical opacity. Nanostructured metal films that exhibit extraordinary optical transmission, while at the same time being electrically continuous, offer considerable flexibility in the design of their transparency and resistivity. Here we present design guidelines for metal films perforated with arrays of nanometer-scale holes, discussing the consequences of the choice of nanostructure dimensions, of the type of metal, and of the underlying substrate on their electrical, optical, and interfacial properties. We experimentally demonstrate that such films can be designed to have broad-band optical transparency while being an order of magnitude more conductive than indium tin oxide. Prototypical photovoltaic devices constructed with perforated metal contacts convert 18% of the incident photons, compared to <1% for identical devices having contacts without the hole array. a) Electronic mail: aliapis@bnl.gov b) Electronic mail: ctblack@bnl.gov 1

2 Many material and device applications require the design of material architectures that simultaneously maximize light transmission and electrical conductivity. For example, a solar cell electrical contact must allow the incident sunlight to access the device interior while also minimizing Ohmic losses in the contact during charge collection. Similar considerations apply to electrical contacts for light-emitting diodes, photodetectors, and thin-film-transistor liquid-crystal displays. Often, designs will compromise one property for the sake of the other. 1 In silicon solar cells, one prevalent design employs a grid of finger electrodes that cover a fraction of the device surface, relying on the long carrier diffusion lengths in crystalline silicon to facilitate charge collection between the fingers. 2 A drawback of this design is the shadowing of 10% of the device area by the metal fingers, through which no light can enter the device a limitation which, to be overcome, necessitates a more complex back contact device geometry. 3 Solar cells based on organic semiconductors, nanocrystals, and perovskites typically use continuous, semi-transparent conducting oxides as electrodes, which are generally not as conductive as metals. 4,5 Other proposed material solutions include carbon-based nanomaterials 6,7 and patterned ultra-thin metal films In this work, we consider the use of nanostructured metal films as transparent electrical conductors, and examine the tradeoffs between optical transmissivity and electrical conductivity dictated by the selection of nanostructure dimensions, the choice of metal, and the choice of substrate. Opaque metal films can become highly transparent when perforated with suitable arrays of nanometer-scale holes. Resonant interactions between the incident light and free surface charges in the metal enabled by the structure s periodicity can enhance the optical transmission through the hole array by orders of magnitude compared to the predictions of standard aperture theory, rendering even thick metal films optically transparent The physical complement to a perforated metal film i.e., an array of plasmonic metal disks has previously been pursued as a means to enhance solar cell performance by providing a mechanism for light trapping in thin-film photovoltaic devices Physically complementary structures have complementary optical properties (Babinet s principle, see Supplementary Information S-1), which implies that both plasmonic particle arrays and hole arrays can be designed to have optical features that are advantageous for optoelectronic devices. However, a perforated metal film has the added advantage of electrical continuity, which allows it to simultaneously serve as an electrical conductor. This dual functionality has led to several successful implementations of perforated metal films as electrical contacts for solar cells, 2

3 (a) h 1 μm (b) (c) (d) (e) 2r α 500 nm Au 500 nm Al 500 nm 500 nm Cu Ag FIG. 1. (a) Cross-sectional scanning electron micrograph of a silver film perforated with an array of nanometer-scale holes fabricated on a silicon substrate. (b) (e) Scanning electron micrographs of hexagonal arrays of nano-holes of lattice constant α = 450 nm and hole radius r in 100-nm-thick gold, aluminum, copper, and silver films fabricated on ITO-coated glass slides. (f) (i) Experimental normal-incidence power transmittances of the samples shown in (b) (e). The dotted lines correspond to the geometric film porosity of each sample. The shaded regions in (f) and (h) represent the wavelength bands where transmission is suppressed due to the onset of interband transitions. primarily those involving organic semiconductors, where they have been proposed as an alternative to conductive metal oxides such as indium tin oxide (ITO) Here, we present general guidelines for the design of transparent plasmonic contacts by exploring the effects of varying the nanostructure dimensions on the optical, electrical, and interfacial properties of thin films of four different metals perforated with hexagonal arrays of nano-holes. We fabricate mm2 area structures having nanometer-scale features using electron-beam lithography, metallization, and lift-off [Figure 1(a), also see Supplementary Information S-2]. The versatility of our fabrication process allows us to straightforwardly vary the geometric film porosity and film thickness, the type of metal, and the type of substrate. The optical transmission spectra of gold, aluminum, copper, and silver films deposited on 3

