Upconverting Electrodes for Improved Solar Energy Conversion

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1 Upconverting Electrodes for Improved Solar Energy Conversion Annual Report, April 22, 2012 Investigators Jennifer Dionne, Assistant Professor Department of Materials Science and Engineering Stanford University Alberto Salleo, Assistant Professor Department of Materials Science and Engineering Stanford University Graduate Student Researchers Diane Wu Department of Chemistry, Stanford University Abstract Upconversion, the combination of multiple low-energy photons to produce a higher energy photon, can significantly decrease transmission losses in photovoltaics. For example, upconverting materials have been shown to theoretically double the efficiency of a single junction solar cell with band gap of 1.7 ev. However, a number of experimental challenges exist to realize such improvement. Wide-spread utilization of solar upconverters requires an improved quantum efficiency and facile device integration into photovoltaic cells. In this proposal, we are developing an upconverting electrode that addresses both challenges. The electrode consists of colloidally-synthesized Ag nanowires decorated with upconverter-doped dielectric nanoparticles, which can be co-deposited over large areas by spray-coating. The upconverter is electrically isolated from the cell, allowing for independent tuning of cell electrical and optical properties. The metallic nanowires provide direct electrical contact to the cell, enabling efficient carrier extraction. By manipulating the geometry-dependent longitudinal and transverse plasmon resonances in the silver nanowires, we can enhance both the absorption and emission of nearby upconvertors and therefore increase upconversion efficiency. Using this strategy, we have been able to enhance the upconversion luminescence of NaYF 4 :Yb 3+,Er 3+ nanoparticles by a factor of 4 with the addition of Ag nanowires in a scalable, spraycoated film. Further, the electrical conductivity is competitive with stateof-the-art Indium Tin Oxide. Our preliminary results are promising to realize a viable upconverting electrode that significantly enhances the cell efficiency of any singlejunction solar cell.

2 Introduction The sun provides over 100 peta-watts of power to the Earth, an amount exceeding the world's energy needs by nearly five orders of magnitude. Unfortunately, current photovoltaic technologies can harvest only a small fraction of this energy, since they are generally unable to utilize photons with energies below the cell bandgap. For example, an ideal single junction solar cell with a bandgap of 1.7 ev wastes approximately 49% of the sun's power because it is not absorbed. Moreover, light with energy just at or above the bandgap is often transmitted as well, due to low absorption efficiencies at these energies. Addressing photovoltaic transmission losses in a scalable, cost-effective manner poses a considerable challenge to high-efficiency, low-cost solar energy conversion. In this proposal, we are developing an efficient upconverting composite for photovoltaic applications, using state-of-the-art synthetic, experimental, and computational techniques. Figure 1 illustrates a schematic of our proposed upconverting solar cell design. An upconverting electrode is placed behind the active semiconducting region of a solar cell to collect transmitted photons. The upconverter transforms the energies of these transmitted photons to energies that can be absorbed by the solar cell. As seen in figure 1b, calculations of cell efficiencies incorporating an upconverting electrode are very promising: the peak efficiency of an ideal single junction cell with an ideal upconverter increases from 30% to 44.4% - an increase in single-junction solar cell performance of nearly 70%. Moreover, efficiency enhancements are seen for all cell bandgaps, even if the upconverter is not ideal (i.e., poor absorption or recombination efficiencies.) Our proposed upconverting electrode consists of colloidally synthesized nanostructures, including upconverter-doped dielectric nanoparticles and Ag nanowires, which can be deposited over large areas by spray-coating. While the upconverter-doped nanoparticles improve absorption of below-bandgap sunlight, the Ag nanowires provide direct electrical contact to the cell, enabling carrier extraction. This upconverting cell design is characterized by three innovative parameters: 1) Decoupled optical and electrical properties. The upconverter material is encapsulated in a host nanoparticle that decorates metallic nanowires. By doping a dielectric nanoparticle with upconverting molecules or ions, the upconverter can be electrically isolated from the solar cell. Accordingly, upconverting solar cells will not be limited by the current-matching constraints seen in multijunction cells. In other words, the electrical and optical properties of the cell can be independently tuned. Our initial calculations indicate cell efficiency enhancements well over 100% for realistic upconverting materials. 2) Enhanced photon absorption above and below the bandgap. The upconverter enables photon absorption of light with energy below the solar cell bandgap, but the proximity of the nanoparticles and active cell region to metallic nanostructures enables substantial plasmonic enhancements. Finite element simulations indicate large-area and remarkably broadband absorption enhancements exceeding 100- fold with tuned nanoplasmonic geometries. These enhancements are equivalent to adding a 100x concentration system to the cell, with the advantage of low-cost, solution-processable synthesis.

