LETTER pubs.acs.org/NanoLett
Plasmonic Energy Collection through Hot Carrier Extraction Fuming Wang and Nicholas A. Melosh* Department of Materials Science and Engingeering, Stanford University, Stanford, California 94305, United States
bS Supporting Information ABSTRACT: Conversion of light into direct current is important for applications ranging from energy conversion to photodetection, yet often challenging over broad photon frequencies. Here we show a new architecture based on surface plasmon excitation within a metalinsulatormetal device that produces power based on spatial confinement of electron excitation through plasmon absorption. Plasmons excited in the upper metal are absorbed, creating a high concentration of hot electrons which can inject above or tunnel through the thin insulating barrier, producing current. The theoretical power conversion efficiency enhancement achieved can be almost 40 times larger than that of direct illumination while utilizing a broad spectrum of IR to visible wavelengths. Here we present both theoretical estimates of the power conversion efficiency and experimental device measurements, which show clear rectification and power conversion behavior. KEYWORDS: MIM devices, hot electron, solar energy conversion, surface plasmons, tunneling
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onversion of electromagnetic radiation into electricity has been demonstrated using a variety of techniques, including photovoltaics, rectannas, and absorber-thermal engines combinations. Rectennas13 are quite appealing for their low cost, simple antenna and metalinsulatormetal diode architecture, and close to 90% efficiency at 2.45 GHz,2 prompting a number of groups to explore higher frequency operation.47 Unfortunately, at IR and visible frequencies the MIM structure must have an RC time constant above ∼1014 Hz with extremely good diode characteristics, placing stringent requirements on the device capacitance and materials that have not been fully realized. However, photoillumination of MIM junctions not only generates AC current but also produces excited electronhole pairs through photon absorption in the metals.8 For IR and optical frequency photoexcitation, the electron mean free path may be larger than the metal film thickness, allowing these electrons to impinge on the buried insulator layer without losing significant energy. If the incident electron energy is larger than the metalinsulator band offset the electrons may travel ballistically across the insulator, generating current.9 This fundamental hotcarrier mechanism has been used extensively to determine Schottky barrier heights, yet has not been considered for energy conversion due to low efficiencies. However, by harnessing the absorption and decay of surface plasmons (SPs) much higher hot carrier densities could be generated, substantially boosting the energy conversion. This takes advantage of the higher absorptivity and spatial localization of SPs relative to linear bulk absorption and could operate across a broad frequency range. A similar hot-electron plasmon excitation effect was recently described for metal/semiconductor near IR photodetectors.10 This counterintuitive use of surface plasmon decay, which is usually considered detrimental, utilizes the metal film as both the absorber and the electron emitter, harvesting the electron energy before it dissipates as heat. In this work, we show excited hot carriers from plasmon absorption can r 2011 American Chemical Society
Figure 1. Solar energy collections by MIM junctions. (a) Schematic of the MIM solar energy converter. With a density difference of hot electrons in the top and bottom electrodes, forward current Itop overwhelms back current Ibtm to output a net DC current. Surface plasmons excited on one electrode can largely increase output current. (b) Energy diagram of a MIM device with hot carriers excited to higher energy levels.
be collected within MIM junctions to realize energy conversion and examine the maximum theoretical efficiency for planar gold and silver MIM devices. Hot electrons in planar MIM multiple-layer devices have been studied for decades as a means to investigate the energy band structure at device interfaces.1113 Upon absorption in the metal, each photon excites an electron into a higher energy state (hot electron) above the Fermi level, leaving an equal number of holes (hot holes) (Figure 1b). Assuming optically excited hot carriers propagate isotropically, half the electrons initially travel toward the metalinsulator interface where they may be collected. Absorption in the upper and lower metal films generates currents with opposite signs (Figure 1a), such that a net DC photocurrent only results if the absorption is larger in one metal than the other electrode. Thus for efficient energy conversion devices photon absorption should be localized in a single electrode. This is quite Received: September 14, 2011 Revised: October 20, 2011 Published: October 24, 2011 5426
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Figure 2. Power generation under direct illumination. Lower: Forward photocurrent (red), backward photocurrent (blue), and total photocurrent (black) as a function of applied bias. The power generation region was marked in brown. Upper: corresponding energy diagram for three different bias regions, I, II, and III.
