Characterizing randomness in photonic glasses using autocorrelation functions of two-dimensional images.

We have developed a simple method to quantify randomness in photonic glasses in relation to the ideal random limit, using autocorrelation functions obtained from two-dimensional images. In our case, the photonic glasses consist of randomly packed silica microspheres which serve as a model system representing isotropic random media. Conventional methods of characterizing randomness in photonic materials often entail technical complexities, such as chemical functionalization, three-dimensional rendering, and particle tracking. Our method circumvents these difficulties based on a mathematical relation that we derive between the autocorrelation function and the radial distribution function. This relation enables us to find the autocorrelation function in the ideal random limit. The autocorrelation function of experimentally fabricated photonic glasses is then obtained from images of a single cross-sectional plane and directly compared to that of the ideal limit. The comparison shows that the autocorrelation function of real structures deviates only slightly from the ideal limit. We find that the deviation can be explained in part by the microsphere polydispersity. Our general method would be useful in characterizing a large class of photonic random media, encompassing biological materials, radiative cooling coatings, and random lasing photonic glasses.

Control of Randomness in Microsphere-Based Photonic Crystals Assembled by Langmuir-Blodgett Process.

Photonic structures in biological systems typically exhibit an appreciable level of disorder within their periodic framework. However, how such disorder within the ordered framework renders unique optical properties has not been fully understood. Toward the goal of improving this understanding, we have investigated Langmuir-Blodgett assembly of microspheres to controllably introduce randomness to photonic structures. We theoretically modeled the assembly process and determined a condition for surface pressure and substrate pulling speed that results in maximum structural order. For each surface pressure, there is an optimum pulling speed and vice versa. Along the trajectory defined by the optimum condition, however, the structural order decreases moderately as the pulling speed increases. This moderate decrease in structural order would be useful for controlled introduction of randomness into the periodic structures. Departing from the trajectory, our experiment reveals that a small change in pulling speed at a given surface pressure can significantly disrupt the structural order. For multilayer assembly, we find that, at a fixed pulling speed, the surface pressure should increase as the number of layers increases to achieve maximum structural order. In totality, we quantitatively present the optimum trajectories for the nth layer assembly relating surface pressure and pulling speed.

Suppression of infrared absorption in nanostructured metals by controlling Faraday inductance and electron path length.

Nanostructured metals have been intensively studied for optical applications over the past few decades. However, the intrinsic loss of metals has limited the optical performance of the metal nanostructures in diverse applications. In particular, light concentration in metals by surface plasmons or other resonances causes substantial absorption in metals. Here, we avoid plasmonic excitations for low loss and investigate methods to further suppress loss in nanostructured metals. We demonstrate that parasitic absorption in metal nanostructures can be significantly reduced over a broad band by increasing the Faraday inductance and the electron path length. For an example structure, the loss is reduced in comparison to flat films by more than an order of magnitude over most of the very broad spectrum between short and long wavelength infrared. For a photodetector structure, the fraction of absorption in the photoactive material increases by two orders of magnitude and the photoresponsivity increases by 15 times because of the selective suppression of metal absorption. These findings could benefit many metal-based applications that require low loss such as photovoltaics, photoconductive detectors, solar selective surfaces, infrared-transparent defrosting windows, and other metamaterials.

Symmetry-breaking nanostructures on crystalline silicon for enhanced light trapping in thin film solar cells.

We introduce a new approach to systematically break the symmetry in periodic nanostructures on a crystalline silicon surface. Our focus is inverted nanopyramid arrays with a prescribed symmetry. The arrangement and symmetry of nanopyramids are determined by etch mask design and its rotation with respect to the [110] orientation of the Si(001) substrate. This approach eliminates the need for using expensive off-cut silicon wafers. We also make use of low-cost, manufacturable, wet etching steps to fabricate the nanopyramids. Our experiment and computational modeling demonstrate that the symmetry breaking can increase the photovoltaic efficiency in thin-film silicon solar cells. For a 10-micron-thick active layer, the efficiency improves from 27.0 to 27.9% by enhanced light trapping over the broad sunlight spectrum. Our computation further reveals that this improvement would increase from 28.1 to 30.0% in the case of a 20-micron-thick active layer, when the unetched area between nanopyramids is minimized with over-etching. In addition to the immediate benefit to solar photovoltaics, our method of symmetry breaking provides a useful experimental platform to broadly study the effect of symmetry breaking on spectrally tuned light absorption and emission.