3 a b Figure 1: Schematic and calculated performance of an upconverting electrode. a) An upconverting electrode can be placed at the back of a solar cell to absorb sub-band gap light (red arrows) that would otherwise be transmitted in a conventional cell. The electrode then re-emits sub-band gap as visible, above-band gap light, increasing the light available to the solar cell. b) An upconverter (UC) can significantly increase the efficiency of an ideal single-junction solar cell. This relative increase is greatest when the upconverter absorption efficiency and cell bandgap are high. The inset shows the absolute efficiency for an ideal solar cell both with and without an ideal upconverter. 3) Excellent electrical conductivity. The metallic nanowires in the upconverting electrode are used to make direct electrical contact to the cell. Our results of oxide-decorated Ag nanowire contacts have indicated sheet resistances less than 10Ω/sq, competitive with standard cell electrodes. Background Several loss pathways limit the efficiency of photovoltaics, including reflective losses, parasitic resistances, and carrier recombination. One loss mechanism inherent to all single junction photovoltaic devices is the transmission of sub- and near-band gap light. Most techniques that aim to reduce transmission losses in single junction solar cells rely on increasing the optical path length in the system, for example by scattering light from metal nanoparticles or coupling incident solar photons to propagating surface plasmons.[1-4] Alternatively, defect levels within the bandgap of the semiconductor can be introduced through the addition of impurities to allow for absorption of photons across multiple energy thresholds.[5,6] However, these techniques introduce additional pathways for recombination in the cell, and have not yet produced cell efficiencies exceeding the Shockley-Queisser limit. Rather than adapting the active cell material to better utilize sub-bandgap light, a separate upconverter can be used to reduce solar transmission losses.[7] The upconverter converts low energy photons to higher energy above-bandgap photons that can then be absorbed by the solar cell and contribute to photocurrent. Because the upconverter can be electrically isolated from the active cell, it need not be current-matched to the cell, nor will it add mid-gap recombination pathways.

4 Many materials upconvert, including lanthanide and transition metal ions,[8] quantum dots,[9] and metal-ligand complexes.[10] Of these upconvertors, the most relevant to solar are those that can upconvert infrared light to the visible (the bandgap for most photovoltaics is ev or 1100 nm 730 nm). Currently, we are focusing on upconverters composed of Yb 3+ and Er 3+ dopants in a NaYF 4 matrix. This material is one of the most efficient near infrared to visible upconvertors to date,[11] and several recent advances have been made in the phase, size and shape controlled synthesis of such NaYF 4 :Yb 3+, Er 3+ upconverting nanoparticles.[12] Incorporation of upconverting materials into solar cells has been shown theoretically to yield significant increases in power conversion efficiency.[13,14] Experimentally, however, the addition of lanthanide upconvertors has yet to show any notable improvement in cell performance, primarily due to the low quantum efficiency of the current best upconvertors [15] and challenges with cell-upconverter compatibility. There are many approaches to internally increasing the upconverter efficiency, including modifying the host lattice and passivating the surface of the upconverter. In addition to exploring these approaches, we aim to enhance upconverter luminescence externally through the addition of metallic nanostructures. These nanostructures tune the absorption, emission, and local density of optical states of the upconverter. Furthermore, they provide a pathway for efficient electrical carrier extraction from the cell in a controlled device architecture. Our upconverting electrode can be generalized to many upconverting materials and photovoltaic designs. Results Our first year of funding has enabled us to develop the first reported upconverting electrode, using a scalable, cost-effective spray-coating method. This electrode is characterized by upconversion optical efficiencies that are four times higher than standalone upconverting materials. Furthermore, the electrical properties of the electrode are competitive with ITO. Successful fabrication of our spray-coated electrodes is illustrated in Figure 2. Using colloidal techniques, hexagonal-phase NaYF 4 :Yb 18%, Er 2% upconverting nanoparticles were synthesized.[16] These particles were then dispersed in cyclohexane at a concentration of approximately 5 mg/ml. We used a low-cost, scalable spray deposition to create films of these nanoparticles on 0.15 mm thick glass substrates (25 mm square, Warner Instrument Corp). For hybrid films, we spray deposited Ag nanowires (~10 m length, ~ 50 nm diameter) onto glass substrates, laser annealed to increase conductivity, and then sprayed a layer of upconverting nanoparticles on top. Films were characterized electrically with a four-point probe. Films were characterized optically using a 980 nm laser diode for excitation followed by collection of UC emission spectra using a spectrometer and CCD. Combining upconverting nanoparticles with Ag nanowires increases the overall upconverted emission by x, as shown in Figure 3. This increase in emission was observed for a film that has yet to be fully optimized for optical properties; with better understanding of the enhancement process, we anticipate higher enhancements. Towards that end, we have studied the power dependence of upconversion emission in these films,