challenging for direct illumination of planar MIM junctions, as a thicker upper metal layers absorb more photons, but fewer electrons reach the interface due to scattering. Thin upper metals allow most hot carriers to reach the interface, but absorb poorly, creating a considerable reverse current from the bottom electrode. Instead, SPs are highly promising as an excitation source, since they may be highly localized to the upper metal-air interface even for very thin films,1419 greatly increasing the net current. Here we calculate the possible conversion efficiency of an MIM hot electron devices based on SP excitation. The energy conversion efficiency is calculated by considering the photon absorption profile within the MIM junction, the probability that the hot carriers reach the metalinsulator interface before thermalizing, the transmission probability through the insulator, and the voltage between the emitter and collector. These processes have been well described for internal photoemission for semiconductors. SP absorption in planar MIM devices using Kretschmann coupling was calculated using a finite-difference frequency-domain (FDFD) method.20 The probability that a hot electron generated from photon absorption reaches the insulator is proportional to exp[d/(λe cos θ)] where d is the distance from the interface, θ is the direction of motion, and λe is the electron mean free path (MFP).21,22 For gold and silver, both common plasmonic materials, the MFP at 1.96 eV energy is 35 and 56 nm,22 respectively. After hot carriers reach the interface, they either tunnel through or traverse over the barrier, Φb, to be collected by the other electrode depending on their energies relative to the barrier height. Here we assume that ifR E > Φb, transmission probability P = 1; if E e Φb, P = exp(2 (k2zE)1/2dz) according to WKB tunneling model,23 where kz(E) is the electron wavenumber along z. However, in almost all cases the power generated from tunneling processes is negligible. The voltage between the emitter and collector can change the barrier profile that changes the transmission probability of hot electrons by tuning Φb. The currentvoltage characteristics of MIM collection devices can be split into different regimes depending on the barrier height and photon energy, as shown in Figure 2. Consider a goldinsulatorgold junction with a goldinsulator barrier of 0.4 eV that is illuminated with 633 nm (1.96 eV) light. After photon absorption, the hot electron energy distribution above the Fermi level is assumed to be uniform with a maximum value equal to the photon energy, implying a uniform
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density of states. The device photocurrent therefore decreases linearly with negative bias (region II), as the increased barrier height cuts-off electrons with insufficient energy. When the bias V is equal to [(Eph Φe)/e], where Eph is the photon energy and Φe is the barrier height, photocurrent decreases exponentially, indicating the onset of tunneling behavior of hot electrons (region III). When V < (Eph/e), there are no available empty states in the bottom metal to accept hot electrons from top metal. As a result, the current from the top electrode, Itop, drops to zero. At positive bias (region I), the maximum barrier height remains Φe at the emitter interface, leading to constant injection current. Positive bias reduces the effective barrier thickness for tunneling, yet this only results in a small additional current. Hot electrons in the bottom metal have similar behaviors and induce a negative photocurrent. As shown in Figure 2, the net current is measured by the difference between the forward (Itop) and backward (Ibtm) photocurrent. The power conversion efficiency can be expressed in terms of net current, photon energy, and insulator barrier height η ¼ FF
Isc Voc 1 Isc Voc ¼ 4 Pin Pin
ð1Þ
Where Isc ¼ Isctop þ Iscbtm Voc
ð2Þ
ðEph Φe Þ I btm 1 sctop ¼ e Isc
! ð3Þ
and 1/4 is the approximate filling factor (FF) for devices with identical metals, Isc and Voc are short circuit current and open circuit voltage, respectively. Pin is the input power of light. If sc Isc btm , Itop, Voc ≈ (Eph Φe)/e. High efficiency is achieved by maximizing forward current while minimizing reverse current. This could be achieved with a very thick upper electrode for example, which would absorb all incident light. However, this is balanced by a reduced number of hot carriers that reach the interface, which decreases exponentially with a characteristic distance roughly given by the mean free path. For direct illumination, this results in a poor efficiency of (∼0.12% for the device shown in Figure 2b). However, SPs excited on the metal surface can efficiently couple light into the upper metal layer (Figure 3a), providing highly anisotropic distributions of hot electrons. Figure 3b shows the photon