Plasmonic Photovoltaics

Transcription

1 Plasmonic Photovoltaics Investigators Harry A. Atwater, Howard Hughes Professor and Professor of Applied Physics and Materials Science, California Institute of Technology Krista Langeland, Ph.D. student, Materials Science, California Institute of Technology Imogen Pryce, Ph.D. student, Chemical Engineering, California Institute of Technology Vivian Ferry, Ph.D student, Chemistry, California Institute of Technology Deirdre O Carroll, postdoc, Applied Physics, California Institute of Technology Abstract We summarize results for the period May 2008-April 2009 for the plasmonic photovoltaics three year GCEP project. We have made progress in two key areas: 1) we have analyzed enhanced optical absorption in plasmonic solar cells theoretically and via full field electromagnetic simulation and coupled full field electromagnetic simulations to semiconductor device physics simulations and design tools 2) we have designed and fabricated a prototype single quantum well plasmonic solar cell with InGaN/GaN heterostructures. These prototype InGaN cells exhibit enhanced absorption and enhanced photocurrent due to plasmonic nanoparticle scattering, in a 2.5 nm thick quantum well. Introduction The plasmonic photovoltaics project is a joint Caltech-Stanford-Utrecht/FOM research program which exploits recent advances in plasmonics to realize high efficiency solar cells based on enhanced absorption and carrier collection in ultrathin film, quantum wire and quantum dot solar cells with multispectral absorber layers. The program has three key focal points: 1) design and realization of plasmonic structures to enhance solar light absorption in ultrathin film and low-dimensional inorganic semiconductor absorber layers; this is the largest area of effort for the proposed effort; 2) synthesis of earth-abundant semiconductors in ultrathin films and lowdimensional (quantum wire and quantum dot) multijunction and multispectral absorber layers; 3) investigation of carrier transport and collection in ultrathin plasmonic solar cells. Background Plasmonic structures can offer the possibility of reducing the physical thickness of the photovoltaic absorber layers while remaining optically thick, in at least two ways i) metallic nanoparticles can be used as subwavelength scattering elements to couple freely propagating plane waves in sunlight into guided modes of an absorbing semiconductor thin film, as depicted in Fig. 1 (top) and ii) a corrugated metallic film on the back surface of a thin photovoltaic absorber film can couple sunlight into surface plasmon polariton modes supported at the metal-absorber interface, shown at the bottom of Fig. 1. If we compare these mechanisms for absorption with conventional light absorption in bulk semiconductor materials, several interesting points emerge. First, the light absorption profile in bulk semiconductor layers is exponential, and the characteristic exponential decay length for light at frequency w is 1/α = c/2ωκ, the optical absorption length at that 1

2 frequency. Thus for a bulk semiconductor layer to be rendered optically thick, its thickness must be L 1/α. By contrast, a semiconductor thin film waveguide need only have a thickness such that it is capable of supporting guided wave modes, and such that the optical mode overlap with the semiconductor absorber layer be high enough to attenuate the incident radiation incoupled to the guided mode over some characteristic lateral distance in the thin film. Enhanced absorption confers several benefits to a solar cell. The reduced thickness requirement implies that fewer Figure 1. At top, nanoparticle array scatters sunlight into guided modes in thin film absorbers; at bottom, corrugated metallic back contact couples sunlight into surface plasmon polariton and photonic modes at metalabsorber interface. grams of absorber material are used in making a cell per generated Watt of electricity. This has very important technical and strategic consequences as photovoltaics scales up in manufacturing capacity from its current level of ~ 8GW in 2008 to > 50 GW by 2020, and eventually to the TW scale. Extension of photovoltaics technology to the TW scale demands that the materials utilized in solar cells be abundant in the earth s crust and amenable to formation of efficient photovoltaic devices. For single junction devices, silicon has proven to be a nearly ideal photovoltaic material. It is abundant, with a nearly optimal bandgap, excellent junction formation characteristics, high minority carrier diffusion length and effective methods for surface passivation to reduce unwanted carrier recombination. Its only shortcoming, low spectral absorption of the AM1.5 terrestrial solar spectrum, gives rise to a relatively high material cost/watt. For silicon, enhanced absorption and light trapping makes it possible to design crystalline silicon thin film cells with acceptably good spectral quantum efficiency, even for absorber layers a few microns in thickness. The lower processing costs for thin film photovoltaics currently result in lower cost/watt than for wafer-based crystalline silicon photovoltaics. While silicon is an abundant material, and the total availability of material does not pose a limitation to expansion of photovoltaics production, material resources are a significant issue for large-scale production of other photovoltaic materials. For example, for copper indium diselenide thin film solar cells, scarcity of indium threatens to limit photovoltaic cumulative production to < 50 GW, and for CdTe thin films, Cd is also a scarce element whose abundance could limit production. If it were possible to reduce the cell thickness for such compound semiconductor cell by x as a result of plasmon-enhanced light absorption, this could significant extend the reach of compound semiconductor thin film photovoltaics towards the TW scale. Earth abundance considerations will also influence plasmonic cell designs at large scale production: although Ag and Au have been the metals of choice in most plasmonic designs and experiments, they are relatively scarce 2