a b c Figure 2: Scanning electron micrographs of films of upconverting nanoparticles (a), conducting Ag nanowires (b), and the composite upconverting electrode(c), all manufactured using low-cost,

5 a b c Figure 2: Scanning electron micrographs of films of upconverting nanoparticles (a), conducting Ag nanowires (b), and the composite upconverting electrode(c), all manufactured using low-cost, scalable techniques. from 1W/cm2-100W/cm2. It is well known that upconversion in Er-doped systems is a two-photon process and consequently has a quadratic power dependence.[17] At high excitation intensities, however, saturation is observed and the power dependence becomes linear. We observe this decrease in slope at lower powers for upconverting nanoparticle films with Ag nanowires than in the control upconverting nanoparticle film (20 vs. 90 W/cm2). This result can be interpreted as higher effective excitation intensity in the Ag nanowire upconverting films. The sheet resistance of the Ag nanowire film before the addition of UCNPs was 6.0±1.7 /sq. Spray deposition of upconverting nanoparticles on top of this film resulted in a small increase in sheet resistance to an average value of 6.5±1.1 /sq, comparable to standard transparent conducting oxides. While it remains higher than metal films commonly used in a back electrode, conductivity could be readily improved by increasing the density of silver nanowires. Studies to lower this sheet resistance are ongoing. Progress Our upconverting electrodes will address photovoltaic transmission losses in a scalable, cost-effective manner. Our upconverting composite is applicable to any photovoltaic technology, ranging from state-of-the-art high-efficiency Si cells to thirdgeneration technologies that have not yet met cost-efficiency milestones. Our experimental efforts are focusing on the use of solution-dispersed materials that are compatible with low-cost and highly-scalable manufacturing technologies. We estimate the cost of raw materials for our upconverting electrodes with 50 nm thick Ag nanowires to be on the order of 10 cents per square meter. Our 4x enhancement in upconversion efficiency is an important first-step towards incorporation of such materials into solar cells. As described below, our upcoming work will address further improvements in upconversion efficiency, enabling photovoltaic device integration.

a b Figure 3: Optical properties of the upconverting electrode. a) Photograph of an upconverting electrode under 980 nm diode laser illumination.

6 a b Figure 3: Optical properties of the upconverting electrode. a) Photograph of an upconverting electrode under 980 nm diode laser illumination. The infrared light is upconverted to green light at the surface of the electrode. b) Emission spectra of upconverting (UC) films with (red) and without (blue) Ag nanowires. In this electrode, the emission intensity is enhanced up to 4.7x upon the addition of Ag. Future Plans The results from the spray-coated films are very promising, but orders of magnitude upconversion enhancement are expected for optimized nanowire geometries. In particular, nanowires of appropriately tuned lengths and diameters can enhance both the absorption and emission of photons in our upconverting films, based on plasmon resonances. We are currently investigating the ideal nanowire geometry for enhanced upconversion using full-field simulations. Further, we are working on synthesis of various length and aspect ratio Ag nanowires. These tuned nanowires will be co-sprayed with upconverting nanoparticles, and we will characterize their upconversion efficiency and conductivity under various illumination conditions. Additionally, we are exploring single nanowire and single upconverting nanoparticle experiments, to isolate the plasmonic enhancement from other effects, such as increased scattering of incident light. Studying the interactions between a single upconverting nanoparticle and a single Ag nanowire both experimentally and theoretically will provide significant insight to the nature and magnitude of the plasmonic contribution to upconversion enhancement. We are also interested in exploring the effects of nanocrystal phase control of upconversion. The crystallographic phase of the nanocrystal is expected to impact the upconversion efficiency, based on coupling to phonon modes. We are conducting upconverting luminescence measurements of our upconverting particles in a diamond anvil cell, where pressures can range up to tens of GPa, inducing a crystallographic phase change of the upconverting nanoparticles. Combined, our understanding of the factors influencing upconversion (i.e., absorption efficiency, emission efficiency, density of optical states, crystallographic phase, and surface functionalization) will enable development of a highly efficient upconverting electrode for incorporation in photovoltaic cells.

