Progress Report on the GCEP Project “Ultra High Efficiency Thermo-Photovoltaic Solar Cells Using Metallic

Photonic Crystals As Intermediate Absorber and Emitter”, Year 1 Investigators Shanhui Fan (Associate Professor, Electrical Engineering, Stanford) Peter Peumans (Assistant Professor, Electrical Engineering, Stanford) Paul Braun (Associate Professor, Material Science, UIUC) Eden Rephaeli (Graduate Student, Applied Physics, Stanford) Nicholas Sergeant (Graduate Student, Electrical Engineering, Stanford) Huigang Zhang (Postdoctoral Scientist, Materials Science, UIUC) Felix Hotama (Undergraduate Student, Materials Science, UIUC) Abstract

Our team aims to combine large-scale numerical simulations, with nanofabrication and characterization to develop the efficient intermediate absorbers and emitters that enables high efficiency solar thermo-photovolatic systems. In this first year, substantial progress has been made in developing designs that enable high efficiency TPV cells, as well as in experimental setup for fabricating and measuring of such devices.

Introduction Thermal Photovoltaic (TPV) solar cells, where solar radiation is absorbed by an

intermediate, which then emits thermal radiation towards a solar cell, is capable of achieving an extremely high efficiency using single-junction solar cells. The theoretical efficiency of 85% far exceeds the Shockley-Queisser limit. In order to approach such efficiency, however, there are very important constraints on the properties of the intermediate absorber and emitter. Until now, there is no known way to meet the requirements on the intermediate that are needed in order to reach efficiency beyond 30%.

The aim of our project is to exploit emerging opportunities in the area of nanophotonic structure such as photonic crystals for TPV applications. It has been recently shown that both the absorption and thermal emission properties of nanophotonic structure can be tailored with appropriate design. Here, we aim to show that such structures can indeed be designed to enable ultra-high efficiency TPV solar cells. Specifically, we will demonstrate that crystal structures can be produced with low-cost self-assembly fabrication techniques using abundant materials such as Tungsten, that the crystals can provide broad-band absorption over the entire solar spectrum, and that the intermediate can be designed to enhance overall cell efficiency.

Background Over the past few years, significant progress has been made on the fabrication of

metallic photonic crystals with sub-micrometer dimensions. The most recent developments were just reported in Symposium J of the April 2009 MRS meeting (www.mrs.org). Braun and co-workers demonstrated that 3D self-assembled and holographically fabricated photonic crystal templates could be converted to refractory

metals such as tungsten. The Norris group at the University of Minnesota presented their results on the 3D direct writing of silica photonic crystal woodpile structures followed by conversion of these structures to tungsten via chemical vapor deposition. Shawn-Yu Lin and his colleagues at RPI described the fabrication of iridium based photonic crystal thermal emitters. Iridium is interesting because of its theoretical potential to operate at shorter wavelengths than tungsten (below 1.8 micrometers) due to its fundamental optical properties. These and other rapid advances in the fabrication of metallic and metalodielectric photonic crystals should pay dividends for thermophotovoltaic energy conversion in the near future.

Results Since the start of the project, we are making significant progresses regarding the

design, measurement and fabrication of devices. Below we discuss in details progress along each directions.

Design

Figure 1: (a) Design of Tungsten absorber. The structure consists of a periodic array of Tungsten Pyramid, with the periodicity of 200nm. (b) Design of a Tungsten emitter. The orange region is a flat Tungsten slab, placed in front of a dielectric multilayer film. (c) The red curve is the emissivity spectrum for the emitter designed in b. The blue curve is the blackbody spectrum at 2000K. The gray curve is the emissivity spectrum for a standard Tungsten emitter design consisting of a cross-grating. Notice that our designed structure features a much stronger suppression of sub-bandgap radiation compared with conventional design. (d) Analysis of system efficiency. The red curve shows the system efficiency of a solar TPV system with the absorber and emitter as shown in (a) and (b), and with a 0.7eV solar cell. The solid and dashed curve shows the efficiency of ideal solar cell with 1.1eV and 0.7eV band gap, respectively, when directly exposed to sunlight.

In our previous work, we have shown that a Tungsten absorber (Fig. 1a) can be designed to provide near unity absorptivity over the entire solar spectrum, and over a wide range of incident angles [1]. Thus such an absorber can satisfy both the spectral and

angular requirement for a black absorber in high-concentration system. Since the start of the GCEP project, we have focused on emitter design for solar TPV system. One design of the emitter is shown in Fig. 1b. Its emissivity spectrum, as shown in Fig. 1c, is optimized for 0.7eV solar cell, and features strong suppression of sub band gap radiation, as well as an emissivity peak at 0.7eV. We have carried out a system analysis to show that such intermediate, when used in conjunction of a 0.7eV cell, can in fact have theoretical efficiency substantially higher than the Shockley-Queisser limit of an optimized 1.1eV cell (Fig. 1d).