15.7% Efficient 10-µm-thick crystalline silicon solar cells using periodic nanostructures.

Only ten micrometer thick crystalline silicon solar cells deliver a short-circuit current of 34.5 mA cm(-2) and power conversion efficiency of 15.7%. The record performance for a crystalline silicon solar cell of such thinness is enabled by an advanced light-trapping design incorporating a 2D inverted pyramid photonic crystal and a rear dielectric/reflector stack.

Two-dimensional metamaterial transparent metal electrodes for infrared optoelectronics.

We examine the optical properties of two-dimensionally nanostructured metals in the metamaterial regime for infrared applications. Compared with straight nanowires and nanogrids, serpentine structures exhibit much lower optical losses of less than 7% even at a large metal area fraction of 0.3. The low loss is primarily due to a small effective conductivity of the meandering structures, and self-inductance plays a modest role in reducing losses in these structures. The high transparency at a large metal area coverage would be useful for transparent electrodes in optoelectronic devices.

Efficient light trapping in inverted nanopyramid thin crystalline silicon membranes for solar cell applications.

Thin-film crystalline silicon (c-Si) solar cells with light-trapping structures can enhance light absorption within the semiconductor absorber layer and reduce material usage. Here we demonstrate that an inverted nanopyramid light-trapping scheme for c-Si thin films, fabricated at wafer scale via a low-cost wet etching process, significantly enhances absorption within the c-Si layer. A broadband enhancement in absorptance that approaches the Yablonovitch limit (Yablonovitch, E. J. Opt. Soc. Am.1987, 72, 899-907 ) is achieved with minimal angle dependence. We also show that c-Si films less than 10 µm in thickness can achieve absorptance values comparable to that of planar c-Si wafers thicker than 300 µm, amounting to an over 30-fold reduction in material usage. Furthermore the surface area increases by a factor of only 1.7, which limits surface recombination losses in comparison with other nanostructured light-trapping schemes. These structures will not only significantly curtail both the material and processing cost of solar cells but also allow the high efficiency required to enable viable c-Si thin-film solar cells in the future.

Toward the Lambertian limit of light trapping in thin nanostructured silicon solar cells.

We examine light trapping in thin silicon nanostructures for solar cell applications. Using group theory, we design surface nanostructures with an absorptance exceeding the Lambertian limit over a broad band at normal incidence. Further, we demonstrate that the absorptance of nanorod arrays closely follows the Lambertian limit for isotropic incident radiation. These effects correspond to a reduction in silicon mass by 2 orders of magnitude, pointing to the promising future of thin crystalline silicon solar cells.

Optical absorption enhancement in silicon nanohole arrays for solar photovoltaics.

We investigate silicon nanohole arrays as light absorbing structures for solar photovoltaics via simulation. To obtain the same ultimate efficiency as a standard 300 microm crystalline silicon wafer, we find that nanohole arrays require twelve times less silicon by mass. Moreover, our calculations show that nanohole arrays have an efficiency superior to nanorod arrays for practical thicknesses. With well-established fabrication techniques, nanohole arrays have great potential for efficient solar photovoltaics.

Efficient low-temperature thermophotovoltaic emitters from metallic photonic crystals.

We examine the use of metallic photonic crystals as thermophotovoltaic emitters. We coat silica woodpile structures, created using direct laser writing, with tungsten or molybdenum. Optical reflectivity and thermal emission measurements near 650 degrees C demonstrate that the resulting structures should provide efficient emitters at relatively low temperatures. When matched to InGaAsSb photocells, our structures should generate over ten times more power than solid emitters while having an optical-to-electrical conversion efficiency above 32%. At such low temperatures, these emitters have promise not only in solar energy but also in harnessing geothermal and industrial waste heat.