7 Publications, Patents, and Presentations 1. Diane Wu, Jennifer Dionne, Alberto Salleo, Efficient upconversion and electrical carrier extraction in Ag nanowire, NaYF4:Yb3+/Er3+ nanocrystal composites, manuscript in preparation 2. Diane Wu, Jennifer Dionne, Alberto Salleo, Composition for upconversion of light and devices incorporating same, provisional patent S12-012, filed April Diane Wu, Jennifer Dionne, Alberto Salleo, Solution-processed Upconverting Electrodes for Enhanced Light Absorption in Photovoltaics, Materials Research Society Spring 2012 meeting, Symposium BB References 1. H. A. Atwater and A. Polman, Plasmonics for improved photovoltaic devices, Nat. Mater. 9, 205 (2010). 2. S. Pillai, K. Catchpole, T. Trupke, and M. Green, Surface plasmon enhanced silicon solar cells, J. Appl. Phys., 101, (2007). 3. V. Ferry, L. Sweatlock, D. Pacifici, and H. Atwater, Plasmonic nanostructure design for efficient light coupling into solar cells, Nano Lett, 8, 4391 (2008). 4. R. Pala, J. White, E. Barnard, J. Liu, and M. Brongersma, Design of plasmonic thin-film solar cells with broadband absorption enhancements, Advanced Materials, 21, 3504 (2009) 5. M. Keevers and M. Green, Efficiency improvements of silicon solar cells by the impurity photovoltaic effect, J. Appl. Phys., 75, 4022 (1994). 6. A. Luque and A. Marti, Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels, Phys. Rev. Lett., 78, 5014 (1997). 7. T. Trupke, M. Green, and P. Wurfel, Improving solar cell efficiencies by up-conversion of subband-gap light, J. Appl. Phys., 92, 4117 (2002). 8. F. Auzel, Upconversion and anti-stokes processes with f and d ions in solids, Chem. Rev. 104, 139 (2004) 9. E. Poles, D. C. Selmarten, O. I. Mii, A. Nozik, Anti-stokes photoluminescence in colloidal semiconductor quantum dots, Appl. Phys. Lett. 75, 971 (1999). 10. T. N. Singh-Rachford, F. N. Castellano, Photon upconversion based on sensitized triplet-triplet annihilation, Coord. Chem. Rev. 254, 2560 (2010) 11. J. F. Suyver, J. Grimm, M. K. van Veen, D. Biner, K. W. Kramer, H. U. Gudel, Upconversion spectroscopy and properties of NaYF 4 doped with Er 3+, Tm 3+, and/or Yb 3+, J. Lumin. 114, 53 (2005). 12. F. Wang, X. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38, 976 (2009) 13. T. Trupke, M.A. Green, P. Wurfel, Improving solar cell efficiencies by upconversion of subbandgap light, J. Appl. Phys. 92, 4117 (2002) 14. A. C. Atre, J. A. Dionne, Realistic upconverter-enhanced solar cells with non-ideal absorption and recombination efficiencies, J. Appl. Phys. 110, (2011). 15. H.-Q. Wang, M. Batentschuk, A. Osvet, L. Pinna, C. Brabec, Rare-earth ion doped upconversion materials for photovoltaic applications, Adv. Mater. 23, 2675 (2011). 16. Z. Li, Y. Zhang, S. Jiang, Multicolor core/shell structures upconversion fluorescent nanoparticles, Adv. Mater. 20, 4765 (2008) 17. M. Pollnau, D. Gamelin, S. Luthi, H. U. Gudel, M. P. Hehlen, Power dependence of upconversion luminescence in lanthanide and transition metal ion systems, Phys. Rev. B. 61, 3337 (2000). Contacts Jennifer Dionne: jdionne@stanford.edu Alberto Salleo: asalleo@stanford.edu Diane Wu: dmw53@stanford.edu

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