absorption cross-section in a 40/3/10 nm Au/Al2O3/Au device as a function of position and wavelength from 400 to 2000 nm calculated in a Kretschmann coupling geometry. Most of the photon absorption occurs in the upper 10 nm of the metal film, with maximum penetration occurring around 700 nm for Au. The coupling efficiency of SPs is wavelength-dependent, decreasing in the long wavelength region as shown in Figure 3a for gold and silver devices. The coupling efficiency for gold devices becomes lower at short wavelength caused by the gold intrinsic interband transition, which leads to an efficiency peak around 700 nm. The number of electrons that reach the metalinsulator interface and are able to inject into the opposite electrode through the conduction band of the insulator is shown in Figure 3c. Despite the decrease of SP coupling efficiency at longer wavelengths, the injected electron current normalized by incident light power is relatively constant due to the longer MFP at longer wavelengths. 5427
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Figure 3. Power generation efficiency enhancement by SPs. (a) Coupling efficiency of gold and silver MIM device as a function of wavelength. (b) Position-dependent absorption in a 10 nm/3 nm/40 nm gold MIM device at different wavelengths. (c) Hot electrons that can reach the metalinsulator interface and inject into the opposite metal as a function of positions and wavelengths. The light power is 1 mW at all wavelengths and the electron barrier is 0.4 eV. (d) Map of power conversion efficiency tuned by metal thickness. (e) Photocurrent versus voltage for gold and silver devices. (f) Power conversion efficiency versus wavelength of silver (blue) and gold (red) devices under cases of direct illumination and Kretschmann excitation of SPs. (g) Enhancement of efficiency by SPs in both silver and gold devices. (h) Efficiency depends on the barrier height for lights at 500 nm (black), 900 nm (red), 2000 nm (blue), and solar irradiance AM1.5 spectrum (green), considering the thermionic reverse current.
Because SP coupling efficiency is also sensitive to metal thickness there should be an optimum for power generation efficiency, as shown in Figure 3d as a function top and bottom metal thickness. For gold, the optimal top thickness is found to be ∼40 nm with as thin a bottom electrode as possible. The optimal value of top metal slightly decreases as bottom thickness increases as SP coupling efficiency is related to the total thickness of the device. The power conversion efficiency decreases from 3.9 to 0.2% as bottom metal thickness increases from 5 to 50 nm due to the increasing reverse photocurrent at a top thickness of 40 nm. Thinner top electrodes decrease the total optical absorption, while thicker ones suffer from relaxation losses during transport to the metalinsulator interface. Photocurrent output from both gold and silver MIM devices using SP excitation is much higher than the output from devices that rely on direct illumination (Figure 3e). At a wavelength of 700 nm, short circuit current of a gold device is 4.7 106 A/mW for direct illumination, while it is 1.0 104 A/mW for SP excitation, a 21x enhancement. As shown in Figure 3f, the power conversion efficiency of silver devices can reach as high as 4.3% at 640 nm, while gold devices have a maximum efficiency of 3.5% at 780 nm. Note that these assume carriers do not reflect from the upper surface; allowing reflection roughly doubles the projected efficiency. Silver devices are almost 28 times more efficient at 400 nm than gold devices due to longer MFPs and higher coupling efficiency. While silver devices show higher efficiency at visible wavelengths, the difference becomes even larger in the infrared, with a maximum enhancement of 38.6 at 1.56 μm. The corresponding enhancements of efficiencies by SPs relative to direct illumination are presented in Figure 3g, which shows the substantial benefit of using SPs. Tuning the energy barrier, Φe, between the metal and insulator is a critical parameter for optimizing conversion. As the barrier is lowered, more hot electrons can go over across the barrier increasing the current, and the output voltage also increases (eq 3).
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Figure 4. Experimental demonstration of power generation. Schematic of hot electron transmission in MIM devices under excitation of SPs (a) and direct illumination (b). (c) Measured photocurrent as a function of voltage corresponds to conditions in (a) (black solid squares) and (b) (red solid squares). Black and red lines are theoretical fits to the data. (d) Reflectivity and photocurrent at 0.3 V as a function of light incident angles at a wavelength of 633 nm with an incident power intensity of 12.1 W/cm2, showing a clear peak of photocurrent at the incident angle of SPPs excitation.