The design that we pursue here also has substantial implications for solar thermal systems in general. We have previously shown that metal-dielectric stacks can be used as selective coatings for use in concentrated solar power systems including solar trough and TPV systems. Despite their simplicity, these stacks can be tuned to selectively emit only a certain spectral range. These stacks have the additional advantage in that they can borrow developments in barrier coatings developed in the microelectronics industry to improve their thermal stability.

We have further optimized these metal-dielectric stacks and studied their behavior when deposited on structure substrates such as a V-grooved substrate (Fig. 2). The V-grooves have periods of the order of 300-700nm and can potentially be fabricated by direct etching of a metal surface, or by templated deposition of metal. The metal-dielectric coating can then be directly deposited onto these substrates.

Figure 2: Metal-dielectric stack deposited onto a V-grooved metal substrate.

We have developed simulation software that evaluates the absorption (i.e. the

emissivity) of coatings in V-grooved structures as a function of wavelength and geometry, as shown in Fig. 3. The stack on a flat substrate yields the lowest emissivity with a sharp threshold between low emissivity at long wavelengths and high emissivity in the visible part of the spectrum. As the angle of the V-groove is increased (increasingly steep V-grooves), the emissivity increases across the spectral range.

Figure 3: Emissivity of a stack (optimized for maximum performance in a solar collector

at 2000K) as s function of the angle of the V-grooved substrate.

By coating the metal-dielectric stacks onto V-grooved substrates, their performance can be increased, as shown in Fig. 4, where the merit of a stack is shown as a function of the angle of the V-grooves (measured in radians). The merit measures the stacks’ performance in a concentrated solar power system and is an indication of the fraction of optical power that can be used to heat the working fluid. The merit of these stacks is compared to that of a V-grooved Mo substrate (black line), clearly showing the performance benefits of metal-dielectric stacks.

Figure 4: Merit of three stack designs as a function of the angle of the V-grooves

Measurements We have designed and built a test system to evaluate the performance of coatings

designed for TPV systems. The system consists of an ion-pumped UHV chamber with a

heated substrate holder that can be rotated to measure emissivity as a function of angle. The substrate is optically accessible via a CaF window (IR transmissive). The emission of the substrates is measured using an FTIR spectrometer. The system was assembled and mated to an optical table that holds the optics and FTIR and we are awaiting installation of the FTIR by the vendor.

Nanofabrication Since joining the GCEP team Braun has obtained promising preliminary data.

Through electrodepostion, Braun has already demonstrated the fabrication of three-dimensional inverse face centered cubic metallic photonic crystals. These structures exhibited rather unique optical properties, which could be tuned as a function of the metal filling fraction (structural openness) of the structures (Fig. 5) [2].

Figure 5. SEM images, optical reflection and emissivity of nickel inverse opal structures as a function of metal filling fraction. The lattice constant in all the SEM images is 2 µm.

Figure 6. Left, SEM of a silica based colloidal crystal partially infilled with tungsten. Middle, SEM of the result after the majority of the tungsten has been removed. The structure now consists of doughnut shaped tungsten disks suspended by the colloidal

template in a 3D array. Right, the optical properties of this structure as a function of the number of layers in the colloidal crystal.

Under GCEP support, Braun has been focusing on increase the upper operating temperature of these structures by fabricating them out of more refractory structures, for

Decreasing Metal Filling Fraction

example tungsten, and on increasing the obtainable structural diversity, since the inverse face centered cubic structures are unlikely to exhibit the optical properties required for TPV applications. The Braun group now has the ability to conformably deposit tungsten inside of a 3D photonic crystal template, as well as to etch away some of the tungsten to create more open structures (Fig. 6). This structural openness is important as Braun first realized in his work on nickel photonic crystals.

Current efforts are focusing on holographic templating methods, which have the potential to create virtually any 3D structure. Once this capability is realized, Braun will be able to fabricate metallic photonic crystals based on Fan’s theoretical designs.

Conclusions Solar thermo-photovoltaics has a theoretical efficiency that is twice the Schockley-

Queisser limit, using single junction cell. Thus it represents a significant opportunity in renewable energy research. In addition, many of the developments that we are pursuing would also have impacts in TPV systems in general, as well as in solar thermal applications.

Publications 1. E. Rephaeli and S. Fan, “Nanostructured Tungsten absorber and emitter for solar thermal

photovoltaics”, OSA Topical Meeting on Solar Energy: New Materials and Nanostructured Devices for High Efficiency, paper STuB, Stanford, CA, 2008.

References

1. E. Rephaeli and S. Fan, “Tungsten black absorber for solar light with wide angular operation range”, Applied Physics Letters 92, 211107 (2008).

2. X. Yu, Y. -J. Lee, R. Furstenberg, J. O. White, and P. V. Braun: Filling fraction dependent properties of inverse opal metallic photonic crystals, Advanced Materials, 19, 1689-1692 (2007).

Contacts

Shanhui Fan, Shanhui@stanford.edu.