4 ITO-coated glass substrates and perforated with hexagonal arrays of nanoscale holes with radius r, lattice constant α, and height h [Figures 1(b) (e)] all contain a similar series of well-defined maxima and minima [Figures 1(f) (i)], with broad Fano-like peaks in the nearinfrared leading to a series of narrower peaks at shorter wavelengths. Although the detailed features in the spectra differ depending on the choice of metal, each spectrum also contains wavelength bands with transmission higher than the geometric film porosity [area of hole / area of unit cell = 2πr 2 / 3α 2, shown as dotted horizontal lines in Figures 1(f) (i)]. For example, nanostructured copper with a geometric porosity of 20% reaches a transmission of 36% at 627 nm. In these systems, it is generally accepted that transmission maxima result from the resonant excitation of surface plasmon modes, while the minima in optical transmission correspond to Wood s anomalies of diffraction, which occur when the diffracted wave-vector becomes tangent to the plane of the grating. 34,35 The resonance wavelengths for both the maxima and the minima are proportional to the array lattice constant, and therefore the transmission spectra of these structures can be tuned in a straightforward manner. Note that because our structures support surface plasmon modes on the metal-dielectric interfaces on both sides of the metal film, the measured transmission spectra contain two sets of resonances. In Figures 1(f) (i), resonances around 450 nm correspond to the excitation of a surface plasmon at the metal-air interface, while resonances at longer wavelengths correspond to the metal-substrate interface. Note also that the useful transparency range of nanostructured metal films is limited by the onset of interband transitions, which for gold and copper occur at 2.38 and 2.1 ev, respectively. To examine the effect of the hole-array design parameters on the structure s optical and electrical performance, we fabricated a series of perforated metal films on glass substrates, systematically increasing the metal film thickness and hole radius, while keeping the holearray lattice spacing constant (α = 450 nm). Figures 2(a),(b) show the experimental normalincidence transmission spectra of these samples. The transmissivity of perforated silver films generally decreases as the film thickness increases [Figure 2(a)], which is in agreement with previous studies of similar structures. 36 The broadening of the resonances and the shift of the transmission maxima to longer wavelengths observed in thinner films is consistent with the increased coupling between the surface plasmons supported by the two interfaces. 32 While thinner films are more transmissive, the use of ultra-thin films as electrical contacts for photovoltaic devices is not ideal, as it creates higher Ohmic losses during charge collection. 4

5 FIG. 2. (a) Transmission spectra of perforated silver films of varying metal thickness h. In all cases, α = 450 nm and r = 126 nm. (b) Transmission spectra of perforated silver films of varying hole radius r. Here, α = 450 nm and h = 94 nm. Scanning electron micrographs of three of these films are shown in (i) (iii). The scale bars correspond to 500 nm. (c) Measured resistivity of perforated aluminum, copper, and silver films as a function of hole radius. In all cases, α = 450 nm. However, even films of thickness many times the metal skin depth can be made to be considerably transmissive: Increasing the hole radius in a 94-nm-thick perforated silver film results in higher transmission across all wavelengths [Figure 2(b)]. Accompanying this increase in transmission is a broadening of the resonances due to the increased radiative 5