3 materials, so scaleable designs will need to focus on earth-abundant plasmonic metals such as Al and Cu. Results Electromagnetic Theory and Simulation Coupled to Semiconductor Device Physics Design Using our understanding of coupling from single grooves and ridges, we extended our model to one dimensional infinite arrays of objects, and examined the effect of incident polarization on the absorption enhancements in our structure. Although a real device could incorporate patterning that is polarization insensitive, the modes launched from the scattering object with the opposite polarization can be effective for increasing optical absorption as well. Figure 2 shows the optical absorption enhancement for an array of 100 nm wide by 50 nm tall Ag ridges, spaced by a center-to-center distance of either 6 μm or 300 nm under both polarizations; only the TM case excites SPP modes. With closer pitch Figure 2. Optical absorption enhancement relative to uncorrugated back reflector for TM polarization and a) 6 micron period and b) 300 nm period corrugation, and for TE polarization and c) 6 micron period and d) 300 nm period corrugation. the absorption enhancements acquire stronger wavelength dependence, with sharper enhancement peaks, but the baseline enhancement is still larger than in the sparse case. Under TE polarization these enhancements can be as large as 20x at 900 nm. Tuning of the pitch and aspect ratio of the scattering objects can shift the position of that largest peak 1. The quantity of greatest interest for determining solar cell photocurrent is the AM1.5G integrated photocarrier generation rate. Figure 3 shows calculated electron generation rate maps, weighted by the solar spectrum. The generation rates assume IQE = 1, i.e, that every photon absorbed produces an electron-hole pair. Figure 3(a) shows a control sample where the back surface is smooth, (b) where the back is air (and so no SPP modes are excited), (c) with an Ag back contact under TM polarization, and (d) with an Ag back contact under TE polarization. The scale is logarithmic in electrons/cm 3. In panel (a) without surface structuring there is only standard thin film reflection and absorption, which produces bands of higher and lower generation rates. The large wavelength range explored washes out many of those features from individual wavelengths, producing a more uniform absorption profile. In (b), (c), and (d) the light scattered by the surface texturing leads to periodic profiles of interference. Panel (c) 3

4 shows much more generation near the metal-si interface due to the SPP modes, and (d) shows higher generation rates overall with more of the generation taking place higher in the semiconductor layer. Figure 3. In (a) photocarrier generation rate for Si film on uncorrugated Ag back reflector; in (b) corrugated structure with dielectric(air) back reflector; in (c) corrugated Ag back reflector under TM polarization and (d) corrugated Ag back reflector under TE polarization. Ultrathin InGaN Quantum Well Plasmonic Solar Cells In this work, we show the effect of nanoparticle scatterers on the photocurrent of an ultrathin photovoltaic cell. We obtained InGaN quantum (QW) based light emitting diodes (LEDs) from Sandia National Laboratories and performed a metallization contact modification after device processing so as render these structures suitable as ultrathin solar cells. The plasmonic InGaN quantum well cells have a p-i-n structure consisting of a 200 nm p-gan window layer, a 2.5 nm InGaN single QW, and a thick n-gan layer (Figure 4). A window is etched into the front metal contact of these devices enabling photogeneration of current. Silver nanoparticles are deposited via thermal evaporation through an anodic aluminum oxide (AAO) template used as a mask for large area nanoparticle array formation. The benefit of using AAO templates is that the particle size, pitch, and height can be controllably varied. In addition, depositing nanoparticles in this way does not affect other parts of 4 Figure 4. InGaN quantum well light emitting diode adapted as quantum well solar cell. the processing of the device and nanoparticles can be deposited over relatively large areas. Initial photocurrent measurements are taken post-etching and prior to nanoparticle deposition. Final photocurrent measurements are taken after the samples have been