However there is a lower limit to the barrier heights due to thermionic current from the bottom electrode. For room temperature applications, the optimal barrier height was found to be ∼0.37 eV for irradiance corresponding to the solar AM1.5 (air mass) spectrum (green curve) (Figure 3h). In this case, the maximum efficiency can reach as high as 2.7%. At the wavelengths of 500, 900, and 2000 nm, the maximum efficiencies are 1.48, 3.83, and 2.26%, respectively. Once the barrier is lower than the optimal value, the efficiency drops quickly due to thermionic reverse current. If the barrier height is greater than the optimal value, fewer hot electrons can inject over the barrier, and the efficiency decreases roughly linearly. A proof-of-concept demonstration of photon power conversion in a large-scale MIM device enhanced by SPs was conducted experimentally as shown in Figure 4. Goldaluminagold devices 1 mm 1 mm area were fabricated with top metal, insulator, and bottom metal thickness of 35, 4, and 30 nm, respectively. The bottom metal was slightly thicker than optimum, however it was chosen to ensure uniform films and low resistance over the large device area. Photocurrent under 633 nm illumination using the Kretschmann plasmon coupling configuration under applied biases from 0.7 to 0.6 V were recorded at room temperature. In this architecture light transmits through the metal layer stack and excites SPs on the top surface. Because of the localized surface plasmon, more hot electrons transmit from the top electrode to the bottom, leading to a positive photocurrent through the load (Figure 4a). On the contrary, under normal incidence illumination on the bottom metal (through the prism), more hot electrons are excited in the bottom electrode, which yields a negative photocurrent (Figure 4b). Experimentally, we observed the sign change of photocurrent (Figure 4c). As shown in Figure 4d, SPs were excited at the incident angle of 44°, corresponding to the minimum reflectivity and maximum photocurrent. The observed photocurrent increased linearly with bias from 0.5 to 0.5 V, reflecting the decreasing electron energy barrier for forward current (region II, Figure 2b). The measured values are smaller than calculated ones, which we largely attribute to the surface recombination at the interface. Compared to the theoretical values under short-circuit conditions, roughly about 0.01% 5428
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’ ASSOCIATED CONTENT
bS
Supporting Information. Theoretical model, sample preparation, and measurement methods. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author Figure 5. Efficiency varies with surface recombination velocity. Power conversion efficiency as a function of surface recombination velocity in the Au MIM devices with barrier heights of 0.4, 0.8, and 1.2 eV, respectively.
excited hot electrons at the interface can escape from the trapping at the interface in the as-fabricated devices. We observe a respectable VOC of 0.5 eV from which the barrier height is estimated to be 1.4 eV, which is in agreement with the trapreduced band gap and barrier height in amorphous thin alumina film.2426 Both the angular dependence and opposite sign photocurrent demonstrate the current is generated by surface plasmon absorption, while the electrical characteristics correspond to a hot carrier mechanism. The efficiency of these devices is currently much lower than theoretical predictions, which we attribute to surface recombination at the metalinsulator interface. Mitigating surface recombination is critical for all energy conversion devices and is often quite high at metal surfaces. The significance of surface recombination in MIM hot-carrier devices was explored by calculating the power conversion efficiency as a function of surface recombination velocity under direct AM 1.5 solar irradiance (Figure 5). The fraction of electrons emitted from the interfaces is calculated from f = Se/(Se + Sr), where Sr is the surface recombination velocity and Se is the effective velocity of hot carrier emission. For one sun concentration the hot carrier emission velocity is approximately 106 cm/s, so that as the surface recombination velocity Sr exceeds this value the overall efficiency decreases rapidly. When the surface recombination velocity reaches 108 cm/s, the average power conversion efficiency is less than 1% of the maximum efficiency, thus for efficient devices recombination at the metal interface must be reduced to the 105106 cm/s range. In this paper, easily fabricated and robust MIM junctions have been used to be a novel photon energy converter. Interestingly, even a simple planar MIM device can be used to harvest solar energy; calculations integrating over the entire solar spectrum estimate the power conversion efficiency of 2.7%. Rather than being limited by the semiconductor bandgap, the performance of the MIM energy converter depends on the interfacial barrier. Compared to rectennas that require high-frequency retification, DC photocurrent is directly generated in the MIM device by hot electrons. Practical devices would likely utilize gratings or antennas rather than the Kretschmann geometry for plasmon generation due to the prism’s size. By using more effectivecoupling methods for SPs, reducing the interface recombination, and optimizing metal thicknesses, the output photocurrent could be further improved. Moreover, if a thin layer of dielectric material with a large interfacial barrier is deposited on the top metal surface, hot electrons that would otherwise be lost can be reflected back toward the metalinsulator interface, increasing the power conversion efficiency further. In addition, our measurements indicated an alternative way to harvest both visible and infrared light without relying on multijunction semiconductor devices.