6 damping of the surface plasmon modes, and slight shifts in the location of the transmission maxima that are attributed to cutoff behavior. 37 We find that our most porous films display broadband transmission approaching that of ITO. Over the wavelength range nm, the average transmission through a 94-nm-thick silver film perforated with 178-nm-radius holes is 66% [red line in Figure 2(b)]. For comparison, a silver film of similar thickness without holes transmits < 3% of the incident radiation, while a 200-nm-thick film of ITO transmits 87% on average. For device applications that operate over a narrow wavelength range, these results can be further improved upon by making the structure symmetric, in which case the optical transmission on resonance can approach unity. 38,39 While the increased transmission and broader resonances observed in highly porous films are advantageous from a light-management perspective, the larger hole diameter also increases the film s resistivity. The resistivity ρ of perforated silver, copper, and aluminum films increases with increasing hole radius r (for constant separation α = 450 nm), in a manner that is consistent with the expression ρ = ρ 0 /[1 βr], where ρ 0 is the resistivity of the metal film in the absence of the hole array, and β is a geometric factor associated with the array [Figure 2(c)]. For all three metals, we determine β nm 1. Nonetheless, we find that even with a geometric film porosity approaching 60%, for r = 178 nm, a perforated silver film remains highly conductive, with a resistivity of Ω m. For comparison, the resistivity of ITO is more than 10 times higher, Ω m. We see therefore that perforated metal films have the potential to have comparable optical properties to ITO while being an order of magnitude more conductive. However, since the surface plasmon resonance wavelength depends on the refractive indices of both the metal and the adjacent dielectric, the transmissivity of perforated metal contacts depends not only on the choice of nanostructure dimensions and the choice of metal, but also on the final device structure. Interfacial electronic properties should also be taken into account when considering the use of a perforated metal film as an electrical contact. For example, a metal-semiconductor contact may be conducting (Ohmic) or rectifying (Schottky), depending on the relative alignment of the energy bands of the two materials. Here, as an illustrative example, we consider metal-semiconductor Schottky contacts, which can drive photovoltaic energy conversion. In these devices, photoexcited charges are separated by a potential barrier with height ϕ B, created due to the difference between the work function of the metal and the electron affinity of the semiconductor [Figure 3(a)]. We find that junctions formed between p-type silicon 6

7 and silver, copper, or aluminum are rectifying and show diode-like behavior, while a gold to p-type silicon junction has a sufficiently small barrier for its current to be Ohmic at room temperature. Note that our silver and gold devices include a thin chromium adhesion layer (see Methods section) which influences the resulting interfacial properties. The barrier heights for non-perforated copper, silver, and aluminum contacts, extracted from the unilluminated forward-bias current-voltage characteristics of the junctions, are ϕ B = 533, 616, and 606 mev, respectively (see Supplementary Information S-3), which is generally consistent with lower work-function materials having larger Schottky barrier heights. Note that while aluminum displays the highest interfacial barrier, which is desirable for photovoltaic applications, it also forms the most resistive contact. We have constructed Schottky solar cells with perforated copper, silver, and aluminum top contacts. In our devices, the metal contacts are 100-nm thick and would completely block incident sunlight in the absence of the hole array. Light absorbed in the surrounding periphery of the device, within a carrier diffusion length from the contact, would result in a small amount of photocurrent, which we estimate to be 1 ma/cm 2. However, the fabricated hole arrays facilitate light transmission through the metal film, and lead to an appreciable increase in photocurrent under simulated AM1.5G illumination (calibrated using a certified silicon reference cell, and at normal incidence), while maintaining the rectifying characteristics of the junction [Figure 3(b), also see Supplementary Information S-3]. These devices support an open circuit voltage of 40, 190, and 260 mv, respectively, which is consistent with the measured Schottky barrier height for each metal. The relatively low device fill factor and large reverse bias saturation current observed are signatures of carrier recombination and large thermally activated leakage currents that are present in low-barrierheight devices. To maximize the photocurrent generated by these devices, it is not sufficient to maximize the transmission of solar radiation through the nanostructured contact, as the absorption depth in the semiconductor must also be considered. To illustrate this, we compare in Figure 3(c) two devices with perforated silver top contacts of equal thickness (h = 100 nm) and hole radius (r = 160 nm), but with different lattice constants (α = 450 and 600 nm). The α = 450 nm device generates more photocurrent and displays a higher open-circuit voltage, but is also slightly more resistive (as evidenced by the decreased slope of the unilluminated forward-bias current-voltage characteristic of the junction), which is consistent with the 7

8 FIG. 3. (a) Energy diagram of a Schottky barrier, showing the process of photoabsorption and carrier separation. ϕ B is the barrier height. (b) Experimental current-voltage characteristics of Schottky solar cells with perforated copper, silver, and aluminum metal top contacts. (c) Experimental current-voltage characteristics of devices with perforated silver top contacts of different lattice constants. increased porosity of the nanostructured metal film. To understand the observed differences in photocurrent, we examine the incident photon to current efficiency (IPCE) of our devices as a function of wavelength. In this experiment, the output of a broadband source is passed through a monochromator and focused 8