5 processed (Figure 5). The sample coated with nanoparticles has a total photocurrent enhancement of 13% 2. The control sample, which is masked during the evaporation step, shows no appreciable change in photocurrent. The third sample, coated with 100 nm of Ag, shows a decrease in photocurrent of 15%. This can be attributed to shadowing of the active region. The photocurrent gains seen at left in Fig. 5 are modest, primarily because the GaN buffer layer beneath the quantum well is quite thick, so that light scattered by the Ag nanoparticles is not tightly confined to the InGaN quantum well region. If the buffer layer thickness can be substantially reduced, truly thin waveguide-based can be realized. Future work will focus on using much thinner AlN buffer layers, and utilizing the nanoparticle arrays to not only scatter light into the absorber but also to incouple light into waveguide modes. This will also require thinning both the n and the p-type cladding layers. Device Current Density (ma/cm 2 ) 6.0x x x x x x10 6 w/ Nanoparticles w/o Nanoparticles w/ Solid Metal Film GaN E g InGaN E g GaN E g InGaN E g GaN E g InGaN E g Wavelength (nm) Wavelength (nm) Wavelength (nm) Figure 5. Photocurrent density for plasmonic InGaN quantum well solar cell (left panel) where red curve is device after Ag nanoparticle array deposition process compared with control cell following processing (red curve in middle panel) and cell coated with continuous thin Ag layer (red curve in right panel) Scientific Meetings: The research partners in the plasmonic photovoltaics project from Caltech, the FOM Institute and Stanford met in August 2008 at the Gordon Conference on Plasmonics (chaired by Harry Atwater). This scientific meeting allowed the PIs, students and postdocs to coordinate and plan technical activities. This year, we have had several graduate student exchanges. Graduate student Rene de Waele of the FOM Institute visited Caltech from January to August 2008, and in May 2009, Caltech graduate student Vivian Ferry will visit the FOM Institute and University of Utrecht to carry out joint research on plasmonic thin film Si solar cell design and fabrication. The PIs have met by videoconference approximately every 3 weeks and met together at the Spring 2009 Materials Research Society meeting in San Francisco. Progress Overall, we have made substantial progress in modeling and simulation of enhanced light absorption in plasmonic thin film solar cells. Of particular note is the coupling of results of two-dimensional and three-dimensional full field electromagnetic simulation to semiconductor device physics models, which will be a powerful tool for quantitative analysis of photocurrents in thin film solar cells. Also, we demonstrated the first single 5

6 quantum well solar cell and realized enhanced absorption in a plasmonic InGaN cell relative to control cells. Contacts Harry A. Atwater: haa@caltech.edu Krista Langeland: langelan@caltech.edu Imogen Pryce: imogen@caltech.edu Vivian Ferry: vivianf@caltech.edu Deirdre O Carroll: doc@caltech.edu References 1. Plasmonic Nanostructure Design for Efficient Light Coupling into Solar Cells, V.E. Ferry, L.A. Sweatlock, D. Pacifici, and H.A. Atwater Nano Letters, 8, (2008) 2. Plasmonic Nanoparticle Enhanced Light Absorption in InGaN Quantum Well Solar Cell, I. Pryce, A.J. Fischer, D.D. Koleske and H.A. Atwater, paper O1.7, Spring 2009 Materials Research Society Meeting, April 12-17,