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This project was supported by U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-AC02-76SF00515. ’ REFERENCES (1) Bailey, R. L. A proposed new concept for a solar-energy converter. J. Eng. Power 1972, 94, 73. (2) Brown, W. C. The history of power transmission by radio waves. IEEE Trans. Microwave Theory Tech. 1984, MIT-32, 1230–1242. (3) Yoo, T.; Chang, K. Theoretical and experimental development of 10 and 35 GHz rectennas. IEEE Trans. Microwave Theory Tech. 1992, 40, 1259. (4) Berland, B. Photovoltaic technologies beyond the horizon: optical rectenna solar cell. ITN Energy Systems, Inc. Final Report; 2003, NREL/SR520-33263, http://www.nrel.gov/docs/fy03osti/33263.pdf. (5) Krishnan, S.; Rosa, H. La; Stefanakos, E.; Bhansali, S.; Buckle, K. Design and development of batch fabricatable metal-insulator-metal diode and microstrip slot antenna as rectenna elements. Sens. Actuators, A 2008, 142, 40–47. (6) Esfandiari, P; et al. Tunable antenna-coupled metal-oxide-metal uncooled IR detector. Proc. SPIE Int. Soc. Opt. Eng. 2005, 5783, 470–482. (7) Dagenais, M.; Choi, K.; Yesilkoy, F.; Chryssis, A. N.; Peckerar, M. C. Solar spectrum rectification using nano-antennas and tunneling diodes. Proc. SPIE Int. Soc. Opt. Eng. 2010, 7605, 76050E-1–12. (8) Shalaev, V. M.; Douketis, C.; Stuckless, J. T.; Moskovits, M.Lightinduced kinetic effects in solides. Phys. Rev. B 1996, 53, 11388–11402. (9) Kovacs, D. A; et al. Photo and particle induced transport of excited carriers in thin film tunnel junctions. Phys. Rev. B 2007, 76, 235408. (10) Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J. Photodetection with active optical antennas. Science 2011, 332, 702–704. (11) Gundlach, K. H.; Kadlec, J. Interfacial barrier height measurement from voltage dependence of the photocurrent. J. Appl. Phys. 1975, 46, 5286–5287. (12) Powell, R. J. Interfacial barrier energy determination from voltage dependence of photoinjected currents. J. Appl. Phys. 1970, 41, 2424–2432. (13) Afanasev, V. V.; et al. Band alignments in metal-oxide-silicon structures with atomic-layer deposited Al2O3 and ZrO2. J. Appl. Phys. 2002, 91, 3079–3084. (14) Raether, H. Surface plasmons on smooth and rough surfaces and on gratings. Springer Tracts Mod. Phys. 1988, 111, 1–133. (15) Stegeman, G. I.; Wallis, R. F.; Maradudin, A. A. Excitation of surface polaritons by end-fire coupling. Opt. Lett. 1983, 8, 386–388. (16) Chen, J.; et al. Surface plasmons modes of finite, planar, metal-insulator-metal plasmonic waveguides. Opt. Express 2008, 16, 14902– 14909. (17) Welford, K. Surface plasmon-polaritions and their uses. Opt. Quantum Electron. 1991, 23, 1–27. (18) Sambles, J. R.; Bradbery, G. W.; Yang, F. Optical excitation of surface plasmons: an introduction. Contemp. Phys. 1991, 32, 173–183. (19) Kovacs, G. C.; Scott, G. D. Optical excitation of surface plasma waves in layered media. Phys. Rev. B 1977, 16, 1297–1311. (20) Veronis, G.; Fan, S. Overview of simulation techniques for plasmonic devices. Surface Plasmon Nanophotonics; Brongersma, M. L., Kik, P. G., Eds.; Springer: New York, 2007; Vol. 131, pp 169182. 5429
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