9 FIG. 4. (a) Experimental (dashed lines) and simulated (solid lines) reflection spectra for Schottky photovoltaic devices with perforated silver top contacts with lattice constants α = 450 nm and α = 600. For both devices, h = 100 nm and r = 160 nm. (b) Simulated optical transmission spectra of the perforated silver contacts (dotted lines) compared to the simulated power absorbed by the active region of these devices (solid lines). (c) Experimental incident photon to current efficiency of these devices compared to the solar spectral irradiance (ASTM G reference spectra). The resonance of the α = 450 nm device is optimized for maximum coverage of the solar spectrum. onto the sample such that the illumination underfills the device. Figure 4(a) shows the experimentally-measured reflection spectra of these two devices. A strong dip in the reflection is seen around 450 nm for the first device, and 600 nm for the second, corresponding to 9

10 the excitation of surface plasmon modes on the metal-air interface. No resonances associated with the metal-substrate interface are seen in this wavelength range due to the high refractive index of silicon. Finite-Difference Time-Domain simulations of this structure accurately reproduce the experimental reflection spectra (see Supplementary Information S-2), and can be used to predict the transmission spectra of these contacts [Figure 4(b), dotted lines], showing that the α = 600 nm contact transmits more light on resonance. However, not all of the photons transmitted through the contact contribute to the measured photocurrent. By monitoring the simulated optical power within the silicon substrate, we extract the fraction of optical power absorbed in the active region of the junctions, which we estimate to be within 1 µm from the metal-semiconductor interface, and predict a higher absorption in the α = 450 nm device [Figure 4(b), solid lines]. The experimental IPCE for our devices display prominent peaks that match well with the predicted absorbed power [Figure 4(c)], confirming that the dominant mechanism for the generation of photocurrent is the transmission of light through the electrode and absorption by the semiconductor. However, we note two additional contributions to the measured photocurrent: Firstly, both devices display a smaller feature in their IPCE spectrum in the vicinity of 310 nm, or 3.98 ev, which corresponds to the interband transition from the L 3 symmetry point to the Fermi surface of silver. Secondly, the IPCE spectra contain a broad background that ranges from 300 nm to 1.1 µm, which is absent in devices fabricated without the plasmonic nanostructure. We theorize that this background is either due to fabrication imperfections in our devices, which may lead to a Purcell enhancement not captured by our idealized model, or due to plasmonic excitation in the metal contact. The latter can contribute to the photocurrent either by energy transfer 40 or by the transfer of energetic charge carriers (hot electrons and holes) across the interface. 41 Further investigations are required in order to distinguish between these possibilities. Through a combination of all these effects, the on-resonance IPCE of the α = 450 nm device is 42%. By comparing the IPCE spectra to the solar spectral irradiance, we find that this device converts 18.5% of the incident photons, compared to 10% for the α = 600 nm device, in agreement with the increased photocurrent observed in Figure 3(c). The equivalent device fabricated without the plasmonic hole array shows almost no photocurrent, with a conversion efficiency of <1%. In designing material architectures for devices with combined electrical and optical functionalities, there are a number of material properties that should be taken into account. 10

11 Nanostructured metal films offer considerable design flexibility, allowing us to control their transparency, resistivity, interfacial properties, and electronic bandstructure. We have reported experimental demonstrations of control of all these parameters, and discussed how they affect the performance of prototypical Schottky photovoltaic devices. For example, we have shown that perforated copper films are highly conductive and can be designed to be moderately transparent. However, the work function of copper is poorly suited to forming a Schottky junction for collecting photoexcited carriers from p-type silicon. Similarly, of the metals studied in this work, nanostructured aluminum forms the highest interfacial barrier to p-type silicon, but is the most resistive and the least transparent. In this case, nanostructured silver offers the best compromise between transparency, conductivity, and barrier height. The same design principles can be applied to electrical contacts for resonant tunneling diodes, 42 and other technological applications that make use of transparent conductors, such as organic thin-film photovoltaics, light-emitting diodes, displays, and touch-screens. SUPPLEMENTARY MATERIAL See supplementary material for a demonstration of Babinet s principle using an array of copper nanoparticles and a perforated copper film of similar dimensions; fabrication, measurement, and simulation methods; current-voltage characteristics of junctions between continuous (i.e. not perforated) copper, silver, and aluminum metal films and p-type silicon. ACKNOWLEDGMENT The authors thank Matthew D. Eisaman and Ahsan Ashraf for assistance in the measurement of external quantum efficiency. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC REFERENCES 1 G. Haacke, Journal of Applied Physics 47, 4086 (1976). 2 L. J. Caballero, Contact Definition in Industrial Silicon Solar Cells, Solar Energy, Radu D Rugescu (Ed.) (InTech, 2010